Papers

AVS2001 Session SC+SS-ThA: Semiconductor Surface Structure

Thursday, November 1, 2001 2:00 PM in Room 111

Thursday Afternoon

Start	Invited?	Item
2:00 PM		SC+SS-ThA-1 Direct Surface Structural Determination using Correlated Thermal Diffuse Scattering T. Abukawa (Tohoku University, Japan) Since the atomic vibration is strongly correlated among neighbor atoms or within very short-range in crystal, the short-range coherency in electron diffraction may survive even if the long-range coherency is destroyed by thermal vibration. The short-range coherency causes broad structures in the thermal diffuse scattering (TDS). The broad diffraction structures due to short range coherency were indeed observed for a Si(001) surface when medium energy electron diffraction was measured at medium scattering angles.¹ The broad features were observed as very simple oscillations of diffracted electron intensity. These features have been termed correlated thermal diffuse scattering (CTDS).¹ Since CTDS is well interpreted as kinematical diffraction among nearest neighbor atoms composing the crystal, the three-dimensional Patterson function can be obtained by simple Fourier transformation of a three-dimensional CTDS pattern. When a medium electron beam is incident at a grazing angle, the surface sensitivity of CTDS is enhanced, and the building blocks of surfaces, i.e. the lengths and the orientations of all the surface related nearest-neighbor bonds, can be directly obtained from the Patterson function analysis. The application of Patterson function analysis of CTDS will be demonstrated for the surface structures of Si(001)2x1,² Si(111)√3x√3-In³ and Si(111)4x1-In. ¹ T. Abukawa, C.M. Wei, T. Hanano, and S. Kono, Phys. Rev. Lett. 82, 335 (1999). ² T. Abukawa, C.M. Wei, K. Yoshimura and S. Kono, Phys. Rev. B62, 16069 (2000). ³ T. Abukawa, K. Yoshimura and S. Kono, Surf. Rev. Lett.7, 547 (2000).
2:40 PM		SC+SS-ThA-3 Atomic Structures of the of InP (111) Surface C.H. Li, D.C. Law (University of California, Los Angeles); L. Li (University of Wisconsin, Milwaukee); R.F. Hicks (University of California, Los Angeles) The atomic structure of the indium-terminated InP (111), prepared by metalorganic vapor-phase epitaxy (MOVPE), has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and x-ray photoemission spectroscopy (XPS). Three reconstructions are observed depending on the MOVPE process conditions and temperature of annealing in vacuum after growth: these are the (rt3xrt3), (2x2) and (1x1). The (rt3xrt3) reconstruction is the most interesting of these and consists of one ad-atom per unit cell resting on a full layer of indium atoms. This structure is stabilized by the incorporation of oxygen atoms, which are gettered from the MOVPE environment by the indium-rich surface. At the meeting, the atomic structure of each InP (111) reconstruction will be described and compared to those observed on the gallium-rich GaAs (111) and GaN (0001) surfaces.
3:00 PM		SC+SS-ThA-4 Surface Stress and the Morphology of Si(111) near Tc J.B. Hannon, J. Tersoff, R.M. Tromp (IBM Research Division) It is well known that surface stress can strongly influence the structure and chemical composition of surfaces. Despite significant theoretical progress, few quantitative experimental investigations have been reported. One reason for this is the long-range of elastic interactions at surfaces. To quantitatively determine the influence of surface stress, the structure of the surface over micron-scale distances must be precisely known. In this talk I describe low-energy electron microscopy measurements of Si(111) surface structure near the 7x7 to 1x1 transition temperature (Tc = 1135 K), where phase coexistence is observed. We find that the equilibrium domain geometry is determined by a competition between the free energy difference between the phases, which favors a single phase, and elastic relaxation at the phase boundaries, which favors phase coexistence.¹ Elastic relaxation occurs because of the difference in surface stress between the two phases. For Si(111), the stress difference has been measured by Twesten and Gibson.² In equilibrium, the force on each phase boundary vanishes. We use this fact to determine the free energy difference between phases, and phase boundary creation energy, from the measured domain configurations. In equilibrium, each phase boundary gives rise to an independent equation relating the free energy difference, the boundary creation energy, and the stress difference. By measuring domain configurations as a function of temperature, we determine the temperature dependence of the free energy difference between phases near Tc. Our measurements correspond to an entropy difference between the phases of only 0.01 kB per 1x1 unit cell, a surprisingly small value given the fact that the 7x7 structure has long-range order and the 1x1 phase does not. These results indicate that the degree of disorder in the two phases is similar. ¹J.B. Hannon, et al., PRL, in press ²PRB 50, 17628 (1994).
3:20 PM		SC+SS-ThA-5 Origin of the Wide Si(7 7 17) Domain Parasitic on Si(5 5 12) Surface S.H. Cho, J. Zhang, J.M. Seo (Chonbuk National University, Korea) Recently the high-index and single-domain Si(5 5 12) has attracted much attention due to its potential application as a template for one dimensional nanowire fabrication. However, another high-index and single-domain (7 7 17) plane has not been reported yet, although they have similar plane directions of only 0.3 degree off and commonly consist of (2 2 5) and (3 3 7) sections [i.e., (5 5 12)=2x(3 3 7)+(2 2 5) and (7 7 17)= (3 3 7)+(2 2 5)]. From the recent STM investigation on Si(5 5 12), we have detected the wide Si(7 7 17) domains parasitic on Si(5 5 12). Most of wide (7 7 17) domains appear in the terrace adjacent to the step parallel to (-1 1 0) row direction. In a single terrace, the (7 7 17) domain extends to a few hundreds Angstrom from the step, then transforms to (5 5 12). Some (7 7 17) domains are also detected from the bent surface without steps. These wide and pure (7 7 17) domains, appearing near the step or on the bent surfaces, are experiencing the compressed stresses, and these excessive stresses replace the compressed stress originating from a (3 3 7) section in (5 5 12). Therefore, the extra (3 3 7) section is not required in a (5 5 12) plane under such stresses, which results in (7 7 17) domain until such local compressed stresses are released.
3:40 PM		SC+SS-ThA-6 First-Principles Study of Thermal Fluctuation of Si(111)√3 X √3 - Ag Surface Appeared in Non contact AFM Images M. Tsukada, N. Sasaki, S. Watanabe (University of Tokyo, Japan) Until recently ''Honeycomb-Chained-Triangle (HCT)" model for the atomic structure of Si(111) √3 X √3 -Ag surface (referred to √3 -Ag surface hereafter) seemed to be accepted. However, a new structural model called ''InEquivalent-Triangle (IET)" model was proposed by Aizawa et al.¹ based on the first-principles calculation as well as the low temperature STM. The IET model is energetically more favorable than the HCT model by about 0.1 eV per √3 X √3 unit cell. In IET model, a mirror plane symmetry is broken and the inequivalent Ag triangles are located in the unit cell. In the present work, we calculated NC-AFM images of IET structure based on a density functional theory. In simulating two-dimensional NC-AFM images, we used a "Fourier expansion method". In this method, the two-dimensional symmetry of the tip-surface system is considered. The thermal averaged images of the fluctuating two different phases of IET structures are calculated in the following way. Namely, at each scanning position of the tip over the surface, the frequency shift is calculated from the force-curve which is obtained from the canonical weighted average of the both phases of IET. The simulated theoretical images reproduces fairly well the experimental observation at room temperature. Therefore our result confirms that the room temperature apparent HCT structure is the thermal fluctuated structures of the two phases of IET. The effect of the tip apex structure on AFM images is also discussed together with the thermal fluctuation. ¹ ¹ H.Aizawa, M.Tsukada, N.Sato and S.Hasegawa, Surf. Sci. 429, L509 (1999).
4:40 PM		SC+SS-ThA-9 Adsorption of C₂H₂ on Si(100) - New High Resolution Core-level Photoemission and Photoelectron Diffraction Results M. Kittel, M. Polcik, J.-T. Hoeft, D.I. Sayago (Fritz-Haber-Institut der MPG, Germany); R.L. Toomes, D.P. Woodruff (University of Warwick, UK) The adsorption of acetylene and ethylene on Si(100) has been subject of a large number of experimental and theoretical studies. Most theoretical studies have favoured a 'di-σ' bonding character for both species, in which the C-C axis lies parallel to the Si dimers with the molecules symmetrically placed atop the dimers. Two independent experimental scanned-energy mode photoelectron diffraction (PhD) studies of the structure of the ethylene adsorption system have provided confirmation of this idea,^1,2 but in the case of acetylene adsorption, one such study³ found this atop dimer configuration, but another found a totally different site between two dimers². In an attempt to resolve this discrepancy we have carried out further experimental studies of the Si(100)/acetylene system at different temperatures and coverages, using high-resolution C 1s photoemission and C 1s PhD. Our new measurements confirm the dominance of the aligned atop-dimer geometry under all conditions studied. We find no direct evidence for a second species, except under conditions associated with fragmentation of the molecular adlayer by the incident soft X-ray beam, although the PhD data could be reconciled with some fractional occupation of other lower-symmetry sites which may contribute only very weak PhD modulations. ¹ P.Baumgärtel et al, New.J.Phys 1 (1999) 20.1 ² S.H.Xu et al., Phys. Rev. Lett. 84, 939 (2000) ³ R. Terborg et al., Phys.Rev.B 61, 16697 (2000).
5:00 PM		SC+SS-ThA-10 The Effect of an As-flux on the Surface Structure During the Growth of Si on GaAs(001) c(4x4) T.A.R. Müller, B.D. Schultz (University of Minnesota); H.H. Farrell (Idaho National Engineering and Environmental Laboratory); A. Franciosi (Universita' di Trieste, Italy and Univ. of Minnesota); C.J. Palmstrom (University of Minnesota) Despite the experimental discovery that two atomic monolayers of Si co-deposited with an As-flux are effective in decreasing the barrier height in the Al/Si/GaAs system by 0.5eV, the detailed interfacial atomic structures have yet to be determined. The present work focuses on understanding the evolution of the structure within the first few atomic monolayers caused by the deposition of Si with and without an As-flux on the GaAs(001) c(4x4) surface by MBE. In-situ Reflection High Energy Electron and Low Energy Electron Diffraction, work function measurements, X-ray photoelectron and Auger electron spectroscopies and Scanning Tunneling Microscopy have been used to determine the properties and composition of the surface as a function of Si coverage. For the co-deposition of Si with As, the measured work function increases up to a Si-coverage of 0.5 ML and the intensity ratio of the Ga and As 3d photoemission peaks decreases. However, for Si deposited at pressures <10^-10 mbar, the work function shows variations of <50meV from the starting surface and the XPS peak intensity ratio increases for a coverage of 0.5 ML. Atomistic models consistent with electron counting, surface symmetry, work function and composition changes will be presented.