AVS2001 Session SC+SS-MoA: Semiconductor Surfaces

Monday, October 29, 2001 2:20 PM in Room 122

Monday Afternoon

Start	Invited?	Item
2:20 PM		SC+SS-MoA-2 Indium-Induced Charge Redistribution and Surface Electronic Structure of the Si(111)-(7x7) Surface M. Yoon, R.F. Willis (The Pennsylvania State University) The adsorption mechanism and the origin of the In-induced surface electronic states of the Si(111)-(7x7) surface have been studied using bias-dependent scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). At very low In coverages, bias-dependent STM images show that some of adatoms have either slightly higher or lower apparent height in the filled-state image, while the empty-state images are completely identical to those of the clean Si(111)-(7x7). These peculiar features are interpreted as being caused by two adsorption mechanisms: charge redistribution due to the adsorption of In atoms on Si dangling bond and the substitution of In for Si atoms in the 7x7 adatom positions, opening the bandgap by removing intrinsic metallic surface states caused by Si adatom dangling bond. As the coverage increases, a self-assembly of a superlattice of two-dimensional non-metallic In nanodots on the Si(111)-(7x7) surface begin to develop on both halves of the 7x7 unit cell which indicates a metal-insulator transition occurring on the surface. This confirms that the origin of the metallicity of the Si(111)-(7x7) surface is the delicate charge transfer between adatom and restatom Si dangling bonds in the center of the 7x7 unit cell.
2:40 PM		SC+SS-MoA-3 Study of GaAs(100) Surface Cleaning Using Synchrotron Radiation Photoemission Spectroscopy Z. Liu, Y Sun, F. Machuca (Stanford University); W.E. Spicer, P. Pianetta (Stanford Synchrotron Radiation Lab); R.F.W. Pease (Stanford University) Atomic surface cleanliness is required for novel NEA electron emission devices fabricated from GaAs and is also very important for GaAs MBE growth. Chemical cleaning of the GaAs (100) surface has been studied with high-resolution photoemission spectroscopy using synchrotron radiation (in the range of 55 eV to 750 eV) at the Stanford Synchrotron Radiation Laboratory. The high surface sensitivity of the technique has allowed us to identify the chemical state of the surface species after both the chemical cleaning and final vacuum processing steps. In order to eliminate contamination from the surroundings, all chemical cleaning steps were performed in an Ar or N₂ purged glove box attached to the load-lock allowing samples to be transferred into the system without being exposed to air. Samples were etched in H₂SO₄:H₂O₂:H₂O solution and then heat cleaned at 500°C (both diluted and concentrated solutions were studied). After chemical etching in the dilute (4:1:100) solution, elemental As (>1ML), As sub oxide (~0.1ML), Ga sub oxide and C (0.5~1ML) are found on the surface. Subsequent annealing at 500°C in UHV produces a stoichiometric, oxide free surface as determined by valence band and core level photoemission. In addition, C has been reduced to less than 10% of its initial level. We believe that the C can be removed by heating because the elemental As from the earlier cleaning step tends to protect the GaAs surface.
3:00 PM		SC+SS-MoA-4 Steady-State Fluctuations of High-T Vicinal Si(111): Investigation of Step-Step Correlation Function of REM Data¹ T.L. Einstein (University of Maryland, College Park); S.D. Cohen (UM,CP); J.J. Métois (University of Aix-Marseilles III, France); H.G. Bantu (UM,CP); H.L. Richards (Texas A&M University); E.D. Williams (UM,CP) To estimate from experimental data the strength of step interactions on vicinal surfaces, one typically studies the terrace-width distribution (TWD), i.e. the separation distance between adjacent steps. From a theoretical perspective, it is simpler to study instead the step-step correlation function h(L), i.e. the probability of finding two steps separated by some distance L, regardless of how many steps lie between them.² In fact, to the extent that traditional 1+1 D fermion models are applicable, exact results are available for h(L).³ We apply this new approach to extensive REM (reflection electron microscopy) data for the intriguing case of vicinal Si(111) at high temperatures T: 1100, 1200, and 1250°C. To compensate for the sublimation in this regime (e.g. 0.015 ML/s at 1100°C), Si is evaporated onto the sample to maintain steady state.⁴ Digitizing the data posed several unusual challenges, some of which made this system particularly appropriate for analysis via h(L) rather than TWD. The key question is whether the step stiffness and step-step repulsion of these steady-state systems are consistent with (higher-T extrapolation) of equilibrium results obtained at lower T. ^5,6 ¹ ¹ Work at UM supported by NSF-MRSEC. ² T.L. Einstein et al., Surface Sci., in press [cond-mat/0012274] & refs. therein. ³ P.J. Forrester, J. Stat. Phys.72 (1993) 39. ⁴ S. Stoyanov, J.J. Métois, & V. Tonchev, Surface Sci. 465 (2000) 227. ⁵ H.-C. Jeong & E.D. Williams, Surface Sci. Reports 34 (1999) 171 & refs. therein. ⁶ C. Alfonso, J.M. Bermond, J.C. Heyraud, & J.J. Métois, Surface Sci. 262 (1992) 371.
3:20 PM		SC+SS-MoA-5 Simplified Bond-Charge Model for the Analysis of Second-Harmonic-Generation Data: Application to Si/Dielectric Interfaces J.F.T. Wang, G.D. Powell, R.S. Johnson, G. Lucovsky, D.E. Aspnes (North Carolina State University) We develop a simplified bond-charge model for the analysis of second-harmonic-generateion (SHG) data, and apply it to study interfaces between singular and vicinal (001) and (111) Si substrates and various oxides, nitrides and oxynitrides. We model SHG microscopically as the 2ωt component of dipole radiation generated by the nonsinusoidal motion of electrons in bonds with anharmonic restoring forces, where the bonding electrons are driven by the ωt incident field. The simplfying assumption, which makes the treatment of SHG simpler than that of linear optics, is that only the anharmoic polarizability along the bond axis need be considered owing to the expected symmetry of the bond. Using this approach we analyze the azimuthal intensity dependence of SHG in the 800 to 900 nm spectral range of Si interfaces of various orientations and surface terminations. We find for example that a knowledge of the azimuthal dependence of the p-p signal for vicinal (111)Si allows us to predict the p-s, s-p, and s-s signals, and to determine the real and imaginary parts of the nonlinear polarizability associated with the different bonds. For interfaces between vicinal (001) Si samples and thin (less than 10 nm) thermally grown oxides, the essential absence of a 4-fold contribution to the azimuthal intensity dependence shows that these interfaces are double-domain. This result allows us to assign both SHG and RD signals of these interfaces to steps. Differences among oxidized samples and samples nitrided in various ways are relatively small, but systematic.
3:40 PM		SC+SS-MoA-6 STM Studies of the Ca/Si(111) System J.W. Dickinson (Virginia Commonwealth University); S.C. Erwin (Naval Research Laboratory); J.A. Carlisle (Argonne National Laboratory); A.A. Baski (Virginia Commonwealth University) The Ca/Si(111) system forms a series of odd-order nx1 (n = 3,5,7,etc.) reconstructions that culminate with a 2x1 phase at 0.5 ML.¹ Similar to group I metals, group II Ca forms a 3x1 phase at low coverage, where a previous model based upon Si honeycomb-chain-channel (HCC) chains accounts well for the observed STM data. In contrast to the group I metals that form the 3x1 phase at 1/3 ML, however, Ca forms this phase at 1/6 ML,² presumably due to the fact that Ca has twice the number of available electrons. Another interesting behavior unique to Ca is the presence of a 2x1 phase that reaches completion at 1/2 ML. We have proposed a model for the 2x1 phase based upon pi-bonded Seiwatz Si chains, where Ca rows form between the Si chains. The variety of odd-order nx1 phases (5x1, 7x1, etc.) that occur between these end-point 3x1 and 2x1 phases are then appropriate combinations of the 3x1 HCC chains and 2x1 Seiwatz chains. For example, a 5x1 unit cell is composed of one HCC chain and one Seiwatz chain, where two Ca rows are found per unit cell. Calculated surface energies based on the HCC and Seiwatz models correctly predict that for increasing Ca coverage, the 3x1, 5x1, and 2x1 phases each appear as stable phases. ¹ A.A. Baski, S.C. Erwin, M.S. Turner, K.M. Jones, J.W. Dickinson, J.A. Carlisle, Surf. Sci. 476, 22 (2001). ² G. Lee, D. Shin, J. Koo, H. Kim, S. Hong, Abstract #S25-8, APS 2001 March meeting.
4:00 PM		SC+SS-MoA-7 Growth of Semiconductor (Si, Ge)/Insulator (CaF₂) Heterostructures on Si(111) A. Klust, M. Bierkandt, C. Deiter (Universität Hannover, Germany); J. Falta (Universität Bremen, Germany); M. Grimsehl (Universität Hannover, Germany); R. Feidenhansl, C. Kumpf (Riso National Laboratory); T. Schmidt (Universität Bremen, Germany); Y. Su (Universität Hamburg, Germany); J. Wollschläger (Universität Hannover, Germany) Silicon-fluorite superlattices exhibit strong photoluminescence at room temperature¹. The origin of this novel behavior, not found in bulk silicon, may be electron confinement and/or interface effects. An important contribution to understanding these effects is detailed knowledge of the epitaxial semiconductor/insulator interface structure, which we find to be different for the Si-on-CaF₂ and CaF₂-on-Si interfaces. The CaF₂/Si(111) interface was investigated using x-ray standing wave (XSW) excited photoelectron spectroscopy (XPS). This combination exploits the large binding energy difference between interface and bulk emission to separately locate these sites relative to the Si substrate. The XSW results show that interface Ca atoms are well ordered even at growth temperatures as low as 370°C. Furthermore, we used surface x-ray diffraction (SXRD) to study growth of Si and Ge on thin CaF₂ films. In contrast to surfactant mediated growth of Si on CaF₂,² Si films grown on pristine CaF₂ films exhibit the same crystallographic orientation as the CaF₂ film. For the CaF₂-on-Si interface, the orientation of the substrate and overlayer are rotated by 180° at growth temperatures used here. These differences in interface structure are attributed due to different interface reaction chemistry. ¹F. Bassani et al., J. Appl. Phys. 79 (1996) 4066. ²B. R. Schroeder et al.. Appl. Phys. Lett. 77 (2000) 1289.
4:20 PM		SC+SS-MoA-8 Structural Relaxation at SiO₂/Si(100) Interfaces Studied by Coaxial Impact Collision Ion Scattering Spectroscopy H. Ikeda, S. Goto, K. Honda, M. Sakashita, A. Sakai, S. Zaima, Y. Yasuda (Nagoya University, Japan) With downsizing the ULSI devices, atomic bonding structures at the SiO₂/Si interface have become a serious factor to determine the device performance. Nevertheless, there are few studies about the interfacial structure. By using coaxial impact collision ion scattering spectroscopy (CAICISS), we have studied the structural changes extending over the first several layers from oxidized Si(100)-2x1 surfaces and claimed that the inward oxidation occurs before the lateral oxidation finishes.¹ In the present study, we clarify Si atomic structures at the SiO₂/Si interface, which depend on the thickness of the oxide films and the oxidation temperature. An ion scattering simulation which is based on the consecutive calculation of the He⁺ ion trajectories was used for the analysis of CAICISS spectra. Below an oxide thickness of 2.3 ML, the structural relaxation of a Si lattice at the SiO₂/Si interface occurs similarly in the range of room temperature to 700°C. The CAICISS spectra show only horizontal displacement of the second-layer Si atoms in these samples. In the following oxidation stage, the oxidation process depends on the temperature. At room temperature, the oxide thickness saturates at 2.3 ML, which is probably due to the restriction of oxygen adsorption on the distorted Si-Si bond sites. The CAICISS spectra of oxidation at above 300°C indicate interface structures that consist of Si atoms existing at normal lattice sites. Furthermore, vertical displacement of the second-layer Si atoms can be detected as well as horizontal one. From these results, it can be concluded that the vertical displacement of the second-layer Si atoms is a trigger of the formation of an amorphous structure and, moreover, a very abrupt SiO₂/Si interface is realized. ¹ ¹ M. Wasekura, M. Higashi, H. Ikeda, A. Sakai, S. Zaima, Y. Yasuda, Appl. Surf. Sci., 159-160 (2000) 35.
4:40 PM		SC+SS-MoA-9 Two Dimensional Scattering-images of Lateral Structured GaAs/AlAs Systems as an Example for Diffuse X-ray Scattering at Interfaces J. Stuempel, I. Busch (Physikalisch-Technische Bundesanstalt, Germany) Two-dimensional X-ray scattering pattern in reciprocal space (space map) is measured by the reflection of X-rays at grazing incidence. These images are composed of specular and diffuse scattered components. The essential information about the statistical parameters of the interfaces (roughness, correlation length, lateral symmetries, etc.) is contained in the diffuse part of the scattered radiation. As an example of highly structured interfaces we use an GaAs/AlAs multilayer system. The interfaces of a multilayer grown on a substrate with a small miscut (angle about 2 °) between the surface and the (100)-plane reconstructs to a lateral correlated system with step hights of some nanometers. The measured space maps of these systems will be presented in detail. Due to a quantitative data interpretation the results will be compared with numerical simulated data. The simulation is based on a modified scattering theory. The underlaying theory will be presented within the talk.