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Publications (18)30.66 Total impact

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    ABSTRACT: There has been significant research in the study of in-plane charge-carrier transport in graphene in order to understand and exploit its unique electrical properties; however, the vertical graphene-semiconductor system also presents opportunities for unique devices. In this letter, we investigate the epitaxial graphene/p-type 4H-SiC system to better understand this vertical heterojunction. The I-V behavior does not demonstrate thermionic emission properties that are indicative of a Schottky barrier but rather demonstrates characteristics of a semiconductor heterojunction. This is confirmed by the fitting of the temperature-dependent I-V curves to classical heterojunction equations and the observation of band-edge electroluminescence in SiC.
    IEEE Electron Device Letters 11/2012; 33(11):1610-1612. · 2.79 Impact Factor
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    ABSTRACT: We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.
    Nano Letters 03/2012; 12(4):1749-56. · 13.03 Impact Factor
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    ABSTRACT: Stacking fault (SF) expansion from basal plane dislocations (BPDs) confined in highly doped 4H-SiC buffer layers is observed under high-power ultraviolet illumination (>1000 W/cm2). Once the SFs reach the active drift layers, grown above the buffer layers, they are seen to rapidly expand up to the sample surface where they can cause device degradation. BPD faulting in the buffer appears to have a carrier injection threshold. Carrier density simulations under various injection conditions and carrier lifetimes are used to establish the conditions of BPD faulting within the buffer layer that could prevent SF expansion into the drift layer.
    Applied Physics Letters 01/2012; 100(4). · 3.79 Impact Factor
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    ABSTRACT: As a wide-bandgap semiconductor, gallium nitride (GaN) is an attractive material for next-generation power devices. To date, the capabilities of GaN-based high electron mobility transistors (HEMTs) have been limited by self-heating effects (drain current decreases due to phonon scattering-induced carrier velocity reductions at high drain fields). Despite awareness of this, attempts to mitigate thermal impairment have been limited due to the difficulties involved with placing high thermal conductivity materials close to heat sources in the device. Heat spreading schemes have involved growth of AIGaN/GaN on single crystal or CVD diamond, or capping of fullyprocessed HEMTs using nanocrystalline diamond (NCD). All approaches have suffered from reduced HEMT performance or limited substrate size. Recently, a "gate after diamond" approach has been successfully demonstrated to improve the thermal budget of the process by depositing NCD before the thermally sensitive Schottky gate and also to enable large-area diamond implementation.
    Device Research Conference (DRC), 2012 70th Annual; 01/2012
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    ABSTRACT: A novel taper-doping anode termination method is introduced for high-voltage silicon carbide devices. The method employs a subresolution two-tone termination mask to achieve a gray-scale exposure and a smoothly tapered photoresist profile. Using the tapered profile as an implantation mask, self-aligned 6-kV SiC PiN diodes are demonstrated with 90% of the parallel-plate breakdown voltage. The avalanche breakdown for the design is controlled and reversible. This one-step technique allows wide design control over the width and shape of the termination profile and has wide device and material applicability.
    IEEE Transactions on Electron Devices 11/2011; · 2.06 Impact Factor
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    ABSTRACT: A new 60 A, 4.5 kV SiC JBS diode is presented, and its performance is compared to a Si PiN diode used as the antiparallel diode for 4.5 kV Si IGBTs. The I-V, C-V, reverse recovery, and reverse leakage characteristics of both diode types are measured. The devices are also characterized as the anti-parallel diode for a 4.5 kV Si IGBT using a recently developed high-voltage, double-pulse switching test system. The results indicate that SiC JBS diodes reduce IGBT turn-on switching loses by about a factor of three in practical applications. Furthermore, the peak IGBT current at turn-on is typically reduced by a factor of six, resulting in substantially lower IGBT stress. Circuit simulator models for the 4.5 kV SiC JBS and Si PiN diodes are also developed and compared with measurements.
    01/2011;
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    ABSTRACT: Shockley Stacking fault (SSF) expansion from basal plane dislocations (BPDs) occurs during forward bias operation in 4H-Silicon Carbide (SiC) and causes forward voltage drift in minority carrier SiC devices [1, 2]. Reverse bias breakdown voltage degradation with SSF expansion has also been reported [3]. The SSFs expansion occurs via the electron-hole recombination enhanced dislocation glide (REDG) process [4]. In order to mitigate the influence of these SSFs in the active drift layer, a high doped buffer layer was grown to convert most of the BPDs to threading edge dislocations (TED) within it. This confines the BPD to the buffer and only the relatively benign TED passes through the drift layer. Previously it was thought that SSF expansion would not occur in these high doped epilayers and propagate into the drift layer. However, this assumption that BPDs within the buffer do not affect the drift layer during carrier injection has not been previously studied. In this work using electron-hole creation by UV excitation, we image the motion and faulting of BPDs buried in the buffer layer and show that SFs originating in that layer expand into the device drift region.
    Semiconductor Device Research Symposium (ISDRS), 2011 International; 01/2011
  • Materials Science Forum - MATER SCI FORUM. 01/2010;
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    ABSTRACT: High voltage GaN Schottky diodes require a thick blocking layer with an exceptionally low carrier concentration. To this aim, a metal organic chemical vapor deposition process was developed to create a (14 μm) thick stress-free homoepitaxial GaN film. Low temperature photoluminescence measurements are consistent with low donor background and low concentration of deep compensating centers. Capacitance–voltage measurements performed at 30 °C verified a low level of about 2×1015 cm−3 of n-type free carriers (unintentional doping), which enabled a breakdown voltage of about 500 V. A secondary ion mass spectrometry depth profile confirms the low concentration of background impurities and X-ray diffraction extracted a low dislocation density in the film. These results indicate that thick GaN films can be deposited with free carrier concentrations sufficiently low to enable high voltage rectifiers for power switching applications.
    Journal of Crystal Growth 01/2010; 312:2616-2619. · 1.55 Impact Factor
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    ABSTRACT: The increase in the forward voltage drop observed in 4H -SiC bipolar devices due to recombination-induced stacking fault (SF) creation and expansion has been widely discussed in the literature. It was long believed that the deleterious effect of these defects was limited to bipolar devices. Recent reports point to similar degradation in 4H -SiC DMOSFETs, a primarily unipolar device, which was thought to be SF-related. Here we report similar degradation of both unipolar and bipolar operation of merged- PiN -Schottky diodes, a hybrid device capable of both unipolar and bipolar operation. Furthermore, we report on the observation of the temperature-mediation of this degradation and the observation of the current-induced recovery phenomenon. These observations leave little doubt that this degradation is SF-induced and that if SFs are present, that they will adversely affect both bipolar and unipolar characteristics.
    Journal of Applied Physics 09/2009; · 2.21 Impact Factor
  • Eugene A. Imhoff, Karl D. Hobart
    Materials Science Forum - MATER SCI FORUM. 01/2009;
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    ABSTRACT: Electrical characterization of 4H-SiC epitaxial MOS devices using current-voltage (I-V), capacitance-voltage (C-V), and thermally stimulated current (TSC) measurements are presented. The effect of gamma ray irradiation on the TSC spectra of epitaxial 4H-SiC MOSCAP devices is discussed. On non-irradiated samples, two TSC peaks are observed near 55 K and 80 K. Due to the generated oxide charge during irradiation, the 80 K emission split into two constituent peaks. These have been attributed to hole traps and Al acceptors.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 01/2009
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    ABSTRACT: A polariton-based light emitter is a extraordinary concept as an alternative to a light-emitting diode (LED) or laser diode. The physics of a polariton laser is fundamentally different from the spontaneous emission process of an LED or the inversion and stimulated emission process of a laser diode. The rapid decay and emission from this polariton–exciton state bypasses the normal irreversible spontaneous emission and associated non-radiative decay mechanisms. An AlGaN/AlN nucleation bilayer was employed on r-plane sapphire to deposit non-polar GaN quantum wells embedded in an AlGaN-based cavity surrounded by top and bottom distributed Bragg reflectors (DBRs). The reflectance data show that the exciton and photon states can be tuned (by changing the angle of the sample) to the same energy. The characteristic strong coupling was observed in the reflectance data where the states of the exciton and photon do not overlap; rather they split into an upper polariton state and a lower polariton state. The photoluminescence (PL) showed a strong emission at a low stimulation level at a similar energy and angle.
    Solid State Communications 01/2009; 149(45):2039-2042. · 1.53 Impact Factor
  • Materials Science Forum - MATER SCI FORUM. 01/2009;
  • Eugene A. Imhoff, Fritz J. Kub, Karl D. Hobart
    Materials Science Forum - MATER SCI FORUM. 01/2009;
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    ABSTRACT: The forward and reverse bias dc characteristics, the long-term stability under forward and reverse bias, and the reverse recovery performance of 4H-SiC junction barrier Schottky (JBS) diodes that are capable of blocking in excess of 10 kV with forward conduction of up to 10 A at a forward voltage of less than 3.5 V (at 25degC) are described. The diodes show a positive temperature coefficient of resistance and a stable Schottky barrier height of up to 200degC. The diodes show stable operation under continuous forward current injection at 20 A/cm<sup>2</sup> and under continuous reverse bias of 8 kV at 125degC. When switched from a 10-A forward current to a blocking voltage of 3 kV at a current rate-of-fall of 30 A/mus, the reverse recovery time and the reverse recovery charge are nearly constant at 300 ns and 425 nC, respectively, over the entire temperature range of 25degC-175degC.
    IEEE Transactions on Electron Devices 09/2008; · 2.06 Impact Factor
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    ABSTRACT: Silicon carbide is a desirable material for high power and temperature bipolar and unipolar electronic devices, such as high blocking voltage pin and Schottky diodes, respectively. However, the presence of electron-hole pair (ehp) recombination at basal plane dislocations (BPDs) in the drift layer of bipolar devices has been observed to create Shockley stacking faults (SSFs). Continued ehp injection causes the SSFs to propagate further, which in turn induces an increase in the forward voltage drop (Vf) [1]. Furthermore, while the effect of SSFs upon SiC-based devices is well known, the driving force for SSF propagation and contraction are still in question. The results presented here provide significant insight into the origin of the SSF driving force, illustrate that both SSF propagation and contraction can be favorable under current injection conditions and that the drift in the specific on-state resistance of DMOSFETS may be recovered via annealing, providing further evidence that SSFs are to blame.
    Semiconductor Device Research Symposium, 2007 International; 01/2007
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    ABSTRACT: Carrier traps in 4H-SiC metal–oxide–semiconductor (MOS) capacitor and transistor devices were studied using the thermally stimulated current (TSC) method. TSC spectra from p-type MOS capacitors and n-channel MOS field-effect transistors (MOSFETs) indicated the presence of oxide traps with peak emission around 55K. An additional peak near 80K was observed due to acceptor activation and hole traps near the interface. The physical location of the traps in the devices was deduced using a localized electric field approach. The density of hole traps contributing to the 80-K peak was separated from the acceptor trap density using a gamma-ray irradiation method. As a result, hole trap density of N t,hole=2.08×1015cm−3 at 2MV/cm gate field and N t,hole=2.5×1016cm−3 at 4.5MV/cm gate field was extracted from the 80-K TSC spectra. Measurements of the source-body n +–p junction suggested the presence of implantation damage in the space-charge region, as well as defect states near the n + SiC substrate. KeywordsThermally stimulated current-hole traps-4H-SiC MOSCAP-4H-SiC MOSFET-threshold voltage-implantation damage-acceptor activation
    Journal of Electronic Materials 39(5):517-525. · 1.64 Impact Factor