AVS1996 Session EM+NS-ThA: Atomic Level Control and Morphology during Growth and Etching

Thursday, October 17, 1996 1:30 PM in Room 204A

Thursday Afternoon

Start	Invited?	Item
1:30 PM	Invited	EM+NS-ThA-1 Theoertical Studies of Diffusion and Growth Phenomena on Semiconductor Surfaces E. Kaxiras (Harvard University); D. Kandel (Weizmann Institute of Science) Growth of semiconductors, both homo- and hetero-epitaxial, represents one of the most challenging problems from a fundamental as well as an applied perspective. Recent successes in altering the mode of growth by carefully selected adsorbates (surfactants) point to the need for understanding the thermodynamics and the kinetics of growth at the microscopic level. We investigate these issues in systems involving Si and Ge substrates and monolayers of representative adsorbates. We use ab-initio Local Density Functional theory to analyze the energetics of different equilibrium adsorbate geometries on these substrates. We also discuss extensive calculations of the activation energies and entropies associated with surface diffusion and with exchange mechanisms operative during growth. Of particular interest is the effect of an external electric field on diffusion (electromigration effect), which leads to morphological instabilities such as step bunching. The peculiar dependence of these instabilites on temperature can be explained by detailed atomistic models that reveal important charge transfer effects during adatom diffusion. The results of our calculations provide fresh insight to surfactant mediated growth and electromigration experiments.
2:10 PM		EM+NS-ThA-3 III-Sb(001) "Odd" Reconstructions: A Violation of the Electron Counting Model? P. Thibado, B. Bennett, B. Shanabrook, S. Erwin, L. Whitman (Naval Research Laboratory) We describe the Sb-rich reconstructions on MBE-grown GaSb and AlSb (001) surfaces as determined in situ with RHEED and STM. GaSb reconstructions observed by RHEED with increasing temperature (and decreasing surface Sb:Ga coverage) are: (2x5), (1x5), c(2x6), and (1x3). STM reveals that both the "(2x5)" and "(1x5)" reconstructions are locally c(2x10). In contrast, on AlSb(001) only two reconstructions are observed: c(4x4), similar to that observed on other III-V(001) surfaces, and (1x3). Given that this material has a lattice constant only 0.7% larger than GaSb, the observation of equivalent surface reconstructions at higher temperatures [i.e. (1x3)] but different ones at lower temperatures [(2x5) vs. c(4x4)] is surprising. We believe the different structures arise from a balance between surface energy reduction associated with multi-layer Sb termination and Sb-dimer formation, and the resulting surface stress (which will differ for the two materials due to their different elastic properties). The apparent structures of the c(2x10) surfaces, as derived from the STM images and supported by first-principles calculations, have more electrons than available bonding and Sb dangling bond (db) states. This is in contrast to the traditional view of III-V(001) reconstructions, where the valence electrons in excess of the bonding states should exactly fill the V dbs and leave all III dbs empty. Our calculations reveal that the excess electrons occupy an Sb dimer antibonding state associated with the multilayer Sb termination. These results suggest that the "electron counting model" requires modification in order to apply to III-Sb surface reconstructions.
2:30 PM		EM+NS-ThA-4 Atomic Layer Epitaxy for II-VI Material Growth Y. Luo, D. Slater, R. Osgood, Jr. (Columbia University) Atomic layer epitaxy (ALE) potentially provides atomic scale control of material growth and is a very promising technique for the fabrication of ultrathin device features. However, despite the wide use of such techniques, few detailed studies of the fundamental chemistry and physics involved have been performed. We present a study of ALE for the heteroepitaxy of II-VI materials using metalorganic molecules in a binary reaction sequence. A primary consideration is to achieve growth at the lowest possible temperature in order to reduce the potential for a loss of interface abruptness due to materials interdiffusion. CdS/ZnSe was chosen as a model for II-VI heteroepitaxy due to the good lattice matching available using this system. Dimethylcadmium and H \sub2\S were employed as precursors for growth on a ZnSe (100) substrate exhibiting a well ordered c(2x2) reconstruction. The supply of source molecules alternately to the growth surface results in self-terminating growth of both Cd and S layers during each cycle of the binary reaction sequence. The grown layer was characterised using AES, XPS and LEIS to monitor chemical composition. LEED was employed to study the surface order. We show that growth can be achieved using this method at substrate temperature as low as 300 K. Strong variations in the quality of the grown layer were observed with variation in substrate temperture, in particular, the results indicate that, at temperatures greater than 300 K, substantial Cd incorporation into the substrate occurs, leading to Zn depletion in the interface region.
2:50 PM		EM+NS-ThA-5 Low-temperature Selective Growth of GaAs by Modulated Flux Chemical Beam Epitaxy J. Ro, S. Kim, E. Lee (Electronics and Telecommunications Research Institute, Korea); S. Park (Kwangju Institute of Science and Technology, Korea); J. Lee (Seoul National University, Korea) We have investigated the modulated flux chemical beam epitaxy (MFCBE) for the low-temperature selective growth of GaAs on a partially masked and non-planar GaAs(100) substrates using trimethylgallium, triethylgallium, arsine and monoethylarsine. In general, selective growth has been achieved at high growth temperatures. However, a reduction of the selective growth temperature is very important to obtain high quality epitaxial films and to fabricate novel device structures. To investigate the effects of flux modulation on the selectivity and growth temperature, GaAs epilayers were grown on a partially masked and non-planar substrate by modulating the beam fluxes or by changing growth mode. To achieve the MFCBE, group III and V source materials were alternately fed into the growth chamber with and without evacuation period between each source supplies. The selectivity of GaAs layer by MFCBE was significantly enhanced compared to that of the layers by continuous beam methods even at low growth temperature. Furthermore, the surface morphology of grown layers was drastically improved depending on the gas supply mode under the same growth condition. Such improvements of selectivity temperature and surface morphology of samples grown by MFCBE seem to be caused by increments of the migration length and desorption probabilty of Ga-adspecies during evacuation and group V source supply periods. Some edge peaking in the epitaxial patterns, which is usually observed for the enhanced-surface diffusion process in the selective growth, was shown depending on the gas supply mode and source materials. This observation suggests that the selective desorption and enhanced migration of Ga-adspecies on the masked surface are the major competing processes. We will present the results which clearly show that modulated flux method plays a crucial role in the reduction of selective growth temperature, the improvement of surface morphology, and the facet formation on the non-planar substrate.
3:10 PM		EM+NS-ThA-6 Positron Annihilation Induced Auger Electron Spectroscopy (PAES) Study of the Structure of Si/H/Ge(100) Interfaces A. Weiss, J. Kim, A. Nangia, E. Jung (University of Texas, Arlington) The effect of adsorbed atomic hydrogen on the stability of Si films grown on a Ge(100) substrate was studied using Positron annihilation induced Auger Electron Spectroscopy(PAES). In PAES, the core-hole excitations that result in Auger electron emission are produced as the result of positron core-electron annihilation. PAES is uniquely sensitive to the topmost atomic layer due to the trapping of positrons in an image potential well just outside the surface before annihilation. This surface specificity was exploited in the study of film stability and interfacial mixing during the growth of Si on Ge. The concentration of Si and Ge atoms in the surface layer were monitored by measuring PAES intensities of the Si LVV and Ge MVV peaks after the deposition of Si on Ge(100) with and without the prior adsorption of H on the substrate. The PAES results indicated that when 8ML of Si was deposited on Ge(100) after introducing 5000L H\sub2\, the surface consisted exclusively of Si atoms. In the absence of H, segregation of Ge atoms to the surface was obseved even after deposition of 8ML of Si at room temperature.
3:30 PM		EM+NS-ThA-7 Etching of Double-Height-Stepped Si(100)-2x1: A Study of Steps and Their Interactions Y. Gong, D. Owens, J. Weaver (University of Minnesota) Bromine etching of double-height-stepped Si(100)-2x1 at 850 K has been studied with scanning tunneling microscopy. Double height B-type steps convert to single height A- and B-type steps (S\sub A\ + S\sub B\), and etching of S\sub B\ steps is preferred at low fluence. The result is an increase in the areal percentage of the minor (1x2) domain and movement of S\sub B\ steps toward their uphill S\sub A\ neighbors. As the mean step-step separation decreases, however, a step repulsion term becomes increasingly important and ultimately establishes the equilibrium S\sub A\-S\sub B\ separation. This balances the energy difference in activation of etching at S\sub A\ and S\sub B\ steps. Terrace pitting is limited, consistent with the very small terrace width on a highly miscut surface.
3:50 PM		EM+NS-ThA-8 Evolution of Step Structures on Vicinal Si(111) during Adsorption and Etching by Bromine and Chlorine\super \* X. Wang, E. Williams (University of Maryland) To investigate the competition between kinetic and energetic effects in etching, vicinal Si(111) surfaces exposed to Cl\sub 2\ and Br\sub 2\ have been studied using STM, LEED and AES. As the atomic structure on the terraces changes from the (7x7) reconstruction to a disordered (1x1) after halogen exposure and annealing below 900 K, triple-layer steps on samples mis-cut towards [2,-1,-1] are converted into single-layer steps, suggesting that halogen alters step energetics. However, under spontaneous etching due to Br\sub 2\ exposure above 900 K, terraces bound by single-layer steps are etched away preferentially, creating more multi-layer steps. Surprisingly, on the equilibrium step bunched structures that formed on surfaces mis-cut towards [-2,1,1], removal of the (7x7) reconstruction by halogen below 900 K does not break the step bunching. Etching this surface with bromine also causes little change in the structure of the step bunches. Thus, at the same etching condition, these two types of vicinal surfaces evolve differently due to differences in reaction kinetics at the distinctively structured steps. Cl\sub 2\ appears not effective in the etching of well-ordered Si(111), implying low probabilities of sticking and dissociation at T > 900 K. Evolution of the step distributions will be used to quantify the effects of halogen adsorption and etching. * Work supported by ONR.
4:10 PM		EM+NS-ThA-9 Control of Atomic Step Distributions on Si(001) by Thermal Processing S. Tanaka, C. Umbach, J. Blakely (Cornell University); R. Tromp, M. Mankos (IBM T.J. Watson Research Center) We have fabricated on Si(001) bi-periodic gratings with spacings ranging between 0.4 and 25 microns and have observed their shapes by AFM and LEEM. By choosing appropriate intitial shapes and the parameters of subsequent thermal processing in ultra-high vacuum (UHV), we have created two types of structures: i) bi-periodic quasi-sinusoids and ii) arrays of large step-free terraces. At temperatures below 1000 C, the quasi-sinuosids exhibit extensive (001) flats at their extrema; the flats at the maxima and the minima are approximately the same size. Between 1050 and 1150 C, the flats at the minima become larger than those at the maxima. At 1200 C the profile becomes sinusoidal without extensive (001) flats at the extrema. These morphologies are controlled by a combination of kinetic and energetic factors. In the intermediate temperature range, the evaporation rate from the geometrically imposed steps leads to the observed asymmetry. In the low temperature range where surface diffusion is the dominant transport mechanism, the symmetrical flats are due to a cusp in the surface energy. In the high temperature range, spontaneuous creation of steps (surface roughening) prevents the formation of extensive terraces and elminates preferential evaporation from the steps that make up the fabricated profile. By exploiting the step-dominated evaporation range, we have been able to fabricate on Si(001) arrays of terraces of large area (10 microns) that are completely free of steps. These may be suitable as substrates for growing epitaxial overlayers or for sub-100 nanometer devices.
4:30 PM		EM+NS-ThA-10 Steps Structures on Passivated Si(111)-1x1 Surfaces J. Boland, B. Itchkawitz (University of North Carolina, Chapel Hill) Upon exposure of the Si(111)-7x7 surface to halogens the 7x7 rest-layer is gradually transformed to a bulk-terminated, halogen-passivated 1x1 structure. This transformation initially occurs at steps and eventually spreads over the entire surface. The 1x1 structure thus obtained shows unexpected step structures. Based on a simple bulk-termination model the principle step directions ( <1,1,2bar> and <1bar,1bar, 2>) are expected to have different step structures (dihalide-like and a monohalide-like steps, respectively). Despite this expectation only dihalide-like steps are observed. To maintain the dihalide step-edge structure at <1bar,1bar,2> steps, the upper terrace is modified by introducing faulted triangular units that span the complete length of these steps. The energy cost of incorporating these faulted structures is offset by the formation of the energetically favored dihalide steps. The significance of these step structures for materials processing is discussed.