GaN epilayers and AlGaN/GaN multiple quantum wells grown on freestanding [1100] oriented GaN substrates
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L4.2.1
GaN epilayers and AlGaN/GaN multiple quantum wells grown on freestanding [1 1 00] oriented GaN substrates C. Q. Chen,* M. E. Gaevski,* W. H. Sun,* E. Kuokstis,* J. W. Yang,* G. Simin,* M. A. Khan,* H.P. Maruska,** D. W. Hill,** M. M. C. Chou,** J. J. Gallagher,** B. H. Chai,** J. H. Song,*** M. Y. Ryu,*** P. W. Yu*** *Depart. of Electr. Engineering, University of South Carolina, Columbia, SC29208, USA **Crystal Photonics, Inc., Sanford, FL 32773, USA ***Dept. of Information and Communications, Kwangju Institute of Science and Technology, Kwangju, 500-712, Republic of Korea
Abstract We report on the homoepitaxial growth of GaN on freestanding [1 1 00] oriented GaN substrates using metalorganic chemical vapor deposition. A proper pretreatment of the substrates was found to be essential for the GaN homoepitaxy. The influence of growth conditions such as V/III molar-ratio and temperature on the surface morphology and optical properties of epilayers was investigated. Optimized pretreatment and growth conditions led to high quality [1 1 00] oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Based on these GaN epilayer, AlGaN/GaN multiple quantum wells have been grown on the freestanding M-plane GaN. Photoluminescence data confirm that built-in electric field for M-plane structures is very weak, and this situation results in a stronger PL intensity in comparison with C-plane multiple quantum wells in tests at low excitation level. Introduction Recently excellent progress has been made in the development of high-quality IIInitride optoelectronic and electronic devices.1,2 Nearly all the reported light emitting diodes (LEDs), laser diodes and high temperature and high power transistors employed III-nitride heterojunctions, which were deposited on the C-plane (0001) sapphire, 6H or 4H-SiC, or free-standing GaN substrates. All of these deposited (III-nitride) films and heterostructures feature the polar (0001) orientation, and hence they exhibit strong piezoelectric and spontaneous polarization induced fields.3,4 These polarization fields give rise to a significant band bending and thus reduce the overlap of the electron-hole wave functions. This situation significantly reduces the optical emission from quantum wells. To avoid these polarization effects, R-plane sapphire substrates have been used for III-nitride optical devices.5,6 Heteroepitaxy on sapphire, however, results in a large number (> 108/cm2) of threading dislocations, which counteract the beneficial effect of polarization field reduction on the optical emission intensity. Homoepitaxy of GaN and III-nitride heterostructures on non-polar M-plane [1 1 00] oriented bulk GaN substrates should in principle eliminate these problems.7-9 This serves as the motivation for our work reported here.
L4.2.2
Experiment Boules of LiAlO2 up to 200 mm long were pulled from the melt by the Czochralski method. GaN bulk cry
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