Research Area - Nitrides
Bulk Growth of GaN
- (C. Hemmingsson, G. Pozina, S. Khromov and B. Monemar )
Despite the rapid progress in the III-nitride technology, freestanding single crystalline GaN substrates are still a big issue. So far the III-nitride devices on the market are mainly grown on foreign substrates, like sapphire, SiC or Si. This leads to very stressed GaN layers with high density of defects due to the large lattice mismatch and difference in thermal expansion coefficient. Thus, freestanding GaN substrates of large area and with low density of defects are highly desirable for the next generation of III-nitride based devices. In order to fabricate freestanding GaN substrates with low stress and defect density, the HVPE growth technique have so far been most successful.
We are studying growth and self-separation of bulk GaN on c-oriented Al2O3 using low temperature GaN buffer layers (LT-GaN). The growth is done in a vertical HVPE reactor at atmospheric pressure where the carrier gases and precursors are delivered from the bottom. GaCl, NH3 and N2 or a mixture of N2 and H2 is used as gallium source, nitrogen source and carrier gas, respectively, in the experiments. The GaCl is synthesized in situ by flowing HCl over a quartz boat containing liquid gallium held at 700-850oC. In order to estimate the partial pressure of process gases at the substrate surface and assure laminar flow conditions, numerical simulations are used.
Before growth of the thick GaN layer, a low-temperature (LT) GaN buffer layers is deposited at 600 oC . The bulk GaN layer is subsequentially grown at 950-1000 oC with a growth rate about 200 µm/hr. We have obtained the highest structural quality of the bulk GaN (XRD w peak with a full width at half maximum of ~450 arcsec for a 20 µm thick GaN layer) using a LT-GaN buffer thickness about 100-200 nm. By studying the in-plane and out-of plane strain versus the LT buffer thickness of the layers, we observe that the strain in the layers is minimized using a LT buffer thickness about 100 nm-250 nm which also is the range where we obtain the highest structural quality of the GaN. By using an optimized LT-GaN buffer layer, thick free freestanding 2’’ GaN substrates can be manufactured by spontaneous self-separation.
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Last updated: 09/28/12