High-quality Zinc Oxide Thin Films Research Articles

Effects of Growth Temperature on Morphological and Structural Properties of ZnO Films

Zinc oxide (ZnO) is one of the most promising oxide possibilities for use in a number of industries due to its unique properties. Because of its broad direct bandgap (3.37 eV) and strong exciton binding energy (60 meV) at ambient temperature, ZnO not only conducts electricity well but also transmits visible light and emits UV light. Here, we investigated the effect of growth temperature on ZnO thin films by changing the growth temperatures from 400 oC to 450 oC. Radio-frequency (RF) magnetron sputtering was used to create ZnO thin films on Si(100) substrates. The atomic force microscopy (AFM) results show that the root-mean-square (RMS) roughness decreases from 6.1 ± 1.0 nm to 4.8 ± 0.6 nm as the growth temperatures increase. XRD patterns display the enhancement of ZnO’s structure when increasing the growth temperature. Our findings indicate that controlling growth temperature is the critical factor in producing high quality ZnO thin films.

ZnO Nanorods Growth and the Parametric Survey of Factors Influencing the Morphologies for DSSC Application-Review

Zinc-oxide (ZnO) nanostructures, such as nanorods, are generally viewed as leading studies of keen interest due to their significant application potential in many applied areas, primarily for the improvement of photovoltaic solar cells. The morphologies of nanorods are linked to the seed layer (SL) optimization. As a result, forming a high-quality ZnO thin film prior to the growth of nanorods is critical. The processing parameters used during the synthesis technique can influence the quality of the synthesized samples. This review article focuses on the outcomes of the process variables responsible for better-synthesized ZnO SL for nanorod growth. The effect of various parameters, such as annealing temperature, precursor solution, and ZnO SL thickness, on the morphology characteristics of ZnO nanorods prepared was reviewed. The study examined numerous published works to find the effect of heat treatment, the thickness of the layer, substrate, and precursor concentration on the morphology of the seed layer in the production of ZnO nanorods.

Effect of thickness and reaction media on properties of ZnO thin films by SILAR

Zinc oxide (ZnO) is one of the most promising metal oxide semiconductor materials, particularly for optical and gas sensing applications. The influence of thickness and solvent on various features of ZnO thin films deposited at ambient temperature and barometric pressure by the sequential ionic layer adsorption and reaction method (SILAR) was carefully studied in this work. Ethanol and distilled water (DW) were alternatively used as a solvent for preparation of ZnO precursor solution. Superficial morphology, crystallite structure, optical and electrical characteristics of the thin films of various thickness are examined applying X-ray diffraction (XRD) system, scanning electron microscopy, the atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet–visible spectroscopy, photoluminescence spectroscopy, Hall effect measurement analysis and UV response study. XRD analysis confirmed that thin films fabricated using ethanol or DW precursor solvents are hexagonal wurtzite ZnO with a preferred growth orientation (002). Furthermore, it was found that thin films made using ethanol are as highly crystalline as thin films made using DW. ZnO thin films prepared using aqueous solutions possess high optical band gaps. However, films prepared with ethanol solvent have low resistivity (10–2 Ω cm) and high electron mobility (750 cm2/Vs). The ethanol solvent-based SILAR method opens opportunities to synthase high quality ZnO thin films for various potential applications.

The effect of ultrasonic wave amplitude on the physical properties of zinc oxide (ZnO) deposited by ultrasonic spray method

In this study, high quality zinc oxide (ZnO) thin films, with improved properties, were prepared by a cost-effective ultrasonic spray pyrolysis technique via a careful optimization of the used ultrasonic wave amplitude. The deposition process was performed on glass substrate and were subsequently annealed at 400 °C. We investigated the effect of various ultrasonic wave amplitude on the structural, surface morphology, optical and electrical properties of the obtained thin films, after varying the applied wave amplitude. Furthermore, deposited thin films were studied by means of XRD, UV–vis spectrophotometer, scanning electron microscope, and four-point probe technique. XRD analysis confirmed that obtained ZnO thin films have polycrystalline structure and a wurtzite (hexagonal) phase, with a c-axis preferred orientation (002). The crystallite size was about 23–30 nm. The SEM micrographs of the surface morphology show uniform, homogenous and dense films with granular structures. The films thicknesses were found to be dependent on the used wave amplitude; they were ranged from 184 to 423.5 nm. In addition, the optical properties of the deposited thin films reveal that the films are highly transparent in the visible region above 80%, while the value of energy band gap varies from 3.24 to 3.27 eV. The Electrical properties investigation revealed a resistivity around 10-3 Ω.cm, showing also a non negligible dependency with the wave amplitude tuning. We obtained an improvement in the carrier concentration (1.6 × 1020–3.9 × 1020 cm−3) and mobility (4.2–15 cm2/V.s) with the ultrasonic wave amplitude rising. High quality ZnO thin films with enhanced properties are in demand and have a large wide of applications in optoelectronics and solar cells.

Synthesis and Characterizations of ZnO Thin Films Grown by Physical Vapor Deposition Technique

In the current study, Zinc oxide (ZnO) thin films have been synthesized over the whole the glass-slide substrate by utilizing the physical vapor deposition (PVD) technique. The Zinc (Zn) seed layer was deposited by heating the high purity Zn powder by using a molybdenum (Mo) boat at 37.503×10-3 Torr vacuum pressure of the PVD chamber. The ZnO thin films were fabricated by oxidation of the Zn seed layer coated glass-slide substrate at 400 °C. The morphological, chemical compositions, crystal quality, structural and optical properties of fabricated ZnO thin film were characterized and studied utilizing several characterization techniques. The results found that the high distribution density, homogenous, uniform, and high-quality ZnO thin film was grown over the entire substrate. The synthesized ZnO thin film with a thickness of 130 nm was grown with high purity and polycrystalline hexagonal-Wurtzite phase of ZnO. The sharp, and dominant diffraction peak was observed at peak position 34.3375 along (002) plane and c-axis. The investigated crystal size, dislocation density, and interplanar spacing were about 13.33 nm, 5.63×10-5 A°, and 2.609 A°, respectively. Also, UV-visible spectroscopy results show the high transmittance and low absorbance in the visible (Vis.) region and were about 90%, and the transmittance decreases sharply near the UV region at a wavelength around 383 nm. Besides, obtained the energy band-gap (Eg) was about 3.24 eV.

Structure and morphology of atomic layer deposition grown ZnO thin film/nanostructure on polymeric template

Abstract Zinc oxide (ZnO) was grown on polymer template, namely Poly (methyl methacrylate) (PMMA), by atomic layer deposition (ALD) technique. The detailed structural characterizations of the composite ZnO/PMMA film, through X-ray scattering techniques, indicate that on the PMMA template crystalline ZnO thin film was formed with its c-axis oriented along the surface normal. Combination of X-ray reflectivity (XRR) and atomic force microscopy (AFM) studies demonstrate the formation of high quality ZnO thin film (about 25 nm thick) on PMMA template, with more than 92% coverage and low surface/interface roughness. AFM results clearly show that the ZnO film consists of almost uniform nanoparticles with ∼24-30 nm in-plane size distribution, which is very much consistent with the observed size of the ZnO crystallites obtained from X-ray diffraction (XRD) study.

Roll-to-roll micro-gravure printed large-area zinc oxide thin film as the electron transport layer for solution-processed polymer solar cells

Large-scale, roll-to-roll (R2R) micro-gravure printing process was developed to deposit the electron transport layer (ETL) using low-temperature, solution-processable zinc oxide (ZnO) nanoparticle ink on flexible substrate for fabricating inverted organic solar cells (OSCs). The properties of micro-gravure R2R printed ZnO thin film was optimized via using web tension, substrate pre-treatment and printing speed, leading to high-quality and thickness controllable ZnO thin films. The inverted OSCs using R2R micro-gravure printed ZnO thin film as the ETL showed performance parameters comparable to those of spin-coated ZnO thin film ETL on the flexible substrate in both P3HT:PCBM and PTB7-Th:PC71BM based devices. The research demonstrated that the potentially commercial large-scale, R2R micro-gravure printing process could be used to produce high-quality ZnO thin film with controllable thickness for efficient inverted OSCs, which would accelerate the development of fully R2R micro-gravure printing OSCs and their commercialization.

Influence of flexible substrates on inverted organic solar cells using sputtered ZnO as cathode interfacial layer

Zinc oxide (ZnO) has recently shown to be of considerable interest for the development of interfacial buffer layers in inverted organic solar cells (OSCs). High quality ZnO thin films can indeed be prepared on large-area ITO-coated flexible substrates, using low temperature deposition techniques such as sputtering, a compatible technique with roll-to-roll process. However, further studies are still needed for a better understanding of the influence of the flexible substrate properties on the photovoltaic performances of those devices. In this work, ZnO films have been sputtered on ITO-coated flexible (PEN) substrates and annealed at different temperatures. The role of the surface morphology and the crystalline quality of ZnO films has been investigated. In the window of flexible compatible process, we found that moderate annealing temperatures of ZnO (⩽180°C) lead to improved structural properties and performances. Interestingly, we achieve optimal performances for an annealing temperature of 160°C, resulting in power conversion efficiency (PCE) equivalent to the highest performances usually achieved on rigid cells.

Improving thin-film zinc-oxide surface acoustic wave device insertion loss using a grooved reflective grating structure

A novel silicon groove reflective grating structure (RGS) to reduce effectively the insertion loss of a high C-axis (002) thin-film zinc oxide (ZnO) surface acoustic wave (SAW) device is proposed. The insertion loss (S 21 parameter) of three types of SAW devices was investigated and compared; one type had no RGS design, and the other two types had either a conventional metal electrode RGS or a novel etched silicon groove RGS. The wave propagation modes, the harmonic response, and the displacement amplitude profile of the proposed SAW devices were simulated by finite element analysis software (ANSYS). The implemented basic SAW device consists of a sputter-deposited high-quality ZnO thin film on the surface of a Si/SiO 2 /Si 3 N 4 /Cr/Au substrate. The insertion loss of the proposed SAW device with silicon groove RGS can be reduced to −19.49 dB , which is much smaller than the insertion losses of the SAW devices with a metal electrode-type RGS (−33.15 dB ) and without an RGS (−47.9 dB ). The resonant frequencies of the SAW devices studied (47.37 to 48.05 MHz) are in close agreement with the simulated results (46.8 to 47.55 MHz).

Crystallinity improvement of ZnO thin film by hierarchical thermal annealing

Molecular beam epitaxy (MBE) was adopted to grow zinc oxide (ZnO) film on sapphire substrate and improve the quality of ZnO thin film epitaxy using a magnesium oxide (MgO) buffer layer and a two-segment temperature scheme for ZnO thin film growth. The influence of thermal annealing of different layers on the optical and crystalline features, stress expression, as well as surface morphology of ZnO thin film was examined. SEM images showed smooth surfaces were formed, and these surfaces allowed the low-temperature ZnO buffer layer to have better epitaxial environment at the very beginning. X-ray diffraction (XRD) analysis revealed that a lower thermal annealing temperature more effectively released the stress of materials. The thermally annealed MgO buffer layer had 26% less strain than the purely thermally annealed, high-temperature-grown ZnO (HT-ZnO), and 33% less strain than the unannealed samples. Atomic force microscopy results showed that the root-mean-square of surface roughness of thermally annealed MgO was 0.255 nm, which was 80% lower than that of thermally annealed HT-ZnO (1.241 nm). Photoluminescence measurement showed that the thermally annealed MgO buffer layer had the highest strength for near-band-edge emission because of improved crystalline quality. HRTEM results showed that the stress caused by the mismatch between the sapphire lattice was effectively released because the MgO buffer layer was annealed at a high temperature. The surface of the MgO buffer layer became smooth and the stress mismatching with the ZnO lattice did not obviously extend upwards. When MBE was used to grow ZnO thin film, a lower thermal annealing temperature for the MgO buffer layer more effectively controlled stress accumulation and produced high-quality ZnO thin film.

High-Performance ZnO Thin-Film Transistors with Location-Controlled Crystal Grains Fabricated by Low-Temperature Hydrothermal Method

In this paper, high-performance bottom-gate (BG) thin-film transistors (TFTs) with zinc oxide (ZnO) artificially location-controlled lateral grain growth have been prepared via low-temperature hydrothermal method. For the proper design of source/drain structure of ZnO/Ti/Pt thin films, the grains can be laterally grown from the under-cut ZnO beneath the Ti/Pt layer. Consequently, the single one vertical grain boundary perpendicular to the current flow will be produced in the channel region as the grown grains from the source/drain both sides are impinged. As compared with the conventional sputtered ZnO BG-TFTs, the proposed location-controlled hydrothermal ZnO BG-TFTs (W/L = 250 microm/10 microm) demonstrated the higher field-effect mobility of 6.09 cm2/V x s, lower threshold voltage of 3.67 V, higher on/off current ratio above 10(6), and superior current drivability, reflecting the high-quality ZnO thin films with less grain boundary effect in the channel region.

Low-temperature growth and characterization of ZnO thin films for flexible inverted organic solar cells

Flexible inverted organic solar cells (IOSCs) with a zinc oxide (ZnO) thin film layer acting as an effective electron transport layer and a low reflective light absorber were fabricated in this study. The high quality ZnO thin films on flexible indium tin oxide-coated polyethersulfone (PES) substrates were grown in a non-vacuum process using mist pyrolysis chemical vapor deposition at a low temperature of 180 °C without any thermal damage to the PES substrate. The power conversion efficiency of the flexible IOSCs with ZnO thin films averaged 3.1% at a simulated air-mass of 1.5 global full-sun (100 mW cm−2) illumination, which was greater than that of flexible IOSCs with a ZnO thin film formed using a sol–gel method (approximately 2.3%).

Structure and Optical Properties of ZnO Thin Film Preparation Using RF Magnetron Sputtering

The aim of this study is to obtain high-quality zinc oxide thin films by reactive radiofrequency (rf) magnetron sputtering. The thin films were prepared at constant total gas pressure, with different oxygen and argon contents. The ZnO samples were characterized by several methods. From XRD measurements it was confirmed that ZnO films are c-axis oriented, the line width and intensity are sensitive to O2/Ar gas pressure. All films exhibited excellent transmission (in excess of 70 %) in the visible range with a steep fall off in transmission at 425 nm. From the absorbance measurements the optical band-gap energy was extrapolated according the transmission spectrum. It shows that the optical band gap of the films increased from 3.233 eV to 3.288 eV with increase in the oxygen concentrations from 20 % to 70 %. Refractive indexes of the obtained thin films were carried out in this study.

Characteristics of Zinc Oxide Thin Film Transistors Fabricated by Location-Controlled Hydrothermal Method

In this paper, high-performance Zinc oxide (ZnO) thin-film transistors (TFTs) with bottom-gate (BG) structure and artificially location-controlled lateral grain growth have been prepared by low-temperature hydrothermal method. As the proper design of source/drain structure of ZnO/Ti/Pt thin films, the lateral grain growth can be artificially controlled in the desired location and the vertical grain boundary perpendicular to the current flow in the channel region can be reduced to single one. As compared with the conventional sputtered ZnO BG-TFTs, the proposed location-controlled hydrothermal ZnO BG-TFTs (W/L = 250 µm/10 µm) demonstrated the higher field-effect mobility of 6.09 cm2/V•s, lower threshold voltage of 3.67 V, larger on/off current ratio above 106, and superior current drivability, which can be attributed to the high-quality ZnO thin films with the reduced vertical grain boundaries in the channel region.

Effects of annealing on properties of ZnO thin films prepared by electrochemical deposition in chloride medium

The development of cost-effective and low-temperature synthesis techniques for the growth of high-quality zinc oxide thin films is paramount for fabrication of ZnO-based optoelectronic devices, especially ultraviolet (UV)-light-emitting diodes, lasers and detectors. We demonstrate that the properties, especially UV emission, observed at room temperature, of electrodeposited ZnO thin films from chloride medium (at 70 °C) on fluor-doped tin oxide (FTO) substrates is strongly influenced by the post-growth thermal annealing treatments. X-ray diffraction (XRD) measurements show that the films have preferably grown along (0 0 2) direction. Thermal annealing in the temperature range of 150–400 °C in air has been carried out for these ZnO thin films. The as-grown films contain chlorine which is partially removed after annealing at 400 °C. Morphological changes upon annealing are discussed in the light of compositional changes observed in the ZnO crystals that constitute the film. The optical quality of ZnO thin films was improved after post-deposition thermal treatment at 150 °C and 400 °C in our experiments due to the reducing of defects levels and of chlorine content. The transmission and absorption spectra become steeper and the optical bandgap red shifted to the single-crystal value. These findings demonstrate that electrodeposition have potential for the growth of high-quality ZnO thin films with reduced defects for device applications.

Structural and Optical Characterization of ZnO Thin Films Deposited by Reactive rf Magnetron Sputtering

The aim of this study is to obtain high-quality zinc oxide thin films at room temperature by reactive radiofrequency (rf) magnetron sputtering in an oxygen environment. The films were deposited on glass and silicon substrates at several temperatures, ranging from ambient temperature to 400 °C. In order to have the best results at room temperature we adjusted several deposition parameters such as the O2/Ar gas pressure, target−substrate distance, rf power, and gas flow rate. The ZnO samples were characterized by several methods. From XRD measurements it was confirmed that ZnO films are c-axis oriented, the line width and intensity are sensitive to the variation of the growth temperature, and the best results were found at room temperature for our growth conditions. From AFM pictures it is seen that the grain size decreases when the temperature is increased to 400 °C and the surface roughness of the as-deposited films increases with the deposition temperature. All films exhibited excellent transmission (in ...

Preferentially oriented ZnO thin films on basal plane sapphire substrates derived from polymeric precursors

Abstract Preferentially oriented zinc oxide (ZnO) thin films on basal plane sapphire have been deposited by spin coating polymeric precursors synthesized via Pechini process. The polymeric precursors were synthesized using citric acid as chelating agent and ethylene glycol as reaction medium. The effects of annealing (from 673 K to 1273 K) on surface morphology, crystallinity, and optical properties were examined. XRD results illustrate polycrystalline ZnO thin films with preferred orientation along (0 0 2) plane that increases with an increase in annealing temperature. The textured ZnO films were obtained on annealing for 10 min at 1173 K without the formation of interfacial spinels. Surface morphology of these films changed from spherical to highly faceted grains. Transmission spectra reveal that all films are >70% transparent except for the sample annealed at 1273 K, which has 45% optical transparency in the visible region. Room temperature photoluminescence spectra revealed broad ultraviolet emission centered at 380 nm without deep-level emission, suggesting high quality ZnO thin films.

Extremely low temperature growth of ZnO by atomic layer deposition

We report on the zinc oxide (ZnO) thin films obtained by the atomic layer deposition (ALD) method using diethyl zinc and water precursors, which allowed us to lower deposition temperature to below 200 °C. The so-obtained “as grown” ZnO layers are polycrystalline and show excitonic photoluminescence (PL) at room temperature, even if the deposition temperature was lowered down to 100 °C. Defect-related PL bands are of low intensity and are absent for layers grown at 140−200 °C. This is evidence that extremely low temperature growth by ALD can result in high quality ZnO thin films with inefficient nonradiative decay channels and with thermodynamically blocked self-compensation processes.

Structural effect on UV emission properties of high-quality ZnO thin films deposited by RF magnetron sputtering

Zinc oxide (ZnO) thin films were deposited on SiO 2/Si(1 0 0) substrates by reactive RF magnetron sputtering technology. The effects of sputtering parameters on the crystalline and luminescent characteristics of ZnO films were investigated. According to the results, the optimal sputtering parameters for the best photoluminescence (PL) characteristics were found to be oxygen concentration, (O 2/O 2+Ar), of 21%, RF power of 100 W, substrate temperature of 500 °C and sputtering pressure of 5 mTorr. The obtained wavelength of light emission was found at 379 nm (ultraviolet—UV region), which may be due to the band-to-band transition. However, the optimal parameters for the best PL characteristics of ZnO thin films were not consistent with those obtained from the (0 0 2) intensities of XRD analyses. As a result, XRD pattern was not considered as the key issue concerning the intensity of PL of ZnO thin film. The intensity of the emitted UV light will correspond to the grain size of ZnO film.

Improved electrical stability and UV emission of zinc oxide thin films prepared by combination of metalorganic chemical vapor deposition technique and post-deposition hydrogen doping

Fourier transform infrared and photoluminescence spectroscopy provide strong evidence that post-deposition hydrogen doping of polycrystalline zinc oxide (ZnO) thin films improves the resistivity by increasing hydrogen-related shallow donors and hydrogen passivation of native defects. Improvement of the electrical stability and UV emission confirm that post-deposition hydrogen doping is a promising method to achieve high quality ZnO thin films for the use as transparent electrodes and/or UV light emitters in thin-film-based optoelectronic devices.