AVS1997 Session SS+NS-MoA: Nucleation and Growth on Semiconductors

Monday, October 20, 1997 2:00 PM in Room A3/4

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	SS+NS-MoA-1 Direct Measurements of Atomic-Scale Surface Kinetics on Si(001) B.S. Swartzentruber (Sandia National Laboratories) Epitaxial growth takes place through a myriad of competing atomic-scale processes including: deposition; surface diffusion; and eventual incorporation in the substrate lattice via the formation and growth of islands or atom attachment at steps. An understanding of the growth process thus requires an understanding of the kinetic and thermodynamic factors affecting atomic interactions on the surface. We have been studying a few of the fundamental mechanisms underlying the initial stages of MBE growth of Si on Si(001) using STM. We perform quantitative measurements of adsorbed dimer diffusion using a novel atom-tracking technique. The atom tracker employs lateral-positioning feedback to lock the STM probe tip into position above selected atoms. Once locked, the STM tracks subtle changes in the dimer configuration as well as its position as it migrates over the crystal surface. By tracking individual dimers directly the time response of the instrument increases by a factor of ~1000 over conventional STM imaging techniques. This increase enables the measurement of every kinetic event, allowing the statistics of processes to be measured explicitly. Analyzing the diffusion statistics yields thermodynamic quantities such as the activation barrier for diffusion and the interaction free energy of the dimers with various surface defects. Increasing the capability of STM to yield quantitative measurements of surface kinetic processes that were previously inaccessible provides additional insight into the complicated interplay between the processes at work during growth and etching. This work was performed at Sandia National Labs supported under US DOE Contract DE-AC04-94AL85000.
2:40 PM		SS+NS-MoA-3 Prenucleation Structures of Ge Adatoms on Si(100)-2x1¹ X.R. Qin, M.G. Lagally (University of Wisconsin, Madison) Many aspects of homoepitaxial growth on Si(100) and the heteroepitaxial growth of Ge on Si(100) have been thoroughly explored. However, a critical element is still not well understood: the atomistic mechanisms of the "lateral condensation" of adsorbed atoms to form ordered islands of a new layer of the deposited material, in particular, the formation of the stable nucleus and the transition from the stable nucleus to the initial small island. Using scanning tunneling microscopy we show that Ge adatoms adsorbed on Si(100) near room temperature arrange themselves into characteristic structures that appear to be precursors to the formation of stable dimers. These prenucleation structures consist of paired adatoms arranged in adjoining substrate troughs in a chain-like array, with the longitudinal axis tilted from the substrate dimer bond direction. It is argued that the pinned buckling of the substrate dimers induced by Ge adatom adsorption plays an important role in the formation of the structures. In addition we observe diluted ad-dimer rows coexisting with the chain structures. The implications of the existence of these structures for the initial formation of two-dimensional islands will be discussed. Supported by NSF and by Sandia National Labs.
3:00 PM		SS+NS-MoA-4 Growth Kinetic Influences on the Morphology and Photoluminescence of Three-Dimensional Islands Formed by Chemical Vapor Deposition in Si_1-xGe_x/Si Multilayers on Si(001)¹ J.S. Sullivan, D.E. Savage, E. Rehder, S. Nayak, T.F. Kuech, M.G. Lagally (University of Wisconsin, Madison) Strain mediated self-organization -- lateral and vertical correlation of position as well as a narrow size distribution -- of coherently strained three-dimensional (3-D) islands has been demonstrated in molecular beam epitaxy (MBE) growth of Group III/V and IV systems with notable results in SiGe/Si multilayers. We now present similar enhancement of self-organization in Si_1-xGe_x/Si multilayers deposited by ultra-high-vacuum chemical-vapor deposition (CVD). Earlier studies of the effect of temperature and alloy composition showed only a small influence on the morphology of a single, CVD grown SiGe layer. These studies illustrate a difficulty with CVD growth: diffusion rate and deposition rate cannot be controlled independently as both are thermally activated processes. We separately alter the growth rate and diffusion rate by substituting Si₂H₆ and Ge₂H₆ for SiH₄ and GeH₄ respectively. By monitoring 3-D island evolution in real time with a unique in-situ reflection high-energy electron diffraction capacity, we prevent the 3-D islands from progressing beyond the initial stages of formation by careful control of the growth. In multilayers we observe ~4 nm tall 3-D islands with (105) facets which are approximately a factor of five smaller in dimension than earlier CVD-grown 3-D islands and a factor of two smaller than the best MBE-grown 3-D islands. Such small islands enhance the possibility of observing quantum confinement of carriers. The morphology of the final strained layer is probed with ex-situ atomic force microscopy and correlated to low-angle x-ray scattering results for the buried interfaces. Optoelectronic properties are examined with continuous-wave photoluminescence (PL). We will discuss results in the context of MBE SiGe/Si multilayer morphological results by presenting correlation functions and comparing them to earlier MBE results. PL results will be discussed in the context of morphology differences among the Si_1-xGe_x/Si multilayers. ¹This work is supported by the NSF.
3:20 PM		SS+NS-MoA-5 Intrinsic Stress upon Stranski-Krastanov Growth of Ge on Si(001) G. Wedler, J. Walz (Freie Universitaet Berlin, Germany); T. Hesjedal, E. Chilla (Paul Drude Institut Berlin, Germany); R. Koch (Freie Universitaet Berlin, Germany) The growth of Ge on Si(001) has been studied intensively in the last years due to the importance for the fabrication of optoelectronic devices (based e.g. on strained-layer superlattices or uniform quantum-dot arrays) as well as of Si-based high speed transistors in GeSi alloys. It is well established that film growth proceeds by Stranski-Krastanov mode, i.e. 3D islands (e.g. ‘hut’-clusters) nucleate on top of a (6-8 ML thick) pseudomorphic layer. By now, however, it is still discussed controversially, whether the transition from 2D to 3D is driven by the energetics - i.e. the large (!) misfit-stress - or the kinetics of the growth process. Here we present in situ intrinsic stress measurements of Ge/Si(001) at 700 - 1000 K in combination with the structural results of AFM and LEED. From the film stress three stages of film growth characterized by different relief of the misfit strain can be discriminated: (i) the pseudomorphic layer-by-layer stage, (ii) nucleation and growth and (iii) coalescence of 3D islands. The dependence of strain relief on the deposition temperature points to the presence of stress-enhanced Ehrlich-Schwoebel barriers and therefore is in favor of a kinetic path for nucleation and growth of the 3D islands.
3:40 PM		SS+NS-MoA-6 Improved Accuracy in Monitoring Si Monolayer Incorporation in GaAs during MBE L. Daeweritz, P. Schuetzenduebe, M. Reiche, K.H. Ploog (Paul-Drude-Institut fuer Festkoerperelektronik, Germany) A microscopic understanding of the incorporation of Si sheets in GaAs in the submonolayer or monolayer range is important for delta-doping and for tuning of band offsets at heterojunctions by insertion of Si interlayers. We use simultaneous RHEED and RDS measurements to monitor characteristic changes in the short- and long-range order that accompany Si incorporation on GaAs(001) and Si segregation during GaAs overgrowth. A new difference-function approach between RD spectra taken from Si-covered and from bare GaAs(001) surface allows us to detect not only As and Ga dimers but also Si dimers and As dimers on Si. Transients measured at photon energies characteristic for the different dimers provide a detailed picture of the surface kinetics and their dependence on the misorientation and preparation of the initial GaAs(001) surface. The growth mechanism is characterized by the evolution of various (3 x 2) and (4 x 2) structures differing in termination. For Si deposition on singular surfaces with (2 x 4)ß reconstruction the nucleation occurs at randomly distributed sites, due to Si incorporation on unoccupied Ga sites in the trenches of this structure, and the phase transitions occur at higher coverages than for pulsed Si supply on a well prepared vicinal surface with (2 x 4)α reconstructed terraces. The latter leads to well ordered structures. These differences in the incorporation kinetics explain the diverging findings of carrier concentration saturation in delta-doped samples reported in the literature. Si incorporation on unoccupied Ga sites in the trenches of the (2 x 4)ß structure promotes clustering that leads to this saturation. The monitoring of Si segregation during GaAs overgrowth by RHEED relies on its dependence of the kink density in As surface dimer rows on the Si concentration.
4:00 PM		SS+NS-MoA-7 Observation of Nanometer Scale Surface Clustering on ZnSe Epilayers Grown by Molecular Beam Epitaxy on GaAs(001) J.B. Smathers, B.T. Jonker, E.M. Kneedler (Naval Research Laboratory) We have used AFM to investigate the surface morphology ZnSe films deposited on GaAs(001) by MBE. We report the first observation of nanometer scale clusters which gradually increase in size and decrease in number at room temperature. Angle resolved XPS and high resolution SAM data indicate the cluster composition to be ZnSe, with no evidence of either surface contamination or elemental Zn or Se clustering. The clusters comprise less than 3% of the film surface area and posses a narrow size distribution. In between the clusters, these films have rms roughnesses under 4Å, producing a surface morphology strikingly similar to that of the "quantum dot" structures recently reported in semiconductor systems characterized by large strain energies. In such systems, nanometer scale clusters are observed only during the initial stages of heteroepitaxy, and they have been explained in terms of surface and strain energies between the substrate and adlayer. In contrast, there is only a 0.25% lattice mismatch in the ZnSe/GaAs system, and we observe clusters on films grown well beyond the critical thickness (t_c=2000Å). Therefore, these ZnSe clusters can not be explained by either lattice strain or interfacial surface energy considerations. This finding is of immediate interest to the ZnSe optical device community where insufficient laser lifetimes result from defects introduced during epitaxial growth. In addition, it raises the possibility for zero dimensional quantum confinement in ZnSe based devices, and suggest the extension of the theories for such surface cluster formation to include homoepitaxial growth. Supported by ONR.
4:20 PM		SS+NS-MoA-8 First Principles Study of AlN Epitaxial Layers on SiC R. Di Felice, J.E. Northrup (Xerox Palo Alto Research Center) We present a microscopic characterization of the initial stages of the formation of AlN thin films on SiC(0001) and SiC(10-10), based on first principles total energy calculations. For the (0001) orientation, we have taken into account many possible epitaxial structures having between 1 and 4 monolayers of AlN, with a wide range of Al and N stoichiometries in the ideal, adatom and vacancy reconstructions. We find that 2D films can wet the substrate in the first stages of deposition in N-rich conditions. A neutral mixed interface, having a 2x2 periodicity, is more conducive to layer-by-layer growth than a charged non-mixed interface. For the (10-10) orientation, we have studied AlN films of different thickness exhibiting two different kinds of interface structure. The interface structure which minimizes the number of non-octet bonds (Si-N and C-Al) has the lowest formation energy. We find that the balance between interface energy and surface energy difference is not sufficient to characterize the energetics of the films. The interaction between the film surface and the film-substrate interface, which rapidly vanishes as the film thickness increases, is shown to be an important factor affecting the energetics and the growth mode of the epitaxial films. We predict that films which are three bilayers thick will wet the substrate in the first stages of Al and N deposition.