S. J. Pearton

University of Florida, Gainesville, Florida, United States

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Publications (1000)1657.12 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The valence band discontinuity (ΔEV) of Y2O3/InGaZnO4 (IGZO) heterojunctions was measured by a core-level photoemission method. The Y2O3 exhibited a band gap of ∼6.27 eV from absorption measurements. A value of ΔEV = 0.44±0.21 eV was obtained by using the Ga 2p3/2, Zn 2p3/2 and In 3d5/2 energy levels as references. Given the experimental bandgap of 3.2 eV for the IGZO, this would indicate a conduction band offset ΔEC of ∼2.63 eV in the Y2O3/IGZO heterostructures and a nested interface band alignment.
    Journal of Nanoscience and Nanotechnology 11/2014; 14(11). · 1.34 Impact Factor
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    ABSTRACT: InAlN/GaN high electron mobility transistors were irradiated from the front side with 340 keV protons to a dose of 5 × 1013 cm−2. Raman thermography showed that the irradiated devices had higher channel temperatures than unirradiated control devices, but only by ∼10% under typical biasing conditions. Accordingly, the irradiated devices have higher thermal resistance (400 °C/W) compared to reference devices (350 °C/W), based on the slope of the power versus channel temperature line. However, increases of 42% in off-state drain breakdown voltage (V BR) and of >92% in critical voltage (V cri) were observed for the proton irradiated HEMT. This is ascribed to the reduction of the peak electric field at the gate edges by ∼50% through the introduction of negative trap charges created from vacancies generated by the proton irradiation.
    09/2014; 32(5):051203.
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    ABSTRACT: The effects of postprocess annealing on the gate leakage current and breakdown voltage characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) was investigated. The fabricated AlGaN/GaN HEMTs were postannealed at 250, 300, 350, 400, or 450 °C under a nitrogen (N2) atmosphere by using rapid thermal annealing, and both direct current (dc) and pulsed measurements were performed to characterize the changes in device performance. The reverse gate leakage current (IG ) at VG = −10 V was reduced by one order of magnitude and the off-state drain breakdown voltage (V off) increased by over three-fold after postprocess annealing at 450 °C. The reverse gate leakage current was found to be independent of gate-to-drain potential after annealing. The gate pulse measurements revealed the activation of deep traps during the postannealing at elevated temperatures.
    09/2014; 32(5):052201.
  • 09/2014; 32(5):051204.
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    ABSTRACT: Deep hole traps were studied in bulk free-standing GaN crystals and in thinner (10–20 μm) GaN films prepared by hydride vapor phase epitaxy (HVPE) on sapphire. Six hole traps in different combinations were detected in these crystals, H1 (activation energy 0.92–0.94 eV), H2 (0.55 eV), H3 (0.65–0.7 eV), H4 (0.85–0.9 eV), H5 (1.1–1.2 eV), and H6 (0.95–1.05 eV). The dominant traps in all samples were the H5 and H6 traps that were attributed, respectively, to gallium vacancy complexes with oxygen (VGa-O) and substitutional carbon related centers. We associate the H5 hole traps with the red luminescence bands, the H4 hole traps with the green luminescence bands, and the H6 hole traps with the yellow luminescence bands often observed in HVPE GaN. These attributions are based on the low energy thresholds of the deep traps optical excitation spectra and the depth of the respective trap levels.
    Journal of Applied Physics 06/2014; 115(22):223702-223702-5. · 2.19 Impact Factor
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    ABSTRACT: AlGaN/GaN High Electron Mobility Transistors (HEMTs) show great promise for applications such as military radar and satellite-based communications systems. Circular devices, with smaller junction areas and higher effective gate width/gate length, are better suited for high power, high frequency application. Since the circular devices have been shown to improve the device performance, the effect of radiation on their reliability is an important topic. There are many studies characterizing the performance under proton, neutron, and electron irradiations for rectangular AlGaN/GaN HEMTs. However, the studies for circular-shaped devices are limited. In this work, we report the effect of proton irradiation on circular-based AlGaN/GaN HEMT. DC performance, breakdown voltage, and gate lag measurement were done to characterize the devices. AlGaN/GaN HEMT heterostructures were grown on c-plane sapphire by molecular beam epitaxy (MBE). The layer structure included a thin AlGaN nucleation layer followed with a 2 µm thick undoped GaN buffer topped by a 25 nm thick unintentionally doped Al0.28GaN layer. Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2-based discharges. The Ohmic contacts were formed by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). Gates with the dimension of 1.5 µm × 314 µm were defined by lift-off of e-beam deposited Ni/Au metallization. The contacts were annealed at 850°C for 45 s under a flowing N2 ambient in Heatpulse 610T system. The final step was deposition of e-beam evaporated Ti/Au (300 Å/1200 Å) interconnection contacts. Devices were exposed to protons with 5 MeV and different doses of 2×1014 proton/cm2. The device DC performance was characterized using an HP 4156 parameter analyzer. The breakdown voltage was conducted with a Tektronix 370A curve tracer. FLOODS TCAD finite-element solver was used to simulate the electrical field around the gate edge to confirm the experimental result. As shown in Figure 1, drain current IVs, IDSS, decreased from 454 to 398 mA/mm after irradiation with a proton dose of 2×1014 cm-2. Moreover, resistance between source and drain increased and extrinsic transconductance decreased after irradiation. However, the drain breakdown voltage increased from 150 to 230 V. The improvement of the breakdown voltage was due to the reduction of electric field at the edge of the gate, caused by the defects generated by proton irradiation at AlGaN/GaN interface, which was confirmed with simulation, as shown in Figure 2. Both gate and drain pulse measurements were also performed to verify the introductions of implanted defects into the HMET structure, which will be discussed in the talk.
    225th ECS Meeting; 05/2014
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    ABSTRACT: AlGaN/GaN high electron mobility transistors are used in high temperature, high power, and high frequency applications due to their superior combination of high breakdown voltage and high frequency performance. Unfortunately, their reliability in the field has warranted further investigation because of the stochastic nature of the variety of defects that arise during their operation. Many analytical techniques are not well suited to this analysis because they sample regions of the device which are either too small or too large for accurate observation. This talk will give a review of recent studies that employ structural, chemical, and electrical device characterization paired with simulation in order to develop structure-property relationships between defects and device performance. This includes the use of a new approach to chemical deprocessing combined with surface-sensitive analysis techniques such as scanning electron microscopy and scanning probe microscopy in the analysis of large-area defect formation. Wet etching of the passivation nitride and metal contacts was used to strip the transistor surface features and remove organic contaminants. This exposed the top surface of the AlGaN epilayer for analysis. This processing revealed and surface analysis revealed three different defects. One of these defects is a nanoscale crack which emanates from metal inclusions formed during alloying of the ohmic contacts of the device prior to use in the field, and could impact the yield of production-level manufacturing of these devices. This defect also appears to grow, in some cases, during electrostatic stressing. Another defect, a native oxide at the surface of the semiconductor under the gate, appears to react in the presence of an electric field, which could play a role in the degradation of the gate contact during high-field, off-mode electrostatic stressing. This defect could also facilitate the formation of the pitting of the AlGaN epilayer beneath the gate contact. The composition of the as-grown gate interfacial layers were characterized using atom probe tomography and are composed of two distinct oxide layers, NiOx and AlOx. Furthermore, using off-state reverse bias electrical stress, Ni-gate metal reactions with AlGaN epilayers emulate the shape and size of the electric field contours between 5 – 6 MV/cm; suggesting a critical field for defect formation. This talk will summarize these finding and place them in the overall context of failure mechanisms in AlGaN/GaN HEMT structures.
    225th ECS Meeting; 05/2014
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    ABSTRACT: The authors report an investigation of the effect of different doses of 5 MeV proton irradiation on circular-shaped AlGaN/GaN high electron mobility transistors. The degradation of saturation drain current (IDSS) was minimal up to an irradiation dose of 2 × 1013 cm−2. By comparison, a dose of 2 × 1014 cm−2 dose produced a 12.5% reduction of IDSS and 9.2% increase of sheet resistance. In addition, the threshold voltage showed larger positive shifts for 2×1014 cm−2 dose compared to 2×1013 cm−2, and both of these doses produced showed larger shifts for smaller gate to drain distances. Increases of 39.8% and 47.1%, respectively, in the breakdown voltage for 6 and 10 μm drain to gate distances (LGD) was observed and was attributed to the creation of a virtual gate at the AlGaN/GaN interface due to the irradiation, which reduced the peak electric field at the drain side of the gate edge.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 05/2014; 32(1):012201-012201-7. · 1.36 Impact Factor
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    ABSTRACT: The changes in direct current performance of circular-shaped AlGaN/GaN high electron mobility transistors (HEMTs) after 60Co γ-irradiation doses of 50, 300, 450, or 700 Gy were measured. The main effects on the HEMTs after irradiation were increases of both drain current and electron mobility. Compton electrons induced from the absorption of the γ-rays appear to generate donor type defects. Drain current dispersions of ∼5% were observed during gate lag measurements due to the formation of a virtual gate between the gate and drain resulting from the defects generated during γ-irradiation.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 05/2014; 32(3):031203-031203-5. · 1.36 Impact Factor
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    ABSTRACT: We demonstrate GaN-based thin light-emitting diodes (LEDs) on flexible polymer and paper substrates covered with chemical vapor deposited graphene as a transparent-conductive layer. Thin LEDs were fabricated by lifting the sapphire substrate off by Excimer laser heating, followed by transfer of the LEDs to the flexible substrates. These substrates were coated with tri-layer graphene by a wet transfer method. Optical and electrical properties of thin laser lift-offed LEDs on the flexible substrates were characterized under both relaxed and strained conditions. The graphene on paper substrates remained conducting when the graphene/paper structure was folded. The high transmittance, low sheet resistance and high failure strain of the graphene make it an ideal candidate as the transparent and conductive layer in flexible optoelectronics.
    Optics express. 05/2014; 22(S3):A812-A817.
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    ABSTRACT: Proton irradiation from the backside of the samples were employed to enhance off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates. Via holes were fabricated directly under the active area of the HEMTs by etching through the Si substrate for subsequent backside proton irradiation. By taking the advantage of the steep drop at the end of proton energy loss profile, the defects created by the proton irradiation from the backside of the sample could be precisely placed at specific locations inside the AlGaN/GaN HEMT structure. There were no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for the irradiated AlGaN/GaN HEMTs with the proton energy of 225 or 275 keV, for which the defects created by the proton irradiations were intentionally placed in the GaN buffer. HEMTs with defects placed in the two dimensional electron gas (2DEG) channel region and AlGaN barrier using 330 or 340 keV protons not only showed degradation of both drain current and extrinsic transconductance but also exhibited improvement of the off-state drain breakdown voltage. The Florida Object Oriented Device and Process Simulator Technology Computer Aided Design finite-element simulations were performed to confirm the hypothesis of a virtual gate formed around the 2DEG region to reduce the peak electric field around the gate edges and increase the off-state drain breakdown voltage.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 05/2014; 32(2):021203-021203-6. · 1.36 Impact Factor
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    ABSTRACT: The energy discontinuity in the valence band (deltaE(v)) of Y2O3-stabilized ZrO2 (YSZ)/InGaZnO4 (IGZO) heterostructures was obtained from X-ray photoelectron spectroscopy (XPS) measurements. The YSZ exhibited a bandgap of 4.4 eV from absorption measurements. A value of deltaE(v) = 0.57 +/- 0.12 eV was obtained by using Ga 2P3/2, Zn 2p3/2 and In 3d5/2 energy levels as references. This implies a conduction band offset (deltaE(c)) of 0.63 eV in YSZ/InGaZnO4 heterostructures and a nested interface band alignment.
    Journal of Nanoscience and Nanotechnology 05/2014; 14(5):3925-7. · 1.34 Impact Factor
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    ABSTRACT: We demonstrate GaN-based thin light-emitting diodes (LEDs) on flexible polymer and paper substrates covered with chemical vapor deposited graphene as a transparent-conductive layer. Thin LEDs were fabricated by lifting the sapphire substrate off by Excimer laser heating, followed by transfer of the LEDs to the flexible substrates. These substrates were coated with tri-layer graphene by a wet transfer method. Optical and electrical properties of thin laser lift-offed LEDs on the flexible substrates were characterized under both relaxed and strained conditions. The graphene on paper substrates remained conducting when the graphene/paper structure was folded. The high transmittance, low sheet resistance and high failure strain of the graphene make it an ideal candidate as the transparent and conductive layer in flexible optoelectronics.
    Optics Express 04/2014; 22(S3). · 3.53 Impact Factor
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    ABSTRACT: The effects of proton irradiation dose on the dc characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with source field plates were studied. The HEMTs were irradiated with various protons doses ranging from 5 × 1012 to 5 × 1015 cm−2 at a fixed energy of 5 MeV. HEMTs irradiated with proton dose below 5 × 1013 cm−2 showed less than 2% degradation of either saturation drain current (IDSS) or transconductance (gm). Significant changes of these parameters were observed for the devices irradiated with doses above 5 × 1013 cm−2. HEMTs irradiated with the highest proton dose of 5 × 1015 cm−2 showed a reduction of IDSS and gm of 86% and 64.7%, and a positive Vth shift of 0.84 V, respectively. Despite the significant IDSS and gm reductions, the off-state drain breakdown voltage (VBR) was improved more than five times at this particular irradiation condition. The significant improvement of off-state drain breakdown voltage was attributed to the formation of a virtual gate at drain side of gate edge, which was the result of the generation of defect centers at AlGaN/GaN interface.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 03/2014; 32(2):022202-022202-6. · 1.36 Impact Factor
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    ABSTRACT: The effect of proton irradiation on the off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates was studied by irradiating protons from the backside of the samples through via holes fabricated directly under the active area of the HEMTs. There was no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for HEMTs irradiated with 275 keV protons, for which the defects created by the proton irradiation were intentionally placed in the GaN buffer. HEMTs with defects positioned in the 2 dimensional electron gas channel region and AlGaN barrier using 330 keV protons not only showed degradation of both drain current and extrinsic transconductance but also exhibited an improvement of the off-state drain breakdown voltage. Finite-element simulations showed the enhancement of the latter were due to a reduction in electric field strength at the gate edges by introduction of charged defects.
    Applied Physics Letters 02/2014; 104(8):082106-082106-3. · 3.52 Impact Factor
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    S.J Pearton, F. Ren
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    ABSTRACT: ZnO and related semiconductors are alternatives to GaN-based compounds for fabrication of UV/blue light emitting diodes (LEDs). Progress in development of ZnO LEDs has been disappointing due to the difficulty of achieving robust p-type doping and the low crystal quality of heterojunctions and quantum wells. We critically review reports of p-type doping using group V impurities and summarize recent progress and prospects for further advancement of ZnO-based light emitters.
    Current Opinion in Chemical Engineering. 02/2014; 3:51–55.
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    ABSTRACT: A combination of TRIM and FLOODS models the effect of radiation damage on AlGaN/GaN HEMTs. While excellent fits are obtained for threshold voltage shift, the models do not fully explain the increased reliability observed experimentally. In short, the addition of negatively-charged traps in the GaN buffer layer does not significantly change the electric field at the gate edges at radiation fluence levels seen in this study. We propose that negative trapped charge at the nitride/AlGaN interface actually produces the virtual-gate effect that results in decreasing the magnitude of the electric field at the gate edges and thus the increase in critical voltage. Simulation results including nitride interface charge show significant changes in electric field profiles while the I-V device characteristics do not change.
    IEEE Transactions on Nuclear Science 12/2013; 60(6):4103-4108. · 1.46 Impact Factor
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    ABSTRACT: We demonstrate AuCl<sub>3</sub>-doped graphene transparent conductive electrodes integrated in GaN-based ultraviolet (UV) light-emitting diodes (LEDs) with an emission peak of 363 nm. AuCl<sub>3</sub> doping was accomplished by dipping the graphene electrodes in 5, 10 and 20 mM concentrations of AuCl<sub>3</sub> solutions. The effects of AuCl<sub>3</sub> doping on graphene electrodes were investigated by current-voltage characteristics, sheet resistance, scanning electron microscope, optical transmittance, micro-Raman scattering and electroluminescence images. The optical transmittance was decreased with increasing the AuCl<sub>3</sub> concentrations. However, the forward currents of UV LEDs with p-doped (5, 10 and 20 mM of AuCl<sub>3</sub> solutions) graphene transparent conductive electrodes at a forward bias of 8 V were increased by ~48, 63 and 73%, respectively, which can be attributed to the reduction of sheet resistance and the increase of work function of the graphene. The performance of UV LEDs was drastically improved by AuCl<sub>3</sub> doping of graphene transparent conductive electrodes.
    Optics Express 11/2013; 21(23):29025-30. · 3.53 Impact Factor
  • Thin Solid Films 11/2013; · 1.87 Impact Factor
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    ABSTRACT: The effects of high energy proton irradiation dose on dc performance as well as critical voltage of the drain-voltage step-stress of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated to evaluate the feasibility of AlGaN/GaN HEMTs for space applications, which need to stand a variety of irradiations. The HEMTs were irradiated with protons at a fixed energy of 5 MeV and doses ranging from 109 to 2 × 1014 cm-2. For the dc characteristics, there was only minimal degradation of saturation drain current (IDSS), transconductance (gm), electron mobility, and sheet carrier concentration at doses below 2 × 1013 cm-2, while the reduction of these parameters were 15%, 9%, 41% and 16.6%, respectively, at a dose of 2 × 1014 cm-2. At this same dose condition, increases of 37% in drain breakdown voltage (VBR) and of 45% in critical voltage (Vcri) were observed. The improvements of drain breakdown voltage and critical voltage were attributed to the modification of the depletion region due to the introduction of a higher density of defects after irradiation at a higher dose.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 10/2013; 31(4):042202-042202-6. · 1.36 Impact Factor

Publication Stats

15k Citations
1,657.12 Total Impact Points

Institutions

  • 1994–2014
    • University of Florida
      • • Department of Materials Science and Engineering
      • • Department of Chemical Engineering
      Gainesville, Florida, United States
  • 2012
    • Korea University
      • Department of Chemical and Biological Engineering
      Seoul, Seoul, South Korea
    • Feng Chia University
      • Department of Chemical Engineering
      臺中市, Taiwan, Taiwan
  • 2007–2012
    • Kyungpook National University
      • School of Materials Science and Engineering
      Daikyū, Daegu, South Korea
  • 2002–2010
    • Inje University
      • Department of Nano Engineering
      Kŭmhae, Gyeongsangnam-do, South Korea
  • 2009
    • Yale University
      • Department of Electrical Engineering
      New Haven, CT, United States
  • 2008
    • National Central University
      • Department of Optics and Photonics
      Taoyuan City, Taiwan, Taiwan
  • 2002–2004
    • Wright-Patterson Air Force Base
      Dayton, Ohio, United States
  • 1998–2001
    • Australian National University
      • Department of Electronic Materials Engineering (EME)
      Canberra, Australian Capital Territory, Australia
    • University of Southern California
      • Department of Electrical Engineering
      Los Angeles, CA, United States
    • Lehigh University
      • Department of Physics
      Albuquerque, NM, United States
  • 1995–2000
    • Sandia National Laboratories
      • Semiconductor Material and Device Sciences Department
      Albuquerque, New Mexico, United States
  • 1997
    • Hampton University
      • Department of Physics
      Hampton, VA, United States
  • 1989–1995
    • AT&T Labs
      Austin, Texas, United States
  • 1993
    • Martin Marietta Laboratories
      Baltimore, Maryland, United States