Optical and Electrical Properties of Low to Highly-Degenerate InN Films
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Optical and Electrical Properties of Low to Highly-Degenerate InN Films D.B. Haddad,1 H. Dai,1 R. Naik,1 C. Morgan,1 V.M. Naik,2 J.S. Thakur,3 G.W. Auner,3 L.E. Wenger,4 H. Lu,5 and W.J. Schaff 5 1 Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201 2 Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, MI 48128 3 Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202 4 School of Natural Sciences and Mathematics, University of Alabama at Birmingham, Birmingham, AL 35294 5 Department of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14583
ABSTRACT The optical and electrical properties of InN films with different levels of carrier concentrations have been investigated. Hall effect measurements at room temperature show that the InN films are n-type with carrier concentration, ne, ranging from ~ 7 ×1017 cm–3 to ~ 3 × 1020 cm–3 and corresponding mobility, µ, of ~ 1300 to 50 cm2V-1s-1. Optical absorption spectra of these films show a bandgap absorption edge ~ 0.6 eV for the InN sample with the lowest ne, and 1.5 eV for the InN sample with the highest ne. However, after corrections for the degeneracy effects, all samples show an intrinsic Eg ~ (0.60 ± 0.05) eV. Temperature dependent (5 – 600 K) electrical measurements show that ne is nearly independent of temperature below 300 K, perhaps due to the presence of donor energy levels resonating with the InN conduction band. However, all the samples show an exponential increase in ne above 300 K due to excitation of other shallow donor like sources. Mobility versus temperature graph shows a maximum ~ 200 K for InN film with ne = 7 × 1017 cm–3 and moves towards lower temperature with increasing ne. INTRODUCTION Theoretical calculations indicate that InN material has excellent electron transport properties.1 As a result InN has been extensively investigated due to its potential applications to a wide range of fast and high frequency electronic devices. Recently, an international interest in the study of InN material has been motivated by the discovery of a narrow energy bandgap, ≤ 0.7 eV,2,3 which is inconsistent with the earlier accepted value of 1.9 eV. Although, it is generally accepted that the lower reported bandgap values (< 0.7 eV) are associated with improvements in the thin-film fabrication techniques leading to higher quality InN films with relatively less disorder, the true value of InN energy bandgap is still under debate. In this paper, we investigate the influence of the electron carrier concentration, ne, on the optical properties of InN films with different levels of carrier concentrations. The temperature dependent (5 – 600 K) carrier concentrations and mobilities for the different InN samples, determined by Hall effect, are also presented and discussed. EXPERIMENT Four samples, labeled Samples A, B, and C were grown by conventional Molecular Beam Epitaxy (MBE) at Cornell University,4 while sample D was grown by Plasma Source Molecular Beam Epitax
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