Thermomodulated Reflectivity Spectra of G An/Sapphire Epilayer
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Y. Lia'b, Y. Lu b, H. Shenc, M. Wrabackc, M. Schurmana, L. KosZia, R. A. Stalla
aEmcore Corporation, Somerset, New Jersey 08873 bThe State University of New Jersey, Rutgers University, Piscataway, New Jersey 08855-0909 cArmy Research Laboratory at Fort Monmouth, New Jersey 07703-5601
ABSTRACT Thermomodulation spectra from the metalorganic chemical vapor deposition (MOCVD) grown GaN have been measured in the temperature range 20 K - 310 K. A theoretical model is established to explain the modulation spectrum by considering the modulation of epilayer thickness and dielectric constant. By performing the lineshape analysis, the bandgap energy and broadening parameter were determined in the temperature range. The nonlinear temperature coefficient (Varshni coefficient) of energy gap is measured to be 5.9x1 0-4 eV/K. The temperature dependence of broadening parameter is also measured for the first time. INTRODUCTION Wide bandgap group I nitrides are strong candidates for blue light-emitting diodes (LED) and lasers intended for high density optical storage and display technologies. GaN has a wurtzite structure in natural form, and has a direct bandgap of 3.4 eV at room temperature[l ]. In the past several years, significant progress has been made on GaN film quality, p-type doping control and growth methods. High efficiency blue LEDs are commercially available[2]. Many optical characterization techniques have been used to determine the optical properties of GaN[313]. The temperature dependence of the energy gap in GaN has been previously evaluated (PR) and photoreflectance experimentally by absorption[3-5], luminescence[6-10], studies[12,13]. Modulation spectroscopy has proven to be a powerful experimental technique for studying and characterizing the properties of group IV and II1-V semiconductors, both bulk and reduced dimensional systems[ 14-21 ]. Recently, W. Shan et al. [ 13] have reported on a PR study of GaN thin film. A. Giordana et al.[24] have performed PR and electrolyte electroreflectance (EER) measurements on GaN. These measurements were made at room temperature. In this paper, we report the first thermomodulation spectroscopy characterization of GaN. The temperature dependence of the energy gap in GaN thin films was obtained. The band gap temperature coefficient in the 20-310 K range is determined. The broadening parameter and its temperature dependence are also determined. EXPERIMENTAL The GaN samples used in this work were grown on c-A120 3 substrates by MOCVD in an EMCORE multi-wafer rotating disk reactor. A detail of growth process can be found elsewhere[25]. The epitaxial GaN layers were Wurtzite in crystal structure. The room 1 2 temperature electron concentration was 2x1016 cm3. The mobility was about 350 cm vAs-l . The
741 Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research Society
thickness was estimated to be about 1[tm from the cross section view of SEM. An interdigital finger pattern with a Ti/Au metallization layer was deposited on the surface of the sample by standard lift-off pro
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