Papers by Author: Tangali S. Sudarshan

Abstract: In this work, the generation of in-grown SFs, half-moon defects, and carrot defects in SiC epilayer is studied by using epitaxy-etch-epitaxy approach. It is found that under our growth conditions, most of these defects have a similar origin, and they nucleate at obstacles to step flow at the SiC substrate/epilayer interface. These obstacles may be micropipes, scratches, or foreign particles on the substrate surface.

Abstract: We investigate molecular adsorption doping by electron withdrawing NO2 and electron donating NH3 on epitaxial graphene. Amperometric measurements show conductance changes upon introduction of molecular adsorbents on epitaxial graphene. Conductance changes are a trade-off between carrier concentration and scattering, and manifest at direct current and optical frequencies. We therefore investigate changes in the infrared (IR) reflection spectra to correlate these two frequency domains, as reflectance changes are due to a change of EG surface conductance. We match theory with experimental IR data and extract changes in carrier concentration and scattering due to gas adsorption. Finally, we separate the intraband and interband scattering contributions to the electronic transport under gas adsorption. The results indicate that, under gas adsorption, the influence of interband scattering cannot be neglected, even at DC.

Abstract: Electrochemical functionalization of treated epitaxial graphene samples on Si-face 6H-SiC are presented in this work. Three semi-insulating 6H-SiC substrates cut from different boules with varying off cut angle (on axis, 0.5° and 1.0° degrees off axis in the [112‾0] direction) were diced into 10mm x 10mm samples and quality EG grown on top. A home-build electrochemical cell was used with current applied though a 10% H₂SO₄ solution, with a Pt wire and exposed graphene as the anode and cathode respectively. Functionalization was determined using Raman spectroscopy and measured by an increase in D/G ratio, increase in fluorescence background and introduction of C-H bond peak at ~2930 cm^-1. Components of the Raman spectra before and after functionalization of all samples used were analyzed to show a substrate dependent effect on functionalization with values such as D/G ratio and normalized fluorescence slope varying between the substrates.

Abstract: SiC epitaxial films grown in an inverted chimney CVD reactor using silane-propane-hydrogen and dichlorosilane (DCS)-propane-hydrogen systems are compared for growth rates and doping concentrations at various growth pressures. Parasitic depositions in the gas injector tube using these precursor gases are also compared for precursor depletion. Virtual Reactor, a commercial software, is employed to predict growth rates and compare them to experimental results for the same growth conditions using DCS and silane gases.

Abstract: Room temperature photoluminescence was obtained by UV excitation of homoepitaxially grown 4H-SiC thin films. A broad band emission from boron deep levels centered at 517nm was observed along with the band-edge emission of 4H-SiC at 391 nm. The wavelength of the excitation was varied and the change in the relative intensity of the two emission peaks was observed. The variation of the relative intensity was correlated with the in-grown stacking fault density in the epilayer. A physical model was developed to explain the correlation in terms of carrier diffusion length. For epilayers with very high density of in-grown stacking faults, a sharp emission was observed at 480nm.

Abstract: We present epitaxial graphene (EG) growth on non-polar a-plane and m-plane 6H-SiC faces where material characterization is compared with that known for EG grown on polar faces. AFM surface morphology exhibits nanocrystalline graphite like features for non-polar faces, while polar silicon face shows step like features. This differing behavior is attributed to the lack of a hexagonal template on the non-polar faces. Non-polar faces also exhibit greater disorder and red shift of all Raman peaks (D, G and 2D) with increasing temperature. This is attributed to decreasing stress with increasing temperature. These variations provide evidence of different EG growth mechanisms on non-polar and polar faces, likely due to differences in surface free energy. We also present differences between a-plane ( ) EG and m-plane ( ) EG in terms of morphology, thickness and Raman characteristics.

Abstract: Formation of particles and their effect on SiC epitaxial growth in the CVD reactor is investigated. Particle induced defects in the epilayer at different gas decomposition conditions are discussed. A higher number of pits with larger diameters are observed in the epilayer for conditions where gases decompose later in the gas injector tube (i.e. nearer to the substrate). On the other hand, the number and size of these pits reduce for the condition where gas decomposes earlier in the tube. To investigate the effect of particles during the growth, various particles with different size, shape and compositions are intentionally placed on the substrate surface before epitaxial films are grown. Samples are mapped and compared at similar locations in the pre-growth, post growth and post-etch (by molten KOH) conditions. It is found that the nature of particle induced defects depends primarily on size and shape of particles.

Abstract: An optimized molten KOH-NaOH eutectic etching method is developed to reveal defects in highly n-doped SiC substrates and to pre-treat the substrate prior to epitaxial growth. Different from the conventional KOH etching method, by way of eutectic method, the basal plane dislocation (BPD) conversion in the subsequent epitaxial growth is independent of the etch pit size pre-generated on the substrate. Even with a short period (~3 minutes) of pretreatment which does not generate any visible etch pits or degradation of surface morphology on the substrate, an epilayer with low BPD density -2 is still achieved. This simple and non-destructive method shows high potential to be practically employed as one of the basic pretreatment steps to the substrates in SiC epitaxial growth in order to achieve very low or free BPD density.

Abstract: In-grown stacking faults (IGSFs) were studied in 4H-SiC homoepitaxial growth from a SiH₂Cl₂-C₃H₈-H₂ system. Most of the IGSFs, start from the epilayer/substrate interface, and exhibit photoluminescence emission peak at 2.58 eV (480 nm) indicating of 8H polytype. The growth parameters, including growth temperature, growth pressure, growth rate, hydrogen etching, et al., varied around the regular growth condition do not show a significant effect on the IGSF generation. Reactor furniture is identified to be a major reason of IGSF formation, especially when the insulation part of the furnace is not completely isolated from the growth zone. Dusting of insulation material is crucial in the formation of IGSFs. When using graphite felt as the insulation material, the IGSF density in the epilayer can be as high at ~10⁴ cm^-2. Improvement of the insulation material by using graphite foil reduces the density to 30-100 cm^-2. Further reduction of IGSF density to less than 10 cm^-2 is achieved by mild pretreatment of the substrate in molten KOH-NaOH eutectic.

Abstract: The effect of various types of in-grown stacking faults and threading screw/edge type dislocations on carrier lifetime and diffusion lengths in 4H-SiC epitaxial films was investigated through cathodoluminescence decays and charge collection efficiencies of electron beam induced current signals at specific defects sites. Most stacking faults yielded ~40% reduction in the carrier lifetime. Moreover, drastic lifetime reductions were observed in regions containing surface triangular defects and bulk 3C polytype inclusions. Dislocations of both types serve as efficient recombination centers, though stronger reduction in diffusion lengths was observed in the vicinity of screw type dislocations.