Gabriel Ferro

Growth by a vapour–liquid–solid mechanism: a new approach for silicon carbide epitaxy

Growing silicon carbide monocrystalline layers at low temperatures (<1400°C) is a challenging problem that requires the development of less conventional epitaxial techniques. Growth from Al-based melts is an interesting alternative that can lead to a more than 300°C temperature reduction compared to that used in the more classical chemical vapour deposition technique. Liquid phase epitaxy under a thermal gradient is often used but this approach has some drawbacks such as the difficulty in controlling the thermal gradient or the presence of the liquid on the seed at the end of the growth. A solution could come from a vapour–liquid–solid (VLS) mechanism in which a Si-based melt is fed by an alkane to form SiC on an immersed seed. The growth is then easily controlled by the alkane flux and the liquid can be eliminated in situ by simply sucking it up. We present in this paper a review on the use of the VLS mechanism for growing SiC at low temperatures. Results of the effect of several metal additives (Al, Ni, Fe and Co) to the melt are shown. These metals incorporate into the lattice and can give specific properties to SiC, such as heavily p-type doping with Al or semi-magnetic properties with Ni, Fe or Co. The VLS technique is shown to be very versatile as it can be applied to selective epitaxial growth of SiC. Finally, perspectives of the VLS technique for solving related problems touching SiC or for growing other semiconductor compounds are given.

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.

Photonic crystal cavities in cubic (3C) polytype silicon carbide films

We present the design, fabrication, and characterization of high quality factor (Q ~10(3)) and small mode volume (V ~0.75 (λ/n)(3)) planar photonic crystal cavities from cubic (3C) thin films (thickness ~200 nm) of silicon carbide (SiC) grown epitaxially on a silicon substrate. We demonstrate cavity resonances across the telecommunications band, with wavelengths from 1.25 - 1.6 μm. Finally, we discuss possible applications in nonlinear optics, optical interconnects, and quantum information science.

Direct synthesis of β-SiC and h-BN coated β-SiC nanowires

Abstract β-Silicon carbide (β-SiC) nanowires (NWs) have been grown by thermal treatment of commercial silicon particles disposed in a graphite crucible under nitrogen atmosphere. By the same way, treatment under argon of a mixture of a boron nitride (BN) based powder and silicon particles led to h-BN coated β-SiC nanowires. The structures of both nanoobjects have been investigated by HRTEM, EDX and EELS.

Evidence for Flat Bands near the Fermi Level in Epitaxial Rhombohedral Multilayer Graphene

The stacking order of multilayer graphene has a profound influence on its electronic properties. In particular, it has been predicted that a rhombohedral stacking sequence displays a very flat conducting surface state: the longer the sequence, the flatter the band. In such a flat band, the role of electron-electron correlation is enhanced, possibly resulting in high Tc superconductivity, magnetic order, or charge density wave order. Here we demonstrate that rhombohedral multilayers are easily obtained by epitaxial growth on 3C-SiC(111) on a 2° off-axis 6H-SiC(0001). The resulting samples contain rhombohedral sequences of five layers on 70% of the surface. We confirm the presence of the flat band at the Fermi level by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy, in close agreement with the predictions of density functional theory calculations.

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.

Hexamethyldisilane/propane versus silane/propane precursors: application to the growth of high-quality 3C–SiC on Si

From a comparative evaluation of hexamethyldisilane (HMDS) and silane–propane (SP) precursor systems, it is shown that HMDS needs a small addition of propane to deposit heteroepitaxial layers of 3C–SiC on Si with superior crystalline properties. In this case, propane compensates for the secondary reactions induced by hydrogen reacting with carbon. Using atmospheric pressure chemical vapour deposition conditions, the new system (HMDS–propane) demonstrates several advantages. It is safer to handle than SP and allows a higher growth rate (up to 7 µm h−1 at 1350 °C) without any degradation of the layer morphology. However, when lowering the deposition temperature, HMDS is revealed to be more stable than silane. This is in contrast to most standard beliefs but explains why a high temperature (~1350 °C) is always necessary to grow high-quality material using HMDS.

Critically coupled surface phonon-polariton excitation in silicon carbide.

We observe critical coupling to surface phonon-polaritons in silicon carbide by attenuated total reflection of mid-IR radiation. Reflectance measurements demonstrate critical coupling by a double scan of wavelength and incidence angle. Critical coupling occurs when prism coupling loss is equal to losses in silicon carbide and the substrate, resulting in maximal electric field enhancement.

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.

Structural properties of undoped and doped cubic GaN grown on SiC(001)

Transmission electron microscopy and x-ray diffraction measurements reveal the presence of stacking faults (SFs) in undoped cubic GaN thin layers. We demonstrate the importance of the defects in the interfacial region of the films by showing that the SFs act as nucleation sites for precipitates of residual impurities such as C and Si present in the GaN layers grown on SiC(001) substrates. We used the imaging secondary ion mass spectroscopy technique to locate these impurities. The systematic decrease of the SF density as a function of the layer thickness is explained by an annihilation mechanism. Finally, the effects of usual dopants on the structural properties of GaN layers are discussed. It is shown that Mg has a tendency to incorporate out of the Ga site by forming Mg precipitates for a concentration higher than 1019 cm−3 in contrast with the results found for heavily Si doped layers.