Molecular Dynamics Simulations of Dislocations in TlBr Crystals under an Electrical Field
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Molecular Dynamics Simulations of Dislocations in TlBr Crystals under an Electrical Field X. W. Zhou1, M. E. Foster1, P. Yang2, and F. P. Doty1 1 Sandia National Laboratories, 7011 East Avenue, Livermore, CA 94550, U.S.A. 2 Sandia National Laboratories, 1515 Eubank SE, Albuquerque, NM 87185, U.S.A. ABSTRACT TlBr crystals have superior radiation detection properties; however, their properties degrade in the range of hours to weeks when an operating electrical field is applied. To account for this rapid degradation using the widely-accepted vacancy migration mechanism, the vacancy concentration must be orders of magnitude higher than any conventional estimates. The present work has incorporated a new analytical variable charge model in molecular dynamics (MD) simulations to examine the structural changes of materials under electrical fields. Our simulations indicate that dislocations in TlBr move under electrical fields. This discovery can lead to new understanding of TlBr aging mechanisms under external fields. INTRODUCTION TlBr crystals have the potential to surpass CdZnTe to become the leading semiconductor for radiation detection [1,2,3,4]. The limiting factor for this transition is the rapid degradation of TlBr under operating electric fields, which gives a device lifetime of only hours to weeks [5,6,7,8,9,10,11]. Interestingly, quantum mechanical studies [12,13,14] indicated that to account for the rapid aging of TlBr using the widely-accepted vacancy migration mechanism, the vacancy concentration must be many orders of magnitude higher than any conventional estimates. The objective of the present work is twofold: (a) incorporate a new analytical variable charge model in MD simulations; and (b) perform MD simulations of dislocations in TlBr crystals under an electrical field to gain understanding of aging mechanisms. THEORY 1. Variable Charge Model Our simulations employ the polymorphic modified Stillinger-Weber potential [15]. The electrostatic forces by the external electric field are separately treated and are superimposed to the atomic forces. These electrostatic forces are defined as the product of the atomic charge and the field. Hence, a model describing atomic charges as a function of environment is needed. Based on the fundamental physics of electronegativity [16,17], atoms can only become positively (or negatively) charged when they approach atoms with higher (or lower) electronegativity. Hence, identical atoms are charge neutral, and magnitude of charges increases when spacing between atoms with different electronegativities decreases. To capture this physics, we first derive an analytical charge expression for an undisturbed (i.e., no thermal vibration and no defects) binary system based on the literature variable charge concepts [18,19], and then generalize the expression for disturbed systems. As will be seen below, this approximate model is good enough to capture accurately the charges from ab initio calculations.
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