Progress in the Preparation of Aluminum Nitride Substrates from Bulk Crystals
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Progress in the Preparation of Aluminum Nitride Substrates from Bulk Crystals J.C. Rojo1, L.J. Schowalter1,2, Glen Slack1,2, K. Morgan1, J. Barani1, S. Schujman2, S. Biswas2, B. Raghothamachar3, M. Dudley3, M. Shur2,4, R. Gaska2,4 , N.M. Johnson5 , M. Kneissl5 1 Crystal IS, Inc., Latham, NY 12110 2 Rensselaer Polytechnic Institute, Troy, NY 12180 3 SUNY, Stony Brook, NY 11794 4 Sensor Electronic Technology, Inc., Latham, NY 12110 5 Palo Alto Research Center Inc. (PARC), Palo Alto, CA 94304 ABSTRACT Large (11-mm diameter) single-crystal AlN boules have been prepared using sublimationrecondensation growth. X-ray topography shows that substrates prepared from those boules have a dislocation density of less than 500 cm-2, while the central region of these substrates was nearly dislocation-free. Rocking curves of less than 10 arcsecs have been obtained indicating the high quality of these crystals. The AlN substrates have been used to growth an AlGaN/AlN multiquantum well structure with excellent crystalline quality and with photoluminescence peaked at around 260nm. In addition, a UV LED with emission wavelength at 360nm has been fabricated. This is the first operating opto-electronic device demonstrated on an AlN substrate. INTRODUCTION Wide bandgap semiconductor devices, based on III-nitride semiconductors, will dramatically recast several opto-electronics and electronics technologies in the areas of short wavelength emission and detection and high-power, high-frequency microwave devices. However, the nitride semiconductor device industry has not been able to fulfill the expectations of a new generation of cost-effective opto-electronic and electronic devices so far. The lack of a highquality bulk nitride substrate has been pointed out as one of the chief factors contributing to the absence of a mature III-nitride technology. The use of commercially available foreign substrates such as silicon carbide and sapphire has been demonstrated to have problems, which would be overcome by a homoepitaxial substrate. Thus, there are many efforts worldwide to grow bulk crystals of gallium nitride (GaN) and aluminum nitride (AlN). While some applications would benefit more from bulk GaN, AlN has also received attention as a candidate for III-nitride epitaxy applications due to its close lattice match, minimal differential thermal expansion, and high thermal conductivity compared to GaN. In addition, AlN is a more desirable substrate than GaN for device structures that require Al-rich nitride epitaxial layers such as solar-blind UV detectors, UV light sources and high-power microwave devices. They also provide good lattice matching particularly for AlGaN active regions with high Al concentrations as required for deepUV LEDs with emission wavelengths below 300 nm. Non-polar substrate orientations are also available using nitride substrates, which may allow improvements in the quantum efficiency of nitride-semiconductor LED’s and laser diodes by eliminating the spontaneous and piezoelectric polarizations. An important application of s
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