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The applicability of using Electron Beam Induced Current (EBIC) measurements on Schottky barriers to obtain the mean electron–hole pair creation energy in β-Ga2O3 is reported. It is shown that, when combined with Monte Carlo simulation, this approach yields for Si, GaN, and 4H–SiC a data set consistent with empirical expressions proposed earlier in the literature for many different semiconductors. The method is then applied to β-Ga2O3, where complications related to hole trapping in the material give rise to a strong gain in EBIC and have to be carefully treated and taken into account. When this is done, the mean electron–hole pair energy formation is found to be 15.6 eV, in reasonable agreement with the values predicted by empirical expressions. [ABSTRACT FROM AUTHOR]

The drain current fluctuation of ungated AlGaN/GaN high electron mobility transistors (HEMTs) measured in different fluids at a drain-source voltage of 0.5 V was investigated. The HEMTs with metal on the gate region showed good current stability in deionized water, while a large fluctuation in drain current was observed for HEMTs without gate metal. The fluctuation in drain current for the HEMTs without gate metal was observed and calculated as standard deviation from a real-time measurement in air, deionized water, ethanol, dimethyl sulfoxide, ethylene glycol, 1,2-butanediol, and glycerol. At room temperature, the fluctuation in drain current for the HEMTs without gate metal was found to be relevant to the dipole moment and the viscosity of the liquids. A liquid with a larger viscosity showed a smaller fluctuation in drain current. The viscosity-dependent fluctuation of the drain current was ascribed to the Brownian motions of the liquid molecules, which induced a variation in the surface dipole of the gate region. This study uncovers the causes of the fluctuation in drain current of HEMTs in fluids. The results show that the AlGaN/GaN HEMTs may be used as sensors to measure the viscosity of liquids within a certain range of viscosity. [ABSTRACT FROM AUTHOR]

The thermal and chemical stability of potential dielectrics for GaN-based devices in a GaN metal organic chemical vapor deposition (MOCVD) environment is investigated, and their suitability for use as a gate dielectric and as a regrowth mask for the selective area growth of GaN is discussed. Thin films of MgO, Sc2O3, and Sc2O3/MgO were grown by molecular beam epitaxy on GaN/sapphire substrates and then annealed in a MOCVD reactor under GaN growth conditions except for the lack of trimethylgallium. All films were processed into metal-oxide-semiconductor diodes and were characterized before and after being annealed using atomic force microscopy, x-ray reflectivity, x-ray photoelectron spectroscopy, current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Gibbs free energy of all possible reactions was calculated, and their probability and possible influence on the characterization results is examined. After being annealed, the atomic force microscopy of the oxide films showed some degree of roughening for all of them. Despite the surface roughening, all the oxide films examined showed potential for use as a regrowth mask. The MOCVD anneal caused the electrical properties of the MgO film to degrade considerably, and the Sc2O3 films were unable to be electrically characterized after annealing due to shorting, which is believed to be caused by the formation of a ScN layer on the surface. The effect of the thickness of the Sc2O3 cap for MgO films was investigated. The characterization results indicate that the Sc2O3 film dissolved into MgO during annealing and that a ScN film did not form on the surface. Of all the oxide films examined in this study, the Sc2O3/MgO stack with the thinner Sc2O3 cap exhibited the greatest stability with respect to its electrical properties. [ABSTRACT FROM AUTHOR]

The performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with either uncapped surfaces or with low-temperature (LT) GaN or SiO2 or SiNx as gate insulators is reported. The sheet carrier concentrations of AlGaN/GaN HFETs with any of these surface insulating layers are similar to each other and in each case about 50% higher than that in an AlGaN/GaN HFET with a free surface. This result is consistent with the insulator layers providing passivation of surface states that cause the depletion of the channel layer. Due to the closer lattice match with the AlGaN surface layer, the HFET with a LT-GaN layer as the gate insulator shows the best dc and rf device performance, demonstrating that this material is an effective insulator for nitride electronic devices. [ABSTRACT FROM AUTHOR]

Investigates the effects of hydrogen on the electrical and optical properties of p-gallium arsenide. Incorporation of hydrogen into the material; Result of the hydrogen passivation of acceptors rather than of increased compensation by donor centers; Increase in the concentration of 0.4 eV hole traps by hydrogen plasma treatment.

Optical transmission spectra, microcathodoluminescence spectra, capacitance-voltage and capacitance-frequency curves, temperature dependence of resistivity and deep level spectra with both electrical and optical injection were measured on n-GaN samples implanted with high doses of Mn (3 × 10[sup 16] and 4 × 10[sup 16] cm[sup -2]) and Co (4 &times ; 10[sup 16] cm[sup -2]). From optical transmission it was found that Mn forms a deep acceptor near Ev + 1.8 eV while the Co acceptor is about 0.1 eV deeper. In addition, Mn and Co form complexes with native defects and these complexes are deep electron traps with a level near Ec-0.5 eV. Such complexes are most likely responsible for a strong blue luminescence band with energy near 2.9 eV. Adjacent to the implanted region a defect region about 1 µm deep is formed, most likely by out-diffusion of point defects from the implanted zone during the 700°C annealing used to partially remove the radiation damage. This region is characterized by a high density of electron traps at Ec-0.25 eV and Ec-0.7 eV and hole traps at Ev + 0.2 eV, Ev + 0.35 eV and Ev + 0.45 eV. [ABSTRACT FROM AUTHOR]

GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. In this paper, we review data on the radiation resistance of GaN-based transistors such as AlGaN/GaN and InAlN/GaN high electron mobility transistors (HEMTs) to different types of ionizing radiation. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects are discussed. The carrier removal rates are a function of initial carrier concentration, dose and dose rate but not of hydrogen concentration in the nitride material grown by metal organic chemical vapor deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two-dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. Neutron irradiation created more extended damage regions and at high doses leads to Fermi level pinning, while Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. [ABSTRACT FROM AUTHOR]

A finite element simulation was used to estimate the temperature distributions within AlGaN/GaN high electron mobility transistors (HEMTs) during the laser lift-off process. The time-dependent simulation showed that a thin layer of GaN at the GaN/sapphire interface was heated up to around 1600 K in less than 25 ns by a pulsed laser exposure with a duration of 25 ns and a fluence of 800 mJ/cm2 to decompose this GaN layer into Ga and nitrogen. Experimentally, there was a threshold fluence around 550 mJ/cm2, corresponding to 1300 K at the GaN/sapphire interface, for partially lifting off the HEMT structure from the sapphire. The simulated temperature at the GaN/sapphire interface with a fluence of 420 mJ/cm2 never reached above 1000 K, however, the HEMT structure was lifted-off by multiple laser exposures at this fluence. Therefore, instead of thermally induced decomposition, the lift-off mechanism could also be through the Ga-N bond breaking during the multiple lower-fluence high-energy 193 nm laser exposures. [ABSTRACT FROM AUTHOR]

A finite element simulation was used to estimate the temperature rise of laser lift-off AlGaN/GaN high electron mobility transistors (HEMTs) mounted on AlN substrates via silicone elastomer under various conditions of power density on the transistor and for a range of elastomer thicknesses. The thermal response was compared to HEMTs fabricated on SiC and sapphire substrates. The maximum temperature of the transistor occurred in the gate area at a power level of 15 W, and was only 25 K higher than HEMTs fabricated on SiC. The transit time required for steady-state temperature of the flip-chip bonding device is in the range of a millisecond, which is faster than that of most power switch applications. [ABSTRACT FROM AUTHOR]

Ti/Al/Ni/Au Ohmic contact metallization on InAlN/GaN heterostructures both with and without a thin GaN cap layer was annealed at different temperatures. The minimum transfer resistance for the contacts of 0.65 Ω mm (specific contact resistivity of 2×10-5 Ω cm2) was achieved after 800 °C annealing for structures without the GaN cap, while those with the cap exhibited their lowest resistance at higher temperatures. The contact morphology showed considerable roughening by 750 °C but the carrier mobility was stable until annealing temperatures of 850 °C. Diffuse scattering experiments showed that the morphological roughness of the InAlN/GaN interface increased as a result of annealing at these temperatures and the data were consistent with outdiffusion of Ga into the InAlN. Unpassivated high electron mobility transistors with a gate dimension of 0.7×180 μm2 were fabricated using these contacts and showed a maximum drain current of 1.3 A/mm and an extrinsic transconductance of 366 mS/mm. The presence of the GaN cap increased the effective barrier height of Ni/Au Schottky contacts from 0.91 to 1.01 eV on the heterostructure. [ABSTRACT FROM AUTHOR]

The effect of device design on maximum channel temperature for AlGaN/GaN high electron mobility transistors was investigated through finite element thermal simulations for dissipated power densities up to 5 W mm-1. The effects of substrate material, die size, and number of gate fingers were examined, as well as the divergence between maximum channel temperatures for two dimensional and three dimensional simulations. The maximum temperature increased as the die size decreased beyond a critical distance from the gates. The critical distance was dependent on the substrate material, with SiC requiring the smallest critical distance. At the maximum dissipated power, the temperature increased above ambient more than 300 °C for devices on sapphire substrates compared to an increase of 44 °C for devices on SiC substrates. As the number of gate fingers increased, the maximum channel temperature also increased, and a temperature gradient was observed along the gate width as well as between gate fingers. [ABSTRACT FROM AUTHOR]

The critical degradation voltage of AlGaN/GaN high-electron mobility transistors during off-state electrical stress was determined as a function of Ni/Au gate dimensions (0.1–0.17 \mu\m), drain bias voltage, and source/drain–gate contact distance. Devices with different gate lengths and gate–drain distances were found to exhibit the onset of degradation at different source–drain biases but similar electric field strengths, showing that the degradation mechanism is primarily field driven. The degradation field was calculated to be \sim1.8 MV/cm by Automatically Tuned Linear Algebra Software simulations. Transmission electron microscopy imaging showed creation of defects under the gate after dc stress. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Pd and Pt Schottky diodes on non-polar a-plane (11–20) GaN layers show large increases in both forward and reverse bias current upon exposure to 4% H2 in N2. The barrier height reduction due to hydrogen exposure is 0.11eV for Pd/GaN and 0.14eV for Pt/GaN, with long recovery times (>25min) at room temperature. The sensitivity to hydrogen is significantly greater than for diodes on conventional c-plane (Ga-polar) GaN, but less than for c-plane (N-polar) material. The diode characteristics remain rectifying after exposure to hydrogen, unlike the case of N-polar GaN where Ohmic behavior is observed. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Ozone treatment of AlN on AlN/GaN heterostructures produces effective surface passivation and chemical resistance to the AZ positive photoresist developer used for subsequent device fabrication. The ozone-passivated AlN/GaN high electron mobility transistors (HEMTs) exhibited low gate leakage currents, high gate modulation voltage, and minimal drain current degradation during gate pulse measurements. With an additional oxygen plasma treatment on the gate area prior to the gate metal deposition, enhancement-mode AlN/GaN high electron mobility transistors were realized. The gate characteristics of the HEMTs treated with the ozone and oxygen plasma behaved in a manner similar to a metal oxide semiconductor diodelike gate current-voltage characteristic instead of a Schottky diode. Drain breakdown voltages of 23 and 43 V for d- and e-mode HEMTs were obtained, respectively. For d-mode HEMTs, there was no reduction in the drain current during the gate pulse measurements at frequencies of 1, 10, and 100 kHz. For the e-mode HEMT, the drain current was reduced 5% at 100 kHz. [ABSTRACT FROM AUTHOR]

Wide band gap semiconductors such as GaN and ZnO have continued to be at the forefront of spintronics research due to the demonstration of room-temperature ferromagnetism in these materials. A goal of this research is the fabrication of a spintronic device that may provide easy incorporation into existing GaN and ZnO technologies, higher integration density, and less power consumption that its electronic counterparts while achieving similar speeds. The progression of the ferromagnetic metals incorporated into GaN has moved from transition metals to rare earth metals such as the lanthanides. The properties of transition metal-doped ZnO thin films for the two most studied transition metal dopants, namely Mn and Co, are also discussed in this review. [ABSTRACT FROM AUTHOR]

We report on the growth and characterization of high-quality GaN nanowires for hydrogen sensors. We grew the GaN nanowires by catalytic chemical vapor deposition (CVD) using gold thin films as a catalyst on a Si wafer with an insulating SiO2 layer. Structural characterization of the as-grown nanowires by several methods shows that the nanowires are single-crystal wurtzite GaN.␣Photoluminescence measurements under 325 nm excitation show a near-band-edge emission peak around ∼3.4 eV. The hydrogen sensors are fabricated by contacting the as-grown GaN nanowires by source and drain electrodes and coating them with a thin layer of Pd. Hydrogen sensing experiments using the fabricated devices show high sensitivity response (ppm detection limit at room temperature) and excellent recovery. This work opens up the possibility of using high-quality GaN nanowire networks for hydrogen sensing applications. [ABSTRACT FROM AUTHOR]

The radiofrequency (RF) performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) on Si-on-poly-SiC (SopSiC) substrates formed by the Smart-Cut™ process is reported. This provides a low-cost, high-thermal-conductivity substrate for power applications. HEMTs with a 0.5 µm gate length show cutoff frequencies (fT) of 18 to 27 GHz for gate-to-drain distances of 3 to 32 µm and a maximum frequency of oscillation (fmax) of 43 to 47 GHz. The fmax values are slightly lower than comparable devices on sapphire, SiC or Si alone. This approach looks promising for applications requiring cheap large-area substrates and better thermal management than provided by pure Si substrates alone. [ABSTRACT FROM AUTHOR]

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n ~ 3 x 1017 cm-3) is reported. The annealing temperature (600-1,000°C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ~6 x 10-5 Ω-cm-2 was obtained after annealing over a broad range of temperatures (600-900°C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Auscheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350°C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350°C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks. [ABSTRACT FROM AUTHOR]

A finite-element simulation was used to quantitatively estimate the heat transfer in a three-dimensional multichip module (MCM) consisting of a GaN power amplifier with solder-bump-bonded Si modulator and integrated antenna on a high-resistivity SiC substrate under various conditions of power density and substrate and epi thicknesses via wire thickness and effective heat transfer coefficient. The maximum temperature in the integrated-antenna approach occurred in the center of the MCM. At a GaN power amplifier power level of 3 W/mm, a steady-state temperature of ∼125 °C was reached in ∼20 s. Bulk GaN substrates were also found to provide good thermal transfer characteristics, while sapphire produced an increase in temperature almost a factor of 3 higher than for SiC. At a power density of 10 W/mm, the steady-state operating temperature was ∼400 °C even with SiC substrates. [ABSTRACT FROM AUTHOR]

Gallium Nitride (GAN) and related materials (especially AlGaN) recently have attracted a lot of interest for applications in high-power electronics capable of operation at elevated temperatures and high frequencies. The A1GalnN system offers numerous advantages. These include wide bandpass, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasisubstrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices. Al[subx]Ga[sub1-x] N (x=0 ∼ .25) Schottky rectifiers were fabricated in a lateral geometry employing F-implanted guard rings and rectifying contact overlap onto an SiO[sub2] passivation layer. The reverse breakdown voltage (V[subB]) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al[sub0.25]Ga[sub0.75]N and 6350 V for GaN, respectively, for 100-[tm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of ≤9.67 x 10[sup5] Vcm[sub-2], which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (V[subB])²/R[subON], where R[subON] is the on-state resistance, was in the range 94 to 268 MWcm[sup-2] for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasibulk GaN substrates grown by HVPE. For small-diameter (75 µm) Schottky contacts, Vs measured in the vertical geometry was -700V, with an on-state resistance (R[subON]) of 3 mωcm², producing a figure-of-merit V[subB][sub2]/R[subON] of 162.8 MW-cm². GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for t [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A subnanometer thick interfacial oxide layer present between the Ni/Au gate metal stack and semiconducting epilayers of an AlGaN/GaN high electron mobility transistor was characterized using high-angle annular dark-field scanning transmission electron microscopy and laser-assisted atom probe tomography. It was revealed that the oxide is composed of distinct Ni-oxide-rich and Al-oxide-rich layers with no Ga-oxide detected. The results provide information that is of potential importance in determining failure mechanisms and improving reliability of AlGaN/GaN high electron mobility transistors. [ABSTRACT FROM AUTHOR]

InGaN/GaN multiquantum well light-emitting diodes (LED) with emission wavelength of 450 nm were irradiated with average energy of 9.8 MeV and dose of 5.5×1011 cm-2 neutrons. Right after irradiation, the forward current of the irradiated LEDs was decreased as a result of the creation of deep levels by the neutron-induced lattice displacement. However, unstable lattice damages resulting from the collisions with the incoming neutrons were removed at room temperature 6 days after the irradiation. The diode turn-on voltage, ideality factor, and optical emission intensity were recovered to preirradiated state by self-annealing process at room temperature. [ABSTRACT FROM AUTHOR]

Multiple GaN nanowires produced by thermal chemical vapor deposition were employed as gas sensors for detection of hydrogen at concentrations from 200–1500 ppm in N2 at 300 K. Palladium coating of the wires improved the sensitivity by a factor of up to 11 at low ppm concentrations relative to uncoated controls. The GaN nanowires showed relative responses of ∼7.4% at 200 ppm and ∼9.1% at 1500 ppm H2 in N2 after a 10 min exposure. Upon removal of hydrogen from the measurement ambient, ∼90% of the initial GaN conductance was recovered within 2 min. Temperature dependent measurements showed a larger relative response and shorter response time at elevated temperature. The adsorption activation energy of the sensor was 2.2 kcal mol-1 at 3000 ppm H2 in N2. These sensors exhibit low power consumption (<0.6 mW) at 300 K. [ABSTRACT FROM AUTHOR]

The detection of lactic acid with ZnO nanorod-gated AlGaN/GaN high electron mobility transistors (HEMTs) was demonstrated. The array of ZnO nanorods provided a large effective surface area with a high surface-to-volume ratio and a favorable environment for the immobilization of lactate oxidase. The HEMT drain-source current showed a rapid response when various concentrations of lactic acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactic acid concentrations from 167 nM to 139 μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN/GaN HEMT based sensors. [ABSTRACT FROM AUTHOR]

ZnO nanowires were grown on 2-μm-thick GaN templates by chemical vapor deposition without employing any metal catalysts. The GaN template was deposited by metal-organic chemical vapor deposition on a c-plane sapphire substrate. The diameters of the resulting nanowires were in the range of 40–250 nm depending on growth time. The ZnO nanowires were vertically well aligned with uniform length, diameter, and distribution density as revealed by electron microscopy. X-ray diffraction spectra showed that ZnO grew in single c-axis orientation with the c axis normal to the GaN basal plane, indicating a heteroepitaxial relationship of (0002)ZnO|(0002)GaN. The lattice constant of the c axis of the ZnO nanowires with diameter of 40 nm was 5.211 Å, which is larger than that of bulk ZnO (5.207 Å). The ZnO nanowires exhibit a residual tensile strain along the c axis, which decreases with increasing diameter. [ABSTRACT FROM AUTHOR]

Antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect c-erbB-2, which is a breast cancer marker. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 s when target c-erbB-2 antigen in a buffer at clinically relevant concentrations was added to the antibody-immobilized surface. We could detect a range of concentrations from 16.7 to 0.25 μg/ml. These results clearly demonstrate the promise of portable electronic biological sensors based on AlGaN/GaN HEMTs for breast cancer screening. [ABSTRACT FROM AUTHOR]

Ohmic contacts to p-type GaN were fabricated using W2B and CrB2 as diffusion barriers for a traditional Ni/Au contact scheme. The annealing temperature dependence (25–1000 °C) of contact resistance and the thermal aging characteristics at 200 °C were examined. A minimum contact resistance of ∼2×10-4 Ω cm2 was achieved after annealing at 700 °C for 60 s. These contacts also showed excellent stability as a function of aging at 200 °C. Auger electron depth profiles reveal a large degree of intermixing at the GaN interface between Ni and Au. [ABSTRACT FROM AUTHOR]

Ohmic contacts to p-GaN using a Ni/Au/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, are reported. The dependence of the contact properties on annealing temperature (25–1000 °C) in N2 is examined. For annealing temperatures greater than 500 °C, the contacts display Ohmic characteristics and reach a minimum of about 2×10-4 Ω cm2 after annealing at 700 °C for 60 s in a N2 ambient. The specific contact resistance is stable on annealing up to at least 1000 °C. However, at high temperatures the morphology of the contacts are very rough and there is a large degree of intermixing between the metallic layers. The thermal stability of these contacts are superior as compared to conventional Ni/Au, which display poor characteristics at annealing temperatures greater than 500 °C. [ABSTRACT FROM AUTHOR]

Schottky contact formation on p-GaN using a W2B-based metallization scheme was investigated using x-ray photoelectron spectroscopy (XPS), current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Schottky barrier height (SBH) determined from XPS is 2.7 eV, whereas fitting of the I-V’s gives 1.2 and 3.8 eV depending on the assumed mechanism of forward current flow. While the C-V’s and the measurement temperature dependence of the I-V’s support tunneling as being the dominant transport mechanism, this latter approach overestimates the true SBH of W2B/p-GaN contacts due to the presence of an interfacial layer acting as an additional barrier to carrier transport. [ABSTRACT FROM AUTHOR]

Au-gated AlGaN/GaN high electron mobility transistor (HEMT) structures were functionalized in the gate region with label-free 3′-thiol-modified oligonucleotides. This serves as a binding layer to the AlGaN surface for hybridization of matched target deoxyribonucleic acid (DNA). X-ray photoelectron spectroscopy shows the immobilization of thiol-modified DNA covalently bonded with gold on the gated region. Hybridization between probe DNA and matched or mismatched target DNA on the Au-gated HEMT was detected by electrical measurements. The HEMT drain-source current showed a clear decrease of 115 μA as this matched target DNA was introduced to the probe DNA on the surface, showing the promise of the DNA sequence detection approach for biological sensing. [ABSTRACT FROM AUTHOR]

An enhancement of ∼0.18 eV (an 18% increase) in Schottky barrier height was obtained for Au deposited at cryogenic temperatures on n-type GaN relative to conventional deposition at 300 K (barrier height of 1.0 eV). Enhancements of 0.04–0.11 eV were achieved for Au deposition on p-GaN under the same conditions. The increase in barrier height on n-GaN persists for annealing temperatures up to ∼200 °C. At higher annealing temperatures, both types of diodes show a deterioration in rectifying behavior. The reverse current of low temperature deposited diodes was approximately two orders of magnitude lower than conventional Au/n-GaN diodes. The ideality factor of the cryogenically processed n-type devices (∼1.06) was similar to that for room temperature diodes (1.13). This simple process method has potential for improving output resistance and power gain and lowering gate leakage current and noise in GaN-based transistors. [ABSTRACT FROM AUTHOR]

Thin films of scandium oxide were epitaxially deposited on GaN via molecular beam epitaxy using elemental Sc and an oxygen plasma. After growth, the Sc2O3 films were annealed at a temperature of 800 °C for 5 min in the growth chamber. The structural quality of Sc2O3 films, before and after annealing, was characterized using high-resolution x-ray diffraction, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM). AFM of the films revealed smooth surfaces with 0.38 nm root mean square roughness and show evidence of step-flow growth. The rocking curve and reflectivity scans of the films reveal that the Sc2O3/GaN interface is abrupt and that it remains so after annealing. Pole figure and grazing incidence θ-2θ measurements show that the films are very textured along the c axis of the GaN substrate. HRTEM produced lattice images of the Sc2O3/GaN interface illustrating the single crystal growth of the Sc2O3 films on the GaN. [ABSTRACT FROM AUTHOR]

MgO is a promising gate dielectric and surface passivation film for GaN transistors but little is known of the band offsets in the MgO/GaN system. X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEv) of MgO/GaN heterostructures in which the MgO was grown by rf plasma-assisted molecular beam epitaxy on top of thick GaN templates on sapphire substrates. A value of ΔEv=1.06±0.15 eV was obtained by using the Ga 3d energy level as a reference. Given the experimental band gap of 7.8 eV for the MgO, this would indicate a conduction band offset ΔEC of 3.30 eV in this system. [ABSTRACT FROM AUTHOR]

Schottky contact formation on n-GaN using a novel W2B/Ti/Au metallization scheme was studied using current-voltage, scanning electron microscopy and Auger electron spectroscopy measurements. A maximum barrier height of 0.55 eV was achieved on as-deposited samples, with a negative temperature coefficient of 8×10-4 eV/°C over the range 25–150 °C. There was also a negative temperature coefficient for the reverse breakdown of the Schottky diodes with W2B contacts. The barrier height was essentially independent of annealing temperature up to 500 °C and decreased thereafter due to the onset of metallurgical reactions with the GaN. The Ti began to out-diffuse to the surface at temperatures of >500 °C. The reverse current magnitude was larger than predicted by thermionic emission alone. [ABSTRACT FROM AUTHOR]

Measurement of the electrical characteristics of 250 devices on the same 2 in. diameter wafer shows that Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors (HEMTs) have lower average specific contact resistance after annealing at 850 °C for 30 s (4.6×10[sup -5] Ω cm[sup 2]) compared to more standard Ti/Al/Ni/Au contacts (2×10[sup -4] Ω cm[sup 2]). HEMTs with these Ir-based contacts also show average interdevice isolation currents approximately a factor of 2 lower, higher peak transconductance (134 mS/mm compared to 121 mS/mm), and higher device breakdown voltage (31 V compared to 23 V) than the devices with Ni-based contacts. This Ir-based contact metallurgy looks promising for applications requiring extended thermal stability of the HEMTs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pt Schottky rectifier arrays were fabricated on 200-μm-thick, freestanding GaN layers. Even with the reduced dislocation density in these layers (∼10[sup 5] cm[sup -2]) relative to conventional GaN on sapphire (>10[sup 8] cm[sup -2]), rectifiers fabricated on the freestanding GaN show a strong dependence of reverse breakdown on contact diameter. We show that by interconnecting the output of many (∼130) smaller (500 μm×500 μm) rectifiers, we can achieve high total forward output current (161 A at 7.12 V), low forward turn-on voltage of ∼3 V, and maintain the reverse breakdown voltage. The on/off ratio of the rectifier array was ∼8×10[sup 7] at 5 V/-100 V. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

p-i-n rectifiers were fabricated on epitaxial layers grown on free-standing GaN substrates. The forward turn-on voltage, V[sub F] was ∼5 V at 300 K and displayed a positive temperature coefficient. The specific on-state resistance (R[sub ON]) was ∼5 mΩ cm[sup 2] at 300 K, with an ideality factor of ∼2 and activation energy for low forward current density of ∼1.6 eV. This is consistent with carrier recombination in the space charge region via a midgap deep level. The figure-of-merit, V[sub B][sup 2]/R[sub ON], where V[sub B] is the reverse breakdown voltage, was 0.32 MW cm[sup -2]. The reverse recovery time was ≤600 ns at 300 K. The improved forward characteristics relative to previous heteroepitaxial p-i-n GaN rectifiers show the advantages of employing a GaN substrate to make a true vertical transport geometry device. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A comparison was made of specific contact resistivity and morphology of Ti/Al/Pt/WSi/Ti/Au and Ti/Al/Pt/W/Ti/Au ohmic contacts to AlGaN/GaN heterostructures relative to the standard Ti/Al/Pt/Au metallization. The W- and WSi-based contacts show comparable specific resistivities to that of the standard contact on similar layer structures, reaching minimum values of ∼10[sup -5] Ω cm[sup 2] after annealing in the range 850–900 °C. However, the W- and WSi-based contacts exhibit much smoother surface morphologies, even after 950 °C annealing. For example, the root-mean-square roughness of the Ti/Al/Pt/WSi/Ti/Au contact annealed at 950 °C was unchanged from the as-deposited values whereas the Ti/Al/Pt/Au contact shows significant deterioration of the morphology under these conditions. The improved thermal stability of the W- and WSi[sub x]-based contacts is important for maintaining edge acuity during high-temperature operation. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Lateral electron mobilities in 0.2-µm-thick n-(Ga, Mn)N films were obtained from Hall measurements, producing values of 116∼ 102 cm²/V s in the temperature range from 298 to 373 K. These values are comparable to, but slightly lower than, electron mobilities in n-GaN of the same electron concentration. By sharp contrast, analysis of the reverse saturation current in mesa Schottky diodes fabricated in the n-(Ga, Mn)N show vertical electron mobilities of 840∼336 cm²/V s in the temperature range from 298 to 373 K. This is consistent with a reduction in electron scattering by charged dislocations for vertical transport geometries [M. Misra, A. V. Sampath, and T. D. Moustakas, Appl. Phys. Lett. 76, 1045 (2000)]. [ABSTRACT FROM AUTHOR]

Sc[sub 2]O[sub 3]-passivated AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with 40 MeV protons to a fluence corresponding to approximately 10 years in low-earth orbit (5 × 10[sup 9] cm[sup -2]). Devices with an AlGaN cap layer showed less degradation in dc characteristics than comparable GaN-cap devices, consistent with differences in average band energy. The changes in device performance could be attributed completely to bulk trapping effects, demonstrating that the effectiveness of the Sc[sub 2]O[sub 3] layers in passivating surface states in the drain-source region was undiminished by the proton irradiation. Sc[sub 2]O[sub 3]-passivated AlGaN/HEMTs appear to be attractive candidates for space and terrestrial applications where resistance to high fluxes of ionizing radiation is a criteria. [ABSTRACT FROM AUTHOR]

The forward current of Pd/GaN and Pt/GaN Schottky diodes is found to increase significantly upon introduction of H[sub 2] into a N[sub 2] ambient. Analysis of the current-voltage characteristics as a function of temperature showed that the current increase is due to a decrease in effective barrier height through a decrease in metal work function upon absorption of hydrogen. The introduction of 10% H[sub 2] into a N[sub 2] ambient was found to lower the effective barrier height of Pd on GaN by 50 ∼70 meV over the temperature range of 298 to ∼423 K and of Pt on GaN by 30∼60 meV over the range of 443 to ∼473 K. The magnitude of the changes increased with temperature due to the effective cracking of the H[sub 2]. The changes in barrier height were completely reversible upon restoration of N[sub 2] ambient. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with different gate length and widths were irradiated with [sup 60]Co γ-rays to doses up to 600 Mrad. Little measurable change in dc performance of the devices was observed for doses lower than 300 Mrad. At the maximum dose employed, the forward gate current was significantly decreased, with an accompanying increase in reverse breakdown voltage. This is consistent with a decrease in effective carrier density in the channel as a result of the introduction of deep electron trapping states. The threshold voltage shifted to more negative voltages as a result of the irradiation, while the magnitude of the drain–source current was relatively unaffected. This is consistent with a strong increase of trap density in the material. The magnitude of the decrease in transconductance of the AlGaN/GaN HEMTs is roughly comparable to the decrease in dc current gain observed in InGaP/GaAs heterojunction bipolar transistors irradiated under similar conditions. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ga[sub 2]O[sub 3](Gd[sub 2]O[sub 3]) was deposited on GaN for use as a gate dielectric in order to fabricate a depletion metal–oxide–semiconductor field-effect transistor (MOSFET). Analysis of the effect of temperature on the device shows that gate leakage is significantly reduced at elevated temperature relative to a conventional metal–semiconductor field-effect transistor fabricated on the same GaN layer. MOSFET device operation in fact improved upon heating to 400 °C. Modeling of the effect of temperature on contact resistance suggests that the improvement is due to a reduction in the parasitic resistances present in the device. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Describes the formation of Schottky contacts on n- and p-type gallium nitride (GaN) after either a conventional surface cleaning step in solvents, HCl and HF or with an additional treatment in (NH[sub 4)[sub 2]S to prevent reformation of the native oxide. Reduction in barrier height; Change in diode ideality factor.

Describes oxygen diffusion into silicon dioxide-capped gallium nitride using plasma chemical vapor deposition during annealing. Evidence that the high dislocation density in gallium nitride strongly affects the effective penetration depth; Absence of V< in the type of gallium nitride upon incorporation of the oxygen.

Examines the effect of post growth rapid thermal annealing of gallium nitride (GaN) films grown on c-sapphire substrates. Influence of the thermally modified, near surface quality on sputtered wurtzite silicon contact to GaN; Microstructure characteristics of the films; Reduction of near-surface defects density by increased annealing temperature.