ZnO nanorods synthesis on a glass substrate coated with ZnO film was performed by means of microwave-assisted heating. Nanorod structure was dependent on the underlying ZnO films; slightly tilted nanorods about 800nm diameter were synthesized on an as-sputtered ZnO film while highly c-axis oriented ones about 100nm diameter were developed on the ZnO film after annealing. Photoluminescence spectra of the ZnO nanorods at 6K showed a badedge excitonic emission with comparable intensity to a visible defect related emission, suggesting the existence of many radiative defects which would be originated from insufficient quality of the underlying ZnO layer. Pattered photoresist layer can successfully be utilized for synthesis of the ZnO nanolods in selective area.

The performance of ZnO metal-semiconductor-metal (MSM) photodetectors can be significantly influenced by modifications in the fabrication process. ZnO thin films were deposited onto silicon substrates by radio frequency magnetron sputtering and later annealed by conventional furnace, rapid thermal anneal or laser anneal. The photoluminescence (PL) analysis revealed that laser annealing at 250 mW/cm2 increased the 370 nm peak from 520 to 1700 a.u. and reduced the defect peak from 380 to 20 a.u. MSM photodetectors were fabricated using an interdigitated and parallel pattern. Values of current responsivity ranged from about 0.025 A/W to above 430 A/W depending upon fabrication conditions and design.

Unintentionally doped bulk ZnO samples were grown by hydrothermal method in Tokyo Denpa Co. Ltd. (Japan) and MTI Corporation. At low temperatures the PL spectrum contained a very broad band with the peak position (between 2.0 and 2.4 eV) depending on the excitation intensity. Evolution of the PL spectrum after a pulse excitation revealed that the broad band is composed of an orange (OL) and green (GL) luminescence bands having maxima at 1.96 and ∼2.35 eV, respectively. The GL band dominated at times up to 1 ms and then disappeared. The OL band decayed as approximately t−1 over a wide time interval, and its spectrum could be recorded even 24 hours after the excitation source was switched off. The slow nonexponential decay of the OL band is attributed to transitions from shallow donors to a deep acceptor (donor-acceptor pair transitions).

The performance of ZnO TFTs fabricated via RF sputtering, with Aluminium Nitride (AlN) as the underlying insulator are reported. The surface roughness of ZnO with AlN is lower than that with SiN by at least 5 times, and that with SiO2 by 30 times. The resulting mobility for the three insulators AlN, SiN, SiO2 using identical process is found to be 3, 0.2-0.7 and 0.1-0.25 cm2/Vs respectively. There does not appear to be any corresponding improvement in the stability of the AlN devices. The devices demonstrate significant positive threshold voltage shift with positive gate bias and negative threshold voltage shift with negative gate bias. The underlying cause is surmised to be ultra-fast interface states in combination with bulk traps in the ZnO.

The first observations have been made on the electronic density of states of Mn, Co and Fe-doped ZnO by tunneling spectroscopy. For Mn- and Co-doped ZnO, rising of DOS were observed around 2.8eV and 3.2eV below the bottom of the conduction band, respectively. In these cases, the Fermi level lay at the bottom of the conduction band. For Fe-doped ZnO, the DOS with about 1.2eV width was observed around the Fermi level in the mid-gap. The result reproduced the recent theoretical calculation.

We observed strong shifts of the blue and yellow luminescence bands with variation of excitation intensity in ZnO films grown on sapphire by MBE using hydrogen peroxide as a source of reactive oxygen. The blue band, having a maximum in the range from 2.85 to 3.15 eV in different samples and different excitation intensities at 10 K, is attributed to diagonal transitions from the conduction band (or shallow donors) to the valence band in realm of potential fluctuations caused by random distribution of charged point defects in a compensated semiconductor. The yellow band is related to an unidentified deep acceptor.

This work emphasizes room temperature deposition and fabrication of top-gated staggered structure ZnO-TFTs and integration of ZnO-TFT based simple logic circuits. We synthesized ZnO thin films by RF sputtering in an Ar/Oxygen ambience with no intentional heating of the substrates. The electrical, optical and structural properties of the ZnO thin films can be well-controlled by altering process parameters such as RF power density and relative Oxygen partial pressure. Typical deposition was carried out at a chamber pressure of 15 mTorr, Ar/Oxygen flow rates of 15 sccm/1 sccm and RF power density of 3W/cm2. The resistivity of the as-deposited films was between 104-106 Ù-cm with high optical transparency (>80%) in the visible spectrum and minimal surface roughness as detected by high-resolution AFM imaging. Gated van der Pauw and Kelvin-bridge structures were lithographically patterned to asses ZnO channel resistance. In the completed devices, a dual-stack (Ta2O5/SiO2) dielectric layer was effective in suppressing gate-leakage current below 10 pA and enabled depletion-mode ZnO-TFT operation exhibiting hard saturation. A Ti/Au metallization scheme was adopted to provide good ohmic contact to ZnO. TFTs retained well-behaved transfer characteristics down to a channel length of 4 ìm with on/off drain current ratio exceeding 105, threshold voltage between -15 V to -5 V and inverse sub-threshold slope of around 1.75 V/decade.

Photoluminescence (PL) of erbium-doped zinc oxide films with nano-sized grains was studied. The films were grown on silicon (100) and fused silica substrates using e-beam evaporation. The evaporating targets used were sintered pellets of ZnO and Er2O3 mixtures with two different Er concentrations. The films were subsequently annealed at 700 °C in air for an hour. PL was measured at two excitation wavelengths, 325 and 488 nm. The 325 nm is used for exciting the host semiconductor ZnO and 488 nm is used for directly exciting Er3+ ions in the ZnO host. Strong Er3+ luminescence of 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions was observed from annealed film with 4.0 % Er2O3 concentration using either 325 or 488 nm excitation. With 325 nm excitation, the Er3+ luminescence observed is attributed to energy transfer from the excitons in ZnO host to the Er3+ ions doped. The effective energy transfer from ZnO host to the doped Er3+ ions is an essential property for the realization of actual current-injection opto-electronic devices operating at wavelengths of Er3+ emission, for example, at 1.54 μm for the erbium-doped fiber amplifier (EDFA). Our PL results indicate that thermal annealing plays an important role for optically activating the doped Er3+ ions in ZnO nano-crystalline grains of the film.

The realization of ZnO random cavities may be an appropriate approach to fabricate low-cost ZnO laser diodes as there is no stringent requirement in the design of the disordered media provided that they can sustain enough optical gain and feedback within the media to achieve closed-loop random lasing. However, ZnO random lasers suffer from some drawbacks such as relatively higher threshold than conventional lasers, lack of directionality and coherency light emission as well as the difficulty to realize electrically pumped ZnO random lasers. In this paper, a short review on the possibilities to realize low threshold directional electrically pumped ZnO random laser diodes.

Zinc oxide was considered to be substitute for indium tin oxide (ITO) as the transparent conductive oxide (TCO) films used in display industry according to indium is a rare and expensive metal. For the flexible display, the polymeric materials used as the flexible substrates are more bendable and lighter weight compared to the glass substrates used in flat panel display. However, the thermal sensitivity of polymer affects the properties of TCOs layers when it was deposited at room temperature. In this study, we doped the different ratio of indium oxide, magnesium oxide and aluminum oxide into zinc oxide to improve the properties of zinc oxide used as the TCOs films deposited by the sputtering method. The polyethylene terephthalate (PET) was used as the flexible substrate without thermal process and the glass was used as the comparison substrate with annealing. The light transmittance and the surface resistivity were measured. The chemical composition and the crystal structure were analyzed by using Rutherford backscattering spectrometry and X-ray diffraction measurement.

Co and Mn ions were implanted into n-type ZnO thin films with different electron carrier concentrations. X-ray diffraction measurements show that the ZnO:Co and ZnO:Mn thin films are of high crystallinity. From magnetization measurements, ferromagnetism was observed in both n-type ZnO:Co and n-type ZnO:Mn thin films with Curie temperatures well-above room temperature. Furthermore, the electron carrier concentration dependence of the saturated magnetization was measured in both types of thin films, and our results support an electron-mediated mechanism for ferromagnetism in ZnO:Co, as predicted by theory. However, our measurements seem to contradict theory for ZnO:Mn, which only predicts long-range ferromagnetism for p-type mediated material.

We studied several photoluminescence (PL) bands in undoped, Li-, Ga-, and N-doped high-quality ZnO bulk crystals and thin films grown by molecular beam epitaxy (MBE). By analyzing PL in a wide range of excitation power densities, sample temperature, and decay time after a laser pulse, we distinguished and analyzed more than 10 broad bands with unique luminescence properties. Among these bands, only the Cu-related green band with a characteristic fine structure and the Li-related orange band were well-studied and reliably identified in the past.

We present an attempt to clarify the valence band order of ZnO and MgxZn1−xO films grown by pulsed laser deposition (PLD) on sapphire substrates. We derive the dependence of spin orbit interaction (Δso) and crystal field interaction (Δcf) on the temperature, strain and cation replacement (Zn with Mg) and compared this with theoretical predictions. The strain was varied by using different orientated sapphire substrates (c-, r-, and m-plane orientations) and by varying the film thicknesses. All these investigations support the conclusion that the symmetry order of the valence band is Γ7-Γ9-Γ7 for ZnO and MgxZn1−xO for x ≤ 0.55.

Erbium-doped polycrystalline ZnO fibers were prepared by electrospinning. Organo-metallic Zn and Er (0.5, 1.0 and 2.0 % Er/Zn ratio) precursors dissolved in methanol, DI water and acetic acid were forced through a syringe needle. An electric field of 20 kV over 16 cm was used to promote the growth of fibers from the needle to a rotating drum. The resulting material consisted of entangled fibers with 0.5 to 1 μm typical diameter. The as-prepared samples present faint Er3+ luminescence at 1.5 μm when excited by the 488 nm line of an Ar+ laser. This luminescence intensity increases up to 25 times when the samples are annealed for 30 min at 500 °C in air. The effect of annealing over the Er environment was determined by EXAFS measurements at the Er L3 edge. In all as-prepared samples Er is coordinated to 8 ± 0.5 oxygen atoms at up to 2.36 ± 0.02 Å interatomic separation. After annealing the coordination is reduced to 5.9 ± 0.5 and the atomic separation becomes 2.30 ± 0.02 Å. This is comparable to Er2O3 where the coordination is 6 and the first neighbor distance is 2.26 ± 0.02 Å. The overcoordination in the as-prepared samples is consistent with the Er L3 edge at 8361.7 ± 0.5 eV compared to 8360.2 ± 0.5 eV in the annealed samples and 8359.9 ± 0.5 eV in Er2O3. In conclusion, the annealing of electrospun ZnO:Er fibers makes the Er environment more similar to that of Er2O3 increasing the luminescence efficiency.

A novel approach for fabricating ZnO crystals using self-encapsulated microwave melt growth technique is developed, which permits idealized temperature profile for the nucleation of the ZnO crystals and their growth in a contamination-free environment. This paper reports that both un-doped ZnO and p-type doped ZnO (by P or Li:N doping) single crystals were successfully fabricated by this technique. The as-grown ZnO crystals were of light brown to clear in color depending on processing conditions. Their transparency and colorless form were restored after post-growth annealing in oxygen.

Unintentionally doped n-type ZnO single crystals were implanted by nitrogen ions with different fluences of 1013, 1014 and 1015 cm−2 respectively. ZnO p-n homojunction was successfully fabricated due to the formation of p-type layer after 650°C post-implantation annealing in air for 30 minutes. Further thermal evolution of deep level defects was studied through thermal annealing up to 1200°C. Electrical characterization techniques including current-voltage (I-V), capacitance-voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and double-correlation DLTS (DDLTS) were used for investigating the control sample, all the as-implanted and annealed samples through Au/n-ZnO Schottky diodes as well as ZnO p-n junctions. Detailed electrical properties of fabricated devices and characteristics of implantation induced defects were analyzed based on plentiful DLTS spectra. Moreover, low-temperature photoluminescence experiments of all the as-implanted and annealed samples were performed and the correlation between results from electrical and optical characterizations was discussed.

Highly conducting and transparent Al-doped ZnO (AZO) thin films, which are oriented along c-axis and have wurtzite structure, were grown on quartz substrate at low temperature by pulsed laser deposition. The techniques of x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), optical transmission spectroscopy (OTS), electrical resistivity, and Hall Effect were used to study the effect of growth temperature and oxygen pressure on the structural, electrical transport, and optical properties of these films. The optical transparency in all the films is high and does not change much with oxygen pressure and growth temperature. However, electrical parameters such as resistivity, carrier concentration, and mobility strongly depend on both oxygen pressure and growth temperature. The temperature dependence resistivity measurement indicates semiconducting behavior of all the films. A detailed study indicates that the films which are highly conducting and transparent correspond to an optimum temperature of 200 °C and an oxygen pressure of 5 × 10−7 bar. Higher transmittance of the AZO films compared with pure ZnO and ITO and comparable mobility make us to suggest that Al-doped ZnO is an excellent material for optoelectronic applications.

Precursor-reduction method is employed to synthesize nanostructures of ZnTe. The shape of zinc blende ZnTe nanocrystals could be influenced by reaction temperature, concentration of the precursors as well as the use of stabilizing agent. By controlling the synthetic conditions, three types of nanoscaled ZnTe, quasi-spherical ZnTe nanocrystals, ZnTe nanobelts and ZnTe nanorods have been observed. Their crystalline structures are also discussed.

Highly c-axis oriented, dense, and fine-grained polycrystalline ZnO films with smooth surface and high resistivity were deposited on 4 inch silicon wafers by employing ZnO targets in a radio-frequency (RF) magnetron sputtering system. By changing applied RF power, substrate temperature and O2/Ar gas ratio, the optimum process parameters were found to be 150 W, 200 °C and 30/70, respectively. Applying the ZnO films deposited under these optimum conditions, surface acoustic wave (SAW) devices of ZnO/IDT/SiO2/Si structure were fabricated by conventional photolithography and etching processes. The interdigital transducers (IDT), made of the aluminum deposited by DC magnetron sputter, were patterned as 2.5/2.5 μm of finger width/spacing. Another type of SAW filter of IDT/ZnO/diamond/Si structure was fabricated. In this structure, high-quality nanocrystalline diamond (NCD) films were deposited on 4 inch silicon wafers by direct current (DC) plasma assisted chemical vapor deposition method using H2-CH4 mixture as precursor gas. On the top of the diamond films, ZnO films were deposited under the optimum conditions. The aluminum IDT pattern was fabricated on the ZnO/diamond layered films. The characteristics of the fabricated SAW devices were evaluated in terms of center frequency, insertion loss, and wave propagation velocity.

Atomic layer deposition (ALD) of Al2O3 is used as a passivation layer on ZnO substrates before nitride growth by metalorganic chemical vapor deposition (MOCVD). This layer is being used to prevent Zn diffusion from the substrate, protect the ZnO surface from H2 back etching, and promote high quality nitride growth. ALD-Al2O3 films were grown at 100°C and then annealed in a furnace at various times at 1100°C for crystallization of the passivation layer. XRD results showed both Al2O3 and ZnAl2O4 phases at different intensities for 50 and 20nm ALD-AlM2O3 films. In addition, the InGaN layer has been successfully grown on the passivated ZnO substrate. Findings show that a short annealing time for the ALD-Al2O3 layer will be optimal for InGaN growth.