Materials Science Forum Vols. 821-823

Abstract: The present communication focuses on the bilayer graphene formation on a Ge doped 4H-SiC surface. The 4H-SiC epilayer was grown by CVD with Germane (GeH₄) as the dopant precursor. This easily leads to the formation of Ge islands as well as graphene on the 4H-SiC surface. The Ge island decorated surface was studied by Raman spectroscopy, XPS, and TEM. It was found that the bilayer graphene is free standing and that the native oxides act as a buffer layer on the surface, covering the Ge islands. The intensity variations of the Ge component in the XPS spectra indicate that Ge atoms can be buried in the SiC surface. The TEM analysis revealed that the graphene layers are in the form of flakes, which can also be rived vertically with the support of the Ge islands on SiC surface.

Abstract: Si nanowires (NWs) samples have been converted to silicon carbide (SiC) NWs at different conditions of substrate temperature in an ultra-high vacuum using a molecular beam epitaxy (MBE) set-up. Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED) have been in-situ carried out to control the growth process. Scanning electron microscopy (SEM) and conventional transmission electron microscopy (CTEM) have been used to characterize the resulting nanostructures. In addition, the samples have been prepared by focused ion beam (FIB) in order to have electron-transparently lamellas for TEM with the interface nanowire-substrate. SiC/Si shell/core NWs free of planar defects have been obtained for conversion tmpratures lower than 800oC.

Abstract: Aluminium is found to play a key role in the process of forming a mechanically stable and highly porous and granular structure of 4H silicon carbide. The material is prepared by a high temperature reaction of the elemental constituents. The reactions are carried out under different background atmospheres, including nitrogen. Ternary carbides containing Al, Si and N, are formed in the process, and are believed to be responsible for the final outcome of the process, at the highest reaction temperatures, in the form of pure, well-connected grains of 4H-SiC forming a strong and rigid structure with high porosity. The Al containing compounds function as structural promoters for the 4H polytype recrystallization. This is expected - and partly shown - to take place through substitution with 4H-SiC and evaporation of all other constituents during the high temperature sintering step. When extruded into honeycomb structures prior to the sintering process this pure mesoporous SiC final product turns out to be ideal for a combined diesel particulate filter with support for catalysts in the pores.

Abstract: High temperature solution growth of Al₄SiC₄ single crystals was carried out from the melt of silicon and aluminium pieces in a graphite crucible used as carbon source (typically 1800°C under 1 bar of argon). The obtained crystals, in the mm scale, were then characterized by a variety of techniques including: Raman spectroscopy, transmission electron microscopy techniques, X-ray diffraction and UV-Vis-NIR spectroscopy. A good structural quality was revealed by TEM, ensuring a well resolved Raman spectrum. The UV-Vis transmission spectrum shows an optical gap located around 2-2.5 eV.

Abstract: We evaluate the influence of the growth parameters on the crystal quality of Si films grown by chemical vapor deposition on 3C-SiC(001)/Si (001) epilayers. It is shown that the pressure plays a major role on the final quality of the films, with two distinct growth regimes. The defects in the films were found to be antiphase boundaries and μ-twins. The influence of the growth parameters as well as the 3CSiC structural properties on these defects are discussed. The impact of a subsequent thermal annealing, under different gas environments, is also investigated and reveals some noticeable differences according to the gas environment used in the annealing process.

Abstract: In this study we present the diamond deposition on AlGaN/GaN substrates focusing on the quality of the diamond/GaN interface. The growth of diamond films was performed using microwave chemical vapour deposition system in different gas mixtures: standard CH₄/H₂ (at low and high ratio of CH₄ to H₂) and addition of CO₂ to CH₄/H₂ gas chemistry. The diamond films were grown directly on GaN films either without or with thin interlayer. As interlayer, 100 nm thick Si₃N₄ was used. Surprisingly, in the case of standard CH₄/H₂ gas mixture, no diamond film was observed on the GaN with SiN interlayer, while adding of CO₂ resulted in diamond film formation of both samples with and without SiN interlayer. Moreover, adding of CO₂ led to higher growth rate. The morphology of diamond films and the quality of the diamond/GaN interface was investigated from the cross-section images by scanning electron microscopy and the chemical character (i.e. sp³ versus sp² carbon bonds) was measured by Raman spectroscopy.

Abstract: A nanoscale electrical characterization of graphene (Gr) contacts to Al_xGa_1-xN/GaN heterostructures has been carried out using conductive atomic force microscopy. The impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface was evaluated considering two Al_0.25Ga_0.75N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, i.e. a uniform and defect-free sample and a sample with a high density of V-defects, that locally cause a reduction of the AlGaN thickness. Rectifying contacts were found on the bare (Gr-free) AlGaN surfaces of both samples, but with a more inhomogeneous and lower Schottky barrier height (Φ_B≈0.6 eV) in the presence of V-defects with respect to the case of the uniform AlGaN (Φ_B≈0.9 eV). Very different electrical behaviour was observed for Gr on the two AlGaN samples, i.e. a low barrier height Schottky contact (Φ_B≈0.4 eV) for the uniform AlGaN and an Ohmic contact for the defective AlGaN. Both Schottky and ohmic Gr/AlGaN contacts exhibit an excellent lateral uniformity, that can be ascribed to an averaging effect of the Gr electrode over the AlGaN interfacial inhomogeneities.

Abstract: We give here an overview of our recent work on growth of rhombohedral boron nitride (r-BN) thin films on SiC substrates by chemical vapor deposition (CVD). We demonstrate the growth of twinned r-BN on various SiC polytypes at 1500 °C, using H₂ as carrier gas and triethyl boron and ammonia as precursors with an N/B ratio of ~ 640. The epitaxial relation with various substrates is determined from XRD and TEM. Adding Si to the gas phase stabilizes the r-BN phase but does not alter the electric properties of the material which remains electrically insulating.

Abstract: GaN High Electron Mobility Transistors (HEMTs) on SiC have gained remarkable attention as these devices are revolutionizing the power and radio frequency (RF) electronics markets. Although significant advances have been made in transistor technology innovations, innovations pertaining to the backside process technology have been noticeably few. This paper will address and focus on innovations in the backside processing of GaN on SiC devices. A series of innovations in backside processing enable through SiC and GaN via etch rates to exceed 1.5 micron/minute. Wafer dicing process innovations afforded a >4x improvement in sawing throughput and a >6x improvement in blade lifetime through the novel addition of ultrasonic power to a conventional sawing tool.

Abstract: AlGaN/GaN heterostructures are important materials for the fabrication of high power and high frequency devices. However, the mechanisms of Ohmic contacts formation on these systems are continuously under scientific debate. In this paper, a structural and electrical investigation of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures is reported. In particular, the behavior of Ti/Al/Ni/Au multilayers was monitored at different annealing temperatures. The contacts became Ohmic after annealing at 750°C and showed a decreasing temperature behavior of the specific contact resistance R_C, described by a thermionic field emission mechanism. On the other hand, annealing at 850°C led to a further reduction of R_C , with a slightly increasing dependence of R_C on the measurement temperature (here regarded as a “metal-like” behavior). The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.