Applied Electric Field Effect on Photorefractive GaAs

Compound semiconductors such as GaAs are known to be very fast photorefractive materials. However, due to their relatively small electro-optic coefficient, their diffraction efficiency is much smaller than that of most oxide materials such as BaTiO3. Even so, easily detectable signal with fairly good signal to noise ratio can be usually achieved in a GaAs based application without the aid of an external electric field due to the cross-polarization coupling1 capability of GaAs and the high sensitivity of modern video cameras. For example, in a recently demonstrated real-time optical image correlator2, the correlation output was normally strong enough to saturate the vidicon camera used. On the other hand, in some applications such as the ring oscillator, the double phase conjugate mirror, and the self-pumped phase conjugator, a sufficiently large net two-beam coupling gain is needed. In compound semiconductors, net gain had been achieved by applied electric field techniques. However, in general, these techniques have two side effects, namely the Schottky-barrier effect3 and the low-frequency current oscillation effect4. Depending on the particular requirement of an application, these effects may or may not be a problem. But, they are not desired in general. In this paper, we report the results of a study on the electrode material dependence of the Schottky-barrier effect in the undoped GaAs crystal. In particular, the widely-used silver-paste electrodes are compared with thermally-evaporated alloy electrodes.