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

Abstract

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 - 100 kV/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 X 107 cm/s, corresponding to a transit time of approximately 2.5 ps across the 0.53 micrometers depletion region, is attained at approximately 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 (approximately 300 kV/cm) transient electron velocity overshoot was also performed using a semi-transparent p-contact AlGaN/GaN heterojunction p-i-n diode. The peak electron velocity of 6.25 X 107 cm/s attained within the first 200 fs decays within 1 ps to a steady-state velocity of 3.2 X 107 cm/s in this improved device.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.