Francis Baillet

Continuous Feed Physical Vapor Transport Toward High Purity and Long Boule Growth of SiC

A new reactor concept for the growth of silicon carbide bulk crystals and/or thick epitaxial layers is presented. A coupled approach involving process modeling and numerical simulation and experimental results and characterization was used. This new process combines both high-temperature chemical vapor deposition (HTCVD) for continuous feeding of the polycrystalline source and physical vapor transport (PVT) for single-crystal growth. A special crucible design was built to perform both steps simultaneously. For the feeding step (HTCVD), tetramethylsilane diluted in argon was used. The typical growth rate obtained by the continuous feed PVT process is 100 μm/h at 1900°C. The growth of thick epitaxial layers is demonstrated with a pure two-dimensional growth regime.

Chromium precipitation by the acidophilic bacterium Thiobacillus ferrooxidans

Thiobacillus ferrooxidans tolerates Cr 3+ up to 75 mM during growth on ferrous sulphate without modification of its bacterial activity (53 μg protein mM -1 Fe 2+ oxidised). At pH 1.4 and in the presence of Cr 2 O 3 , a maximal uptake capacity of 509 mg.g -1 dry weight was obtained with biomass harvested in the middle of the exponential growth phase. Cr 6+ uptake by T. ferrooxidans resulted in the precipitation of a chromium-rich compound on the surface bacterial cells, as observed by transmission electron microscopy.

Raman Imaging Analysis of SiC Wafers

Recent improvements in the implementation of the technique make Rama n spectroscopic imaging possible. Different systems have been developed to reduc e recording and display times to reasonable levels. In this study, we have performed Raman imagin g measurements for different 4HSiC wafers, and some test experiments performed on defective sam ple will be presented. Results were qualitatively interpreted in terms of residual strain fie lds and variations in the carrier concentration in the vicinity of the defects. Introduction In view of its excellent thermal, mechanical, chemical and el ectrical properties, silicon carbide (SiC) is an important semiconductor material for high-temperatur e and high-power devices and for a variety of other applications as well. Although SiC technology has re ched the market-place, the frequent occurrence of defects or nonuniform electrical propert ies is still a serious and persistent problem. Because of these nonuniform properties, some bulk measurements ma y be difficult to interpret, and more local probes are necessary to characterize bot h the structural and electrical properties in SiC wafers. It is known that micro-Raman spectroscopy is a powerful technique for the characterization of SiC: it is non destructive and requires no special preparation of the sam ples. Polytype identification by Raman scattering is possible. Moreover, the Raman parameters suc h a intensity, width, peak frequency and polarization of the strong and weak Raman bands provide fruit ful information both on the structural and the electronic properties of the samples [1]. Finally, recent improvements in the implementation of the technique make Raman spectroscopic imagin g possible. Different systems have been developed to reduce recording and display times to r ea onable levels, and it is now technically possible to produce Raman images with spatial resolut ion better than 1 μm within some hours. Such spectrometers have been used in our group to look for strain fields or impurities in diamond coatings or crystals [2,3]. Other systems have been used to study strain fields or impurities in semi-conducting materials like GaN [4] or SiGe[5]. In this study, we have performed Raman imaging measurements f or different 4H-SiC wafers, and some test experiments performed on defective samples will be presented. First, the method has been used as a full wafer mapping tool, with a rather low spatial resolution ( ≈500μm). Examination of the spectra yielded spatial maps of the carrier concentration. Second, defects (micropipes in particular) were optically detected and studied w ith micron resolution. Results were qualitatively interpreted in terms of residual strain fields and variations in the carrier concentration in the vicinity of the defects. Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 353-356 doi:10.4028/www.scientific.net/MSF.433-436.353

Raman Imaging Characterization of Structural and Electrical Properties in 4H SiC

Raman imaging measurements have been used to determine the spatial distribution of the doping level in n-type 4H-SiC wafers. The carrier concentration and mobility were determined from the line shape analysis of the LO phonon-plasmon coupled mode, using know procedures. The application of the method for mapping of the doping level at the wafer scale as well as in the vicinity of defects, for example micropipes, is demonstrated. Introduction Raman scattering is a powerful technique for the characterization of SiC. It is known that polytype identification by Raman scattering is possible [1]. Moreover, the Raman parameters such as intensity, width, peak frequency and polarization of Raman bands provide fruitful information both on the structural and the electronic properties of the samples [1,2]. The use of modern instrumentation now allows obtaining Raman maps or images within a few hours. Such spectrometers were used in our group to look for parasitic phases, strain fields, impurities... in diamond coatings or crystals [3,4]. They were also used to study the spatial distribution of defects or dopants in 4H SiC wafers [5-7]. As already mentioned, an important feature of Raman spectroscopy in semiconductors is that it provides information on the electrical properties. In this study, n-type 4H-SiC wafers were examined, and the carrier concentration and mobility were determined from the line shape analysis of the LO phonon-plasmon coupled mode, using know procedures [1,2]. The present method will be used as a full wafer mapping tool with a rather low spatial resolution, 100-500 μm, depending on the samples. It will also be used to image the electrical properties of samples in the vicinity of defects, micropipes in particular, with true micron resolution.

Contribution of numerical simulation to silicon carbide bulk growth and epitaxy

High temperature epitaxial processes for SiC bulk and thin films by physical vapour transport and chemical vapour deposition are reviewed from an academic point of view using heat and mass transfer modelling and simulation. The objective is to show that this modelling approach could provide information on fabrication and characterization for the improvement of the knowledge of the growth history. Recent results of our integrated research programme on SiC, taking into account the fabrication, process modelling and characterization, will be presented.

Characterization of Thick 2-Inch 4H-SiC Layers Grown by the Continuous Feed-Physical Vapor Transport Method

We present the first investigation of 2 inch diameter 0.5 mm thick 4H-SiC layers grown by the CF-PVT (Continuous Feed-Physical Vapor Transport) method. From Synchrotron White Beam X-Ray Topography we show that no new defect is generated in the CF-PVT material with respect to the 4H-SiC seed. We also show that a large strain takes place at the layer to seed interface which probably comes from the difference in doping level and thus in lattice parameter between the layer and the seed. From Raman experiments we demonstrate a high structural uniformity and low residual doping level. This is a surprising result which comes despite the lack of sophisticated purification procedure. To get confirmation, we have performed SIMS and LTPL investigations.

Comparison between Various Chemical Systems for the CVD Step in the CF-PVT Crystal Growth Method

We investigated various chemical systems for the CVD step in the CF-PVT bulk crystal growth method, in order to improve the feeding of the SiC polycrystalline source. In this paper, we present experimental results focusing on the comparison of CVD deposition rate and crystal growth rate versus chemical system and experimental parameters. We show the validation of the PVT step under an hydrogen atmosphere or an atmosphere containing chlorinated species. CF-PVT growth rates up to 150 μm/h are demonstrated. Results are discussed on the basis of thermodynamic calculations and numerical simulations of the temperature distribution in the crucible.

Vapor phase techniques for the fabrication of homoepitaxial layers of silicon carbide: process modeling and characterization

High temperature epitaxial growth processes for SiC bulk and thin films are reviewed from an academic point of view using heat and mass transfer modeling and simulation. The objective is to show that this modeling approach could provide further information to fabrication and characterization for the improvement of the knowledge of the growth history and to quantify the different phenomena leading to growth. Recent results of our integrated research program on SiC taking into account the fabrication, process modeling and characterization will be presented.

Coupling between the raman spectroscopy and photoemission microscopy techniques : Investigation of defects in biased 4H-SiC pin diodes

Raman spectroscopy and photoemission microscopy were coupled as two complementary non-destructive optical techniques in order to study biased 4H-SiC pin diodes. These two characterization tools have been largely used for the study of semiconductors but the combination of these two techniques has hardly been reported so far. Some structural defects inducing the same electrical damage could be discriminated and identified. Temperature could be measured in operating devices and the influence of the diode operating mode on the Raman signal could be evidenced.

Characterisation of 4H-SiC PiN Diodes by Micro-Raman Scattering and Photoemission

Structural defects in SiC crystals were investigated and 4H-SiC pin devices were characterized by micro-Raman scattering and photoemission. With the experimental set-up presented, defects could be successfully detected in SiC crystals but stacking faults could not be detected with micro-Raman scattering, although they could be detected by photoemission. Residual stress could be evaluated in 4H-SiC devices, as well as the temperature increase associated with the devices powering. A good correlation was found between the characterization techniques used: micro-Raman scattering and photoemission.