François Cauwet

Infrared kinetic study of ultrathin SiC buffer layers grown on Si(100) by reactive chemical vapour deposition

Abstract A kinetic study was carried out on the growth of an SiC buffer layer on Si(100) by reactive chemical vapour deposition. Experiments were performed at temperatures in the range 1150–1300 °C for 1 to 45 min using C 3 H 8 and H 2 as gas reactants. Infrared transmittance spectrometry was used for accurate film thickness determination (down to 1.2 nm). The growth profiles as a function of time show a four-step mechanism involving the rapid formation of an SiC “thermal layer” by coalescence of SiC islands. The thickness increases by Si out-diffusion through this layer until a critical thickness, controlled by the temperature and Si etching, is reached. Only the initial values of the diffusion profiles can be fitted by Ficks second law. The hypothesis of silicon etching by H 2 is confirmed by thermodynamic calculations. The etching activation energy is E e = 4.4 eV. The temperature dependence of the resulting diffusion coefficients gives an apparent diffusion activation energy of E d = 4.5 eV. The close agreement between these two activation energies illustrates the competition between the two mechanisms deduced from the growth profiles.

A comprehensive study of SiC growth processes in a VPE reactor

Abstract We performed an experimental study of the effect of the gas phase composition on the growth mechanism of 3C-SiC on Si(100) by atmospheric-pressure vapour phase epitaxy at 1350°C. Silane and propane diluted in hydrogen were used as precursors for the growth. We demonstrate the existence of an equilibrium partial pressure of carbon above the growing surface, which ensures a mirror-like morphology. For too low a carbon partial pressure (C/Si ratio in the gas phase lower than 2.7 with a growth rate of 3 μm h −1 ), the layer morphology and crystalline quality quickly degrade. For too high a carbon partial pressure (C/Si ratio higher than 5 with the same growth rate), SiC clusters form on the growing layers. We propose a mechanism of formation for these clusters taking into account the interactions between the C and Si species in the hot boundary layer.

On the growth of 4H–SiC by low-temperature liquid phase epitaxy in Al rich Al–Si melts

Abstract The growth of 4H–SiC by low-temperature liquid phase epitaxy was studied in Al–Si melts. The temperature ranged from 1000°C to 1200°C. Some problems, which were sources of non-homogeneity of the growth or low reproducibility of the process, were identified and reviewed: (1) local delayed wetting of the seed by the melt, (2) morphological (3) presence of alumina particles on the liquid, (4) high reactivity of the melt with graphite at temperature above 1200°C, (5) formation of crystallites on the surface upon cooling. The solutions proposed to avoid or limit these problems are: (1) deposition of a Si layer before the growth, (2) careful backside gluing, (3) a two-step procedure involving the pre-dipping in the melt of a graphite rod on which alumina particles agglomerate, (4) growth at temperature lower than 1200°C. No solution was found to avoid the crystallites formation upon cooling.

Study of the interaction between graphite and Al-Si melts for the growth of crystalline silicon carbide

The chemical interaction between Al-Si melts of different compositions and graphite was investigated in order to clarify the mechanism of spontaneous growth of silicon carbide crystals from these melts. Calibrated graphite small rods were used as carbon source to facilitate comparison between experiments. For a temperature set to 1100°C, the reaction time and Si content of the melt were varied from 1 to 48 hours and 20 to 40 at.%, respectively. It has been found that in a first stage the liquid reacts at a relatively slow rate to form a microcrystalline SiC layer around the graphite rod. When this SiC layer has reached a certain thickness, a violent attack follows in some specific sites by rapid dissolution of the rod. Radial liquid channels progress from the surface of the rod up to its centre and then total conversion of graphite into SiC rapidly occurs. The local Si content of the melt, which controls the carbon solubility in the liquid, governs the overall mechanism. To form faceted β-SiC crystals, the growth mechanism should involve carbon dissolution in one place and supersaturation in another place in relation with local changes of the Si content in the melt.

Comparison of Different Metal Additives to Si for the Homoepitaxial Growth of 4H-SiC Layers by Vapour-Liquid-Solid Mechanism

The vapor-liquid-solid (VLS) mechanism, where a carbon containing gas phase feeds a Si based liquid, is a very interesting approach to grow SiC homoepitaxial layers at low temperature or at fast rate. Indeed, no thermal gradient needs to be controlled and the liquid can easily be removed before cooling by simple sucking up. Different metal (Me) additives to the melt were studied and compared to pure Si : Al, Ni, Fe, Co and Cu. It was shown that SiC growth could be obtained with each metal except Cu. It was found that for too high Me content of the melt, the substrate is partly dissolved in the melt. For lower Me content, growth can occur in a 2D mode. On the other hand, we were confronted with some problems such as the occasional formation of a SiC crust on the liquid or the loss of liquid through wetting on the susceptor.

Low Doped 3C-SiC Layers Deposited by the Vapour-Liquid-Solid Mechanism on 6H-SiC Substrates

We report the results of a systematic investigation performed to reduce the residual n-type doping level of the 3C-SiC layers grown by the VLS mechanism on 6H-SiC(0001) on-axis substrate. This new approach, termed “High purity VLS” leads to low doped and low compensated material, which was confirmed by Raman and Low Temperature Photoluminescence spectroscopy. The resultant 3C morphology remains typical of single-domain layers and the n-type doping level could be estimated around 6x1016 cm-3.

Investigation of 3C-SiC(111) Homoepitaxial Growth by CVD at High Temperature

Starting from 3C-SiC(111) layers grown by Vapour-Liquid-Solid mechanism, homoepitaxial growth by Chemical Vapour Deposition was carried out on top of these seeds. The effect of the growth temperature and of the C/Si ratio in the gas phase was investigated on the surface morphology, the roughness and the defect density. It was found that the initial highly step-bunched surface of the VLS seeds could be greatly smoothen using appropriate conditions. These conditions were also found to reduce significantly the defect size and/or density at the surface.

Growth of Nanocrystalline Translucent h-BN Films Deposited by CVD at High Temperature on SiC Substrates

h-BN layers were deposited on α-SiC substrates by CVD at high temperature (1500-1900°C) using B2H6 and NH3 diluted in Ar. Growth rates were in the 6-10 µm/h range. In all the conditions studied, the BN as deposited layers were found to be translucent to light, some having a light whitish aspect and other a more yellowish one. It was also observed that the deposit was not always adhesive. µ-Raman and TEM characterization showed that the layers were nano-crystalline with crystallite size < 10 nm. The growth rate was found temperature and N/B ratio dependent due to an N limited growth regime which is more pronounced above 1700°C.

Growth and Characterisation of Heavily Al-Doped 4H-SiC Layers Grown by VLS in an Al-Si Melt

We report on a new approach in which the homoepitaxial growth of 4H-SiC layers can be done at low temperature (1100°C). The process involves a VLS (Vapor-Liquid-Solid) mechanism in which propane feeds an Al-Si droplet. Compared to conventional LPE (Liquid Phase Epitaxy) this approach has several advantages. No thermal gradient (vertical or radial) has to be controlled and the liquid can be easily removed before cooling by sucking up the melt. In this way, Al concentrations in the range ~ 1-3.10 20 at.cm -3 have been reached and, both, Raman and TEM characterizations show that no foreign polytype or other material inclusion is formed. The main drawback is a large step bunching, typical of LPE samples.

Ge assisted SiC epitaxial growth by CVD on SiC substrate

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.