Sputtered Zinc Oxide Thin Films Research Articles

Influence of Terbium Doping and Annealing on the Structural and Optical Characteristics of Sputtered Zinc Oxide Thin Films

This paper studied the structural and luminescent characteristics of undoped and doped-with-Tb3+-ions ZnO films of 200 nm and 600 nm thicknesses, grown via RF magnetron sputtering on (100) silicon substrate in Ar and Ar-O2 plasma. X-ray diffraction (XRD) patterns revealed a strong preferred orientation of ZnO and ZnO:Tb crystals of the wurtzite structure along the c-axis, perpendicular to the substrate. In the as-deposited ZnO:Tb films, the additional crystal phases, namely, Tb2O3, TbO2, and an amorphous phase, were revealed. The as-deposited undoped films were under tensile strain, that increased in the doped films. This proved the incorporation of the Tb3+ ions into the ZnO grains, and agreed with the Raman spectra investigation. The XRD data and atomic force microscopy study showed that Tb doping impeded the growth of grains and columns, respectively. The photoluminescence (PL) spectra of the doped films contained the UV band ascribed to exciton PL, a broad intrinsic defect-related band, and the narrow bands caused by the intra-shell transitions of the Tb3+ ions. Terbium doping suppressed ZnO emissions. The post-deposition rapid thermal annealing at up to 800 °C of both the undoped and doped films promoted tensile strain relaxation, grain growth, improvement in the ZnO crystal structure, and an increase in the exciton PL. The intensity of the Tb3+ PL changed non-monotonically, and was the highest for the film annealed at 600 °C. The conventional thermal annealing promoted the non-monotonic changes in the strains and grain sizes in such a way that, after annealing at 900 °C, their values became the same as in the as-deposited ZnO:Tb film. This structural change was accompanied by a decrease in the exciton and Tb3+ PL intensity. The formation of the Zn2SiO4 phase was observed via XRD, and confirmed via scanning electron microscopy. It was attributed to the interdiffusion through the film/substrate interface. The deposition in the Ar-O2 atmosphere is found to be more preferable for the formation of Tb3+ emission centers in the ZnO matrix.

Band-edge emission enhancement in sputtered ZnO thin films with ultraviolet surface lattice resonances

Metallic nanostructures acting as optical nanoantennas can significantly enhance the photoluminescence (PL) of nearby emitters. Albeit luminescence enhancement factors of several orders of magnitude have been reported for quantum dots or molecules, in the case of bulk emitters, the magnitude of the plasmonic enhancement is strongly hindered by the weak spatial overlap between the active medium and the electromagnetic modes of the nanoantenna. Here, we propose a solid-state ultraviolet emitter based on a thin film of zinc oxide (ZnO) coupled with an array of aluminum (Al) nanoparticles. The Al nanorod array is designed to sustain surface lattice resonances (SLRs) in the near ultraviolet, which are hybrid modes exhibiting a Fano-like lineshape with narrowed linewidth relatively to the non-hybridized plasmonic modes. By changing both the period of the array and the dimensions of the nanorods, the generated SLR is tuned either to the near band-edge (NBE) emission of ZnO or to the excitation wavelength. We experimentally demonstrate that NBE emission can be increased up to a factor of 3 compared to bare ZnO. The underlying PL enhancement mechanisms are experimentally investigated and compared with numerical simulations. We also demonstrate that SLRs are more efficient for the ZnO luminescence enhancement compared to localized surface plasmon resonances.

Impact of samarium on the structural and physical properties of sputtered ZnO thin films

Zinc oxide (ZnO) thin films containing samarium (Sm) were prepared by radio frequency (RF) sputtering. The Sm was introduced to ZnO films at different RF sputtering powers. The structure of the prepared films has a hexagonal ZnO phase, and the presence of the Sm in the ZnO lattice led to a decrease in the crystalline nature. The average crystallite size values along the major plane (002) decreased with the increase in sputtering power on the Sm. Spherical grains were observed and their values increased from 74.1 nm for the base sample to 82.57 nm when the Sm sputtered at 70 W. After intercalating Sm in the ZnO lattice, the surface roughness was improved. The absorption edge of the samples shifted to a higher photon energy when the Sm increased from 2.51 to 4.33 at%. The linear and non-linear optical parameters and photoluminescence are discussed in terms of the Sm content.

Effect of Oxygen Flow Rate on the Characteristics of RF Sputtered ZnO Thin Films

Zinc Oxide (ZnO) thin films were deposited on glass substrate by radio frequency (RF)reactive magnetron sputtering technique at variable Oxygen flow rates while Argon flow rates waskept constant. The effect of oxygen flow rate on structural, electrical, optical properties of nanostructured ZnO thin films were investigated by X-ray diffractometer, scanning eletron microscopy(SEM), Hall effect measurements and UV-Visible spectrophotometer. X-ray diffraction (XRD) datareveals films are polycrystalline hexagonal structure with (002) peak as a preferred orientation andcrystallite size was found to be in range12 nm-16 nm.The electrical resistivity of films decreasesfrom 10-1 Ω-cm to 10-2 Ω-cm. All deposited ZnO thin films shows high transmittance above 95% inthe visible range 360 nm-800 nm. The optical band gap and refractive indices have been calculatedusing UV-Vis transmission spectra. Oxygen gas flow rates found to have large impact onoptoelectronic properties of ZnO films.

Structural and optical properties of RF sputtered ZnO thin films: Annealing effect

In this study, the effects of annealing temperature on morphological, optical and structural properties of Radio Frequency (RF) magnetron sputtered Zinc oxide (ZnO) thin films were investigated. X-ray diffraction spectra confirmed the hexagonal wurtzite structure with preferential orientation along the c-axis. The annealing temperature had an important role on the crystallite size and it was calculated as 19.7 nm, 17.2 nm, 15.1 nm and 24.5 nm, depending on the increasing annealing temperature. The optical transmittance of ZnO thin films ranged from 85% to 95% at 550 nm wavelength, and this change occurred linearly with annealing temperature. The optical band gap (Eg) of pure ZnO was 3.25 eV; this first decreased to 3.23 eV at low annealing temperature and then increased linearly to 3.26 eV with increasing annealing temperature.

Effect of oxygen pressure and post-annealing on the properties of reactively sputtered zinc oxide thin films

• ZnO thin films are grown at room temperature on glass using radio-frequency sputtering. • As-grown ZnO films have wurtzite crystalline structure and direct bandgap behavior. • ZnO grown in O-rich and O-deficient plasmas shows annealing-sensitive crystallinity. • Bandgap energy evolution with deposition duration and post-annealing is investigated. • ZnO films with stable morphology and optical behavior are grown under some conditions. Using radio-frequency sputtering in the reactive regime at a fixed radio-frequency power of 200 W, we deposited zinc oxide thin films at room temperature on glass substrates in argon-oxygen sputtering atmospheres with different sputtering durations, and various oxygen contents from 20% to 70%. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and transmittance spectrophotometry were used to probe structural, morphological, chemical and optical properties of as-grown and post-annealed thin films in air at medium temperatures (up to 500 °C). The as-grown samples were found to present a hexagonal crystalline structure with a major (0 0 2) orientation and direct bandgap energies ranging from 3.27 to 3.3 eV. The post-annealing results in the improvement of crystallinity of ZnO thin films deposited at low oxygen contents and in the deterioration then the recrystallization of films deposited in oxygen-rich plasma. The optimal properties and thermal stability were obtained for 30% of oxygen in the sputtering environment, with an annealing-induced shrinkage of the bandgap smaller than 100 meV.

Analysis on Characteristics of ZnO Surface Acoustic Wave with and without Micro-Structures.

In this paper, we fabricate a surface acoustic wave (SAW) device with micro-structures on a zinc oxide (ZnO) thin film and measure its signal response. The manufacturing processes of the SAW device include the fabrication of micro-structures of a SAW element and its interdigital transducer by silicon micro-machining and the fabrication of a thin film of ZnO by RF magnetron sputtering. We, then, measure the SAW properties. This research investigates the properties of sputtered thin films for various amounts of O2/(Ar + O2) using Zn and ZnO targets. Regardless of target, the growth rate of the ZnO thin film decreases as the oxygen content increases. When the SAW is sputtered ZnO thin film using 30% oxygen, the digital signal of the SAW has better performance. The measurement signal of the SAW with micro-structures is similar to that without micro-structures.

Effect of microstructural changes in the biological behavior of magnetron sputtered ZnO thin films

This work is focused on the evaluation of the correlation between the microstructural changes of zinc oxide (ZnO) thin films and its biological behavior. For this study, ZnO thin films were prepared by reactive magnetron sputtering, using different deposition times and reactive gas (O2) flows, and tested against a fungal pathogenic species, Candida albicans. Results showed that the increase of thickness of the films did not affect significantly the surface roughness but changed the crystalline structure of the films (ZnO in the hcp structure), which was followed by a small increase of antifungal properties, leading to a decrease of viable cells. The decrease of O2 flow in the deposition chamber affected the roughness only slightly (roughly 1 nm difference between the different films) but resulted in a clear decrease in the crystallinity of thin films, which improved even further their antifungal activity. These results seem to indicate a correlation between structural features and the antifungal behavior of the ZnO thin films, which will be shown by different analyses. When tested against the fungal species, the films showed to be capable of reducing the growth rate and inhibit their growth, leading to a low number of microbial cells at the end of the experiment. A deeper analysis by flow cytometry on how the thin films affected the microbial cells showed a reduced global enzymatic activity in yeast cells after contact with the materials’ surface.

An insight to origin of ferromagnetism in ZnO and N implanted ZnO thin films: Experimental and DFT approach

In the present study, we have elucidated an effective way to simultaneously tune the optical bandgap and magnetic properties of zinc oxide (ZnO) thin films. This can be achieved by the incorporation of nitrogen ions in the host matrix. We have systematically investigated the influence of N ions in ZnO thin films through experimental techniques and the density functional theory (DFT) calculations to understand the physical mechanism governing the observed magnetic behaviour and the variation in bandgap. For this, RF sputtered ZnO thin films deposited over Si substrates were implanted with N ions by varying the fluences and studied for their optical and magnetic properties. The pristine ZnO film is having saturation magnetization of ∼2.45 emu/cm3 which becomes almost twice for the ion fluence of 1 × 1017 ions/cm2. Furthermore, the optical bandgap is tuned from 3.27 eV to 3.04 eV with N ion implantations. This study provides a new insight to understand the basis of ferromagnetism in non-magnetic ions doped ZnO system.

Characterization of RF sputtered zinc oxide thin films on silicon using scanning acoustic microscopy

Zinc oxide (ZnO) thin films were grown on silicon (100) substrate using radio frequency (RF) sputtering under various processing parameters including deposition time and annealing temperature. A series of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and scanning acoustic microscopy (SAM) have been used to analyze the crystallinity and crystal orientation, structural morphology, surface roughness, and acoustic properties of these films. In particular, quantitative analysis of elastic wave propagation in ZnO thin films by scanning acoustic microscopy has been performed for the first time in the present work. It has been shown that the propagation properties of acoustic waves on the surface of ZnO thin films strongly depend on film thickness, crystallinity, and surface roughness. The dispersion properties of surface acoustic waves (SAWs) are observed as a function of ZnO film thickness. The velocities of SAWs range from 5328.3 m/s to 4245.7 m/s with increasing film thickness from 32.5 nm to 2.04 μm, while smoother surface contributes to faster propagation of SAWs.

Quasi-metallic behavior of ZnO grown by atomic layer deposition: The role of hydrogen

Zinc oxide (ZnO) fabricated by atomic layer deposition (ALD) is intrinsically well-conductive (∼5 mΩ cm), in contrast to the single-crystalline bulk material or sputtered ZnO thin films. There are generally three groups of candidates for the intrinsic n-type conductivity: intrinsic point defects, elemental impurities other than hydrogen, and incorporated hydrogen itself. In this study, we assess the different candidates concerning their impact on conductivity. In the presence of free electron densities of up to 5 × 1019 cm−3, impurities other than hydrogen are ruled out due to their ultra-low concentrations in the ppm range. Intrinsic point defects are also considered unlikely since the evolution of conductivity with deposition temperature is not reproduced in the Zn/O ratio as measured by Rutherford backscattering spectrometry. Hence, the most promising candidate is hydrogen with a concentration of ∼1 at. %, i.e., more than sufficient to account for the free electron density. In addition, we find a correlation between the deposition-temperature dependence of the carrier concentration and the hydrogen concentration. The formation energy of the conductive, hydrogen-related state is determined to be ∼40 meV. Hall measurements down to liquid helium temperatures revealed that the electron densities are constant over the whole temperature range. This constitutes a quasi-metallic behavior of ALD-ZnO for deposition temperatures of ≥150 °C. We propose that the very high concentration of hydrogen-induced donor states causes a vanishing ionization energy so that the donor band merges energetically with the ZnO conduction band. This model is supported by ultraviolet photoelectron spectroscopy measurements.

Thickness uniformity and optical/structural evaluation of RF sputtered ZnO thin films for solar cell and other device applications

Spatial uniformity in thickness and other characteristics over the surface of thin films plays a fundamental role in characterization and device fabrication particularly in the solar cells. We report an investigation in the preparation of zinc oxide (ZnO) thin film on sapphire substrate by RF magnetron sputtering and characterization by spectral reflectance, photoluminescence, transmittance, and atomic force microscopy measurements. Surface of the film is spatially analyzed and a comparison is established with the previously reported results of metal organic chemical vapor deposition (MOCVD) grown ZnO film. Results show that ZnO films prepared by sputtering technique are superior in thickness uniformity for practical applications. Optical quality of the films prepared by sputtering is similar or even shows better quality compared to the films grown by MOCVD. Furthermore, the importance of film thickness uniformity in particular with reference to the solar cell performance is discussed that, so far, could not draw adequate attention from the research community.

An Investigation on Electrical and Hydrogen Sensing Characteristics of RF Sputtered ZnO Thin-Film With Palladium Schottky Contacts

Fabrication and characterizations of a hydrogen sensor with palladium Schottky contacts, grown on radio frequency sputtered zinc oxide thin film is reported. Temperature dependence performance analysis of the proposed sensor was carried out at different hydrogen concentrations (50–1000 ppm) and temperatures (25°C–200°C) using current–voltage ( $I$ – $V$ ) measurements. Lateral shifts have been observed in the Schottky diode $I$ – $V$ characteristic when the device was exposed to hydrogen, and this could be attributed to the reduction of Schottky barrier height (SBH). The optimum performance of the sensor was obtained at 150 °C with maximum SBH variation of 144 meV at 1000 ppm hydrogen. Transient sensor performance at optimum temperature has also been analyzed and sensitivities ranging from 224% to 1125% with a minimum response time of 55 s and a recovery time of 26 s have been obtained. The response of sensor toward methane and nitrogen dioxide is also discussed. The basic $I$ – $V$ and capacitance–voltage characteristics of Pd/ZnO thin film are also reported, which confirmed its excellent rectifying properties. The basic microstructure studies of ZnO thin film were also investigated using X-ray diffraction, scanning electron microscope, atomic force microscopy, energy dispersive X-ray spectroscopy, and photoluminescence measurements. The proposed sensor has proven to be economical, due to its high sensitivity and easy to fabricate structure with a limited number of processing steps.

Influence of iron doping on the structural, chemical, and optoelectronic properties of sputtered zinc oxide thin films

Abstract

Enhancement of the refractive index of sputtered zinc oxide thin films through doping with Fe2O3

Certain optical applications, such as antireflective coatings on solar cells, require transparent films with high refractive indices (>2). Zinc oxide (ZnO) is a transparent semiconductor with exceptional optical properties. However, its refractive index in the transparent dispersionless region of the visible spectrum is lower than 2. In this study, we enhanced the refractive index of sputtered ZnO thin films through doping with iron oxide (Fe2O3), where ZnO targets were doped by 0.5–2.0 wt% of Fe2O3. The films were polycrystalline with smooth surfaces. Differential transmittance spectra were employed to derive the optical band gaps of the films, which showed a minor variation by ±0.03 eV due to doping. The refractive index was extracted from the transmittance spectra using a Cauchy equation and was further fitted by a single-oscillator model. The optimum refractive index was obtained for the films prepared from the target doped with 1.5 wt% of Fe2O3. Enhancement of the refractive index was accompanied by a reduction of optical absorption in the doped films.

Effect of nitrogen doping on structural, morphological, optical and electrical properties of radio frequency magnetron sputtered zinc oxide thin films

Zinc oxide receives remarkable attention due to its several attractive physical properties. Zinc oxide thin films doped with nitrogen were grown by employing RF magnetron sputtering method at room temperature. Doping was accomplished in gaseous medium by mixing high purity nitrogen gas along with argon sputtering gas. Structural studies confirmed the high crystalline nature with c-axis oriented growth of the nitrogen doped zinc oxide thin films. The tensile strain was developed due to the incorporation of the nitrogen into the ZnO crystal lattice. Surface roughness of the grown films was found to be decreased with increasing doping level was identified through atomic force microscope analysis. The presenting phonon modes of each film were confirmed through FTIR spectral analysis. The increasing doping level leads towards red-shifting of the cut-off wavelength due to decrement of the band gap was identified through UV–vis spectroscopy. All the doped films exhibited p-type conductivity was ascertained using Hall measurements and the obtained results were presented.

Synthesis, characterization, and hydrogen gas sensing properties of AuNs-catalyzed ZnO sputtered thin films

Hydrogen present in concentration up to 4 vol.% forms an explosive mixture with air. Its propensity to escape in the event of leak, could lead to quick build-up and formation of an explosive mixture with air in confined spaces, such as an automobile. This necessitates its detection at very low concentration. Zinc oxide (ZnO) is a well-known wide band gap (∼3.37eV) semiconducting oxide that has been widely used for gas sensing applications. This work reports on the fabrication, characterization and gas sensing performance of nanogold decorated ZnO thin films made by DC reactive sputtering. The sensor films were fabricated by depositing a very thin layer of gold on the sputtered ZnO thin film. The as deposited Au@ZnO films were converted into highly crystalline ZnO film covered with gold nanostructures (AuNs@ZnO) by mild heat treatment. The structural and morphological as well as the compositional homogeneity of the as-deposited and heat-treated ZnO, Au@ZnO and AuNs@ZnO thin films were ascertained. The gas sensing behavior of the AuNs@ZnO thin films towards hydrogen as a function of temperature at different H2 concentrations was investigated and compared with that of pure and heat-treated ZnO films. The effect of the presence of gold nanoparticles on imparting improvement (in terms of higher response signal, high reproducibility and complete reversibility) was established; the optimal operating temperature was about 400°C. A plausible mechanism for the observed enhancement in the sensing behavior of AuNs@ZnO films towards H2 is proposed.

Morphological evolution of zinc oxide thin films with variation in sputtering power and substrate temperature

Zinc oxide (ZnO) thin films were deposited on silicon substrates by reactive RF magnetron sputtering technique in order to investigate the evolution of the morphological and the optical properties as a function of different RF power and substrate temperature. Analysis of RF power and the substrate temperature have played a significant role in the morphological and the optical properties of the sputtered ZnO thin films. X-ray diffraction pattern of ZnO thin film has shown the appearance of c-axis-oriented (002) peak for all samples with variation in the degrees of crystallinity. The surface roughness as well as the average grain size of the ZnO films found to be decreased with RF power, where as the films grown at higher temperature has shown the evolution of larger grains. ZnO films, deposited at 150 W RF power and substrate of 200°C, have shown better optical properties.

Morphological evolution of zinc oxide thin films with variation in sputtering power and substrate temperature

Zinc oxide (ZnO) thin films were deposited on silicon substrates by reactive RF magnetron sputtering technique in order to investigate the evolution of the morphological and the optical properties as a function of different RF power and substrate temperature. Analysis of RF power and the substrate temperature have played a significant role in the morphological and the optical properties of the sputtered ZnO thin films. X-ray diffraction pattern of ZnO thin film has shown the appearance of c-axis-oriented (002) peak for all samples with variation in the degrees of crystallinity. The surface roughness as well as the average grain size of the ZnO films found to be decreased with RF power, where as the films grown at higher temperature has shown the evolution of larger grains. ZnO films, deposited at 150 W RF power and substrate of 200°C, have shown better optical properties.

Effect of Aluminum doping on the properties of magnetron sputtered ZnO films

Aluminum doped zinc oxide (AZO) thin films were coated on the glass substrate by diffusing the Al in to the sputtered ZnO thin films. Initially, zinc oxide (ZnO) thin films were deposited on glass substrates using reactive dc magnetron sputtering (DCMS) technique followed by Aluminum (Al) were deposited over the ZnO films by vacuum evaporation technique. The prepared samples were annealed at 400 °C for 3h in vacuum in order to diffuse Al uniformly in to ZnO matrix. The influence of Al doping over the micro-structure, surface morphology, optical and electrical properties of the films were studied by X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), UV-Visible spectroscopy and four point probe (FPP) method respectively. Resistivity of the AZO films decreased with increase in the Al concentration and the film deposited with high Al concentration offered low electrical resistivity of ∼28 Ω.cm and optical bandgap of 3.21 eV.