Steady-State and Transient Electron Transport in ZnO: Recent Progress
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Steady-State and Transient Electron Transport in ZnO: Recent Progress Walid A. Hadi1, Michael Shur2, Lester F. Eastman3, and Stephen K. O’Leary4 1 Department of Electrical and Computer Engineering, University of Windsor, Windsor, Ontario, Canada N9B 3P4 2 Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, U.S.A. 3 School of Electrical Engineering, Cornell University, Ithaca, New York 14853 U.S.A. 4 School of Engineering, The University of British Columbia, Kelowna, British Columbia, Canada V1V 1V7 ABSTRACT We briefly review some recent results on the steady-state and transient electron transport that occurs within bulk wurtzite zinc oxide. These results were obtained using an ensemble semiclassical three-valley Monte Carlo simulation approach. They showed that for electric field strengths in excess of 180 kV/cm, the steady-state electron drift velocity associated with bulk wurtzite zinc oxide exceeds that associated with bulk wurtzite gallium nitride. The transient electron transport that occurs within bulk wurtzite zinc oxide was studied by examining how electrons, initially in thermal equilibrium, respond to the sudden application of a constant electric field. These transient electron transport results demonstrated that for devices with dimensions smaller than 0.1 µm, gallium nitride based devices will offer the advantage, owing to their superior transient electron transport, while for devices with dimensions greater than 0.1 µm, zinc oxide based devices will offer the advantage, owing to their superior high-field steady-state electron transport. INTRODUCTION Zinc oxide (ZnO) is a direct-gap II-VI semiconductor that has become a focus of considerable attention in recent years [1]. With its wide energy gap, large polar optical phonon energy, and large intervalley energy separation, ZnO is expected to exhibit favorable electron transport characteristics [2]. A number of studies of the electron transport that occurs within ZnO have been reported over the years. In 1999, for example, Albrecht et al. [3] reported on Monte Carlo simulations of the steady-state electron transport that occurs within bulk wurtzite ZnO. Further Monte Carlo analyses of the steady-state electron transport that occurs within bulk wurtzite ZnO have been reported by Guo et al. [4] in 2006, by Bertazzi et al. [5] in 2007, and by Furno et al. [6] in 2008. Unfortunately, these Monte Carlo results have yet to produce a satisfactory consensus, there being large differences observed between the obtained velocityfield characteristics. Variations in the material and band structural parameter selections employed for these Monte Carlo simulations are responsible for the differences in the obtained results. In 2010, O’Leary et al. [7] reported further on the nature of the steady-state electron transport that occurs within bulk wurtzite ZnO. An analysis of the transient electron transport that occurs within this material was also presented. These results suggest that for devices with dim
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