Papers by Author: Michel Pons

Abstract: Recently, in some silicon carbide single crystals, some micropipes associated with screw dislocation have been observed by X-ray topography and the strain field around them produced images similar to those of screw dislocations with a very large Burgers vector, about 667 nm. The radius of the hole in the centre of the micropipe is less than 10 'm. This value and the theoretical predictions by Frank (about 7.8 mm) using the Burgers vector magnitude show a large discrepancy. In this paper we present Atomic Force Microscopy experiments around this kind of defects. The Burgers vector magnitude of the screw dislocation and the value of the radius have been measured by this technique. Not only one dislocation, but several have been observed around the micropipe. We concluded that it is in better agreement with the Frank theory modified by Cabrera and Levine concerning kinetic effects during the growth.

Abstract: From the engineering point of view, SiC hot-wall epitaxy is a very important process in SiC semiconductor processes. There are lots of experimental reports on SiC hot-wall epitaxy. They discussed the growth rate, surface morphology, doping concentration, etc. Recently, the effect of face polarity is also made clear. However, each report mentioned the particular results that strongly depend on the experimental conditions and reactor design. In addition, the discussion with inlet condition such as source gas C/Si ratio, not the depositing surface condition, leads to the confusion. In order to understand and try to design and optimize the hot-wall CVD reactor, a numerical approach is attempted. The authors have tried to make it clear that depositing surface condition might be a universal parameter of SiC CVD, and the numerical simulation could predict the growth rate, surface morphology and doping concentration by taking account of the depositing surface condition. In this study, at first, the recent progress of SiC hot-wall epitaxy in experiment is summarized. Then, the present status of its numerical modeling is explained.

Abstract: The growth of SiC crystals or epilayers from the liquid phase has already been reported for many years. Even if the resulting material can be of very high structural quality and the possibility to close micropipes was demonstrated, handling the liquid phase still is a challenge. Moreover, it is highly difficult to stabilize the C dissolution front and then to stabilize the growth front over a long growth time. Based on the Vapour-Liquid-Solid mechanism, we present a new configuration for the growth of SiC single crystal which should allow first to simplify the liquid handling at high temperature and second to precisely control the crystal growth front. The process consists in a modified top seeded solution growth method, in which the liquid is held under electromagnetic levitation and fed from the gas phase. In a Co-Si solution fed from a propane flow at 1350°C, thick epitaxial layers of 4H-SiC have been grown at 28 0m/h. The potentiality of this new process will be discussed in the paper.

Abstract: Because of the formation of DPB (Double Positioning Boundary) when starting from a hexagonal <0001> seed, DPB-free 3C-SiC single crystals have never been reported up to now. In a recent work we showed that, using adapted nucleation conditions, one could grow thick 3C-SiC single crystal almost free of DPB [1]. In this work we present the results of a multi-scale investigation of such crystals. Using birefringence microscopy, EBSD and HR-TEM, we find evidence of a continuous improvement of the crystal quality with increasing thickness in the most defected area, at the sample periphery. On the contrary, in the large DPB-free area, the SF density remains rather constant from the interface to the surface. The LTPL spectra collected at 5K on the upper part of samples present a nice resolution of multiple bound exciton features (up to m=5) which clearly shows the high (electronic) quality of our 3C-SiC material.

Abstract: We have studied the impact of the chemical nature of additional gases fed into the modified physical vapor transport (M-PVT) growth cell. In particular experiments were carried out using helium, argon, nitrogen and propane in the growth setup. Numerical modeling was used to address the underlying physical and chemical effects that impact the global temperature field. It is found that chemical decomposition of complex gases plays a secondary role as heat source or sink. However, temperature variations related to varying gas compositions fed to the systems are primarily induced by changes of the graphite foam isolation properties.

Abstract: The transfer by wafer-bonding of single-crystalline SiC thin films to a polycrystalline SiC support to obtain a “quasi-wafer” is an attractive way for lowering the cost of silicon carbide wafers. Such a process needs high quality polycrystalline substrates, with controlled and high-level bulk properties (thermal conductivity, electrical resistivity) and with very low surface roughness and surface bowing. Currently, polycrystalline SiC wafers which are available are siliconized SiC or CVD processed SiC wafers. Siliconized ceramic wafers are very heterogeneous (mixture of 3C, 6H, 15R and silicon), while CVD ones are of better quality (homogeneous and textured 3C). However neither the siliconized SiC nor the CVD SiC can be CMP polished with low roughness over large dimension. In this paper, wafers with large and textured grains (> 1cm) are processed and characterized. The polishing of such structures is studied and optimized to obtain low surface roughness. To meet these requirements high temperature processes used for single crystal growth were selected. Structural investigations performed on the grown ingots showed an important influence of the used seed since no preferential crystallographic orientation was observed during the growth. The final polishing quality was of high level but step heights were observed between grains.

Abstract: The development of the Continuous Feed Physical Vapour Transport (CF-PVT) process requires a perfect control of each phenomenon in the growth cell. Along this line, the present paper gives some inputs on the CF-PVT mass transfer regimes with respect to the process parameters, both from qualitative and quantitative viewpoints. For example, two boundary cases have been evidenced depending on the temperature. At low temperature, the growth is limited by the sublimation step between the source and the seed. In this case, the CF-PVT process can be roughly assimilated to the classical seeded sublimation technique. At high temperature, the process is limited by the feeding step, i.e. the CVD deposition and infiltration on the lower part of the source. Measurements are correlated to in-situ X-ray imaging. The ability of the X-ray imaging to in-situ qualify and quantify the mass transfer is discussed.

Abstract: Thick (111) oriented β-SiC layers have been grown by hetero-epitaxy on a (0001) a-SiC substrate with the Continuous Feed-Physical Vapour Transport (CF-PVT) method. The growth rate was 68 µm/h at a pressure of 2 torr and a temperature of 1950°C. The nucleation step of the β-SiC layer during the heating up of the process was studied in order to manage first the a to b heteropolytypic transition and second the selection of the b-SiC orientation. With a adapted seeding stage, we grew a 0.4mm thick layer almost free of Double Positioning Boundaries on a 30mm diameter sample. First observations of the layer by cross-polarised optical Microscopy are presented both in planar view and in cross section geometry.

Abstract: We have studied the application of optical techniques for the determination of the spatial distribution of electronic properties of highly aluminum doped p-type SiC wafers. Absorption and birefringence mapping are known to be sensitive characterization methods to determine the homogeneity of charge carrier concentration and defects in n-type SiC. In the case of highly p-type doped SiC these methods fail due to the opaque character of the material. In this paper we show that Raman spectroscopy which is a reflective method can be used in order to address the same materials properties like absorption and birefringence. The study was performed using medium doped p-type SiC:Al where optical transmission and reflection methods can be applied simultaneously.

Abstract: Growth, etching, and doping features of SiC-CVD in a horizontal hot-wall reactor were numerically analyzed using the improved heterogeneous model. The improved model was able to explain the growth and etching features accurately. In addition, we propose the surface flux, surface carbon and silicon concentration, and its ratio as the universal parameter of the SiC-CVD process. Concerning doping features, the improved model showed that nitrogen and aluminum doping incorporation could be explained by the site competition model, while taking into account the amount of surface silicon and surface carbon, respectively.