Magnetron Sputtered ZnO Thin Films Research Articles

Optimization of Post-Annealing Temperature of RF Magnetron-Sputtered ZnO Thin Films for Enhancing Performances of UV Photodetectors

In this study, we examined the surface morphology and crystal structure of RF-sputtered ZnO thin films that were annealed at various temperatures. Also, we fabricated UV photodetectors with an Au-ZnO-ITO sandwich structure, utilizing the thin films annealed at 600 °C. The surface roughness of the film initially increases and then decreases as the annealing temperature rises, and the crystalline quality improves with an increase in the annealing temperature. Due to the Schottky heterojunction formed by the Au-semi contact, the fabricated UV photodetector exhibits a responsivity of 7.91 mA W−1 under 405 nm UV light. And the device demonstrates high response stability and rapid response sensitivity, with a swift rise time of 80 ms at a light intensity of 2.56 mW cm−2.

Exploring the effects of substrate and substrate temperature on the properties of radio frequency magnetron sputtered ZnO thin films

The physical and optical properties of zinc oxide thin films strongly depend on the film deposition techniques. In this work, RF magnetron sputtering was used to deposit ZnO thin films on various substrates (glass, quartz, ITO-coated glass, FTO-coated glass, Si(100), Si(100)/ SiO2) to study the role of substrate and substrate temperature on the morphological, structural and optical properties of the film grown. A detailed discussion on various aspects like surface morphology, strain, photoluminescence, bandgap, surface roughness, and material composition has been done. Compressive stress was present in the films created at room temperature and differed for various substrates based on the lattice mismatch. The nature and temperature of the substrate are found to affect the surface topography of films. Substrate and substrate temperature influenced the emission characteristics of ZnO thin films. In oxygen-rich substrates, defect emissions differed. We found that quartz is a better substrate to fabricate ZnO at room temperature and both Si and Si/SiO2 are preferable at high temperatures due to the lower lattice mismatch and thermal expansion coefficient. Comparing the electrical characteristics of ZnO thin films grown on glass and quartz substrates, ZnO on glass exhibited lower resistivity and higher carrier concentration. The Triboelectric nanogenerator (TENG) combining ZnO and PDMS, was developed to operate in vertical contact-separation mode. The obtained open circuit voltage was 30 V and 33 V for ZnO grown over ITO and FTO respectively, and the slight increase is due to the rougher film's surface on FTO than ITO. The selection of substrate is crucial in many applications and our findings provide an overview of the suitability of substrate and substrate temperature for the fabrication of ZnO thin film-based optoelectronic devices.

High optoelectronic performance of magnetron sputtered ZnO thin films co-doped with F and Ga: Experiment and first-principles calculations

In this study, F-and-Ga co-doped ZnO (FGZO) films were deposited on glass substrates by radio frequency (RF) magnetron sputtering and the effect of film thickness on the structural, morphological, electrical, optical, and chemical properties was systematically investigated. A minimum resistivity of 5.95 × 10−4 Ωcm, carrier concentration of 2.39 × 1020 cm-3, and mobility of 44.0 cm2/Vs were obtained at the film thickness of 1450.4 nm. The transmittance of FGZO films is close to that of FTO and ITO films in the optical range of 400−1000 nm. After 70 s’ etching with diluted HCl, the haze values at 550 nm, 800 nm, and 1100 nm are 82.3 %, 48.2 %, and 22.2 %, respectively. After co-doping with F and Ga, the electron density of states at the Fermi level increased significantly, and the free carriers were mainly attributed to the hybridized F 2p, Ga 4p, Ga 4 s, O 2p, and Zn 4 s electronic orbitals.

Bonding and stoichiometry in low-energy radio frequency magnetron sputtered ZnO thin films on flexible substrate

Spectroscopic and electrical studies of ZnO thin films grown by radio frequency magnetron sputtering at low power without substrate heating revealed improved stoichiometry and reduced free carrier concentration with increasingly energetic growth conditions, which is opposite to what is typically observed. Hall measurements showed a decrease in carrier concentration from 6.9 × 1019 cm−3 to 1.4 × 1019 cm−3 as power increased from 40 W to 120 W, and a decrease in carrier concentration from 8.6 × 1019 cm−3 to 2.6 × 1019 cm−3 as the deposition pressure decreased from 1.2 to 0.4 Pa. These findings correlated well with XRD, x-ray photoelectron spectroscopy and photoluminescence results, which showed that more energetic conditions increased the fraction of oxygen that was stoichiometrically bonded. The change in the absorption edge, likely due to a Burstein-Moss shift, was also in qualitative agreement. In the range of conditions used, energy transfer from the plasma to the growing film governs bonding, stoichiometry and hexagonal nanocrystalline film formation and growth.

Role of oxygen concentrations on structural and optical properties of RF magnetron sputtered ZnO thin films

We report an investigation on the effect of oxygen flow rate on the structural and optical properties of zinc oxide thin films prepared by RF magnetron sputtering. The structural measurements were carried using grazing incidence X-ray diffraction, atomic force microscopy and Raman spectroscopy. The role of oxygen partial pressure on the crystallinity, the surface morphology and vibrational modes has been established. The optical properties of the films were investigated using FR-Basic-VIS/NIR fitted with FR-Monitor software for film thickness, refractive index and color determination. The film thickness is observed to increase when oxygen is introduced at 4 sccm but eventual decrease with increase in the flow rate an indication of initial increase in rate of deposition followed by reduction. Elaborate explanations of these trends are provided.

Study of Structural Properties of BiOI on ZnO Thin Films and ZnO Nanorods

We report the study of ZnO nanorods (ZnO-NR) decorated with BiOI nanoplatelets by the simple precipitation of BiOI in aqueous solution containing ZnO-NR powders. Likewise, as a reference for the decoration of ZnO-NR, BiOI thin films were prepared by the SILAR technique on RF magnetron-sputtered ZnO thin films. The scanning electron microscopy analysis showed that BiOI grew on the ZnO-NR surfaces in nanoplatelet shape with an order of magnitude smaller in size, with respect to the platelets observed in BiOI thin films. Analysis by X-ray diffraction exhibited the hexagonal P63mc phase of ZnO and the tetragonal P4nmm crystal phase of BiOI. On the other hand, BiOI thin films are nonuniform and strongly adherent and, besides they exhibited the P4nmm tetragonal crystal structure of BiOI, they have highly preferential orientation along (001) direction (9.66°). Optical characterization of ZnO-NR/BiOI by absorbance showed a widened peak slightly shifted to infrared compared with the ZnO-NR absorbance peak, suggesting the increase of the absorption wavelength ranges from the ultraviolet to visible for ZnO-NR. Similarly, the results of the optical characterization by absorbance, for BiOI thin films, indicated the bandgap energy of the samples was between 1.78 eV and 1.88 eV, values previously reported for BiOI.

Structural characterization of magnetron sputtered ZnO thin films on Si(100) using RBS, scanning and high resolution transmission electron microscopy methods

Wide band gap materials like ZnO are being studied due to their potential applications in various devices. Here, in this work, ZnO thin films have been prepared using magnetron sputtering technique. Scanning Electron Microscopy (SEM) studies show rough surface. Only partial coverage is obtained for lower thickness and the coverage increases to maximum at higher thicknesses. Rutherford backscattering spectrometry results show the interfacial diffusion at Si–ZnO interface. High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Diffraction (SAD) studies show the polycrystalline nature of the films. As the thickness increases, number of defects also increases. Presence of Moiré fringes and planar defects have been discussed using Fourier filtered HRTEM images and their corresponding FFTs.

Optical Tailoring of RF Magnetron Sputtered ZnO Thin Films

Pure and N2 doped ZnO thin films of thickness ranging ∼300-500nm with 5,10,15,20,25, and 50 sccm inflow ratios of N2 are deposited on soda-lime glass by means of RF magnetron sputtering system, and observed the dependence of optical properties of ZnO by the function of doping with the help of spectroscopic ellipsometer. And found that the N2-inflow highly affects the optical properties of ZnO thin films. Even the high transmittance of about 97% is achieved and absorbance graph also shows that slight variation in N2 inflow affects the absorbance, which is maximum with in UV region. Optical conductivity of ZnO is also observed high with the increase of N2 inflow. With 25sccm N2 inflow conductivity rose to the maximum value of about 1.4 × 107Ω-1cm-1, with 15sccm inflow of N2 conductivity value is 2.0 × 106Ω-1cm-1 in the visible region. This is a strong contribution towards next generation photovoltaic devices.

CO2 gas sensing properties of DC reactive magnetron sputtered ZnO thin film

Nanostructured ZnO thin films were deposited on glass substrates using a DC reactive magnetron sputtering technique. Thin films of three different thicknesses viz 40, 100 and 300nm were prepared and subsequently annealed at 450°C. The structural, topographical, and optical characteristics of all the three annealed films were studied using X-ray diffractometer (XRD), Atomic Force Microscope (AFM) UV–visible and photoluminescence spectrophotometers. The carbon dioxide (CO2) gas sensing behavior of these films was investigated in detail in the concentration range of 500–10,000ppm. The sensing performance was optimized with respect to the ZnO film thickness as well as the operating temperature. ZnO film with 40nm thickness showed better response characteristics at the operating temperature of 300°C than that of thicker ZnO films. A maximum sensitivity (%) of 1.13 with a response and recovery time of 20s was observed towards 1000ppm of CO2.

Thermal stress induced band gap variation of ZnO thin films

The growth temperature and post annealing-dependent optical and structural effect of RF magnetron sputtered ZnO thin films were examined. As the growth temperature increased, the lattice constant increased and approached the bulk value, suggesting a decrease in interfacial strain between the substrate and thin film. For the post annealed samples, the interfacial strain decreased further and was close to the bulk value regardless of the post annealing environments (in air and O2). The optical properties of all ZnO thin films examined and revealed higher transparency (>90%). Furthermore, the optical band gap varied according to the growth temperature and post annealing environments due to a decrease in the interfacial strain effect.

Effect of RF Power on the Structural Properties of Magnetron Sputtered ZnO Thin Films Deposited at Room Temperature

Zinc oxide thin films were prepared at room temperature in pure argon ambient on glass substrates by RF magnetron sputtering. The effect of sputtering power (50~250 Watt) on the structural properties of the film were investigated. The thickness of ZnO thin films was measured using surface profiler (Dektak 150+). Atomic force microscopy machine (AFM-Park system XE-100) was used to characterize the morphology while the crystalinity have been characterized using XRD (Rigaku Ultima IV). It was found that the thickness, growth rate and RMS roughness increases with increasing RF power. All films exhibit the (002) plane which correspond to hexagonal wurtzite structure with the highest peak at 150 Watt.

Grain Boundary Carrier Scattering in ZnO Thin Films: a Study by Temperature-Dependent Charge Carrier Transport Measurements

In the present investigation, the effects of annealing in oxygen atmosphere on the electrical and optical properties of pulsed direct-current (DC) magnetron-sputtered ZnO thin films have been studied. With annealing, the electrical conductivity was found to increase from 2.3 S cm to 123 S cm. The optical transparency was also found to improve from 83% to 90%. The improvement in the electrical properties with annealing is attributed to the reduction of the grain boundary potential from 34 meV to 8 meV. X-ray photoelectron spectroscopy (XPS) measurements revealed oxygen deficiency in as-deposited films, whereas adequate incorporation of oxygen was observed in the annealed films. Ultraviolet photoelectron spectroscopy (UPS) measurements showed a shift of +0.8 eV in the Fermi level with annealing. This shift indicates significant changes in the electrical conductivity with annealing, which is due to an increase in the carrier concentration.

Structural and optical properties of a radio frequency magnetron-sputtered ZnO thin film with different growth angles

This study introduces optical properties of a columnar structured zinc oxide [ZnO] antireflection coating for solar cells. We obtained ZnO films of columnar structure on glass substrates using a specially designed radio frequency magnetron sputtering system with different growth angles. Field-emission scanning electron microscopy was utilized to check the growth angles of the ZnO films which were controlled at 0°, 15°, and 30°. The film thickness was fixed at 100 nm to get a constant experiment condition. Grain sizes of the ZnO films were measured by X-ray diffraction. A UV-visible spectrometer was used to measure the transmittance and reflectance of the ZnO film columnar structures as a function of the growth angles.

The ethanol sensing property of magnetron sputtered ZnO thin films modified by Ag ion implantation

The authors present the gas sensors using magnetron sputtered ZnO films modified by Ag ion implantation. The results indicate the sensing response of ZnO films is improved by Ag ion implantation, and the sensing response increases with the Ag ion implantation doses. The possible reason of the improved sensing property is that the metallic Ag distributed on the nanograins of implanted ZnO films enhances chemical adsorption and promote catalytic reaction at the surface. However, after long-time conservation, obvious response deterioration for the Ag ion implanted ZnO sensor took place.

Effects of UV-ozone treatment on radio-frequency magnetron sputtered ZnO thin films

The effects of UV-ozone treatment on ZnO thin films prepared by using radio-frequency magnetron sputtering are investigated. Decrease in the density of oxygen vacancy as well as increase in the density of oxygen interstitial were inferred from the UV-ozone treated samples. It was also found that a considerable difference in the work function (0.25 eV) is induced by UV-ozone treatment implying a shift in Fermi level. This shift was confirmed by capacitance–voltage measurements, which demonstrated that the boundary between the inversion region and the depletion region of a ZnO-based metal-oxide-semiconductor (MOS) capacitor positively shifts when UV ozone treated. Our results clearly indicate that the threshold voltage of a thin film transistor can be adjusted by modifying the ZnO surface via UV ozone treatment. MOS capacitors fabricated with UV-ozone treated HfO 2 and/or ZnO also yielded a smaller leakage current (~ 73%–90% smaller) and a larger breakdown voltage (~ 8%–11% larger). The physical mechanism behind the effect of the UV ozone treatment is addressed in this study with the help of X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy.

Effect of substrate-induced strain on the morphological, electrical, optical and photoconductive properties of RF magnetron sputtered ZnO thin films

ZnO thin films have been grown by radio frequency (RF) magnetron sputtering on different substrates such as glass, quartz ( z-cut), sapphire (0 0 1) and Si (1 0 0) in order to study the effect of substrate-induced strain along c-axis on the structural, electrical, optical and photoconducting properties of the films. The full width at half maximum for (0 0 0 2) peak increases while the crystallite size decreases with decrease in the compressive strain. The change in resistivity and carrier concentration has been related to the variation of strain. The resistivity of the films increases and the carrier concentration decreases exponentially with increasing the strain. The film on the quartz substrate shows higher UV emission intensity. The ultraviolet photosensitivity decreases with increase in the strain.

Persistent photoconductivity due to trapping of induced charges in Sn/ZnO thin film based UV photodetector

Photoconductivity relaxation in rf magnetron sputtered ZnO thin films integrated with ultrathin tin metal overlayer is investigated. Charge carriers induced at the ZnO-metal interface by the tin metal overlayer compensates the surface lying trap centers and leads to the enhanced photoresponse. On termination of ultraviolet radiation, recombination of the photoexcited electrons with the valence band holes leaves the excess carriers deeply trapped at the recombination center and holds the dark conductivity level at a higher value. Equilibrium between the recombination centers and valence band, due to trapped charges, eventually stimulates the persistent photoconductivity in the Sn/ZnO photodetectors.

Synthesis and characterization of DC magnetron sputtered ZnO thin films under high working pressures

ZnO thin films were deposited on glass substrates using direct current (dc) magnetron sputtering under high working pressures. A pure zinc target was used, and sputtering was carried out in an oxygen atmosphere. The working pressure was varied between 50 and 800 mTorr. XRD characterization showed that for a window of working pressures between 300 and 500 mTorr, the deposited films were polycrystalline, with strong preferential orientation of grains along the c-axis. The film deposited at 400 mTorr had the highest (002) peak with the largest estimated grain size. Outside this window, the crystallinity and c-orientation of grains are lost. The microstructure of the films was investigated by Atomic Force microscopy (AFM). Optical transparency of the films was about 85%. The films produced were highly resistive, which might provide new alternatives for the synthesis of ZnO thin films aimed for SAW devices.

Transition from ferromagnetism to diamagnetism in undoped ZnO thin films

We report a systematic study of the film thickness dependence (0.1–1 μm) of room-temperature ferromagnetism in pure magnetron-sputtered ZnO thin films wherein a sequential transition from ferromagnetism to paramagnetism and diamagnetism as a function of film thickness is observed. The highest saturation magnetization (MS) value observed is 0.62 emu/g (0.018 μB/unit cell) for a ∼480 nm film. On doping the ZnO film with 1 at. % Mn enhances the MS value by 26%. The ferromagnetic order in ZnO matrix is believed to be defect induced. In addition, on doping with Mn hybridization between the 2p states of O and the 3d states of Mn occurs.

Growth, optical and nanostructural properties of magnetron sputtered ZnO thin films deposited on polymeric substrates

AbstractZnO thin films were deposited onto flexible Poly‐Ethylene Terephthalate and Poly‐Ethylene Naphthalate substrates in an ultra‐high vacuum chamber by Pulsed DC Magnetron Sputtering. The process took place at room temperature and in argon atmosphere with fixed pressure, by applying two different power values on the target, in order to investigate their effect on the growth, optical, nanostructural and topographical properties of the films. In‐situ and real‐time Spectroscopic Ellipsometry in the 1.5–6.5 eV energy region has been employed to monitor the ZnO deposition, which revealed information about the growth mechanisms, interface phenomena and optical properties of the films. Transmission Electron Microscopy was used in order to investigate the structural characteristics of the films and to examine the film/polymer interface properties. The X‐ray Diffraction measurements revealed the changes in films structure caused by the change of the target power, nevertheless all films were preferentially grown along the (002) axis. Finally, with Atomic Force Microscopy the surface roughness of the films was defined correlated also with the target power applied. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)