Effect of substrate offcut on AlGaN/GaN HFET structures on bulk GaN substrates

Abstract

Bulk GaN substrates promise to bring the full potential of nitride-based devices to bear since they offer a low thermal and lattice mismatched alternative to foreign substrates for epitaxial growth. However, due to the high cost and low availability of bulk GaN substrates, effects such as surface misorientation (offcut), surface polishing, and preparation of such substrates on subsequent epitaxy are still not well understood. As such, AlGaN/GaN heterostructures with nominal Al compositions of 25% were grown by MOCVD on semi-insulating bulk GaN substrates with offcuts ranging from 0.05 to 1.95° in the m-direction ( ) 0 1 10 to attempt to determine the optimal offcut for bulk GaN substrates for AlGaN-based HFET devices. X-ray diffraction (XRD) studies indicate that the Al composition does not vary with offcut, however reciprocal space mapping shows evidence of strain relaxation of the AlGaN in samples grown on substrates with offcut >1.1°. Additionally, we observed a minimum in sheet resistance of the 2DEGs for substrates with offcuts near 0.5°, arising from higher mobilities in these samples. Evidence of an optimal substrate misorientation is important for AlGaN-based devices grown on bulk GaN substrates.

AlGaN HEMT Structures Grown on Miscut Si(111) Wafers

“…Again, we observed only a slight difference between the samples grown on wafers with miscut angle ≤1°, but a dramatic degradation of both concentration and mobility was observed for sample M20. Sample M5 grown on the substrate with a miscut angle of 0.5° exhibited the highest mobility at both RT and 77 K. These findings are consistent with the results for the similar structures grown on vicinal (0001) sapphire [18] and bulk GaN [47] substrates. It is well known that interface roughness induces potential fluctuations that scatter electrons, resulting in reduced mobility.…”

“…Here, we emphasize that it is the Hall concentration that changes, while the conductivity concentration remains unchanged. However, the reason for the reduced 2DEG concentration in M20 is not completely clear, but it could be due to partial strain relaxation of the barrier layer, as reported in [ 47 ] for the samples grown on bulk GaN substrates with miscut angles >1.1°. The other possible reason is a lower polarization-induced charge density at the interface due to the different strain state of the GaN channel.…”

GaN Single Crystalline Substrates by Ammonothermal and HVPE Methods for Electronic Devices

“…Generally, the variation of off-cut across the surface should not be higher than 0.1 • . This is the main and basic requirement for promoting the step flow, controlling the composition of ternary alloys in the device layers, as well as the incorporation of dopants and unwanted impurities [26][27][28]. For a 2-inch wafer, the required value of radius of curvature should be higher than 15 m. In commercially available HVPE-GaN substrates, the radius is at the level of 5 m, but in the case of 2-inch Am-GaN, it is usually around 30 m. The off-cut uniformity is also important for preparing an epi-ready surface of a substrate.…”