Search

The competitive laser-induced local heating and recrystallization effects in ZnO nanocrystals embedded in a MgO/ZnO stack are reported via resonance Raman spectra. The dependence of the intensity, energy, and resonance effects of the longitudinal optical (LO) phonon on laser excitation condition are discussed in the context of Fröhlich interaction. Redistribution of defects, impurity-diffusion, and grain regrowth caused by thermal and photochemical effects lead to significant changes in coupling strength of electron-phonon interaction, and the resonance behaviors are strongly affected by the interplay of local heating, heat trapping, and local structural modification in such nanostructures.

Antibacterial coating approaches are being investigated to modify implants to reduce bacterial adhesion and viability in order to reduce implant-associated infection. Nanostructured materials possess unique surface properties, and nanotopographic surfaces have been reported to modulate bacterial adhesion. Zinc oxide (ZnO) films presenting well-controlled nanorod surface structures have recently been developed. To assess the efficacy of ZnO nanorod surfaces as an anti-bacterial coating, we evaluated bacterial adhesion and viability, compared to sputtered ZnO substrates (a relatively flat control) and glass substrates (as a reference). Common implant-associated pathogens, Pseudomonas aeruginosa and Staphylococcus epidermidis were investigated. The number of adherent P. aeruginosa on ZnO nanorod surfaces was found to be reduced compared to glass and sputtered ZnO, while the adherent number of S. epidermidis on the ZnO nanorods was equivalent to glass. Regarding bacteria viability, the ZnO nanorod and sputtered ZnO surfaces demonstrated a modest, but significant bactericidal effect on adherent P. aeruginosa, killing 2.5-fold and 1.7-fold more over the number of dead P. aeruginosa on glass, respectively. A greater bactericidal effect of ZnO substrates on S. epidermidis was found, with sputtered ZnO and ZnO nanorod substrates killing -20-fold and 30-fold more over the number of dead S. epidermidis on glass, respectively. These data support the further investigation and optimization of ZnO nanorod coatings with potential for bacterial adhesion resistance and bactericidal properties.

Mesoporous spindlelike iron oxide/ZnO core-shell heterostructures are successfully fabricated by a low-cost, surfactant-free, and environmentally friendly seed-mediate strategy with the help of postannealing treatment. The material composition and stoichiometry, as well as these magnetic and optical properties, have been examined and verified by means of high-resolution transmission electron microscopy and X-ray diffraction, the thickness of ZnO layer can be simply tailored by the concentration of zinc precursor. Considering that both α-Fe2O3 and ZnO are good photocatalytic materials, we have investigated the photodegradation performances of the core-shell heterostructures using organic dyes Rhodamin B (RhB). It is interesting to find that the as-obtained iron oxides/ZnO core-shell heterostructures exhibited enhanced visible light or UV photocatalytic abilities, remarkably superior to the as-used α-Fe2O3 seeds and commercial TiO2 products (P25), mainly owing to the synergistic effect between the narrow and wide bandgap semiconductors and effective electron-hole separation at the interfaces of iron oxides/ZnO.

Macrophages associated with implanted biomaterials are primary mediators of chronic inflammation and foreign body reaction to the implant. Hence, various approaches have been investigated to modulate macrophage interactions with biomaterial surfaces to mitigate inflammatory responses. Nanostructured materials possess unique surface properties, and nanotopography has been reported to modulate cell adhesion and viability in a cell type-dependent manner. Zinc oxide (ZnO) has been investigated in a number of biomedical applications and surfaces presenting well-controlled nanorod structures of ZnO have recently been developed. In order to investigate the influence of nanotopography on macrophage adhesive response, we evaluated macrophage adhesion and viability on ZnO nanorods, compared to a relatively flat sputtered ZnO controls and using glass substrates for reference. We found that although macrophages are capable of initially adhering to and spreading on ZnO nanorod substrates, the number of adherent macrophages on ZnO nanorods was reduced compared to ZnO flat substrate and glass. Additionally adherent macrophage number on ZnO flat substrate was reduced as compared to glass. While these data suggest nanotopography may modulate macrophage adhesion, reduced cell viability on both sputtered and nanorod ZnO substrate indicates appreciable toxicity associated with ZnO. Cell death was apparently not apoptotic, given the lack of activated caspase-3 immunostaining. A decrease in viable macrophage numbers when ZnO substrates were present in the same media verified the role of ZnO substrate dissolution, and dissolved levels of Zn in culture media were quantified. In order to determine long-term physiological responses, ZnO nanorod-coated and sputtered ZnO-coated polyethylene terephthalate (PET) discs were implanted subcutaneously in mice for 14 d. Upon implantation, both ZnO-coated discs resulted in a discontinuous cellular fibrous capsule indicative of unresolved inflammation, in contrast to uncoated PET discs, which resulted in typical foreign body capsule formation. In conclusion, although ZnO substrates presenting nanorod topography have previously been shown to modulate cellular adhesion in a topography-dependent fashion for specific cell types, this work demonstrates that for primary murine macrophages, cell adhesion and viability correlate to both nanotopography and toxicity of dissolved Zn, parameters which are likely interdependent., (Published by Elsevier Ltd.)

The ability to control the behavior of cells that interact with implanted biomaterials is desirable for the success of implanted devices such as biosensors or drug delivery devices. There is a need to develop materials that can limit the adhesion and viability of cells on implanted biomaterials. In this study, we investigated the use of zinc oxide (ZnO) nanorods for modulating the adhesion and viability of NIH 3T3 fibroblasts, umbilical vein endothelial cells, and capillary endothelial cells. Cells adhered far less to ZnO nanorods than the corresponding ZnO flat substrate. The few cells that adhered to ZnO nanorods were rounded and not viable compared to the flat ZnO substrate. Cells were unable to assemble focal adhesions and stress fibers on nanorods. Scanning electron microscopy indicated that cells were not able to assemble lamellipodia on nanorods. Time-lapse imaging revealed that cells that initially adhered to nanorods were not able to spread. This suggests that it is the lack of initial spreading, rather than long-term exposure to ZnO that causes cell death. We conclude that ZnO nanorods are potentially useful as an adhesion-resistant biomaterial capable of reducing viability in anchorage-dependent cells.

A mono-layer of ZnO nanoparticles (NPs), each of which does not mostly overlap with one another, was formed on the surface of a silica glass by implantation with 60 keV Zn+ ions and subsequent thermal oxidation. Then, the sample was irradiated with swift heavy ions (SHIs) of 200 MeV Xe14+ ions in the fluence range of 1 × 1011–5 × 1013 ions/cm2. The X-ray diffraction intensity of the {002} peak from ZnO NPs shows a steep drop to 67% of the unirradiated value at the fluence of 1 × 1012 ions/cm2 but maintains almost the same value up to 50 times higher fluence of 5 × 1013 ions/cm2. The behavior could be ascribed to high susceptibility of this material ZnO for recrystallization in the cooling stage of the thermal spike: While damage remains at the central region of SHI impact, recrystallization is induced in large surrounding regions. The interplay between the damage generation in the core regions and the recovery in the surrounding regions reaches a dynamical equilibrium at the fluence exceeding 1 × 1012 ions/cm2. While it is known that free excitons are sensitive to defect registration, the free exciton photoluminescence (PL) with 20% of the unirradiated intensity still survives up to the highest fluence 5 × 1013 ions/cm2. The stable tolerance of this material in optical absorption and PL against SHI irradiation could be attractive for applications. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and optically detected magnetic resonance (ODMR) techniques are utilized to examine the effects of P implantation and post-implantation annealing on defect formation in ZnO single crystals. From ODMR, the main defects created by ion implantation include oxygen and zinc vacancies as a well as a deep donor labeled as PD. The formation of the PD defect is likely promoted by the presence of P as it could only be detected in the P-containing ZnO. The VO and PD centers are found to exhibit low thermal stability and can be annealed out at 800 °C. On the other hand, a new set of defects, such as Z, T, and D* centers, is detected after annealing. Based on measured spectral dependences of the ODMR signals, the VO, VZn, and PD centers are shown to participate in spin-dependent recombination processes related to red emissions, whereas the Z, T, and D* centers are involved in radiative recombination over a wide spectral range of 1.55-2.5 eV. From the PL measurements, combined effects of implantation and annealing also lead to appearance of a new PL band peaking at ∼3.156 eV, likely due to donor-acceptor-pair recombination. The formation of the involved deep acceptor is concluded to be facilitated by the presence of P. [ABSTRACT FROM AUTHOR]

The transport properties of as-deposited and rapid thermal annealed phosphorus-doped ZnO films grown by pulsed laser deposition are reported. As-grown ZnO:P samples showed n-type characteristics, presumably due to the formation of antisite PZn defects. Rapid thermal annealing yielded a carrier-type conversion from n- to p-type for the ZnO:P films grown at ∼700 °C; samples grown at substantially lower or higher temperatures tended to remain n-type even after the thermal annealing process. The properties and behavior of the n-to-p conversion are most consistent with the formation of PZn-2VZn as the active acceptor state. Variable magnetic field Hall measurements confirmed the p-type behavior. Phosphorus doping concentrations in the range of 0.5-1.0 at. % were considered, with evidence for P segregation in the higher phosphorus concentrations. [ABSTRACT FROM AUTHOR]

We report on the determination of band offsets in rf-sputtered Aluminum Zinc Oxide (AZO)/single crystal β-Ga 2 O 3 (AZO/Ga 2 O 3 ) heterostructures using X-Ray Photoelectron Spectroscopy. The bandgaps of the materials were determined by Reflection Electron Energy Loss Spectroscopy as 4.6 eV for Ga 2 O 3 and 3.2eV for AZO. The valence band offset was determined to be −0.61eV ± 0.23eV, while the conduction band offset was determined to be −0.79 ± 0.34 eV. The AZO/Ga 2 O 3 system has a nested, or straddling, gap (type I) alignment and provides a convenient method for reducing contact resistance on Ga 2 O 3 -based device structures. [ABSTRACT FROM AUTHOR]

Highlights: [•] A critical analysis of the status of p-type doping of ZnO is presented. [•] The performance of ZnO-based light-emitting diodes is reviewed. [•] The role of defects and self-compensation in limiting the maximum p-type concentration in ZnO is emphasized. [ABSTRACT FROM AUTHOR]

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]

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]

The microstructure and growth behavior for vertically aligned Zinc oxide (ZnO) nanowires, synthesized on a ZnO thin film template by pulsed-laser deposition (PLD), is reported. The nanowire growth proceeds without any metal catalyst for nucleation, although an epitaxial ZnO thin film template is necessary in order to achieve uniform alignment. Nanowire growth at argon or oxygen background pressures of 500-mTorr results in nanowire diameters as small as 50–90 nm, with diameters largely determined by growth pressure and temperature. Room temperature photoluminescence show both near-band-edge and deep-level emission. The deep-level emission is believed caused by oxygen vancancies formed during growth. [ABSTRACT FROM AUTHOR]

ZnO is an attractive material for ultraviolet (UV) light emitters and detectors, and transparent thin-film transistors. It is also readily synthesized in the form of nanostructures. In this paper we discuss the fabrication of a number of ZnO thin-film and nanowire devices, including transistors, diodes, and UV and pH sensors. ZnO has been effectively used as a gas sensor material based on the near-surface modification of charge distribution with surface-absorbed species. The large surface area of the nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for high-density sensor arrays. [ABSTRACT FROM AUTHOR]

Enhancement-mode amorphous indium gallium zinc oxide (α-IGZO) channel thin film transistors (TFTs) with a 6 μm gate length and a 100 μm gate width were fabricated on glass substrates by rf magnetron sputtering near room temperature. The resistivities of the α-IGZO films were controlled from 10-1 to 103 Ω cm by varying the deposition power of 75–300 W. The n-type carrier concentration in the channel was 6.5×1017 cm-3. The gate oxide was 90-nm-thick SiNx, deposited by plasma enhanced chemical vapor deposition at 70 °C. The bottom-gate TFTs had saturation mobility of ∼17 cm2 V-1 s-1 and the drain current on-to-off ratio of ∼>105, a subthreshold gate-voltage swing of ∼0.5 V decade-1, and a threshold voltage of 2.1 V. In the TFT with a gate length of 6 μm and a gate width of 100 μm, the relative change of saturation mobility and threshold voltage was less than ±1.5% after 500 h aging time at room temperature. This demonstrates that α-IGZO films are promising semiconductor materials for long-term-stable transparent TFT applications. [ABSTRACT FROM AUTHOR]

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 (N2 ambient). The contacts exhibited ohmic behavior for all temperatures and show a minimum specific contact resistivity of 3.6 × 10-5 Κ cm² 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 O2 ambient led to an increase in contact resistivity by orders of magnitude. [ABSTRACT FROM AUTHOR]

All-ZnO light-emitting diodes (LEDs) offer the promise of a low-cost, brighter alternative to existing GaN-based light emitters for solid-state lighting applications, in part due to the higher exciton binding energy of ZnO. We have used ISE TCAD™ simulations to examine the effect of active, n- and p-layer dopings and thicknesses on the optical output intensity and current-voltage characteristics of both vertical and lateral geometry ZnO LEDs. The latter geometry is attractive for ultralow structures with the ZnO deposited on glass substrates. The current density distribution is more uniform in the vertical structures but there is little difference in optical output power as a function of doping or layer thickness between the two geometries. [ABSTRACT FROM AUTHOR]

Zinc oxide is generally considered to be radiation hard, although there are few experimental reports supporting this assertion. In this paper, we present results on the changes in electrical performance of bulk Pt/ZnO Schottky rectifiers exposed to 40-MeV protons at fluences from 5 x 109 cm-2 to 5 x 1010 cm-2. These doses correspond to more than 10 years or 100 years, respectively, in low-earth satellite orbit. The reverse breakdown voltage of the ZnO diodes increased from ∼3.6 V in unirradiated devices to ∼4 V after the highest proton dose. The effective barrier height decreased with proton dose, while the diode ideality factor increased from 1.8 to 1.9 for the highest dose. These devices appear promising for both aerospace and terrestrial applications where irradiation hardness is a prerequisite. The main degradation mechanism appears to be creation of recombination centers and traps. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The carbon monoxide (CO) detection sensitivities of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors were measured over a range of temperatures from 25-150 °C. Once the sensor was exposed to the CO-containing ambient, the drain current, I, of the high electron mobility transistors increased due to chemisorbed oxygen on the ZnO surface reacting with CO, forming CO2 and releasing electrons to the oxide surface. Although the sensor could detect CO as low as 100 ppm at room temperature, the detection sensitivity, ΔI/I, was only around 0.23%. By increasing the sensor temperature to 150 °C, the detection sensitivity was improved by a factor of over 30% to 7.5%. [ABSTRACT FROM AUTHOR]

Coating silicon dioxide on hydrothermally grown ZnO nanorods is demonstrated using a low temperature plasma enhanced chemical vapor deposition (PECVD) system. Wurtzite structured ZnO nanorods were prepared by spin coating ZnO nanocrystals onto plastics or glass substrates. Then, the nanorods were subsequently grown in a zinc nitrate solution. SiO2 was deposited by PECVD at 50 °C. No current could be measured through the patterned metal dots on the SiO2 coated sample, which indicates that SiO2 was covered seamlessly across the entire substrate. Photoluminescence measurements indicated that the SiO2 layer covering the nanorods did not alter the optical properties of the ZnO. [ABSTRACT FROM AUTHOR]

Thin-film transistors (TFTs) based on amorphous indium gallium zinc oxide (a-IGZO) were fabricated by radio-frequency magnetron sputtering on glass substrates. The TFT device structure was a bottom-gate type, consisting of indium zinc oxide and HfO, as electrodes (gate, source, and drain) and gate dielectric, respectively. The resistivity of the a-IGZO channel layer was ~1 Ω cm. TFTs with a 6 µm gate length and 100 µm gate width displayed a saturation mobility of ~7.2 cm² V-1 s-1, a threshold voltage of 0.44 V, a drain current on-off ratio of ~105, and subthreshold gate-voltage swing of ~0.25 V decade-1. After 1000 h aging time at room temperature, the saturation mobility remained almost constant while the threshold voltage shift was as small as 460 mV. The IGZO TETs based on HfO2 gate dielectrics sputtered near room temperature were found to be good candidates for applications on organic flexible substrates. [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]

Ti(200 Å)/Au(800 Å) Ohmic contacts to n-type amorphous indium zinc oxide (IZO) films with carrier concentrations of (1×1015)–(5×1020) cm-3 showed as-deposited specific contact resistances in the range of (3×10-1)–(1×10-4) Ω cm2. Postgrowth annealing from 200 to 500 °C resulted in significant improvement in contact resistances due to increase of the carrier concentration in the near surface region of IZO layer, which can be attributed to the formation of Ti–O alloy phases that induce oxygen vacancies in the IZO. After annealing at 500 °C, the lowest contact resistance of 8×10-6 Ω cm2 was achieved in the sample with carrier concentration of 5×1020 cm-3. Temperature dependent measurement showed that tunneling was dominant transport mechanism in the contacts on the most highly doped films (n∼5×1020 cm-3) and thermionic emission on the most lightly doped films (n∼1×1015 cm-3). [ABSTRACT FROM AUTHOR]

ZnO-based light emitting diodes were fabricated on c-plane sapphire using ZnO:P/Zn0.9Mg0.1O/ZnO/Zn0.9Mg0.1O/ZnO:Ga p-i-n heterostructures. The p-i-n heterojunction diodes are rectifying and show light emission under forward bias. The electroluminescence spectra shows deep level emission at low bias, but near band edge ultraviolet emission at high voltage bias. A decrease in leakage currents in as-fabricated structures was achieved via low temperature oxygen annealing. [ABSTRACT FROM AUTHOR]

ZnO nanorod-gated AlGaN/GaN high electron mobility transistors (HEMTs) are demonstrated for the detection of glucose. A ZnO nanorod array was selectively grown on the gate area using low temperature hydrothermal decomposition to immobilize glucose oxidase (GOx). The one-dimensional ZnO nanorods provide a large effective surface area with high surface-to-volume ratio and provide a favorable environment for the immobilization of GOx. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 s when target glucose in a buffer with a pH value of 7.4 was added to the GOx immobilized on the ZnO nanorod surface. We could detect a wide range of concentrations from 0.5 nM to 125 μM. The sensor exhibited a linear range from 0.5 nM to 14.5 μM and an experiment limit of detection of 0.5 nM. This demonstrates the possibility of using AlGaN/GaN HEMTs for noninvasive exhaled breath condensate based glucose detection of diabetic application. [ABSTRACT FROM AUTHOR]

Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from -5.5 to -6.5 V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼105. The maximum field effect mobility in the channel was ∼4.5 cm2 V-1 s-1, lower than the Hall mobility of ∼17 cm2 V-1 s-1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates. [ABSTRACT FROM AUTHOR]

The characteristics of a hybrid p-n junction consisting of the hole-conducting polymer poly(3,4-ethylene-dioxythiophene)-poly(styrene-sulfonate) (PEDOT/PSS) and n-ZnO nanorods grown on an n-GaN layer on sapphire are reported. Spin coating of polystyrene was used to electrically isolate neighboring nanorods and a top layer of transparent conducting indium tin oxide (ITO) was used to contact the PEDOT/PSS. Multiple peaks are observed in the electroluminescence spectrum from the structure under forward bias, including ZnO band edge emission at ∼383 nm as well as peaks at 430, 640, and 748 nm. The threshold bias for UV light emission was <3 V, corresponding to a current density of 6.08 A cm-2 through the PEDOT/PSS at 3 V. [ABSTRACT FROM AUTHOR]

N+ ion implantation at moderate doses (1013–1014 cm-2) into nominally undoped (n∼1017 cm-3) bulk single-crystal ZnO substrates followed by annealing in the range 600–950 °C was used to fabricate diodes that show visible luminescence at 300 K and band-edge electroluminescence at 120 K (∼390 nm) under forward bias conditions. The current-voltage behavior of the diodes are characteristic of metal-insulator-semiconductor devices and suggest the implantation creates a more resistive region in the n-ZnO in which holes are created by impact ionization during biasing, similar to the case of electroluminescence in ZnO varistors. The series resistance is only 25 Ω due to the use of the conducting ZnO substrate. [ABSTRACT FROM AUTHOR]

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 500 ppm H2 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 O?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. [ABSTRACT FROM AUTHOR]

Bulk ZnO substrates were rapid thermal annealed in either air or N2 at temperatures up to 500°C. The root-mean-square roughness of the surface as measured by atomic force microscopy begins to increase even after 200°C anneals in N2 or 300°C in air. The Schottky barrier height, [uppercase_phi_synonym]B, obtained from Pt/Au diodes fabricated on these surfaces shows a decrease in effective barrier height upon annealing from 0.35 eV in control samples to ∼0.30 eV in those annealed at 400°C and an increase in the saturation current density (from 4 A cm-2 in the control samples to 17–37 A cm-2 in the annealed samples). These results show that the ZnO surface is degraded by quite low annealing temperatures and care must be exercised in designing the thermal budget for processing of ZnO devices. At much higher annealing temperatures (1000°C), the surface completely decomposes to leave metallic Zn droplets. [ABSTRACT FROM AUTHOR]

The electrical characteristics of Zn0.9Mg0.1O/ZnO p–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4 V for Pt/Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9 V, but displays a small negative temperature coefficient of -0.1–0.2 V K-1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current. [ABSTRACT FROM AUTHOR]

UV-ozone cleaning prior to metal deposition of either e-beam Pt contacts or sputtered W contacts on n-type single-crystal ZnO is found to significantly improve their rectifying characteristics. Pt contacts deposited directly on the as-received ZnO surface are Ohmic but show rectifying behavior with ozone cleaning. The Schottky barrier height of these Pt contacts was 0.70 eV, with ideality factor of 1.5 and a saturation current density of 6.2×10-6 A cm-2. In contrast, the as-deposited W contacts are Ohmic, independent of the use of ozone cleaning. Postdeposition annealing at 700°C produces rectifying behavior with Schottky barrier heights of 0.45 eV for control samples and 0.49 eV for those cleaned with ozone exposure. The improvement in rectifying properties of both the Pt and W contacts is related to removal of surface carbon contamination from the ZnO. [ABSTRACT FROM AUTHOR]

Pt/ZnO Schottky diodes show changes in forward current of 0.3 mA at a forward bias of 0.5 V or alternatively a change of 50 mV bias at a fixed forward current of 8 mA when 5 ppm of H[sub 2] is introduced into a N[sub 2] ambient at 25 °C. The rectifying current–voltage (I–V) characteristic shows a nonreversible collapse to Ohmic behavior when as little as 50 ppm of H[sub 2] is present in the N[sub 2] ambient. At higher temperatures, the recovery is thermally activated with an activation energy of ∼0.25 eV. This suggests that introduction of hydrogen shallow donors into the ZnO is a contributor to the change in current of the diodes. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ti/Al/Pt/Au ohmic contacts on n-type ZnO with a range of carrier concentrations (7.5×10[sup 15]–1.5×10[sup 20] cm[sup -3]) show as-deposited specific contact resistances in the range from 3×10[sup -4] to 8×10[sup -7] Ω cm[sup 2]. Temperature-dependent measurements showed that the dominant transport mechanisms were tunneling in the contacts in the most highly doped films and thermionic emission in the more lightly doped films. After annealing at 200 °C, the lowest specific contact resistance achieved was 2.2×10[sup -8] Ω cm[sup 2]. However, the contacts show evidence of reactions between the Ti and the ZnO film even for this low annealing temperature, suggesting that applications requiring good thermal stability will need metallurgy with better thermal stability. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to [SUP2]H plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼8 × 10[SUP-10] cm[SUP2]/V·s at this temperature. The activation energy for diffusion was 0.17 ± 0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500-600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5 × 10[SUP15] cm[SUP3]). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage in the former case. [ABSTRACT FROM AUTHOR]

Bulk, single-crystal ZnO was etched in Cl[sub 2]/Ar and CH[sub 4]/H[sub 2]/Ar inductively coupled plasmas as a function of ion impact energy. For CH[sub 4]/H[sub 2]/Ar, the etch rate (R) increases with ion energy (E) as predicted from a model of ion enhanced sputtering by a collision–cascade process, R∝(E[sup 0.5]-E[sub TH][sup 0.5]), where the threshold energy, E[sub TH], is ∼96 eV. Band edge photoluminescence intensity decreases with incident ion energy in both chemistries, with a 70% decrease even for low energies (∼116 eV). Surface roughness is also a function of ion energy with a minimum at ∼250 eV, where Auger electron spectroscopy shows there is no measurable change in near-surface stoichiometry from that of unetched control samples. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on catalyst-driven molecular beam epitaxy of ZnO nanorods. The process is site specific, as single crystal ZnO nanorod growth is realized via nucleation on Ag films or islands that are deposited on a SiO[sub 2]-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300-500 °C. The nanorods are uniform cylinders, exhibiting diameters of 15-40 nm and lengths in excess of 1 µm. With this approach, nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics. [ABSTRACT FROM AUTHOR]

Abstract: (0001) α-Al2O3 single crystals (sapphire) were implanted with Zn ions of 60keV at a fluence of 1×1017 ions/cm2. Transmission electron microscopy and optical absorption spectroscopy studies show the formation of ZnO nanoparticles in the sapphire substrate after the implanted sample was annealed at 700°C in oxygen ambient. The photoluminescence spectrum of the annealed sample indicates the formation of ZnO nanoparticles with perfect lattice structure. The selected-area electron diffraction pattern proves that the ZnO nanoparticles have the (0002) orientation which follows the orientation of Al2O3 substrate. The result shows that the crystallographic orientation of nanoparticles obtained through ion implantation is defined by the substrate. [ABSTRACT FROM AUTHOR]

We have measured the band offsets of sputtered ZrSiO 4 on bulk ZnO wafers using X-Ray Photoelectron Spectroscopy and obtained the bandgaps of these two materials using reflection electron energy loss spectroscopy. The valence band offset was determined to be −0.60 eV ± 0.04 eV for ZrSiO 4 on ZnO, while the respective bandgaps were 3.22 eV for ZnO and 5.9 eV for ZrSiO 4 . The conduction band offset for ZrSiO 4 /ZnO was then determined to be 3.28 eV. The ZrSiO 4 /ZnO system has a staggered, type II alignment. This means that while ZrSiO 4 may be useful for surface passivation applications on ZnO, it is not suitable for thin film transistors where a positive valence band offset is needed. [ABSTRACT FROM AUTHOR]

Abstract: ZnO nanowires were grown on a-plane GaN templates by chemical vapor deposition (CVD) without employing a catalyst. The a-plane GaN templates were pre-deposited on an r-plane sapphire substrate by metal-organic CVD. The resulting ZnO nanowires grow in angles off- related to the GaN basal plane. X-ray diffraction (XRD) spectra showed that the ZnO layer was grown with a heteroepitaxial relationship of (110)ZnO||(110)GaN. Photoluminescence spectra measured at 17K exhibited near-band-edge emission at 372nm with a full width at half maximum of 10nm. The growth mechanism on a-GaN was the Volmer-Weber (VW) mode and differed from the Stranski-Krastanow (SK) mode observed for growth on c-GaN. This difference results from the higher interfacial free-energy on the a-plane between ZnO and GaN than that on the c-plane orientation. [Copyright &y& Elsevier]

Abstract: Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900°C for 1h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was ∼200nm with a length of 1μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu3 ions was not present. Transmission electron microscopy shows crystalline Eu2O3 formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp 5DO–7F2 transition of the Eu3+ ion at around 615nm was observed. [Copyright &y& Elsevier]

Abstract: The hydrothermal growths of the ZnO nanorods with the densities ranging from 157 to 73nanorods/μm2 were achieved by diluting the ZnO seed solution. However, the ZnO seed nanocrystals started to agglomerate for the seed solution diluted below 1% of the original nano-crystalline solutions and resulted in the formation of clustered nanorods. With the assistance of a surfactant, Triton X-100, the nanorod density can be further reduced to 4nanorods/μm2. The diameters of the nanorods depended on the concentration of the seed solution and agitation speed of the nanorod growth solution. More diluted seed solution used and less agitation of the growth solution, the larger diameter of the nanorods was obtained. This indicated that the nanorod growth mechanism was controlled by the diffusion of reactants. With sufficient agitation of the growth solution, the nanorod can be uniformly grown with subjects on any arbitrary geometry. We have demonstrated ZnO nanorods growth on both inside and outside of biliary stents as well as on nitinol wires used as metal stents. The effect of nanorod density on the NIH 3T3 and HUVEC cells growth was also investigated in this study and the results suggested nanorod-coating to be a suitable method for controlling cell adhesion and viability on implantable devices. [Copyright &y& Elsevier]

Abstract: The optical properties of N-doped ZnO films grown by pulsed laser deposition are examined for which zinc nitride is used as the source of nitrogen. The motivation for this study is to determine if nitrogen-related acceptor state formation can be achieved in ZnO films using Zn3N2 doping in the ablation target. The films were deposited in oxygen or nitrogen on c-plane sapphire. Photoluminescence measurements at 20K reveal a 3.31eV acceptor-bound exciton emission due to nitrogen substitution on the oxygen site, donor–acceptor pair emission at 3.23±1eV and free electron-acceptor at 3.27eV. The binding energy of the N-related acceptor is estimated to be in the range of 170–15meV. While the as-deposited films were n-type, thermal annealing in oxygen yielded insulating behavior, consistent with compensating acceptor states. [Copyright &y& Elsevier]

Abstract: We report a study of the annealing temperature and time on Ag catalyst size and density for subsequent growth of ZnO nanorods by catalyst-driven molecular beam epitaxy (MBE). Two different substrates (SiO2 and SiN X ) for the Ag deposition were used and the thickness of the Ag held constant at 25Å. Annealing between 600 and 800°C produced Ag cluster sizes in the range 8–30nm diameter on SiO2 and 10–65nm on SiN X with a cluster density from 100 to 2500mm−2 for SiO2 and 30 to 1900mm−2 for SiN X . ZnO nanorods grown on these clusters show single-crystal, wurtzite-phase nature and strong band-edge photoluminescence at 380nm. The nanorods can also be grown selectively on lithographically-patterned dielectric stripes with Ag clusters formed on top by e-beam evaporation and annealing. [Copyright &y& Elsevier]

Abstract: There is a strong interest in developing thermally stable metallization schemes for ZnO and boride-based contact stacks are expected to have potential because of their thermodynamic stability. The contact characteristics on bulk single-crystal n-ZnO of a ZrB2/Pt/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 300–800°C. The contacts were rectifying for anneal temperatures <500°C but exhibited Ohmic behavior at higher temperatures and exhibit a minimum specific contact resistivity of 9×10−3 Ωcm after 700°C anneals. The contact stack reverts to rectifying behavior after annealing above 800°C, coincident with a degraded surface morphology and intermixing of the Au, Pt and ZrB2. The boride-based contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on ZnO. [Copyright &y& Elsevier]

Abstract: Recent efforts on doping ZnO films for charge and spin functionality are reviewed, focusing on chemical doping for charge and spin device formation. Discussion includes the behavior of phosphorus as an acceptor and magnetism in transition metal-doped ZnO. Evidence for p-type behavior in phosphorus-doped (Zn,Mg)O grown by pulsed laser deposition is presented. The magnetic properties of ZnO co-doped with Mn and Sn are also discussed. [Copyright &y& Elsevier]