GaN Room Temperature Exciton Spectra by Photovoltaic Measurement
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ABSTRACT Exciton absorption peak has been clearly observed at room temperature by photovoltaic spectra, on a GaN layer grown on (0001)-plane Sapphire by metalorganic chemical vapor deposition. From the spectra, we obtained A and B exciton transition energies to be 3.401 eV, 3.408 eV, and the energy gap to be 3.426 eV in wurztite GaN, respectively. We have also performed photovoltaic measurements with various incidence angles of light, and observed the polarization behavior of exciton absorption in GaN. Finally, we used UV-polarizer to further confirm the polarization properties of GaN. In conjuction with previous room temperature photoreflectance measurements, this work provide direct and reliable assessment of the excitonic properties and crystal quality of GaN semiconductor layers.
INTRODUCTION GaN and related semiconductors have attracted more and more attention recently1-3. Beside their relative indifference to chemical aggressions and remarkable thermal stabilities, which make them much suitable for the fabrication of high-temperature/high-power devices, operating
in caustic environments, these material family with direct wide band gaps becomes the most distinguished candidate for UV light sources. After the rapid commercialization of GaN-based blue light emitting diodes4 , Nukamura et al. 5 have demonstrated the continuous-wave InGaNbased blue laser diode with lifetime of over 1000 hours and power of more than 2mW at room temperature recently 5 , marking the important breakthrough in the study and applications of nitride family as UV light sources. On the other hand, much efforts have been placed on the study of fundamental properties in GaN and its related compounds and microstructures 6-12 Among these fundamental properties, exciton effect is more important, especially in improving the efficiency of GaN light emitting diodes and laser diodes' 3 . Most of the previous experiments of the excitonic properties in nitrides, were measured at low temperatures - . Only a few authors reported the excitonic transitions in photoreflectance 7 12 at room temperature, by comparing the fitting results using exciton and interband absorption line shape, respectively. Since the exciton binding energy in GaN is more than 20meV 7 , and high quality materials can be grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy(MBE), it is expected to observe exciton absorption directly by optical spectroscopy with high sensitivity, at room temperature. Photovoltaic(PV) spectroscopy is a highly sensitive tool, which has been used successfully in the study of semiconductor materials . PV signal is proportional to a/ho, where a is the absorption coefficient and hu is the photon energy . X. Zhang et al. have studied photovoltaic effects in the GaN structures with p-n junctions . In this paper, room temperature 593 Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
photovoltaic spectra were measured on a GaN film grown on a sapphire substrate by MOCVD, and clear exciton absorption was obser
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