Concentration and Temperature Dependence of Impurity-to-Band Activation Energies

The present paper investigates the cause of the experimentally well-known reduction of impurity activation (ionization) energies in the intermediate doping range. It is shown that this many-body problem can be reasonably approximated by a one-electron screened-impurity approach. Application of this result to literature data on GaP(Zn) gives agreement with the experimental results provided that screening by ionized impurities is included; screening by free carriers alone is insufficient. The inclusion of the ionized impurity screening thus for the first time provides a quantitative explanation for the observed reductions in activation energies. A corollary of the screening effect is that the activation energy decreases with increasing temperature; use of this temperature dependence clarifies previous discrepancies between Hall and neutron activation values for the Zn concentration in GaP.