Deposited ZnO Thin Films Research Articles

Wet chemical etching of ZnO films using NH x -based (NH4)2CO3 and NH4OH alkaline solution

In this paper, we deposited ZnO thin films by RF magnetron sputtering at room temperature from un-doped targets. Wet chemical etching of ZnO films in (NH4)2CO3 and NH4OH solutions were examined. For comparison, hydrochloric acid was also used as an etchant. The NH x -based alkaline solutions provide well-controlled etching rate, and smooth surface and sidewall profiles. Although NH x -based alkaline solution etch rates for ZnO were relatively low, they were enhanced with the use of a H3O stabilizer. In this case, the NH4OH solution went from reaction-dominant mode to diffusion-dominant mode, which is beneficial for smooth surface morphology.

Synthesis and characterization of PbS/ZnO thin film for photocatalytic hydrogen production

Abstract In this work, PbS/ZnO thin film was designed in order to assess the photocatalytic hydrogen production from the water splitting. A p-type PbS thin film was prepared by chemical bath deposition onto a glass substrate, followed by an n-type ZnO thin film deposition by RF magnetron sputtering. The structural and morphological characterization was carried out by grazing angle x-ray diffraction (GA-XRD), and atomic force microscopy (AFM), respectively, while the optical properties were studied using ultraviolet-visible spectroscopy (UV–Vis) and spectroscopic ellipsometry (SE). The photocatalytic activity was evaluated by quantifying the hydrogen production under UV light irradiation using gas chromatography. The results showed a higher photocatalytic hydrogen production of PbS/ZnO thin film (7.38 μmol cm −2 h −1 ) compared to PbS (3.35 μmol cm −2 h −1 ) and ZnO (2.45 μmol cm −2 h −1 ) thin films. Linear sweep voltammetry (LSV) test showed the synergistic effect between PbS and ZnO thin films for the generation and transport of the charge carriers.

Study of structural and electrical properties of ZnO thin film for Thin Film Transistor (TFT) applications

This work reports the room temperature deposition of undoped ZnO thin film using RF magnetron sputtering technique for thin film transistor (TFT) applications. RF sputtered ZnO thin film of thicknesses 100 and 200 nm were deposited over n-type silicon (Si) substrate using 99.9% pure ZnO target. For the deposition of ZnO thin films, a deposition pressure of 28 mTorr and RF power of 50 W was maintained. The gas flow rate was 5 and 20 sccm for oxygen and argon respectively. Structural and surface morphological characterization was done using X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) techniques. XRD analysis of the thin film gives a dominant X-ray diffraction peak corresponding to strong c-axis oriented phase of hexagonal wurtzite structure of ZnO. The XRD results confirm the polycrystalline nature of the thin film and shows crystallinity improvement with increase in film thickness. Further, FESEM results confirm that the grain size increases with the film thickness. The surface roughness is studied through AFM while surface elemental survey is done using XPS. To measure the resistivity of deposited films, four point probe method was used. The resistivity of undoped 100 nm thick ZnO film is found to be 4.47 kΩ cm, while the resistivity of undoped 200 nm thick ZnO film reduces to 724.13 Ω cm because of improved crystallinity.

Deposition of ZnO thin films by RF&DC magnetron sputtering on silicon and porous-silicon substrates for pyroelectric applications

In this study, the temperature response of ZnO thin film is investigated in an attempt to enhance its pyroelectric performance. The film is formed on PS and Si substrates utilizing RF&DC magnetron sputtering deposition technique. The outcome of study reveals a pyroelectric coefficient observed from ZnO film on PS which is 40 times higher than that on Si and a pyroelectric voltage as high as 2.4V due to PS's profound effect on film formation, large surface to volume area and low thermal conductivity. Thus, this study can lead the way to a robust, reliable and more efficient pyroelectric operation of ZnO with employment of PS structure.

Pulsed laser deposition of ZnO thin films decorated with Au and Pd nanoparticles with enhanced acetone sensing performance

We fabricate and compare nanocomposite thin-film ZnO chemoresistive acetone sensors with gold or palladium nanoparticles on the surface, at low operating temperatures. The sensors are fabricated by pulsed laser deposition and operate in the temperature range 159-200oC. The ZnO films are polycrystalline, crystallizing mainly at the (002) and (101) orientations of the hexagonal phase. The nanocomposite ZnO:Au and ZnO:Pd sensors have a lower detection limit and show a response enhancement factor between 2-7, compared with pure ZnO sensors. The ZnO:Pd sensor performs better than the ZnO:Au sensor. The ZnO:Pd sensor sensitivity increases with the amount of palladium on the surface, while it remains roughly unchanged with the ZnO thickness. The lowest acetone concentration we detect is 26 ppm for the operating temperature of 200oC.

Enhanced UV–visible photodetection characteristics of a flexible Si membrane-ZnO heterojunction utilizing piezo-phototronic effect

We report the characteristics of an n-ZnO/p-Si membrane heterojunction flexible photodetector sensitive to UV and visible illumination. A piezo-phototronic effect has been observed for the deposited ZnO thin films on flexible silicon membranes. Si membranes as low as ~3.0 µm thick have been fabricated by the alkaline etching of Si wafers followed by ZnO deposition using RF sputtering for realizing the heterostructure. A peak responsivity of 0.20 AW−1 with a detectivity of 4.8 × 1011 cm Hz1/2 W−1 is found at ~490 nm for zero bias. Strain induced piezo-potential developed in ZnO thin films is found to modulate the transport property of the photo generated carriers, resulting in the enhanced performance of the device. With a gradual increase in the external tensile stress, the photocurrent increases by 22%. The accompanying COMSOL analysis displays the piezopotential distribution developed in ZnO films on application of an external stress to the heterojunction, which is in close agreement with the experimental data.

Seed Layer Assisted Hydrothermal Deposition of Low-resistivity ZnO Thin Films

ABSTRACTIn this work, we describe the combination of hydrothermal and atomic layer deposition (ALD) for growing low-resistivity ZnO polycrystalline continuous films. The effect of the thickness of ALD seed layers on the morphology of the hydrothermal ZnO films was studied. It was shown that ZnO films hydrothermally deposited on very thin seed layer consist of separate nanorods but in the case of 20 nm seed layer ZnO films transform to uniform continuous layers comprising of closely packed vertically aligned crystallites. Photoluminescence spectra were shown to exhibit broad band behavior in the visible range, corresponding to radiative recombination processes via oxygen defects of ZnO crystalline lattice, and narrow band in the UV region, associated with band-to-band recombination processes. It was shown that the resistivity of the obtained ZnO films is decreased gradually with the increase of ZnO films thickness and determined by the presence of crystal lattice defects in the seed layer.

Room-temperature deposition of flexible transparent conductive Ga-doped ZnO thin films by magnetron sputtering on polymer substrates

Flexible Ga doped ZnO transparent conductive films were prepared on polymer substrates at room temperature by RF magnetron sputtering. The effect of the Ar flow rate is more important than that of r.f. power and sputtering pressure. The dependence of the structural, electrical, optical and adhesive properties of flexible Ga doped ZnO transparent conductive thin films on the Ar flow rate was investigated for the first time. When the Ar flow rate is 40 sccm, the residual stress is changed from compressive stress to tensile stress, indicating the crystallinity of the film is best. And the lowest square resistance of 7.9 Ω/sq. is obtained at the Ar flow rate of 40 sccm. Regardless of the Ar flow rate, the optical transmittance in the visible light of all the films is about 90%. The best adhesive behavior is obtained at the Ar flow rate of 40 sccm.

Effect of Phosphate Buffered Saline Solutions on Top-Down Fabricated ZnO Nanowire Field Effect Transistor

ZnO based nanowire FETs have been fabricated by implementing a top-down approach, which uses optical photolithography, atomic layer deposition (ALD) of ZnO thin film, and anisotropic plasma etching. The effects of Phosphate Buffered Saline (PBS) solution on the surface of ZnO nanowire were investigated by measuring the FET characteristics at different PBS dilutions. The drain current, ION, exhibited an increase of 39 times in the highest PBS solution concentration compared to measurement in air. From the measured transfer characteristics and output characteristics in various PBS dilutions, the device was found to maintain n-type behaviour. These results indicate that the device can be effectively used for biomolecules sensing.

Deposition and Optimization of Al-doped ZnO Thin Films Fabricated by In-line Sputtering System

Deposition and Optimization of Al-doped ZnO Thin Films Fabricated by In-line Sputtering System

Role of N doping on the structural, optical and photocatalytic properties of the silver deposited ZnO thin films

In this study, nitrogen doped silver deposited ZnO (N-Ag/ZnO) thin films were successfully prepared at different rate flows of nitrogen via simultaneous co-sputtering of ZnO and Ag in the presence of varying nitrogen flow from 10 to 50sccm by RF sputtering method. The role of N doping on the structural, optical and photocatalytic properties of the silver deposited ZnO thin films was studied. The deposited ZnO based thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Field emission scanning electron microscopy (FESEM), Photoluminescence (PL), and UV–vis absorbance spectroscopy. The photocatalytic degradation efficiency of the prepared thin films was investigated for 2-cholorphenol (2-CP) in aqueous solutions under visible light irradiation. Results obtained revealed that varying the N flow rates resulted in control in the morphology, optical property and photocatalytic activity of the thin films. The particle size of the deposited metals reduced and become spherical as the flow rate of the nitrogen increased from 10 to 50sccm. PL spectra shows that the N-Ag/ZnO thin has low intensity compared to the ZnO, Ag/ZnO thin films suggesting the effect separation of photogenerated electron/hole pairs, which results in the enhanced photocatalytic degradation of 2-CP. The results showed that N-Ag/ZnO (40N-Ag/ZnO>50N-Ag/ZnO>30N-Ag/ZnO>20N-Ag/ZnO>10N-Ag/ZnO) thin films have higher photocatalytic property compared to the pure ZnO, N-ZnO and Ag/ZnO thin films. The regeneration studies showed that after four regeneration cycles, ZnO based thin films are effective and no significant change in the photocatalytic degradation was observed.

Synthesis, structure, vapour pressure and deposition of ZnO thin film by plasma assisted MOCVD technique using a novel precursor bis[(pentylnitrilomethylidine) (pentylnitrilomethylidine-μ-phenalato)]dizinc(II)

A novel binuclear zinc schiff's base complex bis[(pentylnitrilomethylidine)(pentylnitrilomethylidine-μ-phenalato)]dizinc(II) (hereafter referred as ZSP) was prepared and used as a precursor for the deposition of ZnO thin film by MOCVD. The dynamic TG run of ZSP showed sufficient volatility and good thermal stability. The temperature dependence of vapour pressure measured by transpiration technique yielded a value of 55.8 ± 2.3 kJ mol−1 for the enthalpy of sublimation (ΔH°sub) in the temperature range of 423–503 K. The crystal structure of ZSP was solved by single crystal XRD which exhibits triclinic crystal system with the space group of Pī. The molecular mass of ZSP was determined by mass spectrometry which yielded the m/z value of 891 and 445 Da corresponding to its dimeric as well as monomeric form. The complex ZSP was further characterized by FT-IR and NMR. The demonstration of ZnO thin film deposition was carried out by using plasma assisted MOCVD. The thin film XRD confirmed the highly oriented (002) ZnO thin films on Si(100) substrate. The uniformity and composition of the thin film were analyzed by SEM/EDX. The band gap of ZnO thin film measurement indicated the blue shift with the value of 3.79 eV.

An Experimental Design for Processing Parameter Optimization for Cathode Arc Plasma Deposition of ZnO Films

An experimental design utilizing artificial neural networks (ANNs), the Taguchi method, and the genetic algorithm (GA) is proposed to obtain optimal processing parameters for cathode arc plasma deposition of ZnO thin films on a glass substrate. The Taguchi method’s orthogonal array is used to minimize the number of required experiments and to gather the experimental data. An ANN is then used to construct a system model based on the experimental data. Finally, the GA is used to determine the optimal process parameters. The average resistivity obtained from the optimal processing parameters is $3.19 \times 10^{-3} ~\Omega $ -cm and the average transmittance obtained is 86.04%, both of which improve on results obtained using the Taguchi method alone ( $3.69 \times 10^{-3}\Omega $ -cm and 85.41%). This indicates that the proposed design is a viable approach for determining the optimal process parameters.

Influence of triethanolamine in the deposition of Mn-doped ZnO thin films by the successive ionic layer adsorption and reaction process

The successive ionic layer adsorption and reaction method was used to prepare Mn-doped ZnO whose deposition was achieved by the complexation action of triethanolamine with Mn in the adsorption bath. The resulting films had the hexagonal wurtzite structure of ZnO with a remarkable poly-crystallization as consequence of the substitution of Mn in Zn sites, as confirmed by the X-ray Photoelectron Spectroscopy technique. The presence of Mn2+ and Mn4+ produced a change on the growth kinetics of the ZnO films. Undoped ZnO films were formed by flower-like particles. Meanwhile those doped with Mn were formed by the homogeneous distribution of packed spherical particles. These films were specular and had good optical transparency with average transmittances in the range of 79.26–83.35%. In addition, the optical band gap values of the Mn-doped ZnO films were in the range of 3.15–3.32 eV.

Low temperature formation of highly resistive ZnO films using nonequilibrium N2/O2 plasma generated near atmospheric pressure

Low temperature chemical vapor deposition of ZnO thin films was demonstrated using β-diketonate complexes [Bis-2,4octanedionato zinc: Zn(OD)2] and nonequilibrium N2/O2 plasma generated near atmospheric pressure. ZnO films grown at the substrate temperature of 200°C and at the O2 concentration in the N2/O2 mixture gas (O2%) of 0.2, 3.8 and 20% exhibited excellent (0001) preferred orientation on glass substrates, high transmittance of above 85% in the visible region and smooth surface structure with the continuous columnar grains. Electrical measurement revealed that our ZnO films were highly resistive with the specific resistivity exceeding 106Ωcm at room temperature, which is not easily obtained by the conventional high-vacuum processes without intentional acceptor doping. Thermally stimulated current measurement detected two peaks from highly resistive ZnO film grown at the O2% of 0.2%, which correspond to the carrier emission from deep trap levels with the activation energies of 170 and 610meV. The former was assigned as zinc vacancy (VZn) or nitrogen substituting oxygen site (NO), and the latter was assigned as VZn or oxygen interstitial (Oi). The carrier compensation by these deep acceptor levels seems to be the origin of high resistivity.

Chemical Bath Deposited ZnO:Al Thin Films and Their Application to CuInGaSe2 Thin Film Solar Cells

ZnO:Al thin films have been widely used as transparent conductors in variety of optoelectronic devices including CuInGaSe2 thin film solar cells. However, high cost sputtering process has been typically employed to prepare high quality ZnO:Al thin fil simultaneously having both high optical transmittance and low sheet resistance. In this study, we present the ZnO:Al thin films prepared by the chemical bath deposition using NH4OH-based aqueous solution on seeded glass substrates. We also report their properties and application to CuInGaSe2 thin film solar cells as a top transparent electrode. A seed layer on glass substrate was prepared by spin-coating of ZnO nanoparticles with size of a few tens of nanometer. For the chemical bath deposition process, ZnO powder was dissolved in NH4OH-based aqueous solution and un-dissolved ZnO powder was removed by transferring the ZnO solution through 1 um filter. Retrograde solubility of ZnO in NH4OH-based aqueous solution allows us to deposit ZnO thin films by raising temperature of the bath from room temperature to 80 oC. To obtain compact film by enhancing growth rate of crystal in lateral direction, a certain amount (~0.5-2 mM) of ammonium citrate was added into the bath. A various amount of Al(NO3)3 was added into the bath as a aluminum source to maximize electrical conductivity by forming Al substitution in Zn site. The prepared ZnO:Al films were irradiated by UV for 1 min. Optimized ZnO:Al films showed (0002) preferred orientation, optical transmittance of 90% and sheet resistance of 75 ohm/sq. Furthermore, the prepared ZnO:Al thin film was successfully implanted to CIGS thin film solar cells as a top electrode showing power conversion efficiency of 12.5% comparable to the performance (~13.7%) of CIGS solar cells having a sputter-deposited ZnO:Al film transparent electrode.

Nanomechanical and microstructural characterization of sputter deposited ZnO thin films

The nano-mechanical properties of ZnO thin films deposited at different substrate temperature such as (RT) 25°C, 100°C, 200°C, and 300°C using DC-sputtering on Corning glass substrate were investigated. The ZnO thin films are found to be predominately c-axis (002) oriented. The crystal structure is sensitive to increasing substrate temperature and new set of crystal planes become visible at 300°C as thin films become highly polycrystalline. The presence of (103) crystal plane is more pronounced with the increasing substrate temperature. However, high crystallinity and peak intensity ratio I(002)/I(103) (counts) is highest for thin films deposited at 100°C, which is attributed for high hardness and better adhesive properties observed for ZnO thin films. Concomitantly, no major sudden burst of displacement ‘pop-in’ event in load-displacement curve of thin films observed during indentation, indicating the films are dense with low defects and adhered strongly to the substrate.

Effect of copper inclusion on structural, optical and electrical properties of ZnO thin films

In the present investigation, the effect of copper inclusion on structural, optical and electrical properties of spray deposited ZnO thin films is studied. The X-ray diffraction studies showed that the films are polycrystalline in nature with hexagonal structure. The optical studies revealed that the band gap energy of ZnO increases from 3.23 to 3.31 eV as Cu inclusion varies from 0 to 6 at. %. The dark electrical resistivity measurement was done by two probe method. The room temperature resistivity of ZnO increases from 10−2 to 102 O-cm depending on Cu inclusion. The thermoelectric power (TEP) measurements confirmed that the spray deposited ZnO films exhibits n-type conductivity. The Seebeck’s coefficient calculated from TEP was found in the range 30–11 µVK−1 depending on Cu inclusion.

Study of deposition parameters for the fabrication of ZnO thin films using femtosecond laser

Femtosecond (fs) pulsed laser deposition (fs-PLD) of ZnO thin film on borosilicate glass substrates is reported in this work. The effect of important fs-PLD parameters such as target–substrate distance, laser pulse energy and substrate temperature on structure, morphology, optical transparency and luminescence of as-deposited films is discussed. XRD analysis reveals that all the films grown using the laser energy range 120–230 μJ are polycrystalline when they are deposited at room temperature in a ~10−5 Torr vacuum. Introducing 0.7 mTorr oxygen pressure, the films show preferred c-axis growth and transform into a single-crystal-like film when the substrate temperature is increased to 100 °C. The scanning electron micrographs show the presence of small nano-size grains at 25 °C, which grow in size to the regular hexagonal shape particles at 100 °C. Optical transmission of the ZnO film is found to increase with an increase in crystal quality. Maximum transmittance of 95 % in the wavelength range 400–1400 nm is achieved for films deposited at 100 °C employing a laser pulse energy of 180 μJ. The luminescence spectra show a strong UV emission band peaked at 377 nm close to the ZnO band gap. The shallow donor defects increase at higher pulse energies and higher substrate temperatures, which give rise to violet-blue luminescence. The results indicate that nano-crystalline ZnO thin films with high crystalline quality and optical transparency can be fabricated by using pulses from fs lasers.

Large-area thermoelectric high-aspect-ratio nanostructures by atomic layer deposition

We report on the thermoelectric properties of large-area high-aspect-ratio nanostructures. We fabricate the structures by atomic layer deposition of conformal ZnO thin films on track-etched polycarbonate substrate. The resulting structure consists of ZnO tubules which continue through the full thickness of the substrate. The electrical and thermal properties of the structures are studied both in-plane and out-of-plane. They exhibit very low out-of-plane thermal conductivity down to 0.15 W m−1 K−1 while the in-plane sheet resistance of the films was found to be half that of the same film on glass substrate, allowing material-independent doubling of output power of any planar thin-film thermoelectric generator. The wall thickness of the fabricated nanotubes was varied within a range of up to 100 nm. The samples show polycrystalline nature with (002) preferred crystal orientation.