Apr 18, 2019

Origin of a fourfold symmetric (0 0 0 6) Bragg diffraction intensity in -scan mode on a 6H-SiC crystal

The quality of silicon carbide (SiC), when used as a substrate, has profound effects on the growth of the homo/hetero-epitaxial film on it. Here, a fourfold symmetric (0006) x-ray diffraction intensity in a phiv-scan mode is found to have nothing to do with the deformation of wafers or the existence of mosaic domains, regardless of whether it is a double-sided polished 6H-SiC wafer or a thick 6H-SiC slice. The experimental results show that both the diffraction intensity and its full width at half maximum as a function of the azimuth angle phiv exhibit the features of four peaks and four valleys regularly. By measuring the bending of the diffraction planes along the azimuth angles at the peaks and valleys, saddle-shaped deformed (0 0 0 1) atomic planes of the 6H-SiC in macroscopic scale are hypothesized. Based on the hypothesis, a model analysis of the diffraction intensity matched well with the observed anisotropic symmetric x-ray diffraction in a 6H-SiC single crystal.


Source:IOPscience
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Apr 9, 2019

Phase transformation of 6H-SiC at high pressure: An ab initio constant-pressure study

We apply a constant-pressure ab initio technique to investigate the high-pressure phase transformation of 6H-SiC and show that it transforms into a rocksalt structure. This phase change proceeds in two stages: 6H-SiC is first compressed along the c-direction and then it undergoes a shear deformation on the a-b planes. This transformation mechanism is quite similar to that of the wurtzite-to-rocksalt observed in 2H-SiC but there is no metastable phase identified along this path. The 6H-to-RS phase transition is also analyzed from the energy volume calculations. The computed transition parameters agree well with the experimental data.


Source:IOPscience
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Apr 3, 2019

Fabrication of Ti ohmic contact to n-type 6H-SiC without high-temperature annealing

The effect of surface morphology of 6H-SiC substrate on the ohmic contact properties of Ti/6H-SiC structure is studied. The H-terminated surface on Si-face 6H-SiC is obtained by both dipping SiC into HF acid solution for 15 s and thermal heating SiC in hydrogen atmosphere at 1100 °C for 10 min, while the H-terminated surface on C-face 6H-SiC could be obtained only by the latter method. Ti is deposited on Si-face and C-face SiC substrates with H-terminated surfaces and ohmic contact is obtained without high-temperature annealing.


Source:IOPscience
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Mar 25, 2019

Current status of self-organized epitaxial graphene ribbons on the C face of 6H–SiC substrates

The current status of long, self-organized, epitaxial graphene ribbons grown on the (0 0 0 −1) face of 6H–SiC substrates is reviewed. First, starting from the early stage of growth it is shown that on the C face of 6H–SiC substrates the sublimation process is not homogeneous. Most of the time it starts from defective sites, dislocations or point defects, that define nearly circular flakes surrounded by bare SiC. These flakes have a volcano-like shape with a graphite chimney at the centre, where the original defect was located. At higher temperatures a complete conversion occurs, which is not yet homogeneous on the whole sample. This growth process can be modified by covering the sample with a graphite cap. It changes the physics of the surface reconstruction during the Si-sublimation process and, on the C face, makes more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of strongly step-bunched areas with, in between, long and large reconstructed terraces covered by graphitic material. Despite the low intrinsic optical absorption of a few graphene layers on SiC, micro-transmission experiments, complemented by micro-Raman spectroscopy, demonstrate that most of this graphitic coverage is made of one or two homogeneous graphene layers. We show also that most of the thermal stress between the graphene layer and the 6H–SiC substrate is relaxed by pleats or wrinkles which are clearly visible on the AFM images. Finally, the results of transport experiments performed on the graphitic ribbons reveal the p-type character of the ribbons.


Source:IOPscience
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Mar 18, 2019

Stress in (Al, Ga)N heterostructures grown on 6H-SiC and Si substrates byplasma-assisted molecular beam epitaxy

The paper describes experimental results on low temperature plasma-assisted molecular beam epitaxy of GaN/AlN heterostructures on both 6H-SiC and Si(111) substrates. We demonstrate that application of migration enhanced epitaxy and metal-modulated epitaxy for growth of AlN nucleation and buffer layers lowers the screw and edge(total)threading dislocation (TD) densities down to 1.7108 and 2109 cm-2, respectively, in a 2.8-μm-thick GaN buffer layer grown atop of AlN/6H-SiC. The screw and total TD densities of 1.2109 and 7.4109 cm-2, respectively, were achieved in a 1-μm-thickGaN/AlNheterostructure on Si(111). Stress generation and relaxation in GaN/AlN heterostructures were investigated by using multi-beam optical stress sensor (MOSS) to achieve zero substrate curvature at room temperature. It is demonstrated that a 1-μm-thick GaN/AlN buffer layer grown by PA MBE provides planar substrate morphology in the case of growth on Si substrates whereas 5-μm-thick GaN buffer layers have to be used to achieve the same when growing on 6H-SiC substrates.



Source:IOPscience
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Mar 12, 2019

High-quality AlN growth on 6H-SiC substrate using three dimensional nucleation by low-pressure hydride vapor phase epitaxy


There is a method of controlling nucleation and lateral growth using the three-dimensional (3D) and two-dimensional (2D) growth modes to reduce the dislocation density. We performed 3D–2D-AlN growth on 6H-SiC substrates to obtain high-quality and crack-free AlN layers by low-pressure hydride vapor phase epitaxy (LP-HVPE). First, we performed 3D-AlN growth directly on a 6H-SiC substrate. With increasing V/III ratio, the AlN island density decreased and the grain size increased. Second, 3D–2D-AlN layers were grown directly on a 6H-SiC substrate. With increasing the V/III ratio of 3D-AlN, the crystalline qualities of the 3D–2D-AlN layer were improved. Third, we performed 3D–2D-AlN growth on a trench-patterned 6H-SiC substrate. The crack density was reduced to relax the stress by voids. We also evaluated the threading dislocation density by using molten KOH/NaOH etching. As a result, the estimated edge dislocation density of the 3D–2D-AlN sample was 3.9 × 108 cm−2.



Source:IOPscience
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Mar 5, 2019

Electrical properties and microstructural characterization of Ni/Ta contacts to n-type 6H–SiC

 A Ni/Ta bilayer is deposited on n-type 6H–SiC and then annealed at different temperatures to form an ohmic contact. The electrical properties are characterized by I–V curve measurement and the specific contact resistance is extracted by the transmission line method. The phase formation and microstructure of the Ni/Ta bilayer are studied after thermal annealing. The crystalline and microstructure properties are analyzed by using glance incident x-ray diffraction (GIXRD), Raman spectroscopy, and transmission electron microscopy. It is found that the transformation from the Schottky to the Ohmic occurs at 1050 °C and the GIXRD results show a distinct phase change from Ta2C to TaC at this temperature. A specific contact resistance of 6.5× 10−5 Ωcm2 is obtained for sample Ni(80 nm)/Ta(20 nm)/6H–SiC after being annealed at 1050 °C. The formation of the TaC phase is regarded as the main reason for the excellent Ohmic properties of the Ni/Ta contacts to 6H–SiC. Raman and TEM data reveal that the graphite carbon is drastically consumed by the Ta element, which can improve the contact thermal stability. A schematic diagram is proposed to illustrate the microstructural changes of Ni/Ta/6H–SiC when annealed at different temperatures. 



Source:IOPscience
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