Etienne Pernot

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.

Characterization of Bulk 3C-SiC Single Crystals Grown on 4H-SiC by the CF-PVT Method

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.

Tuning the properties of F:SnO2 (FTO) nanocomposites with S:TiO2 nanoparticles – promising hazy transparent electrodes for photovoltaics applications

The appropriate choice of nanoparticles is proved to be essential in tuning the properties of F:SnO2 (FTO) nanocomposites. With the use of more conductive sulphur-doped TiO2 (S:TiO2) nanoparticles, the sheet resistance of S:TiO2–FTO nanocomposites is successfully reduced down to 38% as compared to the standard flat FTO (11.7 Ω sq−1), while the haze factor of the S:TiO2–FTO nanocomposites can be varied from almost zero (reference flat FTO) up to 60%; moreover the majority of 〈110〉 oriented S:TiO2 nanoparticles leads to a strong (110) texture in the resulting S:TiO2–FTO nanocomposites by local epitaxy. Careful morphology analyses and angle-resolved measurements reveal that the haze factor is proportional to the total surface coverage of the S:TiO2 nanoparticle agglomerates, while the feature size of the agglomerates determines the angular distribution of the scattered light – this is confirmed by an angle-resolved Mueller matrix polarimeter which allows obtaining the optical microscopic and angle-resolved images of the exact same textured region. Our work establishes the guidelines to fabricate FTO and other transparent conductive oxide (TCO) nanocomposites as promising electrodes in solar cells with tunable structural, electrical, and optical properties.

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.

Defects in sublimation-grown SiC bulk crystals

In view of its excellent thermal, mechanical and electronic properties, silicon carbide is the reference semiconductor material for high-temperature, high-frequency and high-power devices. Ingots of monocrystalline SiC which are currently grown by the seeded sublimation growth technique have opened the path to the production of large-area SiC wafers. Despite constant progress in crystal growth, the development of industrial applications has been up to now severely limited by the insufficient quality and size of the substrates. This situation results mainly from the fact that the growing process is quite complex. As a consequence, it is quite difficult to control the growth of a given polytype and the doping level while decreasing the number of defects such as misoriented domains, inclusions, macrodefects and micropipes. This paper is mainly devoted to a review of our work on the influence of the seed characteristics and the growth process parameters, such as the thermal field and the pressure, on the occurrence of defects in the as grown ingot and on the enlargement process. Polytype identification and morphology, structural perfection and defect analyses have been carried out using mainly polarized light microscopy and x-ray white-beam synchrotron topography.

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.

Investigation of Defects in 4H-SiC by Synchrotron Topography, Raman Spectroscopy Imaging and Photoluminescence Spectroscopy Imaging

Reflection synchrotron topography, integrated photoluminescence imaging nd Raman spectroscopy imaging have been performed on a 4H-SiC slice. The t hre methods give complementary information on the defects in the crystal. The differ enc s between the observations are discussed.

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.

Structural defects in SiC ingots investigated by synchrotron diffraction imaging

4H silicon carbide as-grown ingots were investigated by diffraction imaging using synchrotron radiation. The white beam section topographs obtained for various sample geometries allowed us to reveal structural imperfections before slicing the bulky ingots to the thin wafers used as electronic device substrates. The systematic investigation indicated that the observed inclusions of different polytypes in 4H-SiC ingots are correlated with the 8° off-axis orientation of the seed. These inclusions, formed at the beginning of the crystal growth, provoke planar defects that propagate along the main vertical axis of the cylindrical crystal. New findings permitted us to understand the inclusion formation with the aim to increase the useful volume.

Growth of AlN and AlN-SiC Solid Solution by Sublimation Method

AlN is considered as the most suitable substrate material for further development of high quality and high performance nitride-based micro- and opto-electronics. AlN ingots are often grown on SiC seeds. To solve the formation of cracks due to the difference in lattice parameters between seed and crystal we chose to “adapt” the lattice mismatch by a buffer layer of the (AlN)x(SiC)1-x solid solution. This paper gives some inputs on the growth of AlN and the solid solution by the sublimation technique, in terms of materials compatibility, hetero- and homo-epitaxial growth of AlN and on the preparation of crack-free solid solution single crystals.