Electronic Transport in Amorphous GaAs and Ga-Mg-As Thin Films

Dc transport and optical measurements have been made on amorphous gallium arsenide and gallium-magnesium-arsenide alloy thin films vacuum deposited by coevaporation of the elements at a substrate temperature of 150°C. The transport properties have been measured on over 40 films over the temperature range 77 T 450 K and for dc fields up to 10⁶V/cm. For dc fields less than 2 X 10⁴V/cm conduction in the GaAs films is Ohmic and bulk-limited. The dc activation energy is 0.50 eV at 300 K which is about half the optical band gap. At high fields and low temperatures the dc current is proportional to exp(V^1/2) and is in good agreement with Poole-Frenkel theory. The thermoelectric power is n-type and relatively temperature independent over the temperature range 300 T 420 K. The addition of magnesium, a p-type dopant in crystalline GaAs, to the amorphous GaAs thin films has no measured effect on the electrical properties of the films in the lowconcentration regime (less than 2 atomic % Mg). At higher magnesium concentrations there is a shift in the optical absorption edge to higher energy, a decreased dc conductivity and an increased activation energy, and a shift in the thermoelectric power toward p-type conduction.