Non-linear transport and structure formation in semi-insulating GaAs

We present an experimental study of slow domains in semi-insulating GaAs using time-resolved, two-dimensional electro-optic voltage probing. A detailed analysis of the shape of the bulk domains proves that they are formed by symmetric charge dipoles made up from ionized traps. The study of the dependence of the domain parameters on the applied voltage allows us for the first time to reconstruct experimentally the bulk j(E)-characteristic of SI-GaAs and to show that prevailing theoretical models fail to explain the observations. The experimentally determined field dependence of the free-carrier concentration reveals a strongly field-enhanced trapping with no critical field. The drift velocity of domains is experimentally found to be exclusively determined by the free-carrier concentration at the centre of the domain.