Generally speaking, hydride vapor-phase epitaxy (HVPE) is the most commonly used technique to obtain free-standing GaN substrates, due to the high growth rate it produces and its low cost (Fujito et al., 2009). For short-wavelength laser diodes and for high-power and high-frequency devices, native GaN substrates have many advantages, such as their low level of current leakage and long lifetimes due to the high quality of the active epitaxial layer, accompanied by low dislocation densities and a lower level of lattice distortion, derived from homo-epitaxy (Liu et al., 2017 Sumiya et al., 2017 Han et al., 2018).
However, these devices are generally constructed on foreign substrates, such as SiC, Si, and patterned sapphire substrates (PSSs). GaN-based devices have experienced important development for applications in light-emitting diodes, radio frequency devices, and electronics (Sandvik et al., 2001 Chai et al., 2018 Li et al., 2018). This method may provide a practical route for fabricating free-standing GaN substrates at low cost with HVPE. The underlying mechanisms for the improvement of crystal quality were assessed. The full width at half-maximums for GaN (002) and GaN (102), respectively dropped from 245 and 412 to 123 and 151 arcsec, relative to those without the 3D GaN interlayer. The 250-μm thick GaN film showed an improved crystalline quality. With the addition of a 3D GaN interlayer, the crystal quality of the GaN epitaxial films was further improved.
It was found that the cracks in the epitaxial GaN layer could be effectively suppressed due to the large size and orderly orientation of the AlN nucleus caused by pre-annealing treatment.
Such combined buffer layer intentionally introduced a thin AlN layer, using a mix of physical and chemical vapor deposition at a relatively low temperature, a 3-dimensional GaN interlayer grown under excess ambient H 2, and a coalescent GaN layer. In this study, 2-inch free-standing GaN substrates with a thickness of ~250 μm were successfully fabricated on double-polished sapphire substrates, by taking advantage of a combined buffer layer using hydride vapor phase epitaxy (HVPE) and the laser lift-off technique. Free-standing GaN substrates are urgently needed to fabricate high-power GaN-based devices.
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