Efthimios Kaxiras, Harvard Univ
Mohan Krishnamurthy, Michigan Technological Univ
Bradford Orr, Univ of Michigan
R. Tromp, IBM T.J. Watson Research Ctr
- Burleigh Instruments, Inc.
- Digital Instruments, Inc.
- IBM Corporation (STSS Unit)
- k-Space Associates, Inc.
- RHK Technology, Inc.
- SVT Associates, Inc.
1997 Spring Exhibitor
In the sessions below, "*" indicates an invited paper.
SESSION B1: STRAINED LAYER GROWTH: MORPHOLOGICAL AND COMPOSITIONAL EVOLUTION
Chairs: Bradford G. Orr and Jerry Tersoff
Monday Morning, March 31, 1997
Golden Gate C3
8:30 AM *B1.1
MECHANISMS OF MORPHOLOGICAL EVOLUTION DURING STRAINED-LAYER EPITAXY, David E Jesson, G. Y. Chen, K. M. Chen, S. J. Pennycook, Oak Ridge National Laboratory, Solid State Div, Oak Ridge, TN; Thomas G. Thundat, R. J. Warmack, Oak Ridge National Laboratory, Health Science Research Div, Oak Ridge, TN.
Strained-layer epitaxy exhibits a rich variety of interesting phenomena reflecting the interplay between surface energy and misfit stress. Utilizing Atomic Force Microscopy and Scanning Transmission Electron Microscopy, we have studied the annealing of SiGe/Si(001) thin films. The experiments provide new insight into the nucleation and subsequent growth of strained facetted islands. We will present a new model for strained facet growth and discuss its relevance to self-limiting growth and kinetic shape instabilities. In the case of thicker films, a new mechanism of stress-driven surface ripple formation is identified which involves the cooperative nucleation of islands and pits . This is distinct from the conventional view of ripple formation as an Asaro-Tiller Grinfeld instability. We will discuss how this new kinetic pathway can be manipulated to facilitate the fabrication of novel nanostructures such as high density quantum dot arrays and provide new insight into the nucleation and multiplication of misfit dislocations . Finally, we will discuss the conditions required for morphological evolution by nucleation or instability.
9:15 AM B1.2
TRANSITION FROM HOMOGENEOUS TO HETEROGENEOUS SURFACE NUCLEATION IN INDIUM GALLIUM ARSENIDE ISLANDS, Rosa Leon, Australian National Univ, Dept of Electronic Matls Engr, Canberra, AUSTRALIA; T. Senden, Yong Kim, Australian National Univ, Research School of Physical Sciences, Canberra, AUSTRAILIA; C. Jagadish, Australian National Univ, Dept of Electronic Matls Engr, Canberra, AUSTRALIA; A. Clark, Australian National Univ, Research School of Physical Sciences, Canberra, AUSTRALIA.
Temperature dependencies of adatom diffusion lengths have been obtained from simultaneous growth of InGaAs islands on different GaAs vicinal substrates. Arrhenius plots show changes in slope that can be understood by a transition from primarily heterogeneous to homogeneous nucleation. Fits to the data can be obtained as the pre-exponential term of homogeneous nucleation overwhelms the exponential term of heterogeneous nucleation. Analogously to nucleation processes In the bulk, where grain boundary nucleation can become dominant, the monolayer steps in vicinal substrates can act as heterogeneous nucleation sites, and depending on growth temperature and surface conditions, strain driven InGaAs island nucleation can be predominantly homogeneous or heterogeneous.
This transition is a key issue in understanding how to achieve the best uniformity, or smaller standard deviation in average dimensions for these strained islands. Detailed statistical analysis of numerous digital images obtained by Scanning Probe Microscopy uncovers a simple general expression that provides a deterministic relationship between miscut angle and island uniformity, for a given island concentration. Optimizaton of size uniformity in these islands, which is of crucial importance in their use as self-organized quantum dots, can thus be understood and obtained reproducibly.
The interplay between temperature activated adatom mobility for group III adatoms and the monolayer steps spacings in various miscut substrates offers varying surface nucleation conditions. It is shown that variations in grown temperature and differences in vicinal orientations can produce wide range mobility of both concentration and average volume for nanometer size InGaAs islands. Results presented here were obtained by vapor phase epitaxy, but the conclusions drawn from this work are generally applicable regardless of growth technique.
9:30 AM B1.3
SELF-ORGANIZATION PHENOMENA IN EPITAXIAL GROWTH OF Ge/Si HETEROSTRUCTURES STUDIED BY TRANSMISSION ELECTRON MICROSCOPY, Oliver Kienzle, Max-Planck-Inst, Inst fur Werkstoffwissenschaft, Stuttgart, GERMANY; Frank Ernst, Max-Planck-Inst, fur Metallforschung, Stuttgart, GERMANY; Manfred Ruhle, Max-Planck-Inst, Inst fur Werkstoffwissenschaft, Stuttgart, GERMANY.
Quantum effect devices based on semiconductor heterostructures have a great potential in microelectronics for devices with low power dissipation and ultra high operating frequency ( 1 GHz). Ge/Si quantum dot structures seem to be the most promising system for realizing such devices, since they can readily be implemented in existing Si-technology. Furthermore, when growing Ge on Si, self-organized formation of Ge-islands starts when the critical layer thickness is exceeded (Stranski-Krastanov growth mode). Well-ordered quantum dot structures can be produced by using self-organization phenomena, controlling the nucleation and the growth of Ge-islands.
To advance the understanding of nucleation and growth processes, we study the early growth stages of epitaxially grown Ge on Si (001) substrates by means of various methods of transmission electron microscopy (TEM). In order to study the morphology of the islands, we analyze cross-sectional specimens using conventional TEM (CTEM). The average height and the average diameter of the islands are determined in dependence of deposition parameters. High resolution TEM (HRTEM) provides insight into bulk and interface morphology, which are important for the electronic and optical properties of the quantum dot structures. The island diameter, density, their two-dimensional spatial distribution, as well as the strain fields they generate are characterized by means of CTEM and electron holography studies of plan-view specimens.
9:45 AM B1.4
MORPHOLOGICAL INSTABILITY AND DEFECT FORMATION IN HETEROEPITAXIAL SiGe THIN FILMS, Cengiz S. Ozkan, William D. Nix, Stanford Univ, Dept of MS&E, Stanford, CA; Huajian Gao, Stanford Univ, Dept of Mech Engr, Stanford, CA.
Heteroepitaxial Si thin films deposited on silicon substrates exhibit surface roughening via surface diffusion under the effect of a. Compressive stress which is caused by a lattice mismatch. In this paper, we investigate and compare the relaxation behavior of capped and uncapped heteroepitaxial Si thin films containing 22 Ge. Films with 50 nm thickness have been deposited on silicon substrates in a LPCVD reactor. For capped films, a capping silicon layer of 150 thickness was deposited on top of Si. Annealing experiments conducted on uncapped films right after deposition in the reactor using a hydrogen ambient resulted in surface roughening in the form of ridges that are aligned along <100> or <110> directions. For capped films, surface roughening was inhibited during annealing in the reactor. X-ray rocking curve measurements were conducted to determine the amount of strain relaxation in both sets of films. Transmission electron microscopy has been used to study and compare the microstructure of these films. In-situ transmission electron microscopy experiments have been conducted on as-grown uncapped films to investigate the dynamics of surface roughening and dislocation formation during annealing. A novel sample preparation procedure for these experiments is also described. Results of these in-situ experiments have shown that the kinetics of surface roughening are quite different in a vacuum ambient.
10:30 AM *B1.5
GROWTH AND ANNEALING OF Rh(111) SURFACES, Roy Clarke, Joanne Wellman, Jun-Hao Xu, Ctirad Uher, Univ of Michigan, Dept of Physics, Ann Arbor, MI; Frank Tsui, Univ of North Carolina, Dept of Physics & Astronomy, Chapel Hill, NC.
We report on the kinetics of growth and annealing of homoepitaxial Rh(111) surfaces. We have observed a morphological transition in the nucleation and growth of this close-packed surface. The transition occurs near 600 K and is characterized by a change in the shape of the surface features from fingered to compact. On both sides of the transition, there lies a regime of persistent layer-by layer growth. The surface morphology is studied using STM and real-time RHEED. If growth is performed at low temperature, we observe non-self-affine 3D multilayered features which on subsequent annealing (at T560K) also exhibit a transition to smooth 2D morphology. The annealing process is a two-step mechanism involving a slow regime limited by adatom detachment from step edges, followed by a fast regime made possible by the formation of a network of ''chain-like'' structures which provide new pathways for surface diffusion.
11:15 AM B1.6
SELF-REARRANGEMENT OF GOLD (100) SURFACES AFTER CONTROLLED DEFECT CREATION, Juan de la Figuera, Ruben Garcia-Martinez, Miguel A. Gonzalez, Rojo M. Juan, Univ Complutense de Madrid, Dept of Materials, Madrid, SPAIN.
We report a STM study of the self-rearrangement of well-characterized Au(100) surfaces after different types of defect generating procedures like argon ion bombardment and scratching along specific directions with the STM tungsten tip. The evolution of the resultant defect microstructures when gold adatoms are further deposited from an evaporation gun has also been investigated. Bombardments have been carried out with both low energy ions (hundreds of eV), leading to independent point defect creation and subsequent clustering and, high energy (5 keV) ions. The stability of the different defects has also been studied following high temperature annealings. The evolution of these defect structures gives valuable insight into the processes underlying homoepitaxial growth.
11:30 AM B1.7
EPITAXIAL STRESS-INDUCED ORDERING AND STRUCTURE MODIFICATION, Ernst G. Bauer, Thomas Duden, Arizona State Univ, Dept of Physics & Astronomy, Tempe, AZ; E. Hueger, H. Wormeester, Technische Univ Clausthal, Physikalisches Inst, Clausthal, GERMANY.
Epitaxial stress causes domain ordering and stabilizes nonequilibrium modifications of crystals. We have studied some prototype systems for the two phenomena, submonolayer Au films on W(110) and thicker Cu and Co films on W(100) with LEEM, LEED, RHEED, XPD, AES, and UPS. Au submonolayers form a striped phase whose periodicity depends strongly on temperature due to the different expansion coefficients of film and substrate, possibly also due to the temperature dependence of the elastic constants.
Cu and Co grow beyond their initial pseudomorphic layer on W(100) in the hexagonal close-packed modification with the (110) plane parallel to the substrate. The two equivalent azimuthal orientations [0001 // ,  form long narrow ribbons which are difficult to detect by LEED and other normal incidence laterally averaging surface analytical tools, but are clearly seen in LEEM and RHEED. The unusual shape of these crystals is also a consequence of stress.
11:45 AM B1.8
LATERAL VARIATIONS IN SURFACE MORPHOLOGY AND ALLOY COMPOSITION IN InGaAsP-BASED STRUCTURES, Rachel S. Goldman, Univ of Michigan, Dept of MS&E, Ann Arbor, MI; Randall M. Feenstra, Carnegie Mellon Univ, Dept of Physics, Pittsburgh, PA; Christofer Silfvenius, Bjorn Stalnacke, Gunnar Landgren, Royal Inst of Technology, Dept of Electronics, Kista, SWEDEN.
Multiple quantum well structures with alternating compressive- and tensile-strained layers are promising for various optoelectronic device applications. Although these structures are intended to be strain balanced, residual-strain related lateral variations in surface morphology and/or alloy composition often occur. Furthermore, the interplay between the morphological variations and alloy decomposition is not well understood. We have investigated these phenomena in a series of 8- and 16-period InGaAsP/InGaP superlattices, grown with and without InP layers in the InGaP barrier. Using cross-sectional scanning tunneling microscopy and spectroscopy, we observe lateral variations in bandgap, due to alloy phase separations in the ternary and quaternary alloys of the superlattice. When the number of Superlattice periods is increased from 8 to 16, the surface develops large undulations involving the top 3-4 superlattices. For structures with InP layers in the InGaP barrier, the effect is drastically reduced, due to the lack of alloy decomposition in the InP interlayers. In this case, only slight undulations occur in the top superlattice. Together, these results suggest that the morphological variations are induced by lateral separation of the tertiary alloy composition occurring during growth.
SESSION B2: DEFECT FORMATION, STRAIN RELAXATION AND GROWTH ISSUES
Chairs: David E Jesson and Efthimios Kaxiras
Monday Afternoon, March 31, 1997
Golden Gate C3
1:30 PM *B2.1
DEMONSTRATION OF A III-V SUBSTRATE WITH A STRETCHABLE LATTICE FOR DEFECT-FREE HETEROEPITAXIAL GROWTH, Felix E. Ejeckam, Y. H. Lo, Cornell Univ, Dept of Electrical Engr, Ithaca, NY; H. Q. Hou, B. E. Hammons, Sandia National Laboratories, Dept of Semiconductor Materials, Albuquerque, NM.
Employing an innovative technology, namely lattice engineering, we demonstrate that a highly effective lattice-compliant substrate can be formed from a conventional GaAs, InP, or Si substrate. To engineer the surface's lattice on which heteroepitaxial films are to be grown, we wafer-bonded a thin layer of GaAs (30 to 100 ) to a GaAs substrate with a high-angle between their respective crystal axes. A planar cross-grid of screw dislocations formed in the twist-boundary that alleviated stress in the 100 film. These screw dislocations consist of very stretched lattice bonds. This in turn allows the film to stretch or contract to whatever mismatched lattice is grown on its surface. By increasing the relative angle between the bonded film and its substrate, the spacing between screw dislocations is reduced and eventually the screw dislocations overlap, yielding a surface lattice structure that is mostly flexible. If an analogy to macroscopic structures is to be drawn, the few inflexible atoms can be visualized as anchors, and the rest of the 100 film as flexible membranes that buckle up or down (over atomic distances) depending on the sense of the strain. We grew a layer of 3000 InP (1 strain) on a 100 twist-bonded GaAs film and TEM images show neither threading dislocations nor stacking faults in the InP film. In sharp contrast, the InP layer grown on a conventional GaAs substrate had many dislocations and stacking faults since the film's thickness is 30 times above the Matthew-Blakeslee critical thickness. Similar promising results have also been observed in other heteroepitaxial materials of much greater lattice-mismatches and will be reported in the conference.
2:15 PM B2.2
PARAMORPHIC GROWTH: A NEW APPROACH IN LATTICE MISMATCHED HETEROEPITAXY TO PREPARE IDEALLY RELAXED MATERIALS, Guy R. Hollinger, Jean Francois Damlencourt, Michel Gendry, Jean-Louis Leclercq, Michel Garrigues, Ecole Centrale Lyon, Dept of Electronics, Ecully, FRANCE.
Lattice mismatched epitaxial growth of semiconductor materials offers extra degree of freedom in design of electronic and optoelectronic devices. The usual approach is to grow plastically relaxed graded buffers (metamorphic growth) with a minimum of threading dislocations. Recently, the use of compliant substrates has also been tried. We propose a new approach for preparation of ideally relaxed thick layers in mismatched heteroepitaxy. The ''paramorphic'' process is based on the use of a thin seed membrane which has the same lattice parameter as the thick mismatched layer to grow. To prepare the seed layer, a pseudomorphically strained layer is first separated by chemical etching from its original substrate and subsequently deposited on the final substrate after having been.
The validity of this approach was demonstrated by growing fully relaxed InGaAs layers on an InP substrate, using molecular beam epitaxy. First, conventional lithography and chemical etching techniques are used to define ''trampoline''-shaped 80 x 80 m relaxed InGaAs platforms. The starting structure was based on a 40 nm pseudomorphic strained InGaAs layer and a 50 nm InAlAs sacrificial layer grown on an InP substrate. The platforms are connected to the rest of the structure by 4 thin arms. Second, a 250 nm thick layer of InGaAs was grown simultaneously on the platforms and on the unprocessed wafer. Relaxation in InGaAs has been measured by spatially resolved photoluminescence at 77 K. The layer grown on the unprocessed substrate shows a peak at 1792 nm which indicates that the layer is only partially relaxed, as expected (the measured value for a fully strained layer is 1752 nm). In contrast, the 1850 nm transition observed for the platform is close to what is theoretically predicted for fully relaxed InGaAs, which demonstrates the validity of the concept.
2:30 PM B2.3
STRAIN RELAXATION AND GROWTH MECHANISM OF HETEROEPITAXIAL GaN LAYERS ON SAPPHIRE SUBSTRATES - TRANSMISSION ELECTRON MICROSCOPY AND FINITE ELEMENT CALCULATIONS, Martin Albrecht, Univ Erlangen-Nurnberg, Dept of Mikrocharakterisierung, Erlangen, GERMANY.
The growth of misfitted heteroepitaxial layers is controlled by the arising misfit strain and the counteracting relaxation processes. These processes were exclusively studied in cubic alloy and compound semiconductors (e.g., Si-Ge and III/Vs) and are fairly well understood. Elastic relaxation due to surface undulation formation or island growth, and plastic relaxation due to dislocation formation depend on each other and change locally the chemical potantial of the growing surface with corresponding influence on the local growth process. The elaborated principles can be applied to analyze the growth of GaN; however, in the case of hexagonal GaN (wurzite structure), additional aspects enter due to a crystal symmetry lower than cubic. We investigate the initial stages of hexagonal GaN films grown by gas source molecular beam epitaxy onto (0001) sapphire substrate. Conventional and high resolution transmission electron microscopy of the grown layers shows that, under certain growth conditions, nanosized three-dimensional inclusions reside at the interface. These misfit "grainlets" have all the same epitaxial relationship differing from that of the surrounding matrix. They are characterized by a tensile misfit strain opposite to the compressional strain in the matrix. Three-dimensional finite element calculations are performed to discuss energetics and kinetics of the initial growth stages and the associated dislocation population. This particular type of growth made can be tailored to produce preferentially misfit dislocations that may be tolerable with respect to the luminescence properties of the grown material.
2:45 PM B2.4
THREADING DISLOCATION DENSITY REDUCTION IN HETEROEPITAXIAL LAYERS, J. S. Speck, Univ of California-S Barbara, Dept of Materials, Santa Barbara, CA; Glenn E. Beltz, Univ of California-S Barbara, Dept of Mechanical & Environmental Engr, Santa Barbara, CA; A. E. Romanov, A.F. Ioffe Phys-Technical Inst, St Petersburg, RUSSIA; W. Pompe, Technische Univ Dresden, Dresden, GERMANY.
This report addresses the evolution of threading dislocation (TD) density during large mismatch heteroepitaxial film growth. The proposed concept for TD reduction is based upon spontaneous pair reactions between TDs resulting from their lateral motion in the film. Reactions between TDs include annihilation, fusion (in which two TDs combine to form one resultant TD), and scattering (whereby TDs change their trajectory of motion). Each of these reactions become favorable when TDs in the pair are separated by less than an appropriate interaction distance. Sources of motions for TDs include lateral motion of an inclined TD due to changing film thickness, TD glide motion due to relaxation of misfit strain by misfit dislocation (MD) formation, and TD climb due to condensation of nonequilibrium point defects. This framework allows for the treatment of TD density changes for prescribed film orientation and crystallography. For homogeneous buffer layers, the theory predicts dependence of TD density for large , and also relates possible saturation behavior with long range fluctuations in the net Burgers vector content of the local TD densities. For strained layers, the approach accounts for the change of strain in the film resulting from MD formation and immobilization of TDs due to blocking by MDs. The effect of selective area growth on TD reduction is explained in the framework of a reaction kinetics approach and supported by computer simulation using a ''dislocation dynamics'' technique. Finally, the enhanced influence of graded layers on TD reduction is associated with unblocking of TDs from MDs in a varying stress field.
3:30 PM *B2.5
A SPM VIEW OF ENHANCED LAYER-BY-LAYER GROWTH OF GAAS(100):SN *, Amir M. Dabiran, Sean M. Seutter, Philip I. Cohen, Univ of Minnesota, Dept of Electrical Engr, Minneapolis, MN.
We have investigated the surface morphology of GaAs(100) during growth by molecular beam epitaxy in the presence of submonolayer coverages of Sn. Submonolayers of Sn segregate to the surface during growth and cause the measured RHEED intensity oscillations to continue long past (roughly twice as long as) where they are observed in the absence of Sn. Atomic force microscopy (AFM) in air and scanning tunneling microscopy (STM) in ultrahigh vacuum have been used to examine 1) surfaces which were annealed after the growth at a temperature of about 600 C, and 2) samples which were rapidly quenched upon the termination of growth, from 600 to 400 C, and then slowly cooled to room temperature. At coverages of less than 0.3 Sn atoms per conventional GaAs(100) surface unit cell, the major effects of the Sn on annealed surfaces are 1) the increased thermal stability of the surface against preferential desorption of arsenic and 2) the appearance of uniformly spaced (about 30 nm) and sized (about 20 nm) islands . The results of rapidly quenched samples indicate that during growth in the presence of Sn, 1) the usual growth anisotropy of GaAs(100) is removed and 2) GaAs growth proceeds by the formation of closely spaced, small islands which coalesce to form complete layers. This last point explains the observed enhancement of layer-by-layer growth, since nucleation is difficult on small islands until they coalesce and form large terraces. An atomic model will be presented to explain the formation of small islands during growth in terms of an island size limitation due to strain build up as Sn is temporarily incorporated by substituting for Ga. The thermal stability of GaAs:Sn surfaces is explained by the presence of a surface interstitial site for Sn. In this model, each interstitial Sn bonds with 4 arsenic atom on the top layer, hindering the preferential desorption of arsenic at elevated temperatures. * Partially supported by the NSF grant DMR- 9307852.
4:15 PM B2.6
ASYMMETRIC STEP ATTACHMENT KINETICS IN STEP FLOW GROWTH, Michael S. Altman, W. F. Chung, Hong Kong Univ Sci & Tech, Dept of Physics, Kowloon, HONG KONG.
Step flow is one of the basic mechanisms of layer-by-layer crystal growth. As an alternative to two-dimensional nucleation and growth, step flow is finding unique application in the fabrication of novel nanostructures. However, step flow motion may become unstable due to a number of driving mechanisms. A step bunching instability in step flow growth of the Si(111) (7x7) surface has been observed with low energy electron microscopy. This instability is shown to be driven by asymmetric step attachment kinetics where step motion results predominantly from attachment of adatoms from the terrace trailing an advancing step. Asymmetric step attachment kinetics have been determined directly by measurement of step flow velocities as a function of unequal leading and trailing terrace widths. These results are corroborated by measurements of the island nucleation position at the critical terrace width for step flow. This nucleation position is a probe of the steday-state, non-equilibrium adatom concentration during step flow, which is very sensitive to the step attachment asymmetry. Atributing the step attachment asymmetry entirely to a step edge diffusion barrier yields an effective barrier of -15 meV, although asymmetry of the step edge diffusion prefactor cannot be ruled out. The impact of surfactants upon the step attachment asymmetry and step passivation will also be discussed.
4:30 PM B2.7
EVALUATION OF LOW-TEMPERATURE HOMOEPITAXIAL GROWTH ON (111) Si USING CONTACTLESS MINORITY CARRIER LIFETIME MEASUREMENTS , Paul G. Evans, Harvard Univ, Engr & Applied Science Div, Cambridge, MA; Stephen A. McDonald, Rowland Inst for Science, Cambridge, MA; John C. Chervinsky, Jene A. Golovchenko, Harvard Univ, Engr & Applied Science Div, Cambridge, MA; Frans Spaepen, Harvard Univ, Engr & Applied Sciences Div, Cambridge, MA.
Recent channeling and TEM studies indicated high quality homoepitaxial crystal growth on Si (111) via monolayer lead and gold mediating layers. The electronic quality of the grown crystal and the amount of the impurity included in the grown layer have remained open questions. In previous work, the lifetime of excess minority carriers in Si crystals has proven to be a sensitive probe of crystal quality. The application of ex-situ contactless minority carrier lifetime measurements to systems grown by mediating layers will be discussed. The possibilities for and importance of in-situ measurement will be explored as well.
4:45 PM B2.8
MULTI-WAFER GROWTH AND NONDESTRUCTIVE CHARACTERIZATION OF III-V EPITAXIAL THIN FILMS GROWN BY LOW PRESSURE METALORGANIC CHEMICAL VAPOR DEPOSITION, Doug Collins, Zhe Chuan Feng, A. Thompson, Richard A. Stall, EMCORE Corp, Somerset, NJ.
Multiwafer epitaxial growth of III-V compound semiconductor materials and structures with high quality and high uniformity is in great demand for the development of electronic and optoelectronic devices. In this study, we demonstrate the low pressure metalorganic chemical vapor deposition (MOCVD), employing a vertical configuration reactor with a high speed rotating disks for the multiple wafer (50-100 mm diameter) growth of various III-V materials, including AlGaAs, InGaP, InGaAlP, InGaAsP and InSb. They are characterized by a series of nondestructive and whole wafer characterization techniques, such as x-ray diffraction (XRD), sheet resistivity (SR), photoluminescence (PL) and other optical spectroscopy. We illustrate a series of mapping distributions of the film thickness, sheet resistivity, surface morphology, XRD peaks and PL spectra within a run and run-to-run. These data show that the grown materials are of high crystalline quality and uniformity. Uniformities of our epitaxial film thickness, sheet resistivity, major PL band peak wavelength, intensity, and peak widths are in the range 1-5. These wafer scale material characterizations were tightly coupled with the epitaxial growth processes for the optimization of growth and processing parameters, and have helped greatly to improve the quality and uniformity of epitaxial films in large scale production.
SESSION B3: SURVEY OF COMMON THEMES IN EPITAXIAL GROWTH - I
Chairs: Eric Ganz and R. M. Tromp
Tuesday Morning, April 1, 1997
Golden Gate C3
8:30 AM *B3.1
DYNAMICAL STM STUDIES OF THE GROWTH OF Si AND Ge ON SILICON BY MBE, Bert Voigtlaender, Forschungszentrum Julich, Inst fuer Grenzflachenforschung & Vakuumphysik, Juelich, GERMANY.
A high temperature scanning tunneling microscope (STM) capable of imaging during MBE-growth is described. We studied the epitaxial growth of Germanium and Silicon at 600-900 K sample temperature ''in vivo''. This technique gives access to the dynamics of the growth process on an atomic scale. The potential of the method is demonstrated by the following results: The layer-by-layer growth of the two-dimensional Stranski-Krastanov layer of Ge on Si(111) and the formation of three dimensional islands during further growth of Ge was observed. An inversion of the aspect ratio of the islands with increasing coverage indicates a transition from coherent to dislocated islands.
In Si(111) homoepitaxy growth was observed along stripes of the width of a (7 x 7) unit cell. Upon coalescence of islands new growth facets with different growth speeds are observed. Nucleation of next layer growth on Si(111) occurs at domain boundaries of the (7 x 7) reconstruction. In Si/Si(100) homoepitaxy the fractional coverage of the nonequivalent terraces was studied as function of coverage and a theoretically predicted transient growth mode was observed. Some of the results will be presented on videotape. This method (MBSTM) opens the possibility to follow MBE growth processes dynamically on a nanometer scale and gives access to the evolution of specific features during growth.
9:15 AM *B3.2
A STUDY OF THE THERMODYNAMICS AND THE KINETICS OF SMALL ADSORBATE ISLANDS BY KINETIC MONTE CARLO SIMULATIONS, Horia Metiu, Univ of California-S Barbara, Dept of Chemistry/Materials, Santa Barbara, CA.
The invention of STM allows us to perform kinetic measurements on one small cluster of adsorbed atoms (an island) in thermodynamic equilibrium. The first part of the talk is concerned with the following question: how much thermodynamic information can be obtained from such kinetic measurements. We use kinetic Monte Carlo simulations to discuss island shape and its fluctuations (caused by atom motion around the island and by evaporation-condensation events), surface tension, the equilibrium of an island with a gas of atoms on the surface, the island diffusion and the evaporation rate of atoms from an island onto the substrate. We show how these purely kinetic measurements can be used to determine thermodynamic quatities. We propose a new method of simulation that can be used to study coarsening in an ensemble of islands and present a new scaling law for island density variation due to island coalescence.
10:30 AM *B3.3
SELF-ORGANIZATION OF 3-D QUANTUM DOT SUPERLATTICES COLLOIDAL CRYSTALS: BUILDING WITH ARTIFICIAL ATOMS, Christopher B. Murray, IBM T.J. Watson Research Ctr, Yorktown Heights, NY; C. R. Kagan, MIT, Cambridge, MA; Moungi G. Bawendi, MIT, Dept of Chemistry, Cambridge, MA.
The self-organization of monodisperse CdSe quantum dots (QDs) into 3D superlattices (colloidal crystals) and randomly close packed colloidal glasses is demonstrated. The individual QDs are slightly prolate fragments of the wurtzite CdSe lattice which resemble ' 'artificial atoms'', their discrete, electronic energy levels can be precisely tuned as the QD size is adjusted from 17 - 150 in steps of less than one atomic plane. The inter-dot spacing is adjustable from intimate contact up to 17 by modifying nature of the organic capping shell surrounding each QD. The colloidal crystals are a regular face center cubic array of QDs in which the internal wurtzite c axes of the individual QDs has preferentially aligned with a single <111> axis of the superlattice producing highly anisotropic mechanical and optical properties, in contrast the close packed glasses display completely isotropic properties. The evolution of new collective electronic phenomena can now be systematically studied in a model system which permits the size, spacing, orientation of constituent QDs to be precisely and independently varied. Comparisons of 3D superlattices and randomly close packed glasses may help differentiate the roles of periodicity and proximity in the development of these new cooperative phenomena.
11:15 AM *B3.4
INSTABILITY STUDY OF STRAINED THIN FILMS AND COHERENT PARTICLES VIA A DISCRETE ATOM METHOD, Jong K. Lee, Michigan Technological Univ, Dept of Metallurgical & Matls Engr, Houghton, MI.
The morphological instability of epitaxially strained thin films and coherent particles is studied by a discrete atom method, which is predicated upon both classical elasticity and statistical mechanics. Under a dislocation-free, plane strain condition, the results show that if the misfit strain is over a critical value, surface undulations in a thin film create growing waves, whose troughs become cracktips. The cracktips advance to the original film-substrate interface, converting a film into islands. Stress analyses reveal that both the island and substrate phases consist of the regions of a compressive as well as a tensional stress state in order to accommodate the strain energy. A three-layer lamellar structure, made of soft-hard-soft phases, is found to be intrinsically unstable if the misfit strain exceeds a critical value. On the other hand, a hard-soft-hard lamellar structure is stable against surface perturbations. In relation to the morphological instability of quantum dots, the shape change of coherent precipitates is also discussed. As in a thin film case, shape evolution proceeds through dynamic activities of coherency induced interfacial waves, which often cause particle splitting. The transition from a coherent to a semicoherent interface begins with the nucleation of misfit dislocations at stress concentrations, and soft precipitates tend to have an equilibrium morphology of low symmetry, whereas hard particles take on a shape of high symmetry.
SESSION B4: SURVEY OF COMMON THEMES IN EPITAXIAL GROWTH - II
Chairs: Roy Clarke and Horia Metiu
Tuesday Afternoon, April 1, 1997
Golden Gate C3
1:30 PM *B4.1
EPITAXIAL GROWTH OF COMPOUNDS AND ALLOYS, J. Tersoff, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.
Most models of epitaxial growth have focused on one-component systems. However, new and important issues arise in growth of compounds or alloys. For alloys, the possibility of spinodal decomposition during growth has long been a concern, but remains poorly understood. It has recently been recognized that strain can also lead to alloy decomposition. I will discuss how these effects are manifested during classic step-flow growth. For compound semiconductors such as GaAs, it is usual to focus on the role of the non-volatile species (Ga); the interplay of the two components remains poorly understood. I will discuss how the As supply controls the Ga chemical potential. This in turn determines the adatom density, diffusion, and general growth quality. Finally, I will outline how these effects can lead to pattern formation during growth, with possible applications for nanofabrication. Part of this work was done in collaboration with Mark D Johnson and Bradford G Orr.al composition.
2:15 PM *B4.2
ATOMIC FORCE MICROSCOPY STUDIES OF SOLUTION CRYSTAL GROWTH MORPHOLOGY AND DYNAMICS, James J. DeYoreo, T. A. Land, J. D. Lee, Lawrence Livermore National Laboratory, Livermore, CA; A. J. Malkin, YU. G. Kuznetsov, A. McPherson, Univ of California-Riverside, Riverside, CA.
The atomic force microscope is well suited for investigating the growth of single crystal surfaces in solutions at the 10 nm to 100 m length scale both in real time and ex-situ. In this talk I will present the results from studies on two systems: the simple ionic salt, KH (KDP) with a molecular size of 3-5 , and the storage protein canavalin with a molecular size of 40-80 . At supersaturations of 0.05 0.3, KDP growth typically occurs by step flow on anisotropic hillocks formed by dislocation sources which generate hollow cores due to strain effects. The terrace width on the growth hillocks is nearly independent of both the Burgers vector of the dislocation source, b, and /kT, in contradiction to the simple BCF model. A model which takes into account the presence of the cores predicts terrace widths in agreement with the measured values. The results provide an explanation for the reproducibility of macroscopic growth rates and lead to a value for the activation energy for elementary step motion of 0.33 eV. At low supersaturation, canavalin growth also proceeds by step flow on complex dislocation hillocks but, in contrast to KDP, agrees well with BCF predictions of morphology. At high supersaturation, dislocation controlled growth gives way to 2D nucleation of islands. The magnitude of the step current and the time dependence of step-pair decay is consistent with a surface diffusion model of growth coupled with a Ehrlich-Schwoebel barrier at the step edge. From the dependence of 2D nucleation on terrace width, we estimate the canavalin diffusion length to be 1-10 m. Analysis of the step dynamics within the Gilmer-Ghez-Cabrerra formalism leads to estimates for the surface diffusion length, activation energy for adsorption and for incorporation of 1 um, 0.1 eV and 0.2 eV, respectively. From the dependence of critical step length and terrace width on /kT we calculate the step edge free energy to be 0.8 eV.
3:30 PM *B4.3
COMPUTER SIMULATION OF HOMOGENEOUS CRYSTAL NUCLEATION, Daan Frenkel, FOM Inst, Amsterdam, NETHERLANDS.
Molecular dynamics simulations can be used to study events that happen on a time scale of nanoseconds or less. Homogeneous crystal nucleation typically happens on a much longer time scale (order milliseconds or more). Still, there are numerical techniques that can be used to study nucleation. In this way, we can estimate the rate of homogeneous nucleation but, more importantly, we can ''see'' the structure of the critical nucleus-and what we saw was not what we expected! Apart from the question HOW crystals nucleate, we should also like to understand WHY or, more precisely, why some materials (e.g., many proteins) are so hard to crystallize. Of course, there is no single answer to this question. However, computer simulations suggest some ways to control crystallization.
4:15 PM *B4.4
A NOVEL BI-DIRECTIONAL STEP-FLOW GROWTH MODE IN MOLECULAR EPITAXY, Dongmin Chen, Rowland Inst for Science, STM Group, Cambridge, MA.
A novel epitaxial step-flow growth mechanism is revealed in the growth of C60 overlayers on Ge(100) and GaAs(110) surfaces by scanning tunneling microscopy. In contrast to the conventional step-flow growth mode, molecules attached to a substrate atomic step protrude well above its top edge and hence create an inverted step. This new step in turn acts as a nucleation site for subsequent growth over the upper terrace, and flows in the direction of ascending steps, in conjunction with the step flow on the lower terrace in the direction of descending steps. This bidirectional step-flow growth mode is a direct consequence of a large out-of-plane lattice mismatch, and thus should exist in many heteroepitaxial systems.
SESSION B5: POSTER SESSION:
EPITAXIAL GROWTH OF SEMICONDUCTORS: FUNDAMENTALS AND APPLIED ASPECTS
Chairs: Efthimios Kaxiras, Mohan Krishnamurthy, Bradford G. Orr and R. M. Tromp
Tuesday Evening, April 1, 1997
REAL TIME MEASUREMENTS OF STRESS RELAXATION BY SURFACE RIPPLE FORMATION DURING SiGe MBE GROWTH , Jerrold A. Floro, Sandia National Laboratories, Org 1112, Albuquerque, NM; Robert Q. Hwang, Sandia National Laboratories, Livermore, CA; R. D. Twesten, Sandia National Laboratories, Dept of Semiconductor & Nanostructure Physics, Albuquerque, NM; L. B. Freund, Brown Univ, Engineering Div, Providence, RI; Eric H. Chason, Stephen R. Lee, Sandia National Laboratories, Albuquerque, NM.
Sensitive, real-time measurements of wafer curvature during Si/Si (001) epitaxial growth are used to determine the time dependent stress evolution. We have measured the reduction in the total film stress resulting from the formation of surface ripples and/or islands during elevated temperature Si deposition. The dynamic stress data is correlated with simultaneously acquired reflection high energy electron diffraction (RHEED) patterns, as well as ex-situ atomic force microscopy (AFM) and transmission electron microscopy (TEM). Finite element calculations relating the stress change to the observed morphologies compare well with the measured relaxation. The morphological instability ''critical'' thickness can be clearly identified by its stress signature. We will examine the dependence of the critical thickness on growth rate and temperature. After a relatively short transient period above the critical roughening thickness, we find that the total film stress becomes constant during further growth. At high growth temperatures, a second sharp transition in the stress behavior is observed in some films. The implications of this surprising behavior will be discussed.
MORPHOLOGICAL EVOLUTION AND PATTERN FORMATION IN SiGe STRAINED, EPITAXIAL LAYERS ON Si, James D. Weil, Mohan Krishnamurthy, Michigan Technological Univ, Dept of M&ME, Houghton, MI.
We report on the microstructural evolution of SiGe strained layers grown on Si(100), by MBE. The Ge content of the alloys ranged from 10-50 and growth temperatures ranged from 550 725C. Characterization was based on RHEED, TEM and AFM. At lower temperatures, densely-packed, elongated, faceted islands (with facet angles of 10 degrees), oriented along <100> directions are seen at the initial stages of growth. With continued deposition, these islands eventually dislocate and the facet planes become dominant. At higher growth temperatures, a bimodal distribution consisting of squarish islands 500 nm on the side, as well as smaller, separated islands 50 nm on the side are observed. Continued deposition show that the larger islands are formed from the coarsening of smaller islands. Growth mechanisms to explain the microstructural transitions from ripple-like morphology to islanded structure will be presented.
GROWTH OF STRAIN-RELAXED PURE Ge FILMS ON Si (001), Akira Sakai, NEC Corporation, Fundamental Res Lab, Ibaraki, JAPAN; Toru Tatsumi, NEC Corporation, Microelectronics Res Lab, Ibaraki, JAPAN; Nobuyuki Ikarashi, Keiko Aoyama, NEC Corporation, Microelectronics Res Labs, Ibaraki, JAPAN.
In lattice-mismatched hetero epitaxial growth, strained films principally relax by two mechanisms: introduction of misfit dislocations and surface roughening of the film. From a technological point of view, the defect and the rough surface are detrimental and thus the growth of the film with low threading dislocation density and the flat surface profile is essential.
We have successfully grown pure Ge films on Si(001) surfaces which were completely strain-relaxed and had flat surfaces , such as compositionally graded layers and highly defective layers, by ultrahigh-vacuum chemical vapor deposition. The procedure consists of the layer-by-layer Ge growth by hydrogen surfactant mediated growth and the post-growth annealing at comparatively higher temperature (600C) for strain relaxation. The key is the formation of a thin (less than 1 nm) capping Si or SiGe layer on the layered Ge film before the annealing. It effectively suppresses islanding of Ge during the annealing even at high temperature. Cross-sectional and plan-view transmission electron microscopy for the annealed sample having a 20 nm thick Ge film clearly revealed the network structure of full edge dislocations with Burgers vector of the a/  type with a spacing of 10 nm at the Ge/Si interface. This dislocation network located in the plane of the interface led to complete strain relaxation of the Ge film which was also confirmed by x-ray diffraction measurement.
SELECTIVITY DEPENDENCE ON Ge COMPOSITION IN SiGe LOW-TEMPERATURE EPITAXIAL GROWTH, Katsuya Oda, Yukihiro Kiyota, Hitachi Ltd, Tokyo, JAPAN.
SiGe low-temperature selective epitaxial growth has been achieved with cold-wall inductively heated UHV/CVD system using high-pressure H pre-cleaning of a substrate. The contaminants such as oxygen and carbon at the interface between epitaxial layer and substrate were successfully reduced to levels less than the detection limit of SIMS by increasing the H partial pressure during pre-cleaning. Selective growth was performed using SiH, GeH, and BH at 575C. The selectivity of SiGe increased with the Ge composition not only on SiO and SiN but also on poly-Si. The growth rates of SiGe layer on SiN, poly-Si, and Si substrate were 2.0, 4.4, and 10.5 nm/min, respectively. The growth rate on SiO could not be determined due to the island growth caused by high selectivity. The selectivity is originated from the reducing reaction on SiO and less number of active sites on SiN than that on Si substrate. The selectivity on poly-Si seems to be influenced by the difference of growth rates on various crystal orientations, which would be enhanced by Ge composition in the SiGe layer.
SOLID-PHASE EPITAXY OF Si ON HYDROGEN-ADSORBED Si(001) SURFACES, Masataka Hasegawa, Naoto Kobayashi, Yasunori Tanaka, Electrotechnical Laboratory, Quantum Radiation Div, Ibaraki, JAPAN.
Solid Phase Epitaxy (SPE) of Si has become an important technique to form such as the silicon on insulator structure and the delta doping layer. The SPE of Si requires clean substrate surface and sufficient levels of vacuum during the deposition. Hydrogen termination of Si surface has been investigated as a method to protect the surface from impurities. In this study we have studied the SPE of Si on hydrogen-adsorbed Si(001) surfaces in-situ by the low-energy time-of-flight Rutherford backscattering-ion channeling spectrometry with the use of 25 keV hydrogen ions, and by the reflection high energy electron diffraction. The hydrogen-adsorbed Si(001) surfaces were prepared by HF treatment, or by exposing the clean surface to atomic hydrogen in vacuum. The amount of the adsorbed hydrogen was controlled by the substrate temperature during the exposure. Amorphous Si layers of several tens monolayers (ML) were deposited on the hydrogen-adsorbed surfaces by the electron beam evaporator which adopts a rod evaporant. The pressure during the deposition was below 7x10 Torr. The SPE was caused by a direct current heating. In the case of the clean surface complete SPE has occurred by 600C heating, and the 2x1 reconstructed surface has appeared. In the case of 2 ML adsorption of hydrogen, the 600C heating did not lead to the SPE at all. Although the SPE has occurred on this surface by 700C heating, a polycrystalline layer was formed at the surface. In the case of 1 ML, and less than 1 ML adsorption of hydrogen, we obtained the same results as in the case of 2 ML. It was found that even if the amount of hydrogen adsorption is less than 1 ML, the hydrogen adsorption will be the obstruction for the SPE of Si on Si(001).
INTENSIVE VISIBLE PHOTOLUMINESCENCE STUDY FROM ANODIZED SiGe LAYER GROWN ON Si BY MOLECULAR BEAM EPITAXY, B. Unal, S. C. Bayliss, DeMontfort Univ, School of Applied Sciences, Leicester, UNITED KINGDOM.
Strong visible photoluminescence (PL) from Molecular Beam Epitaxial B doped porous SiGe grown on p-type Si wafer has been investigated in some aspects. The SiGe porous layer with a starting thickness of 35 nm was formed by an electrochemical etching process with a range of preparation conditions. A significant shift of the emission energy of porous SiGe grown on Si has been observed for various anodization conditions and for the temperature range from 295 K to 78 K. The PL emission energy has been found to remain almost unchanged on varying excitation energy, and to increase linearly with reciprocal temperature. The position of the PL emission was, however, observed to be strongly dependent upon the anodization current density and the duration of the etching process. We interpret the origin of visible Pl of the porous MBE SiGe films by considering the quantum confinement effect, as in the interpretation of PL from Porous Si, and the evolution of the SiGe Si-like band structure.
DIFFUSIONAL NARROWING OF DISPERSION IN Ge/Si (100) COHERENT ISLAND QUANTUM-DOT SIZE DISTRIBUTIONS, Sergio A. Chaparro, Jeff Drucker, Univ of Texas-El Paso, Dept of Physics, El Paso, TX.
MBE growth of Ge/Si (100) coherent island quantum dots has been investigated at various deposition rates and substrate temperatures for fixed Ge coverage. There is a pronounced narrowing of the scaled width of the island size distributions for these samples as the substrate temperature increases independent of the deposition rate. The scaled width of the island size distributions, defined as the quotient of the standard deviation of the island radius and the average island radius (/), allows quantitative comparison of the relative width of distributions of differing average radius. Samples were grown at deposition rates of 1.5 and 4 ML/min at substrate temperatures of 450C and 550C. The 450C samples had scaled widths of between 0.34 and 0.39 while those grown at 550C had scaled widths of 0.25. We interpret these results in terms of increased diffusion kinetics during growth at the higher temperature. Consequences of this result for obtaining ultranarrow quantum-dot size distributions will be discussed.
THE INFLUENCE OF LATERAL DIMENSIONS ON RELAXED, LINEARLY-GRADED SILICON-GERMANIUM BUFFER LAYERS, Richard Hammond, Evan H.C Parker, Univ of Warwick, Dept of Physics, Coventry, UNITED KINGDOM; Hans von Kaenel, ETH Zurich, Dept of Soild State Physics, Zurich, SWITZERLAND; Andrew Shields, Toshiba Corp, Cambridge, UNITED KINGDOM.
Limited-area MBE growth of thin (500nm), linearly-graded Silicon-Germanium (SiGe) buffer layers on Si(100) mesa pillars of lateral dimensions ranging from 3 to 20um has been shown to dramatically reduce the magnitude of the undulating surface cross-hatch characteristic of such relaxed buffer layers. An area dependent transition in the relaxation mechanism has been observed for an identical deposition on a patterned substrate with varying lateral growth dimensions. For lateral growth dimensions of 10um and below, dislocation termination at the edge of the growth zone enables complete misfit extension on different atomic planes within the graded region. In contrast, for limited-area depositions of lateral dimensions exceeding 10um, orthogonal misfit interactions occur and relaxation is dominated by the Modified Frank-Read (MFR) mechanism.
Atomic Force Microscopy has revealed an RMS surface undulation of 0.3nm, with no significant surface cross-hatch for non-MFR relaxed depositions. However, for MFR-relaxed depositions a highly developed cross-hatch is observed with a RMS undulation of 1.2nm. This phenomenon may be attributed to the increased co-operative stress fields of the large dislocation pile-ups inherent to the MFR mechanism. Micro-Raman spectroscopy indicates that all mesa depositions are relaxed to the same extent. By avoiding orthogonal misfit interactions the method of limited-area, linearly graded SiGe buffer layers offers the opportunity of growing high quality, relaxed, defect free virtual substrates to any Ge composition.
NON-DESTRUCTIVE CHARACTERIZATION OF SIGE/SI HETEROSTRUCTURES USING SPECTROSCOPIC ELLIPSOMETRY, Weize Chen, MIT, Dept of MS&E, Cambridge, MA; Richard Westhoff, Lawrence Semiconductor, Tempe, AZ; Rafael Reif, MIT, Dept of EE&C, Cambridge, MA.
Optical characterization of SiGe heteroepitaxial layers has been performed using a phase modulated spectroscopic ellipsometer in the near infrared to the visible range. The layers were grown by CVD, and cover a range of thickness and composition (0 B5.10
COMPOSITIONAL AND CRYSTALLOGRAPHIC STUDY OF SIGE ON SAPPHIRE DEPOSITED BY RAPID THERMAL CHEMICAL VAPOR DEPOSTION DEPOSTION, Wadad Brooke Dubbelday, NCCOSC, RDT&E Div, San Diego, CA; Richard Westhoff, Lawrence Semiconductor, Tempe, AZ; Karen Kavanagh, Univ of California-San Diego, Dept of E&CE, La Jolla, CA; Paul de la Houssaye, NCCOSC, RDT&E Div, San Diego, CA; Fei Deng, Univ of California-San Diego, Dept of E&CE, La Jolla, CA; I. Lagnada, NCCOSC , RDT&E Div, San Diego, CA; R. Scott, J. Huffman, L. Young, Lawrence Semiconductor, Tempe, AZ.
Recent work has demonstrated the great potential for improvement in MOSFET device performance using SiGe alloy structures in various configurations. (1) However, enhancement of NMOS and PMOS devices rely on different film structures such as relaxed, defect free SiGe alloys; these can be difficult to realize on bulk silicon substrates in the same plane on the wafer surface. In addition, the thick layers required to obtain the relaxed SiGe films preclude the fabrication of thin film devices on insulating substrates which are predicted for application in the next generation of deep submicron geometries.
Silicon can be deposited on sapphire and then implanted and regrown to yield device quality films for high speed applications.(2,3) Germanium can also be directly deposited on sapphire.(4) This work represents the first published effort to deposit SiGe films directly on sapphire for microelectronics applications using rapid thermal chemical vapor depostition (RTCVD). A matrix of growth conditions has been used, including low pressure deposition and graded buffer layers. These films have been studied using Rutherford backscattering (RBS), transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic force microscopy (AFM), providing data for the analysis of the growth kinetics and defect structures in the resultant films.
ATOMISTIC FINITE ELEMENT (ATFE) SIMULATIONS OF HETEROEPITAXIAL GROWTH OF SEMICONDUCTOR ISLANDS, Patrick A. Klein, Stanford Univ, Dept of Applied Mechanics, Stanford, CA; Huajian Gao, Stanford Univ, Dept of Mech Engr, Stanford, CA.
We present a methodology for modelling material response generated during heteroepitaxial growth at the smallest length scales for which continuum theory is applicable. Our atomistically based finite element implementation addresses the kinematic nonlinearities arising from large strains, as well as nonlinearities in the material response. The constitutive relations are derived from an atomistic view of the material. The lattice structure and interatomic potentials appropriate for the given materials combine to produce behavior that is consistent with linear elastic approximations of the materials at small strains and transitions to generally anisotropic nonlinear elastic behavior consistent with pure atomistic treatments with increased deformation. Constitutive models have been produced for the following systems: FCC lattices with the Lennard-Jones potential, diamond cubic lattices with the Stillinger-Weber potential, and FCC and BCC lattices with the embedded atom method. The method has been applied to the study of the growth of germanium quantum dots on a silicon substrate in order to shed light on several experimentally observed phenomena, such as the appearance of dislocations at the bi-material interface and favored morphologies. We demonstrate that for this system even uniform film growth cannot be accurately modelled with linear elastic treatments.
GAS SOURCE MBE OF SILICON II:THE EFFECT OF HYDROGEN, James H. G. Owen, Univ of California-S Barbara, Dept of Chemical Engr, Santa Barbara, CA; Kazushi Miki, Electrotechnical Laboratory, Ibaraki, JAPAN; Ilan Goldfarb, David R. Bowler, Chris M. Goringe, G.Andrew D. Briggs, Oxford Univ, Dept of Materials, Oxford, UNITED KINGDOM.
During STM growth studies between 550 K and 650 K, we have found that the surface hydrogen concentration controls the growth morphology. This is because in this temperature range, the disilane deposits hydrogen as well as silicon.Initially, small strings of clean epitaxial dimers form, but at larger total exposures, the shape and size of the epitaxial islands which are produced depend greatly upon the disilane flux which is used at a given temperature. At 550K, low fluxes will allow growth of large islands, but higher fluxes reduce the islands to dimer strings, and ultimately block growth completely. At 650 K, the surface hydrogen is able to desorb slowly. At low fluxes, a whole monolayer may be grown with a low apparent hydrogen coverage. Even so, at high fluxes the hydrogen adsorption rate is greater than the desorption rate, and layer-by-layer growth is blocked.
BORON INCORPORATION WITH AND WITHOUT ATOMIC HYDROGEN DURING THE GROWTH OF DOPED LAYERS ON Si(100), Conrad Lorenzo Silvestre, Phillip Thompson, Glenn Jernigan, Naval Research Laboratory, Washington, DC; David Simons, NIST, Analytical Microscopy Group, Gaithersburg, MD.
Solid Source Molecular Beam Epitaxy (SSMBE) Si growths were done with and without Atomic Hydrogen (AH) to investigate the impact of AH on(1) B incorporation, (2) B activation, and (3) B segregation. A series of 50 or 3 nm thick B-doped Si layers separated by 200 nm of undoped Si were grown at 0.1 nm/s by SSMBE on Si(100). In separate experiments, AH was applied during (at 710C) or after (at 500, 600, and 710C) the growth of the 3 nm B doped layer to determine if AH decreased segregation. AH was applied before the 50 nm B doped layer growth to observe if AH increased B activation between 600 and 800C. Application of 1.3 x 10 Pa of AH at 710 after the 3 nm B doped layer decreased the segregation constant from 0.033 decade/nm to 0.025 decade/nm, as measured by Secondary Ion Mass Spectrometry (SIMS). Application of 1.3 x 10 Pa of AH during B doped Si layer growth had no effect. Application of 1.3 x 10 Pa of AH for 100 s prior to starting the B doped layer growth increased B activation (by a factor of 4 at 800C), as measured by Spreading Resistance Profiling (SRP). Differences in carrier concentration (by SRP) and incorporation by SIMS w & w/o AH are observed indicating two forms of B. We will discuss a model for the increase in B activation with AH implying B monomers, which are electrically active, and B dimers, which are electrically inactive. The use of AH modifies the surface morphology, enhancing the proportion of B in the electrically active form.
SESSION B6: EPITAXIAL GROWTH KINETICS: Si AND SiGe ALLOYS
Chairs: Daan Frenkel and Bert Voigtlander
Wednesday Morning, April 2, 1997
Golden Gate C3
8:30 AM *B6.1
PROGRESS IN SURFACTANT MEDIATED HETEROEPITAXY OF Ge ON Si(111), M. Kammler, Univ Hannover, Inst fur Halbleitertechnologie, Hannover, GERMANY; D. Reinking, K. Hofmann, Univ Hannover, Hannover, GERMANY; F. Meyer zu Heringdorf, M. Horn-von Hoegen, Univ Hannover, Inst fur Festkoperphysik, Hannover, GERMANY.
The deliberate use of an adsorbed monolayer of a group V element as surfactant (here Sb and Bi) during growth on Si hinders islanding and allows to grow smooth and continuous epitaxial Ge films on Si surfaces. On Si(111) the 4.2 lattice mismatch between Si and Ge is accommodated by a quasiperiodic array of dislocations which are confined to the Si-Ge interface. This allows to grow strain relieved, relaxed Ge films of arbitrary thickness, which are almost free of defects.
In this contribution, we present new results on the incorporation of Sb and Bi and the density of threading defects of Ge films grown on a commercial 4-inch MBE machine. In contrast to former estimates Sb and Bi is incorporated to a doping level below the detection limits of 3 x 10 cm, as determined by SIMS. The doping level depends strongly on the growth temperature. The number of stacking faults in the surfactant grown Ge film is below 10 cm and the density of threading defects below 6 x 10 cm as estimated by defects etching and REM.
For the first time, Hall measurements of surfactant grown Ge films on Si(111) yield a carrier density (electrons) of 2-3 x 10 cm, which is compatible with the detection limits of the Sb doping level. The mobility (electrons) at RT is 2700 cm/Vs (11000 cm/Vs at 77 K) as compared to lattice mobility of bulk Ge of 3900 cmVs. Both values belong to the best ever reported for Ge films grown on Si.
9:15 AM B6.2
GROWTH KINETICS OF Si GAS-SOURCE MBE THROUGH EYES OF SURFACE HYDROGENS, Maki Suemitsu, Hideki Nakazawa, Tohoku Univ, Res Inst of Electrical Communication, Sendai, JAPAN; Nobuo Miyamoto, Tohoku-Gakuin Univ, Faculty of Engineering, Tagajo, JAPAN.
Elucidating the behavior of surface hydrogens is key to full understanding of the growth kinetics of Si CVD or GSMBE using Si hydrides. Our recent studies on silane and disilane GSMBE demonstrates that a simple combination of precise measurements on the growth rate, , and the surface hydrogen coverage, Theta, with a help of some basic equations related to the surface kinetics, yields several surprisingly productive information on the growth kinetics in these systems. The knowledge includes (1) the presence of adsorption-limited and desorption-limited growth modes, (2) the temperature-dependent adsorption kinetics of the source gas molecules, and (3) the hydrogen desorption kinetics during growth, as well as effects of hydrides, such as germane or phosphine, on the growth kinetics. The item (1) has always been assumed in previous analyses, but was rarely confirmed directly. This has become possible by use of a hydrogen temperature-desorption measurement on freezed surfaces from the growth. For the item (2), our results on disilane GSMBE indicates a presence of three adsorption modes on Si(100): physisorption, 2-site-2 step chemisorption, and 4-site-1-step chemisorption, which changes in the order listed here as the growth temperature increases. The item (3) concerns with a prevailing erroneous identification of the low temperature growth rate activation energy, , with the hydrogen desorption energy, . A brief discussion on the desorption kinetics is shown to provide a bridge between the two quantities, and its application to a comparative study on silane and disilane GSMBE demonstrates a successful reduction into an identical value from apparently contradicting values. The reaction order of the hydrogen desorption process during growth is also shown to be unity.
9:30 AM B6.3
REVERSIBLE HYDROGEN INDUCED SILICON-GERMANIUM EXCHANGE REACTION ON Ge TERMINATED Si (001) OBSERVED WITH IN-SITU FTIR-ATR SPECTROSCOPY AND REFLECTION HIGH-ENERGY ELECTRON DIFFRACTION*, Eugene Rudkevich, Univ of Wisconsin-Madison, Dept of E&CE, Madison, WI; Donald E. Savage, Univ of Wisconsin-Madison, Materials Science Program, Madison, WI; Feng Liu, Univ of Wisconsin-Madison, Dept of MS&E, Madison, WI; Max G. Lagally, Univ of Wisconsin-Madison, Materials Science Program, Madison, WI; Leon McCaughan, Univ of Wisconsin-Madison, Dept of E&CE, Madison, WI.
The presence of hydrogen on single-crystal surfaces is known to have dramatic consequences on epitaxial growth because it can alter both surface energy and surface kinetic processes. For example, in the SiGe alloy system it has been suggested that hydrogen suppresses Ge segregation leading to a chemically abrupt interface in strained-layer epitaxy. In this work we explore the surface compositional changes induced by exposure to atomic H at different temperatures on 1-2 monolayers of Ge deposited on Si(001) by UHV-CVD. We use Fourier Transform Infrared - Attenuated Total Reflection (FTIR-ATR) spectroscopy combined with dosing of atomic H at room temperature (a “titration” method to decorate surface dangling bonds with H) to obtain the composition of the outermost atomic layer. In addition surface structural changes are followed by RHEED throughout the process. We demonstrate that it is possible to induce Si segregation by exposing the surface to H at temperatures above 200C. The Ge termination can be recovered by heating the surface to temperatures high enough to desorb H. First-principles total-energy calculations show that the bare surface favors Ge termination while the H-covered surface favors Si termination. The temperature dependence of the Si segregation is explained by two activated processes. At low temperature the rate limiting step is determined by the activation barrier for Si-Ge exchange. At high temperature the rate limiting step is H desorption, which depletes the surface of hydrogen causing Ge to segregate. Results will be compared with calculations for the Si-Ge exchange process. * Supported by NSF
9:45 AM B6.4
DRASTIC CHANGES IN THE STICKING AND DESORPTION PHENOMENA OF Sb ON Si SURFACES DUE TO ATOMIC HYDROGEN TERMINATION, Kiyokazu Nakagawa, Yoshinobu Kimura, Masanobu Miyao, Hitachi Ltd, Dept of Electron Devices, Tokyo, JAPAN.
A molecular beam epitaxy (MBE) apparatus is a powerful tool for forming heterostructures. However, selective growth in limited areas of a substrates is difficult and this is an important requirement in the application of MBE to device fabrication. In this paper, we discuss Sb sticking and Sb desorption on hydrogen-terminated and clean surfaces as a function of temperature. We also describe our experimental results and use them to clarify the mechanism of selectivity and whether selective doping is feasible.
Our findings are as follows:
(1) Sb atoms accumulate on clean Si surfaces in proportion to the Sb deposition time, but the accumulation is independent of the substrate temperature.
(2) The concentration of Sb atoms on hydrogen-terminated surfaces saturates at some Sb deposition time due to thermal desorption with an energy of 0.14 eV. This energy appears to correspond to the energy of the van der Waals bond between the Sb atoms and the terminated Si surface.
(3) Sb atoms adsorbed on a hydrogen-terminated surface seem to form Sb clusters and/or islands, and thermal desorption from the states has an activation energy of 1.35 eV which corresponds to the Sb-Sb bond energy.
We also show that the hydrogen treatment enables us to carry out selective area doping. which is not possible with normal MBE.
10:30 AM *B6.5
PATTERN-CONSTRAINED EPITAXY FOR ULTRA-THIN SOI, H.-S. Philip Wong, K. Chang, Y. Taur, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.
Ultra-thin (< 40 nm) films of silicon-on-insulator (SOI) is attractive for use in sub-100nm gate MOSFET's. Gate control of the channel potential is improved with ultra-thin films, thus enlarging the design space of ultra-small MOSFET's. We begin with an overview of the techniques employed to obtain ultra-thin SOI films, including both global and local methods. The main part of the talk will describe a new technique (pattern-constrained epitaxy) which enables the formation of thin (< 40 nm) sheets of single-crystal silicon by silicon epitaxial growth techniques. Applications of the technique to MOSFET fabrication will be demonstrated.
11:15 AM B6.6
SEG Si: FACET CONTROL AND SELECTIVITY VS. NITRIDE AND OXIDE PATTERNS, David J. Howard, IMEC, Leuven, BELGIUM; Matty Caymax, IMEC, Dept of ASPVMT, Leuven, BELGIUM.
As the lateral dimensions and junction depths of devices shrink with every new generation of device, the inclusion of nanometer-precision epitaxial technologies, such as selective epitaxial growth (SEG), provides new options for device design. The utility of SEG Si has been demonstrated in a number of devices, including SEG sacrificial-Si for silicidation, SEG Si capping of ion implanted Si, and doped SEG Si and alloys of SiGe as base materials in transistors. However, the faceting of these Si layers at <110>-oriented pattern edges has been a limiting factor in their application. In addition, a variety of pattern materials, such as thermal SiO TEOS, CVD SiN, PBL, and LOCOS, are used in combination as spacers or for isolation in MOS devices, and thus pose a combined challenge for SEG.
In this paper, a comprehensive SEM study of SEG Si is presented for (I) Si selectivity as a function of process conditions (gas flows, temperature, doping), and in particular (2) Si faceting which results in the SEG Si at spacer material <110>-oriented edges as a function of different process conditions. In addition, the effects of the spacer shape and material (SiO and SiN) on the facets in SEG Si are considered. The results of this study demonstrate ''control'' - and in some cases elimination - of the faceting in SEG Si layers at pattern edges for a range of process conditions and patterned materials.
11:30 AM B6.7
INFLUENCE OF HYDROGEN ON THE INITIAL STAGES OF AL THIN FILM GROWTH, D. P. Adams, T. M. Mayer, B. S. Swartzentruber, Sandia National Laboratories, Dept of Surface & Interface Sciences, Albuquerque, NM.
In this investigation we show that hydrogen can have a large effect on Al thin film structure. Al films have been deposited onto Si(100) and Si(100):H 2x1 surfaces by physical vapor deposition and analyzed using scanning tunneling microscopy (STM). STM demonstrates that Al grown onto clean Si(100) at 100°C develops with a Stranski-Krastanow - like structure.1 In this case the first 0.5 monolayer (ML) of Al was continuous and formed with a 2x2 reconstruction. However, deposition of 0.5 ML of Al onto a monohydride-terminated, (theta = 1) Si surface at 100C resulted in Volmer-Weber growth. In general, three- dimensional islands of Al, with heights >1 ML, formed on a H-passivated surface before coalescence. STM further showed that the passivation layer was not disrupted during Al deposition and subsequent island formation. In addition, this work has investigated the coverage-dependent effects of H on Al film growth kinetics. The role of small H coverages was examined by comparing the Al island number density formed in the presence of 0.1 ML H to that developed during growth on a clean Si surface.
11:45 AM B6.8
MEASUREMENT OF MISFIT DISLOCATION PROPAGATION VELOCITIES DURING UHV-CVD GROWTH OF SiGe/Si(100), Eric A. Stach, Robert Hull, J. C. Bean, Univ of Virginia, Dept of MS&E, Charlottesville, VA; R. M. Tromp, M. C. Reuter, F. K. Legoues, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.
The kinetics of misfit dislocation generation are a central parameter in defining the stability of (Si)/SiGe/Si heterostructures as a function of time at temperature during the growth and annealing cycles. Previous information about such kinetic parameters have generally been derived from ex-situ measurements of heterostructures annealed after completion of the growth process. In this paper we report direct measurements of the propagation velocities of misfit dislocations during growth of thin SiGe epilayers on Si (100), utilizing the unique capabilities of a specially constructed UHV-TEM equipped with UHV-CVD growth capabilities. In contrast with previous reports in the InGaAs/GaAs (100) system, where misfit dislocations were seen to stop during pauses in the growth process (Whaley and Cohen, Appl. Phys. Lett. 57, 144 (1990); Whitehouse, et al., J. Cryst. Growth 150, 85 (1994)), we observe that dislocations continue to propagate at nearly the same velocity during post-growth UHV annealing as they do during growth itself. Additionally, it is found that the measured velocities are several times slower than those observed during post-growth annealing of epilayers which have a native oxide on their surface. The mechanisms for this systematic velocity enhancement as a function of the native oxide cap, and ramifications for predictive strain modeling in the SiGe/Si system will be discussed.
SESSION B7: EPITAXY OF METASTABLE ALLOYS AND OTHER COMPOUNDS
Chairs: Dongmin Chen and Mohan Krishnamurthy
Wednesday Afternoon, April 2, 1997
Golden Gate C3
1:30 PM *B7.1
LOCAL STRAIN AND STRUCTURE OF EPITAXIAL SnGe ALLOYS, Regina Ragan, California Inst of Technology, T. J. Watson Lab of Applied Physics, Pasadena, CA; Gang He, Harry A. Atwater, California Inst of Technology, Dept of Applied Physics, Pasadena, CA.
Metastable SnGe alloys can be grown as homogeneous solid solutions in the diamond cubic phase by molecular beam epitaxy at low temperatures on Ge(001) and Si(001) substrates. Previous theoretical and experimental work treated S SnGe as a virtual crystal and, indeed, we find that cubic lattice parameter measurements by x-ray diffraction are consistent with the virtual crystal approximation for both strain-relieved and pseudomorphic alloy films in the composition range 0 < x < 0.15. However, we have performed the first optical measurements of the direct energy gap in this material which indicate a very strong bandgap bowing (2.8 eV), in marked contrast to virtual crystal approximation predictions of a linear variation of the energy gap with composition. We have also performed total energy calculations of local alloy structure using density functional theory in the local density approximation for 8-atom supercells of SnGe and SnGe, and although the calculated cubic lattice parameter is in good agreement with the virtual crystal approximation, the nearest-neighbor bond lengths of the Ge-Ge, Sn-Sn, and Sn-Ge bonds show significant departures of the bond length and bond angle from the virtual crystal approximation. Significantly, both the close agreement between calculated and measured x-ray reflectivity for 5-70 nm thick pseudomorphic SnGe films on Ge(001) and composition profiles obtained from Rutherford backscattering spectrometry argue against variations in composition on a length scale longer than approximately 3 nm. Thus, local distortions related to the large atomic misfit between Sn and Ge appear to have a profound effect on the structure and properties of SnGe, which is a plausible result in view of the similar phenomena which have been observed for ternary compound semiconductors such as InGaAs.
2:15 PM B7.2
GROWTH OF SINGLE CRYSTAL METASTABLE GeSn ALLOYS AND STRAINED-LAYER SUPERLATTICES ON Ge(001)2x1 BY MOLECULAR BEAM EPITAXY, Osman Gurdal, P. Desjardins, Johan R.A. Carlsson, Nerissa Taylor, Joseph E. Greene, Univ of Illinois-Urbana, Dept of MS&E, Urbana, IL; Henry Radamson, Jan-Eric Sundgren, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN.
Epitaxial metastable Ge alloys with x 0.22 have been grown on Ge(001) by MBE. Sn has a solid solubility in Ge of < 1.0 and lattice constant mismatch of 14.7. Limitations due to kinetic surface roughening and Sn segregation determine the growth temperature, T, chosen in these experiments to be between 70 and 140C at a deposition rate of 0.06 nm s. Based upon RHEED and XTEM analyses, the epitaxial thickness tepi at Tg = 100C was found to range from 100 nm for pure Ge to 4 nm for alloys with x = 0.22. As t is exceeded, extended defects such as dislocations, microtwins, and stacking faults on planes are formed, which eventually lead to amorphous growth. Ge/Ge strained-layer superlattices (SLS) with x 0.24 were also grown, in this case using temperature modulated MBE between 80 and 130C. The SLS structures are commensurate with the substrate for Ge layer thickness less than tepi. High-resolution XRD reciprocal lattice mapping measurements showed that the Ge alloys were fully strained and of high crystalline quality with negligible mosaicity.
2:30 PM B7.3
MOLECULAR BEAM EPITAXY OF GeSn AND GeSnC ALLOYS ON Ge(100), Bi-Ke Yang, Mohan Krishnamurthy, Michigan Technological Univ, Dept of M&ME, Houghton, MI.
We report on the epitaxial growth of Ge and Ge thin films on Ge(100) substrate al low temperatures (150C). The nominal fraction of Sn ranges from 5 30at. and that of C from 5-10at.. These 60 nm thick films were characterized in-situ by RHEED and ex-situ by TEM and x-ray diffraction. After the buffer growth, the Ge(100) surface typically shows a (2x1) reconstruction. During the growth of GeSn alloys, the RHEED changes to (1x1) with some broadening of the streaks indicating disorder in the surface possibly from segregating Sn. For GeSnC alloys, the pattern shows transmission spots with indications of (311) facets. Plan view TEM indicates that GeSn(Sn < 5) thin films are coherent with no defects observed. Cross-section TEM of GeSnC samples shows good epitaxial quality with (311) facets decorated by a Sn rich phase. X-ray diffraction shows that the alloy peak of GeSn shifted to lower nominal Sn content in the presence of C. This suggests that either C helps to compensate the strain in the film or that the amount of Sn incorporated in Ge decreases. Possible growth mechanisms will be discussed in detail.
2:45 PM B7.4
DEPENDENCE OF THE ELECTRONIC PROPERTIES OF InAs/InP SYSTEMS ON SURFACE MORPHOLOGY, Cesar A. C. Mendonca, Edson Laureto, Monica A. Cotta, Maria J. S. P. Brasil, Mauro M. G. Carvalho, Eliermes A. Meneses, UNICAMP, IFGW, Campinas, BRAZIL.
In this work we investigate the electronic properties of InP/InAs/InP systems grown by Chemical Beam Epitaxy, using Atomic Force Microscopy (AFM) and photoluminescence(PL). We show that the morphology and the topography of the InP surface strongly affect the characteristics of the InAs film deposited on top.. Smooth surfaces - typically obtained from two dimensional growth mode - and periodical multi-terrace structures - originated from three dimensional growth - were used as buffer layers to evaluate these effects. Samples grown simultaneously on (100) InP nominally oriented and 2 degrees off towards different directions (A, B and C-surfaces) present remarkably distinct behaviors. These differences are related to the processes involved in the redistribution of the InAs material during growth interruption at the InAs/InP interface. AFM images of samples without the InP capping layer show that the presence of surface steps and corrugations induce early nucleation of InAs at preferential regions of the surface, leading to the formation of three dimensional islands. We foccus our analysis on the PL emission originated at the InAs wetting layer, considering it as a very thin quantum well structure. PL spectra with multiple peaks are observed for samples deposited on nominal and A-surface substrates. These results indicate that the corresponding wetting layer presents different thickness along the structure. On the other hand, samples grown on substrates containing B-steps (B and C-surfaces), present spectra dominated by a single peak which is characteristic of a more uniform wetting layer. The PL line widths ( 15meV) are relatively small and are attributed to short range roughness, i.e., smaller than exciton diameter. We analyse these results considering how the diffusion of surface atoms for different types of surfaces and for different morphologies affect the redistribution of the InAs layer. We also discuss the role of the natural surface steps and the intentional corrugations in producing better uniformity of the thin quantum well structure.
3:30 PM *B7.5
NUCLEATION, DIFFUSION, AND THE INITIAL STAGES OF Si GROWTH ON Si(001), Eric Ganz, Univ of Minnesota, Dept of Physics, Minneapolis, MN.
We will discuss the initial stages of the growth of Si on the Si(001)-2 x 1 surface at temperatures above 400 K. Using hot Scanning Tunneling Microscope (STM) movies, we observe nucleation and metastable states on the path to epitaxial growth. We follow annealing and diffusive behavior on the surface, allowing us to test various growth models. We also use tracking to study the detailed properties of mobile species (Si monomers and dimers). Interactions with surface features and step edges, rotations, and complex diffusional pathways are observed.
4:15 PM B7.6
STRUCTURE OF a-SiC: A MOLECULAR DYNAMICS STUDY, Luciano Colombo, Univ di Milano, Dept of Physics, Milano, ITALY; D. Mura, G. Mula, Univ di Cagliari, Dept of Physics, Cagliari, ITALY; R. Bertoncini, Univ di Cagliari, Centro Ricerche e Svilippo, Cagliari, ITALY.
We investigate a-SiC using classical molecular dynamics (MD) simulations based on the many-body potential by Tersoff . We analyze the network topology as well as the chemical order of the system and characterize the local atomic coordination at different compositions. To these aims, we have improved the original formulation of the potential by introducing, as suggested in the original paper , a new term (never used so far) in the coordination-dependent contribution to the atom potential energy. The exploitation of this new extra term dramatically improves both the accuracy and transferability of the potential, as discussed by comparing present model predictions to recent first-principles data for the x = 0.5 alloy . Our results enlarge the role of classical MD simulations to the modeling of morphological and chemical properties of epitaxially grown multicomponent semiconductor systems.
4:30 PM B7.7
SIC GROWTH BY ENHANCED ENERGY CARBONIZATION OF Si AND SOI, Peter P. Chow, Matthew Rosamond, SVT Associates Inc, Eden Prairie, MN; Ian G. Brown, Lawrence Berkeley National Laboratory, Matls Science Div, Berkeley, CA; Othom R. Monteiro, Lawrence Berkeley National Laboratory, AFRD, Berkeley, CA.
Silicon carbide (SiC) has generated significant attention recently due to its useful properties such as wide energy bandgap, high breakdown field, and physical strength. Technologically it is attractive to be able to grow high quality SiC on large size substrates. Carbonization of silicon and silicon-on-oxide (SOI) has been one of the approaches attempted for nucleation, using hydrocarbon gases as the source for carbon. The SOI as compliant substrate material is particularly appealing because of the possibility of substantially reducing defect density in lattice mismatched heteroepitaxy. We have examined the initial nucleation process and observed that the control of silicon outdiffusion is key to produce high quality, uniform epitaxy. Here the carbonization process has been carved out in a molecular beam epitaxy (MBE) system using low energy carbon ion and thermally evaporated carbon. Both the (100) and (111) oriented Si and (100) SOI substrates have been used and they undergo different transformation during carbonization. The process is followed closely with in-situ electron diffraction (RHEED) in the temperature range of 800 to 1100C, and the samples are analyzed by Auger, transmission electron microscopy (TEM), and optical measurements. The carbon ion species is produced from an arc source and has been characterized to consist of ions with energy in the range of 10 to 20 eV. When compared with thermally evaporated carbon, the ion carbonization exhibits bright and sharp SiC diffraction pattern immediately; x-ray diffraction confirms the higher quality due to energy enhancement. The effect of thin compliant Si layer on the (100) SOI substrate is also discussed.
4:45 PM B7.8
HIGH QUALITY TENSILE STRAINED GaInAsP/GaInAsP MULTIPLE QUANTUM-WELL STRUCTURES FOR 1.3 m LASER APPLICATION, Haiyan An, Shuren Yang, Hongbo Sun, Jilin Univ, Dept of Electronic Engr, Changchun, CHINA; Shiyong Liu, Jilin Univ, Dept of Electrical Engr, Changchun, CHINA; Zhijie Wang, Natl Research Ctr for Optoelectronic Tech,, Beijing, CHINA; Wei Wang, Natl Research Ctr for Optoelectronic Tech, Beijing, CHINA.
For tensile-strained quantum well (QW) lasers emitting at 1.3 m, GaInAsP is preferable to GaInAs since thicker QW can be applicable. However, there are reports on the GaInAsP /GaInAsP strained layer MQW growth, particularly to the tensile strained layers, have been hampered by the layers becoming wavy, rather than abrupt interface. Recent investigations have indicated that the waviness could be related to the GaInAsP miscibility gap which resulting in spinodal-like decomposition of the alloy. In this investigation, we have grown high quality GaInAsP /GaInAsP tensile strained quantum wells at relatively high growth temperature and relatively high V/III ratios which were used to eliminate the wavy layer growth phenomena. Besides, similar to that described by Mircea et al., the same As/P composition in the wells and barriers are used to reduce the driving force for As, P interdiffusion. The growth was performed by low pressure metal organic chemical vapor phase epitaxy(LP-MOVPE). TMIn and TMGa were used as sources of III elements and AsH, PH as sources of V. The active layer are four 11 nm thick tensile strained GaInAsP wells with a strain of -1.2. The presence of more than five order satellite peaks and the presence of pendellosung fringe between them in the (400) double crystal x-ray rocking curve of the LD structure indicates the presence of abrupt interface in the GaInAsP /GaInAsP active layer. The room temperature photoluminescence spectrum exhibits a clear energy separation of 50 meV of electron to light hole and electron to heavy hole transitions. The broad area tensile strained SCH multiple quantum wells lasers have been fabricated. The threshold current density with different cavity-length have been measured. The threshold current density J/N extrapolated at infinite cavity length is 50 mA/cm. To reduce the threshold current, the planar buried heterostructure (PBH) Fabry-Perot lasers with 2.0 m-wide mesas which were chemically etched using a SiO mask have been fabricated. As-cleaved, 350 m cavity length tensile strained lasers show at 20C CW threshold currents of 15 mA . The lasing spectra of the TM polarized light emission demonstrated that lasing indeed occurs from electron to LH recombinations as expected from the tensile strained QW laser.