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SiC epitaxial films grown in an inverted chimney CVD reactor are analyzed and compared for growth rates, doping concentration and surface morphology using silane-propane-hydrogen and dichlorosilane (DCS)-propane-hydrogen chemistry systems. A general 1-D analytical model is presented to estimate the diffusivity of precursor gases, boundary layer thickness and growth rates for both gas systems. Decomposition of precursor gases into Si growth species is investigated by a commercial simulation tool, Virtual Reactor (VR). DCS suppresses the formation of elemental Si at lower pressures, reduces precursor losses, and leads to increased growth rate. However, at higher pressures, even DCS decomposes into elemental Si, which contributes to high Si depletion, limiting the maximum achievable growth rate. Reduction of Si loss using DCS is verified by mass measurements of parasitic depositions in the injector tube. The doping concentration of the epitaxial film is governed by the effective C/Si ratio at the growth surface rather than the inlet C/Si ratio, which is examined at various growth pressures. In addition to the widely known Si-depletion, C-depletion is also shown to exist and it plays a critical role in determining the doping concentration at various growth conditions. Increased roughness for the DCS growth at higher pressures is addressed and attributed to excessive HCl etching at higher pressures., Comment: This paper has been withdrawn since some of the parts are updated from the original submission

Thick, high-purity semi-insulating (SI)homoepitaxial layers on Si-face 4H-SiC weregrownsystematically, with resistivity \geq 109{\Omega}-cmby maintaining high C/Si ratios 1.3-15 during growth.Comparison of secondary ion mass spectra betweenlow-dopedepilayers grown at C/Si ratio<1.3andSI-epilayers grown at C/Si ratio>1.3 showed little difference in residual impurity concentrations. A reconciliation of impurity concentration with measured resistivity indicated a compensating trap concentration of ~1015cm-3present only in the SI-epilayers. High- resolution photo induced transient spectroscopy (HRPITS) identified themas Si-vacancy related deep centers, with no detectable EH6/7 and Z1/2levels. Recombination lifetimes ~5ns suggest application in fast-switching power devices., Comment: Submitted to Applied Physics Letters

A novel electrochemical means to generate atomic hydrogen, simplifying the synthesis and controllability of graphane formation on graphene is presented. High quality, vacuum grown epitaxial graphene (EG) was used as starting material for graphane conversion. A home-built electrochemical cell with Pt wire and exposed graphene as the anode and cathode, respectively, was used to attract H+ ions to react with the exposed graphene. Cyclic voltammetry of the cell revealed the potential of the conversion reaction as well as oxidation and reduction peaks, suggesting the possibility of electrochemically reversible hydrogenation. A sharp increase in D peak in the Raman spectra of EG, increase of D/G ratio, introduction of a peak at ~2930 cm-1 and respective peak shifts as well as a sharp increase in resistance showed the successful hydrogenation of EG. This conversion was distinguished from lattice damage by thermal reversal back to graphene at 1000{\deg}C., Comment: 18 pages, 8 figures, 1 table, revised version submitted to the Journal of Electronic Materials

Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multi-layer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent carrier cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions., Comment: 4 pages, 3 figures

Electron-hole generation and recombination rates for intravalley and intervalley phonon scattering in Graphene are presented. The transverse and the longitudinal optical phonon modes ($E_{2g}$-modes) near the zone center ($\Gamma$-point) contribute to intravalley interband carrier scattering. At the zone edge ($K(K')$-point), only the transverse optical phonon mode ($A'_{1}$-mode) contributes significantly to intervalley interband scattering with recombination rates faster than those due to zone center phonons. The calculated recombination times range from less than a picosecond to more than hundreds of picoseconds and are strong functions of temperature and electron and hole densities. The theoretical calculations agree well with experimental measurements of the recombination rates of photoexcited carriers in graphene., Comment: 6 pages, 9 figures

We report the emission of strong coherent broadband terahertz radiation from 6H-Silicon-Carbide (SiC) excited with optical pulses. The measured terahertz spectral signal-to-noise ratio is better than one thousand. We determine that the terahertz radiation is generated via second order optical nonlinearity (optical rectification). We present a measurement of the ratio of nonlinear susceptibility tensor elements $\chi_{zzz}^{(2)}/\chi_{zxx}^{(2)}$ and the complex index of refraction of silicon carbide at THz frequencies., Comment: 4 pages, 5 figures

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent., Comment: 4 pages, 5 figures

We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz (THz) frequency range. In the THz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of $e^{2}/4\hbar$. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements.

Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.