AVS1996 Session SS1-TuA: Structure of Stepped Surfaces

Tuesday, October 15, 1996 2:00 PM in Room 204C

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS1-TuA-1 LEEM Studies of Stripe Phase and Step Shape Transition on Highly B-doped Si(001) Surfaces Y. Hong, E. Bauer, I. Tsong (Arizona State University); D. Jones, J. Pelz (Ohio State University) We have conducted low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM) studies on highly B-doped Si(001) surfaces, in which an equilibrium "striped" phase was observed. This striped phase was previously predicted by Alerhand et.al., but had never been observed over a large surface area. We report detailed studies of the surface morphology as a function of temperature of the highly B- doped Si(001) surfaces with small (<0.1 degree) misorientation. Real time LEEM images showed that the surface exhibited normal (2x1) and (1x2) domains separated by S\sub A\ and S\sub B\ steps at temperature above 950 centigrade. The step structure transformed into a striking triangular-tiled structure as the temperature was lowered; this structure then evolved into a "striped" phase through bifurcation upon further lowering of the temperature to below 800 centigrade. Different bifurcation mechanisms were revealed by the real-time LEEM images: e.g. new step "finger" formation growing across the terrace, and existing finger tip cracking and splitting into two narrower fingers. Also, isolated islands or depressions split into fine highly elongated islands or depressions during sample cooling. This step shape transition was reversible when the temperature of the surface was increased. These observations can be explained by the surface stress relaxation effects enhanced by the biaxial tensile strain introduced by the B-Si bonds. The biaxial stress varies with temperature due to the temperature dependent segregation of B atoms at the Si surface. Our observations indicate a possibility of obtaining ordered self-assembled nanostructures on Si(001) surfaces.
2:20 PM		SS1-TuA-2 STM Study of Adjustable Nanometer-scale Striped Step Structures on Si(001)-(2x1) Surfaces Due to High B Doping and Strain D. Jones, C. Ebner, J. Pelz (Ohio State University); Y. Hong, E. Bauer, I. Tsong (Arizona State University) In 1988 Alerhand et.al. [Phys. Rev. Lett. 61, 1973 (1988)] predicted that surface stress relaxation effects on Si(001)-(2x1) should lead to the spontaneous generation of an array of periodic step structures ("stripes") at the surface. While evidence of wavy-step and stripe-like step structures have recently been found by Jones et.al. [Phys. Rev. Lett. 75, 1570 (1995)] on Si under surface tensile strain, the large scale periodic structures predicted by Alerhand et.al. have not been observed. Here we present conclusive observations of an equilibrium "striped" phase which completely covers the surface of heavily boron-doped Si(001)-(2x1) wafers, as observed by ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and low-energy electron microscopy (LEEM). This presentation will focus on the STM measurements, where striped step structures as narrow as ~10 nm are found for annealing temperatures below 800\super 0\C, and which evolve into triangular-tiled wavy-step structures above 900\super 0\C. These observations are explained in terms of surface stress relaxation effects, enhanced by strain from temperature-dependent segregation of B atoms at the Si surface. In addition to confirming theoretical predictions of a striped phase on Si(001), these observations indicate a simple method for producing quasi-ordered, nanoscale step structures of variable size and shape on the technologically important Si(001) surface. Recent results on the effect of applied uniaxial stress on step shapes, and modeling of various strained 2-D step structures will also be discussed. Work at The Ohio State University supported by NSF Grants DMR93-57535 and DMR94-06936.
2:40 PM		SS1-TuA-3 Structure of Highly Vicinal Si(001) Surfaces: Si(114) and Si(115) A. Baski, S. Erwin, L. Whitman (Naval Research Laboratory) It is known that vicinal Si(001) surfaces tilted more than ~4\super o\ away from (001) toward (111) consist of B-type (001) terraces separated by rebonded double-layer D\sub B\ steps. These rebonded steps incorporate an extra row of atoms to reduce the number of surface dangling bonds (although at the expense of increased surface stress). We find that this terrace-plus-step morphology can be extrapolated to Si(114), a recently discovered stable surface oriented 19.5\super o\ away from (001). Based on scanning tunneling microscopy (STM) images and first-principles total-energy calculations, we show that Si(114)-2x1 consists of an alternating sequence of single- and double-width B-type (001) terraces, separated by rebonded and NON-rebonded D\sub B\ step edges. This is the first time non-rebonded D\sub B\ steps have been observed on a vicinal (001) surface. We believe these unreconstructed steps are present to relieve the surface stress introduced by the rebonded steps (at the expense of dangling bonds). It is interesting to ask at what step density (i.e. orientation) non-rebonded steps first appear. We have therefore investigated Si(115), which is tilted only 15.8\super o\ away from (001). Our data shows that Si(115) also incorporates non-rebonded steps, although it does not form a stable planar reconstruction. The surface instead breaks up into a complicated arrangement of (117)-, (115)-, and (114)-oriented subunits.*Present address of A.A. Baski: Department of Physics, Virginia Commonwealth University
3:00 PM		SS1-TuA-4 Thermodynamics of Monatomic Steps on Cu (111) Vicinal Surfaces M. Giesen, G. Schulze Icking-Konert, D. Stapel, H. Ibach (Forschungszentrum J\um U\lich, IGV, Germany) We present STM measurements on vicinal Cu\ (\111\)\ surfaces with \(\111\)\ steps as well as \(\100\)\ steps. The Cu\ (\997\)\ surface with \(\111\)\ steps was found to be instable. Large facets with \(\100\)\ steps are created. The Cu \(\21 21 23\)\ surface with \(\100\)\ steps, on the other hand, is stable. The difference in stability of these surfaces is not caused by a difference in step energies. This is shown by an analysis of the equilibrium shape of copper islands on Cu \(\21 21 23\)\. We have also investigated the time dependence of step fluctuations on these Cu \(\111\)\ vicinal surfaces for both step types as a function of temperature using the time correlation function F\(\y\=\0,t\)\ \ =\ \<\ \(\ x \(\ y \=\ 0,t \)\ \-\ x \(\ 0,0 \)\ \)\ \super2\ \>\. Here, x denotes the coordinate perpendicular to the step direction. The coordinate y is held constant. The time dependence of F \(\ 0,t \)\ obeys a power law t \super \alpha\ \. Depending on the mass transport involved, different exponents are found. The exponent \alpha\ equals 1 \/\ 2 if atoms are rapidly exchanged with the terraces, whereas \alpha\ is 1 \/\ 4 if atomic motion is restricted to the step edges. From the analysis of the time dependence of the correlation function we determined the mass transport involved on vicinal Cu \(\ 111 \)\ surfaces. A barrier for atom diffusion alongside the step edges of E \sub d\ \=\ 214 meV was obtained.
3:20 PM		SS1-TuA-5 STM Studies of Equilibrium Step Fluctuations and Step-Step Interactions on a Vicinal Ag(001) Surface P. Wang (University of Maryland); H. Pfn\um u\r (Universit\um a\t Hannover, Germany); S. Khare, T. Einstein, E. Williams, J. Reutt-Robey (University of Maryland) Using STM and LEED we have studied the Ag(1,1,14) surface. We emphasize the importance of proper preparation (and observe that sulfur induces a c(2x4) reconstruction). By analyzing the temporal and the spatial step correlation functions, we deduce the surface mass transport mechanism and estimate the kink energy and the time constant for single-atom diffusion, and the surface stress. By studying the terrace-width distribution[1], we find that we can describe the step-step interaction by a simple repulsion decaying as the inverse square of the step separation, typical of semiconductors but in contrast to vicinal Ag(110)[2]. From the prefactor of the decay, we can estimate [an upper bound of] the surface stress. *Supported in part by NSF MRG DMR91-03031 1. X.-S. Wang et al., Phys. Rev. Lett. 65, 2430 (1990) 2. W.W. Pai et al., Surface Sci. 307-309, 747 (1994).
3:40 PM		SS1-TuA-6 Low Temperature STM Study of Initial Growth of Pb/Cu(211) L. Bartels, S. Z\um o\phel, G. Meyer, E. Henze, K. Rieder (Freie Universit\um a\t Berlin, Germany) We present an STM investigation of the lead adsorption on the Cu(211) surface in the temperature range between 30 K and room temperature. Various superstructures and surface alloys of lead on the low index copper surfaces have been shown previously. The (211) surface is a stepped surface consisting of (111) terraces and (100) steps. This renders the processes of surface reorganisation upon adsorption quasi one-dimensional. With our self-built variable-temperature STM we found the room temperature adsorbed Pb atoms incorporated into the step-edges. This is consistent with previous results on the Cu (111) surface. The first ordered structure consists of Cu-Pb chains and is found at a coverage of 0.5 ML. Increasing the coverage yields a close packed Pb row along the step-edge at a coverage of 0.75 ML. On low temperature prepared samples lead dimers form and the first ordered structure at 0.66 ML consists of rows of dimers at the step-edge. Increasing the coverage leads ag! ain to the same structure at 0.75 ML as described above. Although the formation of the different surface structures depends on temperature, this is no example for a kinetically driven reaction, but rather a consequence of the temperature dependence of the chemical potential of the Pb atoms with respect to intermixing with the top substrate layer. By systematically employing the ability to manipulate single atoms with the tunneling tip, we could determine the formation of the structural elements of the Pb-Cu-layer on the single-atom scale. Furthermore, by measuring the threshold tunneling resistance to detach atoms from different ad-sites (lead/copper at kink-, step-edge- or dimer-site) we can approximate the binding energy and gain some insight into the thermodynamical parameters involved.
4:00 PM		SS1-TuA-7 Double Steps-Single Steps Morphological Transition on Vicinal Surfaces of Alloys: He Diffraction, STM and X-ray Study of Cu\sub 83\Pd\sub 17\(1,1,11) L. Barbier, S. Goapper, B. Salanon (CEA-SACLAY, France) The surface morphology of binary ordered alloys can depend on bulk chemical order : In an He diffraction experiment we have shown that vicinal surfaces of Cu\sub 83\Pd\sub 17\ exhibit double steps below a temperature close to the bulk critical order-disorder phase transition at T\sub c\=778 K and single steps above. The surface morphology in the chemically ordered phase (double steps) has been investigated by STM. Statistical analysis of STM images allowed to analyse the double step disorder by means of measurement of the step edge correlation functions and to deduce kink energies of upper and lower steps. A surface X-ray experiment sensitive both to surface morphology and bulk chemical order has been performed at ESRF in order to tentatively localize in temperature the surface morphological transition and the bulk chemical disordering. At the surface, emergence of antiphase boundaries (APB) between adjacent ordered domains is observable on STM pictures. This allowed us to follow ordered domains growth versus time for T
4:20 PM		SS1-TuA-8 Formation of Atomically Flat Silver Films on GaAs with a "Silver-Mean" Quasiperiodicity C. Shih (University of Texas, Austin); A. Smith (Carnegie Mellon University); K. Chao, Q. Niu (University of Texas, Austin) A flat epitaxial Ag film on a GaAs (110) surface was synthesized in a two step process. Deposition of a critical thickness of Ag at low temperature leads to formation of a dense nanocluster film. Upon annealing, all atoms rearrange themselves into an atomically flat film. This sharply defined critical thickness is so highly preferred that any deficiency from this critical thickness results in completely empty pits in order to accommodate such a deviation. The structure of such silver films was analysized using low energy electron deffraction and scanning tunneling micrscopy. The basic structure of this Ag film is close-packed (111); however, it is modulated by a long range quasiperiodic arrangement along the substrate [001] direction. The observed quasiperiodicity is characterized by the "silver-mean" irrationality, which has been studied extensively in the wake of quasicrystal discovery but has not been realized experimentally before. The ability to grow such epitaxial overlayers of metals on semiconductors, which has been difficult in the past, provides a testing-ground for theoretical models and a connection between metal and semiconductor technologies.
4:40 PM		SS1-TuA-9 Structural Rearrangements in a 2-fold Quasicrystalline Surface Z. Shen, C. Jenks, A. Goldman, T. Lograsso, D. Delaney, P. Thiel (Iowa State University) Little is known about the structure of surfaces of quasicrystalline materials. One basic question is whether the surface retains the structure of the bulk material. With this question in mind, we have studied a two-fold surface of an icosahedral alloy, Al\sub 71\Pd\sub 20\Mn\sub 9\, using (primarily) low-energy electron diffraction (LEED). From previous work, it appears that the five-fold surface, which is the lowest-energy surface in this material, does indeed retain the bulk structure. For the two-fold surface, however, we observe a progression of LEED patterns as a function of annealing temperature, which strongly indicates structural rearrangements. Ion bombardment at room temperature leaves a disordered surface and no discernible LEED pattern. Prolonged heating at 600 K yields a pattern with rhombohedral characteristics. This transforms to a pattern with rectangular symmetry upon heating at 900 K. This progression of patterns suggests an activated surface reconstruction. The development of the rectangular pattern is accompanied by facetting around the edges of the sample. The facets contain three-fold and five-fold surfaces. Their correlation with the sample edges suggests that they are related, kinetically or thermodynamically, to defects or trace contaminants.