S. J. Pearton

University of Florida, Gainesville, Florida, United States

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Publications (965)1345.69 Total impact

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    ABSTRACT: Electrical and luminescent properties and deep trap spectra of Si dopedGaN films grown by maskless epitaxial lateral overgrowth (MELO) are reported. The dislocation density in the wing region of the structure was 106 cm−2, while in the seed region it was 108 cm−2. The major electron traps present had activation energy of 0.56 eV and concentrations in the high 1015 cm−3 range. A comparison of diffusion length values and 0.56 eV trap concentration in MELO GaN and epitaxial lateral overgrowth (ELOG) GaN showed a good correlation, suggesting these traps could be effective in carrier recombination. The doped MELO films were more uniform in their electrical properties than either ELOG films or undoped MELO films. We also discuss the differences in deep trap spectra and luminescence spectra of low-dislocation-density MELO, ELOG, and bulk n-GaN samples grown by hydride vapor phase epitaxy. It is suggested that the observed differences could be caused by the differences in oxygen and carbon contamination levels.
    Preview · Article · Jan 2016 · Journal of Applied Physics
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    ABSTRACT: By varying the photon flux during sub-bandgap optical pumping, the locations of traps associated with sub-bandgap laser pumping in AlGaN/GaN high electron mobility transistors (HEMTs) were identified. Fixed photon flux of sub-bandgap optical pumping was previously employed to determine the activation energies of traps in AlGaN/GaN HEMT; traps with activation energies of 0.73 eV, both 0.73 and 1.91 eV or all three trap states detected for the 671, 532, or 477 nm laser illumination, respectively. However, by illuminating the HEMTs with lower photon fluxes from 532 or 447 nm lasers, the traps with 0.73 eV were absent, and only the traps associated with higher activation energies were activated by the laser light. This indicated that the traps related to the activation energies of 1.91 and 2.35 eV are likely located at the surface of the HEMTs. Photon flux dependent gate-lag measurements were also conducted. Laser illumination of 671 nm did not affect the drain current during the gate lag measurement, implying the traps linked to the activation energy of 0.73 eV are not present in the AlGaN layer of the HEMT structure. On the other hand, the 1.91 and 2.35 eV trap states were distributed in the AlGaN layer of the HEMTs. This approach to selectively exciting the defect states provides a nondestructive method of probing the ionization levels and spatial location of traps in wide bandgap heterostructures.
    No preview · Article · Jan 2016
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    Erin E. Patrick · Mohua Choudhury · Fan Ren · Stephen J. Pearton · Mark E. Law
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    ABSTRACT: AlGaN/GaN high electron mobility transistors (HEMTs) are desirable for space applications because of their relative radiation hardness. Predictive modeling of these devices is therefore desired; however, physics-based models accounting for radiation-induced degradation are incomplete. In this work, we show that a partially ionized impurity scatteringmobility model can explain the observed reduction in mobility. Electrostatic changes can be explained by confinement of negative charge near the 2DEG in the GaN buffer layer. Simulation results from FLOODS (a TCAD simulator) demonstrate that partial ionization of donor traps is responsible for this phenomenon. Compensation of the acceptor traps by the ionized donors in the GaN confine the acceptor traps (negative space charge) to a thin layer near the AlGan/GaN interface. The simulation results show that near equal concentrations of acceptor traps and donor traps of 1 x 10(17) cm(-3) can account for the performance degradation of HEMTs given 5MeV proton radiation at a fluence of 2 x 10(14) cm(-2). Our results imply that device performance can be accurately simulated by simultaneously accounting for mobility and electrostatic degradation in TCAD solvers using the presented approach.
    Full-text · Article · Dec 2015
  • David C. Hays · B.P. Gila · S.J. Pearton · Byung-Jae Kim · F. Ren
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    ABSTRACT: We have measured the band offsets of sputtered ZrSiO4 on bulk ZnO wafers using X-Ray Photoelectron Spectroscopy and obtained the bandgaps of these two materials using reflection electron energy loss spectroscopy. The valence band offset was determined to be −0.60 eV ± 0.04 eV for ZrSiO4 on ZnO, while the respective bandgaps were 3.22 eV for ZnO and 5.9 eV for ZrSiO4. The conduction band offset for ZrSiO4/ZnO was then determined to be 3.28 eV. The ZrSiO4/ZnO system has a staggered, type II alignment. This means that while ZrSiO4 may be useful for surface passivation applications on ZnO, it is not suitable for thin film transistors where a positive valence band offset is needed.
    No preview · Article · Dec 2015
  • Wayne K Morrow · Stephen J Pearton · Fan Ren
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    ABSTRACT: Conventional thin-film diffusion barriers consist of 3D bulk films with high chemical and thermal stability. The purpose of the barrier material is to prevent intermixing or penetration from the two materials that encase it. Adhesion to both top and bottom materials is critical to the success of the barrier. Here, the effectiveness of a single atomic layer of graphene as a solid-state diffusion barrier for common metal schemes used in microelectronics is reviewed, and specific examples are discussed. Initial studies of electrical contacts to graphene show a distinct separation in behavior between metallic groups that strongly or weakly bond to it. The two basic classes of metal reactions with graphene are either physisorbed metals, which bond weakly with graphene, or chemisorbed metals, which bond strongly to graphene. For graphene diffusion barrier testing on Si substrates, an effective barrier can be achieved through the formation of a carbide layer with metals that are chemisorbed. For physisorbed metals, the barrier failure mechanism is loss of adhesion at the metal-graphene interface. A graphene layer encased between two metal layers, in certain cases, can increase the binding energy of both films with graphene, however, certain combinations of metal films are detrimental to the bonding with graphene. While the prospects for graphene's future as a solid-state diffusion barrier are positive, there are open questions, and areas for future research are discussed. A better understanding of the mechanisms which influence graphene's ability to be an effective diffusion barrier in microelectronic applications is required, and additional experiments are needed on a broader range of metals, as well as common metal stack contact structures used in microelectronic applications. The role of defects in the graphene is also a key area, since they will probably influence the barrier properties.
    No preview · Article · Nov 2015 · Small
  • Alexander Y. Polyakov · Nikolai B. Smirnov · I.-H. Lee · Stephen J. Pearton
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    ABSTRACT: A modification of deep level transient spectroscopy which varies the measurement frequency from 10 kHz to 1 MHz and is based on commercially available inductance-capacitance-resistance meters and pulse generators was tested for GaN films and AlGaN/GaN high electron mobility transistor structures with various series resistances. It is demonstrated that the measured spectra at high and low frequency follow the well documented frequency dependences of the stationary capacitance and magnitude of the capacitance transient. Measurements at low frequency allow for accurate determination of the concentration of the traps and, in many cases, detect traps that cannot be observed in the high frequency measurements. This is particularly valuable in materials like GaN where series resistance effects can be significant.
    No preview · Article · Nov 2015 · Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics
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    Full-text · Article · Oct 2015 · ECS Transactions
  • B.-J. Kim · Y.-H. Hwang · Shihyun Ahn · Fan Ren · Stephen J. Pearton · Jihyun Kim · Tae Sung Jang
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    ABSTRACT: The effects of proton irradiation on optical and electrical performances of InGaN/GaN blue light-emitting diodes (LEDs) were investigated. The InGaN/GaN blue LEDs were irradiated with protons at a fixed energy of 340 keV and doses ranging from 5 × 1010 to 1 × 1014/cm2. Both current–voltage (I-V) and light output–current (L-I) characteristics of InGaN/GaN blue LEDs were gradually degraded as increasing the proton doses. The optical performances of LED were much more sensitive to the proton irradiation than that of electrical performances. The electroluminescence spectra and the light output performances before and after proton irradiations had similar trends in degradation. Then, the reverse recovery time before and after 1 × 1014/cm2 proton irradiation slightly decreased from 31.0 to 27.6 ns.
    No preview · Article · Sep 2015 · Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics
  • David C. Hays · Brent P. Gila · Stephen J. Pearton · B.-J. Kim · Fan Ren · Tae Sung Jang
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    ABSTRACT: ZnO is promising for a number of applications in light emission, sensors, and transparent conducting electronics, but its surface is susceptible to instabilities caused by atmospheric exposure. Thus, there is a need for stable passivation or gate dielectric layers that might obviate this issue. One potential candidate is Sc2O3. The authors have measured the band offsets of sputtered Sc2O3 on both Zn- and O-terminated ZnO using x-ray photoelectron spectroscopy and obtained the bandgaps of the materials using reflection electron energy loss spectroscopy. The valence band offset was determined to be ∼1.67 ± 0.16 eV for Sc2O3 on Zn-terminated ZnO (bandgap 3.26 eV) and 1.59 ± 0.16 eV on O-terminated ZnO (bandgap 3.22 eV), i.e., similar within experimental error. The conduction band offset for Sc2O3/ZnO was then determined to be 4.92 eV. The Sc2O3/ZnO system has a staggered, type II alignment, meaning that it is not suitable for thin film transistors but it may still be useful for surface passivation.
    No preview · Article · Sep 2015 · Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics
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    ABSTRACT: The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al2O3 as the gate dielectric were studied. Al2O3/AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5 × 1015 cm−2 at different energies of 5, 10, or 15 MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15 MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127–289 cm−1. These are similar to the values reported for conventional metal-gate high-electron mobility transistors under the same conditions and show that the gate dielectric does not affect the response to proton irradiation for these energies.
    No preview · Article · Sep 2015 · Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics
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    ABSTRACT: The effect of a back gate on the dc performance of AlGaN/GaN high electron mobility transistor was investigated. The back gate was fabricated directly under the device active area by etching off the Si substrate, AlN nucleation layer, and graded AlGaN transition layer and depositing Ni/Au-based gate metal on the exposed GaN buffer layer. The reverse bias gate leakage current decreased from 3.9 × 10−5 to 1.2 × 10−5 mA/mm by applying −10 V at the back gate. Because of the suppression of gate leakage current by the back gate, the drain on/off ratio improved from 1.8 × 105 to 1.2 × 106 and the subthreshold swing from 204 to 137 mV/dec. Moreover, the drain breakdown voltage could be improved by 40% when the back gate was biased at −25 V.
    Full-text · Article · Jul 2015
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    B.-J. Kim · S. Ahn · Y.-H. Hwang · F. Ren · S. J. Pearton

    Full-text · Article · May 2015 · ECS Transactions
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    Y.-H. Hwang · T.-S. Kang · F. Ren · S. J. Pearton
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    ABSTRACT: A novel approach by employing a Cu filled via hole under device active area to improve heat dissipation of AlGaN/GaN high electron mobility transistors (HEMTs) grown on silicon substrate is proposed. In this study, finite element analysis was employed to simulate the temperature distributions of reference HEMT and proposed model. By introducing a Cu filled via underneath the active area and removing a thermal resistive layer resulted from the nucleation layer generated during the HEMT structure grwoth, the maximum junction temperature could be reduced from 147 to 123°C at a power density of 5W/mm.
    Full-text · Article · May 2015 · ECS Transactions
  • Y.-H. Hwang · W. Zhu · C. Dong · S. Ahn · F. Ren · I.I. Kravchenko · D.J. Smith · S. J. Pearton
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    ABSTRACT: A novel structure with backside gate was proposed in this study. The effect of backside gate to the device performance including DC performance and breakdown voltage was investigated. When the back-side gate was biased at -25V, the leakage current could be suppressed to one order lower. As a result, the subthreshold swing (SS) could be improved from 240 to 137 mV/dec. Most important of all, the off-state drain breakdown voltage could be increased by 40%. If the back-side gate is connected to the front-side gate, which could be viewed as a back-side gate field plate structure, the off-state drain breakdown voltage could be improved by 7%
    No preview · Article · May 2015 · ECS Transactions
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    ABSTRACT: The effects of a thermal annealing process on the dc performance of off-state, drain-voltage step-stressed AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. After stress, the reverse bias gate leakage current increased from 7 × 10−3 to 1.96 × 10−1 mA/mm and drain current on–off ratio decreased from 1.9 × 105 to 4.52 × 103. These degradations were completely recovered after a thermal annealing at 450 °C for 10 min. Temperature-dependent drain-current subthreshold swing measurements were employed to estimate the trap densities located in the AlGaN barrier layer near-surface region of the HEMTs before and after off-state drain-voltage step-stressing and also following subsequent thermal annealing. Off-state step-stressing produced a significant increase of trap density from 2.15 × 1012 to 1.63 × 1013/cm2 V. This was reduced to 5.21 × 1012/cm2 V after thermal annealing. These results show that simple thermal annealing can recover much of the degradation caused by step-stressing below the threshold for permanent damage.
    Full-text · Article · May 2015
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    ABSTRACT: The effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs) was studied. AlGaN/GaN HEMT structures with 2 and 5 μm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 μm GaN buffer layer were 7 × 109 and 5 × 108 cm−2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, there was no dispersion observed on the nonpassivated HEMTs with 5 μm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 μm GaN buffer layer.
    Full-text · Article · May 2015
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    ABSTRACT: The degradation mechanism of Ti/Al/Ni/Au-based Ohmic metallization on AlGaN/GaN high electron mobility transistors upon exposure to buffer oxide etchant (BOE) was investigated. The major effect of BOE on the Ohmic metal was an increase of sheet resistance from 2.89 to 3.69 Ω/◻ after 3 min BOE treatment. The alloyed Ohmic metallization consisted 3-5 μm Ni-Al alloy islands surrounded by Au-Al alloy-rings. The morphology of both the islands and ring areas became flatter after BOE etching. Energy dispersive x-ray analysis and Auger electron microscopy were used to analyze the compositions and metal distributions in the metal alloys prior to and after BOE exposure.
    No preview · Article · May 2015

  • No preview · Article · Apr 2015
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    ABSTRACT: The recovery effects of thermal annealing on dc and rf performance of off-state step-stressed AlGaN/GaN high electron mobility transistors were investigated. After stress, reverse gate leakage current and sub-threshold swing increased and drain current on-off ratio decreased. However, these degradations were completely recovered after thermal annealing at 450 °C for 10 mins for devices stressed either once or twice. The trap densities, which were estimated by temperature-dependent drain-current sub-threshold swing measurements, increased after off-state step-stress and were reduced after subsequent thermal annealing. In addition, the small signal rf characteristics of stressed devices were completely recovered after thermal annealing.
    Full-text · Article · Apr 2015 · Applied Physics Letters
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    ABSTRACT: The movement of basal plane segments of dislocations in low-dislocation-density GaN films grown by epitaxial lateral overgrowth as a result of irradiation with the probing beam of a scanning electron microscope was detected by means of electron beam induced current. Only a small fraction of the basal plane dislocations was susceptible to such changes and the movement was limited to relatively short distances. The effect is explained by the radiation enhanced dislocation glide for dislocations pinned by two different types of pinning sites: a low-activation-energy site and a high-activation-energy site. Only dislocation segments pinned by the former sites can be moved by irradiation and only until they meet the latter pinning sites.
    No preview · Article · Mar 2015 · Applied Physics Letters

Publication Stats

15k Citations
1,345.69 Total Impact Points

Institutions

  • 1994-2016
    • University of Florida
      • • Department of Materials Science and Engineering
      • • Department of Electrical and Computer Engineering
      Gainesville, Florida, United States
  • 2008
    • The Electrochemical Society
      Society Hill, New Jersey, United States
  • 2003
    • Inje University
      • Department of Nano Engineering
      Kŭmhae, Gyeongsangnam-do, South Korea
  • 2000
    • Chonbuk National University
      • Department of Chemical Engineering
      Tsiuentcheou, Jeollabuk-do, South Korea
  • 1997-2000
    • Sandia National Laboratories
      • Semiconductor Material and Device Sciences Department
      Albuquerque, New Mexico, United States
    • Carnegie Mellon University
      • Department of Materials Science and Engineering
      Pittsburgh, Pennsylvania, United States
  • 1989-1998
    • AT&T Labs
      Austin, Texas, United States
  • 1993
    • Martin Marietta Laboratories
      Baltimore, Maryland, United States
  • 1992
    • Newark Academy
      Ливингстон, New Jersey, United States