Abstract
Free-standing GaN substrates are urgently needed to fabricate high-power GaN-based devices. 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. 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 H2, and a coalescent GaN layer. 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. With the addition of a 3D GaN interlayer, the crystal quality of the GaN epitaxial films was further improved. The 250-μm thick GaN film showed an improved crystalline quality. 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 underlying mechanisms for the improvement of crystal quality were assessed. This method may provide a practical route for fabricating free-standing GaN substrates at low cost with HVPE.
Highlights
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)
It has recently been reported that the crystal quality of GaN film and related devices were improved when the LT-GaN buffer layer was replaced with a thin AlN buffer layer on both patterned sapphire substrates (PSSs) and plain sapphire substrate, due to the improvement of the crystal quality of the GaN nucleation layer (Chen et al, 2015, 2016)
The intensity of the (002) peak of LT-AlN film showed an obvious increase, which indicates an improvement for the AlN crystal quality, mainly due to obvious grain coarsening and orientation following annealing, as shown in Figure 2B (Okuno et al, 2013)
Summary
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). These devices are generally constructed on foreign substrates, such as SiC, Si, and patterned sapphire substrates (PSSs). 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). The AlN buffer layer can be used in a cost-effective way because it can be deposited either through a sputter or with physical vapor deposition (PVD), using cheaper appliances than MOCVD
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