Papers by Author: Gabriel Ferro

Abstract: Local electrical properties of a 4H-Silicon Carbide SiC(0001) 4°off macrostepped surface, obtained after liquid Si melting in a SiC/Si/SiC sandwich configuration, are investigated by Atomic Force Microscopy (AFM) in both DC and RF modes. On the same sample, macrosteps that are wide enough for allowing spatial resolution of the signal from terraces and step risers, but also some unreacted areas with standard flat surface (without macrosteps) are characterized. Scanning Spreading Resistance (SSRM, DC mode) reveals homogeneous conductivity on the wide terraces of the 4H-SiC(0001) macrosteps. On unreacted areas, which contain many step risers, the resistance is found higher than on the wide terrasses but it is also noisier. In addition, the AFM-RF scanning Microwave Impedance Microscopy (sMIM) mapping confirms the previous results by revealing lower conductivity on the unreacted areas than on the terraces of the macrosteps. Based on these results, some points defects located at the step risers which contribute negatively to the electrical properties of 4H-SiC(0001) surface are identified and electrically characterized.

Abstract: The chemical vapor deposition (CVD) growth of boron carbide (B_xC) 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 BCl₃+C₃H₈ systematically led to polycrystalline B_xC 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 BCl₃ 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_xC, promoting thus the heteroepitaxial growth along this direction.

Abstract: 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.

Abstract: Mesa- and trench-patterned surfaces of 4H-SiC(0001) 4°off wafers were structured in macrosteps using Si melting in a SiC-Si-SiC sandwich configuration. Si spreading difficulties were observed in the case of trench-patterned samples while the attempts on mesa-patterned ones were more successful. In the latter case, parallel macrosteps were formed on both the dry-etched and unetched areas though these macrosteps rarely cross the patterns edges. The proposed mechanism involved preferential etching at Si-C bilayer step edges and fast lateral propagation along the [1120] direction.

Abstract: In this work, the successful heteroepitaxial growth of boron carbide (B_xC) 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 BCl₃ precursor at 1200°C, followed by conventional CVD under BCl₃ + C₃H₈ at 1600°C. Such a procedure allowed obtaining reproducibly monocrystalline (0001) oriented films of B_xC 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_xC layer is formed after boridation but covered with a B-and Si-containing amorphous layer. Upon heating up to 1600°C, under pure H₂ atmosphere, the amorphous layer was converted into epitaxial B_xC and transient surface SiB_x and Si crystallites. These crystallites disappear upon CVD growth.

Abstract: The reconstruction of 4°off 4H-SiC surfaces was investigated using Si melting at 1550°C in a SiC/Si/SiC sandwich configuration. Despite systematically obtaining a macrostepped morphology over the entire areas in contact with the liquid Si, the steps were found wavy when using as-received 4H-SiC wafers. The regularity and straightness of the steps were significantly improved when the surface reconstruction was performed on processed surfaces: on re-polished surfaces the steps were found to be regular and straight in some cases, while this was constantly observed on as-grown epitaxial layers. After a reconstruction process of 2h, the best regularity of the steps was obtained with an average width of ̴ 3-5 μm. Increasing the processed area from 1.44 to 4 cm² did not affect the results which suggests a good scalability of the process.

Abstract: 4H-SiC/Si_(liq)/4H-SiC stacks were treated at 1550-1600°C under H₂ in a RF-heated cold-wall reactor in order to generate macrosteps-structuring of the 4°off SiC(0001) wafers. Using 400 μm thick liquid Si, the observed important matter transport from the edges to the center of the same wafer was attributed to RF-induced convection rolls inside the thick liquid Si. When the liquid thickness was reduced down to 30 μm, the matter transport followed this time the vertical thermal gradient like in the case of liquid phase epitaxy. The dissolution rate of the bottom (hotter) wafer was found to increase from 1.7 μm/h at 1550°C to 3.3 μm/h at 1600°C. The use of H₂ gas was found essential to the system since it does not generate gas trapping (unlike Ar) and it participates to the creation of the vertical thermal gradient.

Abstract: This work investigates the 3C-SiC heteroepitaxial growth on silicon substrates having a wide variety of orientations, i.e. (100) on axis and 2°off, (111), (110), (211), (311), (331), (510), (553) and (995). All the 3C-SiC layers were grown using the same two-step CVD process with a growth rate of 2 μm/h. According to X-ray diffraction characterizations, direct heteroepitaxy (layer having exactly the same orientation as the substrate) was successful on most of the Si substrates except for (110) one which was the only orientation leading to obvious polycrystalline deposit. Each layer led to a specific surface morphology, the smoothest being the ones grown on Si (100)2°off, and (995) substrates. None of these layers cracked upon cooling though those grown on Si (111), (211) and (553) substrates were highly bowed.

Abstract: In view of obtaining a step bunched morphology on large 4H-SiC surfaces, a sandwich configuration is investigated. A piece of silicon is melted between two 4H-SiC 4° off wafers, allowing a better spreading of the liquid than a Si drop approach. This successfully leads to highly step-bunched surfaces, though with irregular steps. The most regular step and terrace stuctures were found to be the result of epitaxial growth via a dissolution-precipitation process occuring from the edges to the center of the wafers. This is probably caused by radio-frequency induced electromagnetic convection within liquid Si. This process is quenched when using smaller liquid thickness.

Abstract: 3C-SiC layers of different microstructures (monocrystalline (100) and (111) oriented and polycrystalline) were implanted with high energy (800 keV) ¹²⁹Xe⁺⁺ ions. Implantations were performed at room temperature (RT) and at 500 °C using two different fluences of Φ₁ = 1x10¹⁶ and Φ₂ = 1x10¹⁷ at/cm². Surface blistering was only observed for RT and Φ₂ implantations into poly-SiC material while mono-SiC kept rather smooth surface. This was due to more homogeneous Xe bubbles distribution (200 nm deep) in the mono-SiC than in the poly-SiC. Xe retention was found to be almost complete for all samples. Some Xe enhanced diffusion was detected in the poly-SiC material which was attributed to grain boundaries. Some irradiation-induced oxidation effect was evidenced, O element being located at the depth where Xe bubbles are accumulating. This was more pronounced for poly than for mono-SiC. These results demonstrate that SiC microstructure affects many aspects of its behavior upon Xe irradiation.