Measurement of carrier transport and dynamics in wide bandgap semiconductors using femtosecond pump-probe techniques

We present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a p-i-n diode, and show how it may be used to determine the electron transit time, velocity overshoot, and velocity-field characteristic in GaN at room temperature. In a GaN homojunction p-i-n diode, the transit time drops with increasing electric field E in the intermediate field regime (50-100kV/cm), and the electron velocity possesses a weak, quasi-linear dependence on E attributed to polar optical phonon scattering. In the high field regime the transit time and the electron velocity gradually become independent of E. The peak electron velocity of 1.9 x10(7) cm/s, corresponding to a transit time of similar to2.5 ps across the 0.53 mum depletion region, is attained at similar to 225 kV/cm. The experimental results are in qualitative agreement with theoretical steady-state velocity-field characteristics found in the literature. A measurement of the high field (similar to 300 kV/cm) transient electron velocity overshoot was also performed using a semitransparent p-contact AlGaN/GaN heterojunction p-i-n diode. The peak electron velocity of 6.25 x 10(7) cm/s attained within the first 200 fs decays within 1 ps to a steady-state velocity of 3.2 x 10(7) cm/s in this improved device.