Materials Science Forum Vol. 954

Abstract: AlN crystals are one of the representative III-V group semiconductor materials. AlN has good electric field characteristics, thermal conductivity and thermal stability. Owing to its wide direct band gap of 6.2eV [1], it can achieve a luminescent wavelength of 210 nanometers in deep ultraviolet, which is an ideal material for UV and deep UV LED devices. But preparation of AlN crystals with PVT for growing conditions demanding, 0.3-0.5 atm of high purity nitrogen atmosphere of growth and the growth of 2100-2400 K temperature [2, 3]. In this paper, two kinds of growth chamber structures are designed and compared. In order to ensure that the temperature gradient between the source material surface and the seed surface satisfy the crystal growth and keep stability in the larger transverse region, [4, 5] the crystal grown on the seed crystal can get better quality.

Abstract: We demonstrate 4^o off-axis 4H-SiC bulk crystal growth using physical vapor transport method (PVT). A radial 6-folded pattern is discovered in the facetted region of the growth surface by differential interference contrast microscopy. SICA88 Wafer Inspection/Review System was used to further inspect such 6-folded pattern with photoluminescence channel for wafers cut from that 4H-SiC crystal. It was found that the 6-folded pattern exists in all the wafers from the bottom to the top of that crystal. Several radial 6-folded patterns are found in the bottom wafer near the seed, but there is only one in the other two wafers. Defects distribution of these wafers after etched were also researched to find out the relationship between the radial 6-folded pattern and the defects. Based on the results, the formation and evolution mechanism of the radial 6-folded pattern on 4o off-axis 4H-SiC crystal is discussed.

Abstract: The effects of annealing on epitaxial graphene on SiC substrates with various conditions are investigated. Results show that high pressure hydrogen atmosphere is more effective to decouple the epitaxial graphene from SiC substrate than that of a relative lower pressure process. Besides, the characteristic 2D-peak of graphene in Raman spectra disappeared with an annealing temperature 1000 °C, which means that the epitaxial graphene layer was decomposed in this condition. The study also shows that the decomposition of graphene can be effectively suppressed by increasing carbon vapor partial pressure through introducing ethylene during high pressure hydrogen annealing at 1000 °C. And the epitaxial graphene is successfully transferred to quasi free standing graphene by the annealing with an appropriate flow of ethylene.

Abstract: The morphology of the in-situ etching process on Si-face and C-face 4H-SiC, by annealing in a hydrogen environment, is studied by atomic force microscopy (AFM). The uniform step-terrace morphology of both the Si-face and C-face 4H-SiC is achieved with the assistance of carbon/silicon flux. The full width at half maximum (FWHM) of Raman 2D peak for the graphene grown on the uniform step-terrace morphology of the Si-face and C-face is 36.2 and 22 cm^-1, respectively.

Abstract: The resonant frequency and Q factor of the SiC microcantilever were theoretically analyzed and calculated based on the stereotyped basic theories of the cantilever beam, and the relationship between the vibration mode and structure geometries was also simulated. Modal analysis by means of finite element method was performed on millimeter-, micron-and nanoscale microcantilevers, and the results showed that the smaller the microstructure was, the higher the resonant frequency can be obtained. The Q factor can be extracted from hamonic spectra after modal analysis, and the amplitude of Q factor was about 10⁵. This paper shows that SiC epitaxial layers have great potential in microcantilevers.

Abstract: Homoepitaxial growths of 4H-SiC were performed on Si-face (0001) on-axis substrates in a SiH₄-C₂H₄-H₂-HCl system by using our home-made vertical hot wall CVD reactor. The influence mechanism of the growth temperature and C/Si ratio on the morphology and growth rate was studied. It is found that the steps in the epilayer become more clear with the increasing temperatures. The result indicates that the C/Si ratio window of on-axis epitaxial growth is very narrow. Only when the C/Si ratio was 1.2, a slightly improved surface morphology can be achieved. The results indicate that 4H-SiC epitaxial layers were obtained on on-axis substrates and the films were highly-oriented 4H-SiC.

Abstract: The research and commercialization of SiC based power device have been burgeoning over the last decade worldwide, which is bringing about an increasing demand on lost-cost and low-defect SiC wafers. To meet this challenge, we have been continuously making efforts on improving the crystal growth and wafer processing techniques. Now, the mass-production of high quality 4-inch, 6-inch n-type and semi-insulating SiC wafers has been realized. Statistically, the micropipe density is lower than 0.5 cm^-2. The resistivity of the wafers is lower than 0.02 Ω·cm and up to 10⁸ Ω·cm for n-type and semi-insulating SiC single crystals, respectively. A state of the art processing technique has been developed to control wafer deformation and thickness within the desired values for subsequent epitaxy. The total defect number of the epitaxial layers grown on the "epi-ready" 4-inch SiC wafer is 63, and the usable area is 97.6%, indicating the high quality of our SiC substrates.

Abstract: A study on carbon particle inclusions during 4H-SiC bulk growth is presented. Special attentions were paid to design of graphite growth compartment, size of SiC source materials, and process of seed crystal handling. It was found that common carbon inclusions with size of 30μm or less were attributed to carbon particles from graphitized SiC source. Less common carbon inclusions with size of over 100μm were also found and were attributed to poor seed crystal mounting process. In order to reduce carbon inclusions, several experiments were designed by using a NAURA Advanced Physical Vapor Transport (PVT) System APS130G. A graphite plate separator was inserted into the growth compartment to prevent the carbon particles from transporting to the growth surface. SiC powder materials with larger diameters were selected to reduce source graphitization. Additional clean process was performed to remove carbon particle residuals on graphite parts during seed mounting. The results showed significant improvement of carbon inclusion problems in SiC ingots and thus high-quality SiC wafers were made successfully.

Abstract: Abstract:Electron mobility limited by surface roughness scattering in free-standing GaAs thin ribbon with an internal parabolic quantum well caused by surface state is investigated in detail. Based on analyzing the parabolic quantum well including the energy subband level, wave function and the confined potential profile in the thin ribbon by solving Schrödinger and Poisson equations self-consistently, the electron mobility could be investigated. Conclusion indicates that remote surface roughness (RSR) of the thin ribbon will change the two dimensional electron gas (2DEG) mobility through the medium of barrier height fluctuation of the parabolic well in atomic scale. Calculation results reveal that the 2DEG mobility decreases with increasing roughness amplitude, which is characterized in terms of the surface roughness height and the roughness lateral size.