[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: 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; DOI:10.1080/10420150.2015.1010170 · 0.60 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 · 1.34 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: 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: 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: 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1116/1.4868632 · 1.36 Impact Factor
[Show abstract][Hide abstract] 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. DOI:10.1116/1.4836577 · 1.36 Impact Factor
[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.
[Show abstract][Hide abstract] 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. DOI:10.1116/1.4864070 · 1.36 Impact Factor