A simple but convenient method for fabricating ZnO micro-sized structure selectively and artificially between two micron-gap electrodes was proposed and investigated. The technique is based on electrolytic deposition in aqueous solution of zinc nitrate or zinc acetate, which selectively occurs between the electrodes being enhanced by electric field. The deposition characteristics were investigated in terms of the deposition conditions, and under the optimized condition ZnO micro-sized structure was obtained with bridging between the electrodes. The gas sensing characteristics of the structure was investigated by exposing to a 500 ppm H2 gas, resulting in 23% variation of the resistance at room temperature. The device dimensions can be downsized to nanometer scale by applying nano-gap electrodes, and therefore this technique is promising for fabrication of highly functional nano-sized sensors at low cost, contributing to mass applications.

The fabrication of high-resistivity ZnO-based thin films lattice-matched to AlGaN/GaN structures has been developed. It relies on low-temperature reactive sputter deposition of ZnO:Sb from ZnSb target. Taking into account the hygroscopic nature of ZnO surface, an additional coating by Si3N4 films is applied to ensure the humidity protecition. The developped passivation suppresses leakage currents in Schottky diods, and substantially improves output characteristics of AlGaN/GaN HEMT.

Zn1−xMgxO films with wurtzite structure were electrochemically grown on the conductive glass substrates from a 0.1mol/L magnesium nitrate aqueous solution containing 0.001mol/L zinc Sulfate kept at 323K. With a decrease in the cathodic potential, Mg/(Zn+Mg) atomic ratios of Zn1−xMgxO films decreased and the lattice parameter for c axes decreased. The band gap energy of Zn1−xMgxO films increased with an increase of Mg/(Zn+Mg) atomic ratio. Thus, Mg/(Mg+Zn) atomic ratio and the bandgap energy of these Zn1−xMgxO films will be controlled by the cathodic potential.

ZnO colloidal nanocrystals have been synthesized using two different approaches and characterized by HRTEM, EDS, and photoluminescence spectroscopy. ZnO nanocrystals synthesized from zinc alkoxy alkyl precursors in the MeIm/H2O coordinating solvent showed only visible surface-defect related emission in their PL spectra. No band-to-band UV emission was observed after ZnS coating of those ZnO nanocrystals. In contrast, a strong band-to-band UV emission dominated PL spectra of ZnO nanocrystals synthesized through wet-chemical acid-catalyzed esterification of zinc acetate.

Polar ZnO(0001)-Zn surfaces can be prepared as very well defined and single crystalline surfaces by hydroxide stabilization simply by introducing hydroxides via a wet chemical cleaning step. Within this proceeding we present an in-situ AFM imaging of the crystallization process. The pH dependent stability of the resulting hydroxide-stabilized surfaces was further investigated by means of an ex-situ LEED approach. These investigations show, that it is possible to obtain high quality single crystalline ZnO(0001)-Zn surfaces in a simple way. Moreover, these surfaces turned out to be very stable within a wide range of pH values between 11 and 3 of NaClO4 based 1mM electrolyte solutions.

Nitrogen (N) is the most promising p-type dopant for zinc oxide (ZnO), and its bonding state must be governed by the substitution of N atoms into anion sites. We have synthesized un-doped ZnO and N-doped ZnO thin films by utilizing a pulsed laser deposition (PLD) method. The N-doped ZnO thin film possessed shorter c-axis length than the un-doped ZnO thin film. This fact seems to be owing to that Zn-N bond length is shorter than Zn-O bond length in wurtzite structure. Besides, from the result of Fourier transform infrared (FT-IR) measurement, the absorption peak of the N-doped ZnO thin film emerged at 406 cm−1, and was attributed to transverse optical (TO) phonon of E1 mode. The infrared lattice vibrations of the N-doped ZnO thin films can be induced by the complex factors consisting of not only the decreases in reduced mass and interionic distance, but also the increase in covalency.

The behavior of threading dislocations (TDs) in ZnO/(0001)sapphire heterostructures grown by a two-step method have been investigated. The initial template, grown by pulsed laser deposition, consisted of a continuous underlayer, which was O-polar and an overlayer comprising a high density of Zn-polar nanorods. High densities (∼7×1010 cm−2) of TDs were found to be restricted to the underlayer, whereas the nanorods were almost defect-free. Subsequent treatments by either hydrothermal growth or chemical vapour deposition (CVD) achieved epitaxial lateral overgrowth of nanorods and led to continuous Zn-polar films. The low TD density of nanorods remained until misoriented grain boundaries and boundary dislocations were generated when neighbouring nanorods become coalesced. The lateral migration of TDs in the overgrowth led to dislocation interactions and reduction of TDs. The total TD density at the top of the overlayer was estimated to be ∼1×109 cm−2 for hydrothermal growth and ∼7×109 cm−2 for CVD growth.

An overview of our recent results on characterization and modification of high-resistivity n-type bulk zinc oxide samples, grown by hydrothermal techniques, is given. Three specific topics are addressed; (i) the role of lithium (Li) as an electrically compensating impurity, (ii) extrinsic n-type doping by hydrogen implantation, and (iii) influence of annealing conditions on deep band emission. In (i), furnace annealing of as-grown samples at temperatures above ∼800 °C is shown to cause out-diffusion of residual Li impurities and concurrently, the resistivity decreases. After annealing at 1400 °C, a resistivity close to 10−1 Ωcm is obtained and the Li content is reduced from above 1017 cm−3 to the mid 1015 cm−3 range, providing evidence for the crucial role of Li as an electrically compensating impurity. For ion-implanted samples, vacancy clusters evolve during post-implant flash lamp annealing (20 ms duration) and these clusters appear to trap and deactivate Li with a resulting improvement of the n-type conductivity. However, these clusters have a limited stability and start to dissociate already after 1h at 900 °C, accompanied by a decrease in the conductivity. For topic (ii), n-type doping by hydrogen implantation is shown to enhance the conductivity by about 5 orders of magnitude already in the as-implanted state. Despite substantial loss of hydrogen, the conductivity remains stable, or even increases, after annealing up to ≥600 °C, and necessary conditions for doping by hydrogen are discussed. In (iii), the origin of the commonly observed deep band emission from monocrystalline zinc oxide is investigated using a concept of annealing as-grown samples in different atmospheres. A strong influence by the atmosphere and temperature is observed and the results can be interpreted in terms of dominant effects on the emission by vacancy-related defects.

To determine donor and acceptor concentrations affecting electrical properties in ZnO crystals, the relaxation time approximation (RTA) has been used to analyze mobility and carrier concentration data measured from 80 to 400 K. Five scattering mechanisms are included: polar-optical-phonon, piezoelectric potential, deformation potential, ionized impurity, and neutral impurity scattering. Temperature dependences of intrinsic mobilities and Hall r factors are determined. Neutral impurity (ni) scattering can play an important role in limiting the total mobility in ZnO single crystals. By including ni scattering, the experimental deformation potential E1 = 3.8 eV for ZnO can be used, rather than treating E1 as a fitting parameter. This approach yields “intrinsic” mobilities for n-type and p-type ZnO, and other II-VI materials. At 300 K, the intrinsic electron Hall mobility in ZnO is predicted to be 230 cm2/Vs. The results of fitting the temperature dependences of mobility and carrier concentrations from representative ZnO and CdSe bulk samples are shown. Intrinsic results for n-type CdS and ZnS will also be discussed. The 300-K intrinsic hole Hall mobility in ZnO is predicted to be about 50 cm2/Vs.

We report on the luminescence of rare earth (RE) (Sm, Er, Tm) ions doped ZnO films grown by the rf-magnetron sputtering technique. Samples were insitu doped with Sm ion or with Sm, Er, and Tm ions simultaneously without any intentional co-dopants and deposited on c-Si or quartz substrates at low temperatures. Selected ZnO:RE samples were thermally annealed in 200ºC-1000ºC temperature range in oxygen or argon gas at ambient pressure. As-grown and annealed samples were amorphous (a-ZnO) as was confirmed by the X-ray analysis. Furthermore, a-ZnO:RE samples were investigated by energy diffraction, photoluminescence and cathodoluminescence. In general, CL spectra of as-grown RE-doped a-ZnO films show characteristic emission lines due to 4f-shell transitions of RE3+ ions. Optical excitation of as-grown a-ZnO doped with RE ions using above the bandgap excitation resulted in strong host emission overlapped with weak RE3+ emission bands. It was observed that a thermal annealing process promotes changes of RE ions' environments resulting in significant 4f-shell transition luminescence intensity quenching.

The influence of zinc acetate concentration in the range 0.3 – 1.3M and annealing temperature in the range 450 – 550°C on the microstructure of zinc oxide films prepared by the sol-gel method was investigated. Zinc acetate concentration of the sol-gel deposited was found to be the major influence on the microstructure and in particular c-axis or (002) preferred orientation of the resulting film, due to its impact on film thickness, for a single sol-gel deposition employing fixed deposition conditions. Preferentially (002) orientated films were obtained from 0.3M zinc acetate sol-gels. However, an increase in zinc acetate concentration resulted in an increase in film thickness and a loss of preferentially (002) orientation. It was found that thick preferentially (002) oriented films could be produced by using multiple depositions of a 0.7M zinc acetate sol-gel using higher drying temperatures.

In this work, various ZnO nanostructures were synthesized and a detailed study on the effect of different process parameters such as temperature, carrier gas flow, inter-electrode spacing, gas concentration and material properties on gas sensitivity was conducted. Initial ZnO nanoparticles were prepared by a simple solution chemical process and characterized by Secondary Electron Microscopy (SEM) and Brunauer, Emmet and Teller (BET) Sorptometer to demonstrate the morphology and surface area respectively. Sensitivity of nano-platelets and porous films was measured for different concentrations of the analytes (H2, CO). High response was observed at room temperature for H2 gas with sensitivities in excess 80% for 60ppm and about 55% for 80ppm of H2 gas at room temperature were observed for the nano-platelets and the porous films respectively with short response and recovery times of about 200 seconds. The sensitivity of the nano-platelets to CO gas was also measured and found to be about near 90% for 80 ppm CO at operating temperatures of 200 °C.

Pt, Ir, Ni, Pd, and silver oxide Schottky contacts were fabricated on the Zn-polar surface of hydrothermally grown bulk ZnO. A relationship was observed between the barrier height of the contact and the free energy of formation of the “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately EC - 0.7 eV, where EC is the conduction band minimum. Valence band x-ray photoemission spectroscopy and the current - voltage characteristics of planar Schottky diodes, measured on similar Zn-polar surfaces, showed the existence of a vacuum activated surface accumulation layer. This is possibly a consequence of the observed OH termination of the Zn-polar surface. The surface accumulation layer is compensated in atmospheric conditions by the presence of acceptor-like adsorbates, such as O2 and H2O. The formation of high quality Schottky contacts to ZnO should therefore involve the reduction of near surface oxygen vacancies and the removal of H or OH from the surface.

Ga-doped ZnO layers were grown on sapphire substrates by molecular beam epitaxy (MBE). Low-temperature photoluminescence (PL) and room-temperature Raman spectra were investigated. Defect-related modes at 277 and 510 cm−1 appeared in the Raman spectrum for Ga-doped layers. The PL spectrum is dominated by a donor-bound exciton peak at 3.356 eV. A weak yellow luminescence (YL) band peaking at 2.1-2.2 eV was studied in detail. It shifted to higher photon energies (up to 0.1 eV) with increasing excitation intensity. The YL band is attributed to transitions from shallow donors to a deep acceptor. The acceptor is thought to be a Zn vacancy-related defect because the intensity of the YL band decreased dramatically with Ga doping.

The characterization of transport properties in Zn0 is known to be challenging, particularly due to surface (in the case of bulk) or interface (in the case of heteroepitaxial layers) conduction channels, which puts severe limitations on the interpretation of Hall Effect measurements. In this communication, we report on the study of transport properties of n-type ZnO bulk material using Hall mobility spectrum analysis estimated through the algorithm known as full Maximum Entropy Mobility Spectrum Analysis, f-MEMSA. The electrical properties of bulk Zn0 are measured using a Hall setup for applied magnetic fields µ0H in the range 0T-9T and for temperatures between 50K and 400K. The f-MEMSA analysis highlights the existence of two types of conduction channels in the considered ZnO substrate. We also show that surface conductive channel can be suppressed using appropriate annealing conditions.

n-ZnO/p-Pentacene and n-ZnO/poly(bis(dodecyl)quaterthiophene) (p-PQT-12) vertical p-n junction diodes were prepared on ITO-coated glass. A continuous film of ZnO nanoparticles was grown on the ITO glass by dip-coating and subsequent heat treatment of a zinc acetate film. Pentacene was then thermally evaporated to form the ZnO/Pentacene diode, whereas PQT-12 was spin coated for the ZnO/PQT-12 diode. Based on the band energies of ZnO, pentacene and PQT-12, for efficient carrier injection, gold was chosen as the top electrode to complement the ITO for both diodes. The microstructures of ZnO and pentacene films are studied by AFM and show a layer of pentacene grains with about twice the extent of the underlying layer of ZnO grains, implying substantial interfacial contact between them The current density-voltage (J-V) measurement shows that the maximum current density for ZnO/Pentacene and ZnO/PQT12 are 160 A/cm2 and 350 A/cm2 respectively. The rectification was characterized by observation of full input-half output waves. Data indicate that these devices can operate up to frequencies of 20 MHz and 9 MHz for ZnO/Pentacene and ZnO/PQT12, respectively, under ambient environment conditions. This rectification frequency is higher than other reported organic and polymer Schottky diodes under these conditions. Turn on voltages of these diodes are also much lower than for the reported organic and polymer diodes.

In this paper, we demonstrate a simple method to synthesize ZnO/TiO2 nanohybrid structure arrays based on the site-specific deposition of titanium oxide on ZnO nanorods under the hydrothermal condition. We have found that the polarity of the ZnO (0001) surface plays an important role in the formation of the nanohybrid structures. Each ZnO nanorod is assembled with one TiO2 particle only at the (0001) end surface. High-resolution transmission electron microscopy study shows that the TiO2 particles that are connected to ZnO nanorods are amorphous. By annealing at different temperatures, these particles can be transformed to nanocrystals of the anatase and rutile phases, which have a particular relationship with the orientation of ZnO nanorods and good interface structures. This work provides a rational approach to the assembly of complex nanohybrids using the intrinsic properties of ZnO nanocrystals.

The effect of growth conditions on the luminescence properties of ZnO films grown on a-Al2O3/GaN(0001)/c-Al2O3 templates by plasma-assisted molecular beam epitaxy has been investigated. We observed that the deflecting of the ions produced by the RF oxygen plasma away from substrate results in improved excitonic emission and modification of the defect-related PL spectrum. The intensity of the near-band-edge lines in photoluminescence spectra from the layers grown with the ion deflector deflection was found to increase as compared to the controls grown without the ion deflector. The yellow-green spectral range was dominated by different defect bands in the films grown with and without ion deflection. The effect of RF power on peak positions of the defect band was studied for the films grown without ion deflection.

Zinc oxide nanocrystals are synthesized using the chemical vapor synthesis (CVS) technique. Diethylzinc is used as zinc precursor. Synthesized zinc oxide nanocrystals are characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption, transmission electron microscopy (TEM) and dynamic laser scattering. As-synthesized zinc oxide has a primary particle size of about 10 nm and an isoelectric point at pH 9.5. The surface of zinc oxide nanocrystals can be modified using short chain organic acids like formic acid and glycine as well as using low molecular weight polyacrylic acid (PAA). The isoelectric point shifts towards acidic pH when glycine is used as surface modifier whereas it shifts towards basic pH with formic acid. The use of PAA eliminates the isoelectric point with very high value of zeta potential over the range of pH studied in these experiments.

Hydrogen has been proposed as one of the contributors to the native n-type doping in as-grown Zinc Oxide and can also be used as an active (intentional) n-type dopant. In this work we have employed Secondary Ion Mass Spectrometry (SIMS) to study deuterium diffusion profiles in single crystalline ZnO. The samples used are hydrothermally grown, high-resistive (10 kΩ cm) monocrystalline ZnO implanted with deuterium to a dose of 1×1015 cm−2 yielding a peak concentration of approximately 5 × 1018 cm−3 at a depth of 2.2 µm. Diffusion profiles have been studied after 30 minutes isochronal heat treatments from 100ºC up to 400ºC in steps of 50ºC. The observed redistribution can be explained by employing a diffusion model which includes trapping of 2H by Li-impurities and an activation energy of 0.85 eV is extracted for the diffusion of 2H.