Showing papers by "Stephen J. Pearton published in 2013"

Review of 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.

Radiation effects in GaN materials and devices

Abstract: This article reviews the effects of radiation damage on GaN materials and devices such as light-emitting diodes and high electron mobility transistors. Protons, electrons and gamma rays typically produce point defects in GaN, in contrast to neutron damage which is dominated by more extended disordered regions. Regardless of the type of radiation, the electrical conductivity of the GaN is reduced through the introduction of trap states with thermal ionization energies deep in the forbidden bandgap. An important practical parameter is the carrier removal rate for each type of radiation since this determines the dose at which device degradation will occur. Many studies have shown that GaN is several orders of magnitude more resistant to radiation damage than GaAs, i.e. it can withstand radiation doses of at least two orders of magnitude higher than those degrading GaAs with a similar doping level. Many issues still have to be addressed. Among them are the strong asymmetry in carrier removal rates in n- and p-type GaN and interaction of radiation defects with Mg acceptors and the poor understanding of interaction of radiation defects in doped nitrides with the dislocations always present.

Flexible graphene-based chemical sensors on paper substrates

Abstract: Graphene-based, flexible NO(2) sensors on paper substrates exhibited an immediate response (32-39%) once exposed to 200 ppm NO(2) gas under a strain of 0.5%. Chemical vapor deposition-grown graphene with a supporting poly(methyl methacrylate) layer was transferred onto paper substrates, followed by formation of two electrodes using silver paste. Current-voltage characteristics and dynamic sensing response were obtained under both relaxed and strained conditions. We demonstrate a facile method without complex photo-lithography and high vacuum processes for fabricating graphene-based flexible NO(2) sensors on paper substrates with high sensing response.

Wide Bandgap Semiconductor One-Dimensional Nanostructures for Applications in Nanoelectronics and Nanosensors:

Abstract: Wide bandgap semiconductor ZnO, GaN and InN nanowires have displayed the ability to detect many types of gases and biological and chemical species of interest. In this review, we give some recent e...

Materials and reliability handbook for semiconductor optical and electron devices

Abstract: Preface Part 1: Materials Issues and Reliability of Optical Devices 1. Reliability Testing of Semiconductor Optical Devices 2. Failure Analysis of Semiconductor Optical Devices 3. Failure Analysis using Optical Evaluation Technique (OBIC) of LDs and APDs for Fiber Optical Communication 4. Reliability and Degradation of III-V Optical Devices Focusing on Gradual Degradation 5. Catastrophic Optical-damage in High Power, Broad-Area Laser-diodes 6. Reliability and Degradation of Vertical Cavity Surface Emitting Lasers 7. Structural Defects in GaN-based Materials and Their Relation to GaN-based Laser Diodes 8. InGaN Laser Diode Degradation 9. Radiation-enhanced Dislocation Glide - The Current Status of Research 10. Mechanism of Defect Reactions in Semiconductors Part 2: Materials Issues and Reliability of Electron Devices 11. Reliability Studies in the Real World 12. Strain Effects in AlGaN/GaN HEMTs 13. Reliability Issues in AlGaN/GaN High Electron Mobility Transistors 14. GaAs Device Reliability: High Electron Mobility Transistors and Heterojunction Bipolar Transistors 15. Novel Dielectrics for GaN Device Passivation And Improved Reliability 16. Reliability Simulation 17. The Analysis of Wide Bandgap Semiconductors Using Raman Spectroscopy 18. Reliability Study of InP-Based HBTs Operating at High Current Density Index

Gamma irradiation impact on electronic carrier transport in AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors were irradiated with 60Co gamma-rays to doses up to 1000 Gy, in order to analyze the effects of irradiation on the devices' transport properties. Temperature-dependent electron beam-induced current measurements, conducted on the devices before and after exposure to gamma-irradiation, allowed for the obtaining of activation energies related to radiation-induced defects due to nitrogen vacancies. DC current-voltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation on transfer, gate, and drain characteristics.

Reliability studies of AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors are gaining commercial acceptance for use in high power and high frequency applications, but the degradation mechanisms that drive failure in the field are not completely understood. Since some of these mechanisms are current or field driven, reliability studies must go beyond the typical Arrhenius-accelerated life tests. In this paper, we summarize recent work on electric field or current driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plates) and the effect of device fabrication processes for both dc and RF stress conditions.

GaN-based ultraviolet light-emitting diodes with AuCl 3 -doped graphene electrodes

Abstract: We demonstrate AuCl3-doped graphene transparent conductive electrodes integrated in GaN-based ultraviolet (UV) light-emitting diodes (LEDs) with an emission peak of 363 nm. AuCl3 doping was accomplished by dipping the graphene electrodes in 5, 10 and 20 mM concentrations of AuCl3 solutions. The effects of AuCl3 doping on graphene electrodes were investigated by current-voltage characteristics, sheet resistance, scanning electron microscope, optical transmittance, micro-Raman scattering and electroluminescence images. The optical transmittance was decreased with increasing the AuCl3 concentrations. However, the forward currents of UV LEDs with p-doped (5, 10 and 20 mM of AuCl3 solutions) graphene transparent conductive electrodes at a forward bias of 8 V were increased by ~48, 63 and 73%, respectively, which can be attributed to the reduction of sheet resistance and the increase of work function of the graphene. The performance of UV LEDs was drastically improved by AuCl3 doping of graphene transparent conductive electrodes.

Defects in N, O and N, Zn implanted ZnO bulk crystals

Abstract: Comprehensive characterization of defects formed in bulk ZnO single crystals co-implanted with N and Zn as well as N and O atoms is performed by means of optically detected magnetic resonance (ODMR) complemented by Raman and photoluminescence (PL) spectroscopies. It is shown that in addition to intrinsic defects such as Zn vacancies and Zn interstitials, several N-related defects are formed in the implanted ZnO. The prevailed configuration of the defects is found to depend on the choices of the co-implants and also the chosen annealing ambient. Specifically, co-implantation with O leads to the formation of (i) defects responsible for local vibrational modes at 277, 511, and 581 cm−1; (ii) a N-related acceptor with the binding energy of 160 ± 40 meV that is involved in the donor-acceptor pair emission at 3.23 eV; and (iii) a deep donor and a deep NO acceptor revealed from ODMR. Activation of the latter defects is found to require post-implantation annealing in nitrogen ambient. None of these defects are detected when N is co-implanted with Zn. Under these conditions, the dominant N-induced defects include a deep center responsible for the 3.3128 eV PL line, as well as an acceptor center of unknown origin revealed by ODMR. Formation mechanisms of the studied defects and their role in carrier recombination are discussed.

Dependence on proton energy of degradation of AlGaN/GaN high electron mobility transistors

Abstract: The effects of proton irradiation energy on dc, small signal, and large signal rf characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. AlGaN/GaN HEMTs were irradiated with protons at fixed fluence of 5 × 1015/cm2 and energies of 5, 10, and 15 MeV. Both dc and rf characteristics revealed more degradation at lower irradiation energy, with reductions of maximum transconductance of 11%, 22%, and 38%, and decreases in drain saturation current of 10%, 24%, and 46% for HEMTs exposed to 15, 10, and 5 MeV protons, respectively. The increase in device degradation with decreasing proton energy is due to the increase in linear energy transfer and corresponding increase in nonionizing energy loss with decreasing proton energy in the active region of the HEMTs. After irradiation, both subthreshold drain leakage current and reverse gate current decreased more than 1 order of magnitude for all samples. The carrier removal rate was in the range 121–336 cm−1 over the range of proton energies employed in this study.

Modeling Proton Irradiation in AlGaN/GaN HEMTs: Understanding the Increase of Critical Voltage

Abstract: A combination of TRIM and FLOODS models the effect of radiation damage on AlGaN/GaN HEMTs. While excellent fits are obtained for threshold voltage shift, the models do not fully explain the increased reliability observed experimentally. In short, the addition of negatively-charged traps in the GaN buffer layer does not significantly change the electric field at the gate edges at radiation fluence levels seen in this study. We propose that negative trapped charge at the nitride/AlGaN interface actually produces the virtual-gate effect that results in decreasing the magnitude of the electric field at the gate edges and thus the increase in critical voltage. Simulation results including nitride interface charge show significant changes in electric field profiles while the I-V device characteristics do not change.

Impact of proton irradiation on dc performance of AlGaN/GaN high electron mobility transistors

Abstract: The effects of high energy proton irradiation dose on dc performance as well as critical voltage of the drain-voltage step-stress of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated to evaluate the feasibility of AlGaN/GaN HEMTs for space applications, which need to stand a variety of irradiations. The HEMTs were irradiated with protons at a fixed energy of 5 MeV and doses ranging from 109 to 2 × 1014 cm−2. For the dc characteristics, there was only minimal degradation of saturation drain current (IDSS), transconductance (gm), electron mobility, and sheet carrier concentration at doses below 2 × 1013 cm−2, while the reduction of these parameters were 15%, 9%, 41% and 16.6%, respectively, at a dose of 2 × 1014 cm−2. At this same dose condition, increases of 37% in drain breakdown voltage (VBR) and of 45% in critical voltage (Vcri) were observed. The improvements of drain breakdown voltage and critical voltage were attributed to the modification of the depletion region due to the intro...

Effect of electron irradiation on AlGaN/GaN and InAlN/GaN heterojunctions

Abstract: AlGaN/AlN/GaN/sapphire, AlGaN/GaN/sapphire, AlGaN/GaN/Si, and InAlN/GaN/sapphire heterojunctions (HJs) were irradiated with 10 MeV electrons to fluences of 2 × 1015 to 3.3 × 1016 cm−2. The main effects on the electrical properties were a decrease in two-dimensional electron gas (2DEG) mobility and the shift of capacitance–voltage (C-V) characteristics to more positive values. The 50% 2DEG mobility decrease occurred at a similar fluence of 3.3 × 1016 cm−2 for all AlGaN/GaN and AlGaN/AlN/GaN HJs, but at a much lower fluence of 1.3 × 1016 cm−2 for InAlN/GaN, which is in line with previous observations for neutron irradiated HJs. The shift of C-V characteristics is due to increased concentration of deep acceptor traps in the barrier/interface region. In AlGaN/GaN/Si transistors, the increase of concentration of deep barrier/interface traps with activation energy of 0.3, 0.55, and 0.8 eV was observed. This increase correlates with the observed degradation of gate lag characteristics of transistors after irradiation with 1.3 × 1016 cm−2 electrons.

Characteristics of carbon monoxide sensors made by polar and nonpolar zinc oxide nanowires gated AlGaN/GaN high electron mobility transistor

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) with polar and nonpolar ZnO nanowires modified gate exhibit significant changes in channel conductance upon exposure to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition, with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change of surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezo-induced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm and 3200 ppm in the open cavity with continuous gas flow using a 50 × 50 μm2 gate sensing area for polar and nonpolar ZnO nanowire gated HEMTs sensor, respectively.

Deep centers and persistent photocapacitance in AlGaN/GaN high electron mobility transistor structures grown on Si substrates

Abstract: Deep trap spectra in AlGaN/GaN high electron mobility transistor structures grown on Si by metalorganic chemical vapor deposition show four major electron traps (Ec—0.15, 0.29, 0.40 and 0.76 eV) in the AlGaN barrier/interface region and three (Ec—0.18, 0.27 and 0.45 eV) in the undoped GaN buffer region. The presence of a high density of deep acceptor traps was observed in the AlGaN barrier region, as determined by hysteresis in low temperature capacitance-voltage (C-V) characteristics. The spectral dependence of persistent photocapacitance shifts showed two optical thresholds of 1.5 V and 3.1 eV, with the second being specific to structures grown on Si substrates. Comparison of results obtained on transistors and on large-area Schottky diodes prepared on heterostructures from which transistors are fabricated show that measurements on test large-area diodes are representative of the main characteristics important for transistor performance.

Effect of temperature on CO sensing response in air ambient by using ZnO nanorod-gated AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) functionalized with ZnO nanorods were used for sensing CO in the concentration range of 50–500 ppm balanced with air at ambient and temperatures from 25 to 250 °C. An increase of the HEMT drain current was observed for exposure to the CO-containing ambient. The sensing limitation was improved from 100 ppm to 50 ppm by increasing the sensor temperature from room temperature to 250 °C. The sensing response was also enhanced from 0.09% to 0.34% by increasing the sensor temperature. Reliable and repeatable current changes with the introduction of a low CO concentration of 50 ppm and also rapid response times of ∼40 s and recovery times ∼15 s were demonstrated.

Sb-based semiconductors for low power electronics

Abstract: Sb-based semiconductors incorporating heterostructures of InP, InAs, AlSb, InSb, GaSb, InGaAs, InGaSb, GaAsSb and InGaAsSb can be used for high speed, low power applications such as wide-bandwidth telecommunications for aircraft, satellites, wireless communication, and global positioning systems, as well as thermophotovoltaic cells, THz medical imaging and remote sensing, IR sensors for space exploration, high resolution biomedical spectroscopy and military systems, including security scanners. Sb-based electronic devices such as heterojunction bipolar transistors (HBTs) offer high speed, low power consumption and good breakdown voltages. High electron mobility InAs/AlSb or InSb/AlSb and high hole mobility InGaSb/AlSb quantum well heterostructure field effect transistors (HFETs) have also been widely pursued for THz amplifiers and high speed complementary logic circuits.

Temperature stability of high-resistivity GaN buffer layers grown by metalorganic chemical vapor deposition

Abstract: Nominally undoped GaN films were grown by metalorganic chemical vapor deposition under three different conditions, namely (1) “standard” growth conditions with growth temperature of 1000 °C and growth rate of 1 μm/h, (2) slightly reduced growth temperature of 975 °C, and (3) standard temperature, but higher growth rate of 2.5 μm/h. The standard sample had a net donor density <1015 cm−3, while the two other samples were semi-insulating, with sheet resistivity ∼1014 Ω/square and the Fermi level pinned at Ec−0.8 eV for the low temperature growth and at Ec−0.9 eV for the high growth rate conditions. The photoconductivity spectra of both of these latter samples show the presence of centers with optical threshold near 1.35 eV commonly attributed to C interstitials and centers with optical threshold near 2.7–2.8 eV and 3 eV often associated with C-related defects. However, no signals that could be attributed to substitutional C acceptors and C donors were detected. Current relaxation spectroscopy revealed deep traps with activation energies 0.2, 0.25, 045, and 0.8 eV. Annealing at 800 °C increased the concentration of these traps. The changes in resistivity induced by annealing in the high-growth rate sample were much stronger than for the low-temperature sample. The authors also observed a strong suppression of the yellow luminescence band intensity in the “standard” sample after annealing, as opposed to a slight increase of this band intensity in the two semi-insulating samples. The role of compensation by native defects and by deep levels related to carbon in the observed changes is discussed.

Effect of buffer structures on AlGaN/GaN high electron mobility transistor reliability

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) with three different types of buffer layers, including a GaN/AlGaN composite layer, or 1 or 2 μm GaN thick layers, were fabricated and their reliability compared. The HEMTs with the thick GaN buffer layer showed the lowest critical voltage (Vcri) during off-state drain step-stress, but this was increased by around 50% and 100% for devices with the composite AlGaN/GaN buffer layers or thinner GaN buffers, respectively. The Voff - state for HEMTs with thin GaN and composite buffers were ∼100 V, however, this degraded to 50–60 V for devices with thick GaN buffers due to the difference in peak electric field near the gate edge. A similar trend was observed in the isolation breakdown voltage measurements, with the highest Viso achieved based on thin GaN or composite buffer designs (600–700 V), while a much smaller Viso of ∼200 V was measured on HEMTs with the thick GaN buffer layers. These results demonstrate the strong influence of buffer structure and defect density on AlGaN/GaN HEMT performance and reliability.

High electron mobility transistors having improved reliability

Abstract: High electron mobility transistors (HEMTs) having improved I-V characteristics and reliability are provided. According to one embodiment, a selective implantation is performed to form a damage region in a gate-to-drain region of, for example, an IηAΓN/GaN HEMT. The selective implantation can be performed by irradiating some or all of a gate-to-drain region of an InAlN/GaN HEMT on a substrate with protons or other ions such as Ge ions, He ions, N ions, or O ions. The damage region can extend in a region below a 2DEG interface of the HEMT.

Field-induced defect morphology in Ni-gate AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors were electrically stressed using off-state high reverse gate biases. In devices demonstrating the largest, most rapid decrease in normalized maximum drain current, defects were found at the gate/AlGaN epilayer interface and characterized using high-angle annular dark-field scanning transmission electron microscopy. These defects appear to be a reaction between the Ni layer of the Ni/Au gate metal stack and the AlGaN epilayer. Additionally, simulations of the electric field lines from the defective devices match the defect morphology. These results provide important insight toward understanding failure mechanisms and improving reliability of Ni-gate AlGaN/GaN high electron mobility transistors.

Electrical characterization of 60Co gamma radiation-exposed InAlN/GaN high electron mobility transistors

Abstract: In0.17Al0.83N/GaN high electron mobility transistors (HEMTs) were cumulatively subjected to 60Co gamma-ray irradiation doses up to 500 Mrad. Source–drain current–voltage characteristics (IDS-VDS) showed little change after lower dose gamma-ray irradiations (<200 Mrad). However, the electrical properties were significantly degraded after 500 Mrad irradiations, indicating that the radiation-induced damage near the active region of the devices was severe. The saturation current level at VGS = 0 V was degraded by 48% after 500 Mrad irradiation. The effective carrier removal rate was not linear with dose, but was 0.54 × 1010 cm−2 Mrad−1 in the range from 200 to 500 Mrad. The cumulative gamma-ray irradiation of In0.17Al0.83N/GaN HEMTs caused much larger reductions in drain–source current compared to AlGaN/GaN devices exposed under the same conditions.

Methane detection using Pt-gated AlGaN/GaN high electron mobility transistor based Schottky diodes

Abstract: Pt-gated AlGaN/GaN high electron mobility transistor based Schottky diodes were employed to detect methane. A detection sensitivity >100 was obtained for the diodes under reverse bias, and this was one order of magnitude higher than the sensitivity of the diodes operated under forward bias. A new method to extract the response time was demonstrated by taking the derivative of diode current, allowing a reduction in the sensor response time by 80%. Methane sensing experiments were conducted at different temperatures, and an Arrhenius plot of the data determined an activation energy of 57 kJ/mol for the sensing process.

A facile method for highly uniform GaN-based nanorod light-emitting diodes with InGaN/GaN multi-quantum-wells

Abstract: We report on a simple and reproducible method for fabricating InGaN/GaN multi-quantum-well (MQW) nanorod light-emitting diodes (LEDs), prepared by combining a SiO2 nanosphere lithography and dry-etch process. Focused-ion-beam (FIB)-deposited Pt was contacted to both ends of the nanorod LEDs, producing bright electroluminescence from the LEDs under forward bias conditions. The turn-on voltage in these nanorod LEDs was higher (13 V) than in companion thin film devices (3 V) and this can be attributed to the high contact resistance between the FIB-deposited Pt and nanorod LEDs and the damage induced by inductively-coupled plasma and Ga + -ions. Our method to obtain uniform MQW nanorod LEDs shows promise for improving the reproducibility of nano-optoelectronics.

Effects of 2 MeV Ge+ irradiation on AlGaN/GaN high electron mobility transistors

Abstract: The dc characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were measured before and after irradiation with 2 MeV Ge+ ions at doses from 5 × 1010 to 5 × 1012 cm−2. The drain current, gate leakage current, and transconductance decreased monotonically with dose, while the drain-source resistance increased to a much greater extent than observed previously for proton irradiation of similar devices. The data are consistent with a strong decrease in electron concentration in the HEMT channel. During off-state electrical stressing of AlGaN/GaN HEMTs, the typical critical voltage for unirradiated devices was ∼13 V. By sharp contrast, no critical voltage was detected for proton irradiated HEMTs up to 35 V, indicating that the Ge irradiation had a strong influence on the electric field distribution near the gate electrode.

Recent advances in wide bandgap semiconductor-based gas sensors

Abstract: We review recent progress in wide bandgap thin-film and nanorod sensors made from GaN or ZnO and related materials for applications in the detection of gases such as oxygen, carbon dioxide and hydrogen. Practical aspects are covered, such as the use of differential sensor pairs to eliminate the effects of temperature variations and of the effect of humidity on the detection sensitivity.

GaN metal–insulator–semiconductor high-electron-mobility transistor with plasma enhanced atomic layer deposited AlN as gate dielectric and passivation

Abstract: AlGaN/GaN based metal–insulator–semiconductor high-electron-mobility transistors (HEMTs) using a plasma enhanced atomic layer deposited 10 nm AlN as the gate insulator and passivation layer were demonstrated. A refractive index of 1.92 for the deposited AlN was measured using an ellipsometer, which was slightly lower than that of bulk AlN. The deviation of the refractive index from the ideal value was caused by AlN surface oxidation, and this was confirmed by X-ray photoelectron spectroscopy and Auger depth profiling analyses. The HEMT drain current was modulated with gate voltages ranging from −3 to +4 V. The HEMT exhibited an on-off ratio of 3.3 × 108 due to the low gate leakage current and a maximum saturation drain current of 600 mA/mm. Beside reducing the gate leakage current, the effectiveness of the HEMT passivation was confirmed by gate pulse measurements, which showed only a 7% decrease of the drain current.

Study of hydrogen detection response time with Pt-gated diodes fabricated on AlGaN/GaN heterostructure

Abstract: The hydrogen detection response time of Pt-gated diode sensors fabricated on AlGaN/GaN heterostructure as a function of the hydrogen concentration was investigated. A new method to extract the response time, taking the derivative of diode current, was proposed and shown to reduce the response time of detecting 1% hydrogen by about 60% as compared to the response time defined as the diode current reaching 90% of its total changes, t90. Hydrogen-sensing experiments were conducted at different temperatures, and an Arrhenius plot of the data determined an activation energy of 17.7 kJ/mole for the sensing process.

Functionalized GaN Based Transistors For Biosensing

Abstract: The US market size of chemical sensors is projected to increase 8.6% annually to reach $6 billion in 2014. This growth will be sustained especially by high demand of biosensors for medical applications such as glucose monitoring, biomarker detection for infectious disease and cancer diagnosis. In addition, there will be strong demand in biodefense, environmental monitoring, food, and pharmaceutical industries. The biosensor market is forecast to reach $4.4 billion in 2014 in the US [1].