Papers by Author: Brenda L. VanMil

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Authors: Jian Hui Zhang, Leonid Fursin, Xue Qing Li, Xiao Hui Wang, Jian H. Zhao, Brenda L. VanMil, Rachael L. Myers-Ward, Charles R. Eddy, D. Kurt Gaskill
Abstract: This work reports 4H-SiC bipolar junction transistor (BJT) results based upon our first intentionally graded base BJT wafer with both base and emitter epi-layers continuously grown in the same reactor. The 4H-SiC BJTs were designed to improve the common emitter current gain through the built-in electrical fields originating from the grading of the base doping. Continuously-grown epi-layers are also believed to be the key to increasing carrier lifetime and high current gains. The 4H-SiC BJT wafer was grown in an Aixtron/Epigress VP508, a horizontal hot-wall chemical vapor deposition reactor using standard silane/propane chemistry and nitrogen and aluminum dopants. High performance 4H-SiC BJTs based on this initial non-optimized graded base doping have been demonstrated, including a 4H-SiC BJT with a DC current gain of ~33, specific on-resistance of 2.9 mcm2, and blocking voltage VCEO of over 1000 V.
Authors: Jun Hu, Xiao Bin Xin, Petre Alexandrov, Jian H. Zhao, Brenda L. VanMil, D. Kurt Gaskill, Kok Keong Lew, Rachael L. Myers-Ward, Charles R. Eddy
Abstract: This paper reports a 4H-SiC single photo avalanche diode (SPAD) operating at the solar blind wavelength of 280 nm. The SPAD has an avalanche breakdown voltage of 114V. At 90% and 95% of the breakdown voltage, the SPAD shows a low dark current of 57.2fA and 159fA, respectively. The quantum efficiency of 29.8% at 280nm and <0.007% at 400nm indicates a high UV-to-visible rejection ratio of >4300. Single photon counting measurement at 280nm shows that a single photon detection efficiency of 2.83% with a low dark count rate of 22kHz is achieved at the avalanche breakdown voltage of 116.8V.
Authors: Robert E. Stahlbush, Rachael L. Myers-Ward, Brenda L. VanMil, D. Kurt Gaskill, Charles R. Eddy
Abstract: The recently developed technique of UVPL imaging has been used to track the path of basal plane dislocations (BPDs) in SiC epitaxial layers. The glide of BPDs during epitaxial growth has been observed and the role of this glide in forming half-loop arrays has been examined. The ability to track the path of BPDs through the epitaxy has made it possible to develop a BPD reduction process for epitaxy grown on 8° offcut wafers, which uses an in situ growth interrupt and has achieved a BPD reduction of > 98%. The images also provide insight into the strong BPD reduction that typically occurs in epitaxy grown on 4° offcut wafers.
Authors: Brenda L. VanMil, Robert E. Stahlbush, Rachael L. Myers-Ward, Yoosuf N. Picard, S.A. Kitt, J.M. McCrate, S.L. Katz, D. Kurt Gaskill, Charles R. Eddy
Abstract: Conversion of basal plane dislocations (BPD) to threading edge dislocations (TED) in 8° off-cut 4H-SiC within an n+ buffer layer would eliminate the nucleation site for Shockley-type stacking faults in active device regions grown on such buffer layers. To that end, the propagation and conversion of BPDs through in situ growth interrupts is monitored using ultraviolet photoluminescence (UVPL) wafer mapping. The optimized growth interrupt scheme lasts for 45 minutes with a propane mass flow of 10 sccm at growth temperature. This scheme has shown a conversion efficiency of up to 99% for samples with electron (hole) concentrations < 5x1014 cm-3 (8x1015 cm-3). Samples subjected to a 45 or 90 minute interrupt under 10 sccm of propane, regardless of conversion efficiency, exhibit a “slit” in the surface morphology associated with each BPD and oriented perpendicular to the off-cut and BPD propagation direction. Repetition of the optimal interrupt sequence with a 5 μm epilayer spacer grown between the two interrupts resulted in the same conversion efficiency as a single optimal growth interrupt. Incorporation of the optimal interrupt into an n+ layer is more complicated as attempts to do so in layers doped with nitrogen to 2x1018, 2x1017 and 2x1016 cm-3 resulted in conversion efficiencies of ~6%.
Authors: D. Kurt Gaskill, Michael A. Mastro, Kok Keong Lew, Brenda L. VanMil, Rachael L. Myers-Ward, Ronald T. Holm, Charles R. Eddy
Abstract: A set of three 4H-SiC wafers with manufacturer specified micropipe density of 0-5 cm-2 were characterized by x-ray diffraction (XRD) maps before and after final chemical-mechanical polish. After final polish, the wafers were also investigated with atomic force microscopy, radius of curvature measurements and cross-polarization (x-pol) mapping. It was found that there was largely a lack of correlation between the XRD and x-pol maps, which strongly suggests that x-pol is insensitive to crystalline imperfections to which XRD is sensitive.
Authors: Virginia D. Wheeler, Brenda L. VanMil, Rachael L. Myers-Ward, S. Chung, Yoosuf N. Picard, Marek Skowronski, Robert E. Stahlbush, Nadeemullah A. Mahadik, Charles R. Eddy, D. Kurt Gaskill
Abstract: The effectiveness of an in-situ growth interrupt in nitrogen doped 8° off-cut epilayers was investigated using ultraviolet photoluminescence imaging. Low-doped n-type epilayers (<1016 cm-3) exhibited an abrupt increase in BPD to TED conversion at the growth interrupt and achieved 96-99% conversion overall (< 10 BPDs/cm-2), while high-doped epilayers had minimal conversion at the interrupt (< 1%) and overall (< 30%). This large discrepancy suggests nitrogen prohibits or alters the conversion mechanism at the growth interrupt. Therefore, a novel SEM technique was developed to "freeze-in" the interface morphology and help elucidate the conversion mechanism. Preliminary results suggest that preferential etching at the point of BPD intersection with the surface is greatly reduced in highly doped layers, which inhibits the conversion mechanism.
Authors: Kok Keong Lew, Brenda L. VanMil, Rachael L. Myers-Ward, Ronald T. Holm, Charles R. Eddy, D. Kurt Gaskill
Abstract: Hydrogen etching of 4H-SiC has been performed in a hot-wall chemical vapor deposition reactor to reduce surface damage and to create a bilayer-stepped surface morphology, optimal for initiation of growth on 4H-SiC substrates offcut 4° and 8° towards the <11-20> direction. To understand how step bunching evolves during the ramp to growth temperature, samples were etched ending at temperatures from 1400 to 1580°C under 0, 2 or 10 sccm of propane (C3H8) addition to hydrogen. Initial exploratory growth of 5 μm thick epilayers on the 4° etched surfaces are also discussed. Atomic force microscopy (AFM) and Nomarski microscopy were employed to investigate changes in the surface morphology. The 8° substrates subjected to H2-C3H8 etching up to growth temperature routinely exhibited bilayer steps. However, when the 4° substrates were etched with a 10 sccm C3H8 flow, considerable step bunching was observed. At 1450°C, with a 10 sccm of C3H8 flow (partial pressure is 1.25x10-5 bar), step bunching started with the formation of ribbon-like steps. Progression to higher temperature etches have shown the coalescence of the ribbons into larger macro-steps up to 30 nm in height. Etching 4° substrates under 2 sccm of C3H8 (partial pressure is 2.5x10-6 bar) or in pure H2 up to 1500°C results in minimal step bunching.
Authors: Robert E. Stahlbush, Brenda L. VanMil, Kendrick X. Liu, Kok Keong Lew, Rachael L. Myers-Ward, D. Kurt Gaskill, Charles R. Eddy, X. Zhang, Marek Skowronski
Abstract: The evolution of basal plane dislocations (BPDs) in 4H-SiC epitaxy during its growth is investigated by using two types of interrupted growth in conjunction with ultraviolet photoluminescence (UVPL) imaging of the dislocations. For the first, each epitaxial growth was stopped after 10-20 μm and a UVPL map was collected. For the second, changing the gas flow interrupted the growth and the BPDs were imaged at the end. The first sequence made it possible to track the formation of half-loop arrays and show that they arise from BPDs that glide perpendicular to the offcut direction. For both types, each interruption causes between 30 – 50% of the BPDs to be converted to threading edge dislocations (TEDs). This result suggests that using interrupted growth may be an alternate method to producing epitaxial layers with low BPD concentration.
Authors: Brenda L. VanMil, Rachael L. Myers-Ward, Joseph L. Tedesco, Charles R. Eddy, Glenn G. Jernigan, James C. Culbertson, Paul M. Campbell, J.M. McCrate, S.A. Kitt, D. Kurt Gaskill
Abstract: Graphene layers were created on both C and Si faces of semi-insulating, on-axis, 4H- and 6H-SiC substrates. The process was performed under high vacuum (<10-4 mbar) in a commercial chemical vapor deposition SiC reactor. A method for H2 etching the on-axis substrates was developed to produce surface steps with heights of 0.5 nm on the Si-face and 1.0 to 1.5 nm on the C-face for each polytype. A process was developed to form graphene on the substrates immediately after H2 etching and Raman spectroscopy of these samples confirmed the formation of graphene. The morphology of the graphene is described. For both faces, the underlying substrate morphology was significantly modified during graphene formation; surface steps were up to 15 nm high and the uni-form step morphology was sometimes lost. Mobilities and sheet carrier concentrations derived from Hall Effect measurements on large area (16 mm square) and small area (2 and 10 m square) samples are presented and shown to compare favorably to recent reports.
Authors: Rachael L. Myers-Ward, Kok Keong Lew, Brenda L. VanMil, Robert E. Stahlbush, Kendrick X. Liu, Joshua D. Caldwell, Paul B. Klein, Ping Wu, Mohammad Fatemi, Charles R. Eddy, D. Kurt Gaskill
Abstract: X-ray diffraction (XRD) rocking curves were mapped across 4H-SiC, 3-inch, 8° off-cut substrates prior to and after epitaxial growth, where a pattern of slightly higher defectivity region was clearly seen. This same pattern was apparent in both cross-polarization images of the epiwafers and microwave photoconductivity decay (μ-PCD) lifetime maps of the epilayers, where the latter shows the lifetime in the high defectivity regions had drastically decreased. Within the short lifetime regions, electron trap concentrations were similar to that as in the long lifetime regions as determined by deep level transient spectroscopy; however, the extended defect density was significantly higher. Consequently, high spatial resolution XRD can be a valuable tool in preselecting substrates for epitaxial growth to produce low defect density material with long injected carrier lifetimes.
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