Jun 14, 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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Jun 5, 2019

Single crystal 6H-SiC MEMS fabrication based on smart-cut technique

A new single crystal silicon carbide (SiC) MEMS fabrication process is developed using a proton-implantation smart-cut technique. A 6H-SiC layer with 1.3 µm thickness has been achieved over an oxidized silicon substrate using the proposed technique. TEM analyses of the silicon carbide thin film reveal single crystal characteristics, which is attractive for potential integration of MEMS devices with high-temperature microelectronics in the same structural layer for harsh environment applications. Implant-induced defect density in the silicon carbide can be substantially reduced to a negligible level through high-temperature annealing. Prototype single crystal 6H-SiC MEMS devices, such as strain sensors, have been successfully fabricated as demonstration vehicles for future harsh-environment micro-system implementation.


Source:IOPscience
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May 28, 2019

The fabrication of suspended micromechanical structures from bulk 6H-SiC using an ICP-RIE system

We have successfully fabricated freestanding single-crystal 6H-SiC cantilevers of several microns size using bulk micromachining with an inductively coupled plasma reactive ion etching system. We have also used these SiC cantilever structures to measure Young's modulus through the use of an atomic force microscope. The measurements were performed on a series of 13 µm thick and 20 µm wide cantilevers with lengths ranging from 100 µm to 350 µm. The average measured Young's modulus of 441 GPa is in excellent agreement with the documented values in the literature.


Source:IOPscience
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May 23, 2019

Temperature-dependent interface reactions and electrical contact properties of titanium on 6H-SiC

n this study the system Ti-Si-C was investigated in terms of two aspects, metallurgical and electrical, in order to understand the formation and the properties of Ti electrical contacts on n-type 6H-SiC. For the metallurgical investigation bulk diffusion couples were prepared from monocrystalline 6H-SiC and Ti and annealed between 700 and 1200 degrees C for different lengths of time. The reaction zones were investigated using a SEM (secondary electron and backscattered electron images as well as energy-dispersive X-ray analysis). For the investigation of the electrical properties Ti contacts were sputter-deposited onto 6H-SiC wafer stripes and annealed at similar temperatures. The contact properties were measured in terms of current-voltage characteristics. We discovered that, over the whole temperature range investigated, the reaction layer growth follows a parabolic growth law which is thermally activated. Above 1200 degrees C the diffusion path from SiC to Ti is SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti5Si3/Ti. The contacts show ohmic behaviour. Between 1000 and 800 degrees C the diffusion path is: SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are also ohmic. Below 700 degrees C the diffusion path is SiC/TiC1-y/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are of Schottky type. It is concluded that the phase in contact with the SiC determines the electrical properties of the junction and the possibility of manufacturing a complete Schottky diode using n-type 6H-SiC and Ti only is demonstrated.


Source:IOPscience
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May 14, 2019

Experimental investigation of slow-positron emission from 4H-SiC and 6H-SiC surfaces

Slow-positron emission from the surfaces of as-grown n-type 4H-SiC and 6H-SiC (silicon carbide) with a conversion efficiency of ~ 10−4 has been observed. After 30 min of 1000 oC annealing in forming gas, the conversion efficiency of the n-type 6H-SiC sample was observed to be enhanced by 75% to 1.9 × 10−4, but it then dropped to ~ 10−5 upon a further 30 min annealing at 1400 oC. The positron work function of the n-type 6H-SiC was found to increase by 29% upon 1000 oC annealing. For both p-type 4H-SiC and p-type 6H-SiC materials, the conversion efficiency was of the order of ~ 10−5, some ten times lower than that for the n-type materials. This was attributed to the band bending at the p-type material surface which caused positrons to drift away from the positron emitting surface.


Source:IOPscience
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May 9, 2019

Model and simulations of the epitaxial growth of graphene on non-planar 6H–SiC surfaces

We study step flow growth of epitaxial graphene on 6H–SiC using a one-dimensional kinetic Monte Carlo model. The model parameters are effective energy barriers for the nucleation and propagation of graphene at the SiC steps. When the model is applied to graphene growth on vicinal surfaces, a strip width distribution is used to characterize the surface morphology. Additional kinetic processes are included to study graphene growth on SiC nano-facets. Our main result is that the original nano-facet is fractured into several nano-facets during graphene growth. This phenomenon is characterized by the angle at which the fractured nano-facet is oriented with respect to the basal plane. The distribution of this angle across the surface is found to be related to the strip width distribution for vicinal surfaces. As the terrace propagation barrier decreases, the fracture angle distribution changes continuously from two-sided Gaussian to one-sided power law. Using this distribution, it will be possible to extract energy barriers from experiments and interpret the growth morphology quantitatively.


Source:IOPscience
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Apr 30, 2019

Characteristics of one-port surface acoustic wave resonator fabricated on ZnO/6H-SiC layered structure

Characteristics of one-port surface acoustic wave (SAW) resonators fabricated on ZnO and 6H-SiC layered structure were investigated experimentally and theoretically. Phase velocities (V p), electromechanical coupling coefficients (K 2), quality factors (Q), and temperature coefficients of frequency (TCF) of Rayleigh wave (0th mode) and first- and second-order Sezawa wave (1st and 2nd modes, respectively) for different piezoelectric film thickness-to-wavelength (h ZnO /λ) ratios were systematically studied. Results demonstrated that one-port SAW resonators fabricated on the ZnO/6H-SiC layered structure were promising for high-frequency SAW applications with moderate K 2 and TCF values. A high K 2 of 2.44% associated with a V p of 5182 m s−1 and a TCF of  −41.8 ppm/°C was achieved at h ZnO /λ  =  0.41 in the 1st mode, while a large V p of 7210 m s−1 with a K 2 of 0.19% and a TCF of  −36.4 ppm/°C was obtained for h ZnO /λ  =  0.31 in the 2nd mode. Besides, most of the parameters were reported for the first time and will be helpful for the future design and optimization of SAW devices fabricated on ZnO/6H-SiC layered structures.



Source:IOPscience
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