Langmuir 1998, 14, 1493-1499
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In Situ Investigation of the Surface Chemistry of Atomic-Layer Epitaxial Growth of II-VI Semiconductor Thin Films Y. Luo, D. Slater, M. Han, J. Moryl,* and R. M. Osgood, Jr. Columbia Radiation Laboratory, Columbia University, 500 West 120th Street, New York, New York 10027
J. G. Chen Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received July 3, 1997. In Final Form: November 26, 1997 Atomic-layer epitaxy (ALE) can provide atomic-scale control of the single-crystal growth of thin semiconducting layers. Despite the widespread investigation of the epitaxial properties of this technique, few detailed studies of the fundamental surface chemistry of the process have been performed. This paper describes an overview of such an in situ study of heterogrowth using a binary reaction sequence with the precursors H2S and dimethylcadmium to grow CdS on ZnSe(100). The surface chemistry was investigated using thermal desorption spectroscopy and near-edge X-ray absorption fine structure (NEXAFS). Epitaxial growth was also characterized in the growth chamber using electron and ion surface probes.
Introduction The development of modern thin-film technology has increasingly focused on the problems of truly atomic-layer control of thin-film processes. Thus, the emphasis is not only to understand the methods of processing an assortment of thin film materials but also to deposit and etch them so that only a single atomic layer is processed. This control is needed in a variety of emerging optical and electronic devices. For example, resonant tunnel junction transistors require blocking tunnel regions only a few monolayers in thickness. At Columbia, Brian Bent was a leader in exploring new chemical routes to such atomic-layer-controlled processes. In the year before his passing, he worked with his group and, in collaboration, with colleagues such as Irving Herman in the Columbia Radiation Laboratory to develop a new method of molecular beam etching of GaAs, which would successively remove first the Ga- and then the Asterminating layers of a (100)-oriented GaAs wafer. His enthusiasm in his work was so great that many of us at Columbia were spurred on to initiate our own new projects in this exciting area of surface chemistry. For example, the collaboration with Brian led Irving Herman and his group to begin a new research initiative in developing ab initio methods for understanding the optical response in the very first few layers of a semiconductor surface, with the objective being to understand the fundamental basis of surface photoabsorption, a new technique for interfacial probing. In their laboratory at Columbia, the authors of this paper have recently begun research on achieving a very basic chemical understanding of the techniques of atomic-layer epitaxy. The goal here is to use the tools of modern surface chemistry to investigate the microscopic chemical mechanisms for this process. In general, atomic-layercontrolled growth relies on the selective reactions of organometallic or hydride precursors with specific surface sites on a compound semiconductor or insulator crystal. Once this layer is reacted, thus forming a new surface
termination, the reaction ceases, and the second precursor must be introduced.1 While atomic-layer epitaxy (ALE) has been investigated extensively by many groups who are interested in growing high-quality single-crystal thin films of group III-V, group VI, or II-VI semiconductors,2-7 it has not generally been viewed as an opportunity for surface chemical investigations. Recently, however, surface chemistry has been used to realize self-limiting growth of amorphous insulators; see, for example, the accompanying paper by George and collaborators.8,9 In addition, ALE presents several intriguing and fundamental questions for surface chemists. For example, while it is generally appreciated that the process of chemisorption itself can result in highly ordered adlayers, the possibility that a macroscopically thick crystalline layer can result from a series of such adsorption reactions has apparently not been considered, and yet this is precisely the result of the ALE process. Second, because surface reactions lead to the deposition of a specific fraction of a monolayer, in our case exactly one monolayer, a sequence of such reactions can in principle lead to the deposition of a specified number of such layers. The chemical forces that control the epitaxial growth and chemical abruptness of such layers, after multiple layers are grown, also need investigation. Finally, while the (1) Suntola, T.; Simpson, M. Atomic Layer Epitaxy; Chapman and Hall: New York, 1990. (2) Suntola, T.; Hyvarinen, J. Annu. Rev. Mater. Sci. 1985, 15, 177. (3) Wu, Y.; Toyoda, T.; Kawakami, Y.; Fujita, Sz.; Fujita, Sg. Jpn. J. Appl. Phys. 1990, 29 (5), L727-730. (4) Yoshikawa, A.; Okamoto, T.; Yasuda, H.; Yamaga, S.; Kasai, H. J. Cryst. Growth 1990, 101, 86-90. (5) Yoshikawa, A.; Kobayashi, M.; and Tokita, S. Appl. Surf. Sci. 1994, 82/83, 316-321. (6) Jow, M. Y.; Maa, B. Y.; Morishita, T.; Dapkus, P. D. J. Electron. Mater. 1995, 24 (1), 25-29. (7) Yarmoff, J. A.; Shuh, D. K.; Durbin, T. D.; Lo, C. W.; LapianoSmith, D. A.; McFeely, F. R.; Himpsel, F. J. J. Vac. Sci. Technol., A 1992, 10 (4), 2303-2307. (8) George, S. M.; Sneh, O.; Dillon, A. C.; Wise, M. L.; Ott, A. W.; Okada, L. A.; Way, J. D. Appl. Surf. Sci. 1994, 82/83, 460-467. (9) Dillon, A. C.; Ott, A. W.; George, S. M. Mater. Res. Soc. Symp. Proc. 1994, 335, 335-340.
S0743-7463(97)00732-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/06/1998
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importance of surface excitations, from electrons, ions, or photons, in enhancing such reactions has been considered by several groups, including earlier work in our laboratory, the detailed chemistry resulting from the excitation process has not been examined. While the goal of ALE is to provide atomic-layer control during the growth process, the reliance on precursor chemistry with a significant thermal barrier to reaction initiation can obviate precise control over atomic-layer thickness. Specifically, in the case of surface reactions at high temperatures, thermal diffusion of the overlayer or the substrate atoms can blur out the interface: This interfacial diffusion is a particularly severe problem with high-bulk-mobility group II atoms such as Cd, Zn, or Hg.10 In the case of organometallic precursors, high reaction temperatures often lead to decomposition of the organic ligands and the incorporation of carbon impurities in the lattice.6,11 In this paper, a study of the surface chemistry for atomiclayer epitaxy of a II-VI heterostructure is presented. For the system chosen it was found that the highest-quality epitaxial growth occurs at room temperature. This roomtemperature epitaxial growth is a dominantly surfacechemistry-driven process; the selective deposition of the appropriate species on the correct crystal sites is determined by chemical driving forces. This situation contrasts with growth at high temperatures where the driving force is mainly diffusion and the sticking/desorption process. Layer-by-layer growth is accomplished by taking advantage of the differential reactivity of the polar ZnSe(100) surface that is terminated with either Zn or Se atoms. Thus, it is convenient to separate one deposition cycle into two single steps, the deposition of one monolayer of Cd and one monolayer of S, each as part of a binary reaction sequence. The crucial condition for ALE growth that distinguishes it from conventional chemical vapor deposition is that a chemical mechanism must be present, typically ligand termination, such that deposition stops after saturation coverage is reached. In addition, there must be a temperature window in which the passivating ligands are thermally stable on the surface, while still being reactive with the appropriate precursor molecules. In this paper, the results of CdS growth via “near-perfect” ALE will be presented in which one full monolayer of both cadmium and sulfur each can be deposited in alternate dosing cycles, and the detailed surface chemistry of this process is studied. Ideal Deposition Sequence The four basic reaction steps which comprise our ideal ALE process are summarized in Figure 1. In the first step, dimethylcadmium (DMCd) reacts with the clean ZnSe surface to fill in the metal atom vacancies in the c(2 × 2) surface and thus complete a full monolayer coverage of metal terminated with CH3 ligands. Ideally in this case, an ordered (1 × 1) surface lattice of alternating Cd and Zn atoms would exist, each of which is methyl terminated. The presence of the surface methyl groups renders the surface passive to further reaction with DMCd. The second reaction step consists of the reaction of H2S with the above surfaces. In this step, CH3 ligands are reactively displaced, yielding desorbed CH4 and Hterminated surface sulfur atoms. Again, ligand passi(10) See for example: Bhargava, R. N.; Ruth, R. P.; Yao, T.; Nurmikko, A. V. J. Cryst. Growth 1994, Proceedings of the Sixth International Conference on II-VI Compounds and Related Optoelectric Materials. (11) Skromme, B. J.; Liu, W.; Jensen, K. F.; Giapis, K. P. J. Cryst. Growth 1994, 138, 338-345.
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Figure 1. Schematic of the proposed surface reactions for the ideal layer-by-layer growth of CdS on a c(2 × 2) ZnSe surface.
vation, in this case H-termination, should cause the reaction to stop after a single monolayer of sulfur is deposited. Note that the effectiveness of this reaction sequence is dependent on the formation of a fully saturated sulfur layer, where H2S has reacted with both the Cd and Zn sites. The third and fourth reaction steps are those which can lead to the growth of the bulk crystal. In the third step, DMCd would react with the H-terminated sulfur surface to form chemisorbed metal atoms, desorbed methane molecules, and a terminating methyl group capping each adsorbed metal atom. This methyl-terminated surface would then react with H2S in a process very similar to that in step 2 except that in the present case only surface Cd is present. Experimental Apparatus The experiments were performed in a UHV chamber pumped by a turbomolecular pump, a titanium sublimation pump, and an ion pump with a base pressure of 5 × 10-11 mbar. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering (LEIS) were done with a hemispherical energy analyzer. These probes were employed to measure the chemical composition of the growth surface, while the surface structure was monitored with a LEED system mounted in a connecting chamber. In addition, thermal desorption spectroscopy was done in situ via a differentially pumped quadrupole mass spectrometer with an entrance aperture located close to the sample. The sample was mounted on a molybdenum foil, which could be resistantly heated and which is also in thermal contact with a copper block for cooling with liquid nitrogen. The precursors, that is, DMCd and H2S, were introduced to the growth surface via two separate dosers, which include leak valves and directed tubing, so as to have precursors impinging on the sample. The dosages presented in this paper are uncalibrated background flux, unless specifically mentioned. The actual dosage on the sample is estimated to be ∼3 orders of magnitude higher than that indicated on the figures. A (100) single-crystal ZnSe substrate was prepared with 1000 eV Ar+ ion sputtering at room temperature followed by annealing at 415 °C. A well-ordered c(2 × 2) reconstruction was obtained after this cleaning procedure. The c(2 × 2) reconstruction of II-VI compound semiconductors is known through both theo-
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Figure 2. AES (MNN at 404 eV) peak-to-peak intensities for deposited Cd versus DMCd dosage on a clean ZnSe c(2 × 2) surface. retical and experimental studies as a stable reconstruction which is terminated by a half monolayer of group II elements.12,13
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Figure 3. XPS peak intensities of Cd 3d5/2, using a Mg anode: (a) consecutive dosing of DMCd onto X-ray-irradiated surfaces; (b) dosing of DMCd on a clean ZnSe surface with no X-ray irradiation.
Characterization of the Growth Process In the first step the c(2 × 2) Zn-terminated surface is dosed with DMCd with the intent of filling the 2-fold vacancies and thereby producing a full monolayer of methyl-terminated metal atoms. Figure 2 shows the measured Cd Auger peak-to-peak intensity versus the relative DMCd dosage, to the c(2 × 2) ZnSe; note that each data point is obtained by exposure of the clean surface to a specific dose of DMCd (see below). The data show that the DMCd reacted with the substrate at room temperature to deposit cadmium on the surface. Furthermore, it is apparent that the amount of the surface Cd also saturated with increasing dosage, indicating that this deposition self-terminated after a certain Cd coverage was attained. At room temperature this passive surface remained inured to further deposition from the dosed precursor for >2 h. LEED measurements after the deposit of one monolayer indicated the formation of a degraded c(2 × 2) pattern. As will be explained below, this degradation is consistent with two-dimensional intermixing of the Cd and Zn surface lattice. The remains of a 2-fold pattern are due to the difference of Zn and Cd electron-scattering cross-sections. Finally, Zn, Se, and Cd AES intensities for the saturated coverage agree with the calculated estimate for a half monolayer of Cd on a ZnSe(100) surface. The assumptions used in the calculation are discussed below. One might expect the clean surface, which has a halfmonolayer Zn-terminated composition, to also be reactive to the H2S precursor. However, despite exposures of the clean ZnSe surface at room temperature to large amounts of H2S, that is ∼105 L (calibrated), no sulfur could be detected on the surface using AES. This experiment demonstrates that H2S does not react with the c(2 × 2) ZnSe surface at room temperature. Recall that, according to the ideal reaction sequence, dissociative chemisorption of DMCd on the c(2 × 2) surface yields a fully methyl-terminated metal surface of alternating Zn and Cd atoms. However, it was found in our studies that dosing of this passivated surface with DMCd following X-ray or e-beam exposure led to non-selfsaturating growth. This observation, to some extent, limits the utility of various surface probes employing (12) Park, C. H.; Chadi, D. J. Phys. Rev. B 1994, 49 (23), 1646716473. (13) Seehofer, L.; Falkenberg, G.; Johnson, R. L.; Etgens, V. H.; Tatarenko, S.; Brun, D.; Daudin, B. Appl. Phys. Lett. 1995, 67 (12), 1680.
Figure 4. (a) Sulfur AES (LMM at 152 eV) peak-to-peak intensities following H2S dosing onto a surface previously dosed with DMCd. (b) AES signal for Cd (MNN at 404 eV), which does not diminish as sulfur exposure is increased.
electrons, for example XPS or AES. Figure 3 shows examples of (a) dosing a clean ZnSe substrate with DMCd with no X-ray irradiation and (b) consecutive dosing of X-ray-irradiated surfaces with DMCd. In the second case deposition did not saturate at one monolayer and, in fact, multilayers of cadmium are deposited. The origin of this effect is believed to be secondary-electron-induced desorption of the methyl groups,14 so as to expose reactive sites on bare deposited metal atoms. Deposition of Subsequent Layers In the second ALE step, H2S is chemisorbed onto a previously “passivated” surface. In the experiment, hydrogen sulfide was introduced onto the methylterminated binary metal surface through a separate dosing line, also at room temperature. AES of the surface showed only Zn and Se from the ZnSe substrate, cadmium from the previous growth step, and the newly deposited sulfur. No carbon was detected within the sensitivity limit of our AES system, which is ∼5% of one monolayer for carbon. Figure 4 shows the AES intensities of cadmium and sulfur versus the hydrogen sulfide dosage. The data clearly indicate that hydrogen sulfide reacts with the methylcovered, mixed Cd and Zn surface and that sulfur has been deposited. In addition, the AES signal for S reaches a saturation limit for long exposures, apparently corre(14) Lasky, P. J.; Lu, P. H.; Khan, K. A.; Slater, D. A.; Osgood, R. M., Jr. J. Chem. Phys. 1997, 106, 6552-6563.
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Figure 5. AES measurements of layer-by-layer deposition of CdS on ZnSe. AES peak-to-peak intensities of Zn and Se from the substrate and Cd and S from the overlayer are plotted versus the deposited overlayer thickness. The dots are the measured data while the curves represent the simulation of ideal layerby-layer growth.
sponding to a self-limiting step for sulfur deposition, that is,
Cd(CH3)* + H2S f CdSH* + CH4 v Zn(CH3)* + H2S f ZnSH* + CH4 v where * denotes the terminating species. After a full monolayer of sulfur was deposited, a full monolayer of cadmium can be deposited onto this hydrogen-terminated sulfur surface at 300 K by dosing with DMCd. Again the variation of the AES signal with dosage indicates that the reaction self-terminates. The selftermination suggests, specifically, that the surface is passivated with a full monolayer of methyl groups. The proposed reaction for this process is as follows:
SH* + Cd(CH3)2 f SCd(CH3)* + CH4 v This reaction would give a full monolayer of cadmium coverage. The deposition of the first complete cadmium layer concludes the first layer-by-layer growth cycle, and from this point on, CdS can be grown in a true binary reaction sequence by employing sequential dosing of the substrate with DMCd and H2S. Thus, each step of the deposition should result in self-terminating growth of a monolayer of cadmium or sulfur. In fact, by repeating this dosing cycle, up to 15 bilayers of CdS were grown layer-by-layer on ZnSe(100) substrate. Figure 5 shows the AES intensities for CdS layers of increasing thickness, grown by the binary reaction sequence. All of the data presented in this chart have been verified to be the result of selfterminated growth. The film at each point is grown from a freshly prepared substrate before measurement, thus eliminating the electron-enhanced chemistry effects mentioned above. In the figure, the AES intensities of Zn, Se, Cd, and S are plotted versus the number of deposited monolayers. The data show the CdS Auger signal saturates as the crystal thickness increases; this saturation is due to the finite mean-free path of Auger electrons. The final ratio of cadmium-to-sulfur Auger-signal intensities can be used to check the stoichiometry of the grown layer. In our case, this final ratio matches very well with the cadmium-to-sulfur Auger intensity ratio measured from a bulk CdS crystal under exactly the same instrumental conditions. This agreement indicates that stoi-
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chiometric growth of CdS has been achieved. The solid lines in Figure 5 present the result of a simulation of relative Auger intensities that would be expected for an ideal layer-by-layer growth. The relative Auger intensities of each species at specific coverages were calculated on the basis of on the assumptions that the flux of each characteristic electron decays exponentially as a function of distance through the solid and that the signal intensity thus depends on the mean-free path of the electron and the relative Auger sensitivities of each different element.15,16 The calculations use the tabulated relative Auger sensitivity of elements16 and take the mean-free paths to be 4.5, 4.0, and 2.0 bilayers for electrons of 376, 152, 59 eV, respectively. One bilayer of CdS in the (100) direction is 2.91-Å thick. Despite some uncertainty in the values of the electron mean-free paths and the instrumental dependence of Auger sensitivities, the overall trend in the simulated Cd, S, and substrate Auger intensity versus layer thickness agrees well with our measured spectra and the decay curve for each substrate element is clearly exponential. This agreement provides support for the facts that the deposition occurs in a layer-by-layer manner and that island growth is not present.15 Deposition has also been studied at different substrate temperatures, and these results will be discussed in a separate publication.17 However, to briefly summarize this work, we found that, at 350 K and above, excess Cd is deposited, with Zn depletion near the interface. This observation is in agreement with our TPD studies which indicate desorption of dimethylzinc (DMZn) starting at 340 K while Cd is left on the surface. Below 160 K, multilayers of DMCd physisorb on the substrate. In the range 160-250 K, the binary reaction sequence is halted after the first dosing cycle. Thus, it appears that the optimum temperature for the growth lies in the window between 250 and 350 K, that is, near room temperature. Characterization of the Growth by LEIS and LEED Low-energy ion-scattering spectroscopy using 5001000-eV helium ions was also employed to characterize the surfaces at different stages of growth. Since LEIS is an extremely surface sensitive probe, all the signals are originated from the very top layer(s).18 Figure 6 presents a series of normalized LEIS measurements from CdS layers after different stages in the reaction sequence: part a corresponds to the clean ZnSe substrate; part b corresponds to the surface after the first bilayer of CdS is deposited; and parts c and d correspond to CdS of two and four bilayers, respectively. The peaks at 840, 896, and 679 eV correspond to He+ ions scattered by the ZnSe substrate, chemisorbed Cd, and sulfur, respectively. The resolution of the measurement does not allow the scattered He+ ion signal from Zn and Se atoms to be separated; our resolution is limited by the relatively large spatial and energetic width of the beam and the large angular acceptance of our energy analyzer. The most striking result shown in Figure 6 is the rapid decay of the signal originating from the ZnSe substrate. In fact, after one monolayer of CdS is deposited, the (15) Feldman, L. C.; Mayer, J. W. Fundamentals of Surface and Thin Film Analysis; Prentice Hall: Englewood Cliffs, NJ, 1986. (16) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger Electron Spectroscopy; Physical Electronics Division, Perkin-Elmer Corporation: 1978. (17) Luo, Y.; Slater, D. A.; Han, M.; Moryl, J. E.; Osgood, R. M., Jr. To be published. (18) Wooddruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science Analysis; Cambridge University Press: Cambridge, 1986.
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present in the topmost crystal layers, resulting in a sharper LEED pattern. More detailed discussion of LEIS and LEED measurements will be presented elsewhere.17 In Situ Thermal Desorption Spectroscopy
Figure 6. LEIS spectra from (a) a clean ZnSe c(2 × 2) surface, (b) one bilayer of CdS, (c) two bilayers of CdS and (d) four bilayers of CdS. The energy of the incident He+ ion is 1 keV. The peaks at 679, 840, and 896 eV correspond to He+ ions scattered off S, Zn/Se, and Cd.
substrate signal has decreased to almost zero. The actual signal corresponding to the substrate atoms would be even smaller except for the presence of a small amount of sputtering by the ion beam during the data collection as well as by diffusional intermixing of the two metal atom species (see below). With thicker CdS layers, the signal from the substrate is completely absent. The nearcomplete extinction of the LEIS substrate signal after the first monolayer growth indicates that CdS is deposited uniformly over the substrate; island formation is not present on the surface. LEED patterns were also observed at various stages of the growth process. These observations were made at room temperature for unannealed surfaces. For the initial stages of the growth process the LEED patterns exhibit increasing diffuse background intensity with CdS thickness, which implies some disorder on the sample surface. As the deposited layer thickened, the background intensity decreased, while the diffraction spots sharpened up with a pattern characteristic of the (1 × 1) structure and orientation of the ZnSe(100) substrate. When the CdS layer thickness exceeded about seven bilayers, the LEED pattern had sharp, bright (1 × 1) spots, with a low background signal, comparable to that of the clean substrate. These very well defined diffraction patterns indicate that the grown CdS thin layer had the same ordered, zinc blende crystal structure as the substrate. The disorder in the first few monolayers is attributed to the diffusion of atoms resulting in intermixing of the two different metallic atoms. LEED, being a diffractive method, will register such intermixing as disorder, even if the metal atoms are bound at the proper “anionic” sites. For the thicker CdS layers there are fewer substrate atoms
The results presented above suggest that the surface reactions are self-limiting under normal room temperature growth conditions. However the details of this selflimiting process, particularly insofar as they relate to the ideal scheme shown in Figure 1, have not yet been examined thus far in this paper. To study the adsorbed ligands which terminate the growing surfaces, it is necessary to resort to the most frequently used probe of reaction chemistry, namely thermal desorption spectroscopy. In this work, the TDS system was mounted in the growth chamber so that the sample could be examined after the surface had been dosed with the appropriate precursor. TDS studies were done for each of the four possible growth surfaces shown in our proposed model for ideal growth (Figure 1). For brevity, only the results for the first two layers are discussed here. Figure 7 shows two sets of thermal desorption spectra obtained after dosing the surface at 100 and 300 K, respectively, with DMCd. In both cases the spectra show the desorption of DMZn from the surface; the peaks at lower mass numbers result from cracking of the DMZn in the mass spectrometer. This indicates the surface is terminated with methyl groups, which is consistent with the ideal model. The desorption of DMZn is not unexpected on the basis of earlier TDS studies of metal alkyls on GaAs surfaces, where it was shown that surface adsorption of such methylated species proceeds by methylization of the surface atoms.19 The desorption of this zinc species is in line with a stronger methyl-metal bond for zinc than for cadmium. Also, note that reaction also appears to occur on the surface dosed at 100 K, prior to the desorption of the physisorbed multilayer. However, no specific measurement has been made to determine the temperature at which the reaction actually occurs, only that it precedes the desorption of the intact molecular precursor at ∼160 K. Experimental results on other surfaces suggest the reaction occurs not at 100 K but during the temperature ramp.19 The metal-covered surface is then dosed by H2S again at the same two temperatures indicated above. In this experiment the reaction does not occur for the lowtemperature surface shown in Figure 8, as low-temperature (∼150 K) desorption of apparently physisorbed H2S is observed. As the surface temperature is ramped to ∼370 K, evolution of fragments corresponding to DMZn is observed, further supporting the low-temperature unreactivity of H2S. On the other hand, if the surface is dosed at room temperature, only the chemisorbed phase of H2S is seen, as indicated by high-temperature desorption, at 485 K, of H2S. Note that in this case no desorption of DMZn or any CHx species is seen, indicating that the H2S chemisorption process has resulted in the complete removal of all the surface CH3 groups, forming a H-terminated surface. More extensive thermal desorption studies have been carried out at higher CdS coverage and are consistent with the ideal reaction-sequence chemistry proposed earlier in Figure 1. These studies are described in detail in another publication.20 (19) Lasky, P. J.; Lu, P. H.; Luo, Y.; Slater, D. A.; Osgood, R. M., Jr. Surf. Sci. 1996, 364, 312-324. (20) Han, M.; Luo, Y.; Moryl, J. E.; Osgood, R. M., Jr. To be published.
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Figure 7. Thermal desorption spectra (TDS) after dosing of DMCd on to ZnSe c(2 × 2) surfaces at 100 K and room temperature. DMCd dissociatively adsorbs on the surface to produce the same products in each case.
Figure 8. Thermal desorption spectra after dosing a methyl-covered, half-monolayer-Zn and half-monolayer-Cd surface with hydrogen sulfide at 100 K and room temperature. No reaction occurs at the lower temperature, while at room temperature H2S reacts with the surface.
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appears at 285.5 eV. The origin of this feature is most likely due to the resonances of the metal-CH3 bonds, which are similar to the X-CH3 resonances in methyl halides in previous NEXAFS studies of surface monomethyl species.23 Finally in the lowest curve, no carbon K edge feature is seen after exposure to H2S, again at room temperature. The absence of any intense carbon K edge features clearly indicates that this exposure has resulted in the desorption of all the chemisorbed carbon species. Thus the data, while still in the process of analysis suggest that the first step in the ALE process proceeds as discussed earlier in the paper in conjunction with Figure 1. Recent data using the sulfur L edge features following the subsequent H2S exposure further support this point.20
Figure 9. NEXAFS spectra from a multilayer of DMCd, a surface which is methyl-terminated and consists of a half monolayer of Zn and a half monolayer of Cd, and one bilayer of CdS.
NEXAFS Measurements At the time of the preparation of this article the authors have just initiated a study of the ALE process with NEXAFS using the NSLS at Brookhaven National Lab. NEXAFS is a particularly powerful technique, since it can, in principle, give information on the chemical (valence) state and the bond orientation of the surface species.21,22 While the data from this experiment are still under evaluation, it is possible to make some preliminary conclusions. The data were taken at the U1A beam line at the NSLS on a substrate which had received the usual sputtering annealing procedure, as discussed above. Figure 9 shows partial electron yield data of the carbon K-edge from a ZnSe c(2 × 2) surface after exposure to DMCd. The upper curve shows a spectrum measured after the surface had been covered at 100 K with multilayers of DMCd, which is in a physisorbed state. The data show a prominent near-edge peak, which is located at 287.8 eV; this is the known position of the carbon edge in the presence of the C-H bond in methyl groups.21 When the c(2 × 2) ZnSe surface is then dosed at room temperature with DMCd, shown in the middle curve, the spectrum corresponding to a saturation dosage undergoes a marked change. First, clearly the presence of CH bonds is still seen on the surface. Second, an additional peak (21) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992. (22) Chen, J. G. submitted to Surf. Sci. Rep.
Conclusions The purpose of the experiments described here is to do an in situ investigation of the surface chemistry of atomic layer epitaxy. This goal appears well on the way to being realized. It is clear that the reaction sequence discussed above leads to chemically driven epitaxy near room temperature. Also, we find growth is achieved in an atomic layer-by-layer fashion with self-limiting surface reactions. The ideal reaction mechanisms hypothesized at the beginning of this paper are consistent with the results thus far obtained by thermal desorption spectroscopy, NEXAFS, and LEIS. Despite these results, many of the details of the reactions and ligand terminations are not yet “tied down”. The uniformity of the surface ligand distribution still has not been examined, nor has the orientation of the surface ligand groups. In addition, key aspects of the surface growth, such as mechanisms for carbon incorporation, still have not been examined. Despite these facts, however, it is clear that classic surface chemistry studies can provide clear insight into the ALE process and be of value for both the chemist and the materials scientist. Acknowledgment. The authors acknowledge the generous help of Brian DeVries and Andrew Mingino at NSLS and Andrew Tepelov at NYU in the initial stages of the NEXAFS work discussed here and thank Theodore Madey for several helpful suggestions regarding the LEIS measurements. Finally, three of us would like to make a special final expression of gratitude to the late Brian Bent for his involvement in the scientific work discussed here. R.M.O. is most thankful for his introduction to NEXAFS by Brian and for his years of unselfish discussionss unfettered by the usual academic concerns when talking to another colleague who happens to be in another department. M.H. would like to thank Brian for introducing her to surface science at Columbia and for his help and support as her advisor. Finally J.G.C. thanks Brian for many years of collaborative work at Exxon. The financial support for this work is provided by the National Science Foundation through Award No. DMR-96-32456 and by the U.S. Department of Energy through Award No. FG02-90ER14104 for partial instrumentation support at Columbia and for general users’ time on the NSLS. LA970732D (23) Lasky, P. J.; Lu, P. H.; Yang, M. X.; Osgood, R. M., Jr; Bent, B. E.; Stevens, P. A. Surf. Sci. 1995, 336, 140-148.
