Papers by Author: D. Kurt Gaskill

Abstract: Quantum technology is a field of significant interest that will benefit many applications including communications and sensing. SiC is a promising material for quantum applications such as quantum memories, due to point defects, specifically V_Si, in the material, which result in long spin coherence times. We have found that no V_Si are present in our epitaxially grown unintentionally and nitrogen-doped 4H-SiC with electron concentrations ranging from 10¹⁴ to 10¹⁸ cm^-3. We create these vacancies using electron irradiation, in concentrations from single defects to ensembles. To utilize the defect luminescence for realistic applications, we have fabricated the SiC into photonic crystal arrays. We present the processing steps required to create photonic crystal cavities in SiC and subsequent challenges.

Abstract: Epitaxial cubic silicon carbide films on silicon have attracted extensive interest for semiconductor device applications such as high-voltage, high-frequency diodes, and hetero-junction bi-polar transistors [1]. This is because they can offer access to the properties of the SiC material such as its wide band gap and high thermal conductivity on the more conventional silicon substrates [2]. Rahimi et al. have shown, however, that the substantial tensile strain generated from the lattice and thermal expansion coefficient mismatch between 3C-SiC and silicon, may reduce the band gap in the SiC epitaxial films [3]. Nevertheless, the impact of this phenomenon on the electrical and electronic performance of the epitaxial SiC films on silicon has not been fully elucidated to date; such information is vital to obtain the optimal performance of devices fabricated from these strained heterojunctions.

Abstract: Lifetime maps for two 4H-SiC epi-wafers (samples 1 and 2) were recorded using microwave photoconductive decay (μPCD) measurements and correlated with the type and distribution of structural defects mapped by synchrotron X-ray topography (white beam and monochromatic). Sample 1 showed lower lifetime inside one of its higher doped facet regions and along its edges. The low lifetime in the facet region was associated with the presence of a high density of multi-layered Shockley stacking faults (SFs) and low angle grain boundaries (LAGBs). These stacking faults are likely double Shockley stacking faults (DSSFs) and probably nucleated from scratches present on the substrate surface and LAGBs present in that region, propagating during epilayer growth. In contrast, sample 2 showed a reduced carrier lifetime in the middle region associated with a network of interfacial dislocations (IDs) and half loop arrays (HLAs) originating from 3C inclusions that are generated during epilayer growth. Along the edges of both samples, overlapping triangular defects, microcracks and BPD loops lowered lifetime.

Abstract: The electrical characteristics of oxygen functionalized epitaxial graphene and Ti/Au metal contact interfaces were systematically investigated as a function of temperature. As the temperature was increased from 300 K to 673 K, the contact resistance and the sheet resistance decreased by 75% and 33%, respectively. The resistance of oxygen functionalized graphene vs temperature exhibited Arrhenius type behavior with activation energy of 38 meV. The results showed no hysteresis effects in resistance measurements over the temperatures studied here, suggesting the contact interfaces remain stable at high temperatures.

Abstract: Graphene, a 2D material, has motivated significant research in the study of its in-plane charge carrier transport in order to understand and exploit its unique physical and electrical properties. The vertical graphene-semiconductor system, however, also presents opportunities for unique devices, yet there have been few attempts to understand the properties of carrier transport through the graphene sheet into an underlying substrate. In this work, we investigate the epitaxial graphene/4H-SiC system, studying both p and n-type SiC substrates with varying doping levels in order to better understand this vertical heterojunction.

Abstract: Epitaxial graphene produced from SiC substrates exhibits a carrier mobility re- duction thought to arise from intercalated silicon. We present the results of density functional simulations and show that individual silicon atoms are highly mobile on and between graphene sheets, suggesting that thermally stable structures involving individual Si impurities are likely to result from the interaction of silicon with defects in the graphene sheets.

Abstract: The growth of epitaxial graphene on C-face 6H-SiC substrates is investigated using pro-cess conditions that can form small, local areas of graphene. The thickness of SiC lost to Si sublimation is not completely countered by the thickness of the resulting graphene and so graphene-covered basins (GCBs) are formed. The GCBs are most likely nucleated at threading dislocations from the substrate. The GCB morphology exhibits ridges, similar to those found on continuous films. The GCBs expand through erosion of the surrounding SiC substrate walls, eventually coalescing into continuous films. The ratio of the Raman D and G peaks was used to estimate the crystallite length scale and it was found to be about 200 nm for small GCBs and > 1 m for continuous films.

Abstract: Doubly-implanted SiC vertical MOSFETs were fabricated displaying a blocking voltage of 4.2kV and a specific on-resistance of 23 mΩ-cm2, on a 4.5mm x 2.25mm device. Design variations on smaller (1.1mm x 1.1mm) devices showed on-resistance as low as 17 mΩ-cm2 with a blocking voltage of 3.3kV. Analysis is presented of the on-resistance and temperature dependence (up to 175°C), as well as switching performance. Switching tests taken at 1000V and 6A showed turn-on and turn-off transients of approximately 20-40ns.

Abstract: The effects of proton irradiation on uv 4H-SiC single photon avalanche photodiodes (SPADs) are reported. The SPADs, grown by chemical vapor deposition, were designed for uv operation with dark count rates (DCR) of about 30 kHz and single photon detection efficiency (SPDE) of 4.89%. The SPADs were irradiated with 2 MeV protons to a fluence of 1012 cm-2. After irradiation, the I-V characteristics show forward voltage (<1.9 V) generation-recombination currents 2 to 3 times higher than before irradiation. Single photon counting measurements imply generation-recombination centers created in the band gap after irradiation. For threshold voltage ranging from 23 to 26 mV, the 4H-SiC SPAD showed low DCR (<54 kHz) and high SPDE (>1%) after irradiation. The SPADs demonstrated proton radiation tolerance for geosynchronous space applications.

Abstract: Homo- and heteroepitaxial 3C-SiC layers were grown on 4H-SiC step-free mesas. The yields of smooth, defect-free mesas were ~ 17% for both intentionally and unintentionally doped films, while those with screw dislocations and multiple stepped surfaces were ~ 22%. The electronic and structural properties of the mesas were found on a micrometer-sized length scale using µ-PL and µ-Raman, respectively. 3C-SiC mesas were found to have complete 3C-SiC coverage with some of the mesas having electronic defects, while other mesas were found to be defect-free.