[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Sub-bandgap optical pumping with wavelengths of 671, 532, or 447 nm was employed to study traps in AlGaN/GaN high electron mobility transistors. The trap energies were determined from the Arrhenius plots of transient drain current at different temperatures. Prominent states were located around 0.7 eV below the conduction band, and these are commonly reported to be nonradiative traps due to defects trapped on dislocations or possibly Ga interstitials. In addition, traps located at 1.9 and 2.35 eV below the conduction band were found, which have been reported as NGa antisite and VGa-ON complexes, respectively. The postillumination drain current decays were analyzed with a persistent photoconductivity method, and time constants were extracted and associated with the recapture process in the AlGaN barrier and GaN channel layers.
Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 11/2015; 33(6). DOI:10.1116/1.4931790
[Show abstract][Hide abstract] 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.
Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 11/2015; 33(6). DOI:10.1116/1.4932013
[Show abstract][Hide abstract] 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.
Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 09/2015; 33(5). DOI:10.1116/1.4928730
[Show abstract][Hide abstract] 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±0.16eV for Sc2O3 on Zn-terminated ZnO (bandgap 3.26eV) and 1.59±0.16eV on O-terminated ZnO (bandgap 3.22eV), i.e., similar within experimental error. The conduction band offset for Sc2O3/ZnO was then determined to be 4.92eV. 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.
Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 09/2015; 33(5). DOI:10.1116/1.4931035
[Show abstract][Hide abstract] 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 340keV 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 27.6ns.
Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 09/2015; 33(5). DOI:10.1116/1.4930297
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: To understand the effects of 60Co gamma-irradiation, systematic studies were carried out on n-channel AlGaN/GaN high electron mobility transistors. Electrical testing, combined with electron beam-induced current measurements, was able to provide critical information on defects induced in the material as a result of gamma-irradiation. It was shown that at low gamma-irradiation doses, the minority carrier diffusion length in AlGaN/GaN exhibits an increase up to ∼300 Gy. The observed effect is due to longer minority carrier (hole) life time in the material's valence band as a result of an internal electron irradiation by Compton electrons. However, for larger doses of gamma irradiation (above 400 Gy), deteriorations in transport properties and device characteristics were observed. This is consistent with the higher density of deep traps in the material's forbidden gap induced by a larger dose of gamma-irradiation. Moderate annealing of device structures at 200°C for 25 min resulted in partial recovery of transport properties and device performance.
Radiation Effects and Defects in Solids 03/2015; 170(5):1-9. DOI:10.1080/10420150.2015.1010170 · 0.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: AlGaN/GaN High Electron Mobility Transistors were exposed to
Co gamma-irradiation to doses up to 300Gy. The impact of Compton- electron injection (due to gamma-irradiation) is studied through monitoring of minority carrier transport using Electron Beam Induced Current (EBIC) technique. Temperature dependent EBIC measurements were conducted on devices before and after exposure to the irradiation, which provide us with critical information on gamma-irradiation induced defects in the material. As a result of irradiation, minority carrier diffusion length increases significantly, with an accompanying decrease in the activation energy. This is consistent with the longer life time of minority carrier in the material’s valence band as a result of an internal electron injection and subsequent trapping of Compton electrons on neutral levels.
[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). DOI:10.1166/jnn.2014.9935 · 1.56 Impact Factor
[Show abstract][Hide abstract] 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 mu m limited by the measurement instrument. The specific on-state resistance was 1.3 and 10.9 m Omega cm(2) for the devices with the gate-drain distance of 7.5 and 37.5 mu m, respectively. The saturation drain current was inversely dependent on source-drain distance and the on-off ratios were in excess of 10(8) due to the low gate leakage current in the MISHEMTs. (C) 2014 American Vacuum Society.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.