Papers by Author: Olivier Dezellus

Abstract: The aim of this study was to find conditions allowing the "natural" formation of a regular and controllable step bunched morphology on a 4H-SiC seed without the need of any SiC deposition. This was performed by melting a bulk piece of Si on a 4°off 4H seed in the temperature range of 1500 - 1600°C, for 15 min. After etching the remaining Si, the 4H surface was found to be successfully highly step bunched with steps very parallel and regular. A mechanism of dissolution-precipitation was proposed, which could occur both on a short (step to step) and long (centre to periphery) range. This process is kinetically limited at low temperature (1500-1550°C) and considered to be close to the equilibrium at 1600°C.

Abstract: Ohmic contacts represent a major technological brick for the development of high power devices on Gallium Nitride. Al(200 nm) Ti(70 nm) metallization on n+-GaN, annealed at 650 °C, provides a “Specific Contact Resistivity” (SCR) in the range mid 10-5 Ω.cm², which is low enough for the main switching power applications. However, the Al-Ti metallic compound phases formed during the annealing step result from solid-solid reactions, which may lead to high stress and / or poor cohesion, possibly deleterious to contact reliability. In this work, we have investigated several configurations of Ti-Al-Si based contacts, aiming at favoring liquid-solid reactions and / or Si element diffusion, in order to get better SCR and / or morphology and cohesion of the metallic phase. Surprisingly, only contacts annealed at low temperature (450 °C) provide low contact SCR, comparable to that of Ti-Al only contact, but systematically higher.

Abstract: The influence of nitrogen impurity on the stabilization of 3C-SiC polytype has been studied during vapour-liquid-solid (VLS) growth on 6H-SiC(0001) seed with Si-Ge melt. By changing the partial pressure of N2 during growth, it was found that the proportion of 3C-SiC inside the grown material increases with N2 partial pressure. 6H inclusions are only found for high purity (low N2 content) conditions. The possible interactions proposed to explain this effect are divided in two effects: i) lattice parameter modification and ii) surface induced lateral enlargement variation. A combination of both effects is suspected.

Abstract: Twin-free 3C-SiC layers were recently obtained by Vapour-Liquid-Solid mechanism on a a-SiC(0001) substrate using Si-Ge melt. The formation of cubic layers is rather unexpected since growth from the melt is known to promote lateral growth and should thus give homoepitaxial layers. The study of the early stage of such growth, after a simple contact between the melt and the substrate (without adding propane), reveals the precipitation of 3C-SiC elongated islands upon the substrate surface. The chemical interactions inside the Ge-Si-C ternary phase diagram suggest an initial dissolution of the SiC seed in contact with a Ge-rich melt (below 1200°C). When the Si content of the melt subsequently increases upon heating, the dissolved carbon atoms precipitate on the seed surface under the form of 3C-SiC islands. When propane is added, these islands enlarge and coalesce to form a complete 3C layer.