How dislocations affect transport

Abstract

Dislocations crossing a junction in HgCdTe have little effect on detector responsivity, but are known to reduce the zero bias impedance R_oA. and increase the leakage current, especially at low temperatures where R_oA is dominated by tunneling and generation/recombination processes. We have calculated the Coulomb and piezoelectric fields associated with dislocations in an attempt to interpret their effect on the junction’s transport properties. Dislocation electric fields can affect transport since they are superimposed on the built-in and applied junction fields which control the currents. The screening of the fields in the neutral region is consistent with the dislocations’ small effect on responsivity. Their impact in the space charge region is found to be significant and consistent with the nonlinear dependence of performance on dislocation density. The piezoelectric potential of the typical 60° dislocation in a sphalerite crystal, and the Coulomb potential 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 of the potential barrier. Because of the nonlinear dependence of junction currents on the barrier (or the junction potential), the angular variation of the currents does not vanish upon averaging. We find that the 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_oA at cryogenic temperatures and low dislocation density, and that the longer range piezoelectric potential may be important. We also find that superposing the potentials of neighboring dislocations, because of the nonlinear dependence of junction leakage currents on junction potentials may account for the observed nonlinearity of performance degradation with dislocation density as measured by etch pit density.