Optical Characterization of GaN Films Grown on (0001) Sapphire Substrate

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ABSTRACT The optical properties of single crystal hexagonal GaN films grown on (0001) sapphire substrate by metalorganic chemical vapor deposition were investigated. The energy gap of hexagonal GaN was determined as 3.39 and 3.400 eV by optical transmission and photoreflectance, respectively. The refractive index of GaN as a function of photon energy was drawn from the transmission spectrum. Furthermore, Raman scattering spectra were employed to study the phonon modes of the GaN film. The properties of LO phononplasmon coupled modes were further studied, and the carrier concentration and damping constant were determined by line-shape fitting of the coupled modes.

INTRODUCTION The wide-gap semiconductors of the nitrides belonging to the III-V family, such as GaN, AIN and their alloys, have received enormous attention for their practical use in both optoelectronic and electronic devices[1,2]. GaN has a direct wide energy band gap as well as notable thermal and chemical stability, and these attractive properties make it not only ideally suitable for fabricating blue and ultraviolet (UV) light emitting diodes and detectors, but also for application in harsh environments, such as at high temperature[3,4]. With the substantial improvement in epitaxy growth technology, high quality GaN films on sapphire substrate have been fabricated by metalorganic chemical vapor deposition (MOCVD)[4-6], and other techniques[7,8]. Therefore, high performance devices, including blue light emitting diodes and UV detectors, have been demonstrated[2]. In spite of impressive technological achievements of the last three years, there are still some problems concerning basic physical properties of this compound which have to be solved. Many studies have been reported on the theoretical calculation of the energy band structure of GaN and on the experimental characterization of its optical properties. However, most experimental studies were still concentrated on the photoluminescence and absorption of GaN[5,7,9]. In this paper, we studied the room temperature structural and optical properties of single crystal hexagonal GaN films grown on (0001) sapphire substrate by MOCVD. Photoreflectance was employed to determine the bandgap of GaN film as an additional optical technique, and the optical absorption edge of 3.39 eV confirmed it. For studying the phonon modes of the GaN film, Raman scattering was performed, and the properties of LO phonon-plasmon coupled modes were further studied. As a result, the carrier concentration and damping constant were determined by line-shape fitting of the coupled modes with considering the dominant scattering mechanisms of deformationpotential and electro-optic interaction. 747 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society

EXPERIMENT In our work, single crystal hexagonal GaN films were grown on (0001) sapphire substrate with an AIN buffer layer by MOCVD. The Ga, Al (for the AIN buffer layer) and N source gases were trimethylgallium (TMGa), trimethylaluminum (TMAI), and ammonia (NH 3), respec