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Investigation of Composition-dependent Optical Phonon Modes in AlxGa1-xN Epitaxial Layers Grown on Sapphire Substrates

Published online by Cambridge University Press:  31 January 2011

Jun-Rong Chen
Affiliation:
jrchen.eo95g@nctu.edu.tw, National Chiao Tung University, Department of Photonics and Institute of Electro-Optical Engineering, Hsinchu, 300, Taiwan, Province of China
Tien-Chang Lu
Affiliation:
timtclu@mail.nctu.edu.tw, National Chiao Tung University, Department of Photonics and Institute of Electro-Optical Engineering, Hsinchu, Taiwan, Province of China
Hao-Chung Kuo
Affiliation:
hckuo@faculty.nctu.edu.tw, National Chiao Tung University, Department of Photonics and Institute of Electro-Optical Engineering, Hsinchu, Taiwan, Province of China
Shing-Chung Wang
Affiliation:
scwang@cc.nctu.edu.tw, National Chiao Tung University, Department of Photonics and Institute of Electro-Optical Engineering, Hsinchu, Taiwan, Province of China
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Abstract

We reported the systematical study of optical properties of hexagonal AlxGa1-xN epitaxial films grown on c-sapphire substrate using metal-organic chemical vapor deposition. By performing Fourier transform infrared spectroscopy measurements, the high-frequency dielectric constants and phonon frequencies can be obtained by theoretically fitting the experimental infrared reflectance spectra using a four-phase layered model. The high-frequency dielectric constant of AlxGa1-xN varies between 4.98 and 4.52 for ε∝,│(polarization perpendicular to the optical axis) and between 4.95 and 4.50 for ε∝,//(polarization parallel to the optical axis) respectively when the aluminum composition changes from 0.15 to 0.24. Furthermore, from experimental infrared reflectance spectra of AlxGa1-xN films, a specific absorption dip at 785 cm-1 was observed when the aluminum composition is larger than 0.24. The dip intensity increases and the dip frequency shifts from 785 to 812 cm-1 as aluminum composition increases from 0.24 to 0.58. According to the reciprocal space map of x-ray diffraction measurements, the emergence of this dip could be resulted from the effects of strain relaxation in AlGaN epitaxial layers due to the large lattice mismatch between GaN and AlGaN epitaxial film.

Type
Research Article
Copyright
Copyright © Materials Research Society 2009

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