Showing papers by "Stephen J. Pearton published in 2014"

Advances in ZnO-based materials for light emitting diodes

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.

Deep hole traps in undoped n-GaN films grown by hydride vapor phase epitaxy

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.

Effect of low dose γ-irradiation on DC performance of circular AlGaN/GaN high electron mobility transistors

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.

Effect of proton irradiation on AlGaN/GaN high electron mobility transistor off-state drain breakdown voltage

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.

Characteristics of gate leakage current and breakdown voltage of AlGaN/GaN high electron mobility transistors after postprocess annealing

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 (Voff) 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.

Study on the effects of proton irradiation on the dc characteristics of AlGaN/GaN high electron mobility transistors with source field plate

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.

Novel approach to improve heat dissipation of AlGaN/GaN high electron mobility transistors with a Cu filled via under device active area

Abstract: A through Si-substrate via-hole under the active area of GaN-based HEMTs grown on Si substrates is proposed to reduce the maximum junction temperature. Due to the large lattice mismatch between Si and GaN, an AlN nucleation layer and an AlGaN transition layer are required to grow GaN layers on Si substrates. This AlN nucleation layer is very defective and thermally resistive. The proposed through Si-substrate via-hole offers access to this AlN nucleation layer from the back side of the wafer. By removing this highly thermally resistive layer and plating the via hole with copper, the maximum junction temperature can be reduced from 146 to 120 °C at a power density of 5 W/mm. Besides reducing the maximum junction temperature of the HEMT, this through Si-substrate via-hole can be electrically connected to the source contact and act as a backside source field plate to reduce the maximum electric field around the gate edges and thereby increase the drain breakdown voltage. If this through Si-substrate via-hole is connected to the front gate pad, it can also behave as a back gate to improve front gate modulation.

GaN-based light-emitting diodes on graphene-coated flexible substrates

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.

Effect of 5 MeV proton radiation on DC performance and reliability of circular-shaped AlGaN/GaN high electron mobility transistors

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.

Enhancement of AlGaN/GaN high electron mobility transistors off-state drain breakdown voltage via backside proton irradiation

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.

Effect of proton irradiation on thermal resistance and breakdown voltage of InAlN/GaN high electron mobility transistors

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 (VBR) and of >92% in critical voltage (Vcri) 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.

High breakdown voltage in AlN/GaN metal–insulator–semiconductor high-electron-mobility transistors

Abstract: The breakdown characteristics of AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) using a 10 nm thick AlN gate insulator and passivation layer deposited plasma enhanced atomic layer deposition. The AlN was effective in significantly reducing gate leakage current relative to Schottky gate devices and showed only small decreases in drain current during gate lag measurements. The devices exhibited a strong dependence of gate breakdown voltage on source–drain distance, reaching a value of 2000 V for a source–drain distance of 40 μm limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 mΩ cm2 for the devices with the gate–drain distance of 7.5 and 37.5 μm, respectively. The saturation drain current was inversely dependent on source–drain distance and the on–off ratios were in excess of 108 due to the low gate leakage current in the MISHEMTs.

Band offsets in YSZ/InGaZnO4 heterostructure system.

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.

Measurement of Band Offsets in Y 2 O 3 /InGaZnO 4 Heterojunctions

Abstract: The valence band discontinuity (ΔE(v)) 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 ΔE(v) = 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 ΔE(c) of - 2.63 eV in the Y2O3/IGZO heterostructures and a nested interface band alignment.

Radiation Damage in GaN-Based Materials and Devices

Abstract: A review of the effects of proton, neutron, γ-ray and electron irradiation on GaN materials and devices is presented. Neutron irradiation tends to create disordered regions in the GaN, while the damage from the other forms of radiation is more typically point defects. In all cases, the damaged region contains carrier traps that reduce the mobility and conductivity of the GaN and at high enough doses, a significant degradation of device performance. GaN is several orders of magnitude more resistant to radiation damage than GaAs of similar doping concentrations. In terms of heterostructures, preliminary data suggests that the radiation hardness decreases in the order AlN/GaN >AlGaN/GaN > InAlN/GaN, consistent with the average bond strengths in the Al-based materials. The radiation damage can alter electric field profiles and drain current due to trapped negative charge and lead to increased breakdown voltages and apparent reliability. Simulations of this effect show good agreement with experiments. We also discuss the literature on effects of different radiation types on the performance of GaN-based high electron mobility transistors

Effect of annealing on electronic carrier transport properties of gamma-irradiated AlGaN/GaN high electron mobility transistors

Abstract: Submitted for the MAR14 Meeting of The American Physical Society Effect of annealing on electronic carrier transport properties of gamma-irradiated AlGaN/GaN high electron mobility transistors ANUPAMA YADAV, CASEY SCHWARZ, MAX SHATKHIN, LUTHER WANG, ELENA FLITSIYAN, LEONID CHERNYAK, Department of Physics, University of Central Florida, LU LIU, YA HWANG, FAN REN, Department of Chemical Engineering, University of Florida, STEPHEN PEARTON, Department of Materials Science and Engineering, University of Florida, DEPARTMENT OF PHYSICS, UNIVERSITY OF CENTRAL FLORIDA COLLABORATION, DEPARTMENT OF CHEMICAL ENGINEERING, UNIVERSITY OF FLORIDA COLLABORATION, DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING, UNIVERSITY OF FLORIDA COLLABORATION — AlGaN/GaN High Electron Mobility Transistors were irradiated with 60Co gamma-ray doses from 100Gy to 1000Gy, in order to analyze the effects of irradiation on the devices’ transport properties. Temperature dependent Electron Beam Induced Current (EBIC) measurements, conducted on the devices before and after exposure to gamma-irradiation, allowed for the obtaining of activation energy related to radiation-induced defects due to nitrogen vacancies. Later, the devices were annealed at 200o C for 25 minutes. All the measurements were performed again to study the effect of annealing on the gamma-irradiated devices. Annealing of gamma-irradiated transistors shows that partial recovery of device performance is possible at this temperature. DC currentvoltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation and annealing on transfer, gate and drain characteristics. Elena Flitsiyan Department of Physics, University of Central Florida Date submitted: 08 Nov 2013 Electronic form version 1.4