J.S. Wright

Room temperature hydrogen detection using Pd-coated GaN nanowires

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.

Nitride and oxide semiconductor nanostructured hydrogen gas sensors

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.

Selective-hydrogen sensing at room temperature with Pt-coated InN nanobelts

The hydrogen sensing characteristics of multiple InN nanobelts grown by metalorganic chemical vapor deposition were investigated. Pt-coated InN sensors could selectively detect hydrogen at the tens of ppm level at 25 °C, while uncoated InN showed no detectable change in current when exposed to hydrogen under the same conditions. Upon exposure to various concentrations of hydrogen (20–300 ppm) in N2 ambient, the relative resistance change increased from 1.2% at 20 ppm H2 to 4% at 300 ppm H2. Approximately 90% of the initial InN resistance was recovered within 2 min by exposing the nanobelts to air. Temperature-dependent measurements showed larger resistance change and faster response at high temperature compared to those at room temperature due to increase in catalytic dissociation rate of H2 as well as diffusion rate of atomic hydrogen into the Pt/InN interface. The Pt-coated InN nanobelt sensors were operated at low power levels (∼0.5 mW).

Pd-catalyzed hydrogen sensing with InN nanobelts

The use of Pd coatings on multiple InN nanobelts is shown to enhance their sensitivity for hydrogen sensing at hundreds of ppm level at 25°C. Without the metal coating to catalyze dissociation of the hydrogen molecules, the InN nanobelts with Ohmic contacts at either end showed no detectable change in current when exposed to hydrogen under the same conditions. Moreover, the Pd-coated InN showed no response to CO2, C2H6, NH3, and O2 (all in N2 ambient). The relative resistance change in the Pd-coated sensors was not linearly dependent on the hydrogen concentration at dilute levels, i.e., 8% at 100ppm H2 and 9.5% at 1000ppm H2. The recovery characteristics of the sensors at room temperature after hydrogen sensing were also examined and ∼50% of the initial InN resistance was recovered 10min after sensor exposure to air. At higher temperatures, larger resistance changes and faster response and recovery were obtained. Pd-coated InN nanobelt sensors displayed much higher relative response than Pt-coated sensors.

W2B and CrB2 diffusion barriers for Ni∕Au contacts to p-GaN

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 ∼2×10−4Ωcm2 was achieved after annealing at 700°C for 60s. 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.

Thermally Stable TiB2 Ohmic Contacts on n-ZnO

The annealing temperature dependence of contact characteristics on bulk single-crystal n-ZnO using a TiB 2 /Pt/Au metallization scheme deposited by sputtering are reported. The contacts are rectifying for anneal temperatures <700°C but transition to Ohmic behavior at higher temperatures and exhibit a minimum specific contact resistivity of 5 X 10 -4 Ω cm after 800°C anneals. Higher temperatures lead to severe contact metallurgy intermixing and a sharp increase in specific contact resistivity associated with an increase in sheet resistance of the ZnO under the contact. Pt, Ti, Zn, and B outdiffuse through the Au layer at 800°C while at higher temperatures the contact morphology is destroyed. These boride-based contacts show much higher thermal stability than previous metal schemes used as Ohmic contacts on ZnO and are promising for high-temperature device applications.

Defects in Electron and Neutron Irradiated n-GaN: Disordered Regions Versus Point Defects

Effects of 10 MeV electron and fast reactor neutron irradiations on carrier removal rate and deep traps spectra were compared for undoped n-GaN samples. It is shown that for electron irradiation the carrier removal rate is well accounted for by the difference in introduction rates of nitrogen-vacancy-related donors with activation energy 0.2 eV and of nitrogen-interstitial-related acceptors at Ec-1.2 eV. In the case of neutron irradiation the introduction rate of all deep traps was much lower than the carrier removal rate indicating that the main contribution to electron removal was due to disordered regions. These regions give rise to a marked persistent photocapacitance signal and a hole-trap-like feature in deep traps spectra. The Fermi level position in the core of disordered regions is located near Ec-(0.85-1) eV.

Ir ∕ Au Ohmic Contacts on Bulk, Single-Crystal n-Type ZnO

The contact characteristics on bulk single-crystal n-type ZnO of an Ir/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 200-1000°C (N 2 ambient). The contacts exhibited ohmic behavior for all temperatures and show a minimum specific contact resistivity of 3.6 X 10 -5 Ω cm 2 after a 1000°C anneal. The contacts transition to rectifying behavior after annealing above 1100°C, coincident with a degraded surface morphology including agglomeration of Ir to the surface and heavy intermixing of the Ir and Au. The Ir contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on bulk n-type ZnO. The contacts showed very little change in resistance after extended aging (30 days) at 350°C. Annealing under O 2 ambient led to an increase in contact resistivity by orders of magnitude.

Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors

The development of new etching and contact metallurgies for the ZnO/ZnMgO/ZnCdO materials system and various approaches for realizing ZnO LEDs are reviewed. ZnO nanorod MOSFETs and pH sensors have been demonstrated. In addition, selective detection of hydrogen with Pt-coated single ZnO nanorods is discussed discussed. The Pt-coated single nanorods show a current response approximately a factor of three larger at room temperature upon exposure to 500ppm H2 in N2 than thin films of ZnO. The power consumption of these sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the μW range. The ZnO nanorods are insensitive to oxygen in the measurement ambient.

Electrical Properties, Deep Levels Spectra and Luminescence of Undoped GaN/InGaN Multi-quantum-well Structures as Affected by Electron Irradiation

Effects of 10 MeV electron and fast reactor neutron irradiations on carrier removal rate and deep traps spectra were compared for undoped n-GaN samples. It is shown that for electron irradiation the carrier removal rate is well accounted for by the difference in introduction rates of nitrogen-vacancy-related donors with activation energy 0.2 eV and of nitrogen-interstitial-related acceptors at Ec-1.2 eV. In the case of neutron irradiation the introduction rate of all deep traps was much lower than the carrier removal rate indicating that the main contribution to electron removal was due to disordered regions. These regions give rise to a marked persistent photocapacitance signal and a hole-trap-like feature in deep traps spectra. The Fermi level position in the core of disordered regions is located near Ec-(0.85-1) eV.