Heteroepitaxial boron carbide (B x C) can be grown on the Si face of 4H-SiC(0001) using a two-step process involving substrate boridation at 1200 degrees C under BCl3 + H2, followed by a chemical vapor deposition (CVD) growth step at 1600 degrees C by adding a C3H8 precursor. However, an in-depth investigation of the early growth stages revealed that complex reactions occur before starting the CVD at high temperature. Indeed, after boridation, the 35 nm B x C buffer layer is covered by an amorphous B-containing layer, which evolves and reacts during the temperature ramp-up between 1200 and 1600 degrees C. Despite the formation of new phases (Si, SiB6), which could be explained by significant solid-state diffusion of Si, C, and B elements through the thin B x C layer, the CVD epitaxial regrowth upon reaching 1600 degrees C does not seem to be affected by these phases. The resulting single-crystalline B x C layers display the epitaxial relationship [1010]B x C(0001)parallel to[1010]4H-SiC(0001). The layers exhibit a B4C composition, e.g., the highest possible C content for the B x C solid solution.

The thermochromic properties (color change with temperature) of n type doped SiC wafers of different polytypes (3C, 4H and 6H) have been investigated up to 500°C under air. It was found that 3C-SiC color passes from bright yellow at room temperature to deep orangeat 500°C leading to a color contrast (ΔE) as high as 64. The hexagonal polytypes undergo also a color change upon heating but far less pronounced, with ΔE values <20. All these semiconductors undergo band gap shrinkage upon heating which effect largely participated to the observed color change. This effect is very sensitive for 3C polytypesince its bandgap is already in the visible energy range at room temperature. The thermochromicity of 3C-SiC was found to be reversible thanks to its thermal stability and its resistance towards oxidation.

The chemical vapor deposition (CVD) growth of boron carbide (B x C) layers on 4H-SiC, 4°off substrates was studied. Depending on the polarity of the substrate, different results were obtained. On Si face, the direct CVD growth at 1600°C under a mixture of BCl 3 +C 3 H 8 systematically led to polycrystalline B x C films, whatever the C/B ratio in the gas phase. On the C face, heteroepitaxial growth was obtained for C/B ratios = 12 or higher with a step bunched morphology. If a boridation step (10 min at 1200°C under BCl 3 flow) was used before the CVD growth, then heteroepitaxy was successful on both substrate polarities. To explain these results, a mechanism is proposed which involves the nature of the chemical bonds at the early stage of nucleation. It is suggested that a full B coverage of the SiC surface should favor the nucleation of the B-rich (0001) plane of B x C, promoting thus the heteroepitaxial growth along this direction.

In this work, the successful heteroepitaxial growth of boron carbide (B x C) on 4HSiC(0001) 4° off substrate using chemical vapor deposition (CVD) is reported. Towards this end, a two-step procedure was developed, involving the 4H-SiC substrate boridation under BCl 3 precursor at 1200°C, followed by conventional CVD under BCl 3 + C 3 H 8 at 1600°C. Such a procedure allowed obtaining reproducibly monocrystalline (0001) oriented films of B x C with a step flow morphology at a growth rate of 1.9 μm/h. Without the boridation step, the layers are systematically polycrystalline. The study of the epitaxial growth mechanism shows that a monocrystalline B x C layer is formed after boridation but covered with a B-and Si-containing amorphous layer. Upon heating up to 1600°C, under pure H 2 atmosphere, the amorphous layer was converted into epitaxial B x C and transient surface SiB x and Si crystallites. These crystallites disappear upon CVD growth.

The structuring of Si face 4 degrees off 4H-SiC surfaces was investigated using Si melting in a SiC/Si/SiC sandwich configuration. The stacks were treated at 1550-1600 degrees C under H-2 in a RF-heated cold-wall reactor. By fixing the liquid Si thickness to 30 mu m, the vertical thermal gradient inside the stack generates carbon transport from the bottom to the top SiC wafer. The constant dissolution of the bottom SiC wafer (1.7 mu m/h at 1550 degrees C) leads to surface structuring in macrosteps. The regularity of these macrosteps can be reproducibly controlled when performing the process on an epitaxial layer thanks to the pre-structuration in parallel microsteps of such kind of surfaces. The best regularity of the steps was obtained after a structuring process of 2 h, with an average terrace width of similar to 3-5 mu m.

The thermochromic properties up to 500 °C of some colored, commercial and non-toxic oxides were studied. It includes Bi2O3, Fe2O3, In2O3, WO3, Er2O3, YAG:Ce (Ce doped yttrium aluminum garnet Y3Al5O12) and YInMn-Blue (Mn doped yttrium indium oxide YInO3). From the CIELAB colorimetric parameters (L*a*b*), evolution of the color contrast ΔE with temperature was calculated. It allowed estimating that most of these oxides change color reversibly with increasing temperature, the strongest thermochromism being found for Bi2O3 (ΔE ∼ 65 at 500 °C). Two exceptions were identified: i) Er2O3 which stayed almost identically pink with ΔE<7 even at 500 °C and ii) WO3 which original color was not recovered upon cooling back to room temperature. While the color change of the other oxides was gradual with increasing temperature, YAG:Ce compound behaved differently since its thermochromicity abruptly started above 300 °C. DSC analyses did not allow detecting any physico-chemical event in relation with these peculiarities. The obtained datasets for all these compounds can be used for anticipating the thermochromic properties of these powders and the blends made out of them.

With a view to designing a homogeneous and controlled step-bunched morphology on large monocrystalline 4H-SiC(0001) surfaces, a sandwich configuration was investigated. A piece of silicon was melted between two 4H-SiC 4 off wafers, allowing a better spreading of the liquid than with a Si drop approach. Using as low liquid Si thickness as 30 mu m, homogeneously highly step-bunched surfaces were obtained, though with irregular step shapes. This was found to be the result of time dependent dissolution of the SiC-bottom wafer under the natural vertical thermal gradient of the setup. The most parallel step and terrace structures were generated using 400 mu m thick liquid Si though this led to long range inhomogeneities associated with complex mass transport. The proposed mechanism includes the combined effects of H2 etching at the edges acting as C pump and electro-magnetic convection rolls for long range transport of these dissolved C atoms.

Because the well-known site-competition and step-controlled epitaxy rules cannot reasonably describe all the incorporation processes of the main impurities (Al and N) into 4H-SiC during epitaxy, the concept of replacement incorporation was proposed and applied to explain the experimental results published so far. In this model, the transient formation of C or Si vacancies at the surface or sub-surface of terraces is proposed to play a key role by destabilizing the impurities sitting on them. In addition to the availability of these vacancies at the surface, desorption was proposed to be a very important limiting process for Al incorporation while only occasionally relevant for N incorporation. The main 4H-SiC epitaxial growth parameters are reviewed and discussed according to the proposed replacement model.

SiO2/SiC interfaces have been under intensive research, because the quality of the interfaces can significantly affect the performance and reliability of SiC power devices. For offering microscopic insights on the generation of interface defects, here we describe the application of our novel scanning probe microscopy method called local deep level transient spectroscopy (local DLTS) to a thermally oxidized Si-face 4H-SiC epitaxial layer. Our sample is an off-axially grown epitaxial layer which is intentionally modified to have wide flat terraces and macro-stepped features instead of typical terraces separated by atomic steps. A spatially resolved map of the interface defect density shows that the macro-steps have a significantly higher interface defect density than the wide flat terraces. The result suggests that high interface defect densities on typical off-axis SiO2/4H-SiC stacks might be related to the steps created during the step-controlled epitaxial growth. Local DLTS is useful for the microscopic evaluation of the interface quality.

The (001) surface of cubic silicon carbide (3C SiC) after cleaning, Ar sputtering and three different wet chemical etching procedures was thoroughly investigated via (angle resolved) XPS, HREELS, and LEED. While Ar sputtering was found to be unsuitable for surface preparation, all three employed wet chemical etching procedures (piranha/NH4F, piranha/HF, and RCA) provide a clean surface. HF as oxide removal agent tends to result in fluorine traces on the sample surface, despite thorough rinsing. All procedures yield a 1 x 1 SiOH/CH terminated surface. However, the XPS spectra reveal some differences in the resulting surface states. NH4F for oxide removal produces a flat band situation, whereas the other two procedures result in a slight downward (HF) or upward (RCA) band bending. Because the band bending is small, it can be concluded that the number of unsaturated surface defects is low. (C) 2017 Elsevier B.V. All rights reserved.

Low temperature (500-800 degrees C) homoepitaxy of not intentionally doped GaN structures on GaN(0001)/Si (111) seed has been investigated by Vapor-Liquid-Solid (VLS) approach. The growth sequence consists in the metalorganic chemical vapor deposition of a network of submicrometric liquid Ga droplets, followed by their nitridation under flowing ammonia diluted either in H-2 or Ar. When nitridation is performed under Ar carrier gas, GaN growth is very difficult to control due to too high N supersaturation within the droplets, despite very low NH3 flows. Nucleation and growth at the droplets periphery are always favored and, in most cases, high growth rates induce a crust-like growth, forming hollow GaN gangues. The use of H-2 as carrier gas is detrimental to GaN (seed and grown material) stability, for nitridation temperatures >= 700 degrees C. But, compared to Ar atmosphere, a pronounced decrease of N supersaturation is demonstrated, allowing a better control of the growth mode. This is probably a consequence of a lower thermal decomposition efficiency of NH3 at the droplets surface. Optimal growth conditions are found at relatively low temperature (600 degrees C) and NH3 flow (20 sccm) for which a network of well-separated and faceted epitaxial GaN dots or rings is obtained. The growth mechanisms allowing these results are discussed. (C) 2017 Elsevier B.V. All rights reserved.