Optical and electrical properties of ZnO films prepared by URT-IP method

Optical and electrical properties were studied on thin polycrystalline ZnO films (200-nm thick) deposited on glass substrates at 200 °C by a DC-arc ion plating method (URamoto-Tanaka-type ion plating method). Effects of the oxygen flow rate (OFR) on film properties were examined. The resistivity of undoped films changed from 4.2×10⁻³ to 9.6×10⁻¹ Ω cm, corresponding to the carrier concentration of 1.0×10²⁰-1.2×10¹⁸ cm⁻³, depending on the increase in OFR from 0 to 40 sccm. The Hall mobility tends to be the maximum value of 28 cm² (V s)⁻¹ at OFR of 10 sccm. Photoluminescence (PL) spectra exhibited a dominant near-band-edge (NBE) emission together with weak PL bands at 2.2 and 3.2 eV. Intensity of NBE was maximum at OFR of 10 sccm. Intensity of the PL band at 2.2 eV increased with increase in OFR. As a result of Ga-doping, the resistivity decreased and the carrier concentration increased by one order of magnitude. The optical transmittance was more than 90% in 400-1200 nm. The ZnO:Ga (3 and 4 wt.% Ga-doped) thin films with the lowest resistivity of 2.6×10⁻⁴ Ω cm, the highest mobility of 25 cm² (V s)⁻¹, and the highest PL intensity were obtained at OFR of 10 sccm. Further increase of OFR led to the decrease in both mobility and PL intensity.     

Influence of thickness and growth temperature on the optical and electrical properties of ZnO thin films

Zinc oxide transparent conductive thin films were prepared using the pulsed laser deposition technique onto Corning glass substrates and the dependences of their optical and electrical properties on the thickness and the growth temperature were investigated. As shown, the films present 90% average transmittance, their energy gap position depending on the film thickness and the growth temperature. An additional absorption band was also observed near 3.44 eV, the position of its maximum also depending on the growth parameters. Finally, the electrical properties of the films were found to be affected mainly by the growth temperature and less by the thickness.     

Effect of alumina doping on structural, electrical, and optical properties of sputtered ZnO thin films

A systematic study of the influence of alumina (Al₂O₃) doping on the optical, electrical, and structural characteristics of sputtered ZnO thin films is reported in this study. The ZnO thin films were prepared on 1737F Corning glass substrates by R.F. magnetron sputtering from a ZnO target mixed with Al₂O₃ of 0-4 wt.%. X-ray diffraction (XRD) analysis demonstrates that the ZnO thin films with Al₂O₃ of 0-4 wt.% have a highly (002) preferred orientation with only one intense diffraction peak with a full width at half maximum (FWHM) less than 0.5°. The electrical properties of the Al₂O₃-doped ZnO thin films appear to be strongly dependent on the Al₂O₃ concentration. The resistivity of the films decreases from 74 Ω·cm to 2.2 × 10^− 3 Ω·cm as the Al₂O₃ content increases from 0 to 4 wt.%. The optical transmittance of the Al₂O₃-doped ZnO thin films is studied as a function of wavelength in the range 200-800 nm. It exhibits high transparency in the visible-NIR wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical bandgap of the Al₂O₃-doped ZnO thin films show a short-wavelength shift with increasing of Al₂O₃ content.     

Influence of thermal annealing on electrical and optical properties of Ga-doped ZnO thin films

Influence of thermal annealing on electrical properties of GZO films has been studied by means of Hall effect measurements and optical characterization based on Drude model analysis for transmission and reflection spectra. Electrical resistivity increased with increasing annealing temperature. Changes of electrical properties were compared between air and N₂ gas atmosphere. Thermal stability in the air was worse compared to the N₂ gas atmosphere. Annealing at rather high temperature caused decrease in the Hall mobility and increase in optical mobility. The difference between the Hall mobility and the optical mobility was attributed to carrier scattering at grain boundaries. Three kinds of deposition method, ion plating using DC arc discharge, DC magnetron sputtering, and RF power superimposed DC magnetron sputtering were compared in terms of the thermal stability.     

Electronic transport and optical properties of thin oxide films

In this work, we present optical characterization of films of two transparent conductive oxides (ITO: indium tin oxide and ZnO: zinc oxide) including absorption coefficient and optical gap energy. We have also investigated the transport properties of ITO and ZnO films through measurements of electrical conductivity and thermoelectric power versus temperature. These measurements enabled us to investigate conduction mechanisms for metal-nonmetal transitions. Undoped ZnO thin films show a metal-semiconductor transition at temperatures beyond 350 K. We have conducted a similar study on ITO films where we demonstrated, for the first time, the existence of a conductivity transition below 400 K, which indicates a high absolute thermoelectric power at temperatures above the transition temperature.     

Dependence of photoluminescence and electrical properties with rapid thermal annealing in nitrogen-implanted ZnO films

Undoped (as-grown) ZnO films grown by pulsed laser deposition on Al₂O₃ (0001) substrates were doped with nitrogen by means of an ion implantation process. Post-implantation annealing behavior in the temperature range between 500 and 700 °C has been studied by photoluminescence and Hall effect measurements. The implanted films show no peak other than the excitonic recombination emission in the as-implanted state, however, after rapid thermal annealing at 700 °C they reveal a nitrogen acceptor related emission at 3.273 eV. The as-implanted ZnO films show more electron concentrations than the as-grown, unimplanted ZnO film. In contrast, after annealing, the electron concentration in the implanted films is significantly reduced, indicating that the incorporated nitrogen becomes activated after the thermal annealing, then produces holes and eventually compensates for certain amount of electrons. The results imply that a proper nitrogen implantation and subsequent annealing may be a way to produce p-type ZnO films.     

Electrical and optical properties of ZnO:Mn thin films grown by MOCVD

Diluted magnetic semiconductor epitaxial thin films of Zn_1 − xMn_xO have been grown on c-sapphire by the MOCVD technique. Variations of a and c lattice parameters follow Vegard's law and attest to the incorporation of substitutional Mn²⁺ ions. Carrier concentration (n-type) and electron mobility were studied versus temperature for different concentrations of manganese. Incorporation of manganese leads to the opening of the band gap, observed as a blue shift in energy regarding pure ZnO.     

Synthesis and characterization of (CuO)x(ZnO)1 − x composite thin films with tunable optical and electrical properties

Mixed (CuO)_x(ZnO)_1 − x composite films have been prepared on glass substrates by a sol-gel spin coating method using copper acetate hydrate and zinc acetate dihydrate as precursors. The surface morphology and crystal structure of the films were investigated by field emission scanning electron microscopy and x-ray diffraction, respectively. It was observed that the crystal structure changed from wurtzite (ZnO) to monoclinic (CuO) as the Cu content increased from 0% to 100% in the films. UV-Vis absorption and photoluminescence measurements indicated that the optical properties can be tuned continuously from pure ZnO to pure CuO as the Cu content was increased, following the expected trends for a transition from ZnO to CuO. The resistivity of the films decreased by three orders of magnitude as Cu increased from 0% to 100%. These semiconducting composite oxides with tunable optical and electrical properties have potential applications in electronics and optoelectronics.     

Growth ambient dependent electrical properties of lithium and nitrogen dual-doped ZnO films prepared by radio-frequency magnetron sputtering

Lithium (Li) and nitrogen (N) dual-doped ZnO films with wurtzite structure were prepared by radio-frequency magnetron sputtering ZnO target with Li₃N in growth ambient of pure Ar and the mixture of Ar and O₂, respectively, and then post annealing techniques. The film showed week p-type conductivity as the ambient was pure Ar, but stable p-type conductivity with a hole concentration of 3.46 × 10¹⁷ cm^− 3, Hall mobility of 5.27 cm²/Vs and resistivity of 3.43 Ω cm when the ambient is the mixture of Ar and O₂ with the molar ratio of 60:1. The stable p-type conductivity is due to substitution of Li for Zn (Li_Zn) and formation of complex of interstitial Li (Li_i) and substitutional N at O site, the former forms a Li_Zn acceptor, and the latter depresses compensation of Li_i donor for Li_Zn acceptor. The level of the Li_Zn acceptor is estimated to be 131.6 meV by using temperature-dependent photoluminescence spectrum measurement and Haynes rule. Mechanism about the effect of the ambient on the conductivity is discussed in the present work.     

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.     

Optical and electrical properties of ZnO nanoparticle thin films deposited on quartz by sparking process

ZnO nanoparticle thin films were deposited on quartz substrates by a novel sparking deposition which is a simple and cost-effective technique. The sparking off two zinc tips above the substrate was done repeatedly 50-200 times through a high voltage of 10 kV in air at atmospheric pressure. The film deposition rate by sparking process was approximately 1.0 nm/spark. The ZnO thin films were characterized by X-ray diffraction, Raman spectroscopy, UV-vis spectrophotometry, and ionoluminescence at room temperature. The two broad emission peaks centered at 483 nm (green emission) and 650 nm (orange-red emission) were varied after two-step annealing treatments at 400-800 °C. Moreover, the electrical resistivity of the films was likely to be proportional to the peak intensity of the orange-red emission.     

Control and characterization of structural and optical properties of ZnO thin films fabricated by thermal oxidation Zn metallic films

The physical properties of ZnO thin films fabricated by controlling thermal oxidation zinc metallic films process have been investigated. Comparative characterization of crystallographical, optical or spectroscopic properties of the samples was performed. The as-oxidized sample by rapid thermal process showed level of crystallinity degraded, higher optical transmittance in the visible and near-infrared region, and lower intensity of the near band edge emission compared to those prepared by conventional thermal oxidation. The overall results suggested that with in-depth understanding of the oxidation mechanism, rapid oxidation process could be employed as an approach to fabricate amorphous transparent oxide thin films from low-melting-point metals, which might have potential advantages in microelectronic and optoelectronic applications.     

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural properties.     

Microstructural and optical properties of Ga-doped ZnO semiconductor thin films prepared by sol-gel process

Transparent thin films of Ga-doped ZnO (GZO), with Ga dopant levels that varied from 0 to 7 at.%, were deposited onto alkali-free glass substrates by a sol-gel process. Each spin-coated film was preheated at 300 °C for 10 min, and then annealed at 500 °C for 1 h under air ambiance. The effects of Ga dopant concentrations on crystallinity levels, microstructures, optical properties, and electrical resistivities of these ZnO thin films were systematically investigated. Photoluminescence spectra of GZO thin films were examined at room temperature. XRD results revealed that the undoped ZnO thin films exhibited a preferred orientation along the (002) plane and that the ZnO thin films doped with Ga showed degraded crystallinity. Experimental results also showed that Ga doping of ZnO thin films could markedly decrease surface roughness, improve transparency in the visible range, and produce finer microstructures than those of undoped ZnO thin films. The most promising films for transparent thin film transistor (TTFT) application produced in this study, were the 3 and 5 at.% Ga-doped ZnO thin films, both of which exhibited an average transmittance of 90.6% and an RMS roughness value of about 2.0 nm.     

Effects of the annealing treatment on electrical and optical properties of ZnO transparent conduction films by ultrasonic spraying pyrolysis

Effects of the annealing treatment on properties of ZnO thin films prepared on silica glass substrates by the ultrasonic spraying pyrolysis process were studied. Zinc acetate dihydrate and methanol were used as a starting material and a solvent, respectively. For ZnO thin films untreated with annealing, the preferred grain growth along the (0 0 2) plane was observed. The electrical resistivity and the direct band gap values of these films decreased with increasing the deposit temperature. By applying the annealing treatment in a reducing atmosphere, while the degree of the preferred (0 0 2) orientation of films decreased, the electrical conductivity of films was improved. When compared with the resistivity values of films without the annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about two orders of magnitude. The lowest resistivity value was 1.62×10⁻¹ Ω cm, which was obtained in the film annealed at 500 °C in nitrogen with 5% hydrogen. The optical transmittances of the films were higher than 80% regardless of the application of the annealing treatment in a reducing atmosphere. The direct band gap values of films annealed in a reducing atmosphere were approximately 3.27 eV.     

Investigation of thin solid ZnO films prepared by sputtering

Zinc oxide (ZnO) thin films have attracted great attention in recent years due to their unique piezoelectric and piezooptic properties, making them suitable for various microelectronics and optoelectronics applications, such as surface acoustic wave devices, optical fibers, solar cells etc. ZnO is a semiconductor with a band gap of 3.3 eV and a large exciton binding energy of 60 meV. Undoped ZnO exhibits intrinsic n-type conductivity and it enables achieving high electron concentration. However, it may be doped to obtain low resistivity p-type thin films. Among group V of the periodic table, nitrogen is used as a popular p-type dopant due to its small atomic size. However, it is difficult to achieve p-type conduction in ZnO films due to the low solubility of nitrogen and its high intensity in self compensating process upon doping.Sputtering techniques enable us to form dense and homogeneous films due to the relatively high energy of the sputtered atoms. Thus we can grow high quality ZnO films with c-axis orientation, low growth temperature, high deposition rate, large area deposition, and availability in various growths ambient. In this work, the zinc oxide films were prepared using various DC sputtering methods in an atmosphere of pure argon and an atmosphere of mixed argon with nitrogen. Optical and electrical properties of the films were investigated.     

High-temperature-dependent optical properties of ZnO film on sapphire substrate

In order to fabricate fiber-optic temperature sensors based on ZnO film, it is important to study the temperature-dependent optical properties of this material. In this work, we deposited ZnO films on c-plane (0001) sapphire substrate at 250 °C. Atomic force microscope and X-ray diffraction measurements show the smooth surface and high orientation along [0001] of ZnO film, respectively. The high-temperature-dependent optical properties of ZnO film were measured by ultraviolet-visible transmission with temperatures ranging from room-temperature to 300 °C and analyzed by theoretically fitting the optical absorption edge curve. It is observed that the band gap energy red shifts nonlinearly from 3.345 to 3.153 eV with increasing temperature. The sharp absorption edge of ZnO films after annealing at 300 °C is almost consistent with that of the as-deposited sample, indicating an excellent thermal stability and the potential application in fiber-optic temperature sensors.     

Microstructure and gas-sensing properties of sol-gel ZnO thin films

The paper presents the properties of zinc oxide thin films deposited on glass substrate via dip-coating technique. Zinc acetate dehydrate, ethanol and monoethanol amine were used as starting materials and N₂ gas was used as thermal annealing atmosphere for film crystallization. The effect of withdrawal speed on the crystalline structure, morphology, zinc and nitrogen chemical states, optical, electrical and gas-sensing properties of the thin films has been investigated using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical transmittance and photoreduction-ozone reoxidation data.     

Growth process observation of homoepitaxial ZnO thin films using optical emission spectra during pulsed laser deposition

Homoepitaxial ZnO thin films were prepared on the Zn-polar or O-polar ZnO substrates by pulsed laser deposition method. Optical emission spectroscopy of the plume was carried out to estimate O/Zn flux ratio under the various deposition conditions such as oxygen pressure, laser fluence, and the distance between target and substrate. It is revealed that the O/Zn flux ratio could be controlled by laser fluence, oxygen pressure, and target-substrate distance. Zn-rich O/Zn flux promotes pit formation and O-rich flux yields the three-dimensional growth. The difference of the growth process on Zn-polar or O-polar substrates is also discussed.     

Morphological, optical and electrical properties of samarium oxide thin films

We present here results on samarium oxide thin films, obtained by pulsed laser deposition and by radio frequency assisted pulsed laser deposition. Three different substrate types were used: silicon, platinum covered silicon and titanium covered silicon. The influence of the deposition parameters (oxygen pressure and laser fluence) on the structure and morphology of the thin films was studied. The substrate-thin film interface zone was investigated; the optical and electrical properties (the losses, dielectric constant and leakage currents) were also determined.