Equations governing scans along arbitrary directions in reciprocal space were developed and used to map reciprocal lattice points (RLPs) with radial raster patterns to study mosaic structure in GaN thin films deposited on semi-insulating 4H-SiC substrates using AlN nucleation layers (NLs). The films were grown by molecular beam epitaxy, keeping the GaN growth conditions the same, but using different AlN NL growth conditions. Mosaic tilt angles determined from symmetric RLP breadth measurements were similar for all samples measured, consistent with screw and mixed dislocation densities determined from transmission electron microscopy (TEM) measurements. Mosaic twist was determined using off-axis skew-symmetric high resolution x-ray diffraction measurements of asymmetric RLP breadths, yielding results consistent with grazing incidence in-plane x-ray diffraction twist measurements. A clear correlation between the twist angle and the edge and mixed dislocation densities determined by TEM was not observed, warranting careful consideration of dislocation structure.
The successful exfoliation of 200 nm GaN layers from 600 nm GaN layers grown on sapphire substrates was achieved using &' implantation and subsequent annealing. We demonstrate that the extent of exfoliation depends not only on the implant dose, but also on the crystallinity of the GaN layer. Extended defects are not uniformly distributed over the wafer, so certain areas showed much more pronounced exfoliation than other areas.The exfoliation of GaN hyers using a light ion (hydrogen or helium) implantation step may offer a means to integrate thin layers of GaN and related materials with other materials. The implantation of hydrogen to split off a thin layer of a semiconductor material has been demonstrated for many different materials, with significant attention paid to silicon and the zinc-blende 111-V ma& rials In such studies, the implant parameters (e.g., dose, energy, temperawe) are assessed in terms of the rate at which blisters form on the surface after subsequent annealing. The blister evolution provides information about conditions for successful layer transfer to another substrate.Previous studies of hydrogemimplanted GaN have demonstrated that the onset of blistering depends strongly on the implant conditions although there is some disagreement about those conditions. The implant temperature has been reported to be > 250 "C for blistering to take place,'whereas blistering was reported for lower implant temperatures (20 "C) but only high doses (2 5x10'' ern.')? Given the vast amount of literature concerning the interaction of extended defects in GaN with hydrogen, we addressed the role of crystallinity on blister fmation.The wafers consisted of 600 nm GaN layers grown on an AIN hffer layer on sapphire substrates by molecular beam epitaxy. Some wafers also included an AlGaN cap layer. With an implantation energy of 60 keV (to provide a projected range depth of about 200 nm), HC doses as low as 2 . 5~1 0 '~ cm-2 were employed and the wafers were held at room temperature. Changes to the strain and to the msaic tilt in the GaN layers were determined using triple axis x-ray scattering and double crystal x-ray topography and the blistering was observed using atomic force microscopy and Nomarski interference contrast microscopy. Subsequent annealing at 400 -500 "C was employed to induce exfoliation.Blistering was observed for a much lower implant dose ( 2 . 5~1 0 '~ cm2, the lowest employed in this study) than had been previously reported' The onset of blistering under these conditions occurred upon annealing at 425 "C. For a slightly higher dose ( 5~1 0 '~ cm-'), the onset of blistering occurred at 300 "C. A compressive strain was measured in the implanted layers after annealing. After blistering, the strain was reduced. However, for samples that did not show blistering (e.g. dose of 2 . 5~1 0 '~ cm-* annealed at 300 "C), the implant-induced strain was not relieved.
This study addressed how defects in SiC substrates influence the crystallographic properties of AlGaN/GaN layers deposited by metallorganic vapour phase epitaxy and by molecular beam epitaxy. We employed double crystal reflection x-ray topography using symmetric (0008) and (00012) reflections with CuKα radiation (λ = 1.54 Å) to image dislocations, micropipes, and low angle boundaries in SiC substrates. Lattice strain near the core of a micropipe defect was estimated to be of the order of 10−7. The substrates investigated exhibited radial patterns of strain and, primarily, of tilt of the order of tens of arcsec.After deposition of the AlGaN and GaN layers, DCXRT images were generated from the substrate (0008) or (00012) and GaN epitaxial layer (0004) reflections. Full-width at half-maximum values ranging from ∼100 to 300 arcsec were typical of the GaN reflections, while those of the 4H–SiC reflections were ∼20–70 arcsec. Micropipes, tilt boundaries, and inclusions in the SiC were shown to produce structural defects in the GaN layers. A clear correlation between SiC substrate defects and GaN defects has been established.
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