Surface characterization

Surface Characterization Philip F. Kane" and Graydon B. Larrabee Materials CharacterizationLaboratory, Texas Instruments Incorporated, Dallas, Texas 75222

Over the past several years, the biennial Fundamental Reviews have featured surveys of the individual characterization techniques applied to surfaces; for example, the 1976 reviews included articles on electron microscopy (48),UPS (33),X P S (245),and the x-ray techniques (62, 483). These have usually included a section on applications but the emphasis generally has been toward instrumentation and techniques. This Application Review is a first attempt to approach the subject of surface characterization from the other end and to collate the literature from the point of view of the user. We were immediately presented with the problem of how to reduce an overwhelming backlog of information to a manageable size. We decided to limit our review to the years from 1970 onward, and to restrict ourselves to English-language journals or translations which were readily available although perhaps not usually read by analytical chemists; in addition, we omitted all abstracts although we did include some meeting material which was available in full. Our materials studies were defined as solid-to-gas interfaces and, in most cases, as bulk material faces. Even with these very drastic limitations, we finished with over 600 references, which certainly did not allow for any great depth if we were to condense this into a reasonable length. We decided to use a number of subheadings for the substrate materials and to indicate the methods which had been applied to the study of the surfaces as they interacted with various gases or otherwise changed under different conditions. While this does not make for particularly stimulating reading, it does allow more easy location of a particular material. T o further condense the text, we have made extensive use of abbreviations for the techniques employed. Most of these are in general use and they are defined in Table I. METALS As might be expected, metals have been the subject of considerable investigation. Being good conductors, they are excellent subjects for electron and ion beam techniques. Their importance as engineering materials has required extensive studies of their mechanical and corrosion properties; some excellent reviews are available (361,527,548,550) which relate such studies with the current characterization tools. Alkali Metals. The adsorption of halogens on sodium films was determined by light emission (311). AES was used to follow the segregation of sodium on a lithium metal surface (400). Light Metals. T h e surface composition of a berylliumcopper alloy was monitored during oxidation by AES (515). AES (554) and X P S (199) were applied to the adsorption of oxygen on magnesium and an AES study enabled estimates of surface charging effects to be made (296);two oxide layers could be detected by SIMS (58). Oxidation of aluminum from the gas phase was studied by ELL (29),by light emission during neon bombardment (320), by SIMS (55,339),by AES (339),by an AES/ELL combination ( I O ) , by X P S (199),by electron-excited x-ray emission (426),by Rutherford backscattering (440),and by determination of the work function using a vibrating capacitor technique ( 7 ) ;segregation of oxygen and carbon a t a rough aluminum surface was demonstrated by X P S (32).Anodic oxide films have been characterized by ELL (150), IR reflection absorptiometry (390),and Rutherford backscattering (579); this last investigation was particularly interesting in that it was able to demonstrate the influence of impurity atoms on the oxidation rate in solution. A relationship between electrochemical polarization of an aluminum brass in seawater and the Mg2+/C1- ratio of the surface was established by X P S (104). The technique of inelastic electron tunneling spectroscopy has been used to follow the reactions of organic materials in solution with an aluminum oxide substrate (316).

AES was used to monitor the adsorption of both HCl(357) and Cl2 (542)gases by aluminum. T h e deposition of sodium on single crystal aluminum was followed by LEED and confirmed by AES to lead to saturation a t about half a monolayer (488). A study of the failure mode when a polymer is mechanically stripped from aluminum was made using X P S (625);it was shown that failure occurred within the polymer rather than at the interface, and scanning electron microscopy indicated a residual layer a few angstroms thick. Changes a t the surface of aluminum alloys following various treatments were followed by ISS, SIMS, and AES (43). Oxide films on titanium have been studied by APS (17, 459),SIMS (55,339,437),and AES (63,214,339,543);additional measurements using ELL allowed one of the studies to demonstrate the various growth processes involved. The aging of titanium-gold metallization on tantalum nitride resistors in air-HC1 environments was followed by electron diffraction and microprobe techniques as well as electrical and mechanical tests (549);the major degradation was a t the bonding layer. ISS was applied to profiling a silicone film on a titanium alloy (42).FEM was used to study the adsorption of chlorine on p-titanium (15). Two commercial titanium alloys were subjected to various mechanical and heat treatments; after fracture, their microstructures could be identified by SEM (114).ISS, SIMS, and AES were used (43) to study surface changes arising from various treatments of titanium alloys and AES to examine fracture surfaces (225). Copper. A structural change in the surface of copper after exposure to nitrogen was detected with ultraviolet spectroscopy (477). Oxide formation has been studied by RHEED (220, 221, 363) and by LEED (402).IMMA was applied to the distribution of oxygen across the surface of copper (182)and SIMS (55,437)to the determination in depth. The presence of oxides of differing valencies was shown by IR emission from the heated metal (318). Oxidation in solution was followed by reduction potential (248) and by photopotential measurements (580).The removal of oxygen and a number of other contaminants by ion bombardment and heating in UHV was followed by LEED and AES (299).The adsorption of carbon monoxide on copper can be determined by LEED/AES (308) and by IR spectroscopy (492, 576, 577), and a number of studies have been made on the effect of chalcogens on the surface (16,69,283,309,517),mostly using LEED and AES; a somewhat different technique utilized heavy ion bombardment to generate x-rays characteristic of sulfur in the surface of copper (and of nickel and steel) (102).The deposition of bismuth (149) and lead (240) was studied using LEED and of gold (443)using AES and SIMS. For copper-nickel alloys, both AES (238)and ISS (87)have demonstrated the surface to be rich in copper. The adsorption of oxygen on such alloys has been studied by LEED and AES (179) and by XPS (405)and of CO on SiOz-supported alloy by IR spectroscopy (142).An AES investigation of treated copper-silver alloy surfaces (81) showed ion sputtering to leave a copper-rich surface whereas cleaving gave a silver-rich surface. Changes in the surface structure of copper-gold alloys were studied with AES (489,555) and LEED (489);stoichiometry of disordered surfaces can be restored by annealing. This alloy surface was shown by AES to be high in gold by some workers (384)but high in copper by others (590);however, these latter also observed sulfur which they believe to account for this variation from theory. Fracture studies of copper and brass have been carried out by SEM ( 1 , 301, 525). Vanadium, Chromium, and Manganese. ISS was used (436)to identify two successive oxide phases on the surface of vanadium; AES (91, 339), X P S (91),and SIMS (55, 339) have also been used to determine oxide films. AES revealed ANALYTICAL CHEMISTRY, VOL. 49, NO. 5, APRIL 1977

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T a b l e I. Abbreviations AES APS ELL EELS EMP EPR ESD ESR FEM FIM IMMA ISS LEED MS NMR RHEED SAM SEM SIMS TEM UHV TIPS XPS XRD XRF

Auger electron spectroscopy Appearance potential spectroscopy Ellipsometry Electron energy loss spectroscopy Electron microprobe analysis Electron paramagnetic resonance Electron stimulated desorption Electron spin resonance Field emission microscopy Field ion microscopy Ion microprobe mass analysis Ion scattering spectroscopy Low energy electron diffraction Mass spectroscopy Nuclear magnetic resonance Reflection high energy electron diffraction Scanning Auger microscopy Scanning electron microscopy Secondary ion mass spectroscopy Transmission electron microscopy Ultra high vacuum UV photoelectron spectroscopy X-ray photoelectron spectroscopy (ESCA) X-ray diffraction X-ray fluorescence

(191)the presence of a sulfur impurity on a specimen of vanadium cleaved in UHV. Chromium oxidation has been studied by LEED and RHEED techniques (169, 319, 421) and by AES (169, 339), APS (4591,SIMS (55,339),XPS (199),and ISS (56).ISS was also used to determine (254)depth profiles of gold ion-plated on chromium; a transition a t the boundary of about 100 atomic layers was observed. Reaction of oxygen and water vapor with manganese was examined by XPS (199). Iron, Cobalt, a n d Nickel. The surfaces of iron and steels have been widely examined for corrosion, wear, and friction properties. For iron, a study was made (317) using LEED, AES, and ELL which suggested hydrogen and organic vapors do not inhibit welding whereas oxides and sulfides do. Almost every technique has been applied to the characterization of oxide films including LEED-AES combinations (369, 370), RHEED (530),APS (17,180,459),XPS (180),AES (528,554, Mossbauer spectroscopy (581),electron-excited x-ray emission (426),SIMS ( 5 5 ) , SEM ( g o ) , and proton-induced xradiation (440).Kinetic studies using surface potential (227) and sticking coefficents (271) as parameters have also been reported for iron and other transition metals. The adsorption of tin on iron has been monitored by AES (526).AES was also applied (371) to the study of surface enrichment in a Fe (84%)-Cr (16%)single crystal. Scratch deformed surface layers of single crystal iron were examined (314) by x-ray diffraction and shown to exhibit features dependent on the orientation with respect to the lattice. The SEM is a valuable tool in the study of fracture mechanisms, fatigue, and wear of steels; several excellent examples can be found in the IIT SEM series ( 1 , 49, 139, 183, 304, 364, 419, 478,479,598) and elsewhere (389).Additional information can be obtained by AES (383) and X P S (465),particularly on the effect of segregation a t fracture sites. Surface segregation, particularly of stainless steels, has been determined by SIMS (524),AES (145,380),a combination of these with ISS (43),and by an emission spectrographic technique using an ion sputtering source (51,597).This last technique was used to profile concentrations into the bulk and ionsputtering is a common tool for this purpose. For example, AES and XPS were used (127)with this technique to profile oxide films on chrome steel. Helium backscattering was similarily applied (147)to the profile of oxygen and heavy metals into a stainless steel. Oxidation of an 18/8stainless was studied by AES ( 9 ) . The thickness and refractive index of a surface oxide film on cobalt were determined by ELL (469). A considerable volume of work has been carried out on the 222R

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surface structure of single crystal nickel. The emphasis has been on LEED studies of chalcogen adsomtion (includina oxidation) (45, 162, 258, 392, 474,-475, 5373, either alone 0"r complemented by other techniques such as AES (259, 260, 270), which has also been used independently (269,503)and combined with ISS (563).Oxide formation on single crystals was also examined by combining LEED with ISS (236) and by ISS alone (89).The adsorption of hydrogen was followed by AES (79)and LEED (121),of carbon monoxide by LEED (353),and LEED/AES (385),of nitric oxide by LEED, AES, and PES (131),and of benzene and cyclohexane by LEED and MS (140). Carbon segregation on single crystal nickel was investigated using AES (269,286,533) and LEED (286,588) and gold using AES (98).LEED was also applied (480)to lead monolayers and EELS (18) to the study of sodium and potassium adsorption. A study of clean nickel surfaces of varying orientations was made with LEED (151) and of clean and oxidized surfaces by AES and X P S (264);interpretation of UPS and X P S data from these surfaces has also been given (418). The growth of electrodeposited nickel was monitored by electron microscopy (223), both T E M and SEM. An AES study of embrittlement in impurity-doped nickel (601) revealed grain boundary segregation of sulfur, bismuth, and tellurium. The surface segregation of carbon was investigated using AES for a relatively pure nickel (128)and oxidation of a similar material was studied by APS (459),electron-excited x-ray emission (426),and SIMS (55,437),adsorption of sulfur by AES (4811, of nitrogen and its oxides by XPS and UPS (92) and of carbon monoxide by IR reflection spectroscopy (401). Surface composition changes were determined using, in conjunction with ion sputtering, XPS (261) for a nickel-aluminum alloy, X P S (561)for a nickel-boron electroless coating, and AES for a Rene alloy (225),a nichrome resistor (470),and Inconel (145). R e f r a c t o r y Metals. Oxygen in Zircaloy surfaces was determined by charged particle activation (374).A more detailed study (156)correlated oxide formation and impurity surface segregation, as revealed by AES, with mechanical properties for two Zircaloy alloys. Oxide formation on niobium was investigated using IR reflectance spectroscopy (564),electron-excited x-ray emission (426),and SIMS (55,438,460)and on single crystal niobium using LEED (184);this last investigation also included nitrogen and this adsorbate was also studied (154) using AES as well as LEED. A star-shaped surface feature found on niobium surfaces after ion implantation with hydrogen was identified as a carbide using TEM, SEM, and SAM techniques (568). The adsorption of oxygen and other gases on single crystal tantalum has been studied using RHEED (266,529),LEED (116),SIMS (436),and AES (116,336);gold condensation on single crystal was also examined ( 173)with LEED and AES. Oxide formation on polycrystalline surfaces was followed by IR reflectance spectroscopy (322), electron-excited x-ray emission (426),IMMA (429),and SIMS (55,460).The surface. diffusion of mercury atoms was demonstrated (66) by field emission microscopy. A large body of literature exists, as might be expected, on surface studies involving refractory metals, and a wide-ranging study of several metals has been made with LEED and AES (157).Oxygen reactions with single crystal molybdenum have been followed by AES (265,504),LEED (504,512),and EELS (504)and with polycrystalline metal by AES (35,257).Carbon monoxide adsorption on single crystal metal has also been studied by LEED (200, 512) and AES (215); radiotracer techniques were correlated with the adsorption and desorption from both single crystal (397) and polycrystalline (159) molybdenum. Some measurements have been made (40)to determine the relative sensitivities of XPS and UPS in detecting carbon monoxide on polycrystalline molybdenum. Other gases investigated on single crystal metal include hydrogen (512), nitrogen (284),and H2S (621),all by LEED, complemented by AES in the last case. AES/LEED studies have also been made on the deposition of alkali metals (571), aluminum (289),silicon (282),and tin (288) on specific crystal faces of molybdenum; FEM has also been applied (129,593)to similar silicon depositions. Sulfur segregation on molybdenum ribbon during heat treatment was examined by AES (347). The surface structure of clean single crystal tungsten was

Phlllp F. Kane's long career with chemical analysis began in 1938 at the quality control laboratories of Standard Telephones and Cables Ltd., London. While there he studied part time at London University and was graduated in 1948. The following year he joined Laporte Chemicals Ltd., Luton, England, where he developed methods for trace determinations in hydrogen peroxide and organic peroxides. In 1957 he joined the Chemagro Corp. in Kansas City, Mo., to direct a group developing methods for the analysis of organophosphorus pesticides. Since 1959 he has been with Texas Instruments in Dallas and has concentrated his research efforts in the characterization of semiconductor materials. He is currently director of their Materials Characterization Laboratory Mr. Kane has published a number of articles in ANALYTICAL Journal of Agricultural and Food Chemistry, and Chemical TechCHEMISTRY, nology. Coeditor of the book, "Characterization of Solid Surfaces", he is also a contributor to "Standard Methods of Chemical Analysis" and "Annual Reviews of Materials Science". Mr Kane is a member of the Society for Applied Spectroscopy and the Chemical Society (London) and is a Fellow of the Royal Institute of Chemistry. He is currently serving on the Advisory Board of ANALYTICAL CHEMISTRY.

Graydon 6.Larrabee has received his MSc. in Chemistry, McMaster University, Hamilton, Ontario, Canada, and B.Sc. in Chemistry, University of Bishop's College, Lennoxville, Quebec, Canada. He is author of articles published in Analyst (London), ANALYTICAL CHEMISTRY, Journal of the Electrochemical Society, and Zeitschrift fur Analytische Chemie; "Surface Contamination" in "Preparation of the Ill-V Compounds", Reinhold Pubiishing Co., New York, 1962; coauthor with P. F. Kane on the "Characterization of Semiconductor Materials". McGraw-Hill. New York. 1970: coeditorwith P. F. Kane on "The Characteization of Solid Surfaces", Plenum Press, New York, 1974; and coauthor with P. F. Kane, "Trace Analysis Techniques for Solids", in "Annual Reviews of Materials Science", Annual Reviews, 1974. Mr. Larrabee is Manager of the Characterization Branch in the Materials Characterization Laboratory of Texa&lnstruments. He joined Texas Instrumentsin 1959 as a radichemist, engaged in the application of radioactive tracer techniques to diffusion, crystal growth, etching, and surface studies of semi-conductors. Current work in this branch is directed toward developing advanced techniques for the characterizationof electronic materials including ion microprobe analysis, Auger spectroscopy, x-ray topography, and optical spectroscopy. The deveiopment of minicomputer automated techniques is an integral part of this program. Mr. Larrabee has worked extensively on silicon and GaAs transistor process development, silicon integrated circuit and tantalum electrolytic capacitors. From 1954 to 1956, he was employed at the Canadian Atomic Energy Establishment at Chalk River, Ontario, where he carried out extensive work on the trans-uranium elements fission product analysis and the use of tracers in analytical studies. As an analytical chemist at Canadian Westinghouse, he worked in the fields of ultraviolet, visible, and infrared spectroscopy: vacuum fusion analysis for gases in metals: and emission spectroscopy. He is a member of: Electrochemical Society, Society for Applied Spectroscopy (North Texas Section President),and Dallas Society of Analytical Chemists.

studied by LEED (589,600),AES (600),and FIM (600).The reaction of oxygen with such material was followed by techniques which have included LEED (26, 177, 268, 276, 349, 472), LEED-AES combinations (27, 34,267),AES (37, 441, 604),UPS (186,187,409,5961,XPS (409,626),ISS (273,451), SIMS (53),ESD (53,386),and FEM (134).An earlier application of the SIMS-ESD combination (54) was to the adsorption of oxygen on polycrystalline tungsten; similar studies have also been made with AES (138)and MS (400,602).Anodic oxide films on tungsten, formed electrolytically, were examined (518) by ELL and TEM. Carbon monoxide adsorption on single crystal tungsten has been studied by RHEED (398),LEED (4,34,276),AES (34,115,541), UPS (187,409,594),X P S (409),ESD (447,628),ELL (541),and by molecular beam scattering (551).CO adsorption on polycrystalline tungsten was monitored by a SIMS-ESD combination (54),XPS (629),and IR spectroscopy (627).Numerous other gas reactions have been studied using single crystal tungsten as the substrate. Hydrogen has been reported ex-

amined by HEED (362),LEED (473,521,630),ESD (54,328) and UPS (186,187),rare gases by LEED (521)and UPS (626), nitrogen by LEED ( 2 , 3 ) ,UPS (187),MS (2,538),and F E M (616),rare gases, nitrogen, and deuterium by molecular beam scattering (551), carbon dioxide by MS (125), oxides of nitrogen by LEED (610),chlorine by FEM (251), iodine by LEED-AES (28),and sulfur by FIM (146);in addition, several hydrocarbons have been studied using FEM (237,534),FIM (65),LEED/XRD (466), and AES (115), and methane and formaldehyde by UPS (166). Polycrystalline surface adsorption of hydrogen was determined by ESD (298) and SIMS-ESD (54)and nitrogen and nitric oxide by XPS (387). The effect of different heating and cleaning treatments on the microstructures of various ribbon, wire, and ingot samples of tungsten was examined by SEM (372) and compared to adsorption theories. An embrittled material was examined by AES (225). A considerable volume of literature related to the diffusion and segregation of metals on single crystal tungsten using FEM and FIM. Included are studies of alkali metals (573), carbon (484),silicon (293,560),titanium (14,61),nickel (303), rhodium (106),iridium (41),copper (105,486),silver (302), and germanium (293). Other single crystal studies involve alkali metals by LEED (407,472,521,572)and AES (572),and mercury by UPS (167). The thermal rearrangement of single crystal rhenium surfaces was studied by FIM (136)and comparisons of clean and oxidized surfaces made using LEED (158, 633). The microstructure of both polycrystalline and single crystal samples was determined after various treatments using SEM (372). AES has been applied (606) to the reaction of oxygen with polycrystalline rhenium ribbon. Carbon monoxide (633) and carbon (635)have both been detected on single crystal rhenium by LEED and adsorption of titanium by FEM (14). Precious Metals. Surface rearrangement by heat of single crystal ruthenium was examined by FIM (136).The reaction of oxygen with single crystal ruthenium was examined using LEED/AES (212),FIM (135),and UPS/XPS (198),of CO and COz using LEED/AES (212),of CO using UPS/XPS (409), and of CO and nitric oxide using UPS (70). Using LEED, the presence of oxygen, CO, and COa (212) and of carbon (583)was detected on single crystal rhodium. The adsorption of hydrogen on palladium single crystal surfaces was studied by LEED (121, 132) and on palladium film by determining surface potential (164);similar studies were performed on electrodeposited metal using a quartz crystal microbalance (97).Surface potential measurements were also used to follow the adsorption of ethylene (165) on evaporated films. LEED was applied (130,353)to examining the reaction of carbon monoxide with different crystal faces. Studies of ion-sputtered and fractured surfaces of alloys with Ag and Ni were made by AES (396). The microstructure of single crystal iridium surfaces was studied by LEED/AES (209) and by LEED using xenon to intensify the scattering (285,453).The adsorption of nitrogen on various crystal faces was determined using the atom probe FIM technique (452) and CO adsorption by LEED (353). The reactivity of specific platinum crystal faces to oxygen has been determined using LEED (72,300)and LEED/AES (332, 356, 445). Anodic oxides were evaluated by ELL (272, 327) and reflectance spectroscopy (442).Oxygen down to 0.01 monolayer was detected (456) on platinum by X P S and the species identified by UPS. The surface composition changes of two Pt/Sn alloys were followed (75, 76) by AES and XPS as a function of annealing in vacuum, oxygen, and hydrogen. The reaction of hydrogen with Pt (111)was studied by LEED and EELS (120)and by LEED/AES (445);this last paper also includes the interaction with oxygen. LEED patterns have been reported (332,353, 446) for carbon monoxide on single crystal faces of platinum and the adsorption has also been studied by AES (354)and UPS (457);this last study also included carbon dioxide. The adsorption of both CO and nitric oxide was examined by AES and UPS (70) and of sulfur by LEED/AES (59,233).LEED has also been used to study the reactions of a number of organic compounds with single crystal faces of platinum, including aromatics (38, 205), saturated hydrocarbons (38),and ethylene (38,355,356,609),and FIM was used to follow hydrocarbon reactions (65).XPS was applied ( 6 )to the determination of mercury on platinum black. The surfaces of platinum-gold alloys were studied by FIM ANALYTICAL CHEMISTRY, VOL. 49, NO. 5, APRIL 1977

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(485,522).The surfaces of platinum electrodes were evaluated by ELL (100). The surface roughness of silver films was determined by scattered light intensity (67).The adsorption of oxygen on the crystal faces of silver has been followed by LEED (235,508, 509), AES (235,508),ISS (235),FEM (137),and FIM (523). A detailed study of silver surfaces of films prepared in a number of ambients correlated optical properties with surface composition as determined by AES (467),and oxidation of polycrystalline metal was followed by EPR (124). Xenon and carbon monoxide structures on Ag(II1) were identified (403) using LEED, AES, and EELS. A LEED/AES investigation determined (507) chlorine monolayers on various low angle silver faces and an infrared ELL method (155) was applied to butanol adsorption. Carbon monoxide adsorbed on epitaxial silver-palladium alloys was the subject of a LEED investigation (119) and silver-tin alloys were examined by X P S (262).

The gold(001) surface was characterized (632) by positive ion channeling spectroscopy, LEED, and AES. Formation of an anodic oxide film on gold was followed by voltage scanning ELL (272),and adsorbed oxygen was characterized (330) by visible reflectance spectroscopy. IR reflectance spectroscopy was applied (342) to determining carbon monoxide on polycrystalline gold. Rapid changes on the surface of gold electrodes were followed (100)by ELL, and specular reflectance was applied (565) to similar changes on a gold electrode by electrosorption of aromatics from solution. The distribution of adsorbed sulfur on different crystal faces of gold was studied (341) by LEED/AES and the surface diffusion of tin (575) by AES. In-situ measurements of reflectivity were used to follow surface changes on a gold electrode as thallium, copper, and lead were electrodeposited (337). Other Metals. The oxidation of indium was followed by ELL (172). Electrodeposited tin surfaces used for solder connections were examined (399)by SEM. The growth of thin oxide films on lead was studied using ELL, XRD, and electron tunneling spectroscopy (170);analysis of the data ( 171) suggested an oxygen vacancy mechanism. Surfaces of lead-acid electrodes were examined (99, 118) by T E M and the lead sulfate crystallites characterized by XRD. The oxidation of polycrystalline strontium and barium was investigated by SIMS (58). LEED patterns have been recorded (176) for arsenic, and carbon contamination was detected by X P S (30). The thicknesses of oxide films on erbium were determined (257) using AES with ion bombardment. The clean (100) crystal face of thorium was studied (567) using LEED and AES, and the chemisorption of oxygen and carbon monoxide monitored by the same techniques. The surface segregation of sulfur, carbon, and phosphorus in polycrystalline thorium was investigated (174) by AES.

SEMICONDUCTORS Silicon. The electrical properties of the silicon surface were determined by measuring the surface photovoltage (234,312), photoelectric effect (344), lifetime (188), acoustic surface waves (144),and surface barrier distribution (614).Abraded silicon surfaces were characterized using E P R (566) and optical techniques (463). Cleaved surfaces were studied using LEED, AES, electron microscopy, and optical reflection (210, 211,213,243). E L L has been widely used to characterize silicon surfaces (280,281,410-412). Surface structure has been investigated using ISS (86,94),LEED (193,226),and RHEED (108,513). Silicon interfaces have been characterized (73,559, 623) by using SEM, AES, XRF, and electrochemical techniques. The presence of impurities and their distribution into the surface have been determined using MS (46,126,496),IMMA (195,204,613), Rutherford backscattering (95,96,417,634), ISS (88,206),AES (107,415,427,454,595),E M P (247,540), LEED (570),and FIM (408).Hydrogen chemisorption (279, 307, 511) and the reactions of PH3 (586) and SiH4 (239) a t silicon surfaces have been studied using AES and LEED. The alkali metals, particularly Na and K, deteriorate the performance of MOS devices. Flame emission spectrometry, with a sensitivity of 1 part in IO9 was used to measure the effects of traces of K and Na on silicon device surfaces (333, 334). These alkali metals were deposited from chemicals used in 224R

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routine chemical treatments. Boron and oxygen surface contamination was measured using the prompt a-particles produced by 685-keV proton bombardment (455).AES has also been used (230) to analyze the top few atom layers of surface after chemical etching. The sensitivity of AES for B and P was established (569) to be -8 X l0ls atoms/cm3 or N 1.6 X 10I2 atoms/cm2assuming a 2-nm escape depth. A quantitative AES study of P on Si was made using a quartz microbalance (365). The Auger yields of C, N, 0, P , S, and C1 were calibrated on Si and Ge (413) by combining AES and ELL. Nitridation of silicon surfaces was studied using AES (230) and LEED-AES (232). Glow discharge ion treatment of silicon (546) was found to ion implant N, Ni, Al, and C into the surface. R F oxygen plasma stripping of organic photoresists was monitored using the optical emission from electronically excited OH and CO species in the UV region of the spectrum (148). Carbon contamination has been confirmed by LEED (231). Adsorbed C02 (4581,CO and 0 2 (305,329,330), and 0 2 (161,277,278, 61 7 ) on silicon surfaces have been investigated using electron spectroscopy. There has been considerable discussion (160, 306, 414, 416, 510) in the literature on the interaction and adsorption of oxygen. Reactive scattering of atomic oxygen was measured from clean Si and Ge surfaces (388).Low energy electron scattering was employed to measure the effect of a thin oxide on the surface plasma loss in Si (608). Surface passivation in silicon surfaces has been characterized using electron spin resonance (535).The optical properties of the clean and slowly oxidized surface of silicon were determined (117). Room temperature oxidation of Si and GaAs was measured (378) and follows an exponential law for GaAs and a cubic law for silicon. Oxygen on silicon surface was determined semiquantitatively using time-of-flight MS with a laser ion source (153)and quantitatively by 3He activation analysis (360). SIMS was used to determine oxygen as well as other elements on silicon surfaces (52,57).Detection limits of well g) were reported. The below of one monolayer ( effect of the substrate on Si02 thicknesses was determined by reflectance ellipsometry (5, 592). The electrical properties of the Si-Si02 interface have been measured by MOS capacitance techniques (103) and electron relaxation time (345,346). Cesiated Si (224) and GaAs and Si (297) surfaces have been investigated. The adsorption and nucleation of indium on clean (111) silicon surfaces was studied (422, 448, 449) by UHV molecular beam MS and a flash desorption technique. Nickel deposits on silicon surfaces were characterized by LEED and AES (111).The possibility of a surface-stabilized compound of Au and Si on Si surfaces was postulated (468). Palladium silicide (359), iron silicide (358), nickel silicide (582), and platinum silicide (13, 430) films on silicon have been identified and well characterized. The diffusion of silicon a t a Si-metal interface has been measured using AES (143, 607) and ion backscattering (252). Germanium. The clean surfaces of a full range of Si-Ge alloys were measured by E P R (425). Correlation of various EPR signals with varying Si and Ge concentrations was made with changes in the bulk band structure which in turn changed the surface states. The roughness of cleaved Ge surfaces was measured optically and correlated with LEED (211, 243). Photoemission and photovoltage of Ge surfaces have been used to measure surface states (93,234),surface barriers (614), surface phonons (234),chemisorption-bond geometry (511), and the effect of cesium and sodium coverage on surface potential (500). The effect of edge dislocations (352) on the electrical properties of the real Ge surface was shown to be a function of the edge dislocation density. The surface recom.bination velocity increased from 100 cm/s for lo4 dislocations/cm2 to several thousands cm/s for lo6 dislocations/cm2. Electron beam bombardment reduced the surface lifetime of minority carriers but did not affect mobility (188).An electron spin resonance study (603)on heat treated Ge surfaces showed four types of centers depending on whether the heating was done in air or vacuum. One center was associated with carbon contamination. FIM and FEM patterns were used to observe surface states on Ge (178).Oxygen adsorption was observed to decrease the surface density of centers above which field ionization occurs (424).ELL in the wavelength region 0.34-1.8 pm was used to study chemical adsorption of gases on clean Ge surface (411).The effect of oxygen on surface structure and surface states of Ge has been determined using LEED (242, 393) and HEED (573). MS (153) and AES (388, 413) have

been used to measure oxygen concentrations on Ge surfaces. The role of water vapor on clean cleaved Ge has been determined (244, 539, 578). Three binding states of water vapor were measured and changes in surface state density correlated with surface coverage. 111-V Compounds. A modulated molecular beam technique was used to determine the interaction kinetics of As4 and Ga on (100) GaAs surfaces (194).It was shown that above 450 K it was possible to produce GaAs from beams of the elements but below this temperature, the compound does not form. SAM was used to obtain surface images a t a rate of