Growth and Characterization of Deep UV Emitter Structures Grown on Single Crystal Bulk AlN Substrates
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Growth and Characterization of Deep UV Emitter Structures Grown on Single Crystal Bulk AlN Substrates X. Hu1, R. Gaska1, C. Chen2, J. Yang2, E. Kuokstis2, A. Khan2, G. Tamulaitis3,5, I. Yilmaz3, M. S. Shur3, J. C. Rojo4, L. J. Schowalter4 1 Sensor Electronic Technology, Inc., Latham, NY 12110, U.S.A. 2 Department of EE, University of South Carolina, Columbia, SC 29208, U.S.A. 3 Department of ECE and CIE, Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A 4 Crystal IS, Inc., Latham, NY 12110, U.S.A. 5 IMSAR, Vilnius University, Sauletekio 9-III, Vilnius, Lithuania ABSTRACT We report on high Al-content AlGaN-based deep UV emitter structures grown over single crystal, slightly off c-axis (5.8 degrees) bulk AlN substrates. AlN/AlGaN multiple quantum well (MQW) structures with up to 50% of Al in the well material were grown by using low-pressure MOCVD and characterized by using X-ray, AFM, SEM and photoluminescence techniques. Two light sources, one at 213 nm wavelength for selective excitation of quantum well layers and another one at 193 nm to excite both wells and barriers, were exploited. A weak temperature dependence (from 8 K to 300 K) of the luminescence intensity and the absence of blue-shift of the luminescence peak with increasing excitation intensity pointed to a low density of localized states, in a good agreement with the X-ray data, which indicated very high quality of these MQW structures. The most striking result was observation of stimulated emission at wavelength as short as 258 nm in Al0.5Ga0.5N/AlN MQWs grown on bulk AlN single crystals. INTRODUCTION Deep-ultraviolet (UV) emitters have potential applications in chemical and biological agent detection, solid-state lighting, lithography and short range telecommunications. III-nitrides are the most promising candidates for the UV emitters. Penetration deeper into the UV region requires high molar fractions of Al in AlGaN structures. However, a higher aluminum content in AlGaN layers causes more defects, which result in the emitter performance degradation. These defects are mainly caused by strain induced by the lattice mismatch at the interfaces between the substrate and the epilayers and between the well and barrier materials in multi-quantum well (MQW) structures. In order to reduce the defect formation in these structures several methods such as growing lattice-matched heterostructures with AlInGaN compounds and using AlN/AlGaN superlattices on c-face sapphire substrate are utilized [1,2]. In this paper, we report on the growth and characterization of structural and optical properties of the AlN/AlGaN MQW structures grown on single crystal bulk AlN substrate. Bulk AlN is a perfect candidate as a substrate for III-nitrides due to its structural, thermal and chemical compatibility. Recently, this material has been used as a substrate in deposition of Al0.5Ga0.5N epilayers of high structural quality [3].
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EXPERIMENTAL DETAILS AlN/AlGaN MQW structures which contain AlxGa1-xN wells with the Al- content up to 50% were grown using low
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