Photoreflectance Study of Gan Film Grown by Metalorganic Chemical Vapor Deposition
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and J.C. Chen**
*Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China "**Department of Electrical Engineering, University of Maryland Baltimore County, Baltimore, MD 21228-5398, USA
ABSTRACT Photoreflectance was used to study the optical properties of single crystal hexagonal GaN film on (0001) sapphire substrate grown by metalorganic chemical vapor deposition. The energy gap of GaN was determined as 3.400 eV, and the possible origin of the PR signal was attributed to the modulation of the surface field and lineshape broadening of defects. Optical absorption and cathodoluminescence of the GaN sample were measured, and the optical absorption edge of 3.39 eV and the cathodoluminescence emission peak of 3.461 eV at low temperature (15.6K) confirmed the results of Photoreflectance.
INTRODUCTION Recently, GaN-based III-V nitride semiconductors attract extensive attention for their potential device applications[1,2]. Among them, gallium nitride (GaN) is one of the most interesting materials which have a wurtzite structure in natural form, and have a wide direct band gap, which provides efficient radiation recombination. The attractive optical properties, together with the outstanding thermal and chemical stability of GaN, make it not only ideally suitable for fabricating the light emitting diodes and detectors operating in the blue and ultraviolet wavelength range, but also for application at high temperature and in hostile environments[3,4]. For most optoelectronic applications, the optical properties of GaN film is the key factor to influence the features of devices. Many studies have been performed on the theoretical calculation of the energy band structures of GaN, and on the experimental characterization of its optical properties. The energy gap of hexagonal GaN at room temperature was determined as -3.4 eV experimentally, while the theoretical calculation values ranged from 2.3 to 3.6 eV[5,6]. However, most experimental studies were still concentrated on the photoluminescence (PL) and absorption of GaN[7-9]. Currently, with the substantial improvement in epitaxy growth technology, high quality GaN films on sapphire have been fabricated by metalorganic chemical vapor deposition (MOCVD)[3,7], vapor-phase epitaxy (VPE)[10] and molecular-beam epitaxy (MBE)[8]. This makes it possible to use additional optical characterization techniques, such as photoreflectance (PR)[10], to study the optical properties of GaN. The derivative nature of PR suppresses uninteresting background effects and emphasizes structure localized in the energy region of interband transitions at critical points. The high sensitivity of PR even at room temperature and nondestructivity make it a powerful tool to study and characterize semiconductor energy band structures. In this paper, photoreflectance was used to study the optical properties near the bandgap of hexagonal GaN. The energy band gap of hexagonal GaN was determined as 3.39 and 735
Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
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