Papers by Author: Davy Carole

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: This study deals with the electrical characterization of PiN diodes fabricated on a 4°off-axis 4H-SiC n⁺ substrate with a n^- epilayer (1×10¹⁶ cm^-3 / 10 µm). Optimized p⁺⁺ epitaxial areas were grown by Vapour-Liquid-Solid (VLS) transport to form p⁺ emitters localized in etched wells with 1 µm depth. Incorporated Al level in the VLS p⁺⁺ zones was checked by SIMS (Secondary Ion Mass Spectroscopy), and the doping level was found in the range of 1-3×10²⁰ at.cm^-3. Electrical characterizations were performed on these PiN diodes, with 800 nm deposit of aluminium as ohmic contact on p-type SiC. Electrical measurements show a bipolar behaviour, and very high sustainable forward current densities ≥ 3 kA.cm^-2, preserving a low leakage current density in reverse bias. These measurements were obtained on structures without any passivation and no edge termination.

Abstract: This work reports on the CVD heteroepitaxial growth of 3C-SiC layers on diamond (100) substrates. To obtain good layer quality, the growth procedure involves a “silicidation” step consisting in depositing a silicon layer by CVD on the diamond substrate, in order to elaborate a very thin SiC buffer layer. 3C-SiC growth is then performed on this SiC seeding layer. Silicidation and growth parameters have been studied in order to improve the quality and the morphology uniformity of the heteroepitaxial layer. The study points out the role of liquid silicon during the growth process.

Abstract: P/N junctions have been fabricated with N⁺ commercial 4H-SiC substrate on which Vapor-Liquid-Solid (VLS) selective epitaxy was used to create a localized p-type doping. The influence of the carrier gas nature (argon or hydrogen) has been investigated in terms of quality of the growth morphology, deposit thickness and electrical behavior of the P/N junction. Distinct results have been observed with a clear improvement when using VLS selective epitaxy under hydrogen.

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: Lateral JFET transistors have been fabricated with N and P-type channels tentatively integrated monolithically on the same SiC wafer. Buried P⁺ SiC layers grown by Vapor-Liquid-Solid (VLS) selective epitaxy were utilized as source and drain for the P-JFET and as gate for the N-JFET. The ohmicity of the contacts, both on VLS grown P⁺ and ion implanted N⁺ layers, has been confirmed by Transfer Length Method (TLM) measurements. A premature leakage current is observed on the P/N junction created directly by the P⁺ VLS gate layer, probably due to imperfect VLS (P⁺) / CVD (N⁺) SiC interface.

Abstract: The interaction of liquid Ge and Si droplets, deposited by CVD, on the surface of 4H-SiC single-crystals is studied. It was found that at 1500 °C Ge forms micrometric droplets while Si forms nanometric dots. While the Si dots do not seem to interact significantly with SiC, the Ge droplets have the tendency to dissolve the Si from the seed. This mechanism not only happens during deposition but also during the cooling. If the cooling rate is too slow, Ge evaporates from the droplets while dissolving Si so that, at the end, the droplets look like to have been fully converted from Ge to Si.

Abstract: Starting from the previously demonstrated twin-free 3C-SiC growth on 4H-SiC when using Ge pre-deposition treatment, this work focuses on the understanding of the growth mechanism that stands behind this result. Toward this end, short growth experiments were performed to allow the investigation of the nucleation stage. Based on the experimental observations, a mechanism is proposed which involves a Ge-induced transient homoepitaxial growth step followed by 3C nucleation when large terraces are formed by step faceting. Lateral expansion of the 3C islands leads to orientation selection and twin boundary elimination. Similar results can be obtained when applying a Si-based pre-deposition treatment so that the crucial transient homoepitaxial step is promoted in fact by the presence of a liquid phase itself, no by its chemical nature.

Abstract: This work reports on the improvement of ohmic contacts made on heavily p-type doped 4H-SiC epitaxial layer selectively grown by Vapor-Liquid-Solid (VLS) transport. Even before any annealing process, the contact is ohmic. This behavior can be explained by the high doping level of the VLS layer (Al concentration > 10²⁰ cm^-3) as characterized by SIMS profiling. Upon variation of annealing temperatures, a minimum value of the Specific Contact Resistance (SCR) down to 1.3x10^-6 Ω.cm² has been obtained for both 500 °C and 800 °C annealing temperature. However, a large variation of the SCR was observed for a same process condition. This variation is mainly attributed to a variation of the Schottky Barrier Height.

Abstract: This work deals with two applications of the Selective Epitaxial Growth of highly p-type doped buried 4H-SiC in Vapor-Liquid-Solid configuration (SEG-VLS). The first application is the improvement of the Specific Contact Resistance (SCR) of contacts made on such p-type material. As a result of the extremely high doping level, SCR values as low as 1.3x10^-6 Ω.cm² have been demonstrated. Additionally, the high Al concentration of the SEG-VLS 4H-SiC material induces a lowering of the Al acceptor ionization energy down to 40 meV. The second application is the fabrication of PiN diodes with SEG-VLS emitter and guard-rings peripheral protection. Influence of some process parameters and crystal orientation on the forward and reverse characteristics of the PiN diodes is discussed.