HOW DISLOCATIONS AFFECT TRANSPORT

Dislocations crossing a junction in HgCdTe have little effect on detec tor responsivity, but are known to reduce the zero bias impedance R(0) A and increase the leakage current, especially at low temperatures whe re R(0)A is dominated by tunneling and generation/recombination proces ses. We have calculated the Coulomb and piezoelectric fields associate d with dislocations in an attempt to interpret their effect on the jun ction's transport properties. Dislocation electric fields can affect t ransport since they are superimposed on the built-in and applied junct ion fields which control the currents. The screening of the fields in the neutral region is consistent with the dislocations' small effect o n responsivity. Their impact in the space charge region is found to be significant and consistent with the nonlinear dependence of performan ce on dislocation density. The piezoelectric potential of the typical 60 degrees dislocation in a sphalerite crystal, and the Coulomb potent ial of a dislocation crossing the junction plane other than normally, are angularly varying in the junction plane. Angular variation of the potentials can be qualitatively interpreted as an angular modulation o f the potential barrier. Because of the nonlinear dependence of juncti on currents on the barrier (or the junction potential), the angular va riation of the currents does not vanish upon averaging. We find that t he range of the Coulomb potential is too small to account for a major portion of the experimentally reported performance degradation but may be responsible for the reduction of R(0)A at cryogenic temperatures a nd low dislocation density, and that the longer range piezoelectric po tential may be important. We also find that superposing the potentials of neighboring dislocations, because of the nonlinear dependence of j unction leakage currents on junction potentials may account for the ob served nonlinearity of performance degradation with dislocation densit y as measured by etch pit density.