Temperature-dependent activation energy and variable range hopping in semi-insulating GaAs

We measured resistivity in the range of 30-390 K on four semi-insulating low-temperature grown molecular-beam epitaxy GaAs samples. The growth temperature range was from 215 degrees C to 315 degrees C. Arrhenius fittings with T-1 and hopping fitting with T-1/4 do not permit us the definition of the temperature ranges controlled by band and hopping conduction, respectively. This leads to major errors in the calculation of both activation energies and hopping parameters. We have used the differential activation energy in order to clearly identify the temperature range for the different transport mechanisms. Hopping dominates at low temperatures and band conduction at high temperatures. In-between, a mixed conduction regime is observed. We introduce a criterion to clearly define the temperature range of hopping, band and mixed conduction. The lower temperature at which mixed conduction is identified decreases for samples with increasing growth temperature. Only the sample grown at 215 degrees C presents both forms of hopping conduction before entering the mixed conduction regime. Hopping parameters were obtained from the fittings of the differential activation energy and the values are in good agreement with the usual method of calculating them if the correct temperature range is used.