Nanocrystalline Zinc Oxide Thin Films Research Articles

Study of birefringence inside nanocrystalline Zinc Oxide thin films using terahertz spectroscopy

We present experimental characterizations of nanocrystalline Zinc Oxide (ZnO) thin films in far-infrared (terahertz) frequency region using terahertz time-domain spectroscopy. A time dependent profile, frequency dependent transmission coefficient, refractive index, absorption coefficient and optical density of ZnO thin films are measured in the terahertz regime. A birefringence is observed as a result of TO–LO phonon splitting took place in the ZnO deposited at 400 °C substrate temperature, while at low substrate temperature of 200 °C there is no sign of birefringence. Thus, it can be clearly evidenced that the presence of birefringence in ZnO is correlated with crystallinity exists inside of the thin films. Absorption behaviour of ZnO nano-crystalline thin films shows the potential use of it for future THz photonics devices. RF magnetron sputtering is the key for the deposition of nanocrystalline ZnO thin films on Si (100) substrate at different temperatures. The X-ray diffraction pattern of ZnO thin films were confirmed the formation of wurtzite hexagonal structure and nanocrystalline nature of the deposited ZnO thin films.

A statistical method to optimize the chemical etching process of zinc oxide thin films.

Zinc oxide (ZnO) is an attractive material for microscale and nanoscale devices. Its desirable semiconductor, piezoelectric and optical properties make it useful in applications ranging from microphones to missile warning systems to biometric sensors. This work introduces a demonstration of blending statistics and chemical etching of thin films to identify the dominant factors and interaction between factors, and develop statistically enhanced models on etch rate and selectivity of c-axis-oriented nanocrystalline ZnO thin films. Over other mineral acids, ammonium chloride (NH4Cl) solutions have commonly been used to wet etch microscale ZnO devices because of their controllable etch rate and near-linear behaviour. Etchant concentration and temperature were found to have a significant effect on etch rate. Moreover, this is the first demonstration that has identified multi-factor interactions between temperature and concentration, and between temperature and agitation. A linear model was developed relating etch rate and its variance against these significant factors and multi-factor interactions. An average selectivity of 73 : 1 was measured with none of the experimental factors having a significant effect on the selectivity. This statistical study captures the significant variance observed by other researchers. Furthermore, it enables statistically enhanced microfabrication processes for other materials.

Raman scattering from irradiated nanocrystalline zinc oxide thin films: Perspective view on effects of energy loss, ion fluence, and ion flux

Raman scattering were used to study lattice damage in nanocrystalline zinc oxide (nc-ZnO) thin films implanted with 300 keV Ar and 1.2 MeV Xe ions at different ion fluences and ion fluxes. Raman spectra of all of the implanted samples show a broad Raman band at the lower Raman shift side along with A1 (LO) phonon mode. The A1 (LO) phonon modes get evolved with an increase in ion fluence with the reduction in the intensity of E2 (high) mode which is the characteristic mode of the wurtzite phase of ZnO. The implanted films show broadening and softening of A1 (LO) phonon mode upon an increase in ion fluences and observed to be more prominent in the case of 1.2 MeV Xe ions as compared to 300 keV Ar ions. The evolution of A1 (LO) phonon mode and broadening of Raman band assigned to the damage induced in the system upon ion-implantation and correlated well with the damage profile, while the peak broadening and softening could be assigned to the stress-induced into the films due to implantation. Thus, for a better understanding, the Raman tensor of first-order optical Raman modes in back-scattering geometry is also presented in the manuscript.

SILAR deposited nanocrystalline ZnO films as LPG sensor

Abstract A facile successive ionic layer adsorption and reaction (SILAR) method is used for the deposition of nanocrystalline zinc oxide (ZnO) thin films. The XRD pattern shows ZnO films are polycrystalline in nature with wurtzite crystal structure. The surface morphology reveals ZnO nanograins embedded conical morphology, which shows hydrophobic behavior. The Liquefied Petroleum Gas (LPG) sensing properties of nanocrystalline ZnO films are investigated. The nanograined ZnO thin film shows maximum gas response of 20% at 673 K upon exposure to 5200 ppm of LPG.

Influence of Spraying Time on the Electrical, Morphological, Optical, and Structural, Properties of Nanocrystalline Zinc Oxide Thin Solid Films Obtained by Ultrasonic Spray Pyrolysis Technique

Influence of Spraying Time on the Electrical, Morphological, Optical, and Structural, Properties of Nanocrystalline Zinc Oxide Thin Solid Films Obtained by Ultrasonic Spray Pyrolysis Technique

Fabrication and characterization of transparent nanocrystalline ZnO thin film transistors by a sol–gel technique

A nanocrystalline zinc oxide (ZnO) thin film-based metal–insulator–semiconductor thin film transistor (MIS TFT) was fabricated by a facile sol–gel technique onto silicon di-oxide/indium tin oxide-coated glass substrates. The microstructural study of the ZnO thin films indicated uniform crystalline growth with typical (002) X-ray diffraction peaks for h-ZnO with a wurtzite structure. The optical transmittance of the ZnO thin films was \({>}80\%\) in the visible region of the electromagnetic spectrum. The field effect transistor (FET) aluminium top contacts were fabricated using suitable shadow masking. The transfer characteristics of a typical ZnO MIS FET revealed nonlinearity in a linear plot. From the slope and crossover, we obtained a first estimate of field effect mobility (\(\mu \)) and threshold voltage (\(V_{\mathrm{T}}\)) of \(0.13~\hbox {cm}^{2}\) \(\hbox {V}^{-1}~\hbox {s}^{-1}\) and 1.03 eV, respectively. The ZnO TFT operated in enhanced mode with n-channel characteristics and the drain current on–off ratio was \(10^{5}\). The deposition parameter needs to be optimized to obtain TFTs with a higher modulation ratio and larger field-effect mobility.

Controlled morphological modifications of ZnO thin films by ion irradiation

Nanocrystalline thin films of zinc oxide (ZnO) of thickness 100 nm are deposited using electron beam (e-beam) evaporation technique. These films are irradiated using 75 MeV Au ion beam at two different fluences namely 1 × 1011 ions cm−2 and 5 × 1011 ions cm−2. GAXRD and Raman spectroscopic studies indicate stability of nanocrystalline phases of ZnO against irradiation. Surface morphology studies using atomic force microscopy show evolution of nanosized hillocks at the surface of the irradiated films. Nano-hillock formation is also confirmed by a blue shift of the UV–visible absorption edge. The electrical conductivity of the films is found to decreases due to irradiation induced morphological modifications. Ion irradiation technique has been effectively used for controlled modification of nanocrystalline ZnO thin film surface. The preliminary studies carried out clearly indicate that the irradiated films are suitable for various applications.

Influence of Oxygen Vacancies on the Frictional Properties of Nanocrystalline Zinc Oxide Thin Films in Ambient Conditions.

Oxygen vacancy is the most studied point defect and has been found to significantly influence the physical properties of zinc oxide (ZnO). By using atomic force microscopy (AFM), we show that the frictional properties on the ZnO surface at the nanoscale greatly depend on the amount of oxygen vacancies present in the surface layer and the ambient humidity. The photocatalytic effect (PCE) is used to qualitatively control the amount of oxygen vacancies in the surface layer of ZnO and reversibly switch the surface wettability between hydrophobic and superhydrophilic states. Because oxygen vacancies in the ZnO surface can attract ambient water molecules, during the AFM friction measurement, water meniscus can form between the asperities at the AFM tip-ZnO contact due to the capillary condensation, leading to negative dependence of friction on the logarithm of tip sliding velocity. Such dependence is found to be a strong function of relative humidity and can be reversibly manipulated by the PCE. Our results indicate that it is possible to control the frictional properties of ZnO surface at the nanoscale using optical approaches.

Regulations of aging time on optical properties of nano-crystalline ZnO thin films fabricated by sol–gel method

Nano-crystalline Zinc oxide (ZnO) thin films on glass substrates have been fabricated by sol–gel method for different aging times. The thermal curve of the dried gel was examined and it was found that evaporation of solvent and the decomposition of the organics have completed before 250[Formula: see text]C. X-ray diffraction patterns and Atomic Force Microscope images of samples only have a small difference of microstructure. The transmittance spectra revealed high transmittance in visible region. The optical band gap of the samples remined stable (3.28[Formula: see text]eV). Also, the Urbach energy was calculated to explain the defect concentration. The Photoluminescence spectra in ultraviolet and visible regions were studied, and the decreased intensity of peaks at 387[Formula: see text]nm and the increased emissions in visible range were found. The results showed that excessive aging time can cause degeneration of optical properties of thin films.

In situ PM‐IRRAS studies of organothiols and organosilane monolayers–ZnO interfaces at high water activities

In situ photoelastic‐modulated Fourier transform infrared reflection absorption spectroscopy has been applied for the investigation of interfacial stability of organothiol and organosilane monolayer films on nanocrystalline zinc oxide thin films. It has been shown that for octadecyltriethoxysilane films, exposure to high water activities results in physisorption of water in the cross‐linked film. This high water activity at the interface leads to a reversible wet de‐adhesion of the interfacial silanol groups from the ZnO surface. However, the organothiol seems to form a denser monolayer and a stable by S–Zn bond that is resistant to the competition with adsorbed water. The reversible attachment for cross‐linked organosilanol films has been demonstrated for the first time by means of an in situ spectroscopic method on model ZnO surfaces. Copyright © 2016 John Wiley & Sons, Ltd.

Nanostructured zinc oxide thin films by spin coating technique

Nanocrystalline zinc oxide (ZnO) thin films were deposited on glass substrates by sol-gel spin coating method. Two different sols were prepared using zinc acetate dihydrate as a precursor material dissolved in isopropyl alcohol as a solvent in the presence of two different aminoalcohols including monoethanolamine (MEA) and triethanolamine (TEA) which act as stabilisers. The molar ratio of MEA/TEA to zinc acetate was maintained at 1·0. The effect of MEA/TEA on the structural, optical, morphological and electrical properties of zinc oxide thin films has been investigated by X-ray diffraction analysis, UV-Vis-spectrophotometer, photoluminescence, scanning electron microscopy with elemental composition analysis and four-probe techniques, respectively. X-ray analysis showed that the crystallinity and the particle size decreases for zinc oxide film with TEA as the stabilising agent. Optical studies revealed that zinc oxide films, with TEA, were highly transparent and exhibited large band gap. Compared to TEA, MEA has a significant impact on the zinc oxide film morphology. The activation energy, Ea, was calculated and was found to be 0·4 and 0·2 eV for zinc oxide thin film with MEA and TEA stabilisers, respectively.

Electrical characterization of Au/ZnO thin film Schottky diode on silicon substrate

Summary An array of Gold (Au) schottky contacts have been deposited on RF Sputtered nanocrystalline Zinc Oxide thin film. A systematic analysis on the electrical parameters of the Schottky diode with the help of current–voltage (I–V) and capacitance-voltage (C–V) measurements has been done, which confirmed its excellent rectifying characteristics. To incorporate the influence of series resistance in the determination of Schottky diode parameters (barrier height, ideality factor and saturation current), Cheung's method along with thermionic emission model has also used. The discrepancy in the value of barrier height determined from C–V characteristics throws light into the presence of interface states.

EPR Spectroscopy of Different Sol Concentration Synthesized Nanocrystalline-ZnO Thin Films

Nanocrystalline zinc oxide (nc-ZnO) thin films were grown on p-type silicon substrate through spin coating by sol-gel process using different sol concentrations (10 wt.%, 15 wt.%, and 25 wt.%). These films were characterized by high resolution nondestructive X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS) attachment, and electron paramagnetic resonance (EPR) techniques to understand variations in structural, morphological, and oxygen vacancy with respect to sol concentration. The film surface morphology changes from nanowall to nanorods on increasing sol concentration. EPR spectra revealed the systematic variation from ferromagnetic to paramagnetic nature in these nc-ZnO films. The broad EPR resonance signal arising from the strong dipolar-dipolar interactions among impurity defects present in nc-ZnO film deposited from 10 wt.% sol has been observed and a single strong narrow resonance signal pertaining to oxygen vacancies is obtained in 25 wt.% sol derived nc-ZnO film. The concentrations of impurity defects and oxygen vacancies are evaluated from EPR spectra, necessary for efficient optoelectronic devices development.

Oriented Zinc Oxide Nanocrystalline Thin Films Grown from Sol-Gel Solution

Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconductor. Thin film ZnO has many attractive applications in optoelectronics and sensors. Recently, nanostructured ZnO (e.g. ZnO quantum dot) has been demonstrated as a hyperbolic material; its dielectric function has opposite signs along different crystal axes within the mid-infrared, making it an interesting material for metamaterials and nanophotonics. Conventional sputtering deposition usually leads to the formation of polycrystalline ZnO films with randomly oriented grains and rough surface. This work demonstrated a solution-based process to grow ZnO thin films with highly oriented nanocrystals. Low-temperature plasmas were employed to modulate the microstructure and optical properties of the films. Such highly anisotropic nanostructured transparent semiconductor films may lead to interesting material properties in developing new optoelectronic devices.

Characterization of nanostructured zinc oxide thin films synthesized at room temperature using low energy plasma focus device

Nanocrystalline zinc oxide (ZnO) thin films were synthesized on glass substrates under extremely non-equilibrium conditions of energetic ion condensation during the focus phase in a low energy plasma focus (PF) device. The samples were deposited using multiple focus shots (5, 10 and 20), at constant axial and angular positions with respect to the tip of anode and anode axis (8cm and 0°), respectively. The argon:oxygen admixture (in 7:3 ratio) was used as a working gas. The structural, morphological, electrical and optical properties of the ZnO deposited thin films were investigated. The results obtained from XRD and SEM analyzes indicated that the size of nanoparticles/agglomerates and the thickness of the ZnO thin films strongly depend on number of shots. AFM analysis revealed that the size of grains and surface roughness of ZnO samples increase with the number of shots. Measurement of the electrical parameters indicated that electrical resistivity is reduced and carrier mobility enhanced with increasing the shots, with no noticeable variations in the carrier concentration. Moreover, the results from optical transmission patterns revealed that optical transmittance and band gap energy decrease with more shots.

Enhanced violet photoemission of nanocrystalline fluorine doped zinc oxide (FZO) thin films

Highly stable fluorine doped nanocrystalline zinc oxide thin films were prepared on corning glass substrates by aerosol assisted chemical vapor deposition (AACVD) at variable deposition temperature of 360°C, 380°C and 420°C. Especially, the optimum deposition temperature was investigated for high intense violet emission. The film crystallinity improved with the increasing deposition temperature and highly textured film was obtained at 420°C. The films exhibited surface morphology variation from spherical to platelets due to deposition temperature effect, analyzed by field emission scanning electron microscope (FE-SEM). Higher growth rate observed at 420°C which leads larger grains and lowest resistivity of ∼5.77Ωcm among the deposited films which may be due to reduction in zinc vacancies and grain boundary area. Zinc vacancies are acts as electron killer centres. UV–visible spectra indicated higher transmittance (83–90%) in the visible region. Red shift of optical absorption edges associated with the increase in particle size consistent well with the XRD results. Reduced E2(high) intensity was observed in Raman spectra, for the film deposited at 380°C which indicates decreased oxygen incorporation confirmed by PL spectra. Especially, enhanced violet emission observed at 3.06eV for the films deposited at 380°C due to electronic transition from the defect level of zinc vacancies to the conduction band, probably attributed to enhanced incorporation of ‘F’ into ‘O’ sites associated with increased Zn vacancies and also decreased oxygen incorporation consistent with the electrical and Raman analyses.

Study of dielectric relaxation processes in printable zinc oxide films on transparent substrates

AC impedance spectroscopic measurements have been performed on sol–gel derived zinc oxide (ZnO) films on transparent fluorine-doped tin oxide coated glass substrates in the frequency range 10−2 to 106 Hz over the temperature range −185 to +25 °C (88–298 K). The relaxation behaviour of the nanocrystal line ZnO thin film can be described in terms of the Debye model giving an interpretation of the semi-circular relaxation phenomenon within the given temperature range. Two different relaxation times were obtained from impedance (Z) and electric modulus (M) studies of the devices and the multiple hopping of charge carriers between trap sites in grain and grain-boundary regions is believed to be responsible for charge transport. The values of activation energies for trap levels obtained from AC conductivity study are 0.0153 and 0.0487 eV which are close to the activation energies obtained from DC electrical measurement for temperature region between 88 and 178 K and 179 and 298 K, respectively.

Photo-induced magnetoresistance of nanocrystalline ZnO thin film

Nanocrystalline zinc oxide thin film has been prepared by conventional spray method. The optical energy gap is about 3.39 eV, which is greater than the energy gap of bulk ZnO. The prepared thin film has semiconductor behavior in dark measurements, while it has metallic behavior under ultraviolet illumination. There is a negative magnetoresistance effect for the prepared film under ultraviolet radiation. Furthermore, there is a magnetic transition from diamagnetic to paramagnetic state under ultraviolet radiation. The results has been explained in terms of the band conduction theory and spin polaron model.

Gas sensing application of nanocrystalline zinc oxide thin films prepared by spray pyrolysis

Nanocrystalline oxygen-deficient ZnO thinfilm sensors were prepared by spray pyrolysis technique using zinc acetate dissolved in propanol and water as precursor. Response of the sensor to target gases NO2 and H2S is studied. At optimum temperature of 200∘C, the sensors have a response of 3.32 to 7 ppm NO2 and 1.4 to 18 ppm of H2S gas. The analytical characterizations of the prepared sensors were performed using X-ray diffraction measurement, scanning electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy. Dynamic response of sensors to different concentrations of NO2 and H2S gas was tested at optimum temperature. Experimental data revealed the sensors to be more selective to NO2 gas with satisfactory response and recovery time.

Thermal post-annealing and gas concentration effect on liquid petroleum gas sensing characteristics of nanocrystalline zinc oxide thin films

Chemical adsorption and sensing reactions of liquid petroleum gas over the surface and between grain boundaries of nanostructured zinc oxide thin films were investigated and monitored under the influence of various post-synthesis thermal annealing and target gas concentration conditions. Prior to sensing tests, zinc oxide thin films were synthesized in three steps: at first, zinc oxide nanoparticles were synthesized by a sol–gel method using zinc acetate dihydrate, ethanol and monoethanolamine as precursor, solvent and stabilizing agent, respectively. Then, untreated zinc oxide thin films were prepared from zinc oxide sol through spin coating procedure and finally, thermal post-annealing treatment with different temperatures was performed to yield heated thin films. The formation of the synthesized nanoparticles, morphology, crystalline size, porosity and transparency of final thin films were characterized by field emission-scanning electron microscopy, X-ray diffraction and ultraviolet–visible spectroscopy techniques. Sensing tests showed the dependence of the liquid petroleum gas detection on the thermal annealing process and gas concentration and indicated a maximum response of 75 % at the operation temperature of 300 °C for the zinc oxide film pre-annealed at 600 °C and 1,000 ppm gas. Moreover, the results exhibit excellent sensitivity for the sensor via short response and recovery times of 34 and 220 s, respectively.