The Influence of Substrate Surface Polarity on Optical Properties of GaN Grown on Single Crystal Bulk AlN
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The Influence of Substrate Surface Polarity on Optical Properties of GaN Grown on Single Crystal Bulk AlN G. Tamulaitis1,5, I. Yilmaz1, M. S. Shur1, R. Gaska2, C. Chen3, J. Yang3, E. Kuokstis3, A. Khan3, J. C. Rojo4, L. J. Schowalter4 1 Department of ECE and CIE, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A 2 Sensor Electronic Technology, Inc., Latham, NY 12110, U.S.A. 3 Department of EE, University of South Carolina, Columbia, SC 29208, U.S.A. 4 Crystal IS, Inc., Latham, NY 12110, U.S.A. 5 IMSAR, Vilnius University, Sauletekio 9-III, Vilnius, Lithuania ABSTRACT Photoluminescence of the GaN layers grown both on N-face and Al-face bulk AlN is studied under CW and pulsed laser excitation in the temperature range from 8 K to 300 K. We compare localization of excitons, residual strain, and activation energies for thermally activated transfer of carriers to nonradiative recombination. At high excitation intensities, conditions for carrier heating, which is important for the threshold of stimulated emission, are also investigated. INTRODUCTION Large lattice mismatch of GaN epilayers with available substrate materials is one of the most problematic issues in GaN/AlN/InN based technology. Growth of GaN on c-plane of the sapphire substrates (lattice mismatch ~15%) is made possible by the deposition of AlN or lowtemperature GaN buffer layers serving for nucleation of the subsequent GaN epilayer of comparatively high structural quality. 6H SiC has a smaller but still substantial lattice mismatch of 3.5%. The most radical solution of the problem is homoepitaxy. Homoepitaxial GaN layers deposited on bulk GaN substrates exhibit outstanding structural and optical quality [1] and allow fabrication of heterostructures with high mobility [2]. The conventional high-pressure method of bulk GaN growth [3] is still an expensive technology allowing production of only small-size single crystals. Recent development of large free-standing GaN substrates grown by hydride vapor phase epitaxy [4] brought in a new impact to homoepitaxy of GaN. The need for deep-UV light emitters and detectors for chemical and biological sensing, photochemistry, and nanotechnology stimulates homoepitaxy on AlN, which has considerably a much larger band gap (6.2 eV) than that of GaN (3.41 eV). Passing more than two decades of stagnation after early development [5, 6], the growth technology for bulk AlN recently advanced its capacity to produce larger-size single crystals. AlN single crystals exceeding 1 cm size and having a density of dislocations less than 104 cm-2 have been produced [7] and utilized as substrates to deposit Al0.5Ga0.5N epilayers of high structural quality [8]. The further development of fabrication of multilayered structures on bulk AlN substrates requires deeper understanding of structural properties of interfaces and study of optical properties of layers deposited on this novel substrate material. The lattices of wurtzite-structure AlN and GaN have the space group P63mc ( C 6v4 ) and are compatible with a spontaneous polari
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