Experimental observation of electron velocity overshoot in AlN

The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on 6H-SiC was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through 80-nm-thick layers indicated the onset of quasiballistic transport for fields greater than 5 10 kV/cm. This was evidenced by a symmetric energy distribution centered at energies above the conduction band minimum. Drifted Fermi-Dirac energy distribution was fitted to the measured energy distribution, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80-nm was deduced. In addition, the velocity-field characteristic was derived from these observations. (C) 2002 American Institute Of Physics. [DOI: 10.1063/1.1534407].