Papers by Author: Mihai Lazar

Abstract: This work deals with the selective heteroepitaxial growth of silicon carbide on (100) diamond substrates using the Vapour-Liquid-Solid (VLS) transport. The morphology, the structure and doping were determined using various characterization techniques. In order to achieve succesful heteroepitaxy, the diamond surface was silicided by solid-state reaction between a silicon layer and the substrate at 1350 °C. This allowed forming a SiC buffer layer on which p-doped 3C-SiC(100) islands elongated in the <110> directions were obtained after VLS growth. The influence of the experimental parameters on the epitaxial growth is discussed.

Abstract: This work deals with the study of the Selective Epitaxial Growth (SEG) of SiC using the Vapour-Liquid-Solid (VLS) transport on diamond (100) substrate with Al-Si as the liquid phase fed by propane. Morphology, structure and doping type of the SiC deposit were determined. Polycrystalline p-doped 3C-SiC was obtained during the growth. Study of the initial step of growth showed that SiC nucleation occurs without any propane addition but just through the interaction of liquid Al-Si and diamond via a dissolution/precipitation process. This explains the random nucleation and the polycrystalline growth. Despite this, preliminary electrical measurements show encouraging results.

Abstract: Transfer Length Method (TLM) based-structures were fabricated on 0.8 µm-thick epitaxial p-type Silicon Carbide (4H-SiC) layers. TLM mesas were defined by a 2 µm height using an SF₆/O₂ reactive ion etching. TLM metal patterns were obtained by a lift-off procedure and electron beam deposition of Ni, Ti, Al and Pt. The patterned samples were annealed in Argon ambient at temperature ranging from 700°C up to 1000°C in a RTA furnace with a rapid heating ramp (up to 50°C/s) to complete the ohmic contact with the p-type SiC layer. Specific contact resistances were extracted from current/voltage measurements. To identify and follow the profile evolution of constituting element in the contacts and at the SiC/contact interface, the ohmic contacts were characterized using Secondary Ion Mass Spectrometry and Energy-Dispersive X-Ray spectroscopy before and after annealing. Ohmic contacts are obtained only for the Ni/Ti/Al and Ni/Ti/Al/Ni stacking layers and not for the Ti/Al/Ti/Ni and Ti/Al/Ti/Pt/Ni compositions. The specific contact resistance of Ni/Ti/Al/Ni stacking layers was observed to decrease from 2.7×10^-4 Ω.cm² at 700°C and 6.3×10^-5 Ω.cm² at 750°C to a minimal value of 1.5×10^-5 Ω.cm² at 800°C. Ohmic contacts are obtained with a reproducibility of 80 %.

Abstract: In this work we report on the study of twin boundary (TB) evolution during heteroepitaxial growth of 3C-SiC on patterned 4H-SiC(0001) substrate by vapour-liquid-solid (VLS) mechanism. Ge₅₀Si₅₀ melt was used at a temperature of 1450°C. 3C-SiC deposit was obtained on top and outside the mesas. Some lateral enlargement of these mesas was observed but it was systematically homoepitaxial. Elimination of TBs inside the 3C-SiC deposit on top of the mesas was observed for specific mesa shape and/or orientation of the sidewalls. Though three–fold or six-fold symmetry mesas are recommended for TB elimination, originally circular mesas lead also to the same result due to initial faceting toward hexagonal shape.

Abstract: This paper presents the design, fabrication, and characterization of micro planar inductors on a microwave magnetic material (YIG). Planar spiral inductors were designed for monolithic DC-DC converters in System-In-Package with 100 MHz switching frequency (1 W, V_in= 3.6 V, V_out= 1 V). A microwave magnetic substrate (YIG) serves as mechanical support, and also presents a double purpose by increasing inductance value and reducing electromagnetic interferences (EMI). This last point is critical to improve the behavior of a switching mode power supply (SMPS). In order to obtain an optimal design for the inductor, geometrical parameters were studied using Flux2D simulator and an optimized 30 to 40 nH spiral inductor with expected 25 mΩ R_DC, 3 mm² footprint area was designed. Subsequently, samples have been fabricated by electroplating technique, and tested using a vector network analyzer in the 10 MHz to 100 MHz frequency range. Results were then compared to the predicted response of simulated equivalent model.

Abstract: Progress in semiconductor technologies have been so consequent these last years that theoretical limits of silicon, speci cally in the eld of high power, high voltage and high temperature have been achieved. At the same time, research on other semiconductors, and es- pecially wide bandgap semiconductors have allowed to fabricate various power devices reliable and performant enough to design high e ciency level converters in order to match applications requirements. Among these wide bandgap materials, SiC is the most advanced from a techno- logical point of view: Schottky diodes are already commercially available since 2001, JFET and MOSFET will be versy soon. SiC-based switches Inverter e ciency bene ts have been quite established. Considering GaN alternative technology, its driving force was mostly blue led for optical drive or lighting. Although the GaN developments mainly focused for the last decade on optoelectronics and radio frequency, their properties were recently explored to design devices suitable for high power and high e ciency applications. As inferred from various studies, due to their superior material properties, diamond and GaN should be even better than SiC, silicon (or SOI) being already closed to its theoretical limits. Even if the diamond maturity is still far away from GaN and SiC, laboratory results are encouraging speci cally for very high voltage devices. Apart from packaging considerations, SiC, GaN and Diamond o ers a great margin of progress. The new power devices o er high voltage and low on-resistance that enable important reduction in energy consumption in nal applications. Applications for wide bandgap materials are the direction of high voltage but also high temperature. As for silicon technology, WBG-ICs are under development to take full bene ts of power and drive integration for high temperature applications.