ELECTROREFLECTANCE STUDIES OF SPACE CHARGE LAYERS ON ZINC OXIDE SURFACES

Using the electroreflectance method space charge layers on crystals of different conductivities have been identified. The space charge layers were formed by adsorption of oxygen or atomic hydrogen. The limit of sensitivity required the irradiation with 5×1013 photons/cm2×sec of band gap energy. After exposure to atomic hydrogen all samples showed accumulation layers. With a partial pressure of oxygen above 350 mm Hg crystals of high conductivity (σ=47 ohm-1 cm-1) exhibit depletion layers, which change into accumulation layers, if the partial pressure is reduced below the limit. Crystals of a lower conductivity (σ=10-3-10-1 ohm-1 cm-1) show accumulation layers up to the highest applied oxygen pressure of 760 mm Hg. The phenomena are attributed to a dynamical equilibrium between adsorption and photo-desorption of oxygen. This equilibrium depends on oxygen pressure and free carrier concentration. By comparing a calculated curve with the experimental results the value of 3.31 ev is obtained for the energy gap, light polarized perpendicular to the c-acis. © 1969 Springer-Verlag.