Onset of GaAs Homoepitaxy and Heteroepitaxy
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ONSET OF GaAs HOMOEPITAXY AND HETEROEPITAXY D K. Biegelsen, R. D. Bringans, J. E. Northrup, and L.-E. Swartz Xerox Palo Alto Research Center, Palo Alto, California 94304
ABSTRACT We present scanning tunneling microscopy images and atomic models for a variety of GaAs(100) reconstructed surfaces. For homoepitaxial material we show the sequence of phases from c(4x4) through c(8x2) as the As surface concentration is reduced. For the heteroepitaxial GaAs/Si(100) growth we show the first two stages of film development, namely, Si(100):As-(lx2) monolayer coverage followed by adsorption of a submonolayer of Ga dimers. The next stages of film development are discussed. INTRODUCTION GaAs is a prototypical, direct-bandgap IIl-V semiconductor. The GaAs(100) surface forms the foundation of existing and future optoelectronic and high speed switching technologies. As device dimensions parallel and/or perpendicular to the plane of the substrate decrease steadily toward interatomic lengths, the control of growth on an atomic scale becomes crucial. To gain such control and understanding of the fabrication processes, atomically-resolved imaging tools are required. In this paper we describe the application of scanning tunneling microscopy (STM) to visualize various GaAs(100) surfaces. We show the bare GaAs substrate reconstructions which are driven by the difference in chemical potentials of Ga and As. The details of the reconstructions are thought to affect such processes as dopant incorporation, adatom diffusion, atomic layer epitaxy, adatom self-organization, and heteroepitaxy. The homoepitaxial surface thus represents the first step in the creation of an interface on GaAs(100). In the second part of this paper we will describe some of our work in characterizing the initial stages of the heteroepitaxial growth of GaAs(100) on Si(100). Unlike the case of homoepitaxy, the heteroepitaxial system seems to evolve in character during many layers of the initial deposition. Again STM provides atomic scale real space information about the structures involved. EXPERIMENT Samples were prepared in a molecular beam epitaxial deposition system and transported through ultra-high vacuum (UHV) to annealing, Auger or STM stations. The substrates were mounted by tungsten wire clips to molybdenum sample holders. The holders in turn contained integral heat sources, either radiant for GaAs substrates or ohmic (direct current) for the Si substrates. GaAs(100) samples were prepared from nominally on-axis wafers by a sequence of cleaning steps. [11 After entry into vacuum via load-lock, the doped GaAs (1018 Si) samples were pre-baked at 500'C for -24 hours. After transfer to the MBE chamber, surface oxides were sublimed off at 6401C [21 in either an As4 or As2 flux. Growth of '-300 nm of nominally undoped GaAs proceeded at 300 nm/hr in an As-stabilized regime. Si samples were cleaned and oxidized by standard solvent and acid dips and the ultra-violet ozone technique. [31 In vacuo cleaning utilized a 24 hr prebake at Ž- 6000 followed by oxide sublimation a
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