Papers by Author: Didier Chaussende

Abstract: Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN₀ from 10¹⁷ cm^-3 to 10¹⁹ cm^-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy E_a = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN₀ 10¹⁹ cm³), both crystals revealed the bipolar diffusion coefficient D_a 4 cm²/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

Abstract: In this paper, an overview of the SiC bulk growth processes is given with a special focus on the most recent results related to growth and modeling. In addition, even if SiC growth is a very old topic and that it is now considered as an « industrial development problem », we will show that there are still many open questions of both fundamental and technological importance related to its crystal growth. Process chemistry and surface mechanisms will be more specifically discussed.

Abstract: We report on polytype destabilization during bulk crystal growth of Silicon Carbide by seeded sublimation method. Polytype transitions are experimentally obtained and a thermodynamic analysis using classical 2D nucleation theory is used towards the understanding of the experimental results. Whether it is a thin lamella or an inclusion, it is found that the polytype transitions systematically occur on the (0001) facet. This suggests that the polytype switch takes place through classical 2D nucleation at the facet. We will show that two criteria must be fulfilled for the occurrence of a foreign polytype: i) minimization of nucleation energy and ii) presence of a facet. This is directly depending on the crystal shape (convex or concave) and its evolution with growth time.

Abstract: Graphite crucible in seeded sublimation growth of Silicon Carbide (SiC) single crystal does not only act as a container but also as an additional carbon source. The modeling of the growth process integrated with the etching phenomenon caused by the interaction between vapor species and the graphite crucible is shown to be able to predict the shape of the crystal front during the growth. The additional fluxes produced at the graphite part are delivered to the growing crystal mainly at the crystal periphery. The results obtained from the modeling are in good agreement with the experimental ones.

Abstract: The numerical modeling of the SiC bulk growth process by physical vapor transport has been established as the essential tool for the process development, especially for understanding and predicting the favorable growth conditions. An accurate computation of mass transfers is strongly dependent on the equilibrium partial pressure calculations. In this paper, we compare the relative impact of the different thermodynamic databases available on the full PVT process modeling. We found that whatever the database used, the trends regarding growth rate calculation, crystal shape, Si/C ratio are correctly described and none of the database would bring about unacceptable errors from the process development point of view even if some discrepancies in the absolute values could be obtained.

Abstract: The carbon distribution and its transport in the liquid from the source to the crystal directly affect the control of parasitic nucleation, the growth front stability, and the growth rate during SiC solution growth. Controlling the carbon transport is one of the key issues for understanding and improving the process. In this paper, numerical modeling by finite element method is used to describe the complex convective flow pattern in the melt. We focus on electromagnetic convection and investigate the effect of coil frequency, keeping a simple and technologically realistic crucible design. We show that below a critical value of frequency, the carbon transport can be controlled by the electromagnetic convection, giving rise to significant growth rate enhancement.

Abstract: This paper presents the results obtained after chemical vapor deposition of SiC with the addition of GeH₄ gas to the classical SiH₄+C₃H₈ precursor system. Epitaxial growth was performed either on 8°off-axis or on-axis 4H-SiC substrate in the temperature range 1500-1600°C. In the off-axis case, the layer quality (surface morphology and defect density) does not change though accompanied with Ge droplets accumulation at the surface. The Ge incorporation level was found to increase with temperature in the 10¹⁷ 10¹⁸ cm^-3 ranges. It was observed that adding GeH₄ leads to the increase of the n type doping level by a factor from 2 to 5 depending on the C/Si ratio. In the on-axis case, GeH₄ was only added to the gas phase before starting the SiC growth. It was found that there is a conditions window (temperature and GeH₄ flux) for which 3C-SiC twin free layers can be grown. Adding this foreign element before SiC growth clearly modifies SiC nucleation on on-axis substrate.

Abstract: Under specific and reproducible conditions we observed that the top bilayer of a spiral step can detach from the remaining bunched step. This dissociated bilayer is located in the middle of a terrace. In this work we detail the parameters for which the spiral step dissociation appears. Furthermore the possible origins of this effect and the reasons why the dissociated step is located in the center of a terrace are discussed.

Abstract: The growth of 3C-SiC on hexagonal polytype is addressed and a brief review is given for various growth techniques. The Chemical Vapor Deposition is shown as a suitable technique to grow single domain 3C epilayers on 4H-SiC substrate and a 12.5 µm thick layer is demonstrated; even thicker layers have been obtained. Various characterization techniques including optical microscopy, X-ray techniques and photoluminescence are compared for the evaluation of the crystal quality and purity of the layers.

Abstract: Large scale, homogeneous quasi-free standing monolayer graphene is obtained on a (111) oriented cubic SiC bulk crystal. The free standing monolayer was prepared on the 3C-SiC(111) surface by hydrogen intercalation of a -reconstructed carbon monolayer, so-called zerolayer graphene, which had been grown in Ar atmosphere. The regular morphology of the surface, the complete chemical and structural decoupling of the graphene layer from the SiC substrate as well as the development of sharp monolayer p-bands are demonstrated. On the resulting sample, homogeneous graphene monolayer domains extend over areas of hundreds of square-micrometers.