Single-crystalline with good optical properties aligned ZnO nanonails were grown on steel alloy substrate without the use of metal catalyst or additives by the thermal evaporation process using high purity metallic zinc powder and oxygen as source materials for zinc and oxygen, respectively. Detailed morphological studies by FESEM revealed that the obtained nanonails are grown in a high density over the whole substrate surface and are exhibiting perfect hexagonal-shaped caps. The diameters of the nanonails at their tops and bases are ranges from 120∼160nm and 50∼70 nm, respectively. The detailed structural characterizations confirmed that the synthesized nanostructures are single-crystalline and grown along the c-axis direction. Raman scattering and room-temperature photoluminescence studies demonstrated the wurtzite hexagonal phase and good optical properties, respectively for the grown nanonails.

The conduction band offset of n-ZnO/n-6H-SiC heterostructures fabricated by rf-sputtered ZnO on commercial n-type 6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and deep level transient spectroscopy measurements showed the conduction band offsets to be 1.25 eV, 1.1 eV, and 1.22 eV, respectively.

For the homoepitaxial growth of ZnO it is inevitable to obtain a regular crystalline single crystal surface prior to growth. Commercially available, hydrothermally grown ZnO single crystals show amorphous surfaces due to mechanical cutting and polishing. Here we present the results of a thermal treatment on these ZnO single crystals. After annealing, a regular crystalline oxygen terminated surface can be obtained. Changes in surface roughness, residual defect concentration and electrical properties can be shown. The bulk crystallinity though was not affected.

We studied photoluminescence from melt-grown ZnO crystals annealed in air ambient at different temperatures. Along with intense and sharp excitonic lines, we observed several broad bands presumably related to deep acceptors. Two luminescence bands peaking at 1.95 and 2.15 eV at 10 K were studied in detail at different temperatures. The 1.95 eV band has been attributed to transitions from shallow donors to yet unidentified deep acceptor. Very slow non-exponential decay of this band at low temperatures supports such an assumption.

Highly structured layers comprising of vertically aligned zinc oxide rods, tripods or platelets were fabricated by spray pyrolysis method at temperatures of 510-550 °C. The zinc chloride solution was pulverized onto the preheated substrates of glass and ITO, SnO2, ZnO covered glass substrates with the help of compressed air as a carrier gas. ZnO layers were characterized by scanning electron microscopy and Raman spectroscopy. C-axis orientated ZnO nanorod arrays of well-developed hexagonal rods with length from some hundreds of nanometers up to some microns and with diameter from 70 nm up to 900 nm . The rise of both the growth temperature and solution concentration increases rod dimensions. Deposition of the solutions with the concentration of 0.05 up to 0.2 mol/l results in structured layers composed of rods on glass substrates. Using ITO, SnO2 and ZnO thin film covered glasses diluted solutions should be used to obtain ZnO nanorods. Alcoholic solutions allow deposit thinner rods and reduce the deposition temperature. Very strong and relatively narrow E2 Raman bands indicate that ZnO rods prepared by spray pyrolysis technique are of high crystal quality.

The development of new etching and contact metallurgies for the ZnO/ZnMgO/ZnCdO materials system and various approaches for realizing ZnO LEDs are reviewed. ZnO nanorod MOSFETs and pH sensors have been demonstrated. In addition, selective detection of hydrogen with Pt-coated single ZnO nanorods is discussed discussed. The Pt-coated single nanorods show a current response approximately a factor of three larger at room temperature upon exposure to 500ppm H2 in N2 than thin films of ZnO. The power consumption of these 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 ZnO nanorods are insensitive to oxygen in the measurement ambient.

Raman scattering measurements were carried out on a bulk, single crystal of wurtzite ZnO over a temperature range from 80 to 760 K and the temperature-dependent shift and broadening of the E2high and A1(LO) modes was analyzed. The E2high mode exhibits a visibly asymmetric line shape that can be related to the interaction with the continuum of acoustic two-phonon density of states. A Fermi resonance model was used to describe the E2high temperature dependence. On the other hand, the anharmonic shift and broadening of the A1(LO) mode are adequately accounted for by a decay model with a dominating Ridley channel involving TO and LA modes. Phonon lifetimes of ∼0.9 and 0.5 ps are found for the E2high and A1(LO) modes, respectively, which corroborates that anharmonic decay involves in both cases a three-phonon process. The A1(LO)lifetime is one order of magnitude lower than that of GaN, which suggests that hot phonon effects should be expected to play a less relevant role in carrier relaxation in ZnO as compared with GaN.

Mg0.15Zn0.85O thin films were grown on fused silica substrates at different substrate temperatures using pulsed laser deposition. X-ray diffraction and transmission electron microscopy were used to investigate the structure of the films. High resolution transmission electron microscopy showed that the film contained small grains with low angle boundaries. The optical properties of the films were investigated using absorption spectra. The bandgap energy values of the films was determined by fitting the absorption data.

The development of reliable and thermally stable Ohmic and Schottky contacts to ZnO is one of the critical issues related to the fabrication of ZnO-based UV light emitters/detectors and field effect transistors. To date, a number of different metallization schemes and surface cleaning procedures prior to metal deposition have been examined for rectifying contacts on n-ZnO. While these reports have shown that low reactive metals such as Au, Ag and Pd form rectifying contacts with Schottky barrier heights in the 0.6-0.8 eV range, the thermal stability of these contacts is usually extremely poor, with degradation occurring even at 60°C for Au/n-ZnO. One approach to achieving increased barrier heights that has proven successful for GaAs, InP, InGaAs and other compound semiconductors is the use of cryogenic deposition temperatures. In this context, we report in this work on the effect of cryogenic temperature metal deposition on the contact properties of Pd, Pt, Ti, and Ni on single-crystal n-type ZnO. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. In comparison, both Pd and Pt contacts showed rectifying characteristics after deposition. All rectifying contacts exhibited barrier heights around 1-2 eV and idealities between 1 and 2. Low temperature deposition gave higher resistances in comparison to room temperature deposition for all cases. Larger contacts also corresponded to an increase in resistance. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300°C, showing an increase in barrier height along with a decrease in ideality with increasing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however low temperature contacts demonstrated some cracking in Pt and Pd, probably due to surface stress.

ZnO nanowires doped with Mn, Fe, Sn, and Li during the thermal growth following direct chemical synthesis were investigated using electric and magnetic measurements. Current-voltage characteristics of individual nanowires configured as a two-terminal device with Al electrodes show apparent rectify behavior indicating the Schottky-like barrier formation and resistivity being less 3 Ω·cm. Reproducible resistance modulation by a dc voltage at room temperature is observed. Magnetic susceptibility of the doped nanowires as a function of temperature demonstrates Curie–Weiss behavior. Magnetization versus field curves show hysteresis with the coercive field of about 200 Oe. The spatially-resolved magnetic force measurements of individual nanowires revealed the magnetic domain structure. The domains align perpendicular to c-axis and can be polarized in the external magnetic field.

Using μ-Raman spectroscopy (μRS) analyses, the impact of hydrogen plasma treatments on sintered zinc oxide (ZnO) samples was investigated. H-plasma exposures (150 W, 13.56 MHz) were carried out for 1 hour at substrate temperatures between 250 °C and 500 °C. μRS reveals that plasma hydrogenated ZnO samples are more defective than non-treated ones. On one hand non-specified defect species are created with a maximal density upon plasma hydrogenation at 350 °C, on the other hand the formation of oxygen vacancies (VO) can be traced. The density of VO defects, appearing upon H-plasma exposure, is not significantly correlated to the applied substrate temperatures. μRS also reveals vibration modes of H2 molecules trapped in nano-voids. The μRS results indicate that those nano-voids are created by the coalescence of VO defects.

A facile and convenient aqueous route has been employed to synthesize well -aligned ZnO nanorods. Field emission scanning electron microscopy studies of ZnO nanorod arrays grown at 70°C on ZnO/Si substrates show fine homogeneous surface. The average diameter of ZnO nanorod is in between 50-60 nm .The length of each nanorod is about 400-500 nm. Structural analysis showed that the ZnO nanorods are single crystalline with wurtzite hexagonal phase. Room temperature photoluminescence spectra of the ZnO nanorod arrays exhibit ultra violet emission and green emission. In addition, selective growth of ZnO nanorods on patterned ITO glass substrate is obtained. Each nanorod has diameter of 50 - 70 nm and their length upto 500 nm. XRD pattern shows that ZnO nanorods on patterned ITO substrate are single crystalline in nature with wurtzite hexagonal phase. The room temperature photoluminescence from the aligned ZnO nanorods showed a strong ultra violet emission at 378 nm and broad deep level visible emission at 580 nm.

Intrinsic room temperature ferromagnetism is reported in sequentially sintered (in air ambient) nanocrystalline ZnO:Co (1 to 10 at% Co) samples prepared by chemical route. The Curie temperature of the ZnO:Co ferromagnetic samples is determined to be ∼495°C. The saturation magnetization of the ferromagnetic phase is found to first increase with Co concentration (up to 5%) and then decrease. It is also found to decrease with increase in sintering temperature up to 800°C; there after it remains unaffected till 1000°C. The d-d band transitions in the optical spectrum confirm that Co2+ substitutes Zn2+ in the ZnO lattice. A plausible explanation of the observed ferromagnetic ordering is presented in terms of Bound Magnetic Polarons model.

We report on visible-light sensitivity in N-doped ZnO (ZnO:N) films that were deposited on ITO/quartz substrates by reactive magnetron sputtering. Colored ZnO:N samples showed enhanced polycrystallization and a significant decrease in optical band gap from 3.1 to 2.3 eV with increasing N doping concentration, as determined by x-ray diffraction and optical absorption measurements. Deep-level optical spectroscopy measurements revealed three characteristic deep levels located at ∼0.98, ∼1.20, and ∼2.21 eV below the conduction band. In particular, the pronounced 2.21 eV band is newly introduced by the N doping and behaves as part of the valence band, resulting in the band-gap narrowing of ZnO. Therefore, this deep level is probably one origin of visible-light sensitivity in ZnO:N.

Hydrothermal ZnMgO crystals were studied by cathodoluminescence. The high energy shift of the excitonic luminescence demonstrates the Mg incorporation in the ZnO lattice in at a few percent. The spectral parameters of the luminescence emission show a marked dependence of the incorporation of defects and Mg on the growth facet. This growth sector selectivity shows similar trends to those observed in hydrothermal ZnO crystals. The in depth distribution of Mg was studied varying the acceleration voltage of the excitation e-beam, showing a slight accumulation of Mg close to the surface.

ZnO(0001) bulk crystals were implanted with 100 keV H2+ ions with various doses in the range of 5×1016 to 3×1017 cm-2. The ZnO crystals implanted up to a dose of 2.2×1017 cm-2 did not show any surface exfoliation, even after post-implantation annealing at temperatures up to 800°C for 1 h while those crystals implanted with a dose of 2.8×1017 cm-2 or higher exhibited exfoliated surfaces already in the as-implanted state. In a narrow dose window in between, controlled exfoliation could be obtained upon post-implantation annealing only. Cross-sectional transmission electron microscopy (XTEM) of the implanted ZnO samples showed that a large number of nanovoids were formed within the implantation-induced damage band. These nanovoids served as precursors for the formation of microcracks leading to the exfoliation of ZnO wafer surfaces. In addition to the nanovoids, elongated nanocolumns perpendicular to the ZnO wafer surfaces were also observed. These nanocolumns showed diameters of up to 10 nm and lengths of up to 500 nm. The nanocolumns were found in the ZnO wafer even well beyond the projected range of hydrogen ions.

Periodically ordered ZnO nanopillar arrays were fabricated by a combination of soft templates created by e-beam lithography and an electrochemical process. Growth at 90 °C in an aqueous solution ensured compatibility with the polymethyl methacrylate used as a template material. We demonstrate that individual ZnO nanopillars with diameters around 100 nm can be precisely placed in desired locations to form two-dimensional periodic structures. This approach provides a new method for design and fabrication of ZnO photonic materials. The process is compatible with standard microfabrication techniques and may have potential applications in the manufacture of photonic and optoelectronic devices.

In this work we investigate the lattice damage induced in ZnO implanted with potential group V acceptors by means of Raman scattering. ZnO samples were implanted with N and P to high doses and Raman spectra were obtained prior and after rapid thermal annealing (RTA). Characteristic disorder-activated modes are observed in the spectra that can be used to assess the degree of lattice damage. ZnO samples were also implanted with native Zn+ and O+ ions under similar conditions to study specific effects of implantation with N+ and P+. As revealed by the intensity of disorder-activated bands, the implantation induced lattice damage is considerably higher for Zn+ than for the lighter O+ ion. In samples implanted with N+ additional Raman peaks emerge that are not observed either in the samples implanted with the native Zn+ and O+ ions or in the samples implanted with P+, thus pointing to a local vibrational mode of N or a N complex as the origin of these modes. Disorder-activated features are fully removed by RTA, indicating a high degree of lattice recovery by RTA even for the heavily damaged ZnO samples implanted with Zn+.

Room temperature photoluminescence (PL) spectra from zinc oxide (ZnO) nanostructures were studied. ZnO samples were produced via thermal chemical vapor deposition (thermal-CVD) and a variety of ZnO nanostructures were synthesized by adjusting the oxygen content during the growth process. All samples exhibit a sharp and strong ultra-violet near-band-edge (NBE) emission at about 3.18 eV. The visible emission from the samples deposited under an oxygen-deficient condition were dominated by blue-green band emission at 2.34 eV. The intensity of the blue-green band was greatly reduced (so-called green band free) for the ZnO deposited at the center of the wafer while strong violet-blue emission bands and broad bands at yellow-orange-red range were collected from the ZnO grown along the edge of the wafer. We believe that the spatial inhomogeniety was caused by turbulent gas flow in the reaction chamber, which resulted in different local oxygen concentration. Origin of visible luminescence from ZnO nanostructures will be discussed and a model to explain the observed visible luminescence process will be presented.

ZnO thin films are of interest for an array of applications, including: light emitters, photovoltaics, sensors and transparent contacts, among others. Production routes for ZnO include sputtering, MBE and MOCVD. This paper focuses on our efforts to produce a large scale MOCVD thin film production tool and the results obtained from the reactor. Specifically, we have constructed a tool with a 16” wafer carrier that uniformly deposits ZnO films on 38×2” wafers simultaneously. The reactor operates at low pressure (<0.1 Atmosphere) and through 700°C. High quality, uniform films have been deposited on an array of substrates. Al-doped films exhibited resisitivities in the 1×10-3 ohm-cm range and transmissivity greater than 80%. Film morphology and crystallinity are a function of process parameters. The large area oxide MOCVD reactor design challenges and results are summarized. Tool performance and ZnO thin film quality are reviewed, as well as preliminary ZnO contact performance on GaN LEDs.