Showing papers by "Stephen J. Pearton published in 2005"

Hydrogen-selective sensing at room temperature with ZnO nanorods

Abstract: The sensitivity for detecting hydrogen with multiple ZnO nanorods is found to be greatly enhanced by sputter-depositing clusters of Pd on the surface. The resulting structures show a change in room- temperature resistance upon exposure to hydrogen concentrations in N2 of 10–500ppm of approximately a factor of 5 larger than without Pd. Pd-coated ZnO nanorods detected hydrogen down to 2.6% at 10ppm and >4.2% at 500ppm H2 in N2 after a 10min exposure. There was no response at room temperature to O2. Approximately 95% of the initial ZnO conductance after exposure to hydrogen was recovered within 20s by exposing the nanorods to either air or pure O2. This rapid and easy recoverability make the Pd-coated nanorods suitable for practical applications in hydrogen-selective sensing at ppm levels at room temperature with <0.4mW power consumption.

Origin of green luminescence in ZnO thin film grown by molecular-beam epitaxy

Abstract: The properties of ZnO films grown by molecular-beam epitaxy are reported. The primary focus was on understanding the origin of deep-level luminescence. A shift in deep-level emission from green to yellow is observed with reduced Zn pressure during the growth. Photoluminescence and Hall measurements were employed to study correlations between deep-level/near-band-edge emission and carrier density. With these results, we suggest that the green emission is related to donor-deep acceptor (Zn vacancy VZn−) and the yellow to donor-deep acceptor (oxygen vacancy, Oi−).

Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods

Abstract: A comparison is made of the sensitivities for detecting hydrogen with Pt-coated single ZnO nanorods and thin films of various thicknesses (20–350 nm). The Pt-coated single nanorods show a current response of approximately a factor of 3 larger at room temperature upon exposure to 500ppmH2 in N2 than the thin films of ZnO. The power consumption with both types of 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 optimum ZnO thin film thickness under our conditions was between 40–170 nm, with the hydrogen sensitivity falling off outside this range. The nanorod sensors show a slower recovery in air after hydrogen exposure than the thin films, but exhibit a faster response to hydrogen, consistent with the notion that the former adsorb relatively more hydrogen on their surface. Both ZnO thin and nanorods cannot detect oxygen.

Hydrogen local modes and shallow donors in ZnO

Abstract: The annealing behavior of the free-carrier absorption, O-H vibrational absorption, and photoluminescence lines previously associated with H-related donors in ZnO has been studied. One set of H-related defects gives rise to O-H local vibrational mode absorption at either 3326 or $3611\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, with the relative intensities of these lines depending on the source of the ZnO starting material. These O-H lines anneal away together at 150 \ifmmode^\circ\else\textdegree\fi{}C along with $\ensuremath{\sim}80%$ of the free carriers introduced by H. The common annealing behavior of the O-H ir lines suggests that they are closely related. An additional defect produced by hydrogenation is thermally stable up to an annealing temperature of 500 \ifmmode^\circ\else\textdegree\fi{}C where a bound-exciton photoluminescence line often associated with H in ZnO is also annealed away. This more thermally stable donor accounts for $\ensuremath{\sim}20%$ of the free carriers introduced by H. These experiments on H in ZnO reveal a complex behavior with several defects being introduced and with properties that depend strongly on the source of the ZnO starting material.

Carbon nanotube films for room temperature hydrogen sensing

Abstract: Thin, uniform, single-walled carbon nanotube films, made by a simple filtration process, subsequently coated with palladium, are shown to be promising detectors of hydrogen. The films detected hydrogen with relative responses of 20% at 100 ppm and 40% at 500 ppm concentrations. Most of the initial film conductance was recovered within 30 s by exposing the samples to air. This quick and easy recoverability make the Pd-coated nanotubes suitable for practical applications in room temperature hydrogen sensing while consuming only ~0.25 mW power. The film fabrication process provides highly reproducible control over the film thickness; an important ingredient for commercial production. In the course of this research strong evidence was obtained indicating that sputter deposition of metal onto the nanotubes, even under very low power, short exposure time conditions, does damage to the nanotubes.

pH measurements with single ZnO nanorods integrated with a microchannel

Abstract: Single ZnO nanorods with Ohmic contacts at either end exhibit large changes in current upon exposing the surface region to polar liquids introduced through an integrated microchannel. The polar nature of the electrolyte introduced led to a change of surface charges on the nanorod, producing a change in surface potential at the semiconductor∕liquid interface. The nanorods exhibit a linear change in conductance between pH 2 and 12 of 8.5nS∕pH in the dark and 20nS∕pH when illuminated with ultraviolet (365nm) light. The nanorods show stable operation with a resolution of ∼0.1pH over the entire pH range. The results indicate that ZnO nanorods may have applications in integrated chemical, gas, and fluid monitoring sensors.

Electrical detection of immobilized proteins with ungated AlGaN∕GaN high-electron-mobility Transistors

Abstract: Ungated AlGaN∕GaN high-electron-mobility transistor (HEMT) structures were functionalized in the gate region with aminopropyl silane. This serves as a binding layer to the AlGaN surface for attachment of fluorescent biological probes. Fluorescence microscopy shows that the chemical treatment creates sites for specific absorption of probes. Biotin was then added to the functionalized surface to bind with high affinity to streptavidin proteins. The HEMT drain-source current showed a clear decrease of 4μA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.

Electrical Detection of Immobilized Proteins With Ungated AlGaN/GaN High Electron Mobility Transistors.

Abstract: Ungated AlGaN∕GaN high-electron-mobility transistor (HEMT) structures were functionalized in the gate region with aminopropyl silane. This serves as a binding layer to the AlGaN surface for attachment of fluorescent biological probes. Fluorescence microscopy shows that the chemical treatment creates sites for specific absorption of probes. Biotin was then added to the functionalized surface to bind with high affinity to streptavidin proteins. The HEMT drain-source current showed a clear decrease of 4μA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.

UV photoresponse of single ZnO nanowires

Abstract: ZnO nanowires grown by site-selected molecular beam epitaxy (MBE) were contacted at both ends by Al/Pt/Au ohmic electrodes. The current–voltage (I–V) and photoresponse characteristics were obtained both in the dark and with ultraviolet (UV, 254 or 366 nm) illumination. The I–V characteristics are ohmic under all conditions, with nanowire conductivity under UV exposure of 0.2 Ω cm. The photoresponse showed only a minor component with long decay times (tens of seconds) thought to originate from surface states. The results show the high quality of material prepared by MBE and the promise of using ZnO nanowire structures for solar-blind UV detection.

Hydrogen and ozone gas sensing using multiple ZnO nanorods

Abstract: ZnO nanorods grown by site-selective molecular beam epitaxy show current–voltage characteristics that are sensitive to the presence of hydrogen or ozone in the measurement ambient for temperatures as low as ∼112 °C for H2 or room temperature for O3. The sensitivity to hydrogen increases sharply with temperature and multiple nanorods contacted at both ends by ohmic electrodes show currents of ∼10-8 A at 200 °C and a differential current change of ∼18% when changing from a pure N2 ambient to 10% H2 in N2. The nanorods are able to detect small concentrations (3%) of O3 in O2, with changes in current of ∼10-7 A at 25 °C. The sensitivity was 21% for O3 at room temperature. The nanorods also show a strong response to above-band-gap illumination with ultraviolet light.

Dry Etching of Electronic Oxides, Polymers, and Semiconductors

Abstract: Summary: A review is given of commonly used plasma etching techniques and typical plasma chemistries for patterning electronic oxides, such as SiO2, HfO2, MgO, Sc2O3, and ZnO, polymers, and several important semiconductors (Si, GaAs, and InP). Issues that affect the reproducibility and manufacturability of etching processes, such as loading effects, endpoint detection, and etch selectivity, are also discussed. Finally, disruption to the etched surface in the form of polymer deposition, plasma residues, ion-induced displacement damage, or preferential loss of one of the lattice elements during the etch process are covered. SEM micrograph of features etched into a ZnMgO/ZnO structure using a BCl3/Cl2/Ar ICP plasma.

Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods

Abstract: A variety of different metal catalyst coatings (Pt, Pd, Au, Ag, Ti and Ni) deposited on multiple ZnO nanorods have been compared for their effectiveness in enhancing sensitivity for detecting hydrogen at room temperature. Pt-coated nanorods show a relative response of up to 8% in room-temperature resistance upon exposure to a hydrogen concentration in N2 of 500 ppm. This is a factor of two larger than that obtained with Pd and more than an order of magnitude larger than that achieved with the remaining metals. The power levels for these sensors were low, ∼0.4 mW for the responses noted above. Pt-coated ZnO nanorods easily detected hydrogen down to 100 ppm, with a relative response of 4% at this concentration after 10-min exposure. The nanorods show a return to their initial conductance upon switching back to a pure-air ambient with time constants of the order of a few minutes at room temperature. This slow response at room temperature is a drawback in some applications, but the sensors do offer low-power operation and ppm detection sensitivity.

Magnetization dependence on electron density in epitaxial ZnO thin films codoped with Mn and Sn

Abstract: The magnetic and transport properties of Mn-doped ZnO thin films codoped with Sn are examined. Superconducting quantum interference device magnetometry measurements indicate that the films are ferromagnetic with an inverse correlation between magnetization and electron density as controlled by Sn doping. Magnetism in low free-electron density material is consistent with the bound magnetic polaron model, in which bound acceptors mediate the ferromagnetic ordering. Increasing the electron density decreases the acceptor concentration, thus quenching the ferromagnetic exchange. This result is important in understanding ferromagnetism in transition-metal-doped semiconductors for spintronic devices.

Mechanical properties of ZnO epitaxial layers grown on a- and c-axis sapphire

Abstract: The University of Sydney, for constructive comments and support. The work at UF is partially supported by the AFOSR under Grant Nos. F49620-03-1-0370 sT.S.d and NSF DMR 0400416.

Comparison of MOS and Schottky W/Pt–GaN diodes for hydrogen detection

Abstract: W/Pt contacted GaN Schottky diodes show forward current changes of >1 mA at low bias(3 V) in the temperature range 350–600 °C when the measurement ambient is changed from pure N2 to 10%H2/90%N2. In this paper we show that use of a metal-oxide-semiconductor (MOS) diode structure with Sc2O3 gate dielectric and the same W/Pt metallization show these same reversible changes in forward current upon exposure to H2-containing ambients over a much broader temperature range (90 to >625 °C). The increase in current in both cases is the result of a decrease in effective barrier height of the MOS and Schottky gates of 30–50 mV 10%H2/90%N2 ambients relative to pure N2 and is due to catalytic dissociation of the H2 on the Pt contact, followed by diffusion to the W/GaN or Sc2O3/GaN interface. The presence of the oxide lowers the temperature at the hydrogen which can be detected and in conjunction with the use of the high temperature stable W metallization enhances the potential applications of these wide bandgap sensors.

Development of MgZnO-ZnO-AlGaN heterostructures for ultraviolet light emitting applications

Abstract: We report on the fabrication and characterization of an ultraviolet (UV) light-emitting diode (LED) based on a p-n junction MgZnO/ZnO/AlGaN/GaN semiconductor triple-heterostructure (THS). Radio-frequency (RF) plasma-assisted molecular-beam epitaxy (MBE) has been employed to grow individual epitaxial layers of ZnO, MgxZn1−xO, and the complete heterostructure on c-plane GaN/sapphire templates. Various growth strategies have been used to optimize the quality of the ZnO layers as well as to precisely control the composition of the MgxZn1−xO compound. Cross-sectional transmission electron microscopy (TEM) study shows the excellent crystalline quality of the pseudomorphically grown ZnO active region of the device. A strong electroluminescence (EL) emission associated with ZnO excitonic transition was observed up to 650 K. The results shown in this paper strongly suggest the viability of RF plasma-assisted MBE in the development of next-generation UV emitters using ZnO-based materials.

Role of growth conditions on magnetic properties of AlCrN grown by molecular beam epitaxy

Abstract: AlCrN layers were grown by gas source molecular beam epitaxy with varying amounts of Cr (up to ∼3at.%) under a broad range of Cr cell temperatures and V/III ratio. Magnetic measurements performed in a superconducting quantum interference device magnetometer showed evidence of ferromagnetism up to 350K in single phase material. Magnetization dependence on dopant cell temperature and V/III was used to optimize the growth conditions of the AlCrN layers. The single-phase material was highly insulating (∼1010Ωcm), while the material containing second phases (predominantly Cr2N and AlxCry) was conducting with resistivity of order 1000Ωcm. High resolution x-ray diffraction rocking curves indicated high crystalline quality in the single phase material.

Transport properties of InN nanowires

Abstract: The transport properties of single InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of both length/square of radius ratio and temperature. The resistivity of the n-type InN nanowires with diameter >100nm was measured by the transmission line method and the value was on the order of 4×10−4Ωcm. The specific contact resistivity for unalloyed Pd∕Ti∕Pt∕Au ohmic contacts was near 1.09×10−7Ωcm2. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN.

Transport properties of p-type phosphorus-doped (Zn,Mg)O grown by pulsed-laser deposition

Abstract: We report on carrier type in phosphorus-doped (Zn,Mg)O films grown by pulsed-laser deposition under a broad range of conditions. For film growth at 500°C, increasing the oxygen partial pressure from 20to200mTorr yielded a carrier type conversion from n to p type. Transport characteristics of as-grown P-doped (Zn,Mg)O films were determined by Hall-effect measurements at room temperature. The P-doped (Zn,Mg)O films grown at 150mTorr oxygen partial pressure were marginally p type and exhibited a hole concentration of 2.7×1016cm−3, a mobility of 8.2cm2∕Vs, and a resistivity of 35Ωcm. The films exhibited good crystallinity with c-axis orientation. These results indicate the importance of oxidation conditions in realizing p-type (Zn,Mg)O films.

Pt-coated InN nanorods for selective detection of hydrogen at room temperature

Abstract: Single crystal InN nanorods were successfully grown on c‐Al2O3 by hydride-metalorganic vapor phase epitaxy. The measured resistance of bare InN nanorods does not change upon exposure to hydrogen ambient. The addition of sputter-deposited clusters of Pt onto the surface of the InN nanorods, however, produced a significant change in the measured room temperature resistance. The measured resistance changed systematically by 0.5%–12% as the ambient hydrogen concentration in N2 was varied between 10 and 250 ppm after 15 min exposure time. Importantly, a relatively low power consumption of ∼0.3mW was measured under these conditions. There was no response at room temperature to O2, N2O, or NH3 exposures.

Characteristics of unannealed ZnMgO /ZnO p-n junctions on bulk (100) ZnO substrates

Abstract: Zn0.9Mg0.1O∕ZnO p-n junctions were grown by pulsed laser deposition at ⩽500°C on bulk n-type, (100), nonpolar, a-plane ZnO substrates. No postgrowth annealing was performed, with the P-doped ZnMgO showing p-type conductivity (hole density ∼1016cm−3, mobility ∼6cm2V−1s−1) in the as-grown state. Front-to-back p-n junctions were fabricated with Ni∕Au used as the p-Ohmic contact and Ti∕Au as the backside n-Ohmic contact. The p contacts showed improved characteristics after annealing up to 400°C, but the n contacts were Ohmic as deposited. The junctions showed rectifying behavior up to 200°C. The forward turn-on voltage was ∼6.5V at 25°C. The simple, low-temperature growth and processing sequence show the promise of ZnO for applications in transparent electronics and UV light emitters.

Comparison of gate and drain current detection of hydrogen at room temperature with AlGaN∕GaN high electron mobility transistors

Abstract: Pt-gated AlGaN∕GaN high electron mobility transistors can be used as room-temperature hydrogen gas sensors at hydrogen concentrations as low as 100ppm. A comparison of the changes in drain and gate current-voltage (I-V) characteristics with the introduction of 500ppm H2 into the measurement ambient shows that monitoring the change in drain-source current provides a wider gate voltage operation range for maximum detection sensitivity and higher total current change than measuring the change in gate current. However, over a narrow gate voltage range, the relative sensitivity of detection by monitoring the gate current changes is up to an order of magnitude larger than that of drain-source current changes. In both cases, the changes are fully reversible in <2–3min at 25°C upon removal of the hydrogen from the ambient.

Ti /Au n-type Ohmic contacts to bulk ZnO substrates

Abstract: Electron-beam-deposited Ti∕Au ohmic contacts on undoped (n∼1017cm−3) bulk ZnO substrates exhibited as-deposited specific contact resistivity of 3×10−4Ωcm2, regardless of the polarity (Zn face or O face) of the ZnO substrate. The annealing environment (air or N2) also had no significant effect on contact properties. The specific contact resistivity slightly decreased after annealing at 300 °C but started to increase above 350 °C. The measurement temperature dependence of specific contact resistivity revealed that the dominant current transport mechanism is field emission even in the moderately doped ZnO. As the annealing temperature increased, some voids were observed on the metal surface, possibly due to reaction of Ti∕Au metallization and the evaporation of the oxygen from the ZnO substrate.

Capacitance pressure sensor based on GaN high-electron-mobility transistor-on-Si membrane

Abstract: The changes in the capacitance of the channel of an AlGaN∕GaN high-electron-mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The capacitance of the channel displays a change of 7.19±0.45×10−3pF∕μm as a function of the radius of the membrane at a fixed pressure of +9.5bar and exhibits a linear characteristic response between −0.5 and +1bar with a sensitivity of 0.86pF∕bar for a 600μm radius membrane. The hysteresis was 0.4% in the linear range. These AlGaN∕GaN HEMT membrane-based sensors appear to be promising for both room-temperature and elevated-temperature pressure-sensing applications.

Neutron irradiation effects on electrical properties and deep-level spectra in undoped n-AlGaN/GaN heterostructures

Abstract: The effect of neutron irradiation on the electrical properties of undoped n-AlGaN∕GaN heterostructures is reported. The two-dimensional electron-gas (2DEG) mobility starts to decrease at neutron doses above 1014cm−2, while the 2DEG concentration slightly increases at low doses and decreases dramatically for doses higher than 2.5×1016cm−2. The result is that the mobility/concentration product (a figure of merit for transistors) starts to decrease appreciably after the dose of 1015cm−2. Capacitance-voltage and admittance spectroscopies, indicate that tunneling of electrons into the states near Ec−0.21eV in AlGaN is a serious factor when cooling down the virgin or lightly irradiated samples. For heavily irradiated samples the states in AlGaN are close to 0.3 and 0.45eV, respectively, from the bottom of the conduction band. Deep-level spectroscopy measurements reveal the presence of hole traps with apparent activation energies of 0.18 and 0.21eV for lightly irradiated samples and deeper hole traps with activa...

Spintronics device concepts

Abstract: Spin-dependent phenomena in semiconductors may lead to devices with new or enhanced functionality, such as polarised solid-state light sources (spin light-emitting diodes), novel microprocessors and sensitive biological and chemical sensors. The realisation of robust semiconductor spin-device technology requires the ability to control the injection, transport and detection of polarised carriers, and to manipulate their density by a field gating. The absence of Si-based or room-temperature dilute magnetic semiconductors has subdued the initial excitement over semiconductor spintronics, but recent reports demonstrate that progress is far from dormant. The authors give examples of a number of different spin-device concepts for polarised light emission, spin field-effect transistors) and nanowire sensors. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field which has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analogue. There have been demonstrations of device-type operation in structures based on GaMnAs and InMnAs at low temperatures. The most promising materials for room-temperature polarised light emission are thought to be GaN and ZnO, but results to date on realising such devices have been disappointing. The short spin-relaxation time observed in GaN/InGaN heterostructures probably results from the Rashba effect. Possible solutions involve either cubic phase nitrides or the use of additional stressor layers to create a larger spin-splitting, to get polarised light emission from these structures, or to look at alternative semiconductors and fresh device approaches.

Detection of halide ions with AlGaN∕GaN high electron mobility transistors

Abstract: AlGaN∕GaN high electron mobility transistors (HEMTs) both with and without a Au gate are found to exhibit significant changes in channel conductance upon exposing the gate region to various halide ions. The polar nature of the halide ions leads to a change of surface charge in the gate region on the HEMT, producing a change in the surface potential at the semiconductor∕liquid interface. HEMTs with a Au-gate electrode not only doubled the sensitivity of changing the channel conductance as compared to gateless HEMT, but also showed the opposite conductance behavior. When anions adsorbed on the Au, they produced a counter charge for electrovalence. These anions drag some counter ions from the bulk solution or create an image positive charge on the metal for the required neutrality. The gateless HEMTs can be used as sensors for a range of chemicals through appropriate modification with covalently bonded halide functional groups on the Au surface. This creates many possibilities to functionalize the surface fo...

Design of edge termination for GaN power Schottky diodes

Abstract: The GaN Schottky diodes capable of operating in the 300–700-V range with low turn-on voltage (0.7 V) and forward conduction currents of at least 10 A at 1.4 V (with corresponding forward current density of 500 A/cm2) are attractive for applications ranging from power distribution in electric/hybrid electric vehicles to power management in spacecraft and geothermal, deep-well drilling telemetry. A key requirement is the need for edge-termination design to prevent premature breakdown because of field crowding at the edge of the depletion region. We describe the simulation of structures incorporating various kinds of edge termination, including dielectric overlap and ion-implanted guard rings. Dielectric overlap using 5-µm termination of 0.1–0.2-µm-thick SiO2 increases the breakdown voltage of quasi-vertical diodes with 3-µm GaN epi thickness by a factor of ∼2.7. The use of even one p-type guard ring produces about the same benefit as the optimized dielectric overlap termination.