Growth kinetics and step density in reflection high-energy electron diffraction during molecular-beam epitaxy

Classified abstracts 7013-7019 33 7013. GaP/Si heteroepitaxy by complex ion beam sputtering We have investigated the sputter deposition using a complex ion beam formed with the mixture o f Xe and PH3 to grow GaP epitaxial films on Si substrates. The complex ion beam was formed by the cold cathode ion source into which Xe and PH3 gases were fed via variable-leak valves. To reduce the ion beam irradiation damage in the deposited film, the Si substrate was arranged in parallel to the polycrystalline GaP target which was sputtered by the oblique irradiation of the ion beam. The ion beam acceleration voltage was kept at 6.5 kV and the ion beam current density was about 100 #A/cm 2 on the GaP target. The GaP films were deposited at a chamber pressure of 2 × l0 -~ Tort, and the substrate temperature was selected between 400 and 600°C. The crystalline quality of the deposited GaP films was examined by the reflection high-energy electron diffraction and the surface morphology was observed by the scanning electron microscope. Single-crystalline GaP films were not obtained by the Ar + or Xe + ion beam sputter deposition, but were obtained by the complex ion beam sputter deposition using the gas mixture o f Xe : PH3 = 1 : 1 as a source gas at a substrate temperature of 500°C, and also by the PH3 ion beam sputter deposition. The surface morphologies of the GaP films grown by the complex ion beam sputtering were smoother compared with those films deposited by the Ar + or Xe ÷ ion beam sputtering. F Ishizuka et al, J appl Phys, 63, 1988, 2091 2093. 33 7014. Growth kinetics and step density in reflection high-energy electron diffraction during molecular-beam epitaxy The kinetics of molecular-beam epitaxy are examined by means of Monte Carlo simulations in combination with a new approach for monitoring surface growth, i.e. by calculating the evolution of the surface step density. The evolution of the step density is shown to have a remarkable correspondence to that of the measured reflection high-energy electron diffraction (RHEED) specular spot intensities for III-V semiconductor compounds. We study growth in a variety of systems, including fiat and stepped surfaces, as a function of substrate temperature and draw several conclusions concerning the relation between R H E E D measurements, kinetics and growth quality. The range of validity of the kinematic approach to R H E E D is discussed and the importance of multiple scattering in the high step density regime is highlighted. Shaun Clarke and Dimitri D Vvedensky, J appl Phys, 63, 1988, 2 2 7 ~ 2283. 34. ION BEAM MIXING, I N T E R F A C E S A N D SOLID PHASE REACTIONS 34 7015. Interfacial reactions in the Ti]GaAs system The reaction between thin Ti films and GaAs, in the temperature range 30(~600°C, was studied using a combination of Auger electron spectroscopy, transmission electron microscopy (TEM), energy dispersive Xray spectroscopy, and electrical measurements. Titanium starts to react with GaAs at ~400°C, producing a layered Ti/TixGal_x/TiAs/GaAs microstructure. The reaction sequence suggests that Ti diffuses into the GaAs and liberates Ga while forming TiAs. Kinetic studies show that the reaction is diffusion controlled with an activation energy of 1.75 + 0.05 eV. The reaction products, for fully reacted films, were identified from electron diffraction patterns as a NiP-type TiAs phase and TixGa~ x (hexagonal structure : a - 0.569 nm, c = 0.932 nm, x = 0.35 ___0.05). Correlation of electrical measurements with TEM results indicates that the formation of TiAs at the interface coincides with an increase in barrier height (0.84 eV) over the as-deposited value (0.72 eV). Ki Bum Kim et al, J Vac Sci Technol, A6, 1988, 1473-1477. 34 7016. Diffusion model for Ohmic contacts to GaAs A diffusion model based on gallium vacancy dependent diffusion assumes grain boundary diffusion of gallium in gold as the principal mechanism of diffusion. This diffusion model is tested with the experimental results on AuGe ohmic contacts to GaAs. The grain boundary diffusion model is modified since the annealing temperature is above the AuGe eutectic point. The diffusion is better characterized by the process of capture by traps (vacancies and dislocations) created during heat treatment. The diffusion parameters for gallium diffusion in gold, and for gold and germanium diffusion in GaAs are reported. The germanium incorporation into the defect lattice structure of GaAs results in donor (Ge~,),

acceptor (Ge~s), neutral pair (Ge~,~3e2,), and acceptor complex states. The increase of the contact resistance beyond the critical annealing temperature is related to the saturation of the gold layer with gallium and excessive arsenic loss. A K Kulkarni and C Lai, J Vac Sci Technol, A6, 1988, 1531 1534. 34 7017. Chemical reactions at the Si/GaAs(110) and Si/InP(110) interfaces: effects on valence-band discontinuity measurements Many photoemission measurements of the valence-band discontinuity are based on the relative positions of the centroids of poorly resolved photoemission peaks. While such measurements are in principle accurate for abrupt, nonreactive interfaces, real interfaces are rarely so simple. We present high-resolution soft X-ray photoemission core level spectra which monitor the band bending and chemistry at two representative heterojunctions. The Si/InP(ll0) interface is highly reactive, has wellresolved chemically shifted In and P components and has preferential outdiffusion of In while the Si/GaS(110) heterojunction has very subtle chemical reactions which are confined to the interracial region. The effects of even such subtle chemical shifts can obscure the determinations of band bending and lead to errors as large as 0.3 eV for the Si/GaAs(110) heterojunction discontinuity. Such results emphasize the importance of obtaining high-resolution photoemission spectra and applying careful analysis for accurate determinations of band bending and valence-band discontinuities. R S List et al, J Vac Sci Technol, A6, 1988, 1543 1547. 34 7018. Metal/semiconductor interfaces on SnO2(ll0) Interfaces formed by tin and palladium deposition on the (110) face of tin dioxide, SnO2 (a semiconductor due to native defects), have been studied from coverages of 0.05 monolayer (ML) to over 10 ML. The structural, chemical, electronic and electrical properties of the surfaces were characterized primarily by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy, and a retractable four-point conductivity probe. Modifications of the substrate by oxidation, annealing and ion bombardment treatments produced three different substrate structures which were used to examine the interface abruptness and the metallization due to Sn and Pd. The deposition of Sn produced an increased surface conductivity on the ordered substrates due to the formation of donor states (oxygen vacancies) at the surface. The deposition of Pd results in the growth of a metallic layer beginning at 1 5 ML, depending upon the surface oxygen concentration. The metallic conductivity rapidly increases with Pd coverage and by 10 ML, LEED reveals microcrystallites in azimuthal registry with the substrate. In contrast, the development of metallic conductivity is relatively slow for the disordered Sn multilayers. J W Eriekson et al, J Vac Sci Technol, A6, 1988, 1593-1598. 34 7019. Tungsten-rhenium alloys as diffusion barriers between aluminum and silicon Tungsten rhenium alloys have been investigated as possible diffusion barriers between aluminum and silicon for high-temperature metallization of microelectronic devices. A1/W-Re/Si structures with the alloy compositions W-30 wt% Re, W ~ l wt% Re and W 72 wt% Re were prepared by electron beam evaporation of W Re and A1 on Si wafers. Thin W Re films, without Al, were also deposited on Si3N4 windows. The samples were annealed at different temperatures and characterized using Rutherford backscattering spectroscopy, X-ray diffraction, and transmission, and scanning electron microscopy (TEM and SEM). The W 3 0 % Re sample was crystalline with a fl-W structure after deposition, while the W 6 1 % Re and the W - 7 2 % Re samples were amorphous and crystallized into the ct-Mn structure (z-phase) at a temperature of 700 and 600°C, respectively. All A1/W-Re/Si structures were stable against A1 penetration up to 500°C. At an annealing temperature of 550°C signs of a reaction leading to the formation of (W, Re)-Al~2 could be observed. At 600°C the barrier layers failed completely due to a reaction between W, Re, A1 and the Si substrate. SEM analyses of the bare Si substrate after the metal layer had been chemically removed showed that etch pits were formed after annealing the diffusion barrier structure at 550°C for 30 min. The failure o f the diffusion barriers is most likely due to local penetration of A1 through defects in the films. S E H6rnstr6m et al, J Vac Sci Technol, A6, 1988, 1650-1655. 993