AlN/GaN Distributed Bragg Reflectors Grown via Metal Organic Vapor Phase Epitaxy using GaN Insertion Layers
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AlN/GaN Distributed Bragg Reflectors Grown via Metal Organic Vapor Phase Epitaxy using GaN Insertion Layers L. E. Rodak, J. Peacock, J. Justice, and D. Korakakis Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506 ABSTRACT Distributed Bragg Reflectors (DBRs) are an important component of various optoelectronic devices for ultra violet and visible wavelengths. In the III-Nitride material system, Aluminum Nitride (AlN) and Gallium Nitride (GaN) offer a large contrast in refractive index and are therefore well suited for fabricating DBRs with high reflectivity and wide bandwidths using relatively few periods. However, the large lattice and thermal mismatch leads to cracking in these heterostructures. In this work short period superlattice layers have been used to fabricate high reflectivity (> 94%) nitride based DBRs via Metal Organic Vapor Phase Epitaxy. Short period AlN/GaN superlattices containing three to four monolayers of GaN have been employed as the low refractive index layer in DBRs to minimize cracking. Using this technique, crack-free DBRs reflecting from 440-475 nm with up to 25 periods have been fabricated. The technique has been proven to be versatile and resulted in large area yield DBRs grown on a variety of different sapphire substrates. INTRODUCTION III-Nitride based Distributed Bragg Reflectors (DBR) remain of interest for optoelectronic device fabrication, including but not limited to, resonant cavity light emitting diodes[1,2] and vertical cavity surface emitting lasers [3]. As the III-Nitride materials continue to emerge as a strong candidate in ultra violet and visible emitter fabrication, integrated DBRs are still required to form small cavities and reduce processing requirements in device fabrication. Although pursued for many years, high quality III-Nitride based DBRs are still challenging to achieve due to variation in material properties and growth conditions [4]. In the III-Nitride system, AlN and GaN are well suited for DBR fabrication due to the large refractive index contrast [4] and well established growth techniques, however, these two binary alloys are also characterized by a large lattice and thermal mismatch [4] which gives rise to cracking in DBR structures. A well defined network of cracks has been observed in DBR growths via MOVPE [5] and also MBE [6] on sapphire substrates. Many techniques have been employed to reduce or eliminate the cracking, including the use of a less lattice mismatched layer such as AlxGa1-xN [7,8] or AlxIn1-xN [7], superlattice insertion layers [3], and surfactants [9]. AlxGa1-xN layers yield promising results in terms of crack elimination [8], however the Al concentration in the AlxGa1-xN layers is typically less than 40% resulting in a small refractive index contrast between the layers. As a consequence, approximately 35 or more pairs are required to achieve 95% reflectivity [8]. AlxIn1-xN is also a potential solution as it can be lattice matched to GaN [7], however the growth is very challenging
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