This PDF file contains the front matter associated with SPIE Proceedings Volume 6474, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We will examine a few of the outstanding new ZnO materials and device developments published in 2006. In the area of bulk crystal growth, high-quality 3-inch ZnO wafers grown by the hydrothermal method have become available, and Bridgman growth has also been developed. In the thin-film area, excellent ZnO layers have been grown by liquid-phase epitaxy. Other types of epitaxial material have also shown improvements, and the quantum Hall effect has now been observed, along with photoluminescence (PL) linewidths as low as 110 &mgr;eV. In the area of impurity characterization, radioactive-tracer methods have been used to make positive identifications of the PL donor-bound exciton lines I₈ and I₉, as due to Ga and In, respectively. Our understanding of the common impurity H has also advanced, because it is now known from both theory and experiment that interstitial H is not stable at room temperature. The same is true of the native interstitials, Zn_I and O_I. New results suggest that the common H-related shallow donor is probably multibonded H substitutional on an O site, and the Zn_I-related shallow donor is probably a complex, such as Zn_I-NO. In the important area of p-type ZnO, it has been demonstrated that Li and N co-doped material has a resistivity as low as 1 &OHgr;-cm and is stable for at least one year. Also, many groups were able to make thin-film and nanowire or nanorod p-n junction light emitting diodes (LEDs). Another very exciting development was the creation of an edge-emitting laser diode, from rows of n-ZnO nanocrystals on a p-GaN thin film. Electronic devices, including transparent transistors, also made great strides, producing record field-effect mobilities.

A quantitative interpretation of the anomalous temperature behavior is presented to describe the so-called "S-shape dependence" of the photoluminescence (PL) in ZnO-related quantum well layers by using an appropriate modeling and simulation. Experimental data for CdZnO/MgZnO and ZnO/MgZnO samples are modeled using Monte Carlo simulations of the involved relaxation mechanisms and thus providing a realistic picture of the excitonic kinetics. The temperature dependence of the PL maximum and full-width at half maximum could be simultaneously reproduced with reasonably good accuracy. Several informations about the distribution of the localization potential wells and identify their hopping transfers between separated states are deduced. We found that the temperature-dependent PL linewidth and Stokes shift is in a qualitatively reasonable agreement with the above-mentioned model, with accounting for an additional inhomogeneous broadening for the case of linewidth for CdZnO quantum wells. The density of localized states used in the simulation for the CdZnO QW was consistent with the absorption spectrum taken at 5 K.

The promising II-VI-semiconductor ZnO has achieved strong interest in research in the past years. Especially, epitaxial growth by metal organic vapor phase epitaxy (MOVPE) is a matter of particular interest due to the large scalability of MOVPE for commercial mass production and its proven high layer quality for other compound semiconductors. In the past years tremendous advance has been made in the field of epitaxial growth. However, due to the lack of epiready ZnO substrates, so far mostly heteroepitaxial growth with a multistep growth process was applied to obtain good surface morphology and until now not all of the physical properties of such multilayers are fully understood. In this paper we present recent results of the electrical behavior of such multiple undoped ZnO layers. Despite numerous efforts one big challenge is the p-type doping of ZnO. Here we present our results to doping experiments with arsenic, nitrogen and as a new approach simultaneous dual doping of nitrogen and arsenic. Homoepitaxial growth offers a great potential for ZnO due to some advantages as the absence of thermal and lattice mismatch and potentially low dislocation density. We present experiments on the thermal treatment of commercial ZnO bulk crystals, which is necessary for subsequent homo- MOVPE.

Ag-doped ZnO thin films have been fabricated by pulsed laser deposition. Thermal analysis and X-ray photoelectron spectroscopy (XPS) were systematically investigated to verify the doping mechanism of Ag doped ZnO thin film depending on deposition temperature. The fabricated p-type Ag doped ZnO films shows the hole concentration in the range from 4.9x10¹⁶ to 6.0x10¹⁷ cm^-3.

Electrical properties of n-ZnO/n-GaN isotype heterostructures prepared by rf-sputtering of ZnO films on GaN layers which in turn grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n- ZnO/n-GaN diodes exhibited highly rectifying characteristics with forward and reverse currents being ~1.43x10^-2 A/cm² and ~2.4x10^-4 A/cm², respectively, at ±5 V. From the Arrhenius plot built representing the 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 and depending on excitation conditions the minority carrier diffusion length in ZnO was estimated in the range 0.125-0.175 &mgr;m. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073 V.

The conduction band offset of n-ZnO/n-6H-SiC heterostructures prepared 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 led to conduction band offsets of 1.25 eV, 1.1 eV, and 1.22 eV, respectively.

We report on the hetero- and homoepitaxial growth of ZnO thin films by the chemical vapor deposition technique. We compare the results obtained on sapphire substrates, on GaN-templates on sapphire substrates and on silicon (111) substrates. Even under optimized growth conditions with the insertion of buffer layers the films tend to grow 3-dimensionally. However, also ZnO substrates, expected to be the best choice, need to be prepared before being used in the epitaxial growth. After mechanical polishing of the ZnO substrates we employed a high temperature annealing step which produced atomically flat surfaces and removed all of the surface and subsurface damage. Thereafter, the two dimensional epitaxial growth was achieved without an additional buffer layer. The substrate had a rocking curve full width at half maximum of 27" which can be compared with that of the film of 22". The films had superior band edge luminescence as compared to the substrate for which the green luminescence band was dominating.

Nitrogen- and phosphorus-doped ZnO thin films were grown by pulsed laser deposition using an electron cyclotron resonance (ECR) nitrogen plasma ion source or a ZnO:P₂O₅ doped target, as the dopant source, respectively. Both types of films were grown on sapphire substrates first coated at low temperature with a ZnO buffer layer. For the N-doped ZnO thin films, temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the measured temperature range of 200-450 K, with typical room temperature acceptor concentrations and mobilities of 5 x 10¹⁵ cm^-3 and 5.61 cm²/Vs, respectively. The room-temperature photoluminescence spectrum of a N-doped ZnO thin film featured a broad near band-edge emission at about 3.1 eV photon energy with a width of 0.5 eV. XPS studies confirmed the incorporation of nitrogen in the samples. The ZnO:P layers (with phosphorus concentrations of between 0.01 and 1 wt %) typically showed weak n-type conduction in the dark, with a resistivity of 70 &OHgr;.cm, a Hall mobility of &mgr;_n ~ 0.5 cm²V^-1s^-1 and a carrier concentration of n ~ 3 x 10¹⁷ cm^-3 at room temperature. After exposure to an incandescent light source, the samples underwent a change from n- to p-type conduction, with an increase in mobility and a decrease in concentration for temperatures below 300K. Electrical measurements showed noticeable differences for both types of doped films when carried out in air or in vacuum. The results are discussed in terms of both the presence of surface conducting channels and the influence of photoconductive effects.

A two-dimensional electron gas (2DEG) was observed in Zn polar ZnMgO/ZnO (ZnMgO on ZnO) heterostructures grown by radical source molecular beam epitaxy. Reflection high energy electron diffraction patterns taken during the growth of the ZnMgO layer remained streaky; x-ray diffraction measurements showed no evidence of phase separation for up 44 % Mg composition. These results shows that the high quality ZnMgO layers up to 44 % Mg composition were obtained without phase separation. The electron mobility of the ZnMgO/ZnO heterostructures dramatically increased with increasing Mg composition and the electron mobility (&mgr;~250 cm²/Vs) at RT reached a value more than twice that of an undoped ZnO layer (&mgr;~100 cm²/Vs) due to the 2DEG formation. The carrier concentration in turn reached values as high as ~1x10¹³ cm^-2 and remained nearly constant regardless of Mg composition. Strong confinement of electrons at the ZnMgO/ZnO interface was confirmed by C-V measurements with a concentration of over 4x10¹⁹ cm^-3. Temperature-dependent Hall measurements of ZnMgO/ZnO heterostructures also exhibited properties associated with well defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value of 2750 cm²/Vs at 4 K. Zn polar "ZnMgO on ZnO" structures are easy to adapt to a top-gate device. These results open new possibilities for high electron mobility transistors (HEMTs) based upon ZnO based materials.

The development of epitaxial growth techniques for the fabrication of nanostructures provides advantages for nanoscale engineering and has yielded many impressive results. ZnO possesses attractive characteristics that include optical, electric and magnetic properties. This material can be utilized to delineate new phenomena through an investigation of surface nanostructures and quantum heterostructures. Homoepitaxy in ZnO can generate specific growth directions in the absence of lattice mismatch at the interface between the film and substrate. Many reports have appeared in the past year concerning the layer growth of nonpolar ZnO. Nonpolar planes are expected to yield large in-plane anisotropy in electrical and optical characterizatics. In nonpolar (10-10) growth using laser-MBE, we found that novel in situ growth techniques allowed for the fabrication of dense arrays of conductive one-dimensional nanostripes with a high degree of lateral periodicity. Highly anisotropic surface morphologies markedly influenced electron transport of ZnO single layers and Mg_0.12Zn_0.88O/ZnO multi-quantum wells (MQWs) with conductivity parallel to the nanostripe arrays being more than one order of magnitude larger than that observed perpendicular to the nanostripe arrays.

The etching characteristics of ZnO epitaxial layers in Oxford Plasmalab 100 ICP 180 and 380 systems are investigated. Etch rates are studied as a function of gas composition, ICP power and RF bias power. Surface profilometry and scanning electron microscopy are used to characterize etch rates and surface morphologies. Highlights from other recently published results are also discussed.

Although ZnO has recently gained much interest as an alternative to the III-Nitride material system, the development of ZnO based optoelectonic devices is still in its infancy. Significant material breakthroughs in p-type doping of ZnO thin films and improvements in crystal growth techniques have recently been achieved, making the development of optoelectonic devices possible. ZnO is known to be an efficient UV-emitting material (~380 nm) at room temperature, optical UV lasing of ZnO has been achieved, and both homojunction and hybrid heterojunction LEDs have been demonstrated. In this paper, processing techniques are explored towards the achievement of a homo-junction ZnO LED. First, a survey of current ZnO processing methods is presented, followed by the results of our processing research. Specifically, we have examined etching through an n-ZnO layer to expose and make contact to a p-ZnO layer.

In this study, we have attempted to enhance the forward emission from metal-capped ZnO mediated by surface plasmon (SP) cross coupling. By using metal alloys and dielectric grating, it is proposed that energy from ZnO can be resonantly coupled to metal/ZnO SPs, transferred to metal/grating SPs and then Bragg scattered to the free space. Although the experimental conditions are not yet been optimized, preliminary results from Al (30 nm) capped ZnO thin film show the forward/backward emission intensity ratio can be increased from 0.1 to 0.6 after the introduction of dielectric grating with periodicity of ~ 800nm on metal side. The ratio is compared favorably with the bare ZnO emission ratio of 0.5. It is thus believed SP cross coupling can be used for fabricating high brightness top-emitting light emitting diodes (LEDs).

The characterization by various experimental techniques of homoepitaxial growth and photonic properties of ZnO epilayers was exhaustively analyzed. The photonic properties of ZnO as promising material for the realization of polariton lasers were investigated by angular dependent reflection spectroscopy. The fitting of the polariton dispersion curve with the experimental results provided us information about the longitudinal-transverse exciton-polariton splitting and damping constants. In addition, the valence band symmetry was examined by angular resolved magneto-optical photoluminescence. From our theoretical and experimental results we extracted evidence that the topmost A valence band possesses Г₇ symmetry. Micro-Raman spectroscopy revealed even in homoepitaxially grown samples the existence of compressive or tensile strain which varied not only in the ZnO layers but also in the templates. In contrast, the untreated substrates were uniformly strained. Sporadically crystal perturbations culminating in the formation of separated growth domains were observed. Additionally, resonant Raman scattering was performed, showing a strong enhancement of the 2E₁(LO) mode for resonant excitation of the I₈ bound exciton complex. We suggest that the resonant Raman scattering led to a longer lifetime of the resonantly excited phonon mode due to a strong exciton-phonon interaction.

The thermal stability of AlO_x and MgO_x on ZnO films has been studied by using photoluminescence, cathodoluminescence and current-voltage measurements. It is found that the interfaces degrade significantly upon thermal annealing, which are evident by the reduction of the band-edge emission as well as the increase of conductance with annealing temperature and duration. By using secondary ion mass spectroscopy and diffusion model, the dependence of luminescence on thermal treatment can be well simulated and the degradation of oxide/ZnO can be attributed to the out-diffusion of Zn into the oxide layer from ZnO. Our studies point out the importance of developing appropriate diffusion barrier for the fabrication of low temperature processed ZnO transistors.

Zinc oxide (ZnO) crystals can be grown by vapor phase transport, hydrothermal solution growth, and high pressure melt growth. Three inch size single crystal has been demonstrated by the hydrothermal growth technique. We will emphasis on the hydrothermal technology of large size ZnO crystal. Material characteristics are discussed and compared to ZnO fabricated from vapor phase and melt growth. A route to obtain high-quality, single-crystalline ZnO films with mechanically untouched surfaces is employed to screen the doping of ZnO under the conditions of growth near the thermodynamic equilibrium. Future trends in ZnO crystal growth technology will be discussed.

We have used positron annihilation spectroscopy to study vacancy defects in ZnO single crystals grown by various methods from both commercial and academic sources. The combination of positron lifetime and Doppler broadening techniques with theoretical calculations provides the means to deduce both the identities and the concentrations of the vacancies. The annihilation characteristics of the Zn and O vacancies have been determined by studying electronirradiated ZnO grown by the seeded vapor phase technique. The different ZnO samples were grown with the following techniques: the hydrothermal growth method, the seeded vapor phase technique, growth from melt (skull melting technique), and both conventional and contactless chemical vapor transport. We present a comparison of the vacancy defects and their concentrations in these materials.

Starting with an introduction to surface acoustic waves, their generation and detection using interdigital transducers (IDTs) on piezoelectric materials (e.g. LiNbO₃ and ZnO) will be reviewed. Then the application of surface acoustic waves in electronic devices will be presented. Moreover, recent studies, using the technique of attaching the material of investigation onto the sound path of the acoustic delay line between the IDTs is discussed.

We report here the synthesis of ZnO films by the pulsed-laser deposition technique using various novel conditions. The dopants are As, Ga, Al and N. The films show excellent crystalline quality with atomically smooth surface morphology. The electrical resistivity was found to be close to 2 x~10^-4 ohm-cm and transmittance >85% with both Ga and Al doping. Doping with As shows several distinct transitions in their electrical resistivity and strong aging effects. On the other hand, doping with Mn in ZnO reduces the grain size. On the other hand, doping with trivalent Er ions in ZnO films causes two effects: for high doping (>8 wt%), a substantial enhancement of diagonal piezo-optic effect (up to 3.7*10^-13 m²/N at &lgr;=633 nm) was observed due to creation of additional dipole moments at the interface of the film and the substrate, and higher electrical conductivity with enhanced 1.54 &mgr;m emission was demonstrated at room temperature for low concentration (<2 wt%) of Er. Furthermore, no quenching effects in emission characteristics at 1.54 &mgr;m were observed up to 2 wt % of Er-doping in ZnO at room-temperature.

We report on the synthesis and processing, and structure - property correlations in gallium doped ZnO films grown on (0001) sapphire and glass substrates by pulsed laser deposition. Films with varying microstructure were grown on amorphous glass by changing the pulsed laser deposition parameters, namely temperature and oxygen partial pressure. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. It is shown that resistivities and transmittance comparable to epitaxial Zn_0.95Ga_0.05O films (&rgr;=1.4x10^-4&OHgr;-cm, %T>80) can be achieved in the nanocrystalline films (&rgr;=1.8x10^-4&OHgr;-cm, %T>80) deposited on glass by carefully controlling the deposition parameters. We have investigated and modeled the conduction mechanisms (carrier generation and carrier transport) in the novel Ga:ZnO films through detailed structural characterization, chemical analysis, and electrical and optical property measurements. The device applications based on these highly conducting and transparent films as electrodes will also be discussed. Our preliminary results have demonstrated that power conversion efficiencies comparable to indium tin oxide (ITO) based organic photovoltaic devices can be achieved using ZnGaO films as the anode.

Scaling behaviors of ZnO transparent thin-film transistors (TTFTs) have been studied by fabricating series of miniaturized ZnO TTFTs having various channel widths and lengths. Mobility of >8 cm²/V.s and on/off ratio of up to 10⁷ are achieved with these TTFTs. Results show that these ZnO TTFTs retain rather well-behaved transistor characteristics down to the channel length of ~5 &mgr;m, rendering possible high-resolution applications. More apparent short-channel effects (e.g., lowering of threshold voltages, degradation of the subthreshold slope with the decrease of the channel length and the increase of the drain voltage, and loss of hard saturation, etc.) are observed when the channel length is reduced below 5 &mgr;m. Influences of parasitic effects on TFT characteristics are also studied by extracting parasitic resistance and channel resistance using devices of various dimensions. The ratio of parasitic resistance to channel resistance at VG = 10 V was increased from 0.04 to 0.36, when the channel length decreased from 20 &mgr;m to 2 &mgr;m. This indicates that parasitic resistance has substantial influences on device performances (e.g., output drain current, apparent field effect mobility, etc.) when the channel length is reduced, and better contact techniques may be required.

Low resistivity and high transparent ITO, RuO_x (1⩽x⩽2) and TiW ohmic contacts to ZnO film was achieved by RF sputter system and annealing treatment. The transmittance of 450°C-annealing ITO, 650°C-annealing Ru and 200°C-annealing TiW were measured to be 94, 68 and 61%, with wavelength of 400 nm, respectively. Moreover, the specific contact resistance of 450°C-annealing ITO, 650°C-annealing Ru and 200°C-annealing TiW on ZnO films was estimated to be 2.15x10^-4, 2.72x10^-4 and 2.56x10^-4 &OHgr;-cm² by circular transmission line model (CTLM) method, respectively. In the study of ZnO-based photodiodes, high quality and vertical well-aligned ZnO nanowires were selectively grown on ZnO:Ga/glass templates by vapor-liquid-solid method. Ultraviolet (UV) photodetectors using these vertical ZnO nanowires were also fabricated by spin-on-glass technology. With 2 V applied bias, it was found that dark current density of the fabricated device was only 3.8x10^-9 A/cm². It was also found that UV-to-visible rejection ratio and quantum efficiency of the fabricated ZnO nanowire photodetectors were more than 1000 and 12.6%, respectively.

Lead zirconate titanate PbZr₅₂Ti₄₈O₃ (PZT) layers were deposited on ZnO layers by rf-sputtering at varying substrate temperatures. The effect of annealing on PZT crystal properties has been studied by X-ray diffraction and atomic force microscopy. It is shown that the annealing in oxygen ambient has significant effect on the quality of the deposited PZT layers. The optimum growth temperature has been found to be 650 C.

SnO₂ and ZnO and metal oxide nanowires were synthesized by vapor transport process in a horizontal tube furnace. The peculiar characteristic of these materials is the emission of visible photoluminescence (PL) when they are excited with UV light. The visible photoluminescence of tin and zinc oxide nanowires is quenched by nitrogen dioxide at ppm level in a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity and other polluting species, such as CO and NH₃. We believe that adsorbed gaseous species that create surface states can quench PL by creating competitive nonradiative paths.

Films of ZnO micro-nano structures were deposited on quartz substrates and subsequently plasma treated in O₂, N₂ and CF₄. It was found that exposure to oxygen plasma enhanced gas response to ethanol vapor of the ZnO films by a factor 2. The effect of surface plasma treatments on the gas response of the ZnO films was discussed in reference to surface morphology observed by high-magnification SEM and surface chemical state determined by XPS. SEM observation revealed that O₂ plasma treatment induced less surface roughening than N₂ and CF₄ plasmas, in agreement with the view that O₂ plasma should reduce preferential sputtering. Deconvolution of the O 1s X-ray photoelectron peak indicated an increase in the Zn-O bond surface density relatively to O-H bond density for the O₂ plasma treated surface, whereas the O-H bond surface density was increased relatively to the Zn-O bond density for the N₂ and CF₄ plasma treated films. The O₂ plasma was found to partially clean the surface from hydroxyl groups and to expose more Zn cations, which might have caused the enhancement of sensor response by increasing the density of active sites for oxidation/reduction reactions.

In this work we investigate the possibility to use Zinc Oxide (ZnO) thin films, deposited by RF magnetron sputtering, for the realization of integrated optical structures working at 1550 nm. Structural properties of sputtered zinc oxide thin films were studied by means of X-ray Diffraction (XRD) measurements, while optical properties were investigated by spectrophotometry and Spectroscopic Ellipsometry (SE). In particular, ellipsometric measurements allowed to determine the dispersion law of the ZnO complex refractive index (see manuscript) = n - jk through the multilayer modeling using Tauc-Lorentz (TL) dispersion model. We have found a preferential c-axis growth of ZnO films, with slightly variable deposition rates from 2.5 to 3.8 Å/s. Conversely, the refractive index exhibits, from UV to near IR, a considerable and almost linear variation when the oxygen flux value in the deposition chamber varies from 0 to 10 sccm. In order to realize a waveguide structure, a 3-&mgr;m-thick ZnO film was deposited onto silicon single crystal substrates, where a 0.5-&mgr;m-thick thermal SiO₂ buffer layer was previously realized, acting as lower cladding. Dry and wet chemical etching processes have been investigated to achieve controllable etching rate and step etching profile, with the aim to realize an optical rib waveguide. The etched surfaces were inspected using scanning electron microscopy (SEM) and optical microscopy. Moreover, we carried out the experimental measurements of the fringes pattern and Free Spectral Range (FSR) of an integrated Fabry- Perot etalon, obtained by cleaving of a single mode rib waveguide.

We report growth of ZnO nanostructures on Au-coated Si substrates using vapor phase transport in the temperature range from 800°C to 1150°C. Nanostructures grown at 800°C are rod-like with diameters of ~ 200 nm. Growth at higher temperature shows a more complex behaviour with 2-D structures connecting 1-D nanorods at intermediate temperatures and 3-D growth at the highest temperatures. Our work indicates that it may be possible to systematically control the growth mode and morphology of ZnO nanostructures by tuning the growth temperature.

ZnO nano-nails which consist of the nanowire at the top of nanorods have been synthesized by the nanoparticle-assisted deposition technique. When the nanowire was excited by the third harmonics of a Q-switched Nd:YAG laser, the ultraviolet stimulated emission was clearly observed from single nanowire, indicating the high crystalinity of the nanowire. The highly sensitive ultraviolet photo-detectors were successfully fabricated by trapping these nanowires between electrodes using the dielectrophoresis technique.

A simple method has been developed for the controlled patterned growth of the ZnO nanorod arrays with different size and shape on substrate. In order to control the position of the ZnO nanorods, exposed ZnO seed is defined, as orderly aligned arrays, with the assistance of photolithography. This technique hinges on the patterning of the seed layer comprised by ZnO sol-gel precursor. The simple way to create patterned ZnO seed array is to use negative photoresist for ZnO seed coating. The UV exposures were performed though mask patterned various shape. The ZnO arrays are synthesized using solution chemical method at normal atmospheric pressure without any metal catalyst. A simple two-step process is developed for ZnO nanorod on substrate at 90°C. The ZnO seed precutsor is prepared by sol-gel process. The ZnO nanorod is grown by solution chemical method. The ZnO nanorod growth was dependent on the ZnO seed layer. The ZnO nanorods have length of 400~500nm and diameter of 25~50nm. The ZnO nanorod is single crystals with wurtzite and grows along the c axis of the crystal plane. The room temperature photoluminescence measurements have shown ultraviolet peaks 378.3nm (3.27eV) with high intensity.

ZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al₂O₃, and bulk ZnO(0001) substrates. Morphology studies as well as X-ray diffraction and transmission electron microscopy showed that ZnO nanorods grew mostly perpendicular to the GaN(0001) and ZnO(0001) substrate surface, whereas a more random directional distribution was found for nanorods on Si(001). Integral optical properties of fabricated nanorods were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was observed from ZnO nanorods on GaN(0001)/c-Al₂O₃ substrates, most likely due to their vertical orientation. Near-field scanning optical microscopy was applied to investigate luminescent properties of individual rods. Raman spectroscopy revealed biaxial compressive strain in the nanorod samples grown on Si(001). Conductive atomic force microscopy showed that nanorods are electrically isolated from each other. I-V spectra of individual nanorods were measured.

One dimensional (1-D) ZnO nanorod structure of hexagonal shape was fabricated on epitaxial GaN layer by hydrothermal method. The growth of GaN epitaxial layer was carried out in a two-flow horizontal MOCVD reactor maintained at a pressure of 200 torr. Firstly, a 25 nm thick GaN buffer layer was grown at 520 °C. Then 2~3&mgr;m thick GaN epilayer was deposited at 1070 °C. Trimethylgallium (TMG) and NH₃ were used as Ga and N source, and H₂ gas was used as carrier gas. After the deposition of GaN epilayer thin-film, single crystalline ZnO nanorod was fabricated in aqueous solution. XRD and FE-SEM results showed ZnO nanorod arrays were oriented highly along the (002) plane. The ZnO nanorod was analyzed to have good quality crystallization by FE-TEM. The SAED pattern has shown that ZnO nanorod was grown in the direction along (002)-plane. Photoluminescence (PL) has shown that the GaN-ZnO hetero-structure has shown ultra-violet lasing action at room temperature. Narrow and strong ultra-violet peak was observed in comparison with PL result from epitaxial GaN layer. The analysis results have proved that aqueous solution growth method developed in the present work can be a good application for optical electronic device.

Recent theoretical predictions of ferromagnetic behavior in transition metal (TM)-doped ZnO have focused significant attention on these materials for use as spintronic materials. Moreover, rare earth (RE) elements in wide bandgap semiconductors would be useful not only in spintronics but also in optoelectronic applications. This work presents results obtained from an investigation into the optical, magnetic, and structural properties of transition-metal (TM)- doped ZnO and rare earth (RE) doped ZnO (TM = Mn, Co, Ni, and Fe; RE = Gd, Eu, and Tb) bulk crystals and thin films. Properties of TM- and RE-doped ZnO bulk crystals and thin films were studied and compared in order to better understand the nature of these dopant centers and their effects on the properties of the host crystal. Optical properties confirm the incorporation of substitutional transition metal ions on cation sites. While most thin film samples show ferromagnetic behavior, the magnetic response of the bulk crystals varies. This suggests that the magnetic behavior of TM-doped ZnO is highly dependent on growth conditions, and growth conditions which favor the formation of grain boundaries and interfaces may be more likely to result in ferromagnetic behavior. Origins of this ferromagnetic behavior are still under investigation. Defect luminescence observed in the RE-doped samples suggests that these materials may prove useful in optoelectonic applications as well.

The integration of ZnO based high mobility transparent semiconductors with perovskites that exhibit a wide spectrum of physical properties (superconductivity, ferroelectricity, ferromagnetism, etc.) may lead to a wide variety of new electronic/optoelectronic devices. Here we present results about the deposition of high crystalline quality Al or Co doped ZnO films grown by pulsed laser deposition on 110 face of strontium titanate single crystals. Field Effect (FE) experiment, allowing to change the carrier concentration of the film by more then 4 orders of magnitude (from ≈10¹⁵ to ≈10²⁰ e-/cm³, estimated by Hall effect measurements under FE), were employed to investigate transport mechanisms in depth. In particular we observed a crossover of low temperature magnetoresistance from a negative behaviour in the accumulation state to a positive one in the depletion state. The measurement of the activation energy as a function of the Gate potential allowed to get information on the density of states.

Strong polarization coupling is expected by combining ferroelectric materials with switchable polarization and wurtzite layers exhibiting a permanent spontaneous polarization. To demonstrate these charge coupling effects, current-voltage, conductivity-frequency and capacitance-frequency (admittance) characteristics have been measured on epitaxial heterostructures grown of ferroelectric BaTiO₃ (001) films on conducting SrRuO₃ layers on SrTiO₃ (100) substrates with oxide SrRuO_x, metallic Pt and semiconducting ZnO top electrodes. The electrical measurements show clear indications for polarization coupling of the ferroelectric perovskite BaTiO₃ and the piezoelectric wurtzite ZnO thin films.