Band Structure Effects on the Transient Electron Velocity Overshoot in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurtzite GaN. The velocity overshoot increases with electric field up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. However, theoretical Monte Carlo calculations incorporating a GaN full-zone band structure show that the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (> 325 kV/cm). Insight into this behavior can be gleaned from the band nonparabolicity deduced from the constant energy surfaces in the Γ valley, which shows that the effective mass in the c-direction can be viewed as becoming larger at high k values.