Showing papers by "Stephen J. Pearton published in 2010"

Hydrogen Sensing Using Pd‐Functionalized Multi‐Layer Graphene Nanoribbon Networks

Abstract: www.MaterialsViews.com C O M M Hydrogen Sensing Using Pd-Functionalized Multi-Layer Graphene Nanoribbon Networks U N IC By Jason L. Johnson , Ashkan Behnam , S. J. Pearton , and Ant Ural * A IO N Sensing of gas molecules is critical in many fi elds including environmental monitoring, transportation, defense, space missions, energy, agriculture, and medicine. Solid state gas sensors have been developed for many of these applications. [ 1–3 ] More recently, chemical gas sensors based on nanoscale materials, such as carbon nanotubes and semiconductor nanowires, have attracted signifi cant research attention due to their naturally small size, large surface-to-volume ratio, low power consumption, room temperature operation, and simple fabrication. [ 4–6 ]

Effects of ambient atmosphere on the transfer characteristics and gate-bias stress stability of amorphous indium-gallium-zinc oxide thin-film transistors

Abstract: We investigated the transfer characteristics and the gate-bias stability of amorphous indium-gallium-zinc oxide thin-film transistors when the channel layer was exposed to hydrogen, oxygen, air, or vacuum at room temperature during measurements. The threshold voltage and the drain current were changed by the ambient atmospheres. The threshold voltage shift (ΔVth) under gate-bias stress was faster in hydrogen than in oxygen and vacuum. It is suggested that hydrogen exposure degrades the gate-bias stress stability due to surface accumulation layer creation. The characteristic trapping times, τ, in H2, O2, air, and vacuum were 5×103, 1.5×104, 2×104, and 6.3×104 s, respectively.

Fabrication of p-channel thin-film transistors using CuO active layers deposited at low temperature

Abstract: We investigated copper oxides for use as an active layer of p-channel field-effect thin-film transistors (TFTs). Cu2O thin films deposited at room temperature using rf magnetron sputtering were transformed to a CuO phase after an annealing treatment in air above 200 °C. The optical bandgaps of the Cu2O and CuO were 2.44 and 1.41 eV, respectively. The bottom gate structured TFTs fabricated using CuO active layers operated in a p-type enhancement mode with an on/off ratio of ∼104 and field-effect mobility of 0.4 cm2/V⋅s.

Low-voltage indium gallium zinc oxide thin film transistors on paper substrates

Abstract: We have fabricated bottom-gate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature (≤100 °C). As a water and solvent barrier layer, cyclotene (BCB 3022–35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2 V−1 s−1, threshold voltage (VTH) of 1.9 V, subthreshold gate-voltage swing (S) of 0.65 V decade−1, and drain current on-to-off ratio (ION/IOFF) of ∼104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat∼2.1 cm2 V−1 s−1, VTH∼0 V, S∼0.74 V decade−1, and ION/IOFF=105–106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. The ability to achieve InGaZnO TFTs on cyclotene-coated paper substrates demonstrates the enormous potential for applications such as low-cost and large area electronics.

Nitride and oxide semiconductor nanostructured hydrogen gas sensors

Abstract: In this paper, we discuss the progress of nitride and oxide semiconductor nanostructures for hydrogen gas sensing. The use of catalyst metal coatings on GaN, InN and ZnO nanowires is found to greatly enhance the detection sensitivity. Pt- and Pd-coated GaN nanowires biased at small voltages show large changes in currents upon exposure to H2 gas at concentrations in the ppm range. Improvements in growth techniques for InN nanostructures have produced nanobelts and nanorods capable of hydrogen detection down to 20 ppm after catalyst coating. Functionalized ZnO nanorods were also investigated for hydrogen detection, but did not generate a relative response as high as that for the nitride-based sensors. All sensors tested exhibited no response at room temperature upon exposure to various other gases including O2, C2H5, N2O and CO2. The high surface-to-volume ratio of nanowires and the ability to use simple contact fabrication schemes make them attractive for hydrogen sensing applications.

Isolation blocking voltage of nitrogen ion-implanted AlGaN/GaN high electron mobility transistor structure

Abstract: Nitrogen ion-implanted AlGaN/GaN high electron mobility transistor structures showed an isolation blocking voltage of 900 V with a leakage current at 1 μA/mm across an implanted isolation-gap of 10 μm between two Ohmic pads. The effect of implanted gap distance (1.7, 5, or 10 μm) between two Ohmic contact pads was evaluated. The isolation current density was determined to be solely dependent on the applied field between the contact pads. A model using a combination of resistive current and Poole–Frenkel current is consistent with the experimental data. The resistance of the isolation implantation region significantly decreased after the sample was annealed at temperatures above 600 °C.

Dip Pen Nanolithography of Conductive Silver Traces

Abstract: We report the first demonstration of subμm, sub-50-μΩ·cm conductive traces directly written by Dip Pen Nanolithography (DPN). We achieved subμm Ag lines with 28.8 μΩ·cm average resistivity after direct-write printing from a silver nanoparticle-based ink suspension and annealing at 150 °C for 10 min. This compares to Ag bulk resistivity of 1.63 μΩ·cm, where the difference is within the range of previously reported variations in conductivity of Ag-based inks due to annealing conditions and larger width scales. We leveraged DPN’s ability to directly place materials at specific locations in order to fabricate and characterize these conductive silver (Ag) traces on electrode patterns and multiple substrates (SiO2, Kapton, mica). The low viscosity of the AgNP ink solution allowed write speeds up to 1600 μm/s, almost 4 orders of magnitude higher than typical thiol-on-gold DPN writing speeds. This direct-write methodology paves the way for site-specific deposition of metallic materials for use in applications suc...

Temperature dependence of current-voltage characteristics of Ni–AlGaN/GaN Schottky diodes

Abstract: Ni–AlGaN/GaN Schottky barrier diodes (SBDs) with lateral geometry were fabricated on sapphire substrates. At 300 K, devices with 500-μm-diameter Schottky contacts exhibited breakdown voltage (VB) of 765 V, forward current (IF) of 0.065 A at 1.5 V, and specific on-resistance (Ron) of 81.3 mΩ cm2, producing a figure-of-merit (VB2/Ron) of ∼7.2 MW cm−2. Measured in multifinger patterns, the same parameters were 420 V, 3.2 A, 4.6 mΩ cm2, and 38.4 MW cm−2, respectively, at 300 K. With the increase in measurement temperature from 300 to 450 K, SBDs with dimensions of 3000×3000 μm2 showed larger effective barrier heights (0.8 eV at 300 K and 1.27 eV at 475 K) and a slightly negative temperature coefficient (−0.48 V K−1) for reverse breakdown voltage, while there was a little change in reverse leakage current. These results show the strong influence of barrier height inhomogeneity on the temperature dependence of apparent barrier heights obtained through current-voltage measurements.

Wireless Detection System for Glucose and pH Sensing in Exhaled Breath Condensate Using AlGaN/GaN High Electron Mobility Transistors

Abstract: Peltier element cooling of ungated AlGaN/GaN high electron mobility transistors (HEMTs) is shown to be an effective method for condensing exhaled breath, enabling the measurement of the pH and glucose of the exhaled breath condensate (EBC). By comparison with standard solutions, the current change measured in the HEMTs with EBC shows that the sensitivity of the glucose detection is lower than the glucose concentration in the EBC of healthy human subjects and the pH of the condensate from the exhaled breath is within the range of 7-8, typical of that for human blood. The HEMT sensors can be integrated into a wireless data transmission system that allows for remote monitoring. Details of the transmitter and receiver design for the transmission system are given. Our work demonstrates the possibility of using AlGaN/GaN HEMTs for extended investigations of airway pathology without the need for clinical visits.

Effects of Photoelectrochemical Etching of N-Polar and Ga-Polar Gallium Nitride on Sapphire Substrates

Abstract: We studied the effects of photo electrochemical (PEC) etching by using various concentrations (1, 2, and 4 M) of KOH solutions on both Ga- and N-face GaN layers on sapphire substrates. The Ga-face was chemically stable for KOH solutions, while by sharp contrast the KOH could etch the N-face, where the 6-fold symmetry was observed after the PEC etching. Surface texturing of GaN-based light emitting diodes and solar cells by KOH-based PEC etch could enhance the efficiency of GaN-based photonic devices by increasing the number of the scattering events and randomly changing the angles of the light.

Aluminum gallium nitride (GaN)/GaN high electron mobility transistor-based sensors for glucose detection in exhaled breath condensate.

Abstract: Background Immobilized aluminum gallium nitride (AlGaN)/GaN high electron mobility transistors (HEMTs) have shown great potential in the areas of pH, chloride ion, and glucose detection in exhaled breath condensate (EBC). HEMT sensors can be integrated into a wireless data transmission system that allows for remote monitoring. This technology offers the possibility of using AlGaN/GaN HEMTs for extended investigations of airway pathology of detecting glucose in EBC without the need for clinical visits.

Defect-enhanced ferromagnetism in Gd- and Si-coimplanted GaN

Abstract: GaN grown by metal-organic chemical vapor deposition was coimplanted with Gd+ ions with energy of 155 keV and dose of 2.75×1010 cm−2 and Si4+ ions with energies of 5 and 40 keV and corresponding doses of 8×1011 and 3.6×1012 cm−2. The implanted samples were not annealed before characterization. X-ray diffraction measurements revealed that the implanted GaN exhibited no secondary phase formation or clustering effects attributable to Gd. Superconducting quantum interference device magnetometer measurements indicated that Gd- and Si-coimplanted GaN exhibited about a 400% higher magnetic moment than a Gd-implanted GaN thin film. This emphasizes the role of defects in producing large saturation magnetization in Gd-doped GaN. Both types of films displayed ferromagnetic ordering and Curie temperatures above room temperature. The higher magnetic moment was displayed when the magnetic field was applied perpendicular to the sample surface (parallel to the c-axis of the sample), suggesting the polarization field in w...

Effect of humidity on hydrogen sensitivity of Pt-gated AlGaN/GaN high electron mobility transistor based sensors

Abstract: The effects of relative humidity on sensing characteristics of Pt-gated AlGaN/GaN high electron mobility transistor diode based hydrogen sensors were investigated. The absorbed water and oxygen molecules blocked available Pt surface adsorption sites for H2 absorption and reduced the hydrogen sensing sensitivity compared to low humidity conditions. The hydrogen sensing sensitivity decreased proportional to the relative humidity. However, the presence of humidity improved the sensor recovery characteristics after exposure to the hydrogen ambient.

Enhancement mode hemt for digital and analog applications

Abstract: An enhancement mode (E-mode) HEMT is provided that can be used for analog and digital applications, In a specific embodiment, the HEMT can be an AlN/GaN HEMT. The subject E-mode device can be applied to high power, high voltage, high temperature applications, including but not limited to telecommunications, switches, hybrid electric vehicles, power flow control and remote sensing. According to an embodiment of the present invention, E-mode devices can be fabricated by performing an oxygen plasma treatment with respect to the gate area of the HEMT. The oxygen plasma treatment can be, for example, an O2 plasma treatment. In addition, the threshold voltage of the E-mode HEMT can be controlled by adjusting the oxygen plasma exposure time. By using a masking layer protecting regions for depletion mode (D-mode) devices, D-mode and E-mode devices can be fabricated on a same chip.

Reverse gate bias-induced degradation of AlGaN/GaN high electron mobility transistors

Abstract: A threshold reverse bias of ∼21 V was observed leading to a sharp increase in the gate current of AlGaN/GaN high electron mobility transistors biased at low source-drain voltage (5 V). The gate current increases by one to two orders of magnitude at this bias, corresponding to an electric field strength around 1.8 MV cm−1. The gate current increased by roughly five orders of magnitude after step-stressing the gate bias from 10 to 42 V in 1 V increments for 1 min at each bias. The drain current was also decreased by ∼20% after this step-stress cycle. The photoluminescence and electroluminescence intensity from the semiconductor is decreased along the periphery of the gate region after stressing and transmission electron microscopy shows a thin native oxide layer under the gate and this disappears as the gate metal reacts with the underlying AlGaN.

Neutron transmutation doping effects in GaN

Abstract: The effects of neutron transmutation doping were studied for undoped (residual donor concentrations <1015 cm−3) GaN films grown by metalorganic chemical vapor deposition After irradiation with reactor neutrons (equal fluences of 15×1017 n/cm2 of thermal and fast neutrons) the sample became semi-insulating, with the Fermi level pinned near Ec−08 eV Isochronal annealing from 100 to 1000 °C showed three stages—slight recovery of conductivity at 200–300 °C, reverse annealing at 300–500 °C, and a broad recovery stage from 600 to 1000 °C After annealing at 1000 °C, the donor concentration in the sample was close to the expected concentration of Ge donors transformed from Ga atoms upon interaction with thermal neutrons (2×1016 cm−3) Admittance spectroscopy showed that the donors had ionization energies ∼Ea=02 eV, much deeper than substitutional Ge donors For intermediate annealing temperatures of 800 °C the donors were deeper (Ea=047 eV), but the proximity of concentrations of all these different center

Proton irradiation effects on AlN/GaN high electron mobility transistors

Abstract: AlN/GaN high electron mobility transistors (HEMTs) were irradiated with 5 MeV protons at fluences from 2×1011 to 2×1015 protons/cm2. Changes from 10% to 35% of the saturation drain current and the source-drain resistances were observed for the HEMTs exposed to the proton irradiations due to radiation-induced carrier scattering and carrier removal. Both forward and reverse bias gate currents were increased after proton irradiation and affected the drain current modulation in the positive gate bias voltage range. There was almost no gate-lag observed for the HEMT exposed to 2×1011 protons/cm2 irradiation and minimal changes for the higher doses, which implied that few surface traps were created by the high energy proton irradiation.

Long-term stability study of botulinum toxin detection with AlGaN/GaN high electron mobility transistor based sensors

Abstract: The long-term stability of antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors for detecting botulinum toxin is reported in this study. The botulinum toxin sensor, which initially showed good data reproducibility and recyclability, was repeatedly tested over a 9-month period. The botulinum sensor was packaged and stored in phosphate buffered saline (PBS) at 4 °C in a refrigerator for long-term storage. The sensor was tested over time at room temperature and we found sensitivity losses of 2%, 12% and 28% after 3, 6 and 9 months, respectively. These results clearly demonstrate a significant step towards the realization of electronic detection of biomolecules by field-deployed sensor chips based on AlGaN/GaN HEMTs.

Ohmic contact properties of non-polar a-plane GaN films on r-plane sapphire substrates

Abstract: The properties of Ti/Al/Ni/Au Ohmic contacts on n-type a-plane GaN epitaxial layers directly grown on r-plane sapphire substrates are reported. The minimum specific contact resistance of ~10?5???cm2 was achieved after annealing at 650?700??C. Ohmic contact properties were measured using transmission line method patterns oriented in both the m- and c-axis directions of a-plane GaN. The sheet resistance of a-plane GaN along the c-axis was two times higher than that along the m-axis, which shows significant electric anisotropy in the two orientations.

Chloride ion detection by InN gated AlGaN∕GaN high electron mobility transistors

Abstract: Real time chloride ion detection using InN gated AlGaN∕GaN high electron mobility transistors (HEMTs) was demonstrated. The InN thin film on the gate area of the HEMT provided fixed surface sites for reversible anion coordination. The drain current of the HEMT sensor exhibited increased a function of chloride ion concentration. The positive ions (Na+, Mg+2, and H+) in the chloride ion solutions showed no effect on the chloride ion concentration detection. The sensor was tested over a range of chloride ion concentrations from 100nMto100μM. The chloride ion HEMT sensors can be integrated with AlGaN∕GaN HEMT based pH and glucose sensors for exhaled breath condensate glucose monitoring technology. The HEMT based sensor can also be integrated into a wireless data transmission system for remote sensing applications.

Detection of an endocrine disrupter biomarker, vitellogenin, in largemouth bass serum using AlGaN/GaN high electron mobility transistors

Abstract: Endocrine disrupters are known to have negative effects on the environment and human health. Real time detection of vitellogenin, an endocrine disrupter biomarker, was demonstrated using AlGaN/GaN high electron mobility transistors (HEMTs). Anti-vitellogenin antibodies were chemically anchored to the gold-coated gate area of the HEMT and immobilized with thioglycolic acid. The potential difference that occurs from the vitellogenin antigen-antibody interaction-induced caused a drain current change in the HEMT. The HEMT sensor was tested for vitellogenin detection both in phosphate buffer saline and largemouth bass serum.

Electrical Characterization of 5 MeV Proton-Irradiated Few Layer Graphene

Abstract: Few layer graphene (FLG) samples contacted were irradiated with protons at an energy of 5 MeV and doses up to 2 × 10 15 /cm 2 . The electrical properties of ungated FLG sheets contacted by Pd/Au in a source (S)-drain (D) configuration, including V DS -I DS , V G -I DS , and the hole mobility, were compared before and after proton irradiation. After irradiation, it is observed that the ambipolar conduction of the FLG sheets was changed to a p-type conduction. The field-effect mobility of the hole carriers and the resistance in the graphene sheets greatly decreased because the proton irradiation increased the number of the surface states.

Oxygen gas sensing at low temperature using indium zinc oxide-gated AlGaN/GaN high electron mobility transistors

Abstract: Indium zinc oxide (IZO)-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect oxygen gas. Amorphous IZO films with high carrier concentration of 1021 cm−3 were deposited on the gate region of the HEMTs by cosputtering from ZnO and In2O3 targets. The changes in IZO gated-AlGaN/GaN HEMT drain current were used to monitor the presence of oxygen. The IZO gated AlGaN/GaN HEMT sensors were tested with O2 at room temperature, 50 °C, and 120 °C. There was no response to O2 at room temperature. At 50 °C, the sensors could sense O2 but gradually saturated. The sensor showed a strong response to the oxygen gas at 120 °C, which is a much lower temperature than with conventional oxide-based oxygen sensors that typically operate in the range of 400–700 °C. This enhanced oxygen sensing sensitivity was due to the amplification effect of the AlGaN/GaN HEMT. A preannealing step at 350 °C was also found to improve the sensitivity and response time of O2 sensing at 120 °C.

Role of Hydrogen in the CVD of Wide Bandgap Nitride Semiconductors

Abstract: Hydrogen is an important component of the gas-phase growth chemistry for GaN, which is typically based on NH3 and (CH3)3Ga, and also the processing environment for subsequent device fabrication (e.g., SiH4 for dielectric deposition, NH3 or H2 annealing ambients), and is found to readily permeate heteroepitaxial material at temperatures ≤200 °C. Its main effect has been the passivation of Mg acceptors in p-GaN through the formation of neutral Mg-H complexes, which can be dissociated through minority-carrier (electron) injection or simple thermal annealing. Atomic hydrogen is also found to passivate a variety of other species in GaN, as detected by a change in the electrical or optical properties of the material. The injection of hydrogen during a large variety of device fabrication steps has been detected by secondary ion mass sprectrometry (SIMS) profiling using 2H isotopic labeling. Basically all of the acceptor species in GaN, i.e., Mg, C, Ca, and Cd, are found to form complexes with hydrogen.

Surface Immobilizations of AlGaN/GaN High Electron Mobility Transistor Based Sensors

Abstract: Recent progress in AlGaN/GaN HEMT sensors is reviewed in this paper. These devices can take advantage of the advantages of microelectronics, including high sensitivity, possibility of high-density integration, and mass manufacturability. HEMT sensors show promising results for protein, DNA, prostate cancer, kidney injury molecules, pH values of solutions, mercury ions as well glucose in the exhaled breath condensate. The method relies on an amplification of small changes in antibody-structure due to binding to antigens. The characteristics of these sensors include fast response (liquid phase-5 to 10 seconds and gas phase- milli-second), digital output signal, small device size (less than 100 × 100 μm 2 ) and chemical and thermal stability.

Spin Dynamics in ZnO-Based Materials

Abstract: In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized, time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.