CHARACTERIZATION OF DEEP LEVELS IN GaAs MIXED CRYSTALS AND PRESSURE INDUCED TRANSPORT IN GALLIUM ARSENIDE

Abstract

Various growth systems for the epitaxial layers of GaLs have been presented. Hydrostatic techniques for trans-mitting high pressures to solid state material samples including epitaxial. films have been discussed. The typical sample preparation for some of the techniques is alo given. The importance of pressure techniques has been demonstrated by presenting some of the results 'obtained from electrical and optical measurements on Si, Ge, Ga:Ls, GaSb and Gats/hlhs mixed crystals. Typical experimental system for Hall measure-ments in the remperature range 400J+°K have also been discussed. The Hall electron mobility in epitaxially grown Galls by VPE, LPE,OMVPE and MBE has been studied both as a function of temperature (27 ( T ( 300°K) and hydrostatic pressure (0-8 Kbar). The analysis of the data show that in LPE and VPE crystals, the mobilities are lowered due to the space charge scattering ( .Csc oc m T ) while in 0-VPE and r BE samples, a space charge like limited mobility (p o( m nT- ) un must be included. The value of the exponent n has been found to be 1 for P4BE and 2 forOMVPE grown sample. It has been concluded that increasing impurity gradation in these layers is responsible for higher value of n. The pressure coefficient of electron mass in GaIIs is determined to be 5.2x15"4 m/Kbar. The computer analysis of the Hall data for Gal _xLLlXAs (x= 0.32) reveals the presence of a deep energy level with an activation energy of (100 ± 5) meV and a shallow level with an (v) energy of 6 meV. The analysis of the Hall and photo Hall data show that the electrons are tightly bound to the deep energy states and the optical ionization energy is much larger than 1.1 eV. It is concluded that the phenomenon of persistent photoconductivity is not due to lattice relaxation as reported earlier in the literature, but is due to the effect of the conduction band structure of the alloys.