Growth of Oriented Thick Films of Gallium Nitride from the Melt
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ABSTRACT While significant strides have been made in the optimization of GaN-based devices on foreign substrates, a more attractive alternative would be homoepitaxy on GaN substrates. The primary motivation of this work is to explore the growth of thick films of GaN from the melt for the ultimate use as substrate material. We have previously demonstrated the synthesis of polycrystalline, wurtzitic gallium nitride and indium nitride by saturating gallium metal and indium metal with atomic nitrogen from a microwave plasma source. Plasma synthesis avoids the high equilibrium pressures required when molecular nitrogen is used as the nitrogen source. Here we report the growth of thick oriented GaN layers using the same technique by the introduction of (0001) sapphire into the melt to serve as a substrate. The mechanism of this growth is not established, but may involve transport of the metal as a liquid film onto the sapphire and subsequent reaction with atomic nitrogen. The films were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. X-ray diffraction showed that the GaN films were oriented with their c-axes parallel to the sapphire c-axis. The TEM analysis confirmed the orientation and revealed a dislocation density of approximately 1010 cm-2. The E2 Raman active phonon modes were observed in the GaN films. INTRODUCTION In spite of remarkable achievement in the growth and processing of gallium nitride and related III-nitride semiconductor devices, improvement of the crystalline quality of heteroepitaxially grown GaN films is still an important issue. Extended structural defects, such as dislocations, are a problem in films grown on sapphire and SiC substrates, even with the aid of buffer layers. Vertical conduction through the substrate and buffer layers is also an issue for devices. Significant reduction in the concentration of threading dislocations has recently been achieved through the use of epitaxial lateral overgrowth (ELOG) on (0001) sapphire and SiC substrates [1,2]. However, the ideal solution to these problems would be homoepitaxy on high quality, bulk GaN substrates. The search for a viable method for production of GaN substrates remains open. Bulk growth techniques have succeeded in growing low-dislocation ( 15 kbar), but the area of the crystallites remains about 1 cm2 [3,4]. This size
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limitation has inspired several groups [5,6] to pursue hydride vapor phase epitaxy (HVPE) as a quasi-bulk approach to GaN growth yielding growth rates as high as 100 µm/hr. However, cracking is a common problem for HVPE growth of thick GaN overlayers over large areas, e.g., 2 inch (0001) sapphire wafers. In our initial experiments, we showed that GaN and InN can be grown at sub-atmospheric pressures without the a
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