Growth and Fabrication of 2 inch Free-standing GaN Substrates via the Boule Growth Method
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Growth and Fabrication of 2 inch Free-standing GaN Substrates via the Boule Growth Method Drew Hanser, Lianghong Liu, Edward A. Preble, Darin Thomas, Mark Williams Kyma Technologies, Inc. Raleigh, NC 27617 U.S.A. ABSTRACT High quality, single crystal GaN substrates have been demonstrated using a boule growth process. Here we report on the crystalline boules that were formed during the growth process and their material characterization. Using hydride vapor phase epitaxy process, GaN crystals were grown at growth rates greater than 200 µm/hr. Boules greater than 3 mm thick were grown and processed into free-standing substrates. Rocking curve measurements using high-resolution Xray diffraction were performed on the substrates with FWHM values of 92 and 146 arcsec for the (002) and (102) reflections, respectively. Atomic force microscope images, etch pit studies, and transmission electron micrographs of the GaN material show high quality material quality with a dislocation density in the range of 5×106 to 1×107 cm-2. INTRODUCTION Gallium nitride based devices have inherent defect populations when grown heteroepitaxially. These misfit defects that are created during GaN growth on lattice mismatched substrates, such as sapphire and SiC, have been shown to diminish device performance.[1-7] Overgrowth techniques have been employed to reduce defect levels [1-11], however it is not clear that the added complexity of these techniques makes it possible to yield a better production process for GaN-based device manufacturing. It is expected that homoepitaxial growth on a native GaN substrate will have advantages by both reducing growth complexity and reducing GaN defect levels, particularly misfit dislocations. Hydride vapor phase epitaxy (HVPE) has been singled out as a promising technique to produce GaN boules because of both the relatively low cost and the high growth rates that are possible.[1,3,4,5] While HVPE GaN growth is also nucleated heteroepitaxially and does have the same defect issues that are described above, it has been shown that these defect levels drop with increasing growth thickness.[12] Therefore, the growth rates possible with HVPE enable higher GaN thicknesses, and the associated low defect levels, that are not economically possible with lower growth rate techniques. Additionally, through the formation of GaN seeds through HVPEgrowth, seeded bulk growth of GaN boules becomes a possibility. Boule growth of thick crystals enables a manufacturing process for GaN substrates that offers good wafer-to-wafer uniformity and can use conventional crystal processing methods, such as sawing, grinding, lapping, and polishing. Here, we present the results of our HVPE grown GaN characterization efforts for thin films and for thick boule growth, as well as growth rate accomplishments. Defect densities where evaluated using a variety of techniques, including atomic force microscopy (AFM), etch pit densities, micro-photoluminescense (micro-PL), transmission electron microscopy (TEM), and high-resolution x-ray diffrac
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