S.J. Pearton

Publications (893)1753.65 Total impact

• Article: Role of Gate Oxide in AlGaN/GaN High-Electron-Mobility Transistor pH Sensors

  B.S. Kang, H.T. Wang, F. Ren, M. Hlad, B.P. Gila, C.R. Abernathy, S.J. Pearton, C. Li, Z.N. Low, J. Lin, J.W. Johnson, P. Rajagopal, J.C. Roberts, E.L. Piner, K.J. Linthicum
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  ABSTRACT: We report on a comparison of different gate oxides for AlGaN/GaN high-electron-mobility transistor (HEMT) pH sensors. The HEMTs show a linear increase in drain-source current as the pH of the electrolyte solutions introduced to the gate region is decreased. Three different gate oxides were examined, namely the native oxide on the AlGaN surface, a UV-ozone-induced oxide and an Sc2O3 gate deposited by molecular beam epitaxy. The Sc2O3 produced superior results in terms of resolution in measuring small changes in pH. The devices with Sc2O3 in the gate region exhibited a linear change in current between pH 3 and 10 of 37μA/pH with a resolution of <0.1pH over the entire pH range. In contrast, the native oxide devices showed a larger change in current, ∼70μA/pH, but with a degraded resolution of ∼0.4pH. Results for the UV-ozone oxide were intermediate in resolution, 0.2pH. These HEMTs have promise for detecting pH changes in biological samples and can be readily integrated into a standard package for wireless data transmission.
  Journal of Electronic Materials 05/2012; 37(5):550-553. · 1.47 Impact Factor
• Article: Shallow and Deep Centers in As-Grown and Annealed MgZnO/ZnO Structures with Quantum Wells

  A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, E. A. Kozhukhova, A. I. Belogorokhov, D. P. Norton, H. S. Kim, S. J. Pearton
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  ABSTRACT: Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied before and after annealing in oxygen atmosphere at high temperatures of 850°C to 950°C. In both as-grown and annealed structures, microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13eV compared with bulk ZnO films, indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of shallow uncompensated donors from ~1017cm−3 to ~1016cm−3 and makes it possible to probe the region of the QW by capacitance–voltage (C–V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are the well-known traps with activation energies of 0.3eV and 0.8eV. After annealing, the electron traps observed in the structure have activation energies of 0.14eV, 0.33eV, and 0.57eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of band-edge luminescence and produce a deep luminescence defect band at 2.2eV. In addition, a defect vibrational band becomes visible in Raman spectra near 650cm−1. No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed. KeywordsZnO-quantum wells-MgZnO
  Journal of Electronic Materials 05/2012; 39(5):601-607. · 1.47 Impact Factor
• Article: Design of edge termination for GaN power Schottky diodes

  J. R. Laroche, F. Ren, K. W. Baik, S. J. Pearton, B. S. Shelton, B. Peres
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  ABSTRACT: The GaN Schottky diodes capable of operating in the 300–700-V range with low turn-on voltage (0.7 V) and forward conduction currents of at least 10 A at 1.4 V (with corresponding forward current density of 500 A/cm2) are attractive for applications ranging from power distribution in electric/hybrid electric vehicles to power management in spacecraft and geothermal, deep-well drilling telemetry. A key requirement is the need for edge-termination design to prevent premature breakdown because of field crowding at the edge of the depletion region. We describe the simulation of structures incorporating various kinds of edge termination, including dielectric overlap and ion-implanted guard rings. Dielectric overlap using 5-µm termination of 0.1–0.2-µm-thick SiO2 increases the breakdown voltage of quasi-vertical diodes with 3-µm GaN epi thickness by a factor of ∼2.7. The use of even one p-type guard ring produces about the same benefit as the optimized dielectric overlap termination.
  Journal of Electronic Materials 04/2012; 34(4):370-374. · 1.47 Impact Factor
• Article: Properties of phosphorus-doped (Zn,Mg)O thin films and device structures

  Y. W. Heo, Y. W. Kwon, Y. Li, S. J. Pearton, D. P. Norton
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  ABSTRACT: The properties of phosphorus-doped (Zn,Mg)O polycrystalline and epitaxial thin films are described. The as-deposited (Zn,Mg)O:P films are n type with high electron carrier density. High resistivity is induced in the films with moderate temperature annealing, which is consistent with suppression of the donor state and activation of the deep acceptor. The resistivity of the as-deposited and annealed film is an order of magnitude higher than similar samples with no Mg, consistent with a shift in the conduction band edge relative to the defect-related donor state. The capacitance-voltage characteristics of annealed metal/insulator/P-doped (Zn,Mg)O structures in which the (Zn,Mg)O is polycrystalline exhibit p-type polarity. In addition, multiple polycrystalline devices comprising n-type ZnO/P-doped (Zn,Mg)O thin-film junctions display asymmetric I–V characteristics that are consistent with the formation of a p-n junction at the interface, although the ideality factor is anomalously high.
  Journal of Electronic Materials 04/2012; 34(4):409-415. · 1.47 Impact Factor
• Article: Synthesis and characterization of phosphorus-doped ZnO and (Zn,Mg)O thin films via pulsed laser deposition

  Y. J. Li, Y. W. Heo, J. M. Erie, H. Kim, K. Ip, S. J. Pearton, D. P. Norton
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  ABSTRACT: The transport and optical properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition (PLD) are studied. The carrier type of as-deposited (Zn,Mg)O:P films converts from n-type to p-type with increasing oxygen partial pressure. All the films exhibit good crystallinity with c-axis orientation. This result indicates the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O3/O2 mixture is two orders of magnitude higher than the films grown in oxygen and O2/Ar/H2 mixture. The room-temperature photoluminescence (PL) of the as-deposited films has been shown that growing in the O2/Ar/H2 mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The enhanced ultraviolet (UV) emission in the films grown in O2/Ar/H2 mixture may result from hydrogen passivation of the deep level emission centers. The annealed ZnO:P films are n-type with nonlinear dependence of resistivity on annealing temperature. The resistivity increases in the films with annealing at 800°C while decreasing with further increasing annealing temperature. Strong visible light emission is observed from the ZnO:P films annealed in oxygen.
  Journal of Electronic Materials 04/2012; 35(4):530-537. · 1.47 Impact Factor
• Article: Spin Dynamics in ZnO-Based Materials

  I. A. Buyanova, A. Murayama, T. Furuta, Y. Oka, D. P. Norton, S. J. Pearton, A. Osinsky, J. W. Dong, C. W. Tu, W. M. Chen
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  ABSTRACT: In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized, time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.
  Journal of Superconductivity and Novel Magnetism 04/2012; 23(1):161-165. · 0.65 Impact Factor
• Article: Synthesis and characterization of single crystalline SnO2 nanorods by high-pressure pulsed laser deposition

  L.C. Tien, S.J. Pearton, D.P. Norton, F. Ren
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  ABSTRACT: Tin oxide (SnO2) nanorods were grown by high-pressure pulsed laser deposition (PLD). The nanorods were grown without the use of a catalyst but required high background pressure growth in order to realize small grain columnar growth and nanorod formation, with nanorod formation most favored on non-epitaxial substrates. The structures and morphology were characterized by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). X-ray diffraction and HRTEM analysis indicate that the as-grown SnO2 nanorods are single crystals with a rutile structure. The nanorods are approximately 50–90nm in diameters and 1.5μm in length. This method provides an approach for large area synthesis of one dimensional SnO2 nanostructure materials.
  Applied Physics A 04/2012; 91(1):29-32. · 1.63 Impact Factor
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  Available from: uweb.txstate.edu

  Article: Growth of the dilute magnetic semiconductor GaMnN by molecular-beam epitaxy

  M. E. Overberg, G. T. Thaler, C. R. Abernathy, N. A. Theodoropoulou, K. T. McCarthy, S. B. Arnason, J. S. Lee, J. D. Lim, S. B. Shim, K. S. Suh, Z. G. Khim, Y. D. Park, S. J. Pearton, A. F. Hebard
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  ABSTRACT: Growth by molecular-beam epitaxy (MBE) of the dilute-magnetic alloy GaMnN is reported. The Mn concentration, as determined by Auger electron spectroscopy (AES), is found to be linear with increasing Mn-cell temperature up to ∼43at.%Mn. No second phases are observed for Mn levels below 9 at.%. The cubic-phase Mn4N is found to be the thermodynamically stable phase at the growth conditions used to produce GaMnN. Hysteresis in M versus H is observed in both GaMnN and GaMnN:C grown on both sapphire and metal-oxide chemical-vapor deposition (MOCVD) GaN at several growth temperatures. Magnetotransport results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis, indicating that Mn is incorporating into the GaN and forming the ferromagnetic-semiconductor GaMNN. Room-temperature hysteresis is obtained in magnetization measurements with an optimum Mn concentration of ∼3 at.%.
  Journal of Electronic Materials 04/2012; 32(5):298-306. · 1.47 Impact Factor
• Article: Size-dependent electrical conductivity of indium zinc oxide deposited by RF magnetron sputtering.

  Young-Woo Heo, S J Pearton, D P Norton
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  ABSTRACT: We investigated the size-dependent electrical conductivities of indium zinc oxide stripes with different widths from 50 nm to 4 microm and with the same thickness of 50 nm deposited by RF magnetron sputtering. The size of the indium zinc oxide stripes was controlled by e-beam lithography. The distance of the two Ti/Au Ohmic electrodes along the indium zinc oxide stripes was kept constant at 25 microm. The electrical conductivity decreased as the size of the indium zinc oxide stripes decreased below a critical width (80 nm). The activation energy, derived from the electric conductivity versus temperature measurement, was dependent on the dimensions of indium zinc oxide stripes. These results can be understood as stemming from surface charge trapping from the absorption of oxygen and/or water vapor, which leads to an increase in the energy difference between the conduction energy band and the Fermi energy.
  Journal of Nanoscience and Nanotechnology 04/2012; 12(4):3264-7. · 1.56 Impact Factor
• Article: Gallium nitride-based gas, chemical and biomedical sensors.

  S. J. Pearton, Fan Ren
  IEEE Instrum. Meas. Mag. 01/2012; 15:16-21.
• Article: Investigation of the effect of temperature during off-state degradation of AlGaN/GaN High Electron Mobility Transistors.

  E. A. Douglas, C. Y. Chang, B. P. Gila, M. R. Holzworth, K. S. Jones, L. Liu, Jinhyung Kim, Soohwan Jang, G. D. Via, Fan Ren, S. J. Pearton
  Microelectronics Reliability. 01/2012; 52:23-28.
• Article: Characteristics of a-GaN films and a-AlGaN/GaN heterojunctions prepared on r-sapphire by two-stage growth process

  A. Y. Polyakov, Lee-Woon Jang, N. B. Smirnov, A. V. Govorkov, E. A. Kozhukhova, T. G. Yugova, V. Y. Reznik, S. J. Pearton, Kwang Hyeon Baik, Sung-Min Hwang, Sukkoo Jung, In-Hwan Lee
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  ABSTRACT: The electrical properties, presence of deep electron and hole traps and photoluminescence spectra were measured for undoped a-GaN films grown by metal-organic chemical vapor deposition (MOCVD) in a two-stage process using a high V/III ratio at the first stage and low V/III ratio at the second stage. Growth was performed on r-sapphire substrates with a high temperature GaN nucleation layer. The films showed a full width at half maximum of 450-470 arcseconds for the (11-20) x-ray rocking curve with little anisotropy with respect to the sample rotation around the growth direction. The stacking fault (SF) density determined by selective etching was ∼5 × 104 cm−1. The residual donor concentration was 1014–1015 cm−3, with a very low density (2.5 × 1013 cm−3) of electron traps located at Ec − 0.6 eV, which are believed to be one of the major non-radiative recombination centers in nonpolar GaN. Consequently, the films showed a high intensity of bandedge luminescence with negligible contribution from defect bands associated with SFs. In contrast to previously studied nonpolar GaN films, the a-GaN layers showed a high concentration of gallium-vacancy-related acceptors near Ev + 1 eV and a strong yellow luminescence band, both indicating that growth conditions were effectively N-rich. a-AlGaN/GaN heterojunctions with thin heavily Si doped AlGaN barriers made on a-GaN substrates showed two-dimensional electron gas (2DEG) concentrations of 1.2 × 1013 cm−3, with 2DEG mobility of 80 cm2/Vs. Capacitance-voltage profiling of Schottky diodes on these HJs suggest that the 2DEG is fully depleted by the built-in voltage of the Schottky diode.
  Journal of Applied Physics 11/2011; 110(9):093709-093709-6. · 2.17 Impact Factor
• Article: Carrier Removal Rates and Deep Traps in Neutron Irradiated n-GaN Films

  In-Hwan Lee, A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, E. A. Kozhukhova, N. G. Kolin, V. M. Boiko, A. V. Korulin, S. J. Pearton
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  ABSTRACT: Carrier removal rates and deep trap spectra were measured in neutron irradiated n-GaN samples grown by metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE, and epitaxial lateral overgrowth (ELOG). The carrier removal rates were found to significantly increase with donor doping and to decrease in the sequence MOCVD/ ELOG/ HVPE. The most prominent traps created by irradiation were quasi-hole traps with energy 0.6–0.7 eV and electron traps with energy 0.45 eV. The former were associated with disordered regions in GaN and determine the carrier removal rate in undoped films. The latter were attributed to radiation defect complexes with shallow donors.
  Journal of The Electrochemical Society. 08/2011; 158(9):H866-H871.
• Article: Comparison of hole traps in n-GaN grown by hydride vapor phase epitaxy, metal organic chemical vapor deposition, and epitaxial lateral overgrowth

  A. Y. Polyakov, I.-H. Lee, N. B. Smirnov, A. V. Govorkov, E. A. Kozhukhova, S. J. Pearton
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  ABSTRACT: Optical deep level spectroscopy (ODLTS) and microcathodoluminescence (MCL) spectra were measured for a large group of n-GaN samples grown via metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth (ELOG), or hydride vapor phase epitaxy (HVPE). In the MOCVD and ELOG samples, the ionization energy of dominant hole traps H1 was dependent on the excitation conditions and was ∼0.9 eV for high injection levels providing saturation of the ODLTS peak magnitude. The trap concentration increased with increasing Si donor concentration and correlated with the yellow band intensity in the MCL spectra. For the HVPE samples, the hole trap spectra were radically different from the MOCVD case: four hole traps—H2, H3, H4, and H5—with activation energies of 0.55, 0.65, 0.85, and 1.2 eV, respectively, were detected. In the MCL spectra, a broad green band that peaked near 2.5 eV was observed in addition to the usual yellow luminescence near 2.3 eV. This green band was attributed to the transitions involving the H4 hole traps. Possible identities of the hole traps detected in the MOCVD/ELOG and HVPE samples are discussed.
  Journal of Applied Physics 06/2011; 109(12):123701-123701-7. · 2.17 Impact Factor
• Article: Measurement of SiO2/InZnGaO4 heterojunction band offsets by x-ray photoelectron spectroscopy

  E. A. Douglas, A. Scheurmann, R. P. Davies, B. P. Gila, Hyun Cho, V. Craciun, E. S. Lambers, S. J. Pearton, F. Ren
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  ABSTRACT: X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEv) of SiO2/InZnGaO4 (IGZO) heterostructures deposited by low temperature plasma enhanced chemical vapor deposition and sputtering at <50 °C, respectively. A value of ΔEv = 1.43±0.15 eV was obtained by using the Ga and Zn 2p3 and In 3d3 and 3d5 energy levels as references. Given the experimental bandgap of 3.2 eV for the IGZO, this would indicate a conduction band offset ΔEC of 4.27 eV in this system.
  Applied Physics Letters 06/2011; 98(24):242110-242110-3. · 3.84 Impact Factor
• Article: Fabrication of InAlAs/InGaAsSb/InGaAs double heterojunction bipolar transistors

  C. F. Lo, F. Ren, C. Y. Chang, S. J. Pearton, S.-H. Chen, C.-M. Chang, S.-Y. Wang, J.-I. Chyi, I. I. Kravchenko
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  ABSTRACT: A trilevel resist system was employed to fabricate self-aligned, submicron emitter finger In _0.52 Al _0.48 As / In _0.42 Ga _0.58 As _0.77 Sb _0.23/ In _0.53 Ga _0.47 As double heterojunction bipolar transistors (DHBTs). Selective wet-etchants were used to define the emitter fingers and to form an InGaAs guard-ring around the emitter fingers. Due to the low energy bandgap of the InGaAsSb base layer and type II base-collector junction, a low turn-on voltage of 0.38 V at 1 A / cm ² and a high dc current gain of 123.8 for a DHBT with a 0.65×8.65 μ m ² emitter area were obtained. A unity gain cutoff frequency (f_T) of 260 GHz and a maximum oscillation frequency (f_max) of 485 GHz at J_C=302 kA / cm ² were achieved.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/2011; · 1.34 Impact Factor
• Article: Electric-Field-Driven Degradation in off-State Step-Stressed AlGaN/GaN High-Electron Mobility Transistors

  Chih-Yang Chang, E.A. Douglas, Jinhyung Kim, Liu Lu, Chien-Fong Lo, Byung-Hwan Chu, D.J. Cheney, B.P. Gila, F. Ren, G.D. Via, D.A. Cullen, Lin Zhou, D.J. Smith, Soohwan Jang, S.J. Pearton
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  ABSTRACT: The critical degradation voltage of AlGaN/GaN high-electron mobility transistors during off-state electrical stress was determined as a function of Ni/Au gate dimensions (0.1-0.17 μm), drain bias voltage, and source/drain-gate contact distance. Devices with different gate lengths and gate-drain distances were found to exhibit the onset of degradation at different source-drain biases but similar electric field strengths, showing that the degradation mechanism is primarily field driven. The degradation field was calculated to be ~ 1.8 MV/cm by Automatically Tuned Linear Algebra Software simulations. Transmission electron microscopy imaging showed creation of defects under the gate after dc stress.
  IEEE Transactions on Device and Materials Reliability 04/2011; · 1.54 Impact Factor
• Article: Finite-element simulations of the effect of device design on channel temperature for AlGaN/GaN high electron mobility transistors

  E. A. Douglas, F. Ren, S. J. Pearton
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  ABSTRACT: The effect of device design on maximum channel temperature for AlGaN/GaN high electron mobility transistors was investigated through finite element thermal simulations for dissipated power densities up to 5 W   mm ^-1 . The effects of substrate material, die size, and number of gate fingers were examined, as well as the divergence between maximum channel temperatures for two dimensional and three dimensional simulations. The maximum temperature increased as the die size decreased beyond a critical distance from the gates. The critical distance was dependent on the substrate material, with SiC requiring the smallest critical distance. At the maximum dissipated power, the temperature increased above ambient more than 300 ° C for devices on sapphire substrates compared to an increase of 44 ° C for devices on SiC substrates. As the number of gate fingers increased, the maximum channel temperature also increased, and a temperature gradient was observed along the gate width as well as between gate fingers.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/2011; · 1.34 Impact Factor
• Article: Annealing temperature dependence of Ohmic contact resistance and morphology on InAlN/GaN high electron mobility transistor structures

  C.-F. Lo, L. Liu, C. Y. Chang, F. Ren, V. Craciun, S. J. Pearton, Y. W. Heo, O. Laboutin, J. W. Johnson
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  ABSTRACT: Ti/Al/Ni/Au Ohmic contact metallization on InAlN/GaN heterostructures both with and without a thin GaN cap layer was annealed at different temperatures. The minimum transfer resistance for the contacts of 0.65 Ω   mm (specific contact resistivity of 2×10^-5 Ω  cm ² ) was achieved after 800 ° C annealing for structures without the GaN cap, while those with the cap exhibited their lowest resistance at higher temperatures. The contact morphology showed considerable roughening by 750 ° C but the carrier mobility was stable until annealing temperatures of 850 ° C . Diffuse scattering experiments showed that the morphological roughness of the InAlN/GaN interface increased as a result of annealing at these temperatures and the data were consistent with outdiffusion of Ga into the InAlN. Unpassivated high electron mobility transistors with a gate dimension of 0.7×180 μ m ² were fabricated using these contacts and showed a maximum drain current of 1.3 A/mm and an extrinsic transconductance of 366 mS/mm. The presence of the GaN cap increased the effective barrier height of Ni/Au Schottky contacts from 0.91 to 1.01 eV on the heterostructure.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/2011; · 1.34 Impact Factor
• Article: Low temperature processing of indium-tin-zinc oxide channel layers in fabricating thin-film transistors

  Ki Chang Lee, Kwang-Min Jo, Sang-Yun Sung, Joon-Hyung Lee, Jeong-Joo Kim, Byoung-Seong Jeong, S. J. Pearton, D. P. Norton, Young-Woo Heo
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  ABSTRACT: The authors report on the fabrication of thin-film transistors (TFTs) using indium-tin-zinc oxides (ITZOs) as active channel layers. Transparent amorphous ITZO semiconductors were deposited at room temperature by rf-magnetron sputtering, followed by an annealing treatment at 100 ° C . The electrical properties of the ITZO channel layers deposited at room temperature using rf-magnetron sputtering were investigated by controlling the oxygen partial pressure during deposition and introducing postannealing treatments. The devices operated in an n -type enhancement mode exhibited a clear pinch-off behavior and had an on-to-off ratio of ∼10⁸ with a low off current of 3×10^-13 A . A field-effect mobility of 17 cm ²/ V   s and a subthreshold slope of 0.5 V/decade were extracted from the device characteristics. These results suggest that ITZO semiconductors show potential as channel materials that are applicable in flexible transparent TFTs.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/2011; · 1.34 Impact Factor