Nonlinear infrared photoconductivity in Ge doped with As or Zn

We observed two kinds of nonlinear excitation intensity dependence of infrared photoconductivity in Ge-doped with As or Zn. The dependence in Ge: As is superlinear because of the impact ionization of As donor induced by the electric field of excitation light, while that in Ge: Zn is sublinear because of saturation of photoionization of Zn acceptors. Fast photoconductivity observed under high excitation is discussed by the model of mobility change caused by intraband excitation. Both spectra reveals phonon structures on the top of monotonic decrease with increasing photon energy.