The effect of doping of transition metal ions (Fe and Co) on transport properties of ZnO has been studied in both bulk and thin films. The solubility limit of these ions have been found to be higher in thin films compared to bulk. Optical measurements reveal the presence of Fe in both 2+ and 3+ state. Co is believed to be in 2+ states. Electrical resistivity measurements show that while for bulk Fe doped ZnO samples there is a decrease in resistivity compared to undoped ZnO, it increases for bulk Co doped ZnO samples. However, thin film samples of both types of doped compounds show a decrease in resistivity compared to undoped ZnO. This difference in bulk and thin film behaviour has been explained on the basis of experimental results.

Here we present a convenient solution-based method, which can afford a procedure to easily fabricate highly oriented ZnO nanorods on substrate at relatively low temperatures. The as-synthesized products have been characterized by scanning electron microscopy (SEM) and X-Ray diffraction (XRD). The results revealed that a densely packed and perpendicularly oriented single-crystalline ZnO nanorod arrays grew vertically on the fluorine-doped SnO2 transparent conducting oxide (FTO) glass substrates. In addition, we found that the length of the nanorod could be freely modified by controlling the solution temperature.

Electroplating of ZnO nanowires was conducted using gold embedded anodized aluminum oxide (AAO) films on Si substrates. For electroplating, insulating layers at the bottom of AAO nanohole structures need to be removed. After electroplating, hexagonal structure of vertical ZnO nanowires was observed, however, they were broken and lied down by thermal annealing process. Photoluminescence (PL) spectra were investigated and that of post annealed ZnO nanowires indicates that nitrogen atoms were incorporated as acceptor.

Ru Schottky barrier diodes (SBD's) were fabricated on the Zn face of n-type ZnO. These diodes were irradiated with 1.8 MeV at fluences ranging from 1 ´ 1013 cm-2 to 2.4 ´ 1014 cm-2. Capacitance and current (I) deep level transient spectroscopy (DLTS) was used to characterise the irradiation induced defects. Capacitance DLTS showed that proton irradiation introduced a level, Ep1, at 0.52 eV below the conduction band at an introduction rate of 13±1 cm-1. A defect with a very similar DLTS signature was also present in low concentrations in unirradiated ZnO. I-DLTS revealed that this proton irradiation introduced a defect with an energy level at (0.036± 0.004) eV below the conduction band. This defect is clearly distinguishable from a defect with a level at (0.033± 0.004) eV below the conduction band that was present in the unirradiated sample. It is speculated that these shallow level defects are related to zinc interstitials or complexes involving them.

The optical and electrical properties of commercially available bulk ZnO samples grown by the hydrothermal, vapor-phase, and melt methods are compared. Low-temperature photoluminescence (PL) is used to identify the presence of common impurities, such as H, Al, Ga, and In, and temperature-dependent Hall-effect (T-Hall) measurements are used to obtain donor energies and to quantify donor and acceptor concentrations. All three types of material produce sharp donor-bound-exciton (D0X) PL lines, I4, I6, I8, and I9, generally associated with H, Al, Ga, and In, respectively. However, the I4 and I9 lines are weak in hydrothermal ZnO, and the I4 line is also weak in melt material. Another D0X line I5, possibly associated with donors near the surface, appears in some hydrothermal samples. Electrically, hydrothermal ZnO has a much lower bulk conductivity than vapor-phase or melt ZnO, partly because the donor concentration is lower, but also because the acceptor concentration is higher. A consequence of the low bulk conductance is that surface conductance becomes much more important and must be included in the Hall-effect modeling. An interesting result of this study is a close correlation between the PL-derived Al (I6) donor energy of 51 meV, and the T-Hall-derived energy of 52 meV.

The electrical properties of epitaxial ZnO thin films grown on sapphire substrates by pulsed laser deposition were investigated by temperature dependent Hall measurents. The thin films investigated were grown at different oxygen partial pressures ranging from 10-2 mbar to 1 mbar. The formation of a degenerate layer, determining the low temperature Hall data, depends on the oxygen partial pressure applied during growth. Further, the formation of such a layer can be correlated to the grain size of the samples. The thermal activation energy of dominant donors decreases in tendency with increasing oxygen partial pressure p(O2); it is about 100 meV for p(O2) ≤ 3 × 10-2 mbar and about 30 meV for p(O2) ≥ 0.1 mar. The concentration of donors and compensating acceptors increases with increasing p(O2).

ZnO as a direct wide-band-semiconductor with its band gap of 3.3 eV at room temperature is a promising optoelectronic material. The main obstacle in the ZnO system is its lack of achieving reproducible p-type conductivity. The main reasons for this are the high residual intrinsic and extrinsic defect concentrations which are still not completely understood.

Homoepitaxial growth of ZnO and thus minimization of intrinsic defects due to lattice mismatch and incorporation of residual substrate species could be a solution to overcome these problems. Despite the availability of ZnO bulk single crystals reports regarding ZnO homoepitaxy are still quite rare. In this paper we report on a successful homoepitaxial growth of ZnO thin films by chemical vapor deposition (CVD).

Atomic force microscopy shows that two-dimensional epitaxial growth was achieved without any additional buffer layer. With a rocking curve full width at half maximum (FWHM) of 17 arcsec the deposited films show a superior crystalline quality compared to its substrate. The optical quality of the epitaxial films has been characterized laterally by cathodoluminescence and spectrally by photoluminescence. Excitonic emissions at 4K are as narrow as 110 μeV. A dependence of the appearance of excitonic emissions from the growth polarity can be shown which is attributed to different incorporation rates of extrinsic defects.

We report the correlations between processing, microstructure and electrical properties of Ga doped ZnO films. Films with varying grain size were grown on amorphous glass by changing the substrate and pulsed laser deposition variables. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. Microstructural characteristics were analyzed in detail by using X-ray diffraction and transmission electron microscopy. Electrical properties were evaluated by resistivity measurements in the temperature range of 15-300K and Hall measurements at room temperature. It was observed that the grain boundaries and orientation of grains (texture characteristics) affected the carrier concentration and the mobility considerably in nanocrystalline films deposited on glass substrates. This effect is envisaged to occur as a result of trapping of electrons and build up of a potential barrier across the grain boundaries. However, the resistivity in nanocrystalline films could be decreased significantly by carefully controlling the deposition conditions. For a film deposited on glass at 2000C and 1 mtorr of oxygen partial pressure, we attained a minimum resistivity value of 1.8 × 10-4Ω-cm. As a comparison, the epitaxial films on sapphire substrates showed a resistivity of 1.4 × 10-4 Ω-cm, deposited at 4000C and pressure of 2.4 × 10-2 torr. Role of grain boundaries and defects in controlling the carrier generation and transport is considered in detail and possible mechanisms limiting the electrical conductivity in films with different microstructures are identified.

Photodetectors based on wide-bandgap semiconductors have demonstrated several advantages over traditional ultraviolet (UV) detectors (photomultiplier tubes and Si-based UV detectors) such as low power consumption, high stability, and no need of other optical filters. ZnO stands a good chance of being a candidate material for solar-blind UV detection because of its direct bandgap of 3.37eV and high photoresponse. In this work, single crystal ZnO microtubes synthesized using a microwave-heating growth method and their UV photodetection properties were studied. The ZnO microtubes exhibited relatively fast UV photoresponse with a cut-off wavelength ∼370 nm, indicating their potential applications as high efficient and low cost UV detectors.

Deposition of Zn1-xMgxO thin films on glass substrates has been investigated by the simple and cost-effective mist CVD technique. A water solution of zinc acetate and magnesium acetate was used as the source of Zn and Mg. The solution was ultrasonically atomized, and the aerosols hence formed were supplied by the N2 carrier gas to the substrates. The band gap energy of ZnMgO was successfully controlled from 3.25 eV (ZnO) to 3.75 eV with the concentration ratio [Mg]/([Zn]+[Mg]) in the solution. The transparency in the visible region was higher than 90% and the surface RMS roughness was 7.5 nm (an example for ZnO) despite the polycrystalline structure; they are satisfactory for the optical applications. A UV photodetctor with interdigital electrode structure on the ZnMgO surface was fabricated, where the photoresponsivity of 2.6 A/W at 350 nm and the lowest detectable power of about 1 μW were obtained. Although these values are satisfactory for the simple UV detection but the existence of deep defects is deteriorating the dynamic response of the detector device.

Zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. Chemical solution deposition (CSD) is attractive technique for obtaining ZnO thin films and has the advantages of easy control of the film composition and easy fabrication of a larger-area thin film at low cost. In this work, epitaxial ZnO thin films on SiC substrate were prepared by using a CSD method with a zinc naphthenate precursor. Precursor films were pyrolyzed at 500°C for 10 min in air and finally annealed at 600°C, 700°C, 800°C and 900°C for 30 min in air. Crystallinity and in-plane alignment of the films were investigated by X-ray diffraction theta-2 theta scan and pole-figure analysis. Scanning electron microscope, scanning probe microscope, and He-Cd laser (325 nm) are used to detect the surface morphology and photoluminescence of the films. The effects of annealing temperature on crystallinity and epitaxy of the films will be fully discussed on the basis of the results of X-ray diffraction analysis.

ZnO nanorods were synthesized by chemical solution method at low temperature. The surface of nanorods was changed to porous by using thermal annealing and chemical etching. Surface morphology and their optical properties were changed according to annealing and etching condition. Photoluminescence from as-grown ZnO nanorods almost showed defect related emission in wide range from 450∼900 nm. After annealing at 500°C, the band-edge emission of ZnO was observed and emission at visible range was changed to green with decreasing red-orange. The surface morphology of ZnO nanorods was transformed to porous by chemical etching and it led to increase the intensity of band-edge emission about three times. The internal quantum efficiency for porous ZnO nanorods, which was calculated from ratio PL intensity at 10K and 300K, is about 21%. Also, the random lasing at porous ZnO nanorods was occurred at high optical excitation by a photon with traveling inside or outside of porous ZnO nanorod gets amplified by injection second photon before leaving porous ZnO nanorods.

In this paper we report the growth of ZnO nanowires (12-60 nm) and nanorods (500 nm) by a method of Catalysis Free Direct Vapor Phase (DVP) technique. The nanowires were grown on c-Al2O3 and pulsed laser deposited ZnO nucleation layer on Al2O3 substrates at 800 °C without employing any metal catalysts that are conventionally used in MOCVD or Vapor-Liquid-Solid phase techniques. The ZnO nanowires are found to emit UV light at 386 nm with considerably lower green band emission.

Vickers indentations of ZnO crystals grown by te hydrothermal method wer studied by cathodoluminescence. The plastic deformation induced defects, which have influence on the luminescence spectrum. The main changes in the luminescence response concern the generation of non radiative recombination centers, but also a band close to the first order phonon replica of the free exciton is observed to appear in the regions deformed by the indentation. Besides, deep levels are also generated. The result of this is a relative increase of the defect related bands respect to the dominant bound exciton emission.

ZnO nanowires are readily fabricated with a novel and unique, high-pressure pulsed laser deposition process developed recently by us. It allows the flexible and reproducible control of morphology, diameter, lenght and composition. Furthermore, we report on the electrical characterization as well as stimulated emission from a zinc oxide (ZnO) microcrystals grown by carbothermal evaporation observed by spatially resolved photoluminescence (PL) and high excitation spectroscopy (HES).

ZnO and ZnO/EosinY hybrid materials were electrodeposited from aqueous zinc salt solutions on (0001) GaN and on (0001) ZnO. Crystalline ZnO was deposited as proven by X-ray diffraction (XRD). The intensity pattern for ZnO/EosinY showed a preferential orientation with the c- plane of ZnO parallel to GaN (0001) or ZnO (0001). XRD rocking curves with FWHM=0.25° indicated a high level of in-plane orientation of the deposited ZnO crystalline domains. The peak position of (0002) ZnO was shifted by 2Θ=1.3°. This difference and the corresponding simultaneous shift of (0004) ZnO were explained by a lattice expansion by 3.6 % in the c- direction. This clearly indicated the strong influence of the Eosin Y molecules adsorbed during the electrodeposition of ZnO. Scanning electron microscopy (SEM) revealed the formation of domains with different crystal sizes pointing at a varying density of nucleation sites on the substrate.

We report on the growth of Zn1-xMgxO nanopillars on a-plane sapphire by the vapor liquid solid (VLS) process. The as-grown nano structures are characterized by scanning electron microscopy (SEM), spatially resolved cathodoluminescence (SEM-CL), and integral photoluminescence (PL). The investigation with SEM applying different secondary electron detectors confirms that the pillars are grown by the VLS process. Integral PL experiments reveal an average value of 7% Mg incorporated in the Zn1-xMgxO nano structures. Both ZnO- and Zn1-xMgxO– related luminescence features are observed. The direct incorpoaration of Mg into single pillars is demonstrated with SEM-CL, and different Mg concentrations are found. First annealing experiments on the sample at 850°C lead to an almost complete breakdown of the ZnO- and Zn1-xMgxO– related near band-edge luminescence, whereas the structural properties of the sample morpholgy remain nearly unchanged.

The Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration self-consistent field (CI/CASSCF/MCSCF) approximations are used to study computationally the electronic properties of zinc oxide artificial molecules whose structure and composition have been derived from those of the symmetry elements of the wurtzite bulk lattice of zinc oxide. Such molecules may provide realistic models for small ZnO quantum dots (QDs) synthesized in “vacuum” or quantum confinement (such as that of well-defined nanopore arrays of silica and alumina membranes) using variety of methods in particular, supercritical fluid deposition. The computational direct optical transition energy (OTE) of the confined molecule appears to be several times smaller than that of the corresponding vacuum cluster. The charge and spin density distributions of these molecules (CDDs and SDDs, respectively) differ significantly, revealing dramatic effects of quantum confinement on electronic properties of Zn-O clusters. The obtained results suggest that manipulations with the electronic properties of the confined clusters by sophisticated design of their quantum confinement may provide means for synthesis of Zn-O – based electronic materials that combine a wide, tunable band gap with large, tunable exciton binding energy.

Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O targets were used to grow thin films by rf magnetron sputtering. XRD patterns of the films showed a strong preferred orientation along c-axis. Zn0.90Co0.10O film showed a transmittance above 75% in the visible range, while the transmittance of the Zn0.85[Co0.50Fe0.50]0.15O film was about 45%; with three absorption peaks attributed to d-d transitions of tetrahedrally coordinated Co2+. The band gap values for Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films were 2.95 and 2.70 eV respectively, which are slightly less than ZnO bulk. The Zn0.90Co0.10O film showed a relatively large positive magnetoresistance (MR) at the high magnetic field in the temperature range from 7 to 50 K, which reached 11.9% a 7K for the magnetoresistance. The lowest MR was found at 100 K. From M-H curve measured at room temperature shown a probable antiferromagnetic behavior, although was possible to observe little coercive field of 30 Oe and 40 Oe for Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films, respectively.

P- and n-type conductivity domains in dual-doped ZnO:As+N layers grown by metal organic vapor phase epitaxy on GaN/sapphire templates were electrically microcharacterized by scanning capacitance (SCM) and scanning surface potential microscopy (SSPM) techniques with respect to their defect states. The p-type domains were found to be dominated by two acceptors with thermal activation energies of about 80 and 270 meV as observed by transient SCM scans at different temperatures. Optically excited SSPM scans revealed defect-to-band-transitions at 400, 459, and 505 nm omnipresent in both domain types as well as a shallower transition at 377 nm exclusively in the p-type regions. According to the similar energy levels the optical transitions at 377 and 400 nm are assigned to acceptor states, whereby the 80meV-acceptor is probably responsible for the conversion from n- to p-type in the domains.