Conduction and Magnetoresistance Mechanisms in Germanium Films Used for Low Temperature Resistance Thermometers

We present the investigation of conductance and magnetoresistance mechanism of Ge-on-GaAs films used for low-temperature resistance thermometers. At temperature below 10 K the main mechanism of conductivity in Ge films is the variable-range hopping. At low temperatures the magnetoresistance can be negative, and its magnitude increases with decreasing temperature. A new giant negative magnetoresistance effect in Ge films at ultralow temperature (T < 0.2 K) and low magnetic fields (H < 1 T) has been observed. This effect is very sensitive to temperature. At 0.03 K the Ge film resistance decreases up to 100 times for H = 1 Tesla. The effect saturates for fields above 1 Tesla. We discuss the physics of this phenomenon and present the results of calculation using a development of the hopping theory of conductivity with the localization corrections.