Variable-range hopping conduction and magnetoresistance in Ge-on-GaAs films at low temperatures and high magnetic fields

The temperature dependence of resistance and magnetoresistance in single-crystal Ge films grown by vacuum deposition on semi-insulating GaAs(100) substrates has been studied. At room temperature, the films have a resistivity of 5⋅10−2 Ω⋅cm, a hole concentration of 5.6⋅1017 cm–3, and a hole mobility of 250 cm2/(V⋅s). In the temperature range of 1.5–20 K, the resistance follows an Efros–Shklovskii variable-range hopping model with an exponent of 0.45. This behavior results from electrostatic potential fluctuations and inhomogeneous impurity distribution, which localize charge carriers and form metallic droplets. The low-temperature transverse magnetoresistance is negative at magnetic fields of 0–3 T, caused by the overlap of electron wave functions in the droplets. At higher fields, positive magnetoresistance, which depends quadratically on the magnetic field, suppresses the negative contribution.