Materials Science Forum Vols. 717-720

Abstract: Hydrogen energy attracts attention as an eco-friendly energy resource. The water splitting by semiconductor materials can generate hydrogen without CO2 emission. However, the hydrogen conversion efficiency using conventional materials is not high enough, or the materials corrode easily even if they show high efficiency. On the other hand, silicon carbide (SiC) is expected to be a water splitting material showing high conversion efficiency without corrosion. In this study, we characterized band edge potentials for 4H-, 6H- and 3C-SiC, and we revealed that they are capable of water splitting. We also estimated conversion efficiencies by photocurrent measurements in electrolytes for bulk 4H- and 6H-SiC.

Abstract: The purpose of the present study is to determine the appropriate physical characterization methods for evaluating material quality changes during the fabrication steps of a typical 4H-SiC Static Induction Transistors (SITs). The most important fabrication step in terms of material quality is the gate implantation and post-implantation annealing. For the purposes of the initial investigation, separate “witness” samples from the processed sample have been used for evaluating implantation and post-implantation annealing. Secondary Ion Mass Spectroscopy (SIMS), optical transmission, room-temperature photoluminescence (RTPL), High Resolution X-Ray diffraction (HRXRD) and C-V measurements with Hg-probe and electrochemical (ECV) cells have been investigated in the frame of the present study. HRXRD and ECV have been proved particularly suitable for characterizing the implanted layers.

Abstract: We propose an atomistic model to study the interface properties of mis-oriented (turbostratic) epitaxial graphene on SiC (000-1) surface. Using calculations from first principles, we compare the energetics, and structural/electronic properties of AB and turbostratic stacking sequences within a model based on the Si adatom surface reconstruction. Our calculations show that the systems with AB and turbostratic sequences are very close in energy, demonstrating the possibility of the observation of Moire patterns in epitaxial graphene on the C-face of SiC. The two-dimensional electron gas behavior is preserved in the epitaxial turbostratic graphene systems. However, there are deviations from the ideal turbostratic epitaxial graphene.

Abstract: We report a new approach to produce high quality epitaxial graphene based on the concept of controlling Si sublimation rate from SiC surface. By putting a mask substrate to suppress Si sublimation from the SiC surface in ultrahigh vacuum, epitaxial graphene growth at 4H-SiC (0001) was locally controlled. Spatially graded surface graphitization was confirmed in a scanning electron microscopy contrast from the outside unmasked region to the inside masked region. The contrast was discussed with Raman characterization as the increase of graphene thickness and the surface compositional change of SiC. Results indicate two types of growth processes of epitaxial graphene at 4H-SiC (0001) step-terrace structures.

Abstract: We report graphene thickness, uniformity and surface morphology dependence on the growth temperature and local variations in the off-cut of Si-face 4H-SiC on-axis substrates. The transformation of the buffer layer through hydrogen intercalation and the subsequent influence on the charge carrier mobility are also studied. A hot-wall CVD reactor was used for in-situ etching, graphene growth in vacuum and the hydrogen intercalation process. The number of graphene layers is found to be dependent on the growth temperature while the surface morphology also depends on the local off-cut in the substrate and results in a non-homogeneous surface. Additionally, the influence of dislocations on surface morphology and graphene thickness uniformity is also presented.

Abstract: The formation of epitaxial graphene on SiC(000-1) in a disilane environment is studied. The higher graphitization temperature required, compared to formation in vacuum, results in more homogeneous thin films of graphene. Some areas of the surface display unique electron reflectivity curves not seen in vacuum-prepared samples. Using selected area diffraction, these areas are found to have a graphene/SiC interface structure with a graphene-like buffer layer [analogous to what occurs on SiC(0001) surfaces].

Abstract: Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp 1x1 patterns in m-LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few mm was obtained on some samples.

Abstract: Large scale, homogeneous quasi-free standing monolayer graphene is obtained on a (111) oriented cubic SiC bulk crystal. The free standing monolayer was prepared on the 3C-SiC(111) surface by hydrogen intercalation of a -reconstructed carbon monolayer, so-called zerolayer graphene, which had been grown in Ar atmosphere. The regular morphology of the surface, the complete chemical and structural decoupling of the graphene layer from the SiC substrate as well as the development of sharp monolayer p-bands are demonstrated. On the resulting sample, homogeneous graphene monolayer domains extend over areas of hundreds of square-micrometers.

Abstract: Structural and electrical properties of graphene elaborated on 3C-SiC(111)/Si and 3C-SiC(100)/Si templates, using propane-argon gas mixtures under CVD environment, are presented. On 3C-SiC(111), the graphitic phase is clearly attributable to graphene and presents good electrical conductivity at the macroscopic scale. The opposite case is observed on 3C-SiC(100), for which the graphitic phase develops more rapidly but with a high degree of disorientation. The graphitization, which can be coupled with 3C-SiC growth stage, is efficient over the whole surface of 2’’ wafer and allows to elaborate, in a single process, Graphene on Silicon wafers.

Abstract: We have grown graphene on SiC(0001) using propane-hydrogen CVD. In this work, we present the effects of growth pressure and temperature on structural and electrical properties. Structural characterizations evidence the formation of graphene with in-plane rotational disorder, except for low growth pressure and high growth temperature which lead to the formation of a (6Ö3´6Ö3)-30° interface between graphene and SiC. Electrical properties of samples presenting different graphene/SiC stacking and interfaces are compared and discussed.