Phase separation in Ga and N co-incorporated ZnO films and its effects on photo-response in photoelectrochemical water splitting

Ga and N co-incorporated ZnO thin films [ZnO:(Ga:N)] with reduced bandgaps were deposited by co-sputtering at different N₂ gas flow rate in mixed N₂ and O₂ ambient at room temperature followed by postannealing at 500 °C in air for 2 h. We found that all of the ZnO:(Ga:N) films exhibited enhanced crystallinity which can suppress the recombination rate between the photogenerated electrons and holes. However, phase segregation of Zn₃N₂ occurred in ZnO:(Ga:N) thin films in nitrogen-rich sputtering ambient. We found that ZnO:(Ga:N) thin films without phase separation of Zn₃N₂ exhibited much better photoelectrochemical (PEC) response, due to the reduced bandgap and better crystallinity. Our results suggest that growth conditions must be controlled carefully to avoid phase separation in Ga and N co-incorporated ZnO thin films to improve PEC response.     

Effects of growth ambient on electrical properties of Al-N co-doped p-type ZnO films

p-Type zinc oxide thin films with c-axis orientation were prepared in N₂O-O₂ atmosphere by an Al-N co-doping method using reactive magnetron sputtering. Secondary ion mass spectroscopy (SIMS) measurements indicate that as-grown ZnO films were co-doped with Al and N. Hall effect measurements show a dependence of types of conduction, carrier concentration and mobility of as-grown ZnO films on N₂O partial pressure ratios. p-Type ZnO thin films deposited in a N₂O partial ratio of 10% show the highest hole concentration of 1.1×10¹⁷ cm⁻³, the lowest resistivity of about 100 Ω cm, and a low mobility of 0.3 cm² V⁻¹ s⁻¹.     

Transparent conducting ZnO thin films deposited by vacuum arc plasma evaporation

A high rate deposition of co-doped ZnO:Ga,F and ZnO-In₂O₃ multicomponent oxide thin films on large area substrates has been attained by a vacuum arc plasma evaporation method using oxide fragments as a low-cost source material. Highly transparent and conductive ZnO:Ga,F and ZnO-In₂O₃ thin films were prepared on low temperature substrates at a deposition rate of approximately 375 nm/min with a cathode plasma power of 10 kW. A resistivity of 4.5×10⁻⁴ Ω cm was obtained in ZnO:Ga,F films deposited at 100 °C using ZnO fragments co-doped with 1 wt.% ZnF₂ and 1 wt.% Ga₂O₃ as the source material. In addition, the stability in acid solution of ZnO films was improved by co-doping. It was found that the Zn/(In+Zn) atomic ratio in the deposited ZnO-In₂O₃ thin films was approximately the same as that in the fragments used. The ZnO-In₂O₃ thin films with a Zn/(In+Zn) atomic ratio of approximately 10-30 at.% deposited on substrates at 100 °C exhibited an amorphous and smooth surface as well as a low resistivity of 3-4×10⁻⁴ Ω cm.     

Structural,morphological, optical and electrical evolution of spray deposited ZnO rods co-doped with indium and sulphur atoms

Undoped and (In–S) co-doped ZnO films were grown by the spray pyrolysis method on glass substrates. The structural, morphological, optical and electrical properties of all the samples were studied in detail. X-ray diffraction results showed that all the samples have a hexagonal wurtzite structure with the preferred orientation that changed from (002) to the random orientation after (In–S) co-doping. From the scanning electron microscopy analysis, it is noted that the morphology of ZnO changed from rods to thin film upon (In–S) co-doping. Compared to undoped ZnO rods, transparency of (In–S) co-doped ZnO thin films significantly increased whereas their band gap values gradually decreased. From photoluminescence measurements, it is observed that the UV peak completely quenched after (In–S) co-doping while the deep level band intensity slightly increased especially for 2 and 4 at.% (In, S) co-doped ZnO samples. Compared with undoped sample, the carrier concentration enhanced with the increase of (In–S) co-doping to 4 at.% and further increase in the co-doping amount results in the decline of the conductivity.     

Characterization of gallium-nitrogen co-doped zinc oxide thin films prepared by RF diode sputtering

We investigated the possibility of achieving p-type zinc oxide (ZnO) by RF diode sputtering and gallium-nitrogen co-doping. ZnO:Ga:N thin films were prepared with a different N₂ content in Ar/N₂ working gas, ranging from 0 to 100%, and at a varying substrate temperature, from room temperature (RT) to 300 °C. A hole conduction with maximum carrier concentration of 2.6 × 10¹⁸ cm⁻³, mobility of 2 cm²/Vs and resistivity of 1.5 Ω cm resulted from deposition at RT with 100% N₂. It arose from N incorporation and formation of N_O acceptors. In the secondary ion mass spectrometry (SIMS) depth profiles of the co-doped films were observed NO/NO₂ negative ions. Average transmittance (including Corning glass substrate) across the visible spectrum varied (60 ÷ 66%) with increasing nitrogen content (50 ÷ 100% N₂). As the substrate temperature increased (RT - 300 °C), highly transparent (T ∼72-83%) and conductive (electron concentrations of 10¹⁷-10¹⁹ cm⁻³) n-type ZnO:Ga:N films were attained. Reduction of optical band gap (E_g) (∼3.13-3.08 eV) was observed for co-doped ZnO films. Atomic force microscopy (AFM) images revealed that the films grown at RT have roughness of approximately 5.3 nm while roughness of those grown at 300 °C is approximately 3.9 nm.     

Power and gas pressure effects on properties of amorphous In–Ga–ZnO films by magnetron sputtering

Amorphous In–Ga–ZnO thin films were deposited on quartz glass substrate at room temperature utilizing radio frequency magnetron sputtering technique. Sputtering power and oxygen flow rate effects on the physical properties of the In–Ga–ZnO films were systematically investigated. It is shown the film deposition rate and the conductivity of the In–Ga–ZnO films increased with the sputtering power. The as-grown In–Ga–ZnO films deposited at 500 W exhibited the Hall mobility of 17.7 cm²/Vs. Average optical transmittance of the In–Ga–ZnO films is greater than 80% in the visible wavelength. The extracted optical band gap of the In–Ga–ZnO films increased from 3.06 to 3.46 eV with increasing the sputtering power. The electrical properties of the In–Ga–ZnO films are greatly dependent on the O₂/Ar gas flow ratio and post-growth annealing process. Increasing oxygen flow rate converted the In–Ga–ZnO films from semiconducting to semi-insulating, but the resistivity of the films was significantly reduced after being annealed in vacuum. Both the as-grown and annealed In–Ga–ZnO films show n-type electrical conductivity.     

Effect of In incorporation on the structural, electrical, and gas sensing properties of ZnO films

In this paper, we report some comparative results on the structural, electrical, and gas sensing properties of undoped, In-doped ZnO, and ZnO–In₂O₃ thin films, respectively. The oxide films were obtained by thermal oxidation (flash oxidation) of metallic films, deposited by thermal evaporation under vacuum. X-ray diffraction patterns reveal that oxidized films are polycrystalline, the crystallites being preferentially oriented with (002) planes parallel with the substrate. It was observed that the films’ morphology, investigated by atomic force microscopy and scanning electron microscopy, is influenced by the In amount. The temperature dependence of electrical conductivity was studied and obtained results indicate that In-doped ZnO and ZnO–In₂O₃ films exhibit an enhancement of electrical conductivity with four orders of magnitude by comparison with undoped ZnO film. Gas sensitivity measurements were performed for four different gases (ammonia, methane, acetone, and ethanol), and it was observed that all investigated films are more sensitive to ammonia. Also, it was observed that gas sensitivity is visibly increased for In-doped ZnO and ZnO–In₂O₃ samples by comparison with undoped ZnO film.     

Structural and electrical properties of p-type ZnO films prepared by Ultrasonic Spray Pyrolysis

Effects of atomic ratio of Zn:N:Al and solution concentration on the structural and electrical characteristic of ZnO films deposited by Ultrasonic Spray Pyrolysis using N-Al co-doping technique were investigated. Hall measurement indicated that with increasing Al doping, conductive type of ZnO thin films changed from n-type to p-type and then to n-type again. However, the solution concentration almost has no effect on the structural and electrical properties of p-type ZnO films. X-ray Photoelectron Spectroscopy indicated that the presence of Al indeed facilitates the incorporation of N through the formation of N-Al bonds in co-doped ZnO films. In addition, Photoluminescence spectra showed p-type ZnO films with a low density of native defects.     

Effects of substrate temperature on dielectric and structural properties of Ti and Er co-doped HfO2 thin films

We report dielectric and structural properties of Ti and Er co-doped HfO₂ (HfTiErO_x) thin films at different substrate temperatures. The film at 400 °C substrate temperatures has the highest k value of 33, improved flat band voltage of −0.3 V, small hysteresis voltage and the significant interface-state density, which shows better dielectric properties for new high-k microstructure. XPS and XRD results reveal that Hf-Ti-Er-O bond may exist in addition with Hf-O, Hf-Er-O and Hf-Ti-O bonds, while the change in chemical structure and degradation of crystallization quality of HfO₂ thin films are directly related to Ti and Er co-doping.     

Influence of preparation condition and doping concentration of Fe-doped ZnO thin films: Oxygen-vacancy related room temperature ferromagnetism

Fe-doped and Fe-Ga co-doped ZnO diluted magnetic semiconductor thin films on quartz substrate were studied. Rapid annealing enhanced the ferromagnetism (FM) of the films grown in Ar/O₂. All the films grown in Ar are n-type and the carrier concentration could increase significantly when Ga is doped. The state of Fe in the films was investigated exhibiting Fe³⁺. Magnetic measurements revealed that room temperature ferromagnetism in the films were doping concentration dependent and would enhance slightly with Ga doping. The origin of the observed FM is interpreted by the overlapping of polarons mediated through oxygen vacancy based on the bound magnetic polaron model.     

The structure and ferromagnetic properties of (Fe0.12CuxZn0.88−x)O thin films deposited on Si substrates by R.F. sputtering

Fe–Cu co-doped ZnO thin films deposited on silicon substrates were prepared by R.F. magnetron sputtering. The effects of various amounts of copper on the microstructure, surface morphology, composition, and magnetic properties of ZnO thin films were examined. The results of the experiments show that the structures of the ZnO thin films grown on the silicon substrate have a preferred orientation of (002). By increasing the copper concentration, the Fe ions exist as Fe²⁺ in the Fe_0.12Cu_xZn_0.88−xO system, but Cu²⁺ and Cu¹⁺ ions coexist when the Cu replaces the Zn. In addition, the ZnO thin films show ferromagnetic behaviour at room temperature and the largest saturation magnetization (Ms) is 5.64 × 10⁴ A/m for the as-grown Fe_0.12Cu_0.02Zn_0.86O thin film.     

Control of ZnO thin film surface by ZnS passivation to enhance photoluminescence

To enhance the optical property of zinc oxide (ZnO) thin film, zinc sulfide (ZnS) thin films were formed on the interfaces of ZnO thin film as a passivation and a substrate layer. ZnO and ZnS thin films were deposited by atomic layer deposition (ALD) using diethyl zinc, H₂O, and H₂S as precursors. Investigations by X-ray diffraction and transmission electron microscopy showed that ZnS/ZnO/ZnS multi-layer thin films with clear boundaries were achieved by ALD and that each film layer had its own polycrystalline phase. The intensity of the photoluminescence of the ZnO thin film was enhanced as the thickness of the ZnO thin film increased and as ZnS passivation was applied onto the ZnO thin film interfaces.     

CO<Subscript>2</Subscript> gas sensitivity of sputtered zinc oxide thin films

For the first time, sputtered zinc oxide (ZnO) thin films have been used as a CO₂ gas sensor. Zinc oxide thin films have been synthesized using reactive d.c. sputtering method for gas sensor applications, in the deposition temperature range from 130–153°C at a chamber pressure of 8·5 mbar for 18 h. Argon and oxygen gases were used as sputtering and reactive gases, respectively. ZnO phase could be crystallized using a pure metal target of zinc. The structure of the films determined by means of X-ray diffraction method indicates that the zinc oxide single phase can be fabricated in this substrate temperature range. The sensitivity of the film synthesized at substrate temperature of 130°C is 2·17 in the presence of CO₂ gas at a measuring temperature of 100°C.     

Preparation and properties of ZnO thin films deposited by sol-gel technique

Zinc oxide (ZnO) thin films were deposited on (100) Si substrates by sol-gel technique. Zinc acetate was used as the precursor material. The effect of different annealing atmospheres and annealing temperatures on composition, structural and optical properties of ZnO thin films was investigated by using Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy and photoluminescence (PL), respectively. At an annealing temperature of 400°C in N₂ for 2 h, dried gel films were propitious to undergo structural relaxation and grow ZnO grains. ZnO thin film annealed at 400°C in N₂ for 2 h exhibited the optimal structure and PL property, and the grain size and the lattice constants of the film were calculated (41.6 nm, a = 3.253 ? and c = 5.210 ?). Moreover, a green emission around 495 nm was observed in the PL spectra owing to the oxygen vacancies located at the surface of ZnO grains. With increasing annealing temperature, both the amount of the grown ZnO and the specific surface area of the grains decrease, which jointly weaken the green emission. Translated from Journal of Lanzhou University (Natural Science), 2006, 42(1): 67–71 [译自: 兰州大学学报 (自然科学版)]     

P-type conductivity in doped and codoped ZnO thin films synthesized by RF magnetron sputtering

N-doped and Al–N codoped ZnO thin films with different volume ratios of N₂ reactive gas were deposited on plane glass substrates using the radio frequency magnetron sputtering method. The phase transition temperature and absorption edge of the ZnO powder were studied by differential scanning calorimetry at different heating rates and with Fourier transform infrared spectroscopy, respectively. The target used for the sputtering was synthesized using a palletize machine. It was sintered at 450 °C for 5 h. The X-ray diffraction results confirm that the thin films have wurtzite hexagonal structures with a very small distortion. The results indicate that the ZnO thin films have obviously enhanced transmittance of up to 80% on an average in the visible region. The Al–N codoped ZnO thin films exhibited the best p-type conductivity with a resistivity of 0.825 Ω-cm, a hole concentration of 6.55 × 10¹⁹ cm^?3, and a Hall mobility of 1.25 cm²/Vs. The p-type conductivity was observed after doping and codoping of the ZnO thin film.     

Effect of the oxygen pressure on the photoluminescence properties of ZnO thin films by PLD

ZnO thin films on Si(111) substrate were deposited by laser ablation of Zn target in oxygen reactive atmosphere; Nd-YAG laser with wavelength of 1064 nm was used as laser source. The experiments were performed at laser energy density of 31 J/cm², substrate temperature of 400 °C and various oxygen pressures (5–65 Pa). X-ray diffraction was applied to characterize the structure of the deposited ZnO films and the optical properties of the ZnO thin films were characterized by photoluminescence with an Ar ion laser as a light source using an excitation wavelength of 325 nm. The influence of the oxygen pressure on the structural and optical properties of ZnO thin films was investigated. It was found that ZnO film with random growth grains can be obtained under the condition of oxygen pressure 5–65 Pa. It will be clearly shown that the grain size and the formation of intrinsic defects depend on the oxygen partial pressure and that high optical quality of the ZnO films is obtained under low oxygen pressure (5 Pa, 11 Pa) conditions.     

Effects of substrate temperatures on the thermal stability of Al-doped ZnO thin films grown by DC magnetron sputtering

In this work, Al-doped (4 at%) ZnO(AZO) thin films were prepared by DC magnetron sputtering using a home-made ceramic target at different substrate temperatures. The microstructure, optical, electrical and thermal stability properties of these thin films were characterized systematically using scanning electron microscopy, UV–Vis-NIR spectrometry, X-ray diffraction, and Hall measurements. It was observed that the AZO thin films deposited at 350 °C exhibited the lowest resistivity of 5.76 × 10⁻⁴ Ω cm, high average visible transmittance (400–800 nm) of 92%, and the best thermal stability. Comparing with the AZO thin films deposited at low substrate temperatures, the AZO thin films deposited at 350 °C had the highest compact surface morphology which could hinder the chemisorbed and diffused oxygen. This was considered to be the main mechanism which was responsible for the thermal degradation of AZO thin films.     

Electrical properties of Na/Mg co-doped ZnO thin films

Conducting and transparent Na-Mg co-doped ZnO thin films were deposited on glass substrates by sol-gel method. Zn (CH₃COO)₂·2H₂O(AR), MgCl₂·6H₂O(AR)) and NaCl (AR) were selected as precursors. They were dissolved in methanol to form a mixed solution with a designed doping ion concentration. Diethanolamine (HN(CH₂CH₂OH)₂, DEA, AR) was added into the mixed solution as a stabilizer in a DEA/Zn(OAC)₂ molar ratio of 1:1. The electrical properties, phase structure, and surface morphology of the films were analyzed via Hall coefficient analyses, X-ray diffraction and scanning electron microscopy, respectively. The films exhibited an obscure (002) preferential growth in all the cases. Surface morphology studies showed that an increase in the films' thickness causes an increase in the grain size. Films with 0.25 μm thickness, prepared under optimal deposition conditions followed by an annealing treatment in vacuum, showed an electrical resistivity in the level of 10² Ω cm and p-type conduction. These results make Na-Mg co-doped ZnO thin films an attractive material for transparent electrodes in thin film devices.     

Spin coating of transparent zinc oxide films using novel precursor

The coating of transparent ZnO films using zinc 2-ethylhexanoate [Zn(OOCH(C₂H₅)C₄H₉)₂] as a novel metal organic monomer is reported. Zinc 2-ethylhexanoate is liquid at room temperature and can be spin-coated on a flat substrate without precipitation of ZnO under ambient condition. The spin-coated films were heated at different temperatures to remove unwanted organic materials from the surface. It was found that transparent ZnO films could be produced on glass substrates at low heating temperature (~400 °C). The ZnO films produced using the new monomer were free of cracks and defects. Also the ZnO films produced using the new monomer have excellent optical transmittance, mechanical properties and small surface roughness. The surface morphology and degree of crystallinity of the films coated by the new monomer were compared with these properties of ZnO films produced using zinc acetate-based sol–gels. The results clearly indicate that the novel monomer is a potential precursor for coating transparent ZnO films at low temperatures.     

RF sputtered piezoelectric zinc oxide thin film for transducer applications

This paper demonstrates the substrate dependency of the c-axis zinc oxide growth in radio-frequency sputtering system. Different deposition conditions were designed to study the influences of Si, SiO₂/Si, Au/Ti/Si, and Au/Ti/SiO₂/Si substrates on the piezoelectric and crystalline qualities of the ZnO thin films. Experimental results showed that the multilayer of Au/Ti/SiO₂/Si-coated silicon substrate provided a surface that facilitated the growth of ZnO thin film with the most preferred crystalline orientation. The 1.5 μm-thick thermally grown amorphous silicon dioxide layer effectively masked the crystalline surface of the silicon substrate, thus allowing the depositions of high-quality 20 nm-thick titanium adhesion layer followed by 150 nm-thick of gold thin film. The gold-coated surface allowed deposition of highly columnar ZnO polycrystalline structures. It was also demonstrated that by lowering the deposition rate at the start of sputtering by lowering RF power to less than one-third of the targeted RF power, a fine ZnO seed layer could be created for subsequent higher-rate deposition. This two-step deposition method resulted in substantially enhanced ZnO film quality compared to single-step approach. The influence of stress relaxation by annealing was also investigated and was found to be effective in releasing most of the residual stress in this layered structure.