Growth of single-phase Mg0.3Zn0.7O films suitable for solar-blind optical devices on RS-MgO substrates

Single-phase rock-salt Mg_0.3Zn_0.7O film was fabricated on MgO (100) substrate by radio-frequency plasma assisted molecular beam epitaxy. A smooth surface was observed by in-situ reflection high-energy electron diffraction and ex-situ atomic force microscopy. X-ray diffraction characterization demonstrated a high-quality single-phase structure with highly (200) orientation and cube-on-cube epitaxial relationship. Zn fraction in the single-phase rock-salt Mg_0.3Zn_0.7O film was determined by Rutherford backscattering spectrometry. Optical property of the film was investigated by reflectance spectroscopy, which indicated a solar-blind band gap of 255.5 nm. The reason why Zn solubility limit is greatly enhanced in non-polar (100) film compared with (111) polar epilayer is tentatively discussed in this work, suggesting MgO (100) is more suitable for the synthesis of single-phase rock-salt MgZnO with high Zn content towards solar-blind opto-device applications.     

Intrinsic room temperature ferromagnetism in Zn0.92Co0.08O thin films prepared by pulsed laser deposition

Zn_0.92Co_0.08O thin films were fabricated on p-type Si (100) and quartz substrates by pulsed laser deposition using a ZnCoO ceramic target. The structural and magnetic properties of the films were characterized by field emission scan electronic microscopy, x-ray diffraction, x-ray photoemission spectroscopy, UV-visible transmission spectra, extended x-ray absorption fine structure spectroscopy and physical property measurement system. Substitutional doping of Co²⁺ in ZnO lattice is demonstrated in the films. The as-deposited Zn_0.92Co_0.08O thin film displayed intrinsic room temperature ferromagnetism with saturation magnetization (M_s) of ~ 0.20μ_B/Co. The corresponding M_s increased to ~ 0.23μ_B/Co when annealed in vacuum and further to ~ 0.42μ_B/Co after annealed in hydrogen. In turn, the M_s dropped to the value of ~ 0.13μ_B/Co after annealed in oxygen. Our studies indicate that oxygen vacancy density plays a key role in mediating the ferromagnetism of the diluted magnetic semiconductor films.     

Characterizations of ferromagnetic Zn1−xCoxO thin films grown on Al2O3 (0001) by reactive radio-frequency magnetron sputtering coupled with post-growth annealing

High-quality ferromagnetic Zn_1−xCo_xO thin films were deposited on a sapphire (0001) substrate at 600 °C by using reactive radio-frequency magnetron sputtering coupled with post-annealing treatment for 1 h at 580 °C under an Ar atmosphere. High resolution X-ray diffraction patterns show that hexagonal wurzite crystal structures of undoped ZnO film were maintained even after Co doping up to 4.5 at.% without forming Co clusters or oxides. X-ray photoelectron spectroscopy spectra represent the energy difference of 15.42 eV between Co2p_3/2 and Co2p_1/2, which is different from 15.05 eV of Co clusters. The characteristic absorption bands near 658, 616, and 568 nm wavelengths out of UV-VIS-IR spectroscopy spectra are correlated with the d-d transitions of tetrahedrally coordinated Co²⁺ ions. The low temperature photoluminescence spectrum for undoped ZnO shows a strong near-band edge (NBE) emission peak of 3.42 eV without deep level emission peaks. But, Co content increases in Zn_1−xCo_xO film, the NBE emission peak intensity decreases and another emission peak at 3.37 eV as well as a broad green emission peak at around 2.5 eV starts to appear with larger intensity due to the more actively creating oxygen vacancies. The emission peak at 3.37 eV proves the interaction between Co ions and the hydrogenic electrons in the impurity band and also supports the typical ferromagnetic hysteresis curves obtained by superconducting quantum interface device magnetometry at 300 K for Zn_1−xCo_xO films. High insulator characteristics are observed for as-grown Zn_1−xCo_xO films whereas it exhibits n-type characteristics with the increased carrier concentration, mobility, and resistivity after post-growth annealing. The spintronic devices could be fabricated with the utilization of Zn_1−xCo_xO films grown by the economically feasible reactive radio-frequency magnetron sputtering coupled with the post annealing treatment.     

Influence of annealing on characteristics of tin disulfide thin films by vacuum thermal evaporation

In this paper, we reported on an approach to prepare tin disulfide (SnS₂) thin films on soda-lime glass substrates by vacuum thermal evaporation using SnS₂ powders as a source. The influence of annealing on the chemical composition, crystal structure, surface morphology, and optical band gap of the SnS₂ thin films was systemically investigated. The as-grown SnS₂ thin film was amorphous, homogeneous, smooth, nearly stoichiometric, with no pinhole and crack free, and with an optical band gap of 2.41 eV. After the SnS₂ thin film was annealed at 300 °C, the crystallization of SnS₂ was demonstrated by X-ray diffraction and scanning electron microscope with a characteristic of a preferred orientation along (001) plane with hexagonal phase and the sheet appearance of the SnS₂ crystals. At the annealing temperature of 350 °C, some SnS₂ crystallites and a few pinholes appeared on the surface of the SnS₂ thin films, though the SnS₂ thin film was not oxidized. When the annealing temperature was increased to 400 °C, SnS₂ was gradually oxidized into an approximate spherical shape of SnO₂ from the top to the bottom of the SnS₂ thin film by trace O₂ in the furnace. Therefore, our experiment suggested that the annealing temperature of the SnS₂ thin film using the vacuum thermal evaporation should not be over 300 °C as a window layer in compound thin film solar cells.     

High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric

A high-performance ZnO thin film transistor （ZnO-TFT） with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer gate insulator is fabricated by sputtering at room temperature. Compared to ZnO-TFTs with sputtering SiO₂ gate insulator, its electrical characteristics are significantly improved, such as the field effect mobility enhanced from 11.2 to 52.4 cm²/V s, threshold voltage decreased from 4.2 to 2 V, and sub-threshold swing improved from 0.61 to 0.28 V/dec. The improvements are attributed to the high gate capacitance (from 50 to 150 nF/cm²) as well as nice surface morphology by using dielectric with high～k Ta₂O₅ sandwiched by SiO₂ layers. The capacitance-voltage characteristic of a metal-insulator-semiconductor capacitor with the structure of Indium Tin Oxide/STS/ZnO/Al was investigated and the trap charges at the interface or bulk is evaluated to be 2.24 × 10¹² cm⁻². From the slope of C⁻² versus gate voltage, the doping density N_D of ZnO is estimated to be 1.49 × 10¹⁶ cm⁻³.     

Microstructural evolution of sol-gel derived ZnO thin films

Zinc oxide thin films, with thicknesses between ∼ 20 and 450 nm, were prepared by spin-coating a sol-gel precursor solution (zinc acetate dihydrate and monoethanolamine in an isopropanol solvent) onto glass substrates, followed by heat treatment at temperatures through 773 K. At 298 and 373 K, the films exhibited the structure of a lamellar ZnO precursor, Layered Basic Zinc Acetate (LBZA). At higher temperatures, LBZA released intercalated water and acetate groups and dehydroxylated to form zinc oxide nanograins with wurtzite structure, which were preferentially oriented in the c-axis direction. Both the degree of the films' c-axis orientation and the topography of their surfaces varied with heat treatment and precursor concentration. For films calcined at 773 K, a minimum of micron-scale surface wrinkles coincided with a maximum in c-axis preference at intermediate concentrations, suggesting that release of mechanical stress during densification of thicker films may have disrupted the ordering process that occurs during heat treatment.     

Crystallization and electrical resistivity of Cu2O and CuO obtained by thermal oxidation of Cu thin films on SiO2/Si substrates

In this work, we study the crystallization and electrical resistivity of the formed oxides in a Cu/SiO₂/Si thin film after thermal oxidation by ex-situ annealing at different temperatures up to 1000 °C. Upon increasing the annealing temperature, from the X ray diffractogram the phase evolution Cu → Cu + Cu₂O → Cu₂O → Cu₂O + CuO → CuO was detected. Pure Cu₂O films are obtained at 200 °C, whereas uniform CuO films without structural surface defects such as terraces, kinks, porosity or cracks are obtained in the temperature range 300-550 °C. In both oxides, crystallization improves with annealing temperature. A resistivity phase diagram, which is obtained from the current-voltage response, is presented here. The resistivity was expected to increase linearly as a function of the annealing temperature due to evolution of oxides. However, anomalous decreases are observed at different temperatures ranges, this may be related to the improvement of the crystallization and crystallite size when the temperature increases.     

Effect of annealing temperature on the tribological behavior of ZnO films prepared by sol-gel method

The tribological behavior of zinc oxide (ZnO) films grown on glass and silicon (100) substrates by sol-gel method was investigated. Particularly, the as-coated films were post-annealed at different temperatures in air to investigate the effect of annealing temperature. Crystal structural and surface morphology of the films were measured by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). XRD patterns and AFM images indicated that the crystallinity and grain size of the films were enhanced and increased, respectively, with temperature. The tribological behavior of films was evaluated by sliding the ZnO films against a Si₃N₄ ball under 0.5 gf normal load using a reciprocating pin-on-plate tribo-tester. The wear tracks of the films were measured by AFM to quantify the wear resistance of the films. The results showed that the wear resistance of the films could be improved by the annealing process. The wear resistance of the films generally increased with annealing temperature. Specifically, the wear resistance of the films was significantly improved when the annealing temperature was higher than 550° C. The increase in the wear resistance is attributed to the increase in hardness and modulus of the film with annealing temperature.     

Thin-film transistors with sol-gel deposited Mg0.1Zn0.9O films as active channel layer

We report on field-effect transistors using Mg_0.1Zn_0.9O thin film as channel layer. The effect of Mg is to increase the band gap and decrease the electron carrier concentration. The Mg_0.1Zn_0.9O film deposited by sol-gel method has a highly c-axis orientation and excellent optical properties. The devices display a channel mobility of 0.76 cm² V^− 1 s^− 1 and an on/off ratio of 400.     

Room temperature deposition of Al-doped ZnO films on quartz substrates by radio-frequency magnetron sputtering and effects of thermal annealing

High-quality Al-doped zinc oxide (AZO) thin films have been deposited on quartz substrates by radio-frequency magnetron sputtering at room temperature for thin film solar cell applications as transparent conductive oxide (TCO) electrode layers. Effects of post-deposition annealing treatment in pure nitrogen and nitrogen/hydrogen atmosphere have been investigated. Annealing treatments were carried out from 300 °C to 600 °C for compatibility with typical optoelectronic device fabrication processes. A series of characterization techniques, including X-ray diffraction, scanning electron microscopy, Hall, optical transmission, and X-ray photoelectron spectroscopy has been employed to study these AZO materials. It was found that there were significant changes in crystallinity of the films, resistivity increased from 4.60 × 10^− 4 to 4.66 × 10^− 3 Ω cm and carrier concentration decreased from 8.68 × 10²⁰ to 2.77 × 10²⁰ cm^− 3 when annealing in 400 °C pure nitrogen. Whereas there were no significant changes in electrical and optical properties of the AZO films when annealing in 300-500 °C nitrogen/hydrogen atmosphere, the electrical stability of the AZO films during the hydrogen treatment is attributed to both desorption of adsorbed oxygen from the grain boundaries and production of additional oxygen vacancies that act as donor centers in the films by removal of oxygen from the ZnO matrix. These results demonstrated that the AZO films are stably suited for TCO electrodes in display devices and solar cells.     

Temperature dependence of conduction mechanism of ZnO and Co-doped ZnO thin films

ZnO and Co-doped ZnO thin films have been deposited on the sapphire substrate by ultrasonic assisted chemical vapor deposition technique. The films were annealed in vacuum at 450 °C. All the films were highly c-axis orientated and contained no impurity phase. The temperature dependence of the electrical conductivity has been measured in order to identify the dominant conduction mechanism. In the higher temperature region the dominance of thermally activated band conduction was observed whereas in the low temperature region the hopping conduction was found to dominate. The hopping conduction mechanism in the lower temperature range in the film was Mott's Variable Range Hopping and not the Nearest Neighbor Hopping. The temperature region, where hopping conduction was dominant found to increase by Co doping in ZnO film. The localization length was found to be larger in the Co-doped ZnO film.     

Influence of VI/II ratios on the growth of ZnO thin films on sapphire substrates by low temperature MOCVD

We investigated the structural, electrical, and optical properties of ZnO thin films grown at different VI/II ratios on sapphire substrates by metalorganic chemical vapor deposition. Transmission electron microscopy and X-ray diffraction revealed the epitaxial nature with a reduced dislocation density of the ZnO films grown at increased VI/II ratios. The carrier concentration of the films increased to 4.9 × 10¹⁸ cm^− 3 and their resistivity decreased to 1.4 × 10^− 1 Ω cm at a VI/II ratio of 513.4 μmol/min. The ZnO films also showed good optical transmittance (> 80%) in the visible and near-infrared wavelength regions. The room temperature PL revealed a strong band-edge emission with a weak deep level emission, suggesting the good crystalline quality of the ZnO films on the sapphire substrates. Furthermore, the intensity ratio of the band-edge emission to the deep-level emission (I_UV/I_Vis) increased with increasing VI/II ratio.     

Activation of phosphorous doping in high quality ZnO thin film grown on Yttria-stabilized zirconia (1 1 1) by thermal treatment

Phosphorous doped (P-doped) ZnO thin films are grown on c-sapphire and Yttria-stabilized zirconia (YSZ) (111) substrates by pulsed laser deposition. Post growth annealing is carried out to activate phosphorous to act as acceptor. The rocking curve of annealed P-doped ZnO films grown on YSZ (111) has full width at half maximum of 0.08°, more than two times narrower than that of the as-grown one. Neutral acceptor bound exciton (A⁰X) is observed from low temperature photoluminescence with estimated activation energy of 11.3 meV. Low carrier concentration of as-grown P-doped ZnO films indicated phosphorous doping creates acceptor states and/or reduced the oxygen vacancies. The carrier concentration of annealed samples is reduced by five order magnitudes from 3.21 × 10¹⁸ cm^− 3 to 1.21-8.19 × 10¹³ cm^− 3. At annealing temperature of 850 °C, the sample has the lowest carrier concentration and highest resistivity. This is an indication that the phosphorous in P-doped ZnO has been activated.     

Effect of annealing temperature on structural, electrical and optical properties of B-N codoped ZnO thin films

The B-N codoped p-type ZnO thin films have been prepared by radio frequency magnetron sputtering using a mixture of nitrogen and oxygen as sputtering gas. The effect of annealing temperature on the structural, electrical and optical properties of B-N codoped films was investigated by using X-ray diffraction, Hall-effect, photoluminescence and optical transmission measurements. Results indicated that the electrical properties of the films were extremely sensitive to the annealing temperature and the conduction type could be changed dramatically from n-type to p-type, and finally changed to weak p-type in a range from 600 °C to 800 °C. The B-N codoped p-type ZnO film with good structural, electrical and optical properties can be obtained at an intermediate annealing temperature region (e.g., 650 °C). The codoped p-type ZnO had the lowest resistivity of 2.3 Ω cm, Hall mobility of 11 cm²/Vs and carrier concentration of 1.2 × 10¹⁷ cm^− 3.     

Structure, luminescence and electrical properties of ZnO thin films annealed in H2 and H2O ambient: A comparative study

Undoped ZnO films were grown on a c-plane sapphire by plasma-assisted molecular-beam epitaxy technique, and subsequently annealed at 200-500 °C with steps of 100 °C in water vapour and hydrogen ambient, respectively. It is found that the c-axis lattice constant of the ZnO films annealed in hydrogen or water vapour at 200 °C increases sharply, thereafter decreases slowly with increasing annealing temperature ranging from 300 °C to 500 °C. The stress in the as-grown ZnO films was more easily relaxed in water vapour than in hydrogen ambient. Interestingly, the controversial luminescence band at 3.310 eV, which is often observed in photoluminescence (PL) spectra of the ZnO films doped by p-type dopants, was observed in the PL spectra of the annealed undoped ZnO films and the PL intensity increases with increasing annealing temperature, indicating that the 3.310 eV band is not related to p-type doping of ZnO films. The electron concentration of the ZnO films increases sharply with increasing annealing temperature when annealed in hydrogen ambient but decreases slowly when annealed in water vapour. The mechanisms of the effects of annealing ambient on the properties of the ZnO films are discussed.     

Influence of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing

ZnO thin films were prepared in Ar and Ar + H₂ atmospheres by rf magnetron sputtering, and then they were annealed in vacuum and Ar + H₂ atmosphere, respectively. The structure and optical-electrical properties of the films were investigated by X-ray diffraction, transmittance spectra, and resistivity measurement, and their dependences on deposition atmosphere, annealing treatment, and aging were studied. The results showed that adding H₂ in deposition atmosphere improved the crystallinity of the films, decreased lattice constant, increased band gap, decreased the resistivity by the order of 10⁴ Ω cm, but exhibited poor conductive stability with aging. After Ar + H₂ and vacuum annealing, crystallinity of the films deposited in Ar and Ar + H₂ was further improved; their resistivity was decreased by the order of 10⁵ and 10¹ Ω cm, respectively, and exhibited high conductive stability with aging. We suggest that the formed main defect is V_O and H_i when H₂ is introduced during deposition, which decreases the resistivity but cannot improve the conductive stability; hydrogen would remove negatively charged oxygen species near grain boundaries during Ar + H₂ annealing to decrease the resistivity, and grain boundaries are passivated by formation of a number of V_O-H complex (H_O) to improve the conductive stability at the same time. Under vacuum annealing, the hydrogen that is introduced non-intentionally from deposition chamber maybe plays an important role; it exists as H_O in the films to improve the conductive stability of the films.     

ZnO/Cu/ZnO multilayer films: Structure optimization and investigation on photoelectric properties

A series of ZnO/Cu/ZnO multilayer films has been fabricated from zinc and copper metallic targets by simultaneous RF and DC magnetron sputtering. Numerical simulation of the optical properties of the multilayer films has been carried out in order to guide the experimental work. The influences of the ZnO and Cu layer thicknesses, and of O₂/Ar ratio on the photoelectric and structural properties of the films were investigated. The optical and electrical properties of the multilayers were studied by optical spectrometry and four point probe measurements, respectively. The structural properties were investigated using X-ray diffraction. The performance of the multilayers as transparent conducting coatings was compared using a figure of merit. In experiments, the thickness of the ZnO layers was varied between 4 and 70 nm and those of Cu were between 8 and 37 nm. The O₂/Ar ratios range from 1:5 to 2:1. Low sheet resistance and high transmittance were obtained when the film was prepared using an O₂/Ar ratio of 1:4 and a thickness of ZnO (60 nm)/Cu (15 nm)/ZnO (60 nm).     

Effect of annealing temperature on ZnO:Al/p-Si heterojunctions

Al-doped, zinc oxide (ZnO:Al) films with a 1.2 at.% Al concentration were deposited on p-type silicon wafers using a sol-gel dip coating technique to produce a ZnO:Al/p-Si heterojunction. Following deposition and subsequent drying processes, the films were annealed in vacuum at five different temperatures between 550 and 900 °C for 1 h. The resistivity of the films decreased with increasing annealing temperature, and an annealing temperature of 700 °C provided controlled current flow through the ZnO:Al/p-Si heterojunction up to 20 V. The ZnO:Al film deposited on a p-type silicon wafer with 1.2 at.% Al concentration was concluded to have the potential for use in electronic devices as a diode after annealing at 700 °C.