AVS1996 Session EM-ThM: Strain-Mediated Kinetics in Epitaxial Growth

Thursday, October 17, 1996 8:20 AM in Room 204A

Thursday Morning

Start	Invited?	Item
8:20 AM	Invited	EM-ThM-1 Nanofabrication by Self-organizing Heteroepitaxy J. Tersoff (IBM T.J. Watson Research Center) Various nanostructures can form spontaneously during heteroepitaxy. These include such potentially useful structures as vertical and horizontal superlattices, quantum dot arrays, and step-bunch arrays (which can serve as templates for growth of quantum wires). I will describe how stress, and perhaps spinodal decomposition, can account for several of these phenomena. This theoretical understanding can provide guidance in tailoring the growth conditions to optimize the properties of these self-organized nanostructures.
9:00 AM		EM-ThM-3 Compositional Stability of Alloy Thin Films J. Guyer, S. Barnett, P. Voorhees (Northwestern University) Lattice mismatch is known to cause nonplanar morphologies during thin film growth, but the composition of alloy films is usually assumed to be uniform. Experimental evidence for inhomogeneous composition is frequently attributed to spinodal decomposition. However, stresses associated with composition variations strongly stabilize a bulk alloy against decomposition. The free surface of a thin film affords some lattice relaxation, leading to decomposition at higher temperatures than within a bulk material, but this effect alone is not enough to account for fluctuations observed in films grown near or above the bulk spinodal. We find that the growth process enables an alternative route to compositional inhomogeneity. If a uniform film develops a surface ripple, regions of compressed and relaxed lattice result. As further material deposits on this perturbed surface, the different alloy components incorporate preferentially where the lattice is most compatible. We thus find that composition fluctuations can arise in the growth of an initially uniform film. When the elastic energy relieved at the film surface exceeds the chemical energy of the composition modulation, compositional and morphological perturbations can arise even in lattice-matched alloy films. We predict this behavior to occur as much as 290 K above the bulk spinodal in In\sub x\Ga\sub 1-x\As. Using the insights provided by the theory, we will discuss recent In\sub x\Ga\sub 1-x\As films grown by molecular beam epitaxy. This alloy system is well suited to test the predictions of this model, due to a large solute expansion coefficient and strong chemical instability.
9:20 AM		EM-ThM-4 Interfacial Properties and Relaxation of Epitaxial InAs Layers Grown on Gaas J. Belk, J. Sudijono, H. Yamaguchi, C. McConville, T. Jones, B. Joyce (Imperial College, United Kingdom) The relaxation of strained layers grown by methods such as Molecular Beam Epitaxy is habitually studied by Transmission Electron Microscopy. In our studies of the compressively strained InAs / GaAs system, we have shown that complementary information can be obtained using Scanning Tunnelling Microscopy (STM). Whereas STM can normally only provide information on the exposed sample surface, it is sometimes still possible to detect dislocations that are buried within the bulk of the specimen. The contrast mechanism of these features is discussed. Another area of interest in material science is within the earliest stages of dislocation nucleation. Detailed imaging of the fully strained material prior to dislocation formation is also possible with STM, allowing some information to be gleaned on this theme. The local atomic, and to some extent, the electronic nature of dislocations which extend to the surface can be probed. The growth behaviour is found to vary strongly with the choice of crystallographic orientation of the GaAs substrate. Thus, despite the magnitude of the misfit remaining the same for all the substrates, growth appears to be highly influenced by the nature of the available strain relief mechanisms. These relaxation processes have different efficiencies, and become active at distinct coverages of the strained material. In particular, InAs deposits on GaAs(110) and (111)A retain a two-dimensional growth mode, making them ideal candidates for inspection by STM. Although dislocation imaging on (001)-orientated GaAs is difficult with STM once three-dimensional growth occurs, an unexpected alloying is found at the heterointerface prior to the growth mode transition.
9:40 AM		EM-ThM-5 Monte Carlo Simulations of the Transition between Step Flow and Island Growth Modes in OMCVD of GaAs R. Venkataramani, K. Jensen (Massachusetts Institute of Technology) Monte Carlo (MC) simulations of the (001) surface of GaAs are used to study the changing surface morphology during growth by organometallic chemical vapor deposition (OMCVD). Growth mechanisms, which include nucleation, Ga and As incorporation, surface reconstructions, and step-edge barriers, are examined to identify the important parameters in controlling the morphology of the growth. The growth models are compared to reported experimental surface roughness measurements by x-ray scattering \super 1\ in order to determine consistent growth mechanisms. The simulations correctly predict the transition between island growth and step flow modes of epitaxial growth as the flux of species to the surface and/or the temperature of the surface is changed. The nucleation rate and mechanism affect the surface roughness in the model. If the model has too high a nucleation rate, the surface becomes very rough with pyramidal islands. If the model has too low a nucleation rate, the surface never undergoes island growth, as all the adsorbates diffuse to the steps. Surface reconstructions affect the measured roughness in the model, especially in the initial stages of growth as the surface smooths while the reconstructions are breaking. The step-edge barrier is a main contributor to the transition between step flow and island growth modes. Computationally, the algorithm is parallelized to be able to simulate large surface sizes in order to model realistic size steps and terraces on the surface. The MC models are shown to match experimental studies, while also extracting important mechanistic details that experiments alone cannot determine. \super 1\ D.W. Kisker, G.B. Stephenson, P.H. Fuoss, and S. Brennan, J. Crystal Growth, 146, 104, (1995).
10:00 AM		EM-ThM-6 Various Factors Influencing the Interfacial Roughness in InGaAs/GaAs Heterostructures: A Cross-sectional Scanning Tunneling Microscopy Study K. Chao, C. Shih, D. Gotthold, B. Streetman (University of Texas, Austin) By using cross-sectional scanning tunneling microscopy, we investigate various factors which influence the interfacial roughness in InGaAs/GaAs heterostructures. Identification of In atoms at the first and second layers in the InGaAs alloys have been achieved. Unlike the result reported by Zheng et al [1], we did not find the tendency for In atoms to correlate along the [001] direction. On the other hand, there appears to be In-In correlation along other crystallographic directions. We further identify three factors which influence the interfacial roughness. The first factor is the surface roughness during the growth. This is a dominant factor for interfacial roughness when both InGaAs and GaAs layers are grown at low temperature (530 C). Growth interrupt up to 120 seconds has limited effect on the interfacial roughness. The second factor is the segregation of In atoms from InGaAs layer into GaAs overlayer. Finally, if the GaAs overlayer on InGaAs is grown at higher temperature (> 600 C) in conjunction with growth interrupt, nearly atomically abrupt interface is produced. In the mean time the apparent InGaAs layer thickness is reduced. We attribute this as due to the In re-evaporation during the temperature ramp before the growth of GaAs layer at higher temperature.[1] J. F. Zheng, J. D. Walker, M.B. Salmeron, and E. R. Weber, Phys. Rev. Lett. 72, 2414 (1994).
10:20 AM		EM-ThM-7 Nanometer-scale Investigations of Alloy Formation in Compound Semiconductor Structures R. Goldman (Carnegie Mellon University); B. Briner (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany); R. Feenstra (Carnegie Mellon University); M. O'Steen, R. Hauenstein (Oklahoma State University) We have investigated alloy formation in compound semiconductor heterostructures using cross-sectional scanning tunneling microscopy and spectroscopy. GaN/GaAs superlattices were produced by exposure of a molecular beam epitaxially grown GaAs surface to a nitrogen flux, without the co-deposition of Ga [1]. Our cross-sectional studies reveal that the nitrided layers are laterally inhomogeneous, consisting of groups of N atoms and larger clusters of GaN. This observed compositional variation indicates that phase segregation has occurred, consistent with the large miscibility gap predicted for the GaAsN system [2]. In the growth direction, the extent of N incorporation is > 20\Ao\. Tunneling spectroscopy on the N atoms reveals a state in the conduction band associated with an acceptor level of N\sub As\ in GaAs. In addition, spectroscopy on the clusters reveals an upward shift of the valence band, possibly due to a high strain in the clusters. Studies of alloy formation in InGaAs-based structures will also be discussed. [1] R.J. Hauenstein, D.A. Collins, X.P. Cai, M.L. O'Steen, and T.C. McGill, Appl. Phys. Lett. 66, 2861 (1995). [2] J. Neugebauer and C.G. Van de Walle, Phys. Rev. B 51, 10568 (1995).
10:40 AM	Invited	EM-ThM-8 Atomic Level Investigation of the Growth of Si/Ge by UHV CVD T. Chiang, T. Miller (University of Illinois, Urbana); D. Lin (National Chiao-Tung University, Taiwan) Si and Ge films can be prepared under UHV conditions by chemical vapor deposition using disilane and digermane as source gases. These gases offer a much higher sticking probability and a lower growth temperature compared to silane and germane. Growth by atomic layer epitaxy can be conveniently carried out. Using synchrotron radiation photoemission spectroscopy and scanning tunneling microscopy, we have examine the heteroepitaxial growth of Ge/Si. The measured core level shifts at surfaces allow us to identify the surface species and surface chemical compositions, and this information can be correlated with the atomic features observed by scanning tunneling microscopy. Issues related to cleavage of precursor molecules, attachment to dangling bonds, diffusion, surface segregation, growth morphology, and pyrolytic reaction pathways will be discussed.
11:20 AM		EM-ThM-10 Ge Growth on Hydrogen Passivated Si(001) and Si(111) Surfaces Y. Khang, J. Lee, Y. Kuk (Seoul National University, Korea) Ge/Si system has attracted special attention because of its importance in fundamental and applicational aspects. It has been widely used as a model system in experimental and theoretical approaches to understand the strained layer epitaxy. Lattice strain from lattice mismatch, surface energy, arrival rate, and surface diffusion are 4 major determining factors for the growth dynamics and kinetics in the Ge/Si system. It has been widely known that less activation energy is needed for overlayer Ge when the substrate Si(001) or Si(111) are passivated. [1] We have studied the growth kinetics of Ge on H passivated Si(001) and Si(111) surfaces by using a scanning tunneling microscope. Ge deposition was made in real time by positioning a gun toward the Si sample while performing an STM scan. By controlling H coverage and Ge arrival rate, we were able to observe various growth modes. In case of a passivated surface, Si step played a much less important role compared to a case of a bare surface. The diffusion anisotropy in Si(001) was less significant and there were many small nucleations < 10x10 \Ao\\super 2\ in whole surface. From these results growth mechanism with and without Si dangling bonds will be discussed. We will also discuss the alloy mechanism of Ge and Si by post annealing with and without H exposure.[1] C.W. Oh, E. Kim, and Y.H. Lee, Phys. Rev. Lett. 76, 776 (1996).
11:40 AM		EM-ThM-11 In Situ Measurements of Temperature-Dependent Strain Relaxation of Ge/Si(111) P. Deelman, L. Schowalter (Rensselaer Polytechnic Institute); T. Thundat (Oak Ridge National Laboratory) We have measured strain relaxation and clustering in Ge films grown by molecular beam epitaxy on Si(111) at substrate temperatures between 720K and 970K in real time with reflection high energy electron diffraction (RHEED). At 720K, we observe an oscillation of the surface lattice constant for the first three bilayers (thicknesses were calibrated by Rutherford backscattering spectrometry), followed by a sharp 2D-3D growth mode transition, when transmission diffraction features appear in RHEED. The surface lattice constant then begins to relax at an initial rate of about 0.5%/BL. The mechanisms of island growth and strain relaxation change with growth temperature. At 770K, the surface lattice constant begins to relax after only 1BL, and at 820K relaxation begins immediately. At both temperatures, however, 3D spots do not appear until after 3BL. The initial rate of strain relaxation decreases with increasing temperature until, at 970K (when 3D spots never appear), it is only 0.04%/BL. This behavior may be explained by a temperature-dependent roughness length scale. At low temperature, atomic force microscope images show the development of small (100nm), faceted islands with aspect ratios (height/width) on the order of 0.07. With increasing temperature, the formation of well-defined facets is inhibited. At 970K, islands grow very large (1\mu\m) from the outset, with aspect ratios less than 0.015. The islands are unable to thicken much, because dislocations can glide in easily at the edges, enabling the islands to grow laterally quickly. The strain in the "new" islands is not substantially less than that in the "old" islands. At 970K, 25% of the Ge/Si misfit strain may be relieved by diffusion.