AVS1997 Session EM+SS-WeM: Semiconductor Surface Reactions and Structures

Wednesday, October 22, 1997 8:20 AM in Room C3/4

Wednesday Morning

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8:20 AM		EM+SS-WeM-1 HREELS Studies of Passivation of Ge(100) using Highly Reactive Nitridation and Oxidation Agents. A. Avoyan (University of California, Irvine); C.S. Tindall (Tohoku University, Japan); J.C. Hemminger (University of California, Irvine) Passivation of the germanium IR detectors is of particular importance due to numerous space applications, which in turn, generate a number of technologically important requirements such as reliability, durability, inertness, noise reduction, ability to withstand highly-stressed applications within a wide temperature range, that these devices have to meet without influencing the performance. Thus, creation of a protective surface layer seems to be an important requirement to increase the lifetime and the performance quality of the device. The reactivity of HNO₃ and HN₃, as prospective precursors of oxide, nitride, and oxynitride growth, was probed on clean Ge(100). Thermal decomposition of HNO₃ and HN₃ (DN₃) on Ge(100) were studied by HREELS, AES, TPD and LEED. HREELS showed that at 170 K HNO₃ adsorbs in predominantly bridge-bonded bidentate configuration, without undergoing decomposition. Following adsorption at 170 K, the thermal decomposition of irreversibly chemisorbed HNO₃ takes place at about 400 K, by liberating H₂O and NO into the gas phase and leaving a monolayer of O_(ads) behind. The thermal stability of O_(ads) did not exceed 700 K. The Auger electron beam assisted decomposition and photodissociation of condensed HNO₃, resulted in more efficient oxidation of the substrate, than thermally induced surface reaction. The reactivity of the passivated substrate was probed by thermal decomposition of both, nitric acid and hydrogen azide on oxidized, N-H_(ads) covered, and N₂⁺ sputtered Ge(100) surfaces. We also studied the influence of preadsorbed potassium on the reactivity of the substrate towards thermal decomposition of HNO₃ and HN₃.
8:40 AM		EM+SS-WeM-2 Adsorption and Decomposition of H₂S on the Ge(100) Surface. L.M. Nelen, C.M. Greenlief (University of Missouri, Columbia) There is interest in developing simple methods for growing magnetic overlayers directly on semiconductor substrates. However, magnetic materials often react directly with the substrate causing an intermixing region that degrades the magnetic properties of the overlayer. The use of passivation layers, such as sulfur, offer a possible way of overcoming this problem. Here we report the investigation of the adsorption and decomposition of H₂S on the Ge(100). H₂S is a simple sulfur containing molecule that decomposes to yield hydrogen gas and deposits sulfur on the germanium surface. The surface reactions of H₂S are investigated by X-ray and ultraviolet photoelectron spectroscopy and temperature programmed desorption. Room temperature exposure of H₂S to Ge(100) and its subsequent surface decomposition is easily followed by ultraviolet photoelectron spectroscopy. Warming the H₂S covered surface results in some molecular desorption along with a substantial amount of decomposition. At saturation, 0.25 ML of H₂S decomposes generating 0.5 ML of atomic hydrogen. Above the hydrogen desorption some etching of the germanium surface is observed by sulfur. The etch product GeS is directly observed in temperature programmed desorption experiments. The overall surface reaction chemistry of H₂S will be presented and discussed in detail.
9:00 AM		EM+SS-WeM-3 TPD and HREELS Studies of the Interaction of Water with GaAs(001)-(4x2) Surface* C.-H. Chung, S.I. Yi, W.H. Weinberg (University of California, Santa Barbara) The adsorption and desorption/dissociation of water on GaAs(001)-(4x2) surface have been studied using Auger electron spectroscopy, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy. We have found that water first adsorbs molecularly at 100 K and is able to dissociate readily upon annealing by virtue of overlapping desorption and dissociation temperatures between 150 K and 200 K. The dissociation probability of water on GaAs surface is about 0.8 at low exposures. However, the decomposition products of water exhibit a high recombination probability, making the oxidation of GaAs difficult. A large fraction of surface hydroxyls are rehydrogenated to produce desorbing water at temperatures between 300 and 700 K. Hence, we have applied a cycling treatment (adsorption of water/annealing) to effect surface oxidation of GaAs. During this cycling, we have monitored GaAs oxide growth in Auger electron spectra. In addition, thermal desorption spectra recorded after exposures of the cycling-treated GaAs surface to water at 100 K represent molecular adsorption and intact desorption of water with little evidence of dissociation, which suggests that the surface has been significantly oxidized by the cycling treatment of water. *Supported by NSF Grant DMR-9504400.
9:20 AM		EM+SS-WeM-4 Chemistry of Hydrogen-Terminated Silicon Surfaces with Halogens M.R. Linford (Stanford University); J.H. Terry (Stanford Synchrotron Radiation Laboratory); R.T. Mo (Stanford University, SSRL); R. Cao (Stanford Synchrotron Radiation Laboratory); H. Luo, C. Wade (Stanford University); P. Pianetta (Stanford Synchrotron Radiation Laboratory); C.E.D. Chidsey (Stanford University) Hydrogen-terminated silicon (111) reacts with chlorine under ultraviolet illumination to form an ideal (1X1) chlorine-terminated surface. Synchrotron photoemission studies show a Si 2p surface peak shifted 0.80 eV to lower kinetic energy, characteristic of silicon in the +1 oxidation state. Spectral deconvolution indicates 0.92 monolayer coverage of chlorine and 0.06 monolayers of oxide. STM confirms that the 1000 Å terraces of the hydrogen terminated surface are preserved upon chlorination. Low Energy Electron Diffraction (LEED) confirms the 1X1 pattern expected from the single monolayer coverage. Traditional photoemission studies suggest a radical mechanism for the chlorine reaction as there is no significant chlorination without light. Bromine shows a slightly reduced reactivity from chlorine. Iodine reactivity will also be reported on.
9:40 AM		EM+SS-WeM-5 Reaction of Chlorine with In- and As-terminated InAs(100) W.K. Wang, J.A. Yarmoff (University of California, Riverside) Much effort has gone into understanding the surface reactions of halogens and halogen-containing compounds with semiconductor surfaces in order to model the processes which occur during device fabrication. Although the reaction of Cl₂ with III-V semiconductor surfaces is not as well understood as with Si, there has been considerable work done with GaAs substrates ¹. This previous work showed that Cl₂ spontaneously etches GaAs, and that there are two regimes with respect to sample temperature. In the present work, we extend the halogen/semiconductor studies to the interaction of Cl₂ with InAs(100), employing low energy electron diffraction (LEED) and surface-sensitive soft x-ray photoelectron spectroscopy (SXPS) to investigate surfaces following reaction in UHV. We examined the initial stages of the reaction of Cl₂ with the In-terminated c(8x2) and the As-terminated c(2x8) surfaces, as well as the surfaces produced following prolonged exposure. Our results suggest that InAs is spontaneously etched by Cl₂ at room temperature in a similar manner as is GaAs ². After reaching steady-state at room temperature, the surfaces are terminated primarily by InCl₂ groups, but also contain some InCl, InCl₃ and AsCl, as well as subsurface tricoordinate defects. ¹W.C. Simpson and J.A. Yarmoff, Ann. Rev. Phys. Chem. 47 (1996) 527. ²W.C. Simpson, W.M. Tong, C.B. Weare, D.K. Shuh and J.A. Yarmoff, J. Chem. Phys. 104 (1996) 320.
10:00 AM		EM+SS-WeM-6 Transitional Phases on the GaAs(001) Surface I. Chizhov, G. Lee, R.F. Willis, D. Lubyshev, D.L. Miller (Pennsylvania State University) The structure of the GaAs(001) surface has been a subject of intensive studies in recent years mainly due to its technological importance as a primary substrate for growth of III-V and II-VI semiconductors. While the most stable reconstructions on this surface, such as As-rich c(4x4), 2x4 and Ga-rich 4x2 have received a lot of attention [1], the behaviour of the surface during transitions between these phases is not well understood. We have studied the transitional phases on the GaAs(001) surface using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). During the c(4x4)-(2x4) transition an intermediate "2x3" phase was observed. STM images show that this phase is in fact composed of 4x3 unit cells and has a mixed 4x3/c(4x6) symmetry [2]. On the contrary, the (2x4)-(4x2) transition proceeds through a succession of multi-domain phases such as 3x6 and 4x6. STM images reveal that the 3x6 phase consists of domains of "2x6" and a disordered phase, while the 4x6 phase in addition contains 4x2 domains. The atomic structure of these phases as well as kinetic aspects of the phase transitions on the GaAs(001) surface will be discussed. This work is supported by the Office of Naval Research, Grant N00014-92- J-1479. [1] Q. Xue et al., Phys. Rev. Lett. 74, 3177 (1995) and references therein. [2] I. Chizhov et al., Phys. Rev. B, in press.
10:20 AM		EM+SS-WeM-7 Hydrogen Adsorption on GaAs (001) Reconstructions H. Qi, L. Li, B.K. Han, S. Gan, R.F. Hicks (University of California, Los Angeles) The adsorption of atomic hydrogen on MOCVD grown GaAs (001) c(4x4), (2x4)/c(2x8), (1x6), (3x2)/c(6x4) and (4x2)/c(8x2) has been studied by multiple internal-reflection infrared spectroscopy (IR), scanning-tunneling microscopy (STM) and LEED. Hydrogen adsorption on As dimers produces a series of vibrational bands between 2170-1930 cm-1 that are detected on all reconstructions. By contrast, hydrogen adsorption on Ga dimers produces a single broad band characteristic of a bridged gallium hydride. The intensity, frequencies and shape of the bridge Ga-H band vary from one Ga-rich reconstruction to the next, indicating that the structure of the Ga dimers changes on these surfaces. The STM images show that the GaAs (001) surface is usually comprised of a mixture of at least two phases. Quantitative comparisons of IR and STM data have allowed us to identify the vibrational spectrum of hydrogen adsorbed on the pure phases. Through this research a comprehensive understanding of hydrogen adsorption on GaAs (001) has emerged.
10:40 AM		EM+SS-WeM-8 Highly Stable Si-Atomic Lines Formation and ß-SiC(100) Surface Reconstructions Studied by Room and High Temperature Scanning Tunneling Microscopy P. Soukiassian, F. Semond, L. Douillard, V.Yu. Aristov, O. Fauchoux (Commissariatà l'Energie Atomique, France); G. Dujardin, A. Mayne, C. Joachim, L. Pizzagalli (CNRS, France) We use atom-resolved scanning tunneling microscopy (STM) to investigate Si-terminated cubic (100) silicon carbide surfaces between 300 K and 1200 K. Flat and high-quality surfaces having a low density of defects are grown with identification of individual Si atoms and dimers. The 3x2 reconstruction (Si-rich) results from Si-dimers rows perpendicular to the dimer direction in a (3x2) atomic arrangement with asymmetric dimers all tilted in the same direction (i.e. not anticorrelated) [1]. The ß-SiC(100) c(4x2) surface reconstruction (Si-terminated) exhibits a centered pseudo-hexagonal pattern giving a c(4x2) array [2]. Calculated STM images using the Elastic-Scattering Quantum Chemistry method are in excellent agreement with experimental topographs. The results suggest a model of dimer rows having alternatively up- and down-dimers (AUDD model) within the row in a unique "undulating" arrangement reducing the high surface stress. We also report the discovery of controlled Si atomic line formation at the phase transition between 3x2 and c(4x2) reconstructions [1,2]. These self-organized atomic channels made of Si-dimers have fascinating characteristics since they are: i) very long (> 100 nm) with their length limited by the substrate only, ii) very straight, iii) regularly spaced, iv) thermally very stable (1200 K), and v) grown on the surface of a wide band-gap material. Their number and spacing could be mediated by annealing time and temperature resulting in arrangements ranging from a very large superlattice to a single isolated atomic line. This investigation shows novel and very interesting aspects of silicon carbide and stresses further the numerous and versatile roles of this advanced electronic material. [1] F. Semond, P. Soukiassian, A. Mayne, G. Dujardin, L.Douillard and C. Jaussaud, Phys. Rev. Lett. 77, 2013 (1996). [2] P. Soukiassian, F. Semond, L. Douillard, A. Mayne, G. Dujardin, L. Pizzagalli and C. Joachim, Phys. Rev. Lett. 78, 907 (1997).
11:00 AM		EM+SS-WeM-9 New Atomic Structure for a Monolayer of Sb on Si(111) N. Takeuchi (Universidad Nacional Autonoma de Mexico) We have performed first-principles total energy calculations to determine the surface atomic structure of a monolayer of Sb on the Si(111) surface. Our calculations show that in the ground state the Sb atoms form chains along the (1-10) directions. However, this chain structure is different from others found in similar systems (for example the (2X1) in the case of a monolayer of Sb on Ge(111)). Instead, it is formed by trimers in opposite orientations: one centered on a T4 site, and the following on a H3. In both trimers, the Sb atoms are placed on top of Si atoms in milk-stool geometries. This chain configuration has lower total energy than the (√3 X sq3) structure, explaining why anneling the (√3 X sq3) surface leads to the diffused (2X2) phase. This (2X2) phase can be undestood as formed by three domains of the new chains.
11:20 AM		EM+SS-WeM-10 Surface Chemistry Study of Atomic Layer Epitaxy for II-VI Semiconductor Growth Y. Luo, M. Han, J.E. Moryl, R.M. Osgood (Columbia University) High quality atomic layer epitaxy (ALE) and in general, MOCVD growth of semiconductors rely on a thorough molecular level understanding of the underlying surface chemistry such as reaction kinetics and identities of surface species. Recently we have shown that atomic layer epitaxy for II-VI materials (CdS/ZnSe) can be accomplished at room temperature using metalorganic and hydride precursors (DMCd and H₂S) in a binary reaction sequence. This low temperature epitaxy is driven by the selective reaction of precursor molecules with the appropriate surface atoms or groups, so as to have the material deposited at the correct crystal sites. In this talk, we will present in situ studies of detailed surface chemistry and growth mechanism underlying the ALE for CdS on ZnSe(100) using TPD, NEXAFS and high resolution XPS. The latter two measurements were done at National Synchrotron Light Source. These methods were utilized to characterize the surface species and their reactions with the precursors at each step of the growth process. The binary reaction sequence consists of alternate supplies of precursors to the growth surface. Dosing with DMCd results in the deposition of a monolayer of cadmium terminating with methyl groups, and dosing with H₂S results in a full monolayer of sulfur being deposited which is terminated with hydrogen atoms. In each case, there exist a temperature range in wh ich the passivating methyl group or hydrogen atom is stable on the surface leading to the self-terminating reaction of that particular precursor with the surface, and then it can be removed with the subsequent dosing of the next precursor. These results agree with our earlier proposed model surface reactions. We will also discuss the results of these measurements with different precursor coverage and at various substrate temperatures.
11:40 AM		EM+SS-WeM-11 Surface Interactions and Energetics of SiH Radicals During Plasma Depsosition of Silicon-Based Materials G.R. Barker, V.A. Venturo, P.R. McCurdy, E.R. Fisher (Colorado State University) Silicon-based materials have been an integral part of the microelectronics industry for several decades. Despite this extensive use, however, the details of the mechanisms for plasma deposition of these materials are still unknown. Surface reactivity measurements can provide much needed information about the molecular level aspects of plasma deposition. Here, we present data for the SiH radical in a number of plasma deposition environments. This work characterizes the behavior of SiH during deposition of amorphous hydrogenated silicon (a-Si:H) from both SiH₄ and Si₂H₆-based plasmas; of silicon nitride (a-SiN:H) from SiH₄/NH₃ plasmas; and of silicon carbide (a-SiC:H) from SiH₄/CH₄ plasmas using the Imaging of Radicals Interacting with Surfaces (IRIS) technique. This technique combines laser-induced fluorescence and molecular beam technology to examine the interactions of small molecules during plasma processing events. In all these systems, SiH appears to have a high reactivity near unity. In the Si₂H₆ plasmas, however, the reactivity drops slightly to about 0.9 +- 0.1. In the SiH₄-based systems, the reactivity does not change appreciably with substrate temperature. In addition to surface reactivity data, the changes in SiH radical translational and rotational energies as a function of plasma power in different plasma systems will be discussed. Comparison to other radicals in these systems such as NH, NH₂ and CH₂ will also be made.