Spatial variation of activation energy in undoped high-resistivity bulk GaAs

In undoped semi-insulating GaAs grown by the liquid encapsulated Czochralsk i or vertical gradient freezing technique mesoscopic electrical nonuniformi ties correlated to the cellular structure of dislocations exist which are m ainly caused by the enrichment of the deep defect level EL2 in the dislocat ion-rich cell walls. In undoped GaAs crystals with a transition semi-insula ting/medium resistivity in the transition region, the resistivity fluctuati ons between cell walls and cell interiors are much more pronounced (up to t hree orders of magnitude) and must be caused by different donors. A point c ontact technique developed for the detection of such nonuniformities was us ed to measure activation energies separately in cell walls and in cell inte riors. In this way, it could be shown that also other defects or impurities than the EL2 are accumulated in the cell walls so that different donor spe cies dominate the electrical properties of cell walls and of cell interiors . (C) 1999 American Institute of Physics. [S0021-8979(99)00902-0].