8855
J. Phys. Chem. 1993,97, 8855-8863
FEATURE ARTICLE Energy-Resolved Studies of Photochemistry on Semiconductor Surfaces Qingyun Yang and Richard M. Osgood, Jr.’ Columbia Radiation Laboratory, Columbia University, New York, New York 10027 Received: April 9, 1993; In Final Form: June 28, I993
The investigation of the dynamics of optically driven chemical processes on surfaces is providing new insight into how the presence of a solid surface alters the mechanisms, energetics, and fragmentation patterns seen in isolated-molecule photochemistry. This paper reviews recent work on the photodissociation dynamics of molecular adsorbates on semiconductor surfaces, emphasizing energy-resolved measurements of the desorbed fragments. The results of this research show that the mechanisms and energetics for photofragmentation for molecules a t the top of thick condensed layers are very close to those of the gas phase. At low coverages, however, electrontransfer reactions dominate and direct photon-assisted chemistry is quenched.
Introduction
Over the past two decades, studies of the photodissociation of isolated molecules have provided a powerful and scientifically rewarding insight into the half-collision dynamics following electronic excitation.*-’ These studies have used the trajectories and the internal-state distribution of photofragments to probe not only the nascent excited states but, in addition, also the potential surfaces and interlevel crossings in the excited complex as it dissociates. Recently, the techniques of these gas-phase studies have been applied to the case of a molecule adsorbed on a single-crystal surface? These dynamical studies have shown that even for weakly adsorbed molecules a much wider variety of photochemical phenomena are present than for isolated molecules. These new effects arise from the introductionof adsorbatesubstrate bonding as well as new excitation channels stemming, for example, from excitation of the crystal surface or bulk. Many of the initial studies of laser-initiated photochemical dynamics concentrated on single-crystal metals,41 primarily because of the seminal importance of such surfaces in studies of fundamental surface chemistry.6 In fact, measurements of fragment energies of molecules on metal surfaces have been instrumental in elucidatingkey questions regarding the molecular photoresponse? For example, recent work has shown the effect of the metal-crystal surface in altering the postdissociation trajectories of the molecular fragments,*in aligning the parent molecule on the surface? reducing the yield via quenching processes,”* and trapping products on the surface.13 Furthermore, the research has shown that a surface can provide new channels for enabling UV photodissociation or photoreactions. In particular, the results on metal surfaces have suggested that photoemitted electrons or hot carriers may be a major surface dissociation ~ h a n n e l . 1 ~Such ’ ~ effects are particularly important as the irradiating photon energy approaches 4-5 eV, since the work function of many metal surfaces falls in this range. Semiconductors present particularly varied and interesting surface systems for photochemical studies. While their preparation may be difficult, their detailed and, sometimes, complex surface structure and reconstruction which result from the directional covalent bonding are often well-known.20 In contrast to metals, semiconductor nonequilibrium carrier lifetimes are very long, 10-9-10-3 s versus s in metals.21 Further, the availability of variously doped samples as well as high carrier mobilities and variable near-surface fields make them ideal
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candidates for the study of photochemical mechanisms involving electron-hole pairs. For example,it is known that the adsorption of an electronegativespecies such as oxygen, in either its atomic or molecular form, on a semiconductor surface, is often accompanied by capturing an electron from the bulk and transforming the species into an ion.22
Background The photochemistry occurring in adsorbate films on semiconductor surfaces is controlled by the response of the molecular system and the crystal surface to the incident optical field. The intraadsorbate optical response of physisorbed molecules, which are weakly bonded in adlayer film up to tens of angstroms from the surface, might, initially, be anticipated to resemble that of the isolated gas-phase molecule.23 For chemisorbed species, the adsorbate response should become more tightly coupled to that of the solid and will thus deviate significantly from the free molecule limit.24 Much of the photochemistry occurring on semiconductor surfaces is seen at visible or ultraviolet photon energies. In this spectral region, semiconductor crystals adsorb light strongly in contrast to their transparency for photon energies below the bandgap. In the near-ultraviolet, other interband transitions contribute; optical absorption lengths become extremely short, and strong spectral dependencies in the real part of the dielectric constant occur as well. These features are illustrated by the measured optical dielectric and absorptionconstantsas for GaAs given in Figure 1. Note that for the mid-ultravioletregion, 250 nm, the absorption coefficient is such that most absorption is within 100 A of the surface. This distance is sufficiently short that many electrons are created at less than a scattering length from surface. Also, throughout most of the spectral range of interest, the surface reflectivity at normal incidence is relatively high, i.e., (n - I)*/@ 1)2 = 0.5-0.6, where n is the refractive index. The effect of both of these phenomena will be seen below to play an important role in photoreactions on semiconductor surfaces. The covalency of semiconductor bonding leads to highly structured surfaces which are subject, in some cases, to complex and distinctive surface reconstruction.20 Figure 2 shows the equilibrium surfaces for the semiconductorsurface, GaAs( 1 IO), used extensively in the experiments described here. In contrast to the complex structure seen, for example, in 7 X 7 Si(l1 I), the
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8856 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 25
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u's Figure 1. A schematic side view of the relaxed (1 10) cleavage face of the zincblende structure compound semiconductors. Note, in this case, the top layer rotates so as to thrust the As atom ( 0 )outward and the Ga atom (0)inward. Therelaxationin the second layer is greatlyexaggerated for visibility. (From Duke, C. B.; Wang, Y. R. J . Vuc. Sci. Technol. 1988, Bb, 1440.)
surface structural change in GaAs( 110) is rather modest; only a tilting of the topmost layer is seen. Similar relaxation is seen for other members of the 111-V compound semiconductorfamily. These and more complicated surface structures present a variety of surface sites, which can respond differently to photoexcitation. In addition, the character of the surface reconstructioncan change dramatically in the presence of adsorbate-substrate bond formation. Thus, the adsorption of atomic As on GaAs( 100) gives five different surface reconstructions in going from an As-rich to a Ga-rich surface.26 Finally, the rich variety of atomic bonding on reconstructed semiconductor surface makes it possible to investigatesite-selective photochemical reactions. In particular, Avouris has recently utilized the varied surface sites on 7x7Si( 111)to show the existenceof site-selectivethermal chemistry." Subsequently, B O S Z Ohas ~ ~ reported that the photochemical response to these different sites may also be distinct. Figure 3 shows a schematic sketch of an adsorbate molecule on a semiconductor surface including a set of representative cuts through the potential energy surfaces. The sketch illustrates some of the important phenomena for interfacial charge-transfer photochemistry; electrons may be transferred to an adsorbate either by tunnelingor by photoemi~sion.2~ In the caseof tunneling, both the physical distance between the semiconductor surface atoms and the adsorbate molecules and the orientation of those molecules are critical parameters. Note that unoccupied energy levels in the acceptor molecule must be energetically accessible for tunneling to occur. For the transfer of thermalized electrons, Le., those at a semiconductor conduction band minimum, this condition implies that the acceptor levels of the ion must be energetically in resonance with the conduction band minimum of the semiconductor. For the case of photoemission from semiconductorsurfaces,i.e.,ejection of hot electronswith energies above the surface barrier, it is clearly important to have a photon energy which is greater than the photoemission threshold of the semiconductor surface. Since this threshold is sensitive, through the dipolar surface layer, to surface coverage, this minimum photon energy will change as the coverage and type of adsorbate are altered.30131 The finite scattering and penetration depth of emitted electrons implies that reactions will be confined to the first few monolayers of the adsorbate film.5~3~ Note that, in both cases of electron-transfer reactions mentioned above, the bondcleavage step results from the addition of an electron to the adsorbate molecule. Thus, the illuminating light functions only to release the electronsat the molecule-crystal interface. Despite this, however, the magnitude of the photon energy can be important in enabling a surface barrier to be surmounted as well as in allowing different adsorbate levels to be accessed.30J3 The photochemistry of adsorbates on semiconductor surfaces has been found to be unusually rich in new and unexpected chemical phenomena. These include the importance of photogenerated mobile carriers in the bulk and on the surface,34
0
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Figure 2. (top) Real and imaginary parts ~1 and ~2 of the GaAs dielectric constant (from Philipp, H. R.; Ehrenreich, H. Phys. Reu. 1963, Z20, 1550). (bottom) log-log plot of the absorption coefficient a(hu) from the intrinsic edge to 25 eV, after Casey, H. C.; Sell, D. D.; Wecht, K. W. J. Appl. Phys. 1975,44,250.For a summary of the optical response of GaAs, see: Blakemore, J. J . Appl. Phys. 1982, 53, R123.
photochemical changes in the surface structure and compositi0n,3~J~ photoreactions which are dominated by specific surface quenching phenomena for optically excited molecules,37 and optical field enhancement of surface reaction rates.38 Based on the experience with gas-phase photochemistry, a complete understanding of the process will clearly require years to develop. In this article, we will review the physical picture that has emerged thus far. In our review we will concentrate on the understanding that has been gleaned by recent applications of energy-resolved techniques to the study of photon-stimulated reaction^.^^^^^ In addition, we will not include a discussion of phenomena seen at ultrafast time scales since this elegant work has been reviewed in a separate, recent article in this same journal.40
Experimental Techniques To date, two techniques have been used to measure ejected reaction products: time of flight (TOF) using a quadrupole mass
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Figure 3. Schematic diagram showing possible substrate-electron-
mediated processes for an adsorbed molecule on GaAs following optical excitation of the substrate. Note the shift in unoccupied levels of the molecule at smaller moiecule-substrate distances due to image-charge interaction (see ref 57 or 58). spectrometer (QMS) and pulsed-laser probing of the internalenergy distributions of desorbed fragments. The former relies on accurate knowledge of the flight time of the fragments in both their neutral and ionized forms: the latter uses measurement of the optical signal as a function of the delay between the desorption and probe pulse. Time of flight is particularly useful since it can, in principle, measure the translational energy of virtually all small ions or neutral species as the particle is ejected from the surface. The yield into various solid angles can be obtained by any of a variety of angularly oriented measurement techniques. Optical measurements of the desorbing fragments have thus far been done by laser-induced fluorescence (LIF), although resonant multiphoton ionization (REMPI) is clearly possible and, in fact, has been used very successfully to investigate molecular surface scattering and laser-induced desorption from metals. While somewhat limited in its ability to detect a wide variety of species, the technique provides unmatched insight into the internal coordinates of the desorbed fragments. The above measurements require the use of well-characterized bare surfacesof known reconstructiontoobtain repeatable surface chemistry and the use of low pulse fluence to obtain precise dynamical information. For example, since the experiments described here are to observe chemical reactions, in many cases the surface will degrade by the formation of the products from those very same reactions, as the surface illumination continues. As a result, a freshly prepared surface can only be exposed to a limited number of laser shots before the surface must once again be reprepared. In addition, the laser energy per pulse must be minimized, typically to energiesless than mJ/cm2 for nanosecond pulses; otherwise,surface heating or postdesorptioncollisionswill render the experimental measurement meaningless.41 All of the above considerations put maximum emphasis on extracting the best signal-to-noise ratio from the measurements for each laser pulse. Each of these techniques has been used recently to study the dynamics of light-induced reactions on semiconductor surfaces. For example, recently Richter et a1.42 have used LIF to study opticallyinduceddesorptionofNOfrom7X7-Si(111).Theresults of this work were consistent with a desorption mechanism initiated by generation of a hole in the silicon surface band and followed by electron capture of this hole and ejection of the neutral NO molecules. In this case, the state resolution of the measurement technique was used to clarify the energetics of the dissociation process. The site specificity of this reaction, as well as its identification, was shown clearly by the disappearance of the desorbed signal via the known titration procedure for certain surface states on 7X7-Si(lll). Note that in this experiment optical absorption from occupied surface bands gave the process a threshold with photon energies less than the bandgap.
Figure 4. Potential curves of CH3Br for the first absorption continuum. The insert shows the decompositionof the total cross sections of CHlBr into partial cross sections of the IQ, 3Qb and 3Q1 states. The partial cross sections are given by the dashed lines. The sum of these cross sections is the dotted line; the total cross section including other electronic transitions is given by the solid line. From ref 46.
In a second experiment, TOF was used to compare the translational energies of normal and deuterated ammonia which weredesorbed froma4X6-GaAs (100) surfacewith6.4-eVlight.43 In this experiment, the TOF signatures indicated that while both species were desorbed with equal translation energies. The much greater magnitude of the TOF signature for NH3 in comparison to ND3 was suggestive of a new vibrationally mediated desorption mechanism.
Thick Adsorbed Layers Photodissociation of molecules in thick adsorbed molecular layers should most closely approximate the photochemistry in the isolated molecule. In this case, the intermolecular interaction is relatively weak, and theimportanceof thesubstrateis minimized for molecules at the adlayer-vacuum interface because of the thickness of the adsorbate layer. Despite these facts, it is found that intermolecular interaction is still sufficient to introduce new molecular adsorption bands, and optical interaction with the surface can still significantly modify the total optical field. Photodissociation at the Surface of Condensed Layers. By detecting fragments of molecules which have been irradiated in the topmost layer,@we can determine whether photodissociation of these moleculesdoes, in fact, resemble the process in the isolated molecules. Only photofragments from molecules in the outer layer will be detected strongly by the mass spectrometer, since fragments produced in lower-lying layers will either be broadly scattered into the entire 4 r angular space or, particularly for the case of deeply buried parent molecules, be caged or trapped. Results with our model system of CH3Br on GaAs(ll0) illustrate the physics of this process clearly.4s The appropriate starting point for examining these results is the well-known gasphase photochemistry of CH,Br, which is summarized by the molecular data shown in Figure 4, namely, the relevant potential curves of the CH3-Br bond as well as the measured optical spectrum for the middle-ultraviolet region.& Note the large difference between the optical cross section at 193 nm and that
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0 0.2 0.4 0.6 0.8 Neutral Velocity [cm/psec] Figure 5. Time of flight spectra obtained at mass 15 at 30-ML coverage on GaAs( 110). Shown are the data converted to flux in velocity space. The photon energies used to initiate the desorption are 6.4 eV (193 nm) and 5.0 eV (248 nm) shown by the top and the bottom curves, respectively. The high-velocity peaks are due to direct dissociation, whereas the lowvelocity peaks stem from collisionally ejected CH3Br. The spectra are vertically not to scale.
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of 248 nm, although in both cases the lowest spin-orbit state of bromine fragment is 0btained.~6 UV irradiation of thick condensed layers on GaAs( 110) causes desorption of Br, CH3, and CH3Br, each of which is detected by our TOF system. For example, Figure 5 shows typical TOF spectra of CH3 fragments (mass 15) desorbed from a sample with 30-ML coverage and obtained with s-polarized irradiation at 248 and 193 nm and measured with the mass spectrometer aligned so as to detect CH3desorbingnormal to thesample surface. Note, again, that the data are not perturbed by postdesorption collisions. The fast feature is due to CH3 formed from direct dissociation of CH3Br molecules, while the slow feature is due to secondary desorption of CH3Br molecules, which are ejected in part by collisions of photofragments with the adlayer; the detection of CH3Br molecules at mass 15 results from cracking in the ionizer of the mass spectrometer. The fast CH3 desorption peak has an averageenergy of 1.6 and 2.5 eV as well as a maximum kinetic energy of 1.9and 3.3 eV for 248- and 193-nmillumination, respectively. Comparison of these kinetic energies with those expected from the isolated molecule indicates that the physisorbed molecule dissociates in the manner of a loosely constrained molecule. The measured maximum kinetic energies are very close to the values expected from a straightforward subtraction of the bond energy (2.87 eV46947) from the incident photon energy (5.0 eV for 248 nm and 6.4 eV for 193 nm). The peak position for 248 nm (1.6 eV) is 0.3 eV lower than the maximum kinetic energy, probably due to fragment vibrational excitation, collisionsby the departing fragment with surrounding molecules,44and (variable) amounts of recoil in the Br atom. Concerning the first point, Van Veen et al.46 reported an average vibrational energy of the umbrella mode of 0.277 eV at 222 nm and of 0.317 eV at 193 nm in their gas-phase studies. At 193 nm, our measured peak position (2.5 eV) is 0.8 eV lower than the maximum energy, showing that the above-mentionedeffects play an even more important role. Note that all of these energy-loss processes would be expected also to broaden the translational energy distributions.4 Br TOF signals were only obtained for 193-nm irradiation. The peak position is at an energy of -0.5 eV. The high-energy cutoff extends to about 1.7 eV, a value considerably higher than that seen in gas-phase dissociation. However, this energy is very reasonable if some molecules are oriented such that the Br atom
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Figure 6. (top) Schematic diagram defining various parameters relevant to the calculations of a CH3Br layer of thickness, d, on top of a GaAs substrate, including the angles and the orientations of the p and s-polarizations. (bottom) Integrated TOF signal fluxes of the direct dissociation peak in Figure 5 versus exposureand also overlayer thickness. The lines represent least-squares fits to the electromagnetic model. The s-polarized laser light impinged on the sample at a 60' incidence angle. The mass spectrometer was aligned with the sample normal.
is pointing toward the vacuum in these thick adlayers.4 Note that while this orientation is not seen at very low coverage (1-2 ML coverage), it is consistent with the known crystallography of bulk CH3B14*as well as structures deduced from measurements described below. Optical Interference in the Adsorbed Film. For even thicker films, the adsorbed layer is best viewed optically as an adsorbing dielectric layer, covered with a monolayer of the adsorbed molecules. In this instance, optical effects important in thin-film physics will manifest themselves in the photoejection process. We illustrate these effects45 again using the case of CH3Br on GaAs( 110). In Figure 6 we show the dependenceof the intensity of the direct photodissociation feature as a function of coverage, a quantity which is directly proportional to the thickness of the condensed layer. The most striking feature of these data is that the number of desorbing CH3 fragments oscillates as the thickness of the layer is increased. In order to understand the oscillatory behavior, consider photodissociation of molecules at the solidvacuum interface. Our photofragment signal samples the optical intensity essentially at the film surface49since any fragments from lower layers will have to lose a substantial amount of energy in order to escape. The local optical field just below the film surface is determined by both the incident wave and the light reflected from the semiconductor surface.50 Interference between these two fields results in oscillation of the electric field near the surfaceof the adsorbatelayer as a function of the adlayer thickness.
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 8859
Feature Article Sincedirect dissociation is proportional to the square of the optical electric field, the fragment intensity will also oscillate with film thickness. This oscillatory behavior can be quantified by considering the optical propertiesof our sample system, sketched in the top panel of Figure 6. In particular, the total electric field vector, just ourside the CH3Br film is the sum of the field of the incident beam, 8rwEaa,and the field of the reflected beam, given by
a&,
where p,s designate the components of the electric field parallel and perpendicular to the plane of incidence, f p , s is the complex reflectivity of three-layer dielectric stack, theAsymbol designates the directional unit vector for the incident, i, and reflected r, wave, and where we have neglected any anisotropy in the refractive index of the adlayer. In order to obtain the field just inside the layer, it is necessary to take into account the continuity of the parallel component of E and the perpendicular component of D across the boundary.49 For the s-polarization, the electric field is continuousacross the interface; thus, eq 1 may be used directly to obtain the total electric field just inside the molecular adlayer. The 'inside" p-polarization field, on the other hand, is more difficult to calculate since it includes components both parallel and perpendicular to the surface. In this case, the reflection coefficient will oscillate as a function of film thickness as in the s-polarized case; however, now, the orientationof the total electric field will also vary with thickness. The desorption signal will sample this surface electric field through the square of its projection onto the molecular transition dipole.' This projection as well as the limited detection angle of the TOF technique makes it important in some cases to include consideration of any ordered orientation of the film molecules. For example, in comparing signals at different wavelengths, the character, parallel or perpendicular, of the transition must be considered. The gas-phasemeasurementsreferred to above show that at 193 nm both a perpendicular and a parallel dipolar transition are present, whereas at 248 nm only a perpendicular dipole transition is in effect. The oscillation in the desorption signal with film thickness, which is seen clearly in the experimental data shown in Figure 6 for the case of s-polarized light, is clearly in agreement with eq 1. These plots show oscillations in the desorbed CH3 signal for two different wavelengths, 248 and 193 nm. In fact, fitting such data with the theoretical model outlined above enables the extraction of the optical properties of the film, including the real and imaginary dielectric constants and the physical thickness of the film. While all three quantities enter into the fit interactively, predominantly, the order of the interferencemaximum determines the thickness, the depth of the modulation determines the real index, and the damping of the modulation determines the imaginary index. The calculated fit for both 248 and 193 nm are also shown in Figure 6,thus yielding a set of optical constants for these wavelengths, which are weakly and strongly absorbing in the adlayer, respectively. The wavelength-dependent absorption coefficient obtained in this manner is in good agreement with extrapolations from the gas phase.45 Finally, we note in passing that the angular variation of the electric field with thickness forp-polarized light makes it possible, in principle, to determine the structure of the underlying film, if the molecular orientation remains, in fact, constant with film thickness. This determination is possible because the projection of the thickness-varying orientation of the electric field on the molecular substructure gives a fragmentation yield which is characteristic for a particular structure. Preliminary results of such a determination are described in ref 5 1 for the case of thick CH3Br adlayers. Adsorbate Photochemistry through Interadsorbate Charge Transfer. As is seen in the case of the gas-to-liquid-phasechange,
the formation of the adsorbed phase might be expected to induce alterations in the adsorbate electronic spectrum. This change can, in turn, affect the surface adsorbate photochemistry. These effects are shown clearly in a recent TOF study of the photochemistry of Clz on GaAs( 110).3s Recently, several groups have shown the importance of intermolecular charge-transfer processes in introducing a strong mid-UV absorption band in halogen-rare gas matricesSZand condensed Br2.S3 Cousins and Leones4 showed that the same process occurs in condensed-liquid films of pure halogens, albeit at somewhat shorter wavelengths than in the mixed liquids. Specifically,in the case of Clz layers, this charge-transfer process
2C1,
+ hv
-
C1;
+ C1:
(2) can be calculated to give rise to a continuous band centered at -6.7 eV,35 a value in close agreement with that actually seen at 6.2 eV.54 Because one of the products of the charge-transfer process is the unstable molecular chlorine negative ion, prompt dissociation will occur upon formation of the negative ion to yield Clz- C1 C1- in the absence of caging and recombination. TOF studies35in our laboratory have shown that such a process does occur for Clzmultilayers on both GaAs and insulator surfaces and that this behavior persists to coverages as low as a few monolayers. Intermolecularcharge-transfer bands cause a much larger cross section for C1atom production around 200 nm relative to that at the peak of the gas-phase spectrum (-320 nm).ss This behavior is shown clearly in the TOF study of Clz on GaAs. In particular, the TOF signal at 351 nm is an order of magnitude smaller than that at 193 nm, a result in agreement with the intermolecular charge-transfer reaction but opposite to that expected on the basis of the gas-phase cross section for intramolecular photodissociation. Note that in this experiment the electron-mediated origin of the dissociation process is also suggested by the very weak, if any, fragment translational energy dependence on the excitation wavelength.
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Low Coverage Photochemistry Under low-coverage conditions, interaction of the adsorbed molecules with the substrate becomes pronounced. This interaction leads both to the modification of the photochemistryseen in isolated molecules and to the introduction of new mechanisms for photochemical reactions. An important feature for adsorbatemediated photochemistry is the nearly continuous electronic band structure of semiconductorsubstrates, thus providing adsorption for virtually any excitation photon energy exceeding the bandgap (see Figure 1). Thus, photochemistryof admolecules on surfaces (metals or semiconductors)for coverages of I1 ML is typically dominated by photoabsorptionwithin the substrate and electron transfer between the substrate and the adsorbates. The understanding of these processes benefits from previous studies of gasphase, electron-molecule reactions. However, near metal surfaces, or those of semiconductorswith high dielectric constants, image-charge interactions can cause sizable shifts, 1-2 eV, in the adsorbate orbital energies, an effect shown clearly in the resonant-level m0de1.5~~57This interaction leads to the increase in the electron affinity and decrease in the ionization energy of the adsorbate, that is, shifting of an occupied or an unoccupied electronic level of the adsorbates toward the Fermi leve1.58 These level shifts, in combinationwith the continuousadsorption of the substrate, result in a significant red shift in the threshold for which photochemistry is observed. Mechanisms of Light-Induced Reactions. In the absence of simple, light-induced, surfaceheating, reactions on semiconductor surfaces can be initiated by intraadsorbate or substrate excitation.59 Light-induced charge transfer between the adsorbed species and the substrate can also result in indirect excitation of thesubstrateadsorbate bond by changes in the electrostatic forces,
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8860 The Journal of Physical Chemistry, Vol. 97,No. 35, 1993
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Yang and Osgood
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Figure 8. TOF spectra of the photoejected CHa measured at 193 nm for 2-ML CH3Br coverage. The data were normalized by the actual amount
of substrate absorption. 120 180 240 Temperature [K] Figure 7. TPD spectra of CH3Br on GaAs( 110) taken at a ramp rate of 5 K/sfor successivelylarger exposure. Layer-by-layergrowth is clearly
evident.
a process first described by Antoniewicz for electron-stimulated desorption.60 To illustrate the photon-initiated processes which lead to bond cleavage in adsorbed molecules, we again draw upon the wellIn this instance we studied system of CH3Br on GaAs(1 will describe the physics at 1- and 2-ML coverages since they illustrate the full range of phenomena seen for this system. Because a well-characterized surface condition is crucial in order to develop a sound understanding of the low-coverage chemistry, the surface must be restored to its stoichiometric, fully crystalline condition after each experimental run. For the (1 10) plane, this is convenientlydone by a series of sputter/annealing cycles, where the maximum annealing temperature (550 "C) is chosen so as not to disproportionate the surface by As evaporation. After the surface is thus prepared, dosing with CH3Br leads to a clear layer-by-layer growth for the first two monolayers. As is shown in Figure 7, thermal desorption spectroscopy of submonolayer coverage shows that the desorption temperature lowers as the coverage increases.61 In addition, the cracking pattern of the desorbedspeciesindicatesthat CHsBr is molecularly adsorbed, even in the first monolayer. Thereduction in desorption temperature with increasing coverage indicates that the interadsorbate interaction is repulsive and thus that the molecules are aligned with their dipoles, and hence their molecular orientation, in thesamedirection. On the basis of the relative electronegativity of the Br and the GaAs surface, it is reasonable to assume that the molecule is oriented with the Br down.62 This expectation is borne out by more recent angular-resolved TOF data in our laboratory. When 1-2-ML layers of CH3Br are irradiated with laser radiation, superthrmal CH3 radicals are ejected from the surface. A typical time-of-flight signal is shown in Figure 8 for the case of 2-ML coverage and irradiation at 193 nm and where the TOF signal has been converted to velocity distribution by means of the usual Jacobian transformation. Four separate velocity peaks are seen: 0.1, 0.25, 1.1, and 2.5 eV. However, the peak at 0.1 eV is, in fact, not due to desorption resulting from CH3-Br bond cleavage but rather the result of desorption of intact CH3Br molecules from the surface, as a result of either quenched adsorbate excitation or collisional ejection of adsorbed molecules by energetic photofragments. These desorbed molecules are then cracked in the OMS.
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The two CH3 peaks at higher, i.e., 2.5 and 1.1 eV, kinetic energy are due to direct interadsorbate excitation and electroninduced reactions, respectively. In particular, the peak at 2.5 eV is identical to that observed for thick adlayers and identified as direct photodissociation,analogous to the same process in thegas phase (see previous section). The feature at 1.1 eV results from substrate-electron-induced dissociation. In this case electrons are emitted into the adlayer by photoemission; that is to say, a valence electron is excited by the incident light to a state in the GaAs substrate lying above the surface barrier for the GaAsCH3Br interface. This electron then moves ballistically into the CH3Br layer where it is eventually scattered and captured by an isolated molecule. This process is relatively surface sensitivesince the electron capture length is generally only a few monolayers.63 This process has been observed on metal surfacesby severalgroups using a variety of experimental techniques.5J.64 The threshold for this process, termed internal photoemission,6s can be several electronvolts lower than that for photoemission into vacuum due, in part, to the lowered surface barrier.16." In fact, the threshold for the internal photoemission in the CH3Br systemscan be measured by observing the illumination wavelength at which the 1.1-eV peak disappears.29 This experiment which was done with a tunable dyelaser system showed that the threshold was approximately 3.5 eV. This threshold is in reasonable agreement with that expected from the anticipated change in the CH3Br-covered surface barrier. After an electron is captured by the adsorbate molecule, a temporary negativeionisformed. In thegasphase,sucha negative ion in some cases, e.g. CH3Br-, dissociates promptly after electron attachment, a process known as dissociative electron attachment (DEA):6'
CH,Br,,
+e
-
CH,?+ Br,,
The energy in the desorbed CH3fragment can be estimated from the known gas-phase potential curves of CH3Br and CH3Br-.67 Because of the weak bonding and concomitant loose molecular packing in the physisorbed layer, the potential curves should be relatively unchanged in shape at the molecular equilibrium internuclear distance. (This distance is approximately the same for the free molecule and the solid.48) Note, however, that the ion curve is shifted downward due to the small, but nonnegligible, imagecharge interaction for charged species at distances less than 10 A from the surface; this shift is proportional to the inverse of the distance to the substrate (see Figure 3) and thus is 1-2 eV for molecules in the first
[email protected]* While this shift does affect the vertical electron affinity of the molecule-ion transition, it does not change the point at which the ion is formed on the ion repulsive curve and, as a result, the gas curves should give a reliable estimate of the fragment kinetic energy. In
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20 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Neutral Velocity [cm/psec] Figure 9. TOF spectra of the photoejected CHI measured at various wavelengths for 1-MLCH3Br coverage. The data were normalized by the actual number of photons adsorbed by the substrate. Different total irradiation was used in order to compensate for the low signal at longer wavelengths. fact, the free molecule c ~ r v e s 6give ~ an anticipated energy of 1 eV. This value compares very well with the measured value of 1.1 eV for the adlayer, in which the Br-down orientation of the CH3Br ensures that most of the energy released is imparted into the desorbing methyl fragment. This photoemission-induced reaction is a form of hot-electron chemistry69 which has been of interest for many years in conjunction with semiconductor electro~hemistry.'~While generally in electrochemistry the interest is on the tunneling of hot electrons through a surface barrier, our basic observation here, even though based on a photoemission channel, indicates clearly that hot-electron chemistry occurs and is important on semiconductor surfaces,a conclusion made possible by the translational energy resolution of the experimental technique. For surfaces with only 1-ML CH3Br coverage, the time-offlight spectrum simplifies considerably. Figure 9 shows TOF traces taken at a coverage of 8 = 1 ML for CH3 desorption using several excitation wavelengths. The figure shows that for all excitation wavelengths only one TOF peak is seen and that the CH3 kinetic energy is 0.6 eV. The fact that direct dissociation is not seen for 193nm is a result of the quenching of the molecular excitation for molecules directly adsorbed on the GaAs surface. This process is most probably due to energy loss by resonant electron transfer to and from the surface, a process also operable for metal surfaces.71.72 The photon-energy threshold of the 0.6-eV peak along with its excitation wavelength-insensitive fragment energy strongly suggests that the dissociation originates from thermalized electrons in the conduction band of the GaAs. In particular, the 840-nm photon energy threshold of the peak seen in Figure 9 corresponds to the bandgap of GaAs. For photon energies near this threshold level, the efficiency of surface-electron production is much reduced, a result in accord with the increasing absorption depth for photon energies just above the crystal bandgap and, hence, loss in collection efficiency at the semiconductor surface. In contrast to the hot-electron or photoemission mechanism described earlier, the 0.6-eV feature originates from electrons injected from the GaAs conduction band minimum (CBM). Thus, in this case, while the initial photoabsorption event creates electrons at varying energies above the CBM, depending on the optical wavelength, these excited electrons rapidly (
