Temperature- and illumination-dependence of the work function of gallium arsenide

Abstract

Contact potentials of vacuum-cleaved (110) surfaces of GaAs have been measured as functions of temperature (77 < T < 400 K) and illumination intensity by means of a transparent vibrating capacitor. Surface photovoltage as a function of wavelength and intensity of light, as well as relaxation times, have been measured at various temperatures. Samples of greater than 10 18 cm −3 doping exhibit photovoltages of 1–2 mV or less and little variation of work function with temperature. At a doping level of 10 17 cm −3 p-type samples display photovoltages of more than 50 mV at 77 K and up to 10 mV at room temperature. Moderately doped n-type samples (1.8 × 10 16 and 2.7 × 10 17) exhibit photovoltages up to 0.25 eV at room temperature. At low temperatures, their work function can have two apparently stable values: upon rapid cooling in total darkness, the work function decreases about 80 mV, but during illumination at 77 K, however weak, it decreases by an additional 0.5 V to a new value at which it remains in the dark. This behavior can be accounted for by surface photovoltage according to a simple model not requiring a temperature dependence of surface state spectra. Light is absorbed through interband transitions in the bulk, the surface charge is altered by surface recombination and the return to equilibrium occurs by thermal excitation and tunneling of carriers.