Pressure Dependence of Optical Transitions in In-rich Group III-Nitride Alloys
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Pressure Dependence of Optical Transitions in In-rich Group III-Nitride Alloys S. X. Li1,2, J. Wu1,2, W. Walukiewicz1, W. Shan1, E.E. Haller1,2, Hai Lu3, and William J. Schaff3 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, 2 Department of Materials Science and Engineering, University of California, Berkeley, California 94720 3 Department of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853 ABSTRACT The hydrostatic pressure dependence of the optical transitions in InN, In-rich In1-xGaxN (0 < x < 0.5) and In1-xAlxN (x = 0.25) alloys is studied using diamond anvil cells. The absorption edges and the photoluminescence peaks shift to higher energy with pressure. The pressure coefficient of InN is determined to be 3.0±0.1 meV/kbar. Together with previous experimental results, our data suggest that the pressure coefficients of group-III nitride alloys have only a weak dependence on the alloy composition. Photoluminescence gives much smaller pressure coefficients, which is attributed to emission involving highly localized states. This indicates that photoluminescence might not be an accurate method to study the pressure dependence of the fundamental bandgaps of group III-nitrides. INTRODUCTION In recent years, developments in short-wavelength light emitting devices have stimulated great interest in the properties of group-III nitrides. The recent discovery of the narrow bandgap of InN at 0.7 eV [1-5] has extended the range of the direct bandgaps of group III-nitride alloys into the near infrared. In the view of the newly determined InN band gap it has become necessary to reevaluate the composition dependencies of the major material parameters of group III-nitride alloys. The pressure coefficient is one of the parameters important for fundamental material physics and for device applications. To our knowledge, there have been only a few experimental studies on the pressure behavior of GaN [6-8], Ga-rich In1-xGaxN alloys [7, 9] and AlN [10]. A wide range of band gap pressure coefficients has been found even in the most extensively studied Ga-rich In1-xGaxN alloys [7,9]. There has been no report on hydrostatic pressure studies on the bandgap of InN and In-rich In1-xGaxN and In1-xAlxN alloys. In this paper we report investigations of the hydrostatic pressure behavior of the fundamental bandgaps of InN, In-rich In1-xGaxN (0 < x < 0.5) and In1-xAlxN (x = 0.25) alloys based on optical absorption and photoluminescence (PL) measurements. Together with previous experimental results, our data provide a general picture of the hydrostatic pressure behavior of the fundamental bandgap of group III-nitride alloys over a large range of compositions. EXPERIMENTAL DETAILS The samples used in this study were all Molecular-Beam Epitaxy (MBE) grown. The InN sample is a 7.5-µm thick single crystal film deposited on (0001) oriented sapphire with a GaN buffer layer (250 nm). In1-xGaxN (x = 0.17, 0.32, and 0.50) and In1-xAlxN (x = 0.25) films, 240 ~ 300 nm thick, were grown with an AlN bu
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