AVS1998 Session SS1+NS-ThM: Growth and Thin Films

Thursday, November 5, 1998 8:20 AM in Room 308

Thursday Morning

Start	Invited?	Item
8:20 AM	Invited	SS1+NS-ThM-1 Peter Mark Memorial Award Address - Morphology of Epitaxial Films during Low Temperature Growth D.G. Cahill (University of Illinois, Urbana-Champaign)
9:00 AM		SS1+NS-ThM-3 The Influence of Dislocations on the Intermixing Kinetics of Pd-Au Monolayer Films O. Schaff, A.K. Schmid, M.C. Bartelt, R.Q. Hwang (Sandia National Laboratories) The kinetics of surface alloying differ dramatically from the bulk counterparts. This is due to the fact that the dominant diffusion mechanisms are fundamentally different on surfaces. In this work, we used classical bulk studies as a guide to construct an effective experimental approach to directly determine such atomic mechanisms controlling intermixing kinetics in two-dimensional samples. On a clean Ru(0001) substrate, we prepared prototypical two-dimensional diffusion couples, consisting of monolayer regions of Au adjacent to monolayer regions of Pd. Between carefully controlled annealing steps, atomic resolution scanning-tunneling microscopy was used to image changes in the adlayer structure, and thereby identify and characterize the phenomena leading to the formation of a Pd-Au surface alloy. We report several striking observations: (I) The Pd-Au surface alloy forms only on the Au-rich side of the diffusion couple, reflecting strong asymmetry in the diffusion constants of the two metals across the Pd-Au seam. (II) Diffusion constants, and thus alloying, in this system are strongly anisotropic. Specifically, the rate of interdiffusion at a given Pd-Au boundary, measured from changes in the average position of the alloy interface, depends strongly on the orientation of the boundary with respect to the underlying Ru lattice. We relate these observations to the energetics of exchange of surface atoms with the "gas" of Au and Pd adatoms, as well as to the dynamical properties of the dislocation network present in the Au side of the diffusion couple.
9:20 AM		SS1+NS-ThM-4 The Kinetic Nature of Slope Selection during Unstable Growth S. van Dijken, L.C. Jorritsma, B. Poelsema (University of Twente, The Netherlands) At temperatures below 320 K, the Cu(001) transforms into an arrangement of facets upon epitaxial growth, resulting in a pyramidlike surface morphology. The sides of the pyramids all have the same facet orientation, selected by the growth temperature. With increasing growth temperature, [113], [115] and [117] facet faces are obtained.^1,2 Up to now, it is not clear whether this slope selection is determined by local equilibrium or by the kinetics of the growth process. We present evidence for the kinetic origin of this phenomenon. At the same temperature various facet orientations can be obtained, depending on the angle of incidence of the impinging adatoms. We will show that the observed slope selection can be explained by the refraction of atoms above the surface. ¹ ¹H.J. Ernst, F.Fabre, R. Folkerts and J. Lapujoulade, Phys. Rev. Lett. 72, 112 (1994) ²L.C. Jorritsma, M. Bijnagte, G. Rosenfeld and B. Poelsema, Phys. Rev. Lett. 78, 911 (1997)
9:40 AM		SS1+NS-ThM-5 Growth on Cu(100) Using Improved Simulation Algorithm¹ J.G. Amar (University of Toledo); M.R. D'Orsogna, T.L. Einstein (University of Maryland, College Park); I. Beichl (National Institute of Standards and Technology); F. Sullivan (Center for Computing Sciences) We have developed a novel Monte Carlo scheme to simulate homoepitaxial growth on (100) surfaces of sc and fcc crystals, using tree and list structures. We have applied it to the specific case of Cu, in both the submonolayer and multilayer growth regimes. Energy barriers were calculated using Effective Medium Theory, and diffusive processes were grouped into 4 classes. The effect of an Ehrlich-Schwoebel barrier was also considered. For the submonolayer regime at 213 K, we find reasonable agreement with experimental results² for the scaling of the island density as a function of the ratio of diffusion and deposition rates. For multilayer growth at 160 K we find good quantitative and qualitative agreement with experimental results³ for the width as a function of coverage. In particular, the width exponent ß agrees with the experimentally reported value. At higher temperatures (T=200 K), our simulations underestimate the exponent ß. Presumably a new diffusion channel becomes important. We have tried several single-atom processes, e.g. up-stairs climbing and biased upward and downward funneling, but none improve agreement significantly. The initial surface morphology may also exert an important influence on ß at high coverages. We are currently investigating the effects of different initial growth conditions such as a slightly rough or stepped substrate. ¹ ¹Work partly supported by DoD; MRD and TLE primarily supported by NSF MRSEC grant DMR 96-32521. ²A. Swan, Z.P. Shi, J.F. Wendelken, and Z. Zhang, Surface Sci. 391, L1205 (1997). ³H.J. Ernst, F. Fabre, R. Folkerts, and J. Lapujoulade, Phys. Rev. Lett. 72, (1994); JVSTA 12, 1809 (1994).
10:00 AM		SS1+NS-ThM-6 The Atomistics of Homoepitaxial Growth on bcc(110)-Surfaces U.K. Koehler, C. Jensen, A. v. Stockhausen (Ruhr-Universitaet Bochum, Germany) Time lapsed STM-movies, which allow a direct observation of the kinetics of growth processes on an atomic level up to 500°C, SPA-LEED and Monte-Carlo simulations are used to study the nucleation and growth behavior of homoepitaxy on the bcc(110)-surface. For Fe on Fe(110) and W on W(110) a strongly anisotropic growth with islands elongated in [001]-direction is found. A very effective Schwoebel-barrier leads to a nearly perfect statistical growth at RT with increasing island anisotropy in upper layers. At higher coverage a complete facetting of the surface is found and analyzed with SPA-LEED. A quantitative analysis of the STM-movies together with a kinetic Monte-Carlo simulation, which includes the full crystallographic symmetry of the bcc(110)surface, is used to extract information on the atomic diffusion behavior governing growth. A variety of growth features like the rugged appearance of the island edges and changes in the island shape with temperature are correctly reproduced in the simulation and can be assigned to the local diffusion energetics at step edges. Especially an anisotropic diffusion, which strongly suppresses diffusion along [001], is needed to reproduce the observed island anisotropies. A comparison of the layer distribution in the simulation with the one found with STM is used to determine the Schwoebel-barrier. Lateral island coarsening and an atom flux across the step edge following a temperature increase after growth is directly observed in STM.
10:20 AM	Invited	SS1+NS-ThM-7 Probing the Forces Stabilizing Self-Assembled Structures: Dynamics of Vacancy Island Lattices in Ag films on Ru(0001) K. Pohl, M.C. Bartelt, J. de la Figuera, N.C. Bartelt (Sandia National Laboratories); J. Hrbek (Brookhaven National Laboratory); R.Q. Hwang (Sandia National Laboratories) Nature exhibits processes that rival our most advanced patterning technologies used to create ordered lattices of nanoscale structures. Such self-organized phenomena have the potential to revolutionize materials performance, leading to higher density information storage and high-speed nanoscale electronics. Though many observations of self-organization have been reported, the fundamental mechanisms underlying such behavior remain unclear. The commonly accepted source of such mesoscopic-scale forces is the stress field mediated by the substrate which supports the grown structures. This, however, has not been confirmed, nor have such interactions been directly measured. In our work we have taken the approach of using observations of thermal fluctuations of an ordered array of surface defects to probe the interactions between the defects. In particular, we have used STM to study the array of vacancy lattice islands which forms upon exposure of a monolayer of Ag on Ru(0001) to sulfur. This is an extremely well-defined example of an ordered "mesoscopic" surface structure. At room temperature, each island is observed to vibrate around its equilibrium lattice postion. These vibrations appear to be harmonic and by performing a normal mode (phonon) analysis of the vibrations we can determine the elastic constants of the island array. The magnitude of the interactions is consistent with theories of elastic step-step interactions in strained films. This work was supported by the Office of Basic Energy Sciences of the U.S. DOE, Division of Materials Science (Contract No. DE-AC04-94AL85000).
11:00 AM		SS1+NS-ThM-9 STM Study of Ultrathin NaCl(111) Layers on Aluminum W. Hebenstreit, J. Redinger (TU Vienna, Austria); R. Podloucky (University Vienna, Austria); M. Schmid, P. Varga (TU Vienna, Austria) Polar surfaces like NaCl(111) are electrostatically unstable and cannot be found as terminating surfaces of crystals. But we can grow NaCl islands with (111) surface orientation on Al(111) and Al(100) single crystals. The (111) structure is revealed by atomically resolved Scanning Tunneling Microscopy (STM). The maxium coverage we could achive was 0.3 monolayer. The islands are triangular shaped, located at the lower side of substrate step edges and in the case of the Al(111) substrate alinged with the closed packed directions of the substrate. The islands consist of two Na layers with one Cl layer in between. We performed ab initio calculations with the FLAPW (full potential lineraized augumented plane waves) method of the electronic structure of a free standing Na-Cl-Na sandwich structure. These calculations reveal that the Na 3s level is filled with half an electron. The sandwich consists of two +0.5 charged Na metallic layers with a -1 charged ionic Cl layer in between, so the whole film ist neutral, free of dipoles, and electrostatically stable. The film is 4.6% laterally contracted and 5.3% expanded in vertical direction with respect to NaCl bulk distances.
11:20 AM		SS1+NS-ThM-10 Three-Dimensional SiGe Island Density on Si(001) and Morphology After Si Overgrowth¹ J.S. Sullivan, E. Mateeva, H. Evans, D.E. Savage, M.G. Lagally (University of Wisconsin, Madison) Thin films of SiGe deposited on Si(001) can form three-dimensional (3D), coherently strained islands via a modified Stranski-Krastanov growth mode. Single films as well as highly ordered 3D superlattices with specific island sizes and densities may exhibit unique electronic and optoelectronic properties. In order to investigate how common process variables in epitaxial multiple-layer film growth influence 3D island density and morphology, we deposited SiGe films on Si(001) using low-pressure chemical vapor deposition and varied alloy composition, substrate temperature, and deposition rate. Films containing {105} faceted SiGe islands were overgrown with and embedded in Si at various substrate temperatures. Film growth and morphological evolution were monitored with in-situ, real-time reflection high-energy electron diffraction. Atomic force microscopy was performed ex-situ to characterize film morphology, and buried-island morphology was determined with cross-sectional transmission electron microscopy. The 3D island number density exhibits an Arrhenius-type dependence on substrate temperature, a power law relationship with deposition rate, and an inverse proportionality to Ge mole fraction in the alloy. Islands broaden during overgrowth and embedding due to thermally activated mass transport and Si interdiffusion; such that the {105} facets grow outward producing a (001) mesa at the apex. We will discuss our results in the context of simple thermodynamic and kinetic models and describe possible methods of obtaining and maintaining a specific size and size distribution of 3D islands. ¹ ¹Supported by the NSF.
11:40 AM		SS1+NS-ThM-11 Effects of Ion Pretreatments on the Nucleation of Silicon on Silicon Dioxide C Basa (University of North Carolina, Chapel Hill); Y.Z. Hu (AG Associates Inc.); M.T. Tinani, E.A. Irene (University of North Carolina, Chapel Hill) It is well known that the silicon (Si) surface condition is crucial for low temperature Si expitaxy.¹ In particular, hydrogen has been implicated as an important factor inhibiting Si nucleation on Si,² and SiO₂ surfaces.³ In addition, nuclei densities can change depending upon the preparation of the surface film.³ We previously studied the effects of pretreatments of various forms of hydrogen (molecular and ionic) on nucleation parameters (nuclei density, and incubation time (t_inc)).⁴ We found that H₂ pretreatments passivated the surface causing longer t_inc, lower nuclei density, larger nuclei, and rougher final films. H⁺ pretreatments increased the number of nucleation sites resulting in shorter t_inc, higher nuclei density, smaller nuclei, and smoother final films. However, the mechanism for H⁺ effects on nucleation was not elucidated in that study. Therefore, the study was extended to include the effects of (1) ion dose and energy, (2) other ions (Ar⁺, He⁺, N⁺) and (3) temperature of the pretreatment on the nucleation of poly-Si on SiO₂. There are three major results from the surface pretreatment experiments. First, in the range tested, ion dose has more influence on nucleation parameters than ion beam energy. Second, results with different ionic species indicate a physical, rather than chemical mechanism, for creating nucleation sites. Third, high temperature ionic pretreatments damage the surface less than room temperature treatments. ¹T. Yamazaki, M. Miyata, T. Aoyama, and T. Ito, J. Electrochem. Soc., 139, p. 1175 (1992). ²K. Tsubochi and K. Masu, Mat. Res. Soc. Symp. Proc., 315, p. 59 (1993). ³J. T. Fitch, J. Electrochem. Soc., 141, p. 1046 (1994). ⁴Y. Z. Hu, C. Y. Zhao, C. Basa, W. X. Gao, and E. A. Irene, Appl. Phys. Lett., 69, p. 485 (1996).