Materials Science Forum Vols. 679-680

Abstract: Epitaxial graphene produced from SiC substrates exhibits a carrier mobility re- duction thought to arise from intercalated silicon. We present the results of density functional simulations and show that individual silicon atoms are highly mobile on and between graphene sheets, suggesting that thermally stable structures involving individual Si impurities are likely to result from the interaction of silicon with defects in the graphene sheets.

Abstract: In this study we examined the structural evolution of graphene grown on 8° off-axis 4H-SiC(0001) substrates at temperatures from 1600°C to 1700°C in Ar ambient. Morphological transformation of SiC substrate after annealing was examined by Tapping Mode Atomic Force Microscopy. Moreover, by etching-out graphene layers from graphitized SiC substrates in selective trenches we determined the number of graphene layers. Numbers of graphene layers were then independently confirmed by Transmission Electron Microscopy imaging.

Abstract: Gallium nitrides are primarily used for their excellent light emission properties. GaN LEDs are mostly grown on foreign substrates, essentially sapphire and SiC, but more recently, also on Si substrates. In this paper, we will demonstrate that the high structural quality of InGaN/GaN multiple quantum wells can be deposited on 3C-SiC/Si (111) substrate using MOCVD. This demonstrates that 3C-SiC/Si is a promising template for the epitaxial growth of InGaN/GaN multiple quantum wells for LEDs.

Abstract: Gallium nitride (GaN), grown by HVPE, was implanted with 300 keV Eu ions and then annealed at 1000 oC . Deep level transient spectroscopy (DLTS) and Laplace DLTS (L-DLTS) were used to characterise the ion implantation induced defects in GaN. Two of the implantation induced defects, E1 and E2, with DLTS peaks in the 100 – 200 K temperature range, had DLTS signals that could be studied with L-DLTS. We show that these two defects, with energy levels of 0.18 eV and 0.27 eV below the conduction band, respectively, are two configurations of a metastable defect. These two defect states can be reproducibly removed and re-introduced by changing the pulse, bias and temperature conditions, and the transformation processes follow first order kinetics.

Abstract: In this work the electrical and structural properties of AlGaN/GaN heterostructures grown onto 8°-off-axis 4H-SiC epilayers were investigated. A morphological and structural analysis of the samples showed the presence of “V-shaped” near-surface defects in the AlGaN layer, with a preferential orientation along the miscut direction [11-20]. In the presence of these defects an anisotropy of the current-voltage characteristics of high electron mobility transistors (HEMTs), fabricated with two different orientations, was observed. The sheet carrier density ns and the channel mobility n were determined from the device characteristics. The results were discussed considering the possible implications for AlGaN/GaN HEMT technology.

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 temperature dependence of Ohmic contacts to GaN devices is investigated in this paper via by measuring TLM contact resistances TLM vs Tas a function of temperature. measurements. In particular, the two types of Ohmic contacts are considered: (1) Contacts to highly doped implanted regions (such as the MOSFET drain/source contacts or the back contact of Schottky diodes) and (2) contacts to the 2 dimensional electron gas (2DEG) of an AlGaN/GaN heterojunction.

Abstract: Wide band gap semiconductors have been attracted as the material for fabricating power switching devices to obtain lower power conversion loss in high voltage circuit, and to operate harsh environment of high temperature. This paper focuses on diamond as the wide band gap semiconductor material and elucidates the dynamic characteristics in switching operation. To this end, Schottky barrier diode (SBD) is fabricated with p type diamond semiconductor and static I-V characteristics is evaluated. Then, the switching operation of diamond SBD is demonstrated, and forward current dependency of the recovery phenomena is characterized. The diamond SBDs show superior fast switching capability with low reverse recovery current, which is inherent in uni-polar switching device.

Abstract: Crystalline silicon carbide (SiC) and silicon (Si) biocompatibility was evaluated in vitro by directly culturing three skin and connective tissue cell lines, two immortalized neural cell lines, and platelet-rich plasma (PRP) on these semiconducting substrates. The in vivo biocompatibility was then evaluated via implantation of 3C-SiC and Si shanks into a C57/BL6 wild type mouse. The in vivo results, while preliminary, were outstanding with Si being almost completely enveloped with activated microglia and astrocytes, indicating a severe immune system response, while the 3C-SiC film was virtually untouched. The in vitro experiments were performed specifically for the three adopted SiC polytypes, namely 3C-, 4H- and 6H-SiC, and the results were compared to those obtained for Si crystals. Cell proliferation and adhesion quality were studied using MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays and fluorescent microscopy. The neural cells were studied via atomic force microscopy (AFM) which was used to quantify filopodia and lamellipodia extensions on the surface of the tested materials. Fluorescent microscopy was used to assess platelet adhesion to the semiconductor surfaces where significantly lower values of platelet adhesion to 3C-SiC was observed compared to Si. The reported results show good indicators that SiC is indeed a more biocompatible substrate than Si. While there were some differences among the degree of biocompatibility of the various SiC polytypes tested, SiC appears to be a highly biocompatible material in vitro that is also somewhat hemocompatible. This extremely intriguing result appears to put SiC into a unique class of materials that is both bio- and hemo-compatible and is, to the best of our knowledge, the only semiconductor with this property.

Abstract: An ever-increasing demand for biocompatible materials provides motivation for the development of advanced materials for challenging applications ranging from disease detection to organ function restoration. Carbon-based materials are considered promising candidates because they combine good biocompatibility with high chemical resistance. In this work we present an initial assessment of the biocompatibility of epitaxial graphene on 6H-SiC(0001). We have analyzed the interaction of HaCaT (human keratinocyte) cells on epitaxial graphene and compared it with that on bare 6H-SiC(0001). We have found that for both graphene and 6H-SiC there is evidence of cell-cell and cell substrate interaction which is normally an indication of the biocompatibility of the material.