GaAs thin film epitaxy and x-ray detector development

Abstract

We report on the growth of high purity n-GaAs using Liquid Phase Epitaxy and on the fabrication of Schottky barrier diodes for use as x-ray detectors using these layers. Our epilayers are grown form an ultra-pure Ga solvent in a graphite boat in a hydrogen atmosphere. Growth is started at a temperature of approximately 800 degrees C; the temperature is ramped down at 2 degrees C/min. to room temperature. Our best epilayers show a net-residual-donor concentration of approximately 2 X 10<SUP>12</SUP> cm<SUP>-3</SUP>, measured by Hall effect. Electron mobilities as high as 150,000 cm<SUP>2</SUP> V<SUP>-1</SUP> s<SUP>-1</SUP> at 77K have been obtained. The residual donors have been analyzed by far IR photothermal ionization spectroscopy and found to be sulfur and silicon. Up to approximately 200 micrometers of epitaxial GaAs have been deposited using several sequential growth runs on semi-insulating and n<SUP>+</SUP>-doped substrates. Schottky barrier diodes have been fabricated using this epitaxial material and have been electrically characterized by current-voltage and capacitance-voltage measurements. The Schottky barriers are formed by electron beam evaporation of Pt films. The ohmic contacts are made by electron beam evaporated and alloyed Ni-Ge-Au films on the backside of the substrate. Several of our diodes exhibit dark currents of the order of 0.3-3.3 nA/mm<SUP>2</SUP> at reverse biases depleting approximately 50 micrometers of the epilayer. Electrical characteristics and preliminary performance results of our Schottky diodes using <SUP>109</SUP>Cd and <SUP>241</SUP>Am gamma and x- ray radiation will be discussed.