Materials Science Forum Vols. 556-557

Abstract: Vacancies, divacancies and carbon vacancy-carbon antisite pairs are found by electron paramagnetic resonance (EPR) to be dominant defects in high-purity semi-insulating (HPSI) 4HSiC substrates having different thermal activation energies of the resistivity ranging from ~0.8 eV to ~1.6 eV. Based on EPR results and previously reported data, the energy positions of several acceptor states of the vacancies and vacancy-related complexes are estimated. These deep levels are suggested to be associated to different thermal activation energies and responsible for the semiinsulating behaviour in HPSI SiC substrates. Their role in carrier compensation is discussed.

Abstract: Scandium can be used to influence the stoichiometry of SiC during growth of the hexagonal polytypes. Using PL-EPR and total energy calculations in the framework of density functional theory, scandium is predicted to be built in predominantly at the Si-sublattice in form of ScSi acceptors with acceptor levels at 0.55 eV (6H-SiC) and 0.48 eV (4H-SiC). In addition, new PL-EPR spectra are found with a large anisotropy in the g-tensor suggesting defect pairs as an origin.

Abstract: Halide chemical vapor deposition (HCVD) allows for rapid growth while maintaining the purity afforded by a CVD process. While several shallow and deep defect levels have been identified in 6H HCVD substrates using electrical techniques, here we examine several different point defects found in 4H n-type HCVD SiC using electron paramagnetic resonance (EPR) spectroscopy. One spectrum, which exhibits axial symmetry and broadens upon heating, may represent a collection of shallow defects. The other prominent defect has the g tensor of the negatively charged carbon vacancy, but additional hyperfine lines suggest a more complex center. The role of these defects is not yet determined, but we note that the concentrations are similar to those found for the electrically detected defect levels, making them a reasonable source of electrically active centers.

Abstract: We report the results of a SIMS and micro-Raman investigation performed on cubic (3C) SiC crystals grown on hexagonal SiC seeds using a Ge-Si bath and the so-called Vapor Liquid Solid growth technique. From SIMS measurements, we find a Ge concentration which, roughly, scales like the Ge concentration in the melt and, in term of micro-Raman measurements, explains the presence of weak but discernable Ge-Ge peaks around 300 cm-1. Since no similar Si-Si vibrations are found, this discard the possibility of having at the same time both Ge and Si constitutional super-cooling with two separate Ge and Si phases.

Abstract: Atomic structure and morphology of 6H-SiC(0001) and 3C-SiC(100) surfaces are studied by scanning tunneling microscopy (STM), synchrotron radiation-based !-spot x-ray photoemission spectroscopy (!-spot XPS) and low energy electron microscopy (LEEM). STM shows very high quality Si-rich 6H-SiC(0001) 3x3 surfaces with less than 2% of atomic defects. Si removal upon annealing leads to atomic crack defects formation with a novel 2"3x2"3-R30° reconstruction coexisting with few 3x3 domains having no crack, suggesting important stress relief during the phase transition. LEEM also shows cracks formation on cubic 3C-SiC(100) surfaces and gives insights about surface morphology with large faceting and mesa (!m) formation. These defect fractures developing upon Si removal are likely to be also generated during initial oxidation since the initial oxygen interaction tends to relieve surface strain on SiC in contrast to Si surfaces. These atomic crack defects could be related to the interface electronic states recurrent at SiO2/SiC interfaces.

Abstract: We demonstrate how sodium enhanced oxidation of Si face 4H-SiC results in removal of near-interface traps at the SiO2/4H-SiC interface. These detrimental traps have energy levels close to the SiC conduction band edge and are responsible for low electron inversion channel mobilities (1-10 cm2/Vs) in Si face 4H-SiC metal-oxide-semiconductor field effect transistors. The presence of sodium during oxidation increases the oxidation rate and suppresses formation of these nearinterface traps resulting in high inversion channel mobility of 150 cm2/Vs in such transistors. Sodium can be incorporated by using carrier boats made of sintered alumina during oxidation or by deliberate sodium contamination of the oxide during formation of the SiC/SiO2 interface.

Abstract: Using density functional theory, we investigate the 6H-SiC{0001} surfaces in the unreconstructed 1 × 1 and the H-passivated configuration. The strong correlation effects of the dangling bonds at the surface are treated by spin-polarised calculations including the Hubbard-U parameter. We find that the clean surfaces are semiconducting with surface states in good agreement with experimental data. The impact of the Hubbard-U is stronger on the C-terminated face. For the H-passivated surfaces we find resonances in the valence band. The antibonding C−H state is located in the upper part of the bandgap around the ¯􀀀-point.

Abstract: In 4H silicon carbide MOSFETs, threshold voltage varies with temperature. It is believed that this is caused by trapping of inversion electrons at high density of interface-traps (Dit) present at the SiC/SiO2 interface in 4H-SiC MOSFETs. In this work, we present an approach to model the interface trap density as a function of temperature that includes the effect of band gap narrowing. Using the temperature dependent trap charge density, we can estimate the variation of mobile inversion layer charge density, which in turn, explains the threshold voltage behavior with temperature in 4H-SiC MOSFETs.

Abstract: The nano-characterization of thermal oxides grown on 4H-SiC is for the first time presented and analysed to derive its reliability. The dielectric breakdown (BD) kinetics of silicon dioxide (SiO2) thin films thermally grown on 4H-SiC has been determined by comparison between I-V measurements on large-area (up to 1.96×10-5 cm2) metal-oxide-semiconductor (MOS) structures and conductive atomic force microscopy (C-AFM) with a resolution of a few nanometers. C-AFM clearly images the weak breakdown single spots under constant voltage stresses. The stress time on the single C-AFM tip dot has been varied from 1×10-3 to 1×10-1 s. The density of BD spots, upon increasing the stress time, exhibits an exponential trend. The Weibull slope and the characteristic time of the dielectric BD events were so determined by direct measurements at nanometer scale demonstrating that the percolation model is valid for thin thermal oxide layers on 4H-SiC (5-7nm), but it fails for larger thicknesses (10 nm).

Abstract: This paper studies the electronic properties of MOS capacitors fabricated on double positioning boundary free 3C-SiC/6H-SiC where the 3C-SiC films were grown using the Vapour- Liquid-Solid mechanism. The temperature- and frequency-dependent electrical properties of SiO2/3C-SiC/6H-SiC structures have been studied. Capacitance measurements indicate that the single-domain 3C-SiC film is doped near the surface with an average concentration of 8.3 × 1016 cm-3. The measured interface state density near the conduction band edge of 3C-SiC is below 1011cm-2⋅eV-1 and increases towards mid-gap as obtained from conductance and capacitance measurements. Our results are consistent with the assumption that the interfaces of SiO2/ n-type SiC consist of two different kinds of interface traps – the carbon clusters located at the interface and the intrinsic defects located within the oxide layer.