Scattering of Carriers from Doubly Charged Impurity Sites in Germanium

Abstract

The effect of doubly charged impurity sites on the mobility of carriers in Ge has been observed in both equilibrium and photo-conductivity measurements. For equilibrium studies Zn-doped samples were used. Zn is a double-acceptor impurity in Ge, introducing levels at about 0.03 ev and 0.09 ev from the valence band. Studies of the magnitude of mobility indicate that the scattering cross section for holes from doubly charged Zn sites is approximately four times that for holes from singly charged Ga sites.For $n$-type Ge crystals containing Fe, Ni, Mn, or Zn impurity sites, charged doubly negative by compensation, high light-level photoconductivity experiments indicate that an increase in the steady-state free electron concentration is associated with a corresponding increase in mobility. This mobility increase is attributed to a decrease in charge from two to one units at the sites of trapped holes. Its magnitude is consistent with a ${Z}^{2}$ dependence for charged impurity scattering. In heavily doped crystals, the mobility change represents an appreciable fraction of the conductivity change. On saturation of hole trapping (approximated in Fe- and Mn-doped samples), the concentration of trapped holes is observed to be approximately equal to the known double-acceptor impurity concentration. Thus hole trapping may be unambiguously identified with specific double-acceptor impurities.