Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

Abstract

High-resistive GaN ([10 8 X cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and Al x Ga 1-x N (x & 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.

Effects of the AlN nucleation layer thickness on the crystal structures of an AlN epilayer grown on the 6H-SiC substrate

“…The much higher values for 1 ⊥ in the AlN epilayer for sample B and sample E were measured as −22.0 × 10 −4 ± 1 × 10 −5 and −26.4 × 10 −4 ± 6 × 10 −5 . This measured negative slope of the plots indicates the compressive strain experienced in a smaller grain size in the AlN epilayers for all of the samples [40]. From Table 3, the highest L ⊥ value (30.3 ± 0.3 nm) for AlN epilayers was measured on layers with the LT-AlN(NL) growth times of 120 s (sample C).…”

“…These imperfect layers manifest a mosaic structures consisting of many small hexagonal grains and the mosaic structures of the layers can be characterised by means of mean tilt (a tilt ), mean twist (a twist ) angles, and the average size of the mosaic blocks with lateral (L ) and vertical (L ⊥ ) coherence length [27][28][29][30][31][32][33][34][40][41][42][43][44]. The a tilt of the mosaic blocks are defined as the rotation of the mosaic blocks out of the blocks perpendicular to the surface normal, and the a twist as the inplane rotation around the surface normal [27][28][29][30][31][32][33][34][40][41][42][43][44]. The mosaic structure model of the crystals has been applied several times to III-nitride based material films [27,28,[40][41][42][43][44].…”

“…The a tilt of the mosaic blocks are defined as the rotation of the mosaic blocks out of the blocks perpendicular to the surface normal, and the a twist as the inplane rotation around the surface normal [27][28][29][30][31][32][33][34][40][41][42][43][44]. The mosaic structure model of the crystals has been applied several times to III-nitride based material films [27,28,[40][41][42][43][44]. The degree of mosaicity expressed by lateral, vertical coherence length, heterogeneous strain (1 ⊥ ), tilt and twist angle are important parameters in characterising the quality of the epitaxial films [31][32][33][34].…”

“…The mosaic structures parameter of mean tilt, twist angle, the average size of the sub-grains, and the inhomogeneous strain causes some broadening in the FWHMs of the rocking curve of an imperfect films. The mean a twist of the mosaic blocks can be obtained using FWHMs of ω-scan or Φ-scan of the XRD measurements [28,40,44].…”

“…The a twist values were determined using direct measurements or extrapolation methods. These methods consider the simultaneous presence of tilt and twist in the structures [28,40,42,44]. The a twist value was determined using some complicated calculations and fitting methods in which the functions are fitted to the data obtained from the measurement of ω-scans in skew geometry from reflections with increasing lattice plane inclination.…”