Papers by Keyword: Bulk Growth

Abstract: Close Space PVT (CS-PVT) is a modification of standard PVT exhibiting a short source-to-seed-distance and enabling a large variety of growth process variations to meet the specific requirements of the SiC material (i.e. special polytype and/or doping) to be grown. In this work, we study the growth of 4H-SiC p-i-n structures exhibiting thick SiC layers to be used as SiC photovoltaic cells for remote power transfer in space. Nevertheless, the found results are also applicable (i) to the SiC thick layer growth of power electronic devices and (ii) SiC pucks with a thickness of up to 10mm. In addition, we present the new type of growth machine TableTopCS^TM in its design being dedicated for the special crucible configuration of CS-PVT.

Abstract: This work discusses three aspects of the PVT growth process to reach a higher SiC crystal yield: (i) Type of carbon isolation material and procedure to maintain reproducible growth conditions from one process to the next. (ii) The pros and cons of temperature and power control (or a mixture of both) during the SiC crystal growth phase of the PVT process; and (iii) the selection of a set of process parameters and the specifications of the grown SiC crystal, which serve as a fingerprint of reproducible growth conditions (related to the selection and design of hot zone isolation components).

Abstract: The review on bulk growth of SiC includes a basic overview on the widely used physical vapor transport method for processing of 4H-SiC boules as well as the discussion of three current research topics: (a) Sublimation bulk growth of large area, freestanding cubic SiC, (b) in-situ Visualization of the PVT Process using 2D and 3D X-ray based imaging and (c) prediction of dislocation formation and motion in SiC using a continuum model of dislocation dynamics (CDD).

Abstract: The growth conditions of 75 mm SiC crystals in the PVT process is varied by different methods while the temperature field is kept constant. The addition of graphite into the source material leads to the formation of an ordered step flow with step heights of 0.014 µm, while the addition of graphite into the source together with N₂ doping changes the step kinetics on the main facet, leading to very large, bunched steps of 0.17 µm. When elemental Si was added into the source material large macro steps are formed on the whole crystal surface. While the doping induced step bunching is related to the incorporation kinetics, the large steps induced in Si-rich conditions are attributed the reduction of surface energy. With the variation of the inert gas pressure the morphology of the surface is altered, similarly. Under low pressure conditions (0.2 mbar) a fine step structure evolves, while at a high pressure (40mbar) large surface steps are formed on the whole growth interface. Large surface steps are strongly impeded in their lateral motion at defects permeating the growth interface. At these sites the formation of foreign polytypes is facilitated.

Abstract: We report the study of the effect of the growth rate and of the doping on the stress and the defect density of a Cubic Silicon Carbide (3C-SiC) bulk layer grown at low temperature on a silicon substrate. After the growth process, the silicon substrate was melt inside the CVD reactor used for the deposition and then the intrinsic stress was measured by the curvature of the wafer without influence of the thermal stress between silicon and 3C-SiC. A considerable increase of the curvature was observed increasing the doping of the layer. The average stress is compressive and then produces a convex bow. At the same time, the average quality of the grown material deteriorates increasing the doping concentration. Using μ-Raman measurement in cross-section of the 3C-SiC grown samples, it was possible to observe the dependence of the stress and of the quality of the material as a function of the thickness and of the growth rate, due to the variation of the growth rate during the process. In particular, the increase of the growth rate produced both an increase of the stress and a decrease of the material quality. Furthermore, the increase of the doping concentration produced both an increase of the stress and a further deterioration of the crystal quality.

Abstract: The process conditions for fast growth of 4 in. 4H-polytype SiC (4H-SiC) single crystals were studied for high-temperature gas source method. Prior to experiments, crystal growth simulations were conducted to investigate the influence of vertical gas-flow velocity on the radial distribution of the growth rate. Crystal growth experiments were performed using the crucibles designed for 4 in. crystal growth following the simulation studies. By investigating growth rate as functions of the input partial pressure of source gases and temperatures of growing surfaces, expressions for the growth rate of 4-in. crystals were derived. We also clarified the optimal conditions for single-crystal growth. Finally, fast growth of 4 in. 4H-SiC crystals with uniform shape was demonstrated.

Abstract: The growth of n-type 4H-SiC crystal was performed by physical vapor transport (PVT) growth method by using nitrogen and aluminum (N-Al) co-doping. Resistivity of N-Al co-doped 4H-SiC was as low as 5.8 mΩcm. The dislocation densities of N-Al co-doped substrates were evaluated by synchrotron radiation X-ray topography (SXRT). In addition, epitaxial growth was performed on the N-Al co-doped substrates by chemical vapor deposition (CVD). No double Shockley type stacking fault was observed in the epitaxial layer.

Abstract: The presence of threading mixed dislocations (TMDs) (with both edge and screw component) in 4H-SiC crystals grown by PVT method has been reported both from axial slices (wafers cut parallel to the growth axis) and commercial offcut wafers (cut almost perpendicular to the growth axis). In this paper, a systematic method is developed and demonstrated to unambiguously determine the Burgers vectors of TMDs in 4H-SiC commercial offcut wafers using both Synchrotron Monochromatic X-ray Topography (SMBXT) and Ray Tracing Simulations. The principle of this method is that the contrast of dislocations on different reflections varies with the relative orientation of Burgers vectors with respect to the diffraction vectors. Measurements confirm that in commercial offcut wafers the majority of the threading dislocations with screw component are mixed type dislocations.

Abstract: In order to diffuse the use of SiC, mass-production technologies of SiC wafers are needed. It is easy to be understood that high-speed and long-sized growth technologies are connected directly with mass-production technologies. The gas source growth method such as HT-CVD has the possibilities and the potential of the high-speed and long-sized growth. In this article, it was clarified that the high growth rate were achieved by the control of the source gas partial pressures and by the gas boundary layers. The average growth rate was 1mm/h on the f4 inch-diameter crystal, and the maximum growth rate reached 3.6 mm/h on the 12.5x25 mm tetragon by the above gas control. The crystal qualities of the gas source methods were also evaluated the equivalent level in comparison with the sublimation method. Concerning the 1mm/h-growth f3 inch crystal, the densities of TSDs were kept in the 10² cm^-2 levels from the seed to the upper-side of the ingot. Moreover, the ingot size increased year by year and a f4 inch x 43 mm sized ingot has been developed.

Abstract: Limitations in the very fast growth of 4H-SiC crystals are surveyed for a high-temperature gas source method. The evolution of macro-step bunching and void formation in crystal growth is investigated by changing the partial pressures of the source gases and crystal rotation speeds. The variation in macro-step formation depending on radial positions, where step-flow or spiral growth governs, of a grown crystal is also revealed. Based on the relation between growth conditions and macro-step bunching, a trade-off between growth rate enhancement and crystal quality and a method to improve such trade-off are discussed. Nitrogen at a high concentration under very high growth rates in the high-temperature gas source method is also investigated.