The evaluation of Mott parameters by using the field dependent conductivity in amorphous germanium

The dc conductivity of amorphous germanium films, prepared by thermal evaporation in high vacuum, was investigated between 77 and 300 K as a function of applied electric field. The dc conductivity of these films obeys Mott's T^{- 1 4} variable range hopping conductivity law below 200 K for low (F < 10³ V/cm) applied electric fields. The electric field dependent variable range hopping conductivity data was analyzed using the models developed by Apsley and Hughes (1975) and by Pollak and Riess (1976). The two important parameters of hopping conduction, the localized wave function exponent α^-1 and the density of localized states at the Fermi level N(E_F), were calculated by using these models.