Effects of adding methane on the growth and electrical properties of GaN in oxide vapor phase epitaxy.

GaN grown via oxide vapor phase epitaxy (OVPE-GaN) can produce free-standing substrates with ultra-low resistivity because of the high doping concentration of oxygen. The bulk growth of OVPE-GaN is hindered by polycrystals generated during long-term growth. We have previously reported that thicker films can be grown by reducing the partial pressure of water vapor in the growth atmosphere with CH₄. However, as CH₄ is a dopant of carbon, a compensating acceptor, its addition may increase electrical resistance. In this study, we further investigated the effect of reducing water vapor partial pressure on polycrystals by combining Ga₂O production (reaction of Ga and water vapor: a Ga–H₂O system), which can reduce water vapor, with CH₄ addition. However, CH₄ addition to the Ga–H₂O system increased polycrystal generation, possibly owing to the thermal decomposition of excess CH₄. The properties of OVPE-GaN with CH₄ addition were also evaluated. Although the CH₄ addition resulted in high carbon doping, the carbon-doped OVPE-GaN maintained low resistivity. This is because the OVPE method involves three-dimensional growth with growth pits, and the growth pits leave behind low-resistivity high-oxygen-concentration regions. As the resistivity remains low even when CH₄ is added in the OVPE method, both polycrystallization suppression and low resistivity can be achieved by selecting an appropriate CH₄ flow rate. [ABSTRACT FROM AUTHOR]