Showing papers by "Stephen J. Pearton published in 2012"

GaN and ZnO-based materials and devices

Abstract: 1. UV LEDs-Michael Shur (RPI, email shurm@rpi.edu) 2. Non-Polar GaN Growth-Jung Han (Yale, email jung.han@yale.edu) 3. High Quality AlGaN Alloys-Hongxing Jiang and Jingyu Lin (Texas Tech, email hx.jiang@ttu.edu and jingyu.lin@ttu.edu) 4. Bulk AlN for UV LEDs -L.J. Schowalter (Crystal IS, email Leo@crystal-IS.com) 5. Enhancement of the Light-Extraction Efficiency of GaN-Based Light Emitting Diodes-Jihyun Kim (Korea University, Seoul 136-701, Korea, email: hyunhyun7@.korea.ac.kr ) 6. GaN-Based Sensors-F. Ren (Univ. Florida, email ren@che.ufl.edu) 7. III-N Alloys for Solar Power Conversion-Wladek Walukievicz (LBL, email W_Walukiewicz@lbl.gov) 8. GaN HEMT Technology-Wayne Johnson (Kopin, email Wayne_Johnson@kopin.com) 9. GaN Power Devices-John Shen (Univ.Central Florida, email johnshen@mail.ucf.edu)- 10. Nitride Nanostructures-Li-Chyong Chen (National Taiwan University, email chenlc@ntu.edu.tw) 11. Radiation-Induced Defects in GaN-Alexander Polyakov (IRM, Russia, email aypolyakov@gmail.com) 12. Electron Injection Effects in GaN-L. Chernyak (Univ. Central Florida, email chernyak@physics.ucf.edu) 13. Progress and Prospect of Rare-Earth Nitrides- R. Palai ( Department of Physics, University of Puerto Rico, San Juan, PR, 00931, USA, email: r.palai@uprrp.edu)- 14. Advances in PLD of ZnO and related compounds-Ashutosh Tiwari, (Dept. of Materials Science & Engineering, University of Utah, E-mail: tiwari@eng.utah.edu) 15. ZnO nanowires and p-type doping-Deli Wang (UCSD, Electrical and Computer Engineering, email dwang@ece.ucsd.edu) 16. Multifunctional ZnO Structures-Yicheng Lu (Rutgers, email ylu@ece.rutgers.edu)- 17. ZnO/MgZnO Quantum Wells-C. Jagadish (ANU, Australia, email cxj109@rsphysse.anu.edu.au) 18. GZO TFTs-E.Fortunato(CENIMAT, FCT-UNL, Campus de Caparica, 2829-516 Caparica, Portugal, email elvira-fortunato@fct.unl.pt )

UV ozone treatment for improving contact resistance on graphene

Abstract: Optimized UV ozone cleaning of graphene layers on SiO2/Si substrates is shown to improve contact resistance of e-beam evaporated Ti/Au contacts by three orders of magnitude (3 × 10−6 Ω-cm2) compared to untreated surfaces (4 × 10−3 Ω-cm2). Subsequent annealing at 300 °C lowers the minimum value achieved to 7 × 10−7 Ω-cm2. Ozone exposure beyond an optimum time (6 min in these experiments) led to a sharp increase in sheet resistance of the graphene, producing degraded contact resistance. The UV ozone treatment is a simple and effective method for producing high quality contacts to graphene.

Degradation Mechanisms for GaN and GaAs High Speed Transistors

Abstract: We present a review of reliability issues in AlGaN/GaN and AlGaAs/GaAs high electron mobility transistors (HEMTs) as well as Heterojunction Bipolar Transistors (HBTs) in the AlGaAs/GaAs materials systems. Because of the complex nature and multi-faceted operation modes of these devices, reliability studies must go beyond the typical Arrhenius accelerated life tests. We review the electric field driven degradation in devices with different gate metallization, device dimensions, electric field mitigation techniques (such as source field plate), and the effect of device fabrication processes for both DC and RF stress conditions. We summarize the degradation mechanisms that limit the lifetime of these devices. A variety of contact and surface degradation mechanisms have been reported, but differ in the two device technologies: For HEMTs, the layers are thin and relatively lightly doped compared to HBT structures and there is a metal Schottky gate that is directly on the semiconductor. By contrast, the HBT relies on pn junctions for current modulation and has only Ohmic contacts. This leads to different degradation mechanisms for the two types of devices.

Electrical properties and radiation detector performance of free-standing bulk n-GaN

Abstract: Electrical properties and deep electron and hole trap spectra were measured for undoped n-GaN cut from a thick boule grown by hydride vapor phase epitaxy (HVPE). The material is characterized by a very low concentration of residual donors (1013–1014 cm−3) in the first 30 μm near the growth surface. The bulk electrical properties were similar to those of standard high quality undoped bulk HVPE n-GaN, with a net donor concentration of ∼1016 cm−3 and mobility ∼1000 cm2/V s. The strong decrease of electron concentration in the surface region of the high resistivity GaN was caused by the compensation of shallow residual donors by a high density (∼6 × 1015 cm−3) of hole traps with activation energy of 0.2 eV, confined to the compensated region. In addition, other hole traps H5 with activation energy 1.2 eV and concentration 5 × 1015 cm−3 were present. These latter traps had similar concentrations in both the high resistivity and standard conducting HVPE GaN. Radiation detectors prepared on the high resistivity ...

Comparison of neutron irradiation effects in AlGaN/AlN/GaN, AlGaN/GaN, and InAlN/GaN heterojunctions

Abstract: Neutron irradiation effects were compared for AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with Al composition in the AlGaN barrier ranging from 20% to 50%, “standard” Al0.25Ga0.75N/GaN HEMTs and for InAlN/GaN HEMTs with InAlN barrier lattice matched to GaN (17% In in the barrier). These samples were exposed to fast reactor neutrons with average energy ∼2 MeV and fluence of 1–3 × 1015 cm−2. The main effect of irradiation was the decrease of two-dimensional electron gas (2DEG) mobility and a positive shift in the threshold voltage corresponding to 2DEG depletion in capacitance–voltage characteristics. For the highest fluences, there was a decrease in both 2DEG concentration and accumulation capacitance, with the effect being strongest for AlGaN/AlN/GaN HEMTs with the highest Al composition and for InAlN/GaN HEMTs. The results correlate with the increase in concentration of deep negatively charged traps in the AlGaN or InAlN barrier with neutron dose. For applications in which tolerance...

Highly sensitive AlGaN/GaN diode-based hydrogen sensors using platinum nanonetworks

Abstract: AlGaN/GaN based diode sensors incorporating platinum nanonetworks for hydrogen sensing were demonstrated. Platinum nanonetworks with 2–3 nm diameter were synthesized by a simple solution phase method, and uniformly deposited on the semiconductor surface by spin-coating. The density of Pt nanonetworks was controlled by the number of the spin coating cycles. Selective-area deposition of the Pt nanonetworks was achieved by the standard photoresist lift-off technique. Compared to conventional Pt thin film diode sensors, the Pt nanonetwork sensor showed remarkably larger current change of 2.3 × 10 7 % at 1 V for 4% H 2 in N 2 exposure, which resulted from the larger effective barrier height reduction due to the increased surface area of the Pt nanonetworks.

Band offsets in HfO2/InGaZnO4 heterojunctions

Abstract: The valence band discontinuity (ΔEV) of sputter deposited HfO2/InZnGaO4 (IGZO) heterostructures was obtained from x-ray photoelectron spectroscopy measurements. The HfO2 exhibited a bandgap of 6.07 eV from absorption measurements. A value of ΔEV = 0.48 ± 0.025 eV was obtained by using the Ga 2p3/2, Zn 2p3/2, and In 3d5/2 energy levels as references. This implies a conduction band offset ΔEC of 2.39 eV in HfO2/InGaZnO4 heterostructures and a nested interface band alignment.

Effects of proton irradiation energies on degradation of AlGaN/GaN high electron mobility transistors

Abstract: The authors report the proton energy dependence of the degradation of AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap layers from 5 to 15 MeV at a fixed dose of 5 × 1015 cm−2. All the samples degraded after proton irradiation. However, higher damage in dc electrical properties was observed at lower proton energies. Saturation currents at VDS = 6 V and VGS = 0 V reduced by 47% after proton irradiation at 5 MeV energy, but the reduction was less by 25% and 9% at 10 and 15 MeV, respectively. Similar trends were observed in other electrical properties [transconductance (gm) and gate leakage currents]. This energy dependence from 5 to 15 MeV can be explained by the energy-dependent penetration depth of the proton. Protons with higher kinetic energy can penetrate deeper while creating less numbers of defects at shallow depths where the active layers of the HEMTs are located. These results are in good agreement with stopping and range of ions in matter results. The optimization of the AlGaN/GaN...

Buried graphene electrodes on GaN-based ultra-violet light-emitting diodes

Abstract: We report that the oxidation of graphene-based highly transparent conductive layers to AlGaN/GaN/AlGaN ultra-violet (UV) light-emitting diodes (LEDs) was suppressed by the use of SiNX passivation layers. Although graphene is considered to be an ideal candidate as the transparent conductive layer to UV-LEDs, oxidation of these layers at high operating temperatures has been an issue. The oxidation is initiated at the un-saturated carbon atoms at the edges of the graphene and reduces the UV light intensity and degrades the current-voltage (I-V) characteristics. The oxidation also can occur at defects, including vacancies. However, GaN-based UV-LEDs deposited with SiNX by plasma-enhanced chemical vapor deposition showed minimal degradation of light output intensity and I-V characteristics because the graphene-based UV transparent conductive layers were shielded from the oxygen molecules. This is a simple and effective approach for maintaining the advantages of graphene conducting layers as electrodes on UV-LEDs.

GaN-based light-emitting diodes on origami substrates

Abstract: GaN-based light-emitting diodes (LEDs) were transferred to paper substrates after a laser lift-off (LLO) process with an ArF excimer laser system (λ = 193 nm) to remove the sapphire substrate and produce freestanding blue LED templates. The threshold voltage (∼2.7 V), current-voltage characteristics, and peak emission wavelength (442 nm) were not changed after the paper substrate was subsequently wrinkled. We were able to demonstrate transfers to both planar and folded (origami) paper structures, showing the promise of the LLO process for transferring LEDs to arbitrary surfaces.

Proton-irradiated InAlN/GaN high electron mobility transistors at 5, 10, and 15 MeV energies

Abstract: InAlN/GaN high electron mobility transistors (HEMTs) grown on SiC substrates were subjected to 5-15 MeV high energy protons with a fixed 5 × 1015 cm−2 fluence. The saturation currents and gate leakage currents of all the proton-irradiated InAlN/GaN HEMTs were degraded. Proton irradiation at lower energy was found to degrade the direct current (DC) current-voltage (I-V) characteristics more severely than higher-energy irradiation, because the energy loss component of the lower energy protons was larger than those of higher-energy protons in the vicinity of the 2-dimensional electron gas conducting channel. Our experimental results were consistent with stopping and range of ions in matter simulation results of the energy deposition profile by the protons.

Sensors using AlGaN/GaN based high electron mobility transistor for environmental and bio-applications

Abstract: AlGaN/GaN high electron mobility transistor (HEMT) based sensors have been developed to analyze a wide variety of gases and biological agents for bio medical applications. The conducting 2DEG channel of GaN/AlGaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. Examples of detecting breath cancer marker, carbon monoxide carbon dioxide, kidney injury molecule, and botulinum toxin are discussed in this paper.

Gallium nitride-based gas, chemical and biomedical sensors

Abstract: Semiconductor-based sensors are widely used for applications in detection of particular gases and liquids, fire detection, liquid quality monitoring, biosensing and medical sensing. As an example, our group has installed hydrogen sensors at a car dealership in Orlando, FL, USA, that houses a fleet of hydrogen-fuelled buses. These sensors are platinum (Pt)-coated aluminum gallium nitride (AlGaN) or gallium nitride (GaN) differential diodes whose output can be remotely monitored via an Internet connection and which provide an alarm both locally and to phones of relevant personnel. A screen-shot of the output of the six detector array is shown in Fig. 1.

Transmission electron microscopy characterization of electrically stressed AlGaN/GaN high electron mobility transistor devices

Abstract: A set of AlGaN/GaN high electron mobility transistor devices has been investigated using step-stress testing, and representative samples of undegraded, source-side-degraded, and drain-side-degraded devices were examined using electron microscopy and microanalysis An unstressed reference sample was also examined All tested devices and their corresponding transmission electron microscopy samples originated from the same wafer and thus received nominally identical processing Step-stressing was performed on each device and the corresponding current–voltage characteristics were generated Degradation in electrical performance, specifically greatly increased gate leakage current, was shown to be correlated with the presence of crystal defects near the gate edges However, the drain-side-degraded device showed a surface pit on the source side, and another region of the same device showed no evidence of damage Moreover, significant metal diffusion into the barrier layer from the gate contacts was also observed, as well as thin amorphous oxide layers below the gate metal contacts, even in the unstressed sample Overall, these observations emphasize that gate-edge defects provide only a partial explanation for device failure

Effect of buffer layer structure on electrical and structural properties of AlGaN/GaN high electron mobility transistors

Abstract: AlGaN/GaN high electron mobility transistors (HEMTs) with similar active layers structures were grown on SiC or sapphire substrates using different buffer layer structures, including GaN of different thickness (1 or 2 μm) or composite AlGaN/GaN buffers. The highest density of hole traps was observed in the buffer on sapphire, while the lowest density of hole traps was obtained in the thick (2 μm) GaN buffer on SiC. The reverse leakage currents in HEMTs were lower in the devices grown on SiC substrates and the on-off ratios improved by two orders of magnitude for thicker GaN buffers or composite AlGaN/GaN buffers compared to a standard 1 μm GaN buffer. The maximum drain-source currents and tranconductances were all larger for the devices on SiC compared to sapphire.

Metastable centers in AlGaN/AlN/GaN heterostructures

Abstract: A set of AlGaN/AlN/GaN high electron mobility transistor structures with Al composition in the AlGaN barrier changing from 20% Al to 50% Al was grown by metalorganic chemical vapor deposition on sapphire and studied by capacitance-voltage (C-V) measurements, admittance spectroscopy, and deep level transient spectroscopy. C-V and admittance measurements were performed in the dark and after illumination. The results suggest the presence of high concentrations of deep negatively charged traps in the AlGaN barriers, producing shifts of the C-V characteristics to more positive voltages. The density of negatively charged centers can be increased by cooling at high reverse bias. These centers have a high barrier for the capture of electrons. Their thermal activation energy is estimated as 0.85 eV, while the optical ionization energy is ∼1.7 eV.

Low-pressure inductively coupled plasma etching of benzocyclobutene with SF6/O2 plasma chemistry

Abstract: Highly anisotropic features of benzocyclobutene, a promising low stress layer for microelectromechanical systems, were achieved by inductively coupled plasma (ICP) etching with fluorine/oxygen based chemistry. The effects of chamber pressure, platen power, ICP power, and SF6/O2 relative concentration were studied on etch rate, selectivity, and sidewall morphology. Highly anisotropic features were achieved with sidewall slopes ∼88° for almost all conditions. Grass-like residue, due to redeposition of etch material, was observed under certain conditions and is shown to be dependent on pressure, ICP power, and feature dimensions.

GaN-Based Sensors

Abstract: Recent progress in the use of surface-functionalized GaN for sensing of gases, heavy metals, UV photons and biological molecules is reviewed. The use of such sensors for monitoring nerve cells is also explored. Additionally, we briefly review progress with InN-based chemical sensors. For the detection of gases such as hydrogen, the GaN is coated with a catalyst metal such as Pd or Pt to increase the detection sensitivity at room temperature. Functionalizing the surface with oxides, polymers, and nitrides is also useful in enhancing the detection sensitivity for gases and ionic solutions. The use of enzymes or adsorbed antibody layers on the semiconductor surface leads to highly specific detection of a broad range of antigens of interest in the medical and homeland security fields. We give examples of recent work showing sensitive detection of glucose, lactic acid, prostate cancer, and breast cancer markers. Nerve cell-GaN surface coupling allowed for the analysis of cell reactions to different neuroinhibitors. InN chemical sensors have also been shown to be viable for sensing anions, pH, and polarity.

Carbon monoxide detection sensitivity of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors in different temperature environments

Abstract: The effect of ambient temperature on the detection sensitivity of carbon monoxide (CO) using ZnO nanorod-gated AlGaN/GaN high electron mobility transistor (HEMT) sensors was studied over a range of temperatures from 25 to 400 °C. An increase of the HEMT drain current was observed for exposure to the CO-containing ambients, due to chemisorbed oxygen on the ZnO surface reacting with CO to form CO2 and releasing electrons to the oxide surface, increasing the counter charges in the two-dimensional electron gas channel of the HEMT. By increasing the detection temperature from 25 °C to 150 °C, the CO detection sensitivity, ΔI/I, and detection limit were significantly improved from 0.23% to 7.5% and from 100 ppm to ∼30 ppm, respectively. However, the sensitivity of the CO detection was degraded by the decrease of mobility and saturation drain current of HEMT at temperatures higher than 200 °C.

Effects of Semiconductor Processing Chemicals on Conductivity of Graphene

Abstract: Graphene layers on SiO2/Si substrates were exposed to chemicals or gases commonly used in semiconductor fabrication processes, including solvents (isopropanol, acetone), acids, bases (ammonium hydroxide), UV ozone, H2O, and O2 plasmas. The recovery of the initial graphene properties after these exposures was monitored by measuring both the layer resistance and Raman 2D peak position as a function of time in air or vacuum. Solvents and UV ozone were found to have the least affect, while oxygen plasma exposure caused an increase of resistance of more than 3 orders of magnitude. Recovery is accelerated under vacuum but changes can persist for more than 5 h. Careful design of fabrication schemes involving graphene is necessary to minimize these interactions with common processing chemicals.

Effects of P implantation and post-implantation annealing on defect formation in ZnO

Abstract: Photoluminescence (PL) and optically detected magnetic resonance (ODMR) techniques are utilized to examine the effects of P implantation and post-implantation annealing on defect formation in ZnO single crystals. From ODMR, the main defects created by ion implantation include oxygen and zinc vacancies as a well as a deep donor labeled as PD. The formation of the PD defect is likely promoted by the presence of P as it could only be detected in the P-containing ZnO. The VO and PD centers are found to exhibit low thermal stability and can be annealed out at 800 °C. On the other hand, a new set of defects, such as Z, T, and D* centers, is detected after annealing. Based on measured spectral dependences of the ODMR signals, the VO, VZn, and PD centers are shown to participate in spin-dependent recombination processes related to red emissions, whereas the Z, T, and D* centers are involved in radiative recombination over a wide spectral range of 1.55–2.5 eV. From the PL measurements, combined effects of implanta...

Proton irradiation energy dependence of dc and rf characteristics on InAlN/GaN high electron mobility transistors

Abstract: The effects of proton irradiation energy on dc and rf characteristics of InAlN/GaN high electron mobility transistors (HEMTs) were investigated. A fixed proton dose of 5 × 1015 cm−2 with 5, 10, and 15 MeV irradiation energies was used in this study. For the dc characteristics, degradation was observed for sheet resistance, transfer resistance, contact resistivity, saturation drain current, maximum transconductance, reverse-bias gate leakage current, and sub-threshold drain leakage current for all the irradiated HEMTs; however, the degree of the degradation was decreased as the irradiation energy increased. Similar trends were obtained for the rf performance of the devices, with ∼10% degradation of the unity gain cut-off frequency (fT) and maximum oscillation frequency ( fmax) for the HEMTs irradiated with 15 MeV protons but 30% for 5 MeV proton irradiation. The carrier removal rate was in the range 0.66–1.24 cm−1 over the range of proton energies investigated.

Graphene-based metal diffusion barrier

Abstract: Contacts for semiconductor devices are formed where a barrier layer comprising graphene is situated between a first layer comprising a conductor, and a second layer comprising a second conductor or a semiconductor. For example, a metal layer can be formed on a graphene layer residing on a semiconductor. The barrier layer can be directly formed on some second layers, for example, graphene can be transferred from an organic polymer/graphene bilayer structure and the organic polymer removed and replaced with a metal or other conductor that comprises the first layer of the contact. The bilayer can be formed by CVD deposition on a metallic second layer, or the graphene can be formed on a template layer, for example, a metal layer, and bound by a binding layer comprising an organic polymer to form an organic polymer/graphene/metal trilayer structure. The template layer can be removed to yield the bilayer structure. Contacts with the graphene barrier layer display enhanced reliability as the graphene layer inhibits diffusion and reaction between the layers contacting the barrier layer.

SnO2-gated AlGaN/GaN high electron mobility transistors based oxygen sensors

Abstract: Hydrothermally grown SnO2 was integrated with AlGaN/GaN high electron mobility transistor (HEMT) sensor as the gate electrode for oxygen detection. The crystalline of the SnO2 was improved after annealing at 400 °C. The grain growth kinetics of the SnO2 nanomaterials, together with the O2 gas sensing properties and sensing mechanism of the SnO2 gated HEMT sensors were investigated. Detection of 1% oxygen in nitrogen at 100 °C was possible. A low operation temperature and low power consumption oxygen sensor can be achieved by combining the SnO2 films with the AlGaN/GaN HEMT structure.

Degradation of dc characteristics of InAlN/GaN high electron mobility transistors by 5 MeV proton irradiation

Abstract: The dc characteristics of InAlN/GaN high electron mobility transistors were measured before and after irradiation with 5 MeV protons at doses up to 2 × 1015 cm−2. The on/off ratio degraded by two orders of magnitude for the highest dose, while the subthreshold slope increased from 77 to 122 mV/decade under these conditions. There was little change in transconductance or gate or drain currents for doses up to 2 × 1013 cm−2, but for the highest dose the drain current and transconductance decreased by ∼40% while the reverse gate current increased by a factor of ∼6. The minority carrier diffusion length was around 1 μm independent of proton dose. The InAlN/GaN heterostructure is at least as radiation hard as its AlGaN/GaN counterpart.