Oriented Zinc Oxide Thin Films Research Articles

Structure and defects-related optical properties of highly (002)-oriented zinc oxide thin films

We report on the highly (002)-oriented ZnO thin films deposited on glass substrates at different substrate temperatures using the recently developed pulsed laser deposition (PLD) facility at our institution. Effects of the substrate temperature on the structural, compositional, morphological, and optical properties of the ZnO thin films are studied. The XRD studies revealed the deposition of high-quality ZnO thin films with preferred hexagonal wurtzite structure along the (002) plane above 200 °C substrate temperatures. An increase in the crystallite size estimated using the Scherrer formula from 9.58 nm to 13.25 nm of the deposited thin films reflects the enhancement of the crystallinity with the increasing substrate temperature. Scanning electron microscope (SEM) micrographs revealed a smooth surface morphology with small grains for the thin film deposited at room temperature. The thin films deposited at high substrate temperatures showed agglomeration of nanoparticles resulting in nano rod-like structures. The optical band gap energy was found to be increased with an increase in the crystallinity of the thin films. In the photoluminescence (PL) spectra; near band edge (NBE) emissions were observed in all of the deposited thin films. The deep-level emissions of ZnO thin films were found to be highly dependent on the substrate temperature during film deposition. The thin films grown at 500 °C showed the visible emission in blue, green, and yellow-orange regions, which corresponded to zinc vacancy (VZn), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. These crystal defects in the deposited ZnO thin films are responsible for the deep level emission linked to the visible emission.

Synthesis and Characterization of c-Axis Oriented Zinc Oxide Thin Film and Its Use for the Subsequent Hydrothermal Growth of Zinc Oxide Nanorods

ABSTRACTOriented ZnO seed layers were deposited by three different techniques, namely, simple drop casting (DC), sol-gel derived dip coating (DPC) and spin coating of ball-milled ZnO powder solution(BMD) for the subsequent growth of vertically aligned ZnO nanorods along the substrate normal. X-ray diffraction (XRD) analyses revealed that ZnO(DC) seed layer exhibit the highest preferential c-axis texturing among the ZnO seed layers synthesized by different techniques. The Scanning Electron Microscopy (SEM) analysis evident that the morphology of ZnO seed layer surface is compact and coherently carpets the underlying substrate. ZnO nanorods(NRs) were then grown by hydrothermal method atop the ZnO seeded and non-seeded substrates grown by different techniques to elucidate the best ZnO seed layer promoting well-aligned ZnO Nanorods. The presence of c-axis oriented ZnO(DC) seeding layers was found to significantly affect the surface morphology and crystallographic orientation of the resultant ZnO NRs films. The optical band gap of ZnO(DC) seed and ZnO NRs were estimated to be 3.30 eV and in the range of 3.18 – 3.25 eV respectively by using UV-VIS-NIR diffuse reflection spectroscopy. The room temperature photoluminescence analyses revealed that nanostructured ZnO films exhibit a sharp near-band-edge luminescence peak at ∼380 nm consistent with the estimated optical band gap and the ZnO nanorod arrays are notably free from defect-related green-yellow emission peaks.

Low-Temperature ZnO Thin Film and Its Application in PbS Quantum Dot Solar Cells

Zinc oxide (ZnO) has been widely deployed as electron conducting layer in emerging photovoltaics including quantum dot, perovskite and organic solar cells. Reducing the curing temperature of ZnO layer to below 200 oC is an essential requirement to reduce the cell fabrication cost enabled by large-scale processes such as ink-jet printing, spin coating or roll-roll printing. Herein, we present a novel water-based ZnO precursor stabilized with labile NH3, which allow us to spin coat crystalline ZnO thin films with temperatures below 200 oC. Thin film transistors (TFTs) and diode-type quantum dot solar cells (QD SCs) were fabricated using ZnO as electron conduction layer. In the QD SCs, a p-type 1,2-ethylenedithiol treated PbS QDs with a bandgap of 1.4 eV was spin-coated on top of ZnO layer by a layer-by-layer solid state ligand exchange process. Electron mobility of ZnO was about 0.1 cm2V-1s-1 as determined from TFT measurements. Power conversion efficiency of solar cells: FTO/ZnO/PbS/Au-Ag was 3.0% under AM1.5 irradiation conditions. The possibility of deposition of ZnO at low temperatures demonstrated herein is of important for solution processed electronic and optoelectronic devices. 
 Keywords
 ZnO, low-temperature, quantum dots, solar cells, TFTs
 References
 [1] A. Janotti, A. Janotti, C.G. Van De Walle-fundamental of ZnO as a semiconductor, Reports on Progress in Physics, 72 (2009) 126501.[2] H. You, Y. Lin-investigation of the sol-gel method on the flexible ZnO device, International Journal of Electrochemical Science, 7 (2012) 9085–9094.[3] Y. Lin, C. Hsu, M. Tseng, J. Shyue, F. Tsai-stable and high-performance flexible ZnO thin-film transistors by atomic layer deposition, Applied Materials &Interfaces, 7(40) (2015) 22610–22617.[4] C. Lin, S. Tsai, M. Chang-Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells, Organic Electronics, 46 (2017) 218-255.[5] H. Park, I. Ryu, J. Kim, S. Jeong, S. Yim, S. Jang-PbS quantum dot solar cells integrated with sol−gel-derived ZnO as an n‑type charge-selective layer, Journal of Physical Chemistry C, 118(2014) 17374−17382.[6] Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger-inverted polymer solar cells integrated with a low- temperature-annealed sol-gel-derived ZnO film as an electron transport layer Advanced Materials, 23(2011) 1679–1683.[7] V.A. Online, R. Suriano, C. Bianchi, M. Levi, S. Turri, G. Griffini-the role of sol-gel chemistry in low-temperature formation of ZnO buffer layers for polymer solar cells with improved performance, RSC Advances, 6(2016) 46915-46924.[8] X. D. Mai, J. An, H. Song, J. Jang-inverted Schottky quantum dot solar cells with enhanced carrier extraction and air-stability, Journal of Materials Chemistry A, 2 (2014) 20799–20805.[9] H. Choi, J. Lee, X.D. Mai, M.C. Beard, S.S. Yoon, S. Jeong - supersonically spray-coated colloidal quantum dot ink solar cells, Scientific Report, 7(2017) 622.[10] C.R. Newman, C.D. Frisbie, A. Demetrio, S. Filho, J. Bre- introduction to organic thin film transistors and design of n-channel organic semiconductors, Chemistry Materials, 16(2004) 4436-4451.[11] M. Asad, N. Abdul, Chapter 9: Sol-Gel-Derived Doped ZnO Thin Films: Processing, Properties, and Applications, in Recent Applications in Sol-Gel Synthesis, Edt:C. Usha. InTech, Rijeka, Croatia, 2017. [12] D. Guo, K. Sato, S. Hibino, T. Takeuchi, H. Bessho, K. Kato, Low-temperature preparation of (002)-oriented ZnO thin films by sol–gel method, Thin Solid Films, 550 (2014), 250-258. [13] S. T. Meyers, J. T. Anderson, C. M. Hung, J. Thompson, J. F. Wager, D. A. Keszler, Aqueous Inorganic Inks for Low-Temperature Fabrication of ZnO TFTs, J. Am. Chem. Soc, 130 (2008), 17603-17609.

Investigation of structural, optical and vibrational properties of highly oriented ZnO thin film

This article deals with poly crystalline Zinc Oxide (ZnO) thin film fabricated on ultrasonicated clean glass substrate by sol-gel spin coating. X-Ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and UV–Visible spectroscopy (UV–Vis.), have been utilized to describe the fabricated ZnO film. The X-Ray diffraction (XRD) pattern infers that the ZnO thin film is polycrystalline. It is found that the prepared thin film, has hexagonal wurtzite phase with a better orientation of (002) plane at 2θ = 34.76°. The Raman spectra confirms the presence of C46v space group for the ZnO film with hexagonal wurtzite structure. The E2 (high) mode peak at 433 cm−1 in Raman spectra signifies that the ZnO thin film distinctly leans along c-axis. The A1 (TO) mode corresponding peak at 391 cm−1 could have arisen from the imperfections such as oxygen vacancy, zinc interstitial or their complexes and free carriers. The maximum absorbance has been observed in between 200 and 280 nm in the absorbance spectra of the synthesized ZnO thin film. The direct optical band gap of ZnO thin film is found to approximately 3.23 eV.

Evaluation of the optical characteristics of c-axis oriented zinc oxide thin films grown by sol gel spin coating technique

In this work we have systematically studied the optical characteristics of synthesized wurzite zinc oxide thin films exhibiting (002) orientation. Using sol gel spin coating technique zinc oxide thin films are grown on pre cleaned fused quartz substrates. Structural properties of the films are studied using X-ray diffraction analysis. Micro structural analysis and thickness of the grown samples are analyzed using field emission scanning electron microscopy. With an aim to investigate the optical characteristics of the grown zinc oxide thin films the transmission and reflection spectra are evaluated in the ultraviolet-visible (UV-VIS) range. Using envelope method, the refractive index, extinction coefficient, absorption coefficient, band gap energy and the thickness of the synthesized films are estimated from the recorded UV-VIS spectra. An attempt has also been made to study the influence of crystallographic orientation on the optical characteristics of the grown films.

Crystal facet engineering of ZnO photoanode for the higher water splitting efficiency with proton transferable nafion film

Zinc oxide (ZnO) is considered as an extraordinary photoanode with direct band gap and high charge mobility for solar light water splitting reaction, but it suffers from proton (H+) produced by the photo-oxidation reaction. In the current work, we demonstrate that the long term-stability of ZnO photoanode with the introduction of proton transfer media. Proton transferable polymer such as nafion allows favorable transport of the generated H+ ions from the surface of ZnO photoanode to electrolyte and results in an enhanced its stability. Nafion polymer film was coated on the surface of ZnO thin film photoanode with each selectively exposed (002), (100) and (101) facet. Especially, nafion film with (002) crystal facet ZnO photoanode showed 3.9mA/cm2 of the high photocurrent density and 0.3–0.6μmol/cm2 of high oxygen and hydrogen gas evolution for 2h. The photoelectrochemical performance of nafion coated (002), (100) and (101) facet oriented ZnO thin film photoanode increase the performance of water splitting reaction and stability of ZnO thin film were demonstrated as a development of high efficient ZnO photoelectrode. Present work suggests the strategy to fabricate the stable metal oxide photoelectrode with the high photoelectrochemical performance.

Laser ablated ZnO thin film for amperometric detection of urea

Zinc oxide (ZnO) thin films deposited onto indium tin oxide (ITO) coated corning glass substrates using pulsed laser deposition (PLD) technique at two different oxygen pressures (50 mT and 100 mT) have been used as efficient matrix for realization of efficient urea biosensors after immobilization of urease (Urs) enzyme onto ZnO film surface. The bioelectrode Urs/ZnO/ITO/glass having ZnO matrix grown at 100 mT is found to be exhibiting an enhanced sensitivity of 36 μΑ mΜ−1 cm−2 for urea over a wide detection range of 5–200 mg/dl. The relatively low value of Michaelis–Menten constant (Km = 0.82 mM) indicates high affinity of the immobilized urease towards the analyte (urea). The prepared sensor exhibits high selectivity towards detection of urea and retains 90% of its activity for more than 12 weeks. The observed enhanced response characteristics of bioelectrode is attributed to the growth of highly c-axis oriented ZnO thin film by PLD at 100 mT oxygen pressure with desired rough and porous surface morphology besides high electron communication feature. The results confirm the promising application of PLD grown ZnO thin film as an efficient matrix for urea detection.

Preparation of oriented zinc oxide thin films by firing Zn–Al layered double hydroxide thin films

The Zn–Al layered double hydroxide (LDH) particles were prepared by heating aqueous solutions of Zn(NO3)2 and Al(NO3)3 with ethylenediamine at 348K for 24h. The obtained particles, which had sheet morphology, were well dispersed in water. The sol of LDH nanoparticles was dried on an alkali-free glass substrate and LDH thin films were obtained easily. Firing the LDH films at temperatures higher than 573K produces a highly oriented zinc oxide film. The obtained films had orientation of zinc oxide (002) on the glass substrate. This orientation reflected that of the LDH films. The zinc oxide films showed more than 80% of the optical transmittance when the wavelength was 400nm.

Effect of R.F Power to the Properties of ZnO Thin Films Deposited by Magnetron Sputtering

The effect of radio frequency (R.F) power to the properties of zinc oxide (ZnO) thin films deposited by magnetron sputtering is presented. This project has been focused on electrical, optical and structural properties of ZnO thin films. The effect of variation R.F power at 100 watt ~ 400 watt on the ZnO thin films has been investigated. The thin films were examined using current-voltage (I-V) measurement, UV-Vis-NIR spectrophotometer, x-ray diffraction (XRD) and atomic force microscope (AFM). ZnO thin films were prepared at room temperature in pure argon atmosphere by a R.F magnetron sputtering using ZnO target. I-V measurement indicates that at 300 watt R.F power show the highest conductivity. All films have showed high UV absorption properties using UV-VIS spectrophotometer (JASCO 670). Highly oriented ZnO thin films [002] direction was obtained by using Rigaku Ultima IV. The root means square (rms) roughness for ZnO thin film were about (<2nm) was measured using AFM (Park System XE-100). Keywords-ZnO thin films, R.F power, electrical properties, optical properties, structural properties

Growth of Multi-Shaped Zinc Oxide Nanostructures Using C-Axis Oriented Zinc Oxide Thin Film as a Seeded Catalyst via Hydrothermal Aqueous Chemical Growth Method

Zinc oxide (ZnO) nanostructures with different kind of morphologies were synthesized on glass substrates via the hydrothermal aqueous chemical growth method utilizing c-axis oriented ZnO thin film as seeded catalyst. By preparing ZnO thin film at different molar concentrations between 0.2~1.0 M, oval shaped ZnO nanostructures mixed with ZnO nanowires and rod shaped ZnO nanostructures mixed with ZnO nanowires were produced after immersion process into 0.0002 M zinc nitrate solution for 24 hour. The XRD spectra show synthesized ZnO nanostructures were ZnO hexagonal wurtzite crystalline. The photoluminescence (PL) measurement indicates the luminescences of the samples were depending on the shapes of ZnO nanostructure.

Effects of substrate parameters on structure and optical properties of ZnO thin films fabricated by pulsed laser deposition

Abstract Highly oriented zinc oxide thin films have been grown on quartz, Si (1 1 1) and sapphire substrates by pulsed laser deposition (PLD). The effect of temperature and substrate parameter on structural and optical properties of ZnO thin films has been characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), optical transmission spectra and PL spectra. The experimental results show that the best crystalline thin films grown on different substrate with hexagonal wurtzite structure were achieved at growth temperature 400–500 °C. The growth temperature of ZnO thin film deposited on Si (1 1 1) substrate is lower than that of sapphire and quartz. The band gaps are increasing from 3.2 to 3.31 eV for ZnO thin film fabricated on quartz substrate at growth temperature from 100 to 600 °C. The crystalline quality and UV emission of ZnO thin film grown on sapphire substrate are significantly higher than those of other ZnO thin films grown on different substrates.

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.

Zinc oxide thin film-based MEMS acoustic sensor with tunnel for pressure compensation

An acoustic sensor exhibiting enhanced sensitivity has been fabricated after integrating the piezoelectric zinc oxide (ZnO) thin film with silicon micro-electro-mechanical system (MEMS) having pressure compensation tunnel. The sensor structure comprises of silicon diaphragm loaded with a layered structure where a highly c-axis oriented ZnO thin film (3 μm thick) fabricated by RF magnetron sputtering, has been sandwiched between PECVD deposited SiO 2 layers. The pressure compensation in the developed acoustic sensor is achieved using a tunnel in the MEMS structure. The response characteristics of the acoustic sensors are reproducible for the devices prepared under similar processing conditions under different batches. The measured value of central capacitance and dissipation factor of the sensor is about 65 pF and 0.003, respectively, whereas the value of ∼144 pF is obtained for the rim capacitance with a dissipation factor of 0.005. The sensor was found to exhibit enhanced sensitivity of about 300 μV/Pa (rms) under varying acoustic pressure and can be used effectively over a wide frequency range (30 Hz to 8 kHz).

Cholesterol biosensor based on rf sputtered zinc oxide nanoporous thin film

Cholesterol oxidase (ChOx) has been immobilized onto zinc oxide (ZnO) nanoporous thin films grown on gold surface. A preferred c-axis oriented ZnO thin film with porous surface morphology has been fabricated by rf sputtering under high pressure. Optical studies and cyclic voltammetric measurements show that the ChOx∕ZnO∕Au bioelectrode is sensitive to the detection of cholesterol in 25–400mg∕dl range. A relatively low value of enzyme’s kinetic parameter (Michaelis-Menten constant) ∼2.1mM indicates enhanced enzyme affinity of ChOx to cholesterol. The observed results show promising application of nanoporous ZnO thin film for biosensing application without any functionalization.

Growth of Highly Oriented Zinc Oxide Thin Films by Plasma Enhanced Chemical Vapor Deposition

In the paper, zinc oxide (ZnO) thin films are deposited by plasma enhanced chemical vapor deposition (PECVD) at different substrate temperatures. The ZnO films are characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The analysis results indicate that highly crystalline films with high orientation can be obtained at a substrate temperature of 300 oC with 50 ml/min flow rate from Diethylzinc (DEZ). Furthermore, the investigation of optical property shows that ZnO films are transparent, and the peak transmittance in the visible region is as high as 85%.

Highly c-axis oriented ZnO film on silica glass using zinc naphthenate precursor

High transmittance and good electrical conductivity thin conducting oxides (TCOs) have potential use in a variety of technological applications including plasma display panels (PDPs), electroluminescent devices, flat cathode ray tube (CRTs) and solar cells [1–4]. Recently, zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. ZnO is a wide band gap (3.3 eV) semiconducting, piezoelectric, and photoconducting material. In particular ZnO films with preferential orientation along the c-axis have been shown to work as SAW devices because of large piezoelectric constant. Ohyama et al. [5] prepared ZnO films on silica glass substrates from zinc acetate sol containing 2-methoxyethanol and monoethanolamine (MEA) with extremely preferred orientation along (002) plane. Ohya et al. [6] reported transparent, electrically conductive ZnO thin films with polycrystalline structure from zinc acetate and alkoxide. Recently Wessler et al. [7] reported textured ZnO thin films on (0001) sapphire by spin-coating a sol of acetate and ethanolamine. In this letter, the preparing of highly c-axis oriented ZnO thin film on amorphous silica substrates from a zinc naphthenate precursor is reported. Crystallinity, surface morphology, surface roughness, and optical behavior were studied. A coating solution was prepared using zinc naphthenate (Nihon Kagaku Sangyo Co., Ltd., Japan) and by diluting the solution with toluene (concentration: 4 wt% metal/100 mL sol). Metal naphthenates are more advantageous than metal alkoxides as starting materials, in terms of cost, stability in air, and ease of handling. Preparation of the coating solution using zinc naphthenate was easily performed by the addition of toluene, while complicated procedures were needed for the preparation of sol from zinc alkoxide [6]. The starting solution was spin-coated onto the cleaned substrate at 1500 rpm for 10 s in air. The asdeposited film was prefired at 500 ◦C for 10 min in air. The coating process was repeated five times to prepare

Sol-gel-derived c-axis oriented ZnO thin films

c-Axis oriented zinc oxide thin films were prepared by sol-gel process on fused quartz substrates. The structure, optical and electrical properties of ZnO films were investigated. Growth of the thin films strongly depends on heat-treatment conditions. The c-axis lattice constants of the thin films and the band gap are a little bigger than that of ZnO crystal. The differences between the thin film and crystal might be attributable to the grain boundaries and imperfections in thin films.

Characterization of high quality c axis oriented ZnO thin films grown by metal organic chemical vapor deposition using zinc acetate as source material

High quality c axis oriented zinc oxide (ZnO) thin films have been deposited on glass substrates by low pressure, metal organic vapor deposition, using zinc acetate (Zn(CH 3COO) 2) and H 2O as source materials. Films have been characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and Auger electron spectroscopy.

Studies of MZS and MZOS Structures with Zinc Oxide Deposited by Conventional Rf Diode and Magnetron Sputtering Techniques

ABSTRACTHighly crystalline and c-axis oriented zinc oxide thin films were sputter deposited from a ceramic target using rf diode and magnetron sputtering techniques. A comparative evaluation of structure and electrical characteristics of ZnO films in the MZS and MZOS configurations is presented and the results are discussed. The physical and electrical properties were significantly influenced by highly energetic particles originating from the presence of oxygen neutrals in the plasma during the growth process and the behavior differed between the diode and magnetron sputtering processes.