Observations of electron velocity overshoot during high-field transport in AlN
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Observations of electron velocity overshoot during high-field transport in AlN Ramón Collazo, Raoul Schlesser, Amy Roskowski, Robert F. Davis and Z. Sitar Department of Materials Science and Engineering, North Carolina State University, Raleigh, N.C. 27695-7919
ABSTRACT The energy distribution of electrons transported through an intrinsic AlN film 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. The electron energy distribution featured kinetic energies higher than that of completely thermalized electrons. Transport through 80 nm thick layers indicated the onset of quasi-ballistic transport. This was evidenced by symmetric energy distributions centered at energies above the conduction band minimum for fields greater than 530 kV/cm. Drifted FermiDirac energy distributions were fitted to the measured energy distributions, 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. This is the first experimental demonstration of this kind of transport in AlN. INTRODUCTION Transient transport is characterized by the onset of ballistic or velocity overshoot phenomena. It takes place in spatial lengths in the sub-micron range, and occurs in rapidly changing applied fields or immediately after the application of a field. In AlN, this phenomenon is expected to occur for transport lengths of less than 100 nm, and field strengths greater than 450 kV/cm.1 Ballistic transport can be simply described by the acceleration of an electron immediately after the application of a field: inertial effects limit the acceleration of the electron, while subsequent scattering events start randomizing the momentum and limiting the energy, thus causing the system to eventually reach a steady state. Nevertheless, a velocity overshoot with respect to the steady-state velocity is obtained before the steady state is reached. The energy relaxation time characterizes the time scale of this overshoot 2, which is usually of the order of tenths of picoseconds. Transport with velocity overshoot occurs between the limit of the ballistic transport and before it reaches a steady-state transport condition. The velocity in this regime is still higher than the steady-state velocity but slightly smaller than the ballistic velocity. In this transient regime, the energy increases progressively from the equilibrium energy (thermal energy) towards its steady-state value.2 The overshoot effect can be explained in the following way: the electron mobility decreases with increasing average
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carrier energy for most semiconductors; as a result, the instantane
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