In this paper we describe gate bias and temperature induced device instabilities of inverted-staggered ZnO-TFTs. It is shown that low positive and negative gate bias results in the transfer characteristics shifting in a positive and negative direction respectively. It is suggested that this is a consequence of temporary charge trapping at or close to the channel/insulator interface. The degradation of device parameters such as the threshold voltage, subthreshold slope and effective channel mobility is demonstrated at elevated measurement temperatures, suggesting the generation of defects and/or trap states in the interfacial region. In addition, it is postulated from the extracted activation energy of the drain current that the Fermi-level is pinned during the operation of the devices due to the high level of states close to the conduction band edge. These results highlight the relatively ease with which defects could be created at the interface, indicating a high concentration of weak or strained bonds. Both charge trapping and defect creation-induced instabilities appear to be reversible, as all devices regain their original characteristics after a period of relaxation at room temperature.

Bulk ZnO grown by the hydrothermal technique was investigated using electron paramagnetic resonance (EPR), photoluminescence (PL), and infrared absorption (FTIR) techniques. Isolated subsitutional lithium is the dominant acceptor and could be detected using EPR or PL. A large concentration of neutral Li+-OH− centers were observed using FTIR data. EPR spectra assigned to Mn, Co, Ni, Fe, and Group III (Al, Ga) donors were also observed. Photoinduced changes in the charge states of the different deep and shallow centers were produced using 325 nm light, and the stability of these changes were monitored with EPR during pulsed thermal anneals. The charge-state changes for some defects were persistent and remained up to 300 K. These impurities, when present in device structures, may act as stable charge trapping sites.

Epitaxial ZnO films are successfully grown on Al2O3 substrates with phase controlled CrN buffer layer using Zn and O-plasma pre-exposures on CrN layers by plasma assisted molecular beam epitaxy (P-MBE). The Zn exposures on CrN layers prior to ZnO film growth result in the formation of rocksalt CrN without surface oxidation. On the other hand, the surface of the initially deposited CrN layers with rocksalt structure changes into hexagonal structured Cr2O3 after O-plasma exposure as confirmed by reflection high-energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HR TEM). Etching studies show that the ZnO films grown on CrN have +C polarity, while the polarity of ZnO on Cr2O3/CrN double buffer is -C polarity. The interdiffusion of Zn and Cr occurs at the ZnO/CrN interface, while the interdiffusion is negligible at the ZnO/ Cr2O3 interface. The interdiffusion of Cr and Zn can be suppressed by inserting a low-temperature ZnO buffer layer in between ZnO and CrN layers, which helps improve the crystal quality of ZnO layers grown with CrN buffer.

MgxZn1-xO/ZnO/MgxZn1-xO quantum wells (QWs) (0.12 ≤ x ≤ 0.15) have been grown on a-plane sapphire substrates by pulsed laser deposition. The nominal ZnO well layer thickness lies between 1.2 nm and 6 nm. Atomic force microscopy (AFM) investigations at ZnO/MgxZn1−xO heterostructures show the film-like structure of the ZnO layers. Their root mean square surface roughness of ∼ 0.5 nm gives information about the interface roughness in the QWs. AFM results from the MgxZn1−xO barrier layers show the same surface structure and roughness. We confirmed the lateral homogeneity of the Mg distribution in the MgxZn1−xO barrier layers by scanning cathodoluminescence measurements. The QWs show a bright and laterally homogeneous luminescence, suggesting good crystalline quality of the ZnO wells. The measured QW photoluminescence energies agree well with calculated values and display the presence of the quantum-confined Stark effect. As a result of quantum confinement a high-energy shift of the ZnO excitonic photoluminescence of 222 meV is observed in the thinnest QW.

The xZnO-(1-x)α-Fe2O3 nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 hours. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.

We report the elaboration of Ni doped ZnO nanoparticles prepared by a sol-gel processing technique. In our approach the water for hydrolyse was slowly released by esterification of the metal acetate with methanol followed by a supercritical drying in ethyl alcohol. Doping concentrations between 5 and 25 at% have been investigated. In the as-prepared state the powders with an average particle size of 30nm present ferromagnetic properties; thermal annealing in the 500°C to 700°C temperature range in air or oxygen modifies the magnetic properties. We ascribe the observed ferromagnetism to the presence and transformation of Ni based secondary phases.

Size dependence of the low frequency vibrational spectra of ZnO nanocrystals prepared using chemical method has been investigated. Optical transmission spectra of the ZnO colloid solution exhibit a shift in the onset of the absorption band edge from 332 to 350 nm due to particle growth. X-ray diffraction analysis of the prepared ZnO nanocrystals exhibit peaks corresponding to the hexagonal wurtzite structure. Two peaks with unusually very high intensity were observed in the low frequency (∼ 10- 25 cm-1) Raman spectra of these nanocrystals. The peak position of these phonon modes shifted towards lower frequencies as the size of the nanocrystals increases and assigned to the confinement of acoustic phonons in ZnO nanocrystals.

Electrical properties of n-ZnO/n-GaN isotype heterostructures obtained by rf-sputtering of ZnO films on GaN layers grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-ZnO/n-GaN diodes revealed highly rectifying behavior with forward and reverse currents ∼1.43×10-2 A/cm2 and ∼2.4×10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built using temperature dependent current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current measurements the minority carrier diffusion length in ZnO was estimated in the range 0.125-0.175 mm, depending on excitation conditions. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073 V.

We have studied by electron paramagentic resonance spectroscopy the Mn2+ EPR spectra in low doped (x=0.03) Zn1-xMnxO films of n-type conductivity prepared by metalorganic chemical vapour deposition. Contrary to higher doped films with x=0.07, the x=0.03 film shows a weak ferromagnetic Mn-Mn interaction which we attribute to the presence of moderate deep donors leading to magnetic polaron formation.

ZnO thin films are of interest for an array of applications; including: light emitters, photovoltaics, sensors and transparent contacts among others. Of particular interest is the potential to produce p-type layers from which p-n junction devices could be routinely produced. While it is fairly routine for MOCVD to produce n-type films with doping concentrations in the 10E20 cm-3 range and resistivities below 10E-3 ohm-cm; it is very difficult to produce measurable p-type ZnO. We report on our efforts with doping films p-type using N gas sources and metalorganic sources of P, As, and Sb. Films showing acceptor bands by photoluminescence have been demonstrated; however reliable electrical measurements remain difficult. Specific problems include achieving low resistance ohmic contacts, accounting for the photo-responsiveness of ZnO films and sensitivity limits in Hall measurements of low-doped and compensated materials. The presentation will review deposition parameters, produced and processed films and material characteristics.

Excitation of a luminescence by highly exothermic chemical reaction on the surface of a luminophore provides a unique opportunity to separate surface luminescence from the bulk luminescence. This enables studies of the electronic properties of the semiconductor surfaces even if the surfaces are of complicate shapes. We have studied heterogeneous chemiluminescence (HCL) of ZnO powders. The luminescence was excited by a release of chemical energy, namely by catalytic recombination of hydrogen atoms. The HCL spectra were compared to the photoluminescence (PL) spectra. The HCL spectra were sensitive to the details of preparation and treatment whereas PL spectra almost did not change. HCL spectra of powder samples pretreated for enhancing “green” luminescence exhibited long-wavelength tail (up to 800 nm) and their maximum was blue-shifted as compared with PL spectra. Different HCL bands forming long-wavelength tail were isolated by changing the temperature of the samples. Additional milling of ZnO led to amplification of the HCL-specific surface bands. Pure ZnO showed neither PL nor HCL; however we were able to observe HCL surface bands with maxima at 610 nm and 730 nm after treatment of the sample in atomic hydrogen atmosphere at 570 K. Remarkably, such treatment did not cause appearance of the PL. The HCL in the presence of atomic hydrogen was steady in time and was caused by an abstraction of adsorbed hydrogen by incident hydrogen atoms, i.e. the reaction followed Eley-Rideal mechanism. The HCL can be utilized for in situ monitoring of the growth and evolution of ZnO in controlled atmosphere.

Highy-quality nano-structured ZnO samples have been synthesized by simple chemical solution and post-thermal treatment. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), temperature-dependent photoluminescence (PL) spectra measurements. XRD patterns illustrated that there were no second phases in these ZnO samples, and the TEM results indicated that the ZnO samples are single crystalline with a hexagonal structure. Room-temperature PL spectra of ZnO thin films showed a strong UV near-band-edge (NBE) emission located at about 390 nm and a green defect-related (G) emissions, where the intensity ratio (INBE/IG) varies with the annealing temperatures. Meanwhile, the ZnO samples exhibited free exciton and very sharp exciton emissions at low temperatures. Particularly, room-temperature UV random lasing characteristic of ZnO films has been observed as well. It is shown that these nano-structured ZnO samples can exhibit random laser action depending on the growth condition. The threshold intensity for the lasing is comparable to earlier reported data. These results indicate that nano-structured ZnO samples prepared by simple techniques may be a promising material for further photonic devices. Possible lasing mechanism is discussed and further investigation to clarify the mechanism between the nano-structured ZnO samples is still underway.

The structure and magnetism of ZnO-based solid solutions, dilute magnetic semiconductors, with nickel solute were obtained via a solvothermal method. Compared with previous methods for solid solution DMSs, our synthesis method was really facile and economical. The one-dimensional solid solution of Zn1-xNixO nanostructures were grown in a alcoholic solution. Moreover, the percentage of doped nickel can be easily controlled. The X-ray diffraction, transmission electron micrograph and magnetization hysteresis loops of nickel-doped ZnO nanocrystals were presented to confirm that the nickel impurities are embedded inside the nanocrystal.

In this paper, ZnO thin films deposited by two methods have been studied. Specifically, the films were grown using i) Laser Assisted Molecular Beam Deposition (LAMBD) and ii) RF sputtering. Subsequent to film deposition, a subset of samples deposited using LAMBD were laser annealed. An additional set of samples (from LAMBD and RF sputtering) were annealed with N2 or forming gas at 600°C for 30mins.

After deposition, optical and electrical properties of ZnO thin films have been studied. The application of ZnO to optical devices, including Metal-Semiconductor-Metal Photodetectors (MSM-PD) and solar cells, has been made. Several deposition experiments recently demonstrated that the thin films of RF-ZnO and LAMBD-ZnO have near ZnO parameters including refractive index close to 2, 1:1 stoichiometry ZnO, and 3.3 eV ZnO bandgap. Mixtures of single crystal and polycrystal grains were observed by Transmission Electron Microscopy (TEM) from LAMBD ZnO thin films. MSM current-voltage data show symmetrical photo current behavior. High ratio of photocurrent to dark current, good responsivity and fast pulse response of LAMBD-ZnO MSM were observed. ZnO/Si heterojunction solar cell result has been demonstrated and improvement in the ultraviolet light spectrum of spectral response has been shown in this paper.

Nanoindentation studies were conducted on a-axis oriented ZnO single crystals. The mechanical properties and deformation mechanisms were monitored and compared to previously determined data from c-axis material. Hardness and modulus values reveal that a-axis ZnO is significantly softer than c-axis material (measured hardness of 2 ± 0.2 GPa) and behaves much more plastically. Additionally, the influence of contact induced damage upon the defect structure of a-axis material was also examined using cathodoluminescence spectroscopy and monochromatic imaging to monitor the luminescence from indent sites. Deformation directly under the indent site enhanced the occurrence of red defect luminescence, and was attributed to a native defect in ZnO that has a higher formation energy than the defects responsible for the green and yellow visible defect bands, which were present in the ZnO during growth and were found to cluster to the indent site during annealing.

We fabricated well-aligned ZnO nanorods with thin diameter by two-step chemical vapor deposition method combined with laser ablation of a sinterd ZnO target. Firstly, well-aligned ZnO nanorods with thick diameter of about 110 nm are grown on an n-Si (111) wafer. Next, a thin ZnO nanorod with about 30 nm diameter is grown on the center of the flat tip of each well-aligned and thick ZnO nanorod by controlling a flow rate of oxygen. Although thick ZnO nanorods do not emit a recognizable field emission current, thin ZnO nanorods with 1500 nm length show a field emission current of 500 μA under an electric field of 31 V/μm.

Nanocrystalline transition metal (TM=Co, Mn, Ni, Fe) doped zinc oxide powders have been elaborated by a new protocol based on slow hydrolyse of zinc acetate dissolved in methanol and supercritical drying in ethyl alcohol. The powders have a narrow size distribution with an average value of ∼ 25nm. Electron microscopy characterization showed that the size of the ZnO:TM particles did not change significantly for the different dopants. High doping levels of up to [TM] =0.25 have been investigated. X-ray diffraction studies showed the formation of the ZnO wurtzite phase for all dopants but secondary phases are equally detected. High temperature ferromagnetism was observed for Ni and Co doped powders whereas Mn doped powders showed only antiferromagnetic interactions. EPR spectroscopy indicates that the magnetism is related to the presence of extrinsic phases.

ZnO is grown by chemical vapour deposition on {1 1 -2 2} GaN planes formed by epitaxial layer overgrowth. Window stripes in a SiO2 mask are oriented in the <1 -1 0 0> direction of the GaN film. Triangular GaN ridge are formed during ELO growth by metal organic chemical vapour deposition. A flat (0 0 0 1) ZnO plane is grown on each triangular cross-section ridge and it is found that the ZnO film has dislocation density reduced by two orders of magnitude compared to that of the GaN substrate.

ZnO thin films were prepared on borosilicate glass from both single- and multi- step coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH ] and isopropanol. ZnO films prepared over a range of zinc acetate concentrations, for a fixed annealing temperature, showed that sol-gels prepared with a 0.3M zinc acetate concentration resulted in the formation of films with the greatest degree of c-axis orientation. In this study, a detailed investigation of the influence of process annealing temperature over the range 450 – 550°C on the microstructural, physical, electronic and optical properties of these single and multi-step ZnO thin films around this 0.3M zinc concentration set point is presented. X-ray analysis showed that all single-step deposition thin films were preferentially orientated along the [002] c-axis direction of the crystal. In contrast, only the multi-layer film annealed at 550°C showed similar preferential orientation. All single step deposited films showed a similar average optical transmittance above 87%, independent of annealing temperature. The transmittance of the multi-step films was shown to be strongly correlated to the degree of c-axis orientation. The optical band-gap energy was evaluated to be 3.298 – 3.316 eV for all samples. The photoluminescence spectra of the single layer ZnO films showed a strong emission centred at ca. 405 nm, which blue shifted with increasing annealing temperature. The multi-layer ZnO samples emitted throughout the UV and the visible range, with the samples prepared at 500 and 550°C showing the expected ZnO emission peak at 380 nm. Despite being thicker, the emission from the multi- layer samples was less than measured for the single layer samples. The effect of sol-gel annealing temperature and deposition process on film microstructure, morphology, electrical resistivity and optical transparency is detailed.

Transition metal-doped ZnO bulk crystals and thin films have been investigated to determine the effects of transition metal incorporation on optical, magnetic, and structural properties of ZnO. A modified melt growth technique was used to grow bulk Zn1-xMnxO, Zn1-xCoxO, and Zn1-xFexO. Optical transmission measurements show an apparent shift in absorption edge with increasing transition metal incorporation. Raman spectroscopy also shows increasing lattice disorder with increasing transition metal concentration. ZnO thin films doped with Ni, Co, and Gd were grown by metalorganic chemical vapor deposition (MOCVD). While the Co-doped thin films showed antiferromagnetic behavior, magnetic hysteresis was observed in the Ni-doped and Gd-doped thin films. Structural quality was verified with X-ray diffraction (XRD), and optical properties were investigated using room temperature photoluminescence (PL) and optical transmission measurements. Properties of ZnO:TM bulk crystals and thin films are compared and used to discuss possible origins of ferromagnetism in these materials.