We employ an optically detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in an AlGaN/GaN heterojunction p-i-n diode to measure the electron velocity overshoot in GaN at room temperature. It has been found that electron velocity overshoot occurs at electric fields as low as 105 kV/cm, with the peak transient velocity increasing with E up to $320 kV/cm, at which field a peak velocity of 7.25 ร 10 7 cm/s is attained within the first 200 fs after photoexcitation. At higher fields, the increase in transit time with increasing field suggests the onset of negative differential resistance due to intervalley transfer. The existence of transient velocity overshoot at fields lower than the calculated peak steady-state velocity suggests that it occurs while the electrons are primarily in the G valley. Full zone Monte Carlo calculations imply that the overshoot is associated more with band nonparabolicity in the G valley than with intervalley transfer at fields less than 325 kV/cm, and, in conjunction with theoretical calculations employing a semiclassical transport model, confirm the importance of this nonparabolicity for the determination of the temporal shape of the transient velocity overshoot curves.
Transient electron velocity overshoot is often linked with negative differential resistivity in direct bandgap III-V semiconductors through the transferred electron effect, in which electrons initially accelerated in the high mobility central valley achieve sufficient energy to encounter the larger density of final scattering states in the higher mass satellite valleys that effect a drop in both, the transient and steady-state electron velocities. This description is well suited to semiconductors such as GaAs, for which the intervalley energy separation is small and the effective masses in the satellite valleys are much larger than that in the central valley, and transient velocity overshoot has been observed in such materials [1]. While multivalley analytical Monte Carlo calculations predict that transient velocity overshoot and negative differential resistivity involving primarily intervalley scattering should occur in GaN [2], it has also been shown theoretically that for semiconductors in which the energy difference between the G valley minimum and that of the lowest satellite valley is large, band nonparabolicity in the G valley may play an important role in the negative differential resistivity [3], as has been suggested for cubic GaN [4]. Moreover, transient electron velocity overshoot [5,6] and evidence of negative differential resistivity [5,7] have been observed experimentally for transport in the c-direction in wurtzite GaN at fields for which both, band nonparabolicity in the G valley and intervalley scattering, are expected to contribute.