Diffusion lengths in p-type MOCVD GaAs

Abstract

In developing MOCVD GaAs/GaAlAs technology for photocathode applications, the minority carrier diffusion length ( L) of p-type GaAs has been studied. Homo- and heteroepitaxial GaAs with hole concentrations in the range 1 × 10 15−8 × 10 19 cm -3 has been compared with similar liquid phase epitaxial material. L was assessed using two techniques which were in substantial agreement: (1) the spectral response of an electrolytic barrier (“Water Drop”) applied to the sample surface, and (2) EBIC measurements using a scanning electron microscope. The diffusion length data show a high doping region where L is controlled by the free hole concentration, and a low doping region where L attains a value of about 10 μm, independent of the hole concentration. This low doping limit could be interpreted as the result of bulk lifetime limitation due to a concentration of residual recombination centres, but analysis show that the surface recombination could also produce an artificial limitation in both the assessment techniques used. Direct minority carrier lifetime measurements were used to resolve the position and show that the surface recombination effects are indeed compromising the L data at low hole concentrations. A value of 100 cm 2 s -1 for the electron diffusion coefficient at a hole concentration of 2.2 × 10 18 cm -3 is directly deduced from the lifetime data.