Papers

AVS1996 Session SS+EM-MoA: Hydrogen on Semiconductor Surfaces

Monday, October 14, 1996 1:30 PM in Room 204C

Monday Afternoon

Start	Invited?	Item
1:30 PM		SS+EM-MoA-1 First Principles Predictions of Reaction Mechanisms on Si(100)-2x1 D. Doren, R. Konecny, A. Robinson Brown (University of Delaware) Density functional calculations on cluster models of Si(100)-2x1 have been used to predict reaction mechanisms and energetics for several reactive adsorption processes. A discussion of dissociative adsorption and recombinative desorption of H\sub 2\ will focus on reconciling apparently conflicting dynamical measurements. Surface electronic excitations appear to play a critical role in determining the energy distribution of desorbing molecules. Mechanisms predicted for dissociative adsorption of H\sub 2\O, BH\sub 3\, SiH\sub 4\ and disilane, Si\sub 2\H\sub 6\, will also be described and compared to available experimental data. Based on these examples, a qualitative theory of reaction mechanisms for hydrides on silicon surfaces will be proposed. These reactions can largely be understood in terms of the electron density distributions in the molecule and surface dimer. The presence of both electron-rich and electron-deficient sites on a buckled dimer permits reaction by either electrophilic or nucleophilic attack mechanisms. Finally, a novel surface Diels-Alder reaction will be discussed, for which symmetric addition to an unbuckled surface dimer is allowed by orbital symmetry. This reaction creates new reactive surface sites that may be useful for subsequent chemical surface modification.
2:10 PM		SS+EM-MoA-3 Hyperthermal H Atom Interactions with D/Si(100) S. Buntin (National Institute of Standards & Technology) The interactions of H atoms having hyperthermal energies with a monodeuteride-terminated Si(100) surface are investigated. H atoms are generated by laser photolysis of a pulsed free-jet expansion of HI. By varying the laser wavelength and polarization H atoms having a relatively narrow energy distribution with mean values ranging from about 1 to 3 eV are selected. Full characterization of the photolysis conditions allows the determination of relative, as well as absolute, H atom exposures at the surface. The depletion probability of adsorbed D per incident H atom is identical for species having kinetic energies of 1.0 and 2.9 eV, and has an absolute value of 0.3(+/-0.2). These results, together with previous studies, indicate that the rate of depletion of surface D, which is likely due to abstraction, is nominally independent of incident H atom kinetic energy over the range of about 0.3 to 3 eV. The dynamical implications of these results will be discussed.
2:30 PM		SS+EM-MoA-4 Effects of Impurities on Hydrogen Desorption from Si(100) J. Ekerdt, N. Russell (University of Texas, Austin) Codeposition of impurities, especially p- and n-type dopants, during chemical vapor deposition of silicon is of great importance in tailoring the electrical properties of the deposited films. We demonstrate that the increase in the low-temperature growth rate of silicon with in situ diborane doping is accompanied by a comparable acceleration of the hydrogen desorption kinetics from p\super +\ Si(100), despite the low level of surface impurities present (B undetectable by AES). A similar reduction in the growth rate, observed with phosphine doping, is accompanied by a retarded rate of hydrogen desorption from n-type material. Recent interest in the growth of Si/Ge and Si/Ge/C alloys for bandgap tailoring has shown an acute dependence of the growth kinetics on the film composition, especially in the desorption-limited growth regime. We compare the effects of surface germanium and carbon on the hydrogen desorption kinetics. A detailed, quantitative evaluation of various models proposed to explain the dependence of the hydrogen desorption kinetics on the germanium coverage is presented.
2:50 PM		SS+EM-MoA-5 The Effect of Foreign Adatoms on Silicon Surface Chemistry J. Crowell, B. Ning, M. Jacobson, D. Wang, G. Batinica (University of California, San Diego) The surface chemistry of silicon can be significantly modified by foreign adatoms such as germanium, phosphorous, and boron. One of the most important reactions to occur on silicon surfaces involves the desorption of hydrogen. For instance, the rate limiting step in silicon deposition involves hydrogen desorption. We have used temperature programmed desorption, Auger electron spectroscopy, and multiple internal reflection infrared spectroscopy to study the effect of adatoms such as germanium and phosphorous on the surface chemistry of silicon. For example: germanium reduces the desorption energy of hydrogen from silicon surfaces, shifting the desorption maxima to lower temperatures. Phosphorous has the opposite effect, shifting the desorption maxima to higher temperatures. Examples citing representative reactions altered by these adatoms on silicon surfaces will be described. Furthermore, the mechanism by which these and other adatoms modify the surface chemistry of silicon will be discussed.
3:30 PM		SS+EM-MoA-7 In Situ Studies of Growth and Reaction of Amorphous Silicon Hydride Films: Abstraction, Insertion and Etching by Atomic Deuterium S. Lee, M. Kong, S. Bent (New York University); C. Chiang, S. Gates (IBM T.J. Watson Research Center) We report spectroscopic and kinetic studies of the reactions of atomic deuterium with amorphous silicon hydride films. Two types of films are investigated: polysilane (a heavily hydrogenated form of a-Si:H) and a monohydride-terminated silicon surface. Both films are prepared by filament-assisted chemical vapor deposition from disilane. The films are exposed to atomic deuterium at temperatures ranging from -110 \super o\C to 200 \super o\C. Multiple-internal reflection infrared spectroscopy is used to obtain Si-H and Si-D bonding information, and direct recoiling methods are used to measure reaction rates. Exposure of both films to deuterium atoms results in the replacement of silicon hydride with adsorbed deuterium. The resulting deuteride bonding depends strongly on temperature, however, with lower temperature favoring formation of di- and tri-deuteride. At 200 \super o\C, etching of polysilane is observed. The results demonstrate the importance of abstraction, insertion, and etching reactions in this system, and the rates of abstraction and insertion are reported.
3:50 PM		SS+EM-MoA-8 Hydrogen Desorption from Ion-roughened Si(100) G. Hess, N. Russell, B. Gong, P. Parkinson, J. Ekerdt (University of Texas, Austin) Desorption kinetics of hydrogen on smooth silicon surfaces have been widely studied and related to silicon film growth via CVD. The surface morphology of CVD grown films, especially plasma-enhanced CVD films, can differ substantially from the smooth, singular surfaces employed in previous studies of hydrogen desorption. We have simulated these conditions, by exposing Si(100) surfaces to low doses (up to 10\super 17\ cm\super -2\ ) of 500 eV Kr\super+\ ions at a 30 degree angle of incidence, followed by exposure to atomic hydrogen at 425 K. The effects of increasing ion exposure and hydrogen coverage were investigated using temperature programmed desorption (TPD) and vibrational spectroscopy (HREELS). Our vibrational data are in agreement with previous studies of hydrogen covered silicon surfaces with a high density of monoatomic steps. We therefore assume high step density and no dimers as the main characteristics of ion-roughened surfaces. A TPD coverage series showed a dramatic increase in the coverage of dihydride species with increasing ion dose (saturating at 2 ML hydrogen for an ion dose of 10\super 16\ cm\-2\), as monitored by the area under the \beta\\sub 2\ TPD feature of the sputtered surfaces, but the desorption peak temperatures for \beta\\sub 2\ and \beta\\sub 1\ desorption remained unchanged. We find that on exposure to atomic hydrogen the monohydride and dihydride state are populated simultaneously rather than sequentially (as observed on the smooth surface), suggesting that the surface mobility of hydrogen is substantially hindered because of the lack of surface dimers. A quantitative analysis of these data supports a mechanism for \beta\\sub 2\ hydrogen desorption from two adjacent Si atoms.
4:10 PM		SS+EM-MoA-9 Mechanism in the Growth Related Photodecoposition of Disilane on Si(100) O. Dippel, S. Wright, E. Hasselbrink (Fritz-Haber-Institut, Germany) Photo=ADinduced homoepitaxy of silicon using disilane is widely studied in the hope of developing low temperature fabrication techniques. Such processes promise smaller thin layer structures than can be achieved using conventional chemical vapor deposition (CVD). Mesoscopic growth parameters such as roughness and growth rate have been studied extensively in recent years. In contrast, very little is known about the microscopic reaction mechanisms. Using pulsed excimer laser light (193 nm) we carried out time-of-flight measurements (ToF) and high resolution electron energy loss spectroscopy (HREELS) to investigate the photo=ADinduced reactions of disilane physisorbed on a deuterium terminated Si(100) surface. Deuterium rather than hydrogen is used in order to study the role of abstraction and insertion reactions. Deuterium containing desorption products in the ToF spectra such as HD and SiH\sub 3\D yield information about possible abstraction reactions while the vibrations of surface SiH\sub x\D groups indicate the insertion of SiHx radicals. The experimental results will be used to suggest possible mechanisms for the reactions of the primary photodissociation products with the surface.
4:30 PM		SS+EM-MoA-10 Diborane Reactions on Si(001) Investigated by Temperature Programmed Desorption H. Kim, G. Glass, S. Park, T. Spila, J. Greene, J. Abelson (University of Illinois, Urbana-Champaign) We evaluate the reaction of diborane with the Si(001)2x1 surface using TPD, AES and LEED as part of an effort to understand GS-MBE growth of p\super +\ layers. Hydrogen TPD is performed on the diborane-adsorbed surface, then atomic D is adsorbed to saturation and deuterium TPD is performed. The adsorption of diborane increases with substrate temperature, and hydrogen desorption occurs over the range from 520 to 870 K with two peaks. Adsorption at T\sub s\ = 873 K to a dose of 2.64X10\super 19\ molecules/cm\super 2\ leads to a thick amorphous boron layer. Deuterium TPD for this surface shows one peak at 570 K with a tail extending to 1000 K. When less than 1ML boron is adsorbed at T\sub s\ < 673 K, LEED shows the 2x1 pattern after hydrogen desorption. However, deuterium TPD shows only half the saturation coverage compared with an undoped Si surface, indicating a decreased dangling bond density. The monohydride ( \beta\\sub 1\ ) peak appears at 780K, similar to an undoped surface, but the dihydride ( \beta\\sub 2\ ) peak has vanished. Instead, a broad feature appears which begins at 570 K. This lower temperature desorption is due to the weakening of the Si-H bond strength, in the presence of Si-B backbonds associated with B occupying subsurface positions. As the boron concentration increases, the lower temperature peak increases in intensity while the \beta\\sub 1\ peak decreases. Continuous boron indiffusion is observed during annealing at temperatures as low as 973 K. The above results, showing a reduced H desorption temperature and surface dangling bond density in the presence of high B doping, we correlated with Si GS-MBE growth kinetics.