Cathodoluminescence and Photoluminescence Spectroscopy Study of Low Temperature Molecular Beam Epitaxy GaAs
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CATHODOLUMINESCENCE AND PHOTOLUMINESCENCE SPECTROSCOPY STUDY OF LOW TEMPERATURE MOLECULAR BEAM EPITAXY GaAs R. Enrique Viturro*, Michael R. Melloch**, and Jerry M. Woodall*** *Xerox Webster Research Center, 114-41D, Webster, NY 14580 **School of Electrical Engineering, Purdue University, West Lafayette, IN 47907 ***IBM Research Division, P. 0. Box 218, Yorktown Heights, NY 10598
ABSTRACT We report a cathodoluminescence (CL) and photoluminescence (PL) study of molecular beam epitaxy grown GaAs at low substrate temperatures (LT GaAs), and semi-insulating LEC GaAs. The as grown LT GaAs material shows intense deep level emissions which can be associated with an excess concentration of Arsenic. These emissions subside with annealing for a few minutes at temperatures above 450 0 C. CL measurements clearly show an extremelly reduced concentration of traps in the post-growth 600 0C annealed material. These results account for a diminished role of electronic point defects in controlling the insulative behavior of LT GaAs and strongly support the "buried" Schottky barrier model. INTRODUCTION Recently, a new kind of GaAs, named low temperature (LT) GaAs,' was shown to have extremely high resistivity, high crystalline quality, and high carrier mobility. 22 All these properties make LT GaAs an excellent material for device applications. 46 The origin of the insulative behavior in LT GaAs is still a subject of controversy. Two dissimilar mechanisms of insulation have been proposed. One model assumes dopant compensation by point defects in the semiconductor, 42 and it is similar to the model proposed for explaining insulation in semi-insulating (SI) melt-grown GaAs. A second model suggests that As precipitates act as buried Schottky6 interfaces whose overlapping depletion regions render the material semi-insulating. A brief summary of the properties of LT GaAs follows. During MBE growth of GaAs at low substrate temperatures, in the range 200-250 0 C, large amounts of Arsenic are incorporated within the epilayer. 2 This excess As, of the order of 1-2% or mid 1020 cm-3 ,2 was shown to be mostly in the form of AscQ antisite defects. 2' 7' 8 X-ray rocking curves indicate a lattice expansion of the LT GaAs of about 0. 1 %.2 This expansion may be caused by the excess As arranged as interstitials, and was shown to scale with the amount of excess As. 2,7 These epilayers are conductive, and the temperature dependence of the conductivity follows the 1/4th power law characteristic of hopping conduction.5, 9 Raising the substrate temperature to normal growth temperatures, typically 600 0C, results in the excess arsenic forming clusters, called As precipitates, 8,10,11 and in a decrease of the lattice constant to values closer to the characteristic value of GaAs.2' 7 Transmission electron microscopy (TEM) studies of LT GaAs grown at 250 0 C and annealed at 600 0 C indicate a density of As precipitates of the order of 1017 cm-3, with an average diameter in the range 3 - 10 nm and average spacing of about 20 nm.8,10,11
Mat. Res. Soc. Symp. Pr
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