Papers by Author: A.Y. Polyakov

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Authors: E.R. Glaser, B.V. Shanabrook, W.E. Carlos, Hun Jae Chung, Saurav Nigam, A.Y. Polyakov, Marek Skowronski
Abstract: We have employed low-temperature photoluminescence to estimate the total residual N concentration in semi-insulating (SI) SiC substrates where all N shallow donors are compensated in the dark. The ratio of the nitrogen-bound exciton line (Qo) to the free excitonic emission (I77) as a function of excitation power density (Pexc) was tracked for several SI 4H-SiC samples with varying residual N concentration (~ 7x1014 – 5.2x1016 cm-3). Most notably, a linear relationship was found between Qo/I77 and [N] for [N] < 1x1016 cm-3 while a sub-linear behavior was observed for samples with higher N levels. This technique should be particularly valuable to map [N] where the levels are close to or below the present SIMS detection limit of ~ 5-7 x 1014 cm-3. Results obtained for a limited number of low n-type and SI 6H-SiC substrates are also presented.
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Authors: Sung Wook Huh, A.Y. Polyakov, Hun Jae Chung, Saurav Nigam, Marek Skowronski, E.R. Glaser, W.E. Carlos, Mark A. Fanton, N.B. Smirnov
Abstract: Deep electron and hole traps were studied in a series of high purity 6H-SiC single crystals grown by Halide Chemical Vapor Deposition (HCVD) method at various C/Si flow ratios and at temperatures between 2000 oC and 2100 oC. Characterization included Low Temperature Photoluminescence (LTPL), Deep Level Transient Spectroscopy (DLTS), Minority Carrier Transient Spectroscopy (MCTS), and Thermal Admittance Spectroscopy (TAS) measurements. Concentrations of all deep traps were shown to strongly decrease with increased C/Si flow ratio and with increased growth temperature. The results indicate that the majority of deep centers in 6H-SiC crystals grown by HCVD are due to native defects or complexes of native defects promoted by Si-rich growth conditions. The observed growth temperature dependence of residual donor concentration and traps density is explained by increasing the effective C/Si ratio at higher temperatures for the same nominal ratio of C and Si flows.
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Authors: Sung Wook Huh, Joseph J. Sumakeris, A.Y. Polyakov, Marek Skowronski, Paul B. Klein, B.V. Shanabrook, Michael J. O'Loughlin
Abstract: Carrier lifetimes and the dominant electron and hole traps were investigated in a set of thick (9-104mm) undoped 4H-SiC epitaxial layers grown by CVD homoepitaxy. Deep trap spectra were measured by deep level transient spectroscopy (DLTS) with electrical or optical injection, while lifetimes were measured by room temperature time-resolved photoluminescence (PL). The main electron traps detected in all samples were due to Ti, Z1/Z2 centers, and EH6/EH7 centers. Two boron-related hole traps were observed with activation energies of 0.3 eV (boron acceptors) and 0.6 eV (boron-related D centers). The concentration of electron traps decreased with increasing layer thickness and increased toward the edge of the wafers. PL lifetimes were in the 400 ns-1800 ns range with varying injection and generally correlated with changes in the density of Z1/Z2 and to a lesser extent the EH6/EH7 electron traps. However, the results of DLTS measurements on p-i-n diode structures suggest that the capture of injected holes is much more efficient for the Z1/Z2 traps compared to the EH6/EH7 centers making the Z1/Z2 more probable candidates for the role of lifetime killers. A good fit of the thickness dependence of the measured lifetimes to the usual analytical form was obtained assuming that Z1/Z2 is the dominant hole recombination center and that the surface recombination velocity was 2500 cm/sec.
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Authors: Mary Ellen Zvanut, G. Ngetich, H.J. Chung, A.Y. Polyakov, Marek Skowronski, N.Y. Garces, E.R. Glaser
Abstract: The understanding of the structure and associated defect level of point defects in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy-carbon antisite pair, − C Si V C . Samples containing this defect exhibit two different photo thresholds, which depend on the resistivity activation energy, Ea. For samples with Ea less than 0.8 eV, a photothreshold at 0.75+/- 0.05 eV is observed, but for those with Ea greater than 0.8 eV, the threshold is between 2 and 2.5 eV. Previous work focused on the former case. Here, the SiC substrates with the larger Ea are emphasized, showing that the photo-threshold likely measures the neutral to negative defect level, − / 0 C Si V C .
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Authors: E.M. Omeljanovsky, A.V. Pakhomov, A.Y. Polyakov, O.M. Borodina
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Authors: Hun Jae Chung, Sung Wook Huh, A.Y. Polyakov, Saurav Nigam, Qiang Li, J.R. Grim, Marek Skowronski, E.R. Glaser, W.E. Carlos, Jaime A. Freitas, Mark A. Fanton
Abstract: Undoped 6H- and 4H-SiC crystals were grown by Halide Chemical Vapor Deposition (HCVD). Concentrations of impurities were measured by various methods including secondary-ion-mass spectrometry (SIMS). With increasing C/Si ratio, nitrogen concentration decreased and boron concentration increased as expected for the site-competition effect. Hall-effect measurements on 6H-SiC crystals showed that with the increase of C/Si ratio from 0.06 to 0.7, the Fermi level was shifted from Ec-0.14 eV (nitrogen donors) to Ev+0.6 eV (B-related deep centers). Crystals grown with C/Si > 0.36 showed high resistivities between 1053 and 1010 4cm at room temperature. The high resistivities are attributed to close values of the nitrogen and boron concentrations and compensation by deep defects present in low densities.
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Authors: Saurav Nigam, Hun Jae Chung, Sung Wook Huh, J.R. Grim, A.Y. Polyakov, Mark A. Fanton, B.E. Weiland, David Snyder, Marek Skowronski
Abstract: Growth rates and relative stability of 6H- and 4H-SiC have been studied as a function of growth conditions during Halide Chemical Vapor Deposition (HCVD) process using silicon tetrachloride, propane and hydrogen as reactants. The growth temperature ranged from 2000 to 2150 oC. Silicon carbide crystals were deposited at growth rates in the 100-300 μm/hr range in both silicon- and carbon-supply limited regimes by adjusting flows of all three reactants. High resolution x-ray diffraction measurements show that the growth on Si-face of 6H- and C-face of 4H-SiC substrates resulted in single crystal 6H- and 4H-SiC polytype, respectively. The growth rate results have been interpreted using thermodynamic equilibrium calculations.
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Authors: A.Y. Polyakov, Mark A. Fanton, Marek Skowronski, Hun Jae Chung, Saurav Nigam, Sung Wook Huh
Abstract: A novel approach to the high growth rate Chemical Vapor Deposition of SiC is described. The Halide Chemical Vapor Deposition (HCVD) method uses SiCl4, C3H8 (or CH4), and hydrogen as reactants. The use of halogenated Si source and of separate injection of Si and C precursors allows for preheating of source gases without causing premature chemical reactions. The stoichiometry of HCVD crystals can be controlled by changing the C/Si flow ratio and can be kept constant throughout growth, in contrast to the Physical Vapor Transport technique. HCVD was demonstrated to deposit high crystalline quality, very high purity 4H- and 6H-SiC crystals with growth rates comparable to other bulk SiC growth techniques. The densities of deep electron and hole traps are determined by growth temperature and C/Si ratio and can be as low as that found in standard silane-based CVD epitaxy. At high C/Si flow ratio, the resistivity of HCVD crystals exceeds 105 _cm. These characteristics make HCVD an attractive method to grow SiC for applications in high-frequency and/or high voltage devices.
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Authors: Mark A. Fanton, Qiang Li, A.Y. Polyakov, R.L. Cavalero, R.G Ray, B.E. Weiland, Marek Skowronski
Abstract: The effects of H2 addition to the growth ambient during physical vapor transport (PVT) growth of 6H and 4H SiC were investigated using SIMS, DLTS and Hall effect measurements. Using this hybrid physical-chemical vapor transport (HPVT) approach, boules were grown using Ar-H2 and He-H2 mixtures with H2 concentrations up to 50 at%. Thermodynamic modeling suggests that addition of H2 improves the carbon transport in HPVT compared to standard PVT. This should lead to a substantial decrease in the concentration of residual N donors and the concentration of electron traps. This is confirmed by the experimental results. As expected, the source transport rate increased as H2 was added to the growth environment due to increased C transport. The background nitrogen concentration and the free electron density decreased significantly with increasing H2 concentration. The formation of electron traps (activation energies of 0.4 eV, 0.6-0.65 eV, 0.7 eV, 0.9 eV and 1 eV) was also strongly suppressed. These changes were observed for H2 concentrations as low as 4 at%. The decreased N concentration improves the ability to produce high resistivity SiC material, and for H2 concentrations as high as 10-25%, the very first wafers cut from the seed end of the boules have a resistivity exceeding 106 cm.
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