To study the effects of degradation of phonon distribution on the high frequency response in nano structures

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URI: http://hdl.handle.net/2042/6803
Title: To study the effects of degradation of phonon distribution on the high frequency response in nano structures
Author: Sarkar S.-K; Sarkar S.; Sahu P.K.; Kumar K.-S.; Rani S.
Abstract: low dimensional semiconductor nano structures, Frolich coupling, LO phonons , phonon lifetimeThere has been an intense interest in low dimensional semiconductor nano structures, because of subtle and profound changes that occurs, as length scale becomes comparable to the exciton Bohr radii and the dimensionality is reduced significantly.[1] In nano structures, hot electron conditions are developed if the applied electric field is sufficiently high so as to cause a pronounced deviation from Ohm’s law. At these fields, drift energy of electrons may be compared with the thermal energy at the lattice temperature. The average electron energy is also much higher than that in the thermal equilibrium with the lattice. Under such high electric fields, the carrier mobility exhibits a complicated dependence on the applied electric field. Under hot electron condition, the hot carriers, photon excited by ultra short pulses in polar semiconductors, initially loose energy rapidly by emitting longitudinal optic phonons via dominant Frolich coupling. Thus, the energy supplied to the carriers by the high electric field goes into phonon generation. As the phonon lifetime is long enough, phonon distribution is disturbed and a nonequilibrium population of LO phonons or hot phonons are produced, leading to their reabsorption by the carriers. The measurement of energy loss rate in low dimensional nano structures and other analysis show that the nonequilibrium LO phonons play an important role in slowing down the carrier cooling rate[2]. In the present work the high frequency performance of GaN nano structures is studied in the framework of heated, drifted, Fermi-Dirac distribution function incorporating the relevant scattering mechanisms and the influence of nonequilibrim LO phonons. The effects of large q-LO phonons and electron-LO phonon interaction at low qLZ (Lz is channel length ) are also studied. The ac mobility normalized by dc value is found to decrease with increasing frequency of the applied field beyond about 80GHz while the drift velocity lags behind the applied field above around about 20GHz. The f3dB cutoff frequency at which the ac mobility drops to 0.707 of its low frequency value is found to decrease with increasing LZ and carrier concentration. The weakening of scattering for a wider channel width, combined scattering mechanisms and the increasing degeneracy of the carriers at higher concentration account for such behaviour. However, it is always higher when degradation of phonon distribution is incorporated. This behaviour is attributed to the slowing down of the carrier energy and momentum loss rates in the presence of non equilibrium phonons[2]. Thus, we find that the degradation of phonon distribution enhances significantly the 3-dB cut-off frequency thereby makes the high frequency response flatter reflecting that high frequency response is better if effects of nonequilibrium phonon distribution are included in the calculations. References : 1. A. Majumdar, L. P. Rokhinson and D. C. Tsui : - Appl. Phys. Lett - 76, 3600 (2000) 2. A. S. Vengurlkar, S. S. Prabhu, S. K. Roy and J. Shah : Phy. Rev .B-50,461(1994)
Subject: low dimensional semiconductor nano structures, Frolich coupling, LO phonons , phonon lifetime
Publisher: TIMA Editions , Grenoble, France
Date: 2006

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