AVS1996 Session SS+EM-TuA: Reactions on Semiconductor Surfaces

Tuesday, October 15, 1996 2:00 PM in Room 203B

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS+EM-TuA-1 Reactions of Diethylgermane and Ethyl Groups on Ge(100) J. Chen, C. Greenlief (University of Missouri, Columbia) The adsorption and decomposition of diethylgermane on the Ge(100) surface was investigated with the intent of elucidating the surface processes leading to the deposition of epitaxial Ge films. Room temperature adsorption of diethylgermane leads to the formation of surface germanium hydrides and ethyl groups. The ethyl groups decompose at higher temperatures and form ethylene via a \beta\-hydride elimination reaction. Isotopic labeling experiments are used to confirm this reaction step. This is in contrast to the Si(100) surface where both \alpha\- and \beta\-hydride elimination is observed for the decomposition of surface ethyl groups. Low energy electron diffraction is used to evaluate the quality of the deposited germanium films. The surface reaction of diethylgermane on the Ge(100)) will be compared with that observed previously on Si(100) and will be discussed in detail.
2:20 PM		SS+EM-TuA-2 Arsine Decomposition on Ge(100) Surfaces S. Gan, T. Nguyen, R. Hicks (University of California, Los Angeles) Solar cells for satellite power stations consist of epitaxial thin films of GaAs and AlGaAs deposited on Ge(100) substrates. The films are deposited on the germanium by MOCVD. We have investigated the surface chemistry of the first few steps in the solar cell manufacturing process, namely the etching and annealing of Ge(100), and the arsenic passivation of the surface with arsine. Samples were either etched with 1:1:30 HF:H\sub2\O\sub 2\:H\sub 2\O, or oxidized with UV/ozone, and then annealed in ultrahigh vacuum to 700\super o\C. Analysis of the surfaces by x-ray photoelectron spectroscopy (XPS) revealed that the oxide desorbs from Ge(100) at 550\super o\C. Carbon, on the other hand, is more difficult to remove. After HF etching and annealing to 600\super o\C, the C/Ge atomic ratio was 0.60, while after UV/ozone oxidation and annealing to 600\super o\C, the C/Ge atomic ratio was 0.15. Scanning-tunneling and atomic-force micrographs (STM and AFM) taken after heating show that there is a narrow temperature window for preparing a smooth surface. At 550\super o\C, the surface contains main pits and hillocks 5-10 nm in diameter. Heating to 600\super o\C removes these features. However, further heating to 650\super o\C causes surface melting, and on vicinal surfaces, a loss of the step structure. We have also investigated the reaction of AsH\sub 3\ with the Ge(100) surface. At -100\super o\C, arsine adsorbs molecularly, whereas at 0 and 200\super o\C, it dissociates into AsH\sub 2\ and As, respectively, with the transfer of H atoms to nearby Ge sites. When the crystal is exposed to AsH\sub 3\ at 400\super o\C, the molecule decomposes with desorption of H\sub 2\ and deposition of one monolayer of As. This As finally desorbs as As\sub 2\ above 450\super o\C. STM images of the adsorbed arsine and arsenic reveal a variety of interesting adsorbate structures that depend strongly on the substrate structure, temperature, and AsH\sub 3\ dosage. These images as well as vibrational data on the adsorbed molecules will be presented at the meeting.
2:40 PM		SS+EM-TuA-3 Thermal Decomposition Reactions of Acetaldehyde and Acetone on Smooth and Sputtered Si(100) J. Armstrong (University of Texas, Austin); M. Langell (University of Nebraska); J. White (University of Texas, Austin) Carbonyl containing molecules have been used as chelating agents for delivering metals in CVD processes to form interconnects on silicon, for example copper (II) bis(acetylacetonate), and copper (II) bis(1,1,1,5,5,5)-(hexafluoropentanedionate). We have studied the chemistry of molecules possessing carbonyl functionality on smooth and sputtered Si(100). The goals are to understand at a molecular level how these adsorbates bond to Si(100) and build models that help elucidate the decomposition pathways over a wide temperature range (110 K-1000K). High resolution electron energy loss spectroscopy (HREELS), x-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) experiments were performed using acetone (ACE) and acetaldehyde (ALD) as adsorbates. The results show features common to each adsorbate system. First, lack of a carbonyl stretch in HREELS and large chemical shifts in the C 1s and O 1s XPS peaks (1 to 3 eV) suggest an adsorption state in which the carbonyl is both reduced in bond order and suffers complete bond cleavage. Second, TPD shows large hydrogen desorption features near 840-850 K. Third, a significant mass 44 peak occurs in the TPD spectrum around 1050 K while XPS shows total loss of oxygen along with no decrease in the C 1s signal thereby indicating SiO desorption. This leads to 34% and 62% of the monolayer decomposing on smooth Si(100) forming carbide for ALD and ACE respectively. On a sputtered Si surface, the reaction products diminish to near negligible levels and significantly less carbon (18 to 28 % of the equivalent monolayer of the smooth surface) appears as silicon carbide.
3:00 PM		SS+EM-TuA-4 Scanning Tunneling Microscopy of N\sub 2\H\sub 4\ on Silicon Surfaces C. Tindall, L. Li, Y. Hasegawa, T. Sakurai (Tohoku University, IMR, Japan) The chemistry of N\sub 2\H\sub 4\ on Si(100)2x1 and Si(111)7x7 has been studied using scanning tunneling microscopy. At low coverages, adsorption of the molecule on Si(100)2x1 at room temperature leads to moieties which are symmetrically bonded across the Si dimers. This is consistent with an N\sub 2\H\sub 4\ geometry in which the molecule has adsorbed dissociatively across the Si dimer. At one monolayer coverage, the STM topographs appear similar to those of the hydrogen terminated Si(100) monohydride phase. This is consistent with an initial bonding geometry in which the N-N bond of the molecule is parallel to the surface, and is thus in agreement with previously published HREELS data. Upon annealing the substrate at 600K, well-ordered clusters of reaction products are formed on the surface which are imaged as bright spots situated symmetrically on top of the dimer rows. On Si(111)7x7, the molecule behaves in manner similar to that of NH\sub 3\. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.
3:20 PM		SS+EM-TuA-5 Infrared Spectroscopy Study of the Interactions of Tert-butyl Containing Compounds with Si (100) J. Shan (University of Wisconsin, Madison); R. Hamers (University of Wisconsin, Madison, U.S.A) The surface chemistry of organic- based precursors was studied on the silicon (100) surface using surface infrared spectroscopy. Tert- butyl phosphine and tert- butyl chloride are found to give very different surface species upon adsorption and subsequent thermal dissociation on Si (100). We have identified the intermediate species by comparing the infrared spectra with those obtained from adsorption of other possible intermediates such as ethylene, as well as isotropic labeling studies. Very different reaction mechanisms are proposed, based on their adsorption and thermal decomposition behavior. The implications of the different reaction mechanisms for the use of organic precursors for silicon CVD processing are discussed.
3:40 PM		SS+EM-TuA-6 Adsorption and Interaction of Ammonia Molecules on GaAs(110) Studied with Scanning Tunneling Microscopy G. Brown, M. Weimer (Texas A&M University) We have previously used scanning tunneling microscopy to examine the room-temperature chemisorption of submonolayer amounts of ammonia on GaAs(110). Atomic-resolution topographs confirm that reaction occurs between lone-pair electrons of the free molecule and cation dangling bonds at the surface, in agreement with expectations based on Lewis acid-base chemistry [1]. Reaction sites on both p- and n-type substrates are uncharged, indicating that adsorption-induced bonding and antibonding levels are degenerate with the bulk valence and conduction bands, respectively. As a consequence, the microscopic screening produced by a single adsorption-induced dipole is directly observed. We have recently studied the interactions between adsorbates through the distribution of molecule pairs as a function of their separation vector in the surface plane. This pair correlation function reveals a strong, short-ranged repulsion along the surface chains that completely excludes reaction at adjacent cation sublattice sites. The pair potential also displays an unusual dependence on azimuth that is not explained by the dipole-dipole forces between individual molecules. Tunneling spectroscopy indicates that modest ammonia exposure significantly disrupts the GaAs(110) dangling-bond surface states. [1] G. Brown and M. Weimer, J. Vac. Sci. Technol. B13, 1679-1683 (1995).
4:00 PM		SS+EM-TuA-7 Reaction of Zincalkyls on Ga-Rich and As-Rich GaAs(100) Surfaces H. Lam, N. Venkateswaran, J. Vohs (University of Pennsylvania) Temperature programmed desorption and high resolution electron energy loss spectroscopy were used to study the reaction of dimethylzinc (DMZ) and diethylzinc (DEZ) on the c(4x4), (2x4), and (4x2) reconstructions of GaAs(100). The zincalkyls dissociated on all three surfaces to form adsorbed alkyl groups and Zn atoms. The Zn desorption temperature was a function of the surface Ga/As ratio, while the alkyl group desorption temperature was independent of the surface reconstruction. Based on the experimental results, specific adsorption sites for Zn atoms and alkyl groups on the three reconstructions have been proposed. Zn atoms interact with surface As dimers on the As-rich c(4x4) and (2x4) surfaces and with Ga dimers on the Ga-rich (4x2) surface. The alkyl groups appear to preferentially bond to surface As atoms on all three reconstructions, while surface Ga atoms are involved in dehydrogenation reactions. The implications of these results for the growth of heterostructures on GaAs using metalalkyl precursors will be discussed.
4:20 PM		SS+EM-TuA-8 The Adsorption State of Hydrogen Sulfide on GaAs(001) Surfaces C. Chung, S. Yi, W. Weinberg (University of California, Santa Barbara) Using HREELS after adsorption of H\sub 2\S on GaAs(001)-(4X2) surface at 100K, we observed the H\sub 2\S scissoring mode at 1170cm\super -1\ due to weak molecular adsorption which disappeared after annealing to 250K, the S-H stretching mode at 2250cm\super -1\, and the As-H stretching mode at 2110cm\super -1\ without a Ga-H stretching mode. This suggests that dissociative adsorption occurs producing mercaptan (HS-) and H-, with the latter bonded to only As surface sites. Upon annealing to 700K, the thermal desorption of AsH\sub 3\, H\sub 2\S, and H\sub 2\ is observed, which frees vacant sites of gallium which are available for further sulfur chemisorption. These vacant sites were monitored by atomic hydrogen post-exposure. When a GaAs surface which had been saturated with H\sub 2\S at 100K and annealed to a temperature just below that of Ga\sub 2\S desorption is exposed to atomic hydrogen at 100K, only a Ga-H stretching mode at 1870cm\super -1\ appears. This occurs without any apparent hydrogenation of As or S, suggesting that only the Ga sites made available due to desorption of AsH\sub 3\ or H\sub 2\S are available for post-hydrogenation. To build up more sulfur on the surface, we repeated cycles of 3L H\sub 2\S exposure at 100K followed by heating to 700K. After repeated cycles, atomic hydrogen exposure resulted in sulfur hydrogenation as shown by the appearance of the S-H stretching mode and the H\sub 2\S scissoring mode without the appearance of Ga-H or As-H stretching modes. This result shows that complete sulfur passivation of GaAs surface using H\sub 2\S is accomplished after the cycling treatment. *Supported by NSF-DMR Grant, QUEST, and Keck Foundation.
4:40 PM		SS+EM-TuA-9 Dissociative Adsorption of Hydrogen Sulfide on GaAs(100)-(2x4) and (4x2) Surfaces S. Yi, C. Chung, W. Weinberg (University of California, Santa Barbara) The adsorption of H\sub 2\S on various GaAs(100) surfaces, ranging from the As-rich (2x4) to the Ga-rich (4x2) reconstruction, has been examined, using High Resolution Electron Energy Loss Spectroscopy, Temperature Programmed Desorption, Auger Electron Spectroscopy, and Low Energy Electron Diffraction. Both molecular adsorption of H\sub 2\S and decomposition of H\sub 2\S into HS and H occurred at a surface temperature of 100 K. Dissociative adsorption of H\sub 2\S is strongly affected by the availability of molecularly adsorbed H\sub 2\S, and the behavior suggests a precursor mediated mechanism. The dissociative adsorption probability of H\sub 2\S below 150 K, the desorption temperature of the molecularly adsorbed state, is near unity on GaAs(100)-(4x2) surface and decreases with increasing As surface coverage and surface temperature. Preferential decomposition of H\sub 2\S into As-H and Ga-SH was inferred from observation of the As-H stretching vibrational mode at 2110cm\super -1\ and the absence of a Ga-H stretching vibrational mode on Ga-rich (4x2) surface reconstruction. In contrast, on the As-rich (2x4) reconstructed surface, the decomposition of H\sub 2\S into As-H and As-SH occurs dominantly. Upon thermal treatment, on the As-rich (2x4), S\sub 2\ desorption occurs dominantly whereas on the Ga-rich (4x2), Ga\sub 2\S desorption was found. *Supported by NSF-DMR Grant, QUEST, and Keck Foundation.