Anal. Chem. 1997, 69, 231R-250R
Surface Characterization G. E. McGuire,* P. S. Weiss, J. G. Kushmerick, J. A. Johnson, Steve J. Simko, R. J. Nemanich, Nalin R. Parikh, and D. R. Chopra
Center for Microelectronics, MCNC, 3021 Cornwallis Road, P.O. Box 12889, Research Triangle Park, North Carolina 27709 Review Contents Proximal Probes Scanning Tunneling Microscopy Atomic Force Microscopy Near-Field Scanning Optical Microscopy Other Scanning Probe Microscopies Electron Spectroscopy X-ray Photoelectron Spectroscopy Auger Electron Spectroscopy Optical Characterization of Surfaces Ion Beam Analysis Rutherford Backscattering Spectrometry Microbeam Analysis Elastic Recoil Detection Particle-Induced X-ray Emission Nuclear Reaction Analysis Channeling Applications Nontraditional Applications of Ion Beams Summary X-ray Techniques Appearance Potential Spectroscopy Glancing-Angle X-ray Diffraction Extended X-ray Absorption Fine Structure Literature Cited
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Reviews of surface characterization have appeared in Analytical Chemistry every two years since 1977 (1-10). During this time, the field has grown significantly in the volume of papers published as well as the number of applications and diversity of surface characterization tools. This review is similar to the last one in this series, being written by multiple authors with specialties in an attempt to highlight advances in each of these areas. This review begins with literature from January 1995 and ends with literature from approximately October 1996. PROXIMAL PROBES A review of proximal probe techniques and applications is presented. Proximal probes have become well established as surface characterization tools over the 16 years since the initial development of the scanning tunneling microscope (STM) (A1). As proximal probe techniques have matured and become more accessible, the volume of literature has skyrocketed. Due to the large volume of scanning probe microscope (SPM) literature, this review attempts to highlight important advances in the field as opposed to providing a complete bibliography. Specifically, reports solely on surface topography and structural determinations, as well as papers on superconducting substrates, are not included. Since the Journal of Vacuum Science and Technology B published the complete proceedings of the Eighth International Conference on Scanning Tunneling Microscopy/Spectroscopy and Related Techniques, papers from that conference will be excluded from this review (A2). Means of chemical identification and differentiation remain paramount issues in developing these S0003-2700(97)00009-7 CCC: $14.00
© 1997 American Chemical Society
instruments as analytical tools. While some progress on this front has been made, as described below, no general methods of identification have yet been demonstrated. Scanning Tunneling Microscopy. In the last two years, several books and reviews have been published on scanning tunneling microscopy, including Martin’s Selected Papers on Scanning Probe Microscopies, Design and Application (A3) and Magonov and Whangbo’s Surface Analysis with STM and AFM: Experimental and Theoretical Aspects of Image Analysis (A4). Rohrer discussed the history as well as techniques and applications of the STM (38 references) (A5). Freenstra reviewed scanning tunneling spectroscopy techniques and data interpretation (51 references) (A7). Avouris discussed atomic-scale manipulation with the STM (72 references) (A7). Hamers reviewed the theory and applications of the STM, the atomic force microscope (AFM), and the near-field scanning optical microscope (NSOM) as they relate to chemical processes (280 references) (A8). Lieber and co-workers reviewed atomic manipulation of inorganic materials with a SPM (117 references) (A9). Weiss and co-workers discussed methods of observing motion on surfaces with the STM (55 references) (A10). Delamarche et al. reviewed characterization of self-assembled monolayers (SAMs) by the STM (108 references) (A11). Bottomley et al. reviewed scanning probe microscopy for the 1996 Analytical Chemistry Fundamental Reviews issue (>500 references) (A12). Due to their many possible applications, including as chemical sensors, optical devices, and lubricants as well as many others, self-assembled films continue to be intensely investigated. Even for the best characterized systems, alkanethiols on Au(111), the imaging mechanism remains incompletely understood. The films form an insulating layer, as seen from electron transfer studies of SAM-coated Au electrodes; however, resolution of the molecular lattice is possible in STM images. Takami et al. reported molecularly resolved differentiation of individual terminal functional groups in multicomponent SAMs on Au(111) (A13). Methyl-, hydroxyl-, and azobenzene-terminated alkanethiol and dialkyl disulfide SAMs on Au(111) were investigated with a STM operating at high tunneling gap imepdance (large tip-sample separation). Topographic features in bicomponent SAMs were correlated with the exposed terminal functional groups, thus establishing that topographic features observed with the STM relate to the molecules making up the film and not just their surface attachment (A13, A14). Poirier and Tarlov studied the temporal evolution of solutiondeposited butanethiol SAMs on Au(111) using a room temperature ultrahigh-vacuum (UHV) STM (A14). Poirier and Tarlov observed that the surface evolved from a large number of small pits to a small number of large pits, indicating Au migration and an Ostwald ripening mechanism of pit size distribution. McDermott et al. investigated the structural origins of the pits in alkanethiol SAMs on Au(111) with both scanning tunneling and atomic force Analytical Chemistry, Vol. 69, No. 12, June 15, 1997 231R
microscopies (A15). McDermott et al. determined that the pits resulted from a reconstruction of the topmost layer of the Au(111) surface and were not due to the structure of the SAM. Poirier and Pylant used a room temperature UHV STM to observe the nucleation and growth of SAMs of vapor-deposited mercaptohexanol on Au(111) (A16). Poirier and Pylant found that, at low coverages, the alkanethiols adopt a lattice gas phase, typified by rapid diffusion. When the surface concentration of alkanethiol molecules reached a critical density, stable alkanethiol islands nucleated preferentially in areas of unfaulted Au stacking. Poirier and Pylant also observed that island growth is accompanied by a compression of the Au(111) herringbone reconstruction. Methyl-terminated SAMs are known to pack in (x3 × x3)R30°-based hexagonal arrangement on Au(111). The packing of ω-substituted alkanethiol SAMs shows deviation from this standard. Wolf et al. examined the packing of 6-[4-(phenylazo)phenoxy]hexane-1-thiol on Au(111) using an AFM in ethanol and STM operated at high gap impedance (A17). Wolf et al. found that steric repulsions due to the bulky end groups cause incommensurate lattice packing of the molecules. Poirier and coworkers examined the packing of vapor-deposited hydroxyfunctionalized hexanethiol SAMs using a room temperature UHV STM (A18). Poirier and co-workers found that mercaptohexanol forms a commensurate (3 × x13) overlayer on Au(111). Poirier and co-workers also studied the effect of adsorbed H2O on the mercaptohexanol SAM and found that, over time, H2O caused a disordering of the film into a polymorphic phase interspersed with linear features. Venkataraman et al. imaged alcohols, alkanethiols, alkyl chlorides, and dialkyl disulfides on graphite with a STM (A19). The S-H and S-S functional groups appeared as protrusions in the images, whereas the O and Cl were indistinguishable from the hydrocarbon backbone. Cyr et al. studied monolayer films of primary substituted hydrocarbons CH3(CH2)nCH2X (X ) CH3, OH, NH2, SH, Cl, Br, I; n ) 16-30) on graphite (A20). The molecules packed into well-ordered two-dimensional films with their molecular axes parallel to the graphite surface. The terminal functional groups NH2, SH, Br, and I again appeared as protrusions in the STM images. Cyr et al. proposed a model based on adsorbate polarizability to explain the image contrast. Andres et al. employed fully conjugated aryl R,ω-dithiols to tether gold nanocrystals to Au(111) substrates (A21). Current-voltage measurements performed using a room temperature UHV STM revealed a “Coulomb staircase” indicative of a Coulomb blockade. In order to make atomic and molecular electronic devices a reality, a means to form electrical contacts between various components will be necessary. Along these lines, molecular wiresssingle molecules capable of conducting currentshave received considerable recent attention. Bumm et al. used dodecanethiol SAMs to isolate single ethyl-substituted 4,4′-bis(phenyleneethynylene) benzothiolate, a molecular wire candidate (MWC), in order to investigate its properties with a STM and a microwave frequency alternating current STM (A22). The highfrequency conductivity of the tunnel junction was measured by a microwave frequency difference technique. The difference frequency signal was larger over the MWC, which may be related to higher conductivity of the MWC compared to that of the surrounding dodecanethiolate film. Yazdani et al. measured the resistance of a single Xe atom and two Xe atoms in series between a W STM tip and a Ni(110) surface at 5 K (A23). The measured 232R
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resistance, as well as that calculated for a jellium surface, was higher than that predicted for ideal one-dimensional conduction. The higher resistance is attributed to the Xe 6s resonance, through which tunneling occurs, which lies well above the Fermi level of the Ni substrate. SAMs of alkanethiols are stabilized by van der Waals interactions among the neighboring hydrocarbon backbones. There are other mechanisms by which adsorbate-adsorbate interactions can have important directing effects for film growth. Kamna et al. reported that benzene adsorption at step edges of Cu(111) could be explained in terms of substrate-mediated adsorbate-adsorbate interactions (A24). Adsorbed benzene molecules perturb the surface electronic structure, thereby directing the adsorption of subsequent molecules. Trost et al. found, through analysis of STM images and Monte Carlo calculations, that for N on Ru(0001), nearest-neighbor and next-nearest-neighbor repulsions existed while the third-nearest-neighbor site was weakly attractive (A25). The presence of short-range repulsive interactions explains the lack of island formation for this system. Successive adsorption of binary adsorbate systems also produced interesting results. Stensgaard et al. studied the interaction of benzene with a Ni(110) surface with preadsorbed oxygen using a room temperature UHV STM (A26). Stensgaard et al. found that benzene adsorption compressed the -Ni-O-added rows from a (3 × 1) to a (2 × 1) structure and occupied the troughs created by the compression. Batteas et al. found that the coadsorption of S and CO greatly affected the structure of the stepped Pt(111) surface (A27). Upon initial adsorption of S, step bunching occurs, and the p(2 × 2) S phase is formed on terraces separated by biatomic height steps. Subsequent adsorption of CO displaces S, compressing it to 31/2 times the lattice spacing of Pt(111), and forms new terraces with exclusively CO adsorbed, thus breaking the biatomic steps into monatomic height steps between alternating S- and CO-covered Pt terraces. Such restructuring could have important implications for catalysis due to the generation of step sites. Crew and Madix investigated the reaction kinetics and active sites for oxidation of CO on Cu(110) at 400 K with a variable temperature UHV STM (A28). Crew and Madix found that there is a strong preference for CO to react with oxygen in the p(2 × 1) oxygen rows which form on this surface. The reaction initiates at the edge of an oxygen island, creating a kink in the superstructure. These kinks then become the dominant reaction site for subsequent CO oxidation. Buisset et al. used a low-temperature UHV STM to image isolated features on the unreconstructed regions between p(2 × 1) oxygen regions on Cu(110) at 4K (A29). These features, which were assumed to be oxygen atoms, occupied hollow sites in the troughs between the p(2 × 1) reconstruction. These individual oxygen adatoms are proposed to be an active form of oxygen known to participate in deprotonation reactions on copper surfaces. Probing adsorbate diffusion and surface dynamics with atomic resolution is of prime importance for understanding the mechanisms of thin-film growth and surface catalysis. However, since the time scales for diffusion are often too rapid for direct observation, experimental conditions have to be tailored to enable accurate measurements. To this end, many creative experiments for monitoring adsorbate motion have been undertaken. Zambelli et al. determined that step edges are the active site for NO dissociation on Ru(0001) using a room temperature UHV STM (A30). Zambelli et al. then exploited this system to enable direct
measurement of N diffusion on Ru(0001) (A31). STM images of NO dosed onto atomically clean Ru(0001) substrates revealed that dissociation occurs only at step edges. O showed up as streaks in the STM images due to rapid diffusion, but the N atoms were well resolved. By monitoring the concentration profile of N over time with respect to the step edge, Zambelli et al. determined the activation energy (0.94 eV) and preexponential factor (2 × 10-2 cm2 s-1) for diffusion. Measurements of the hopping rate of N were also taken, and the calculated diffusion constant was identical to the Fick’s law value. It is important to understand the role the STM tip plays in adsorbate motion, in order to eliminate artifacts from the measurement process. Li et al. investigated the influence the STM tip has on atomic motion of Ag on Ag(110) at 50 and 295 K using an UHV STM (A32). Li et al. found that, at 295 K, the STM tip can significantly perturb the equilibrium configuration of step edges, even when low-field conditions are employed. At 50 K, Ag atoms could be manipulated preferentially in the [110] direction. Swartzentruber used an atom-tracking technique to increase the temporal resolution of the STM in order to monitor Si dimer diffusion on Si(001) using a room temperature UHV STM (A33). The STM tip was dithered in a circle, after locking onto a dimer, in order to generate a two-dimensional error value by which the tip position above the Si dimer was maintained. The atom-tracking method enabled continuous monitoring of diffusion of Si dimers. Bott et al. reported combining STM imaging and Monte Carlo calculations to determine the activation energy and attempt frequency for diffusion of adsorbates (A34). Diffusion is often modeled as consisting of hops between adjacent adsorption sites. In contrast to this, Wolkow and Moffat observed that benzene diffusion on Si(111)-(7 × 7) consisted of long jumps farther than nearest-neighbor site distances using a room temperature UHV STM (A35). A “break-before-make” mechanism for diffusion, where the benzene molecule is in a weakly bound excited state, was proposed to account for the longrange diffusion. Go´mez-Rodrı´guez et al. studied the dynamics of Pb atoms on Si(111)-(7 × 7) (A36). Pb atoms diffuse rapidly, appearing as blurs, inside each half (7 × 7) unit cell. By recording individual jumps between neighboring unit cells, the activation barrier (0.64 ( 0.07 eV) and prefactor (106(1 s-1) for diffusion between half cells was determined. Morgenstern et al. studied the motion of vacancy islands on Ag(111) with a STM and theoretical calculations (A37). It was determined that vacancy island motion is dominated by adatom diffusion across the vacancy island rather than around the edge. Transient mobility of adsorbates on solid surfaces has also been studied due to its possible role in catalytic processes. Transient mobility is nonequilibrium motion on a surface due to energy released from surface processes such as adsorption or dissociation. Wintterlin et al. studied the dissociation of O2 on Pt(111) using a variable temperature UHV STM (A38). The relative spacing of oxygen atoms, formed from dissociative adsorption of O2 on Pt(111), was determined with a STM at temperatures from 150 to 160 K, where thermally activated diffusion is quenched. The observed distribution was consistent with a hot atom mechanism whereby some of the dissociation energy was imparted as kinetic energy to one or both of the O atoms. The length scale of transient mobility observed was two to three lattice constants. Manipulation of matter on the atomic scale and the construction of nanometer-scale devices by the STM is being actively
explored. Shen et al. used inelastically tunneling electrons to desorb H selectively from the Si(100)-(2 × 1):H surface (A39). Electrons tunneling inelastically between the probe tip and sample desorbed H with atomic resolution, creating dangling bonds on the surface. Martel et al. probed the molecular precursor to stable Si oxidation by selectively cleaving oxygen-oxygen bonds through resonant capture of tunneling electrons in empty energy levels of O2 on Si(111)-(7 × 7) (A40). The resonantly trapped electrons couple to vibrational excitations of the O2 molecules and, depending on bonding site, can lead to dissociation or desorption. Persson and Avouris discussed the theoretical aspects of localized bond breaking induced by vibrational excitation due to inelastically tunneled electrons (A41). Positioning of individual adsorbates on surfaces has been possible for some time. The actual mechanisms involved, however, are not completely understood. Jung et al. studied the mechanism for tip-induced positioning of Cu-TBP-porphyrin on Cu(100) at room temperature (A42). In this instance, movement was achieved through “pushing”, repulsion between tip and adsorbate, of the molecule with the STM tip. Molecular mechanics simulations suggest that flexure of the molecule precedes molecular translation, and this adjustment of molecular conformation effectively lowers the barrier to motion. The ability to position adatoms with a STM tip can be used to create a map of surface sites. Meyer et al. dosed a Cu(211) surface with CO and then mapped out the surface sites near an adsorbed CO molecule with a native Cu adatom removed from a nearby step edge using the STM probe tip (A43). From this map, Meyer et al. were able to determine the adsorption site of CO, even though the Cu(211) lattice could not be imaged directly. Knowledge of the adsorption site of CO then allowed Meyer et al. to determine the adsorption sites of Pt and C2H4, which were coadsorbed with CO on Cu(211). Adsorption site information can be obtained directly by simultaneously imaging the adsorbate and underlying lattice. Bradshaw and co-workers determined the adsorption site of C2H4 on Cu(110) at 4 K with a low-temperature UHV STM (A44). The advantage of determining the adsorption site directly is that any influence of the coadsorbate is removed. The STM is inherently a spectroscopic probe. Although many studies are carried out at single bias voltages or only over a narrow range, exploration of the current-voltage characteristics of systems is often required in order to understand STM images fully. Avouris et al. investigated the manner in which steps and adsorbates modify the electronic surface states of Au(111) and Ag(111) by scanning tunneling spectroscopy (STS) (A45). STS revealed enhancements at the bottom of steps and depressions on top of steps in the local density of filled states for Au(111). Spectra of S on Ag(111) revealed different structural features at different bias voltages, reinforcing that structure in constant current STM images cannot always be attributed to topographic features. Spectroscopy can be used to differentiate between chemically distinct surface species. Stroscio et al. reported an intense feature in the tunneling spectra of both Fe(001) and Cr(001) (A46). Band calculations determined that the features were due to a nearly unperturbed d orbital intrinsic to bcc(001) metal surfaces. Davies et al. then exploited this spectroscopic feature to monitor the growth of Cr thin films on Fe(001) with chemical specificity (A47). Jung et al. were able to differentiate between Cu and Mo with ∼1 nm spatial resolution by using a sample bias of +5 V or greater Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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(A48). The contrast was attributed to the difference in work functions of Cu and Mo which affects their relative image state energies. Knowledge of the tunneling barrier is required in order to enable quantitative analysis of STM images. To further the understanding of the tunneling barrier in STM experiments, Olesen et al. measured the apparent tunneling barrier height (ATBH) between a W tip and Au(110), Ni(100), and Pt(100) samples (A49). Unlike previous studies, Olesen et al. found that the ATBH does not decrease at small tip-sample separations but remains constant until point contact. Olesen et al. measured current, voltage, and impedance directly as the tip was pushed toward the surface. The bias voltage between the tip and sample did not remain constant but decreased at small tip-sample separations due to the finite input impedance of the current preamplifier. If the change in bias voltage had not been accounted for, Olesen et al. would have measured a decreasing ATBH in accord with previous studies. Preparation of a well-defined atomically sharp STM tip is paramount to enable well-resolved images. To this end, experimentalists go to great lengths to develop techniques which reliably form high-quality STM tips (A50). One approach is to attach a foreign species to the end of the tip. This species then defines the physical and electronic structure of the tip. Smalley and coworkers reported attaching nanotubes to the ends of tips and AFM cantilevers (A51). The AFM nontube tip yielded exceptional lateral resolution of trenches etched in a TiN-coated aluminum film due to its high aspect ratio. A nanotube attached to a STM tip was capable of imaging the charge density waves on a freshly cleaved 1T-TaS2 surface. Kelly et al. adsorbed a C60 molecule onto the apex of a STM tip in order to image threefold symmetric electron scattering from point defects on a graphite surface (A52). The scattering pattern had been theoretically predicted yet was previously unobserved. Kelly et al. proposed that C60 alters the density of states near the Fermi level of the tip, thus enabling the scattering to be observed. Atomic Force Microscopy. The AFM has been used to characterize the surface morphology of a wide variety of samples. These include Langmuir-Blodgett (LB) films, SAMs, surfactants, and a plethora of biological materials. The capabilities of the AFM have also been extended to nanofabrication. Louder and Parkinson reviewed AFM instrumentation, operation, and application to surface characterization (A53). The formation and stability of LB films have been studied. Sikes et al. imaged three fatty acid LB monolayers on mica and SiO2 deposited at various pH values (A54). All three adsorbates formed islands when the substrate surface was negatively charged. However, at low deposition pH, the images appeared smooth and featureless. Sikes et al. concluded that substrate-induced interactions were significant in LB film organization (A54). The morphology and structure of single-layer and multilayer LB films were determined after different annealing cycles from 30 to 130 °C. Eleven-layer films agglomerated into micrometer-size crystals, and the domain packing changed in the annealing processes (A55). Stability of LB bilayer films of phospholipids were dependent on the pH of the solution in the AFM fluid cell upon scanning (A56). Vollhardt et al. imaged the surface pressure relaxation process of a monolayer of arachidic acid. Data were in accord with a nucleation-growth model based on a twodimensional (2D) to three-dimensional (3D) phase transition 234R
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(A57). Arachidic acid/cadmium arachidate multilayers were imaged in different pH solutions with an AFM by Kurnaz et al. Only films at higher pH were robust and molecularly resolved (A58). Scanning force microscopy and contact angle measurements of arachidic acid and derivatives were also performed. Penacorada et al. concluded that strong intermolecular forces dominated any influence of the underlying substrate on film morphology: the stronger the interlayer forces, the more stable the arachidic acid LB film (A59). Self-assembled film properties and structure have also been studied with the AFM. Jaschke et al. synthesized 10 different thiols and disulfides and imaged the resulting structures after selfassembly on Au(111) (A60). Samples included azo-functionalized SAMs and disulfide with hydrocarbon and fluorocarbon chains that were imaged in height, friction, and deflection modes. Individual lattice constants, lattice types, and commensurability of the monolayers were determined. When the azobenzenethiols were functionalized with ether-terminal functional groups, the lattice constant increased and the 2D film structure changed (A60). Caldwell et al. concluded that densely packed azobenzenethiol films exhibited limited electrochemistry (A61). However, coadsorption increased the free volume and enhanced the film’s redox activity. Liu et al. furthered this study by synthesizing a mixed SAM-LB film with enough free volume to be electrochemically active. The composite film also exhibited a reversible cis-trans conformation change when irradiated with ultraviolet light. This photochemistry had been previously unattainable in single-component SAM or LB systems. The AFM images revealed that codeposited SAM and LB films were unstable, but the SAMLB composite film was uniformly distributed on the gold surface (A62). The AFM, with appropriate tip preparation, can image surfactants. When the tip is coated with surfactant prior to imaging, double-layer forces repel the similar material on the surface. This soft force feedback allows the discrimination of surface structure without disturbing the adsorbate. Surfactants on silica, mica, and hydrophobic surfaces such as graphite and MoS2 have been studied. Manne and Gaub studied the role of the surface in interfacial interactions by varying aggregation geometry and relating pH to adsorbate density (A63). An interesting hemicylindricalshaped film on graphite, reported by Manne et al., was reproduced with sodium dodecyl sulfate. Wanless and Ducker also induced desorption of a surface contaminant using the AFM tip (A64). Advances in sample preparation and the tapping mode AFM have accelerated the progress of imaging biological samples. There have been several reviews on the subject (A65-A67). Bezanilla et al. imaged DNA with an AFM in tapping and contact modes (A68). The negatively charged mica substrate needed a buffer with a divalent cation to bind the negatively charged DNA. Minerals containing divalent cations in their lattices were tested as substrates, but the interactions were weaker and the results were difficult to explain. DNA was stretched and strongly adsorbed to mica-coated (3-aminopropyl)triethoxysilane (APS) because the APS had a positive surface charge at neutral pH. This electrostatic interaction immobilized the DNA effectively, but there were difficulties in achieving appropriate surface smoothness (A68). Hu et al. adsorbed DNA on APS-mica using a “molecular combing” sample preparation technique (A69). Nucleation processes of proteins were studied by Malkin et al. (A70). Thaumatin commonly formed 2D islands but also grew
at spiral dislocations under low supersaturation conditions. Island growth kinetics were estimated for this system (A70). A lipid bilayer was also removed by an AFM tip, but its interior protein remained bound to the mica substrate. Schabert et al. measured a conformational change of the protein and imaged other surface phenomena (A71). Another conformational change was imaged in real time in living pancreatic cells by Schneider et al. (A72). Morphological transformations in the plasma membrane were observed during cellular activity. To image biological materials, a low-temperature AFM has been developed (A73). The low temperatures reduce surface contamination and increase the mechanical strength of biological samples (A74). The force amplified biological sensor has also been designed to perform an assay using an antibody-coated cantilever and magnetic particles that will bind to analytes (A75). The displacement of the cantilever with the magnetic particles and detectable species on them is then measured in a magnetic field (A75). The AFM has been used to measure surface forces. The elastic properties of human platelet cells were estimated using the force-dependent cantilever deflection and the deviation of the deflection that constituted the sample indentation (A76). From these two variables, Radmacher et al. mapped out force curves and could determine the elastic moduli of the local areas of the cell (A76). The AFM tip has also been modified to monitor adhesion and frictional forces. Adhesion was measured by contacting the tip to a surface and slowly lowering the sample until the tip disconnected. Green et al. used a SAM-covered tip on a SAM monolayer while electrochemically monitoring a redox reaction. The tip adhered strongly while the reaction was active but felt weaker forces when only one redox component was present (A77). Hudson and Abruna used the same experimental design with an electroactive polymer on the tip that interacted with several substrates. Thus, chemical interactions could be characterized as the tip was electrochemically controlled (A78). Adhesive molcules from a marine sponge were analyzed for their binding forces (A79). Noy et al. studied the adhesive and frictional forces of different functionalized SAMs with a chemical force microscope capable of measuring frictional forces via lateral force analysis. Both forces exhibited the same dependence on functionality, highlighting the strength of hydrogen bonding in adhesion and friction (A80). Less conventional samples that have been characterized by an AFM include C60, zeolites, and polymers. A fullerene film exhibited an irreversible rearrangement to porous, rodlike structures upon compression (A81). Komiyama et al. estimated the pyridine ring tilt angles on a zeolite from AFM images (A82). Kumaki et al. imaged a copolymer on mica where single particles could be discriminated from longer stringlike chains (A83). Hu et al. demonstrated a new imaging technique to study water films (A84, A85). The scanning polarizable force microscope (SPFM) exploits a charged AFM tip and long-range electrostatic forces to allow imaging at tens of nanometers separation, thus preventing wetting of fluid to the tip. Dielectric differences between liquid thin films and their underlying solid substrates account for image contrast. The water adsorption process on mica was imaged over time while the ambient humidity was increased. Two phases were observed. The first (I) was completed at 2228% humidity, and the second (II) was completed at 40-50% humidity. The dielectric constant was lower in phase II than in
I, indicating a lower surface ion mobility (A84, A85). A reaction was imaged by depositing sulfuric acid on aluminum, forming the aluminum sulfate corrosion product. At 30% relative humidity, the product was still a solid, and only local regions were affected. The solution product mixed with the acid to swell and to spread at higher relative humidity, signifying the onset of reaction (A86). The SPFM has been used to manipulate liquid droplets by increasing the tip force and maintaining a constant tip-sample distance, and also by decreasing the distance at constant bias voltage. KOH droplets adsorbed preferentially on terrace steps and were irreversibly pinned at those sites. On flat terraces at higher humidity, however, droplets could be moved controllably across mica and graphite surfaces (A87). The SPFM has also been used to measure dielectric polarization forces on insulators (A88). Other instrumental modifications to the AFM include replacing the cantilever on the AFM with a 50 nm diameter glass pipet to create a tapping mode scanning ion conductance microscope (SICM). Proksch et al. fit the capillary with an electrode to detect ionic currents through its aperture. Feedback was accomplished by monitoring the ionic signal in constant voltage mode. The SICM was used to image a polycarbonate surface (A89). Sugawara et al. imaged defect motion on InP(110) with an UHV AFM (A90). Jarvis et al. reported a modified UHV AFM with magnetically controlled feedback to counteract any “jump to contact” of the tip. The cantilever stiffness was changed with force control so as to measure smooth, reversible, and stable potentials of the local bonding environments. Applications of this instrument include measuring interatomic potentials, studying different tip elasticity and forces, and tip crash suppression (A91). AlGaAs/GaAs multilayers have been frequently studied in UHV, but Prohaska et al. designed a new fluid cell to image such samples in toluene. The sample holder was made without polymer components that swell in organic solvents. The degradation of an oxide layer with acid was recorded (A92). Nanofabrication has been performed with the AFM. Snow and Campbell made a titanium wire constrained to 5-10 nm via an AFM anodic oxidation (A93). The feedback loop to the AFM tip was the electrical resistance of the sample. This allowed control of the width of the wire while simultaneously measuring tip current and voltage. Undesirable Ti-O-Ti junctions, formed via oxide swelling, were also reported (A93). Mu¨ller et al. coated a tip with platinum to catalyze hydrogenation of surface azide groups of a SAM to amine groups. Further surface modification selectively attached a fluorescent label or aldehyde-modified latex bead to the amine-terminated SAMs (A94). This surface was then imaged with a scanning laser confocal microscope. Mu¨ller reported converting 10% of the azide using the AFM catalysis (A94). Near-Field Scanning Optical Microscopy. Applications of the stand-alone NSOM have been limited since the best far-field instruments yield nearly as high resolution but perturb the system under study to a lesser extent. There are several exceptions where the NSOM has proven uniquely able to make relevant measurements. Hwang et al. imaged pressure-induced phases of lipid monolayers with the NSOM and found local florescence differences omitted by far-field epifluorescence microscopy (A95). Grabar et al. compared and contrasted the instrumental capabilities of the NSOM, AFM, TEM, and FE-SEM in imaging gold Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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colloid surfaces (A96). Bian has computed tip-sample interaction effects on the emission of molecules measured with the NSOM, accounting for perturbations due to the coated NSOM probe (A97). A pseudoisocyanine (PIC) crystal was studied by a NSOM that probed to 50 nm below the PIC surface. Vanden Bout et al. concluded that small PIC crystals or films differ in structure from larger samples previously studied by X-ray crystallography (A98). The NSOM spectra demonstrated line narrowing instead of the inhomogeneous broadening found in far field (A98). Higgins et al. imaged excitons in a J-aggregate of PIC (A99). The molecular orientation of the film, as well as an exciton migration distance of 50 nm, was determined. Hybrid NSOM techniques include the scanning interferometric aperatureless microscope (SIAM), the low-temperature NSOM, and the polarization NSOM. SIAM operates in transmission mode by back-illuminating a thin sample with laser light that is focused on the vibrating tip of an AFM located on the sample’s opposite side. The scattered electric field due to dipole-dipole coupling from the tip and sample is detected with an optical interferometer. SIAM yields resolution similar to that of the AFM but images different surface defects (A100). Harris et al. constructed a lowtemperature NSOM with a fiber capable of simultaneous optical excitation and detection and measured the photoluminescent excitation (PLE) and decay of InGaAs strained layer quantum wires (A101). PLE spectra were comparable to far-field measurements. However, the signal from the quantum well component of the cleaved edge overgrowth layers could be distinguished from the quantum wire with the NSOM (A101). A polarizationdependent NSOM sums the fluorescence signal from each dipole illuminated by the tip (A99). A polarization-modulated NSOM that rotates the light polarization at high frequencies to detect optical anisotropies of samples has also been demonstrated (A102). Other Scanning Probe Microscopies. Other scanning probe techniques include electrochemical techniques, such as the scanning electrochemical microscope (SECM). The SECM uses a very small electrode to probe the faradaic currents from tip and sample interactions. The SECM measured heterogeneous electron transfer rates and interfacial properties using ultramicroelectrodes (UMEs) to probe the interface between two immiscible electrolyte solutions (ITIES). Tsionsky et al. demonstrated that a platinum UME oxidized a species in the organic phase that was reduced at the ITIES with a reducing agent in the aqueous phase (A103). The redox components were found not to penetrate into the solvents. In situ measurements indicated the formation of an insoluble complexation thin film at higher salt concentrations (A103). Wei et al. described the preparation of a gallium electrode for an amperometric UME combined with an ion selective electrode (A104). The density of electrochemically active bound antigens to antibodies has been explored by Wittstock et al. using the SECM in collection mode (A105). Shiku et al. performed two redox reactions in two separate solutions on an enzyme substrate to control and image local surface features (A106). The SECM has also been integrated with an AFM to image the dissolution of KBr crystals in solution (A107). Nanotribological uses of the AFM have been reviewed (A108). Further progress in this field requires better definition and control of interaction parameters such as contact size and probe asperities (A109). 236R
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ELECTRON SPECTROSCOPY X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are mature techniques for studying the surface composition and surface chemistry of solid materials. As such, they are basic tools used in a wide variety of applications. Use of these techniques to study material problems continues to grow. Over 5000 English language papers related to XPS and AES have been published since the last review. This report concentrates on recent trends in the use of these techniques to solve materials problems. A review of the basic fundamentals of XPS and AES has been published recently by Turner and Schreifels (B1). The Handbook of Surface Imaging and Visualization, edited by Hubbard, has been published (B2). Chapters devoted to XPS and AES are included as well as chapters covering several related surface imaging techniques. Work continues in the surface analysis community to produce standard reference materials, collections of reference data, and standard procedures for reporting and sharing data. Brandl et al. have developed a toolkit for converting surface analysis data to a standardized format (B3). Seah and Gilmore are developing a database of AES spectra (B4). Data are collected using a new instrument calibration procedure which removes instrument artifacts from the data. Strohmeier proposed a Cu/Ag/Au alloy for calibrating the energy scale of XPS instruments (B5). This material replaces the three separate metal specimens commonly used to calibrate XPS spectrometers. Superlattices composed of AlAs/GaAs were proposed as reference materials for sputter depth profiling (B6). X-ray Photoelectron Spectroscopy. XPS continues to be an important tool for measuring surface chemistry and surface composition in support of a wide variety of technical areas. Review articles on many applications of XPS have recently been published. The contribution of XPS to adhesion science has recently been reviewed (B7, B8). Studies of buried metal/polymer interfaces by XPS were reviewed by Wu (B9). An important aspect of adhesion science is an understanding of the acid-base properties of a material’s surface. Recent progress in the use of XPS to make surface acid-base measurements was reviewed by Chehimi and Delamar (B10). XPS analyses of carbon fibers for composites (B11), polymer weathering (B12), self-assembled monolayers (B13), Galena surfaces (B14), and zeolites (B15, B16) have been reviewed. A major disadvantage of the XPS technique is its inability to directly detect hydrogen. However, recent progress on indirect detection methods that systematically measure the chemical shifts of the elements that hydrogen is bound to was reviewed by Kerber et al. (B17). XPS characterization of catalyst surfaces after treatment in a high pressure reactor was reviewed by McIntyre et al. (B18). The spatial resolution of XPS continues to improve, which is increasing the applications of this technique in small-area analysis, elemental mapping, and chemical state imaging. Work is progressing in three distinct areas. In the first approach, conventional XPS instruments are now available with high spatial resolution (spot size in the 10 µm range). The use of these modern instruments to study microscopic contamination (B19) and for failure analysis has been reviewed (B20). Evans et al. used XPS imaging to study patterned self-assembled monolayers on gold surfaces (B21). Hallam and Wild report on the use of imaging XPS to study grain boundary segregation in metals (B22). The
advantage of XPS over the more traditional AES technique for this work is the ability of XPS to provide chemical state information. Imaging XPS has also been used to study adhesive joint failures (B23) and transfer films in tribological studies (B24). Zupp et al. reported on attempts to obtain quantitative results from XPS imaging experiments (B25). In the second approach to XPS imaging, several groups are using synchrotron X-ray sources to perform photoemission microscopy at very high spatial resolution (up to 0.1 µm). Recent progress in photoelectron microscopy has been reviewed by several authors (B26-B28). Schneider et al. used this technique to study magnetic domains in multilayer sandwich structures (B29). Ko et al. studied patterned Si/SiO2 with photoelectron microscopy (B30). The capabilities of a new photoelectron microscopy system at ELETTRA in Europe were described by Casalis et al. (B31). In the third approach to XPS imaging, a laser-plasma X-ray source is used to produce a focused X-ray spot on the specimen (B32, B33). A spatial resolution of 20 µm was demonstrated in photoelectron images from laser-ablated silicon, which show that the oxide was removed from ablated regions. Newer XPS instruments with brighter X-ray sources and higher count rates collect data much faster, which is allowing scientists to study dynamic events using XPS. Gantner et al. have modified an XPS system to allow time-resolved measurements during laser melting of surfaces (B34). Reduction and reoxidation of bismuth molybdate surfaces was studied by Uchida and Ayame (B35). Activation energies for reoxidation of bismuth and molybdenum were reported. Jeong et al. studied electrochromic HxWO3 films as a function of injected charge (B36). Gengenbach et al. studied the surface of a plasma-deposited polymer over an extended period of time (B37). They developed a systematic curve-fitting routine to measure the complex chemical changes that occurred as the polymer aged. Scientists continue to develop new XPS measurement techniques to study the mobility of polymer surfaces. Yasuda et al. measured the redistribution of chains in a Teflon-perfluorovinyl ether copolymer immersed in water (B38). Ether groups in the perfluorovinyl ether chains migrated to the surface after water immersion. Lukas et al. studied the effect of water immersion on two methacrylates using a different approach (B39). Comparisons were made between polymers in the dry state and materials studied in a frozen hydrated state. Hong and Boerio measured the ability of epoxies to displace or absorb oil residues on metal surfaces (B40). The distributions of functional groups in a liquid perfluoro ether were measured by Pradeep (B41). XPS continues to be an important technique for studying microelectronic devices and materials. XPS contributions to microelectronics (B42) and to the characterization of the SiO2/Si system have been reviewed (B43). As the feature size on microelectronic devices continues to shrink, the chemical composition of the sidewalls of features is becoming more important. Several groups have developed angular resolved XPS measurement techniques to characterize these hard-to-measure regions (B44B46). The methods take advantage of geometric shadowing of photoelectrons to differentiate the signals that emanate from the top, sidewall, and bottom of features. The interface states in an oxide layer were measured by XPS as a function of bias in metal oxide semiconductor (MOS) devices by Kobayashi et al. (B47). Auger Electron Spectroscopy. A special issue of Microscopy, Microanalysis, Microstructures was dedicated to Pierre Auger, the discoverer of the Auger effect (B48). Review articles covering
AES imaging, AES of insulators, AES analysis of local electronic structure, and the use of AES in the semiconductor industry are included. Applications of AES to geochemistry were reviewed by Nesbitt and Pratt (B49). Improved methods of AES imaging were reviewed by Prutton et al. (B50). AES continues to be an important technique for studying surface phenomena on metals. Slavin reviewed the growth modes of metal films on dissimilar substrates (B51). Thermal and ion irradiation-induced segregation in metal alloys was reviewed by Igata (B52). Stress voiding and electromigration in aluminum alloys were reviewed (B53). Electrochemical oxidation of noble metals was reviewed by Conway (B54). Analysis of thin films and buried interfaces by sputter depth profiling continues to be an important application of AES. Recent work is aimed at improving the quantitation and depth resolution of the technique. Mathieu reviewed progress in these areas (B55). Garten and Bubert used a systematic set of depth profile analyses on three binary alloy systems to develop matrix effect factors for improving the quantitation of depth profiles (B56). Mixing simulations were used to model the effects of atomic mixing and preferential sputtering on depth profiles from a GaAs/ AlAs multilayer (B57). Wenham et al. used principal component analysis and target factor analysis to examine two multilayer metal systems (B58). They identified extra factors which were due to the depth dependence of inelastic scattering, which affects the accuracy of quantitation. Neural pattern recognition was proposed as a simpler alternative to target factor analysis for analyzing depth profiles (B59). The proposed method requires no standards, and a fully automatic algorithm is presented. Sputter-induced artifacts were eliminated from depth profiles of CdHgTe films by making beveled sections and measuring AES line scans across the specimen (B60). Inoue et al. achieved high depth resolution by eliminating crater edge effects from a depth profile (B61). They prepared the analysis area as a plateau by capping the region with an erosion-resistant film, followed by chemical etching to remove the surrounding material. An interesting application of AES is the study of dynamic surface events. Reichl et al. studied the diffusion of sulfur, oxygen, and nitrogen in iron as it was heated from 200 to 830 °C (B62). Li et al. measured different diffusion rates of O along different crystallographic directions in single crystal Zr (B63). Chambers et al. developed a fast AES to measure the surface composition of films as they were being grown in a molecular beam epitaxy (MBE) system (B64). The high spatial resolution that can be achieved with AES makes it an extremely useful technique for studying problems in the microelectronics industry. Kibalov and Smirnov described a method for characterizing the etch residues at the bottom of high aspect ratio via holes (B65). They used an electrostatic deflector on the front of a cylindrical mirror analyzer (CMA) to collect and analyze Auger electrons from the bottom of the pits. A different approach to analyzing etch residues at the bottom of via holes was proposed by Hoener et al. (B66). They sectioned the specimen using a focused ion beam to allow AES analysis at the base of the via hole. The ability of AES to characterize submicrometer-size particles was examined by Ito et al. (B67). They determined the electron beam conditions which yielded optimum spatial resolution. The ability of AES to characterize submicrometer-size particles was compared to those of two other techniques by Childs et al. (B68). AES was superior to scanning energy Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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dispersive X-ray spectroscopy and SIMS for elemental mapping and chemical identification at high spatial resolution (0.1 µm particles). Yamada et al. described an AES system with improved beam optics and a probe tracking system that yielded clear images of a 40 nm thick gate oxide (B69). Childs et al. have developed an automated high spatial resolution AES that can be used to characterize submicrometer-size defects on microelectronics circuits (B70). Rosen determined the mechanism of hillock growth on aluminum interconnects (B71). The growth of Al2Cu precipitates on Al-Cu interconnects was imaged using AES by Barkshire and Prutton (B72). Zhou et al. determined the local density of states at the GaAs/Si interface using AES (B73). Factor analysis was used to correct distortions in the peak shapes caused by ion sputtering. New applications of AES for the analysis of insulating or poorly conducting materials continue to appear. Sekine et al. employed a cold stage to minimize electron irradiation damage during AES analysis of polymers (B74). AES was used to characterize the surface composition changes caused during ion beam irradiation of a SiC ceramic (B75). Eick et al. studied the interface between a dental adhesive and dentin using AES (B76). OPTICAL CHARACTERIZATION OF SURFACES Optical measurements have proved to be a valuable method of surface characterization. Raman scattering and infrared (IR) spectroscopy measurements of the vibrational modes have become the most commonly used optical methods of characterizing surface properties. The application of these techniques to thin-film and surface structures is often difficult because of sensitivity limitations of the techniques. Other techniques, such as ellipsometry, reflectance difference spectroscopy, and nonlinear laser techniques, are also finding application in the characterization of surfaces. Continued advancement of the optical measurements has been stimulated by the advantage of nondestructive and noncontact measurements which are often relatively straightforward to interpret. This review will highlight some of the applications of these techniques. While disordered and microstructured surfaces are important for many applications, the fundamental processes are most often demonstrated at ordered surfaces, and as such, the structures at ordered surfaces will be emphasized. In prior reviews, the application of Raman and IR spectroscopy to the characterization of diamond films, fullerene materials, and high-temperature superconducting materials has been noted, and these techniques continue to be critical in the evaluation of these and other materials and the growth processes. This review will highlight new developments of optical characterization methods in semiconductors, insulators, and molecular adsorbates. While surfaces of silicon have been well studied, the importance of integrated circuit (IC) technology has led to renewed interest in the Si surface under various process-related conditions. The structure of the HF etched surfaces have been studied by Raman spectroscopy (C1). Polarization- and angle-dependent measurements were employed, and the results were supported by theoretical modeling. The results suggested that there is stress relaxation at the steps and that this effect contributes to the etching process. Other Raman studies indicated an unusually large Raman cross section of hydrogen vibrations for hydrogenterminated Si(111) surfaces (C2). The effect was not due to resonance, and the Raman transition dipole matrix elements are 3 times larger than those for molecular SiH4. One advantage of 238R
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IR spectroscopy is that the technique can be employed for realtime in situ analysis of the surface etching process (C3). The results confirmed the presence of surface-bonded F if a concentrated HF etch is employed, while water rinsing or use of dilute HF results in a H-terminated surface. For the (111) surface, only Si-F2 bonding (no Si-F) was identified (C4). The same technique has been employed to examine the etching in an ammonium fluoride solution (C5). For (100) surfaces, the dihydride termination was observed, but continued etching resulted in exposure of monohydride (111) microfacets. Another novel process being considered to engineer Si substrates is to employ wafer bonding. The technique involves joining two wafers with appropriately prepared surfaces and then annealing at temperatures above 1000 °C. To explore the effects of various surface terminations and to examine the bonding process, IR spectroscopy has been employed (C6, C7). The measurements focused on the role of water and Si-O, SiO-H, and Si-H bonding. One of the key steps in multilevel interconnects in IC fabrication is chemomechanical polishing. Recent IR results have reported the interplay of local oxidation by OH and passivation by hydrogen (C8). The active region of the field effect transistor relies on the oxide-silicon interface. The kinetics of the oxidation of hydrogen-terminated surfaces have been studied by IR spectroscopy (C9). The results suggest that the stability in air or the initial oxidation rate is greatly dependent on the humidity of the air. Optical anisotropy measurements have also been used to explore the clean surface, the hydrogenpassivated surface, and the initial oxidation (C10). The adsorption of phosphine on Si(100) has been studied with IR spectroscopy (C11). The results indicated the presence of surface P and H after phosphine adsorption. A new development for improved IR sensitivity is the use of a buried metal layer substrate (C12). The technique involves implantation of Co to produce a buried layer of CoSi2. Raman spectroscopy was employed to study defects after ion implantation. In one study, the reduction of Raman intensity was related to the formation of vacancies in the near-surface region (C13). In studies of ion implantation of Si and GaAs, a phonon confinement model or phonon localization was used to analyze the Raman line shape of the LO mode (C14, C15). In studies of MBE growth, Raman scattering was applied as an in situ probe of the growth of CdTe on InSb (C16). The modulation of the intensity was related to the optical interference and could be used to monitor the film thickness, while the Raman spectra were used to observe the interface bonding. Similar techniques were employed to examine the formation of the ternary compound ZnSxSe1-x on GaAs substrates (C17) and the formation of Sb contacts on ZnSe epitaxial layers (C18). The free carrier concentration at the interface of ZnSe on GaAs was studied from the Raman line shape of the GaAs-coupled LO phonon-plasmon mode using different exciting wavelengths (C19). IR spectroscopy has been used to monitor molecular adsorbate processes at compound semiconductor surfaces. The sites for arsine adsorption on GaAs have been determined for different surface reconstructions (C20). Similarly, the adsorption sites of trimethylgallium on c(2 × 8) GaAs(001) surfaces have been explored (C21). Multiple internal reflection was used to determine the sites for H adsorption on c(2 × 8) and -(1 × 6) GaAs(001) (C22). Real-time in situ techniques have been developed for monitoring plasma exposures of GaAs surfaces. IR spectroscopy has identified O and H bonding configurations (C23), and optical reflection spectroscopy has been used to study the damage and other properties of the surface during etching (C24). Raman spectros-
copy was also applied to examine properties of films. The strain at the top surface and substrate interface was explored for GaAs grown on CaF2 (C25). The large band gap of the CaF2 allowed measurement through the substrate. The properties of oval defects for Si-doped GaAs were studied with microscale Raman (C26). The technique was used to determine the carrier concentration through analysis of the LO phonon-plasmon-coupled mode frequency. Theoretical calculations along with Raman measurements were used to analyze the lattice dynamics of sulfurpassivated InP(001) (C27) and Sb-terminated GaAs or InP (C28). The results identified surface layer vibrations in both cases. Spectroscopic ellipsometry and Raman scattering were used to examine the hydrogen interaction with Sb-terminated GaAs or InP surfaces (C29). The results suggested etching that was initiated at defect sites. The strain in InAs islands on InP was studied by Raman and TEM (C30). With the recent demonstration of blue light emission from diodes based on GaN, there has been significant interest in optical characterization of these materials. The surface reflectance was used as an in situ monitor of the nitridation of GaAs (C31). Raman spectroscopy has been studied as a potential technique to characterize these films. Results have studied the interface structure (C32), the strain (C33), and the free carrier concentration (C34). Many researchers have now adopted these approaches. The techniques can also be extended to examine InN (C35). The initial stages of diamond film growth on Si substrates may involve the formation of a SiC layer. Recent IR measurements have demonstrated the ability to detect ultrathin layers of SiC (C36). As noted above, Raman spectroscopy is often used to characterize the quality of diamond films. A recent microscale Raman study found very large stress differences between adjoining crystalline domains (C37). The properties of diamond surfaces may prove critical to many potential applications ranging from abrasion to electron emission. The C-H surface bonding was explored using visible-IR sum frequency generation spectroscopy (C38). In other studies, IR spectroscopy was employed to characterize the H termination of the surfaces (C39). In one study, the vibrations of physisorbed CO2 were employed to observe the presence of defect sites (C40). IR was also employed to examine the structure of C2H2 adsorbed on diamond. This molecule is often considered as the precursor to diamond film growth (C41). The material boron nitride bears many similarities to diamond in both structure and properties. For this material, IR measurements are most often used to distinguish between the various phases that can occur in film growth (C42). Surface studies of amorphous thin films are also critical for understanding how they are affected in various processes. Recent IR measurements have established that the surface of amorphous Si is terminated by a dimerized monohydride structure similar to that of Si(100) surfaces (C43). IR reflectance has also been used as an in situ probe of the H surface bonding in the deposition of hydrogenated amorphous Si (C44). The technique has demonstrated that the H incorporation is different for the initial stages of a Si-H film growth. To obtain higher electron mobility semiconducting films, microcrystalline Si films are often considered. Raman spectroscopy is commonly used to characterize these films, and the technique has recently been applied to very thin films to study the initial stages of film growth (C45, C46). In situ IR has also been used to study the surface reactions during
plasma deposition of SiO2 (C47), and the results indicate that O-H coverage occurs for low-temperature plasma oxide deposition. The properties of an oxide surface exposed to CH3OH have also been related to the O-H surface bonding (C48). The oxidation at metal surfaces continues to be an important area for many applications. For Mo(100) surfaces that had been oxidized, it was found that Raman spectra were obtained after only ∼0.6 nm and indicated the presence of MoO2 features (C49). In situ FT-IR has been employed to examine the oxidation of organic molecules on single-crystal TiO2 (C50). It was found that the reconstruction of the TiO2 affected the oxidation of the organic molecules. In a very novel technique, thin high-temperature superconductor films were characterized in a Febry-Perot resonator (C51). The system exhibited sharp resonances below 6 × 1012 Hz. The interactions of organic molecules on surfaces of high-temperature superconducting materials were studied using an FT-Raman system with 1064 nm excitation (C52). The technique of reflection-absorption IR spectroscopy (RAIRS) has proved particularly useful for characterization of molecular adsorbates on metal and other surfaces. An electrochemical cell can be used to obtain RAIRS from Au(100) surfaces under different potentials (C53). The results indicate variations in the surface reconstruction. The site-dependent vibrational couplings of CO adsorbates on stepped Pt surfaces were measured for different coverages (C54, C55). In situ IR was used to explore electrolytes adsorbed on a Pt(111) electrode in acid solution (C56). The RAIRS technique was combined with work function measurements to characterize interfacial cation solvation by ammonia on Pt(111) surfaces (C57). While the results explored solvation effects, the measurements were carried out in an UHV environment. In studies of CO adsorption on Cu(100), an IR laser diode was employed for the reflection-absorption spectroscopy (C58). An unusual mode softening of the C-H vibration was observed for cyclohexane on Mo(110) surfaces (C59). Other measurements indicated intense Fermi resonances in the C-H stretch region of adsorbed methoxide on Mo(110) (C60). Low-frequency IR measurements of isotopic mixtures of CO adsorbed on Cu surfaces displayed features attributed to hindered rotations and carbonmetal modes (C61). Time-resolved measurements were used to examine the kinetics of the decomposition of methoxide on Ni(100) surfaces (C62). The IR measurements of H on Cu(111) showed strong features related to the parallel vibrational modes (C63). The enhancement was attributed to excitation of electronhole pairs. The vibrational spectra of CO adsorbed on Ru(001) indicated a two-phonon bound state of the internal stretching mode (C64). This unusual feature is related to the anharmonicity of the vibrations. More complex photoinduced reactions of methane on Pt(111) were excited with an ArF laser and measured by RAIRS (C65). The results are related to the symmetry of the surface. Reversible phases of chemisorbed and physisorbed CO on Cu(100) were also identified (C66). Steric effects and rotational isomerization were recognized in studies of isomerization of ethyl formate on Ni(111) (C67). In IR studies on NaCl substrates, an amorphous monolayer was identified for adsorption of SO2 (C68). In contrast, N2O adsorption leads to ordered monolayer structures (C69). There continue to be many advances in the techniques themselves. Methods to enhance the IR absorption spectra have been quantitatively analyzed (C70). Sum frequency generation is being employed in UHV and underexposure conditions (C71Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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C73). In laser ablation deposition techniques, the process itself generates large pressures that can be measured with coherent anti-Stokes Raman spectroscopy (CARS) (C74). In addition, waveguide CARS can be used to explore monolayer films on waveguide structures (C75). The waveguide structure was also employed to enhance the Raman signal in planar glass waveguides (C76). Contactless electroreflectance is increasingly being used to characterize III-V semiconductor device structures (C77). The highest lateral spatial resolution Raman measurements reported have been obtained using a near-field scanning optical microscope (C78). Raman microscopy has also been obtained with plasmon surface polaritons (C79). ION BEAM ANALYSIS The main areas of ion beam analysis (IBA) can be divided into Rutherford backscattering spectrometry (RBS), elastic recoil detection (ERD), particle-induced X-ray emission (PIXE), nuclear reaction analysis (NRA), accelerator mass spectrometry (AMS), and channeling applications of these various methods. Whereas the first four of these methods are generally restricted to determinations of elemental composition-depth profiles of nearsurface regions of materials, channeling applications of these methods are often included for determinations of structures, lattice imperfections, and lattice sites. For example, the quality of epitaxial layers, the structure of surfaces, the nature, depth, and annealing of radiation or implantation damage, and the lattice position of impurity atoms or impurity-defect complexes have been studied. Low-energy elastic scattering of ions from surfaces has also been used extensively for surface structure studies. In the following review, these studies will be grouped with the RBS or channeling data. The various IBA topics form the basis of a long-standing series of international conferences and also are often included within more topical conferences. In the years 1995-97, proceedings of the following conferences were published: Second International Conference on Computer Simulation of Radiation Effects in Solids, Santa Barbara, CA, July 24-29, 1994 (D1); Thirteenth International Conference on the Application of Accelerators in Research and Industry, Denton, TX, Nov 7-10, 1994 (D2); Twelfth International Conference on Ion Beam Analysis, Tempe, AZ, May 22-26, 1995 (D3); Seventh International Conference on PIXE and its Applications, Padua, Italy, May 26-30, 1995 (D4); Sixteenth International Conference on Atomic Collisions in Solids, Linz, Austria, July 1721, 1995 (D5); Fourth European Conference on Accelerators in Applied Research and Technology, Zu¨rich, Switzerland, Aug 29Sept 2, 1995 (D6); Eighth International Conference on Radiation Effects in Insulators, Catania, Italy, Sept 11-15, 1995 (D7); Seventh International Conference on Accelerator Mass Spectrometry, Tucson, AZ, May 20-24, 1996 (D8); E-MRS 1996 Spring Meeting: New Trends in Ion Beam Processing of Materials, Strasbourg, France, May 28-31, 1996 (D9); Tenth International Conference on Ion Beam Modification of Materials (IBMM), Albuquerque, NM, Sept 1-6, 1996 (D10); and MRS 1996 Fall Meeting Symposium A, Materials Modification and Synthesis by Ion Beam Processing, Boston, MA, Dec 1-6, 1996 (D11). In 1992, a valuable reference book was published, the Encyclopedia of Materials Characterization (D12), which describes ion beam spectroscopies as well as optical, electron beam, X-ray emission, nuclear, and mass spectroscopies. A major new publication in the field of IBA has appeared in the Handbook of Modern Ion Beam Materials Analysis, edited by Tesmer et al. (D13), 240R
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published by the Materials Research Society (MRS). This book is a greatly expanded version of the Catania Handbook, which has been the standard reference in IBA for many years. Also, a new book, Forward Recoil Spectrometry: Applications to Hydrogen Determination in Solids, edited by Tirira et al. (D14) has been published recently. Rutherford Backscattering Spectrometry. In the last two years, further developments in the areas of microbeam analysis, high detection sensitivity, and microelectronics materials have been made. Microbeam Analysis. At present more than 40 microbeams or microprobes are in operation with lateral resolution of 10-6 or smaller. Takai et al. (D15) give applications of nuclear microprobes in semiconductor industries. Doyle of Sandia National Laboratory (D16) presented a summary of the current status of nuclear microprobe techniques, describing facilities, techniques, applications, and insights into requirements of the technique in the 21st century. Secondary electrons emitted from the microbeam gave a SEM-like image of the sample, while other detectors used PIXE and RBS for elemental analysis. Breese et al. (D17) described ion beam-induced charge microscopy for analysis of integrated circuits. It was shown that ions which are stopped within the depletion layers generated charge pulses which are much more sensitive to ion-induced damage than longer range ions which are stopped in the substrate. King et al. (D18) showed that ion channeling can be used to produce images of individual crystal defects, such as dislocations and stacking faults, using a scanned focus ion beam. Scho¨ne and co-workers (D19) at Sandia reported the first results of time-resolved ion beam-induced charge collection (TRIBICC) using ∼1.5 MeV/amu He, C, and Si ions incident on Si integrated circuits. Ion beam-induced charge collection (IBICC) is developed to elucidate the process involved in single-event upset (SEU) of integrated circuits. With electronics devices, dimensions are advancing from micro- to nanometer scale: new submicrometer resolution probes are being exploited by Butz and Legge (D20), who give auspices and horizons of nanoprobe technology. Elastic Recoil Detection. Barbour and colleagues (D21) at Sandia applied low-velocity, high-Z projectiles for elastic recoil detection analysis and showed that this, in combination with a time-of-flight technique, can greatly improve depth sensitivity compared to those of conventional RBS and ERD. ERD with extremely heavy ion beams (230 MeV Bi) was explored by Foster (D22). Due to Zi4 dependence of the ERD cross section, only a small beam current is required, which helps in minimizing ion damage of the sample. Also, with this heavy projectile, almost all elements can be detected simultaneously. Interdiffusion in polymer films was studied by Endisch et al. (D23) using an 8.5 MeV C beam. The measured depth resolution was better than 15 nm, and the total depth range for hydrogen was more than 500 nm. Schiettekatte et al. (D24) showed that quantitative depth profiling of light elements by means of the ERD and the E × B technique considerably enhances the resolution by eliminating the straggling induced by the absorber. Also, the E × B filter allows the simultaneous detection of different particles such as H, D, and He. Walsh and Doyle (D25) enhanced the sensitivity of traditional ERD by combining the well-known resonance technique in the 1H(12C,p)12C reaction to profile hydrogen in materials. Parikh et al. (D26) showed that depth resolution in ERD of H and D can be improved by using deflecting electric and magnetic fields in a forward-scattered path.
Particle-Induced X-ray Emission. In its approximately 24 year history, the PIXE technique has reached a maturity stage and penetrates diverse fields of research, giving multielemental analysis of a variety of samples. The Seventh International Conference on PIXE and its Applications was held in Padua, Italy, May 26-30, 1995, and the proceedings were published in Nuclear Instruments and Methods in Physics Research B 118 (D4). PIXE and particle-induced γ ray emission (PIGE) techniques are applied in a variety of fields to determine trace impurities. Cohen et al. (D27) applied PIXE in conjunction with the IBA technique to study elemental analysis of fine atmospheric particles. The multielemental capability of PIXE enabled authors to define fingerprints of various fine-particle sources. Afarideh et al. (D28) employed multielemental determination of deferroxamine drug (DPO), a widely used during therapy for patients with bthalassemia-Major and elemental variations in blood serum in hyperbiliru binamia newborns before and after blood transfusion. PIXE with complementary ion beam analytical techniques was used Swietlicki et al. (D29) to analyze the elemental composition of atmospheric aerosols. The application of PIXE to geological and mineralogical samples was shown by studying volcanic sediments by Khandaker et al. (D30). Determination of elemental compositions of papers, inks, and pigments using external PIXE is a well-established technique for the study of ancient manuscripts. Jarjis (D31) introduced a new field of study encompassing standard PIXE, proton backscattering, and imaging using scanning proton beams in both in-vacuum and external modes. Radiationinduced segregation in 304 stainless steel by PIXE and RBS channeling was studied by Kawatsura et al. (D32). Nuclear Reaction Analysis. Lennard (D33) employed the 14N(d,a)12C nuclear reaction in conjunction with a step-etching technique to depth profile nitrogen in the near-surface region (510 nm) of silicon oxynitride samples, obtaining a detection limit of ∼4 × 1013 cm-2. Harmon et al. (D34) applied newly developed neutron elastic recoil detection (NRED) to detect hydrogen isotopes in samples which cannot be put in the vacuum. Combining RBS with NRA and/or ERD is commonly done to enhance depth resolution and or elemental sensitivity. This was shown by simultaneous NRA and RBS measurements on oxygen stoichiometry in oxide samples using precision stoichiometry standards by Amsel et al. (D35), using 16O(d,p)17O NRA at 150° and RBS with deuterium beam at 165°. The results obtained from these measurements are independent of film thickness and beam doses, with precision in the 1% range. Terwagne et al. (D10) showed that carbon and hydrogen can be measured simultaneously using combined NRA and ERD techniques induced by 3He particles. Ila and co-workers (D36) applied the NRAchanneling technique to investigate formation of stress fields and their effects on various constituents of the LiNbO3 crystals in addition to RBS. Anderson and colleagues (D37) used 7.6 MeV 16O(4He,4He) resonance to study the anomalous channeling behavior of YBa2Cu3O7-x near Tc. They observed narrowing of oxygen dips higher than Tc, indicating either an increase in oxygen vibration amplitude or oxygen displacement. Poker et al. (D38) used d(3He,p)4He nuclear reaction to measure surface coverage of deuterium on clean Cu surfaces. They claimed that a few hundredths of a monolayer of deuterium can be detected using this technique. Hofsaess (D39) wrote a chapter on elastic recoil coincidence spectroscopy in the recently published book, Forward Recoil Spectrometry: Applications to Hydrogen Determination in
Solids. Simultaneous multielement depth profiling by R-γ coincidence particle spectroscopy, which retains the quantitative depth profile capability of RBS and is capable of simultaneously profiling many elements that are lighter than the matrix, has been developed by Correll et al. (D40). This technique uses 9 MeV R particles, which gives poorer depth resolution (a factor of 3) compared to 2 MeV RBS; however, the depth over which the impurity can be profiled is greater by the same factor. This was demonstrated by profiling 400 keV Fe implanted into Co. Channeling Applications. The channeling mode of backscattering spectroscopy has been widely used in many IBA techniques. Some of these applications are noted for their special significance. Williams et al. (D41) applied NRA-channeling to estimate the depth and width of damage created by ion implantation in LiNbO3. Meyer and colleagues (D42) used medium energy ion scattering (MEIS) combined with channeling to study the initial growth phenomena and the crystalline quality of YBaCuO ultrathin films deposited on SrTiO3 and MgO substrates. The determination of the coverage as a function of depth revealed that, on both substrates, the films grow in blocks of one unit cell. However, on SrTiO3, the growth of additional layers is initiated only after the completion of the preceding layer, while on MgO, island growth is observed, with different coverage values on three layer levels appearing simultaneously. Structure and composition of heteroepitaxial Si1-x-yGexCy films grown on Si were analyzed by combining ion channeling and NRA at 4.265 MeV C beam by Hearne et al. (D43). Quintel et al. (D44) studied annealing of heavy ion implantation damage in diamond by emission channeling produced by implantation of radioactive 111In and 73As. Authors observed two distinct stages for annealing, the first between 300 and 600 K, and the second at 1100 K. Wahl et al. (D45) employed emission channeling from implanted 8Li to determine lattice location of Li in Si and Ge. In both substrates, Li implantation at room temperature leads to the occupation of the tetrahedral sites, and that at ∼400 K leads to the incorporation on bond-center sites. Nontraditional Applications of Ion Beams. In materials science and semiconductor research, ion beam techniques have become a major tool for research and manufacturing. Now these techniques are expanding steadily into diverse fields like biomaterials, environment, archaeology, geoscience, etc. In addition, novel IBA methods are being applied to these new studies. A review of innovative IBA methods for studying the environment was published recently by Malmqvist (D46). (See also Maenhaut (D47) for a general survey of nuclear methods for environmental studies.) In it, he describes “ion beam thermography”, in which the temperature-dependent decomposition of chemical compounds present in aerosols is combined with PIXE or NRA to identify the compounds (D48). Coincidence methods, by which an emitted charged particle from a nuclear reaction is detected in coincidence with an emitted photon (giving a “photontagged” nuclear reaction), can reduce background radiation by orders of magnitude (D49). The lack of chemical information via traditional IBA can also be overcome by combining IBA with ion-induced luminescence (D50). The use of bolometers, or calorimetric radiation detectors, was reviewed by Broniatowski (D51). They have potential application to high depth resolution studies. Summary. Novel trends in the analytical applications of ion beam backscattering (also forward-scattering) spectrometry emphasize the point that it is possible to “tailor” the experimental conditions for a wide range of analytical problems. This is Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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achieved by considering the relevance of backscattering cross section data, the use of different incident projectiles having different energies, sample-detector and incident beam geometry, application of absorber, electric and/or magnetic fields, time-offlight on emerging particles, and the applications of broad, focused, and extracted ion beams. Both Rutherford and resonant scattering techniques are utilized in order to address questions related to diverse materials problems. X-RAY TECHNIQUES Appearance Potential Spectroscopy. For the study of the electronic structure of the surface, analytical techniques typically employ particles of low enough energy to probe only a few tenths of a nanometer into the solid, investigating the surface atoms rather than the bulk atoms. The techniques employed to investigate the distribution of density of states (DOS) above the Fermi level (EF) are X-ray absorption spectroscopy (XAS), inverse photoemission (IPE), Bremsstrahlung isochromat spectroscopy (BIS), and appearance potential spectroscopy (APS). This review focuses on the various aspects of APS which apply to surface characterization of materials. Several review articles have been published highlighting the application of this technique (E1). Appearance potential spectroscopy measures the probability for electronic excitation of a core level as a function of incident electron energy. It is based on two-electron excitation-relaxation process, and spectra yield information on the convoluted density of unoccupied states above the Fermi level. In an APS experiment, the energy of the incident electrons is linearly increased. As the incident electron energy exceeds the excitation threshold of a core level, the incident electron can lose energy to the core electron as the result of inelastic collision. The final energy states of the incident and core electrons will be above the EF. The core level DOS is considered to have a negligible energy width. The final states must take into consideration all possible combination of energies of both electrons that are consistent with the conservation of energy. The excitation is followed by radiative (X-ray fluorescence) and nonradiative (Auger electron) processes, which compete for the deexcitation of the core holes. The event is signaled by the “appearance” of a small bump in the signal strength versus incident electron energy curve. The sharp fluctuations in the total X-ray intensity corresponding to the appearance potentials of characteristic X-rays are extracted nondispersively from the Bremsstrahlung background by modulation techniques. Since APS represents the differential of the convoluted density of unoccupied states, it is particularly suitable for studying materials that have high DOS above EF. If the DOS is low or of disadvantageous character, APS cannot be utilized as a universal analytical technique. There are three main groups of elements that provide a strong signal: 3d transition metals, alkaline earth, and rare earths. When the intensity of the emitted soft X-ray is measured, the technique is called soft X-ray APS (SXAPS). Measurement of the total secondary electron yield is termed Auger electron APS (AEAPS). In another modification of the technique, the current of the elastically reflected electrons is measured. At the threshold of core level excitation, the number of electrons that undergo inelastic scattering disappears from the measured current. The method is appropriately called disappearance APS (DAPS). The excitation process is the same for all APS spectra. It seems that differences in the decay step are responsible for the differences in the features among SXAPS, AEAPS, and DAPS. Based on this model, the information obtained from DAPS 242R
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is free of the relaxation complications. SXAPS and AEAPS, however, include additional information dependent on their respective decay steps. Because of the threshold energies used in APS, the inelastic interaction excludes the incident electron from the complications of excitation and transport processes owing to many possible electron interactions. Another important aspect of APS is that it reveals a localized DOS because the matrix element governing the core hole production involves the very short range wave function of the initial core electron state. The applicability of AEAPS as an analytical tool for determining surface composition and depth profiling in a manner similar to AES is limited, since AEAPS is quite insensitive to a number of elements. APS provides information regarding the elemental identification, chemical bonding, density of unoccupied states, nearest-neighbor configuration in the surface layer, and the mechanism of excitation and transport processes occurring in the near-surface region of the sample. In this respect, APS, AES, and XPS are complementary techniques. Osiceanu and Vass (E2) reported the experimental and design parameters of a simple APS system. The noise sources are identified, and the signal-to-noise ratio is analyzed. A comparison between different types of analyses is made, and additional improvements are suggested. Further, an appearance potential study of titanium, titanium-hydrogen, and titanium-oxygen systems has been reported by Osiceanu and Vass (E3). The binding energies of 2p3/2 states are shifted for titanium hydride and titanium oxide. The density of unoccupied states above the Fermi level was obtained by spectral deconvolution. The authors propose a transition model for the deexcitation of the threshold excitation states in titanium oxide. Chourasia et al. (E4) have studied the GdMn2 compound by soft X-ray appearance potential spectroscopy. The spectra of the M4,5 levels of Gd and L2,3 levels of Mn in the intermetallic compound exhibit positive chemical shifts. Assuming the core level shifts result from charge transfer and hybridization, the authors obtained the hybridization effects between the 3d and 5d bands in the intermetallic compound. The 3d-5d hybridization plays a significant role in the magnetism observed in the intermetallic compounds. Suga et al. (E5) have discussed the UV and X-ray photoemission spectra, X-ray inverse photoemission, and X-ray absorption spectra of valence-fluctuating Sm3Se4 in the Sm 4d-4f excitation region in terms of the 4d104f5-4d94f6 and 4d104f6-4d94f7 multiple structures. The Sm 3d absorption spectra support the bulk valence fluctuation. The effective hybridization of Sm 4f states with conduction electron states is found to be large and much different from the 3d photoemission, 4d photoemission, and 4f BIS final states, resulting in the different intensity ratio of Sm2+ and Sm3+ signals in these spectra. Lu et al. (E6) have investigated the Coulomb correlation effects resulting from the oxidation of strongly correlated materials La, Ce and narrow-band materials Cr and Ti using ionization loss spectroscopy (ILS) and disappearance potential spectroscopy (DAPS). They report that chemical shifts in peak energies of La, Ce, and Cr observed by DAPS are greater than those observed with ILS during oxidation. The results are interpreted in terms of the differences in Coulomb correlation energies. Chourasia et al. (E7) have employed SXAPS to study the changes in the electronic structure of PrMn2 and SmMn2 intermetallics as a result of alloying. The SXAPS spectra of the M4,5 levels of Pr and Sm and L2,3 levels of Mn in these compounds exhibit chemical shifts when compared with corresponding elemental spectra. The
observed shifts in the core levels result from the charge transfer and hybridization effects between the Mn 3d and the rare earth 5d bands. The data yield values of hybridization shifts which show a decreasing trend attributed to the decrease in the number of electrons in the 3d-5d hybridization band with increasing Z of rare earths. Their results correlating the Mn magnetic moment with the number of electrons are consistent with the experimental data. Glancing-Angle X-ray Diffraction. X-ray diffraction (XRD) provides information regarding the crystal structure of bulk threedimensional solids. The information includes not only the atomic arrangement but also the electron distribution of the individual atoms. However, for near-surface analysis, the penetration of the X-ray beam is controlled by employing a grazing-angle geometry. This configuration enhances the surface sensitivity of the materials under investigation and allows thin films to be studied. Glancingangle XRD is a powerful tool to determine the structure of adatoms on the surface of the material. The strengths and limitations of glancing-angle XRD for surface characterization have been reviewed by several authors (E8). Liu et al. (E9) have investigated the relationship between the surface structure and the ferroelectric properties of PbTiO3 thin films using glancing-angle XRD. The data show a correlation between surface lattice distortion and spontaneous polarization. The structure properties of short-period MBE-grown p-i-n-doped Si17Ge2 superlattices were studied by Plotz et al. (E10) using X-ray diffractometry and X-ray reflectivity at glancing incidence. Their data show that the structures are pseudomorphic and yield information on the interface roughness parameter in terms of electron density profile. Zymierska (E11) proposed a theoretical model for determining the surface roughness by grazing-angle incidence X-ray reflectivity. His calculations are based on Fresnel theory and take into account damping. The data are presented for a whole range of wavelengths and different distributions of roughness on Fe and Ni surfaces. The structural and photocurrent-voltage characteristics of WO3 thin films on p-GaAs have been investigated by Yoon et al. (E12). Glancing-angle diffraction identified the formation of a monoclinic phase at 400 °C. Above 400 °C, small traces of Ga2O3 were observed, along with higher intensities of the monoclinic peaks. Electrical resistivity measured by the van der Pauw method was influenced by the crystalline nature and the interfacial states induced by the diffused atoms from each side of WO3 and GaAs. Miyagawa et al. (E13) have investigated the formation of Si3N4 by nitrogen implantation into SiC. They have studied the structure and chemical bonding state of the implanted layer by glancing-angle XRD and XPS. It was found that the maximum concentration and half-width of the nitrogen profile implanted at 1100 °C were strongly decreased in comparison with those at room temperature; nitrogen implantation into SiC at 1100 °C resulted in a composite layer of β-Si3N4 and SiC. Kimura et al. (E14) have studied surface imperfections of Si-on-insulator wafers by glancingangle X-ray reflection topography using tunable wavelength synchrotron radiation. The experimental topographs reveal a characteristic comparison of samples which cannot be observed by conventional X-ray topography. Oyanagi et al. (E15) describe a new apparatus for structural studies of surfaces and burried interfaces using synchrotron radiation. As a performance test, they report studies of the structure of Ge overlayers on Si(001) and a probe of the local structure of adatoms with ∼0.1 monolayer
sensitivity. Structural properties of RMn2Hx (R ) Y, Gd, or Dy) hydrides were studied by Przewoznik et al. (E16) using powder XRD at 300 K. Their data show that the latter parameters increase continuously upon H absorption, and the cubic crystal structure is preserved up to x ) 3.5 for all three systems. For the fully charged RMn2H4.3, the same type of rhombohedral distortion of the crystal lattice was found. For the H concentration range 3.5 < x < 4.3, coexistence of cubic and rhombohedral phases with various contents was found. A common systematic phase diagram was constructed for three RMn2Hx systems. Saito et al. (E17) have reported a new method for determining the atomic number density of a near-surface element in materials using grazing-angle X-ray reflection and anomalous dispersion effect. They have obtained atomic number densities of near-surface elements in ZrO2-Y2O3 crystals, Cr thin films grown on a glass substrate, and passivated stainless steel. The variation of the critical angles of total external reflection through the anomalous dispersion effect was clearly detected in their measurements. Nakaura et al. (E18) have carried out XRD of YBa2Cu3Oy compounds at several points near the Cu K-edge. In both compounds, the absorption edge of Cu ions in superconductor Cu(1) y ) 6.90, and that in nonsuperconductor Cu(2) y ) 6.07 is nearly equal to that of CuO. Changes in the O content of YBa2Cu3Oy produce no significant changes in the valence state of Cu ions in Cu(1) and Cu(2) sites. Data show that the energy position of the edge of Cu(1) sites in the nonsuperconductor is lower than that in the superconductor. The nature of the phase transitions of Bi2MoO6 has been investigated by Sankar et al. (E19) using combined X-ray diffraction and X-ray absorption spectroscopy. The distorted MoO6 octahedra in the low-temperature form are shown to undergo further distortion in the intermediate-temperature form before transforming to MoO4 tetrahedra in the high-temperature phase. Hayashi et al. (E20) have studied the single-crystal thin film of Nb and Ti nitrides on MgO substrate of (100) orientation using glancing-angle XRD. Their data revealed that the cubic B1 phase of Nb and Ti nitrides forms predominantly at 600 °C, while epitaxial layers formed at temperatures g400 °C. The epitaxial degree of TiN thin films increased as the temperature increased, with the best results of xmin ) 7.3% obtained for the film deposited at 600 °C. Ogale et al. (E21) have systematically studied 57Fe ion implantation in cupric and cuprous oxide films by Moessbaur spectroscopy and glancing-angle XRD. For the as-implanted CuO films, the Cu6Fe3O7, CuFeO2, β-CuFeO2, and Cu2O phases are detected. The same phases are present in Cu2O-implanted films. Annealing at 400 °C under Ar atmosphere produces in both cases a strong increase of the CuFeO2 and β-CuFeO2 phases at the expense of the Cu6Fe3O7 phase. The latter phase completely vanished after annealing at 500 °C, and new phases R-Fe2O3 and small Fe clusters appeared. The amount of CuxFe3-x04 phase continues to increase after annealing at 500 and 600 °C, whereas CuFeO2 decreases. The R-Fe2O3 contribution disappears after annealing at 600 °C. Wang et al. (E22) have investigated the phase formation of Co/SiGe structures at several annealing temperatures using EXAFS and XRD techniques. For the Co/ Si0.80Ge0.20 samples annealed from 400 to 600 °C, Co(Si1-yGey) phases with y ≈ 0.10 were identified, and the coordination number of Si around Co was found to increase as the annealing temperature increased. For the sample annealed at 700 °C, only the CoSi2 phase was formed. The results indicate a preferential Co-Si Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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reaction when annealing the Co/SiGe structure. Combined X-ray absorption and X-ray diffraction studies of CuGaS2, CuGaSe2, CuFeS2, and CuFeSe2 under high pressure have been performed by Tinoco et al. (E23). The combination of both techniques allowed complete determination of the structure in the whole stability range. The equation of state of these compounds were determined as well as the volume compression at the transitions. Liu et al. (E24) report the study of lead titanate ferroelectric thin films by glancing-angle XRD. They find differences between the surface and bulk structures. The lattice constants of the surface layers of thin films differ from those of the inner part, and crystallite size and lattice strain increase toward the substrate. Plotz and Lischka (E25) characterized the submicrometer thick films and multilayer stacks by the application of glancing incidence X-ray reflectivity measurements. The authors discuss different setups for X-ray reflectivity measurements and computer methods which allow the extension of various layer parameters from the experimental data. Hara et al. (E26) measured the orientation of Al 1.0% and Si 0.5% Cu layer by glancing-angle XRD. Only the (111) Al peak is observed in this layer by conventional XRD. Their data show that the (111) plane is tilted to the random direction ranging from 0 to 16°. A fine grain Al layer is nucleated at the first step of sputtering. Grain size increases gradually, and the tilt increases with increasing thickness. Glancing-angle X-ray scattering studies of the premelting ice surfaces have been reported by Dosch et al. (E27). The experiments give evidence that all the high-symmetry surfaces exhibit surface melting with onset temperatures Ts ≈ -13.5 °C for the basal and Ts ≈ -12.5 °C for the nonbasal surfaces. The temperature dependence of the thickness of the quasiliquid is presented and discussed in view of various theoretical predictions. Kojima et al. (E28) have carried out the time-resolved X-ray diffraction measurement of silicon surfaces during laser irradiation under grazing incidence conditions. Their data showed that the lattice expansion in the lateral direction was negligible. The data on the shift of peak position for the skew 422 reflection yielded information on temperature rise in silicon thin surface layers. Extended X-ray Absorption Fine Structure. The phenomenon of fine structure in the absorption coefficient in the first few hundred electronvolts energy range above the absorption edge has been exploited to determine atomic structure of ordered and disordered materials. Structures near the absorption threshold are known as X-ray absorption near-edge structures (XANES), while the features at higher energies are called extended X-ray absorption fine structures (EXAFS). EXAFS techniques have had a major impact on the understanding of the structure of biological molecules, the structure of amorphous semiconductors and insulators, the structure of supported catalysts, and local structural arrangement in complicated crystals. There are many aspects of EXAFS techniques, relative to X-ray and neutron diffraction techniques, that should yield unique information about materials. Core level ionization in an atom by absorption of a photon with sufficient energy creates a photoelectron which propagates as a spherical wave. Scattering of this wave by the neighboring atoms causes interference effects which depend on the local geometry and the incident X-ray energy. This interference is reflected as a modulation in the absorption coefficient on the high-energy side of the absorption discontinuity in the X-ray absorption spectrum. Fourier transform analysis of EXAFS yields structural information regarding the nearest-neighbor distances, coordination numbers, and types of nearest-neighbors. Contrary to XRD and low-energy 244R
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electron diffraction (LEED) methods, which are only applicable to structure with long-range order periodicity, the EXAFS can be used for structure analysis in short-range order and disordered systems such as real catalysts. Information from the atoms near the surface can also be obtained with surface EXAFS (SEXAFS) for systems exhibiting a large surface-to-volume ratio, e.g., small catalyst particles. An EXAFS experiment requires a very accurate determination of the X-ray absorption coefficient. High-intensity and tunable X-ray sources must be utilized in order to achieve this determination. The advent of synchrotron radiation and rotating anode X-ray tubes have speeded up the progress in the field. Application of EXAFS has been extended to metastable systems and diluted systems having a very low concentration of elements of interest. As the availability of synchrotron sources increases, EXAFS will take its place alongside X-ray, neutron, and low-energy electron diffraction techniques as another powerful tool for determining atomic structure of ordered and disordered materials. Several authors have recently reviewed the application of EXAFS to study the geometric and electronic structures of materials (E29). In recent years, the local structures of high Tc superconductors have been probed using EXAFS spectroscopy. Bridges et al. (E30) have studied the distortion about CO in YBa2Cu3O7 (YBaCO) and about the O(4) atom in thin films and single crystals of YBCO. Song et al. (E31) have investigated the pre-edge features of oxygen in XANES spectra of Y(Ba1-xSrx)Cu3O7-δ (x ) 0.5). They observed no substantial change of the relative intensity of pre-edge peaks. The suppression of Tc due to an oxygen deficiency was ruled out. The redistribution of hybridization of the valence band and the conduction band by the Sr substitution on the Ba sites cannot fully account for the suppression of superconductivity. Overford et al. (E32) have studied the influence of O 2p holes in the single-crystal, high-Tc superconductor Bi2Sr2CaCu2O8 upon the interface formation to Cu, Ag, and Au using O K-edge X-ray absorption measurements. They report that Cu reduces the amount of doping-induced O 2p holes significantly in the vicinity of the interface, whereas Ag and Au gave a much smaller reduction of these states. The results support the view that the Bi-O layers are needed for the doping of the Cu-O2 layers in Bi2Sr2CaCuO8. Choy et al. (E33) have studied a new high-Tc superconducting intercalation compound, AgI/Bi2Sr2CaCu2Oy. EXAFS results show that there is no charge transfer between the guest species (AgI) and host lattice. The data confirm that interblock coupling has little effect on the superconductivity, since the Tc is not significantly evolved upon AgI intercalation with a large lattice expansion along the C-axis, which is perpendicular to the Cu-O superconducting layers. Oyanagi (E34) has studied the role of oxygen atoms in high-Tc superconductivity by EXAFS. Polarized EXAFS data reveal that the outof-plane and in-plane anomalies in YBaCO occur at Tc, while the in-plane anomaly initiates at ∼120 K, well above Tc. The lattice anomalies may be related to a strong spin-lattice interaction. Er et al. (E35) have investigated the local structure of infinite layered superconductor Sr1-xLaxCuO2 in the low-temperature region by EXAFS, XRD, and Raman spectroscopy. EXAFS showed that the Cu-O bond distance slightly expanded in a range 10% and the structural changes that are measured exclusively around the Ba atoms. These findings yield an understanding of the complex glass systems. Capelletti et al. (E38) have studied the glassy system of xBi2O3(1-x)GeO2 with high bismuth content (x ) 0.9) by EXAFS at the Ge absorption K-edge. They report a tetrahedral coordination for Ge atoms at all Bi concentrations. The role of the preparation procedure on the glass IR transmission is discussed. Henderson (E39) obtained EXAFS on a series of sodium silicate glasses containing 15-40 mol % Na2O. EXAFS data indicate that the Si-O bond distance increases from 1.61 Å for amorphous SiO2 to 1.66 Å for 30 mol % added Na2O. For Na2O > 30 mol %, the Si-O bond distance decreases. The Si-O bond distance changes indicate that, for e30 mol % Na2O, network depolymerization effects on the Si-O bond dominate any effect from increased nonbridging oxygen formation. Armand et al. (E40) report the EXAFS study of (1y)GeS2+yAg2S(y e 0.5) using synchrotron radiation. Their Ge K-edge EXAFS measurements of binary glass GeS2 at 35 K have determined the local medium-range order around Ge atoms, based on R-GeS2 crystalline structure. The results show significant differences from those of previous models for chalcogenide glasses. The local structural modifications in Ar ion-damaged InGaAs have been studied by Yu and Hsu (E41). The In-As nearestneighbor distance remains close to its crystalline value, even when the layer is heavily damaged by Ar ion implantation. Once the Ar dose exceeds the threshold for amorphizing in the InGaAs layer, the In-As bond distance relaxes to that of the pure crystalline InAs. The sudden change in local structure as the material transforms from crystalline to amorphous suggests that the transition is due to simultaneous amorphous nucleation rather than the accumulation and overlapping of isolated amorphous regions. EXAFS measurements at the Zn, Mn, and Te K-edges of the diluted magnetic semiconductors Zn1-xMnxTe(x e 0.65) were performed by Happo et al. (E42) to study the local coordination around each of the atoms. The nearest-neighbor and next-nearest-neighbor distances do not undergo any significant change on changing x. The tetrahedral ZnTe4 and MnTe4 clusters are embedded in the alloy with a well-preserved form. Maeyama et al. (E43) have investigated the surface structure of (NH4)2Sxtreated GaAs(111)A, (111)B, and (001) by S K-edge X-ray absorption fine structure measurements using synchrotron radiation. The S-Ga bond lengths ordering on the three GaAS surfaces have been determined to be different, supporting three kinds of adsorption sites on the three GaAS surfaces. Lebeder et al. (E44) have studied the noncentrality of Pb and Sn atoms in the samples of Ge0.9Pb0.1Te and Ge0.85Sn0.15Te, substituting Ge atoms in the GeTe lattice, by the EXAFS method. They have explained the
transition of impurity atoms into a noncentral position under conditions of strong local stress to result from the formation of a chemical bond by an unshared electron pair of impurity atoms. Hasnaoui et al. (E45) have utilized SEXAFS to study the Si/GaAs system at the Si K-edge from 0.5 to 3 monolayer coverage. At very low coverages, the first Si absorption site is above the center of the triangle formed by two As and one Ga surface atoms. Fernandez et al. (E46) have used EXAFS to study the structural modifications in Ta metal resulting from the implantation of N+ ions. In the implanted samples of Ta, data show the formation of Ta-N bonds at approximately 2.9 Å and the amorphization of the structure for a dose of 3.46 × 1017 ions cm-2. A reordering of the network occurs at higher doses, as revealed by the increase in intensity of EXAFS oscillations and the appearance of new TaTa and Ta-N distances of second and higher shells. Koyano et al. (E47) have investigated the structure of cobalt oxide overlayers on the surface of ZrO2 powder, i.e., CoOx/ZrO2 prepared by lowtemperature plasma oxidation using EXAFS, XRD, XPS, and IR spectroscopy. They found that CoOx on ZrO2 was present as fine particles having the structure of Co3O4(spinal). The dispersion of CoOx was higher for CoOx/ZrO2 prepared by plasma oxidation than that prepared by the impregnation. The high dispersion is responsible for the higher activity of CoOx/ZrO2 prepared by plasma oxidation. Le Fevre et al. (E48) have utilized the SEXAFS of an anisotrope single crystal, cadmium (0001), for the crystallographic characterization of adsorbed atoms, layers, and thin films. They found the polarization dependence of EXAFS to be simple at the K absorption edge. For L2,3 absorption edges, the polarization dependence is more complicated, since the initial p state can be excited to a final state of s or d symmetry. The authors have studied the bulk anisotropic system, Cd, and have shown that neglecting the p-to-s transition in the analysis of angular-dependent spectra leads to errors in bond length determination. Lindsay et al. (E49) have employed SEXAFS to determine the adsorption geometry of K on Si(100)-(2 × 1) in the single-site regime at 0.5 monolayer coverage. Their data show that K adsorbs in a low-symmetry dangling bond site, with a K-Si bond length of 3.20 ( 0.03 Å. Moreover, normal incidence SEXAFS data evidence that K backscatters at 3.81 ( 0.04 Å, which is consistent with K adsorption in adjacent sites along the dimer rows. Boscherini et al. (E50) have carried out the K-edge EXAFS study on InAsxP1-x/InP buried epitaxial interfaces in strained-layer InAsxP1-x/InP superlattices. Their data show that the first shell environment of As at these interfaces is similar to that found in bulk InAsxP1-x alloys of similar composition, as determined experimentally and by comparison with recent theories of bond lengths in semiconductor alloys. Nitsche et al. (E51) report the K-edge EXAFS study on nanostructured zirconia and yttria powders of grain size 5-30 nm. The effective coordination number of the Zr and Y atoms is reduced up to 40% in the powders. The decrease of the coordination number is most obvious in the cation-cation distances. Fully dense ceramic samples with a grain size of 80 nm have nearly the same coordination number compared to samples with conventional sizes of about 1 µm. Hubble et al. (E52) report the EXAFS studies of amorphous WS5,WSe5, and WS3. They find, in the cases of WS5 and WSe5, the metalmetal distances of ∼2.75 Å to be consistent with the formation of metal-metal bonds between d1 metal centers; the metal-metal bond would also explain the observed diamagnetism of these compounds. The observed Se-Se bond length of 0.34 Å is typical Analytical Chemistry, Vol. 69, No. 12, June 15, 1997
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of a diselenide group. Comparison of the S K-edge absorption spectrum of WS6 with those of the related compounds suggests that the S is in the -1 oxidation state. The S K-edge absorption spectrum of WS3 suggests that this compound contains both S-I and S-II. The observed diamagnetism of WS3 can be explained by the formulation of W-W bonds of ∼2.75 Å, which are found in the EXAFS studies. Carmalt et al. (E53) have investigated the structure of amorphous Ph3SbO. Their EXAFS data are consistent with a structure in which trigonal-bipyramidal units SbPh3O2 share their axial oxygens to form a chain linked at the O atoms. Vlasenko et al. (E54) report the EXAFS study of cobalt π complexes with polystyrene-polybutadiene block copolymer. The data show evidence that Co atoms in the polymer form dimers, where each metal atom interacts with one butadiene fragment of the block polymer and three carbonyl groups. The thermal decomposition of the metallocomplex polymer leads to the formation of a phase of metallic Co clusters with an average diameter of 10 Å. Moscovici et al. (E55) have determined the X-ray absorption near-edge structure (XANES) spectra of hexagonal boron nitride at the B and N K-edges to probe the local symmetry of empty states at each atomic site in the lamellar compound. Peak behaviors lead to the identification of π* and σ* resonances similar to those shown by graphite.
Robert J. Nemanich is Professor, Department of Physics, and associate member of the Department of Materials Science and Engineering at North Carolina State University. He received a Ph.D. in physics from The University of Chicago in 1976. He then joined the Xerox Palo Alto Research Center. In 1986 he moved to NCSU. He has a long involvement with the Materials Research Society and is currently Vice President. He is a Fellow of the American Physical Society and has served on the Executive Committee of the Division of Materials Physics. His research has been primarily in the area of of electronic materials and has focused on characterization and properties of surfaces interfaces and thin films. Nalin R. Parikh is Research Associate Professor of Physics and Astronomy and Adjunct Professor with the Applied Science Curriculum at the University of North Carolina at Chapel Hill. He received a Ph.D. in Metallurgy and Materials Science from McMaster University, Hamilton, Canada, in 1985. He joined UNC-CH in August 1985 and since has expanded the ion beam characterization and modification of materials facilities. His research interests include studying implantation doping in semiconductors and radiation damage and recovery of semiconductors; growing/etching of diamond films by the electron cyclotron resonance microwave chemical vapor deposition method; and developing/improving new characterization techniques using ion beam and thermal neutrons such as coincidence spectroscopy, neutron depth profiling, and charge parcel energy filter. D. R. Chopra received his M.S. in physics from the University of Nebraska and Ph.D. in physics from New Mexico State University in 1964. Currently he is Regents Professor of Physics at Texas A&M UniversitysCommerce. He has over 30 years of experience in teaching and research. His fields of research include surface physics, materials science, and chemical physics. He utilizes appearance potential, X-ray photoelectron, and soft X-ray spectroscopies for the characterization of materials. He is a manuscript referee for Journal of Vacuum Science and Technology, Surface Science Letters, Journal of Less Common Metals, and Applied Surface Science. He serves on the executive committee of the Texas Chapter of the American Vacuum Society and is a member of the American Physical Society and Society of Sigma Xi.
ACKNOWLEDGMENT
LITERATURE CITED
P.S.W., J.G.K., and J.A.J. thank NSF, ONR, and the Alfred P. Sloan Foundation for support. R.J.N. gratefully acknowledges the research support of the ARO, NSF, and ONR. D.R.C. acknowledges support from the Robert A. Welch Foundation and Texas A&M UniversitysCommerce Organized Research.
(1) (1) Kane, P. F.; Larrabee, G. B. Anal. Chem. 1977, 49, 221R230R. (2) Kane, P. F.; Larrabee, G. B. Anal. Chem. 1979, 51, 308R-317R. (3) Larrabee, G. B.; Shaffner, T. J. Anal. Chem. 1981, 53, 163R174R. (4) Bowling, R. A.; Larrabee, G. B. Anal. Chem. 1983, 55, 133R156R. (5) Bowling, R. A.; Shaffner, T. J.; Larrabee, G. B. Anal. Chem. 1985, 57, 130R-151R. (6) McGuire, G. E. Anal. Chem. 1987, 59, 294R-308R. (7) Fulghum, J. E.; McGuire, G. E.; Musselman, I. H.; Nemanich, R. J.; White, J. M.; Chopra, D. R.; Chourasia, A. R. Anal. Chem. 1989, 61, 243R-269R. (8) Ray, M. A.; McGuire, G. E.; Musselman, I. H.; Nemanich, R. J.; Chopra, D. R. Anal. Chem. 1991, 63, 99R-118R. (9) McGuire, G. E.; Ray, M. A.; Simko, S. J.; Perkins, F. K.; Brandow, S. L.; Dobisz, E. A.; Nemanich, R. J.; Chourasia, A. R.; Chopra, D. R. Anal. Chem. 1993, 65, 311R-333R. (10) McGuire, G. E.; Swanson, M. L.; Parikh, N. R.; Simko, S. J.; Weiss, P. S.; Ferris, J. H.; Nemanich, R. J.; Chopra, D. R.; Chourasia, A. R. Anal. Chem. 1995, 67, 199R-220R.
Gary E. McGuire received his Ph.D. in inorganic chemistry from the University of Tennessee. He is presently the Director of the Electronic Materials and Device Technology Laboratory of MCNC in the Electronic Technologies Division. Active programs under his direction include field emitter devices, ferroelectric materials, materials characterization and display technologies. He is Editor of the Journal of Vacuum Science and Technology B and the Journal of Electron Spectroscopy and Related Phenomena, and Series Editor for Noyes Publication of the Materials Science and Process Technology Series. Prof. Paul S. Weiss is an Associate Professor of Chemistry at The Pennsylvania State University, University Park, PA. He received his Ph.D. in chemistry from the University of California at Berkeley. He and his group study the chemistry and physics of atoms and molecules on surfaces using scanning tunneling microscopy/spectroscopy and other techniques. He is currently a Visting Professor in the Department of Molecular Biotechnology at the University of Washington as a John Simon Guggenheim Memorial Foundation Fellow. James G. Kushmerick is completing his doctoral degree in physical chemistry at The Pennsylvania State University. He received his B.S. degree in chemistry from the University of Delaware in 1994. His research interests include extending the capabilities of the scanning tunneling microscope to record the vibrational spectra of isolated adsorbates through inelastic electron tunneling spectroscopy. Jennifer Johnson is a graduate student at The Pennsylvania State University studying surface chemistry with Dr. Paul Weiss. She utilizes a low-temperature UHV STM to study adsorption dynamics on metal surfaces. She graduated with a B.S. degree from the College of William and Mary in Virginia in 1995, and is a member of the American Chemical Society and the Society of Sigma Xi. Steven J. Simko is a Staff Research Scientist at the General Motors, NAO Research and Development Center. He received a B.S. degree in chemistry from the University of Delaware in 1980 and his Ph.D. in analytical chemistry from the University of North CarolinasChapel Hill in 1985 under the direction of Richard W. Linton. His research interests include characterization of insulating materials surfaces, ion-solid interactions, failure analysis, and characterization of electrical contacts. He serves as chairman of the Michigan Chapter of the American Vacuum Society and is a member of the American Chemical Society and Sigma Xi. 246R
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