This PDF file contains the front matter associated with SPIE Proceedings Volume 6895, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Two dimensional homoepitaxial growth of high quality ZnO epilayers was achieved by chemical vapor deposition techniques without a buffer layer. We report on the optical and structural properties of these epilayers with particular focus on the polarity of the surface of the substrate. Photoluminescence spectra exhibit strong dependence of the bound exciton recombinations on the termination of the substrate. This is particularly pronounced in the large variety of transition lines in the O-face terminated sample with values for the full width at half maximum as low as 80μeV. Cross-sectional micro Raman spectroscopy and high resolution transmission electron microscopy reveal the presence of strain in the epilayer grown on O-face ZnO by a shift of the non-polar E₂(high) mode and a variation in the lattice constant ratio. Still, the crystal quality of the films is further increased compared to the substrate, which is shown be a half-width of 17" of the XRD rocking curve in both epilayers on Zn-face and O-face terminated ZnO substrate.

We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of 650°C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates. Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 10¹⁵ cm^-3, whereas the phosphorous doping increases the free electron concentration at room temperature to 10¹⁷ cm^-3 and above. Temperature dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.

Plasma-assisted epitaxy has been demonstrated as one possible process for the ZnO growth, which is processed at low temperatures below 400^oC using oxygen-plasma excited by radio-frequency power at 13.56MHz. This paper is focused on the plasma-assisted epitaxial growth of ZnO layer concerned with shallow acceptor doping using N₂+O₂ gas plasma and high-quality undoped-ZnO growth using a novel buffer layer consisted of very thin (below 1nm-thick) strained Ti₂O₃ on A-sapphire. Donor-acceptor pair emission was clearly observed at 3.273eV in low temperature photoluminescence spectrum of nitrogen-doped PAE-ZnO layer grown in Zn-rich supply condition and the activation energy of the shallow acceptor was determined about 132meV. The doping feature of nitrogen related with the oxygen vacancy is also expected by the photoluminescence study. High-quality ZnO layer with smooth surface including fine hexagonal pyramids was grown on the buffer layer at the growth temperature as low as 340^oC with the photoluminescence spectrum dominated by free-exciton emissions at 10K. RHEED observations indicated the ZnO on the thin buffer layer was epitaxially grown toward c-axis with an in-plane relation ship of [-12-10]ZnO//[1-104]Al₂O₃ without any rotational domains.

Molecular-beam epitaxial growth far from thermal equilibrium allows us to overcome the standard solubility limit and to alloy ZnO with CdO in strict wurtzite phase up to mole fractions of several 10%. In this way, a band-gap range extending from 3.3 eV down to 2.3 eV can be covered. Strong improvement of the crystalline quality indicated by a rocking curve width of only 45 arc sec is achieved when growing the ternary on ZnO substrates. Despite very low growth temperatures (~150 °C), layer-by-layer growth indicated and controlled by RHEED oscillations is accomplished. This enables us the fabrication of atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition range. Post-growth annealing increases the radiative efficiency up to two orders of magnitude and demonstrates thermal stability of the structures with respect to phase separation even up to temperatures of about 500°C. Low-energy shifts of the photoluminescence features reaching the order of 1 eV as well as a dramatic increase of the lifetime from the sub-ns to the 100-μs time-scale uncover the presence of huge polarization-induced electric fields of some 10⁸ V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical excitation in the 10 kW/cm² screens these fields and recovers practically the bare quantum-confined energy transitions. On appropriately designed structures, laser action from the UV down to the green wavelength range is observed under optical pumping. The threshold at low temperature is only 60 kW/cm² and increases only moderately up to room temperatures. All these findings make ZnO-based heterostructures a promising alternative to group-III-nitrides for opto-electronic applications in the short-wavelength range.

Thin films of ZnO and Mg_xZn_1-xO were epitaxially grown on Zn-polar ZnO substrates by plasma assisted molecular beam epitaxy. The miscut of c-plane ZnO substrates toward the [1-100] axis direction leads to a flat substrate surface with straight step edges. The growth mode of epitaxial ZnO films significantly depended on the growth temperature, and a substrate temperature over 800°C was needed for flat film surfaces with monolayer-height steps. Photoluminescence (PL) peak originating from the n = 2 state of A-free excitons was observed at 12 K for the ZnO films grown under stoichiometric and O-rich growth conditions. Mg_xZn_1-xO films were also fabricated under Zn-rich conditions. The film surface exhibited a step-and-terrace structure. The effective PL lifetime of Mg_0.08Zn_0.92O film was as long as 1.9 ns, which is the highest value ever reported, presumably due to a high purity level of the film.

Strong coupling between the exciton and cavity modes were demonstrated in a bulk ZnO-based hybrid microcavity The hybrid microcavity consisted of a λ-thick bulk ZnO cavity layer sandwiched between a 29 pair Al_0.5Ga_0.5N/GaN bottom distributed Bragg reflector (DBR) and an 8 pair SiO₂/Si₃N₄ top DBR grown by molecular beam epitaxy, metalorganic chemical vapor deposition, and ultra high vacuum plasma-enhanced chemical vapor deposition, respectively. All layer interfaces were sharp and optical reflectivity measurements were performed to characterize the DBRs. The anti-crossing behavior in the polarton dispersion, which indicates the system is in the strong coupling regime was observed in angle-resolved photoluminescence measurements at room temperature, and a vacuum Rabi splitting of ~50 meV in the ZnO-based hybrid microcavity was obtained.

An optically pumped ZnO distributed feedback laser operating at 383 nm has been designed, fabricated and characterized. Single mode operation was observed for a wide temperature range between 10 and 270 K. In order to avoid technologically difficult etching of ZnO, a 3rd order diffraction grating was dry-etched into an additional 120 nm-thick Si₃N₄ layer deposited on the ZnO active region. The spectral linewidth of the laser emission was 0.4 nm, whereas an optical pump threshold intensity of 0.12 MW/cm² and a peak output power of 14 mW were seen. The temperature tuning coefficient of the ZnO refractive index was determined from wavelength vs. temperature measurements; a value of 9 × 10^-5 K^-1 was found, in good agreement with literature values.

We report on ZnO wafer bonded III-nitride light emitting diodes (LEDs). ZnO wafer was selectively etched to form a hexagonal pyramid shape. For enhancing light extraction and heat conductivity, two types of LED structures were demonstrated: one is having patterned GaN at the wafer bonded interface between GaN and ZnO, the other is the LED having no sapphire. The experimental results of electrical and optical characteristics indicate the high potential of these types of LEDs for solid state lighting sources.

After our recent successful demonstration of high brightness white light emitting diodes (HB-LEDs) based on high temperature grown n-ZnO nanowires on different p-type semiconductors, we present here LEDs fabricated on n-ZnO nano-wires and p-type organic semiconductors. By employing a low temperature chemical growth (≤ 90 °C) approach for ZnO synthesis combined together with organic p-type semiconductors, we demonstrate high quality LEDs fabricated on a variety of different substrates. The substrates include transparent glass, plastic, and conventional Si. Different multi-layers of p-type organic semiconductors with or without electron blocking layers have been demonstrated and characterized. The investigated p-type organic semiconductors include PEDOT:PSS, which was used as a anode in combination with other p-type polymers. Some of the heterojunction diodes also contain an electron blocking polymer sandwiched between the p-type polymer and the n-ZnO nano-wire. The insertion of electron blocking layer is necessary to engineer the device for the desired emission. Structural and electrical results will be presented. The preliminary I-V characteristics of the organic-inorganic hybrid heterojunction diodes show good rectifying properties. Finally we also present our findings on the origin of the green luminescence band which is responsible of the white light emission in ZnO is discussed.

Vertically aligned ZnO nanowires have been successfully synthesized on c-cut sapphire substrates by a catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD) in Ar and N₂ background gases. In NAPLD, the nanoparticles formed in a background gas by laser ablation are used as a starting material for the growth of the nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which are accomplished by changing the energy of the ablation laser, the repetition rate of the laser and so on. When single ZnO nanowire synthesized in a N₂ background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated emission was clearly observed, indicating high quality of the nanowire. These nanowires were used as building blocks for an ultraviolet light emitting diode with a structure of n-ZnO/ZnO nanowire/p-GaN.

We report on the epitaxy of vertically aligned ZnO nanowires (NWs), the collective integration technology of these nanowires and their optical and electrical characterizations. ZnO based nanowires are grown mainly on sapphire substrates by metal-organic vapour phase epitaxy (MOVPE). Photoluminescence spectra at 4 K exhibit strong excitonic peaks at around 380 nm without green luminescence band, showing the low deep radiative defect density. Technological processes have been developed both for mineral and organic integrations of the as-grown nanowires. Photoconducting properties in the UV-visible range have been investigated through collective electrical contacts. The electrical transport properties of vertically integrated single nanowires have also been investigated by current sensing AFM measurements. A comparison of the PL spectra at 300 K of the as-grown and integrated nanowires has shown no significant impact of the integration process on the crystal quality of the nanowires.

Phosphorous-doped ZnO (ZnO:P) nanowires were prepared by a high-pressure pulsed laser deposition process. To extend the size range of available wires, μm-thick ZnO:P microwires were grown additionally by a direct carbothermal deposition process. Low-temperature cathodoluminescence of single ZnO:P nanowires grown by both processes exhibit characteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission ((A⁰, X), 3.356 eV), free-electron to neutral-acceptor emission ((e, A⁰), 3.314 eV), and donor-to-acceptor pair emission (DAP, ~3.24 and ~3.04 eV). This proves that stable phosphorus acceptor levels have been induced into the ZnO:P nano- and microwires. From the quantitative evaluation of the spectroscopic features we deduct an acceptor binding energy of 122 meV. The ZnO:P microwires were used as channels in bottom-gate field effect transistors (FET) built on Si substrates with SiO₂ gate oxide. The electrical FET-characteristics of several wires show reproducibly clear qualitative indication for p-type conductivity for variation of gate voltage. This behavior is opposite to that of nominally undoped, n-type conducting wires investigated for comparison. The p-type conductivity was found to be stable over more than six months.

ZnO is a wide bandgap semiconductor that exhibits properties that are near-ideal for light-emitting diodes, but presents materials challenges that must be overcome in order to achieve highly efficient light emission. The most significant issue with ZnO is p-type doping. Related materials issues include understanding electron-hole transport across pn junctions, as well as understanding and minimizing leakage current paths within the bulk and on the surface. In this paper, the formation and properties of phosphorus-doped Zn_1-xMg_xO films, ZnO-based pn homojunctions and heterojunctions is discussed. The behavior of phosphorus within the ZnO and ZnMgO matrices will be described. Comparisons with other acceptor dopants will be made. Discussion will include stability of transport properties, stabilization of surfaces, and device characteristics.

The ability to externally control the properties of magnetic materials would be highly desirable both from fundamental and technological point of views. In this respect, dilute magnetic semiconductor (DMS), in which a fraction of atoms of the nonmagnetic semiconductor host is replaced by magnetic ions, have recently attracted broad interest for their potential application in spintronics. In this work, we focused on transition metal (TM) (Co, Mn and Cu) doped Zinc oxide (ZnO) because room temperature ferromagnetism was both theoretically predicted and experimentally observed. However, the origin of such ferromagnetism, in particular whether it is a signature of a true DMS behaviour (long range magnetic interaction between the doping ions) or it arises from the formation of secondary phases, segregation or clustering is still under debate. Measuring the dependence of the magnetic properties on the carrier concentration can clarify the underlying physics. The samples were characterized by resistivity, Hall effect, magnetoresistance, Seebeck effect, synchrotron X-ray adsorption spectra (XAS) and magnetic dichroism (XMD) while modulating the carrier density by electric field. The insulating-gate field-effect transistor structures are realized in ZnO/Strontium Titanate (SrTiO₃) heterostructures by pulsed laser deposition. These devices offers the capability to modulate the carrier density of a probe accessible (light, AFM tip, ...) channel, by more than 5 orders of magnitude (from ≈10¹⁵ to ≈10²⁰ e^-/cm³, estimated by Hall effect measurements under FE). The Co and Mn films measured by DC SQUID magnetometer result ferromagnetic and anomalous Hall effect was observed at low temperature but nor ferromagnetic nor antiferromagnetic signal was detectable in the XMD spectra. Cu doped films are insulating and nonmagnetic. Photo Emission Electron Microscopy (x-PEEM) and magnetic force microscopy (MFM) showed that the sample are homogeneus and no clustering of TM were detected. A large effect of the magnetic ions, strongly dependent on the carrier concentration, was observed on the transport properties and this effect according can be explained by a giant s-d exchange leading to spin splitting of the s-type conduction band. Since the filling of such band can be modified by field effect a electric field control of the spin polarization can be achieved.

The effect of plasma-induced ion damage on the optical properties of ZnO films grown by plasma-assisted molecular beam epitaxy on a-sapphire substrates and GaN(0001)/c-sapphire templates prepared has been studied using steady-state and time-resolved photoluminescence. We observed that the deflecting 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 the photoluminescence spectra from the layers grown with the ion deflection was found to increase by factors 7 to 20 for the layers grown on GaN(0001)/c-sapphire at a plasma power of 350 W and by 3 to 4 times for ZnO grown on a-sapphire substrates at a plasma power of 265 W as compared to the controls grown without the ion deflection. 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. For the ZnO films grown on a-plane sapphire substrates, time-resolved photoluminescence showed a significant increase in luminescence decay times both at RT and 89 K. However, for ZnO on GaN(0001)/csapphire substrates, virtually no improvement in decay time was found at 89 K with only a moderate increase in decay constant at room temperature.

ZnO nanostructures were synthesised by Metal Organic Chemical Vapor Deposition growth on Si (100) and c-Al₂O₃ substrates coated with a 5nm thick layer of Au. The Au coated substrates were annealed in air prior to deposition of ZnO so as to promote formation of Au nanodroplets. The development of the nanodroplets was studied as a function of annealing duration and temperature. Under optimised conditions, a relatively homogeneous distribution of regular Au nanodroplets was obtained. Using the Au nanodroplets as a catalyst, MOCVD growth of ZnO nanostructures was studied. Scanning electron microscopy revealed nanostructures with various forms including commonly observed structures such as nanorods, nanoneedles and nanotubes. Some novel nanostructures were also observed, however, which resembled twist pastries and bevelled-multifaceted table legs.