ZnO Film Thickness Research Articles

The effect of thickness on the optical, structural and electrical properties of ZnO thin film deposited on n-type Si

In this study, we investigated the effect of thickness on the optical, structural and electrical properties of ZnO thin film deposited on n-type Si wafer. Three groups of transparent conductive ZnO layers were successively deposited by Radio Frequency (RF) magnetron sputtering. Structural properties of ZnO thin film layers were examined by using X-ray diffraction (XRD). Particle sizes of ZnO samples were investigated by the Atomic Force Microscope (AFM) measurements, and the results compared with XRD measurements. The optical characteristics of thin films were obtained via using UV–Vis spectrometer. It was tried to determine the effect of film thickness on electrical and optical properties. Moreover, the devices with ZnO interfacial layer were fabricated using the deposited ZnO films having thickness of 5, 50, and 250 nm. The electrical characteristics of the devices were investigated by capacitance/conductance-voltage (C/G-V) measurements. It was seen that both the band gap and capacitance values decrease as the ZnO film thickness increases.

Elucidation of ALD MgZnO deposition processes using low energy ion scattering

Low energy ion scattering (LEIS) provides an analytical tool for probing the surface composition and structure on the angstrom to nanometer scale. These length scales are central to the growth and processing of ultrathin films produced by atomic layer deposition (ALD). Here, the authors present the application of LEIS to the elucidation of ALD deposition processes and in particular how it provides information about growth parameters including the growth per cycle (GPC), the nature of the film–substrate interfaces, and adatom incorporation into the growing film. The deposition of varying thickness zinc oxide films and the composition of magnesium-doped zinc oxide films are used as model systems. LEIS has been used to investigate the GPC of ZnO using two approaches, namely, static and dynamic measurements. The static approach exploits inelastic energy loss processes to estimate the GPC of different thicknesses of ZnO films. The dynamic approach measures the GPC via a combination of LEIS surface analysis and sputter depth profiling. The measurement of GPC using these two methods is compared with spectroscopic ellipsometry. The adatom incorporation of Mg into the ZnO matrix is measured using a dynamic LEIS process, and the variation of Mg incorporation is discussed as a function of the varying ALD cycle fractions of Mg and ZnO used to deposit MgxZn1-xO films in the range of 0 < x < 1.

Surface and interface properties of polar thin films on a ferroelectric substrate: ZnO on LiNbO3 (0001) and (0001¯)

The impact of ferroelectric polarization on film growth has been studied through the deposition of polar ZnO thin films on ferroelectric LiNbO3 (0001) and (0001¯) surfaces. The growth was monitored by reflection high energy electron diffraction and characterized by x-ray photoelectron spectroscopy (XPS), x-ray diffraction, and x-ray reflectivity. The evolution of the XPS peak intensities and x-ray reflectivity data suggest that the growth was Stranski–Krastanov with a two-dimensional to three-dimensional transition and a small degree of roughness at the interface. The film maintained the substrate surface crystallography initially, then transitioned to an ordered ZnO (0001) phase after passing through a disordered regime. Higher Zn 2p XPS core level binding energies were observed on positively poled LiNbO3 and were attributed to the negative compensating charges at the positive surface n doping the ZnO film, thereby the Fermi level is pinned at the bottom of the ZnO conduction band. In addition, the reaction of 2-propanol was used as a probe to identify the polarity of thick ZnO films. The results indicate that ZnO films grown on either LiNbO3 (0001) or (0001¯) polar surfaces ultimately develop a negative polarization. Therefore, it is concluded that the LiNbO3 polar substrate has a more obvious impact over a short range near the ZnO/LiNbO3 interface but this does not translate into directing the polarization direction of thicker ZnO films.

Self-absorption of violet radiation in ZnO thin films produced on ZnSe crystal surfaces by isovalent substitution method

Photoluminescence (PL) spectra of ZnO thin films produced by isovalent substitution method by means of thermal treatment of zinc selenide single crystal substrates in air are studied in 350 – 750 nm range at room temperature. Irrespective of annealing temperature (500 – 870 °C) and annealing time (20 – 120 min), PL spectra consist of a short-wavelength violet radiation with maximum localized between 396 and 423 nm and a long-wavelength yellow-orange radiation with maximum localized between 575 and 600 nm. Both PL bands are attributed to donor-acceptor (DA) pairs. It is shown that the violet PL band intensity decreases and the yellow-orange PL band intensity increases with increasing ZnO film thickness that is caused by self-absorption of the short-wavelength radiation and energy transmission to DA-pair centres of the long-wavelength radiation.

Electrochemical Deposition of ZnO Porous Nanoplate Network for Dye-Sensitized Solar Cells.

Mesoporous ZnO films composed of interconnected porous nanoplates were prepared by an electrochemical deposition-pyrolytic conversion approach and constructed into the photoanodes of dyesensitized solar cells (DSSCs). Precursor nanoplates grown on conducting glass substrates were transformed into ZnO porous nanoplates by calcination at 400 °C for 1 h. Correlations between the ZnO film thickness and the electrochemical deposition time were determined in order to prepare ZnO films of various thicknesses and to study the effect of the film thickness on the photovoltaic performance of DSSCs. The optimal film thickness was determined to be approximately 27 μm, and the best performing cell reached an energy conversion efficiency of 2.91%. The results show that the ZnO porous nanoplate network so prepared is suitable for DSSC applications.

Ammonia and DMMP Sensor based on Nanostructured ZnO Thick Films

Ammonia and DMMP Sensor based on Nanostructured ZnO Thick Films

UV-Enhanced Ethanol Sensing Properties of RF Magnetron-Sputtered ZnO Film.

ZnO film was deposited by the magnetron sputtering method. The thickness of ZnO film is approximately 2 μm. The influence of UV light illumination on C2H5OH sensing properties of ZnO film was investigated. Gas sensing results revealed that the UV-illuminated ZnO film displays excellent C2H5OH characteristics in terms of high sensitivity, excellent selectivity, rapid response/recovery, and low detection limit down to 0.1 ppm. The excellent sensing performance of the sensor with UV activation could be attributed to the photocatalytic oxidation of ethanol on the surface of the ZnO film, the planar film structure with high utilizing efficiency of UV light, high electron mobility, and a good surface/volume ratio of of ZnO film with a relatively rough and porous surface.

The electrophoretic deposition of ZnO on highly oriented pyrolytic graphite

Intensive research has been conducted on ZnO thin and thick films in recent years. Such layers, used in different electronic devices, are deposited utilizing various methods, but electrophoretic deposition (EPD) has been chosen because of the advantages like low energy consumption, economical superiority, ecofriendliness, controllability, and high deposition rate. Here, we report electrophoretically depositing ZnO layers onto highly oriented pyrolytic graphite. Well-dispersed and stable ZnO suspensions are used for the deposition of continuous and even layers of ZnO on the substrate. ZnO powder is dispersed in acetone. The electric field applied is in the 250 V/cm to 2000 V/cm range. The morphology of the deposits are studied by SEM at the different stages of the deposition process.

Hysteresis-free DC conduction in zinc oxide films with a conducting polymer counter electrode

A conducting polymer, namely poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been employed as an alternative metal counter electrode to study steady state charge conduction through a spin-coated 5.2 µm thick ZnO film in a sandwich structure on a fluorine doped tin oxide coated glass substrate. The room temperature current–voltage characteristics exhibit rectifying behaviour without hysteresis as the bias voltage is cycled between ± 10V for varying voltage sweep rate from 100 to 1000 mV/s. Thermionic emission is believed to be the dominant conduction mechanism at forward bias voltage V ≤ 2.5 V whereas the space charge limited current conduction becomes effective in a higher voltage region V > 2.5 V. The barrier height (φb), ideality factor (n) and series resistance (Rs) are found to be strongly temperature dependent parameters showing the increase of {phi _b} and simultaneous decreases of n and Rs within the range of 218–298 K. The behaviour of the temperature dependent charge carrier mobility in the higher voltage region has also been discussed. The use of PEDOT:PSS is quite promising as an alternative to metal electrodes in semiconductor devices.

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.

Opto-capacitive study of n-ZnO/p-Si heterojunctions elaborated by reactive sputtering method: Solar cell applications

Thin films of ZnO were deposited on glass and p-Si substrates by DC reactive sputtering, at different deposition times. The physical properties of the films were investigated by X-ray diffraction, UV–visible transmittance and four-probe measurements. The results show the improvement of the crystallinity, the transparency and the conductivity of films with increasing the thickness. The dark current-voltage, the capacitance-voltage and the spectral response characteristics of n-ZnO/p-Si heterojunctions revealed that the potential barrier profile and the interface state change strongly with the film thickness, which leads to an important difference in the carriers transport phenomena. The good rectifying behaviors were obtained for the thinner ZnO films (16–50nm). Under illumination, the hetero-junctions with thinner films show a good photosensitivity in the visible range wavelength (~425nm), while, the hetero-junctions with thick ZnO films exhibit a good photosensitivity in the visible and NIR regions (~1100nm). The least thick ZnO film (16nm) shows a photovoltaic behavior with a short circuit current density (Jsc) of ~1mA/cm2 and an open voltage (Voc) of ~0.4V.

ZnO thin film piezoelectric micromachined microphone with symmetric composite vibrating diaphragm

Residual stress is an important factor affecting the sensitivity of piezoelectric micromachined microphone. A symmetric composite vibrating diaphragm was adopted in the micro electro mechanical systems piezoelectric microphone to decrease the residual stress and improve the sensitivity of microphone in this paper. The ZnO film was selected as piezoelectric materials of microphone for its higher piezoelectric coefficient d31 and lower relative dielectric constant. The thickness optimization of piezoelectric film on square diaphragm is difficult to be fulfilled by analytic method. To optimize the thickness of ZnO films, the stress distribution in ZnO film was analyzed by finite element method and the average stress in different thickness of ZnO films was given. The ZnO films deposited using dc magnetron sputtering exhibits a densely packed structure with columnar crystallites preferentially oriented along (002) plane. The diaphragm of microphone fabricated by micromachining techniques is flat and no wrinkling at corners, and the sensitivity of microphone is higher than 1 mV Pa−1. These results indicate the diaphragm has lower residual stress.

Investigation of rheological, structural, surface, optical and thermal properties of low temperature produced silver doped ZnO thick film as thermal interface material in lighting application

Effects of screen-printed AgxZn1−xO thick film deposited on Al substrate with composition 3 wt% ≤ x ≤ 9 wt% on the rheological, surface, vibrational and thermal properties were investigated in this paper. AgxZn1−xO powder with different doping concentration was synthesized through co-precipitation method and further mixed with solvent to form a thixotropic paste. The paste was screen-printed on Al substrate. Rheological behavior of pastes showed shear thinning phenomenon with increment of shear rate. Structural properties of pure and doped thick films were investigated by X-ray diffraction (XRD). It showed that (101) was a strong preferred orientation among other peaks. Field emission scanning electron microscopy (FESEM) results indicated the aggregated flake-like structure that become more prominent with increase of doping from 3 to 7 wt%. Surface roughness of the films exhibited a decreasing trend with the increase of silver content whereas the step analysis facilitated to identify the contact surface point of the film for thermal conduction in application purposes. By increasing the dopant’s concentration, peak position of ZnO had shifted within the range from 503 to 506 cm−1. FTIR spectra of the films showed no vibrational peaks related to AgO or Ag2O. Thermal characterization revealed a decreasing trend of film’s resistivity when the doping was increased to 7 wt%. The lowest achieved junction temperature with tested LED was measured at 47.32 °C at 7 wt% of silver doping.

Oxygen Effect on Structural and Optical Properties of ZnO Thin Films Deposited by RF Magnetron Sputtering

ZnO thin films with wurtzite structure have been grown on Si (100) and glass substrates using radio frequency (rf) magnetron sputtering at room temperature. The ZnO thin films have been characterised by XRD. The (002) orientation is observed at zero Oxygen flow after the (100) developed with increasing oxygen ratio. Usually, this orientation (100) is difficult to obtain. The thickness of ZnO films was confirmed by cross-section SEM, and their stoichiometry was measured by Energy Dispersive X-ray Spectroscopy (EDX) and Rutherford backscattering spectroscopy (RBS). The optical band gaps have been determined using UV spectra and found to be varied from 3.24 to 3.29 eV as a function of the oxygen ratio. Moreover, photoluminescence (PL) spectra showed more defects at higher oxygen flow. The crystalline quality of the deposited film degrades with oxygen enhancements.

Comparison of Deposition Methods of ZnO Thin Film on Flexible Substrate

This paper reports the effect of the different deposition methods towards the ZnO nanostructure crystal quality and film thickness on the polyimide substrate. The ZnO film has been deposited by using the spray pyrolysis technique, sol-gel and RF Sputtering. Different methods give a different nanostructure of the ZnO thin film. Sol gel methods, results of nanoflowers ZnO thin film with the thickness of thin film is 600nm. It also produces the best of the piezoelectric effect in term of electrical performance, which is 5.0 V and 12 MHz of frequency which is higher than other frequency obtained by spray pyrolysis and RF sputtering.

Application of ZnO micro rods on the composite photo-electrode of dye sensitized solar cells

In this study, we prepared low-density ZnO micron rods (MRs) on ITO/PEN substrate by hydrothermal method, and used the scraper to fill the ZnO nanoparticles into the micron rods to prepare ZnO composite structure as the photo-electrode of dye sensitized solar cells (DSSCs). The nanostructures and photo-electrodes were investigated by field emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), incident photon conversion efficiency (IPCE) and I–V characteristics analyses. In the results of FE-SEM, they show that the average length of the ZnO MRs and the thickness of ZnO film are 14 and 33.1 μm, respectively. EIS analyses show that there is a minimal RK and the longest electron lifetime of DSSCs with the ZnO composite structure, which means less recombination of the electrons in the ZnO composite structure. The results of IPCE indicate that the photo-electrode of ZnO composite structure exhibits the best quantum efficiency about 81.7%. It may be due to ZnO MRs play as a light scattering layer resulting in the increase of path length for the incident light, thereby increasing the collision probability between the incident light and the ZnO nanoparticles. In addition, we also compared the I–V characteristics of three different photo-electrodes with ZnO NPs, ZnO NPs pressed 418.8 kg/cm2 and ZnO composite structure. The best structure is ZnO composite structure, and it has the best current density (JSC = 10.59 mA/cm2) and overall conversion efficiency (η = 3.46%) of dye-sensitized solar cells.

Random Lasing in wide-gap semiconductor thin films under deep UV pulsed laser pumping

We present a comparative study of spectral features of random lasing in GaN and ZnO semiconductor films. Numerical calculations of the complex susceptibility are carried out for different carrier concentrations. The absorption coefficients, refractive indices, and theoretical Mott densities are deduced from these calculations in the vicinity of the optical bandgap. As expected in such materials, above the Mott density, calculations highlight the specific spectral domain for each semiconductor where an optical gain may take place. Semiconductor films are optically pumped using a pulsed NdYAG laser at 266 nm, and photoluminescence spectra of the near band edge are investigated at low and high spectral resolution (down to 50 pm). Dependence of the random laser emission upon the thickness of ZnO films is studied. It is demonstrated that random lasing is sustained by waveguiding propagation in the plane of films.

Modeling the sensing characteristics of chemi-resistive thin film semi-conducting gas sensors.

For chemi-resistive thin film gas sensors a generic theoretical model is proposed to predict the variation of sensor response with the operating temperature and thickness of the sensing film. A diffusion equation is formulated assuming that inflammable target gases move through the sensing film by Knudsen diffusion and react with the adsorbed oxygen following first-order kinetics. We have assumed a realistic non-linear variation between the conductance and test gas concentration and derived a general expression relating the sensor response to the operating temperature and thickness of the film. Assuming Langmuir adsorption kinetics, we have theoretically predicted the response and recovery transients during gas sensing using thin film sensing elements. It is predicted that for irreversible type sensing, the response time is reduced with an increase in test gas concentration, whereas for reversible sensing, the response time is independent of test gas concentration. For zinc oxide thin film sensors, an excellent match is obtained between the model prediction and experimental data for their thickness (122 nm to 380 nm) and temperature variation (200 °C to 325 °C) in 500 ppm carbon monoxide (CO) sensing. The maximum CO response% (∼53%) was achieved in 320 nm thick ZnO films. The conductance transients for response and recovery for CO sensing closely follow Langmuir adsorption kinetics and as predicted theoretically, indeed for irreversible sensing, the response time reduces from 350 s to 220 s with an increase in test gas concentration from 20 to 550 ppm. In the case of reversible sensing we found that the response time is ∼55 s irrespective of the CO gas concentration in the range of 5-500 ppm. The models developed in the present work are quite generic in nature and we have discussed their applicability to a wide variety of sensing materials with various types of surface morphologies.

Gas Sensing Performance of Al2O3 Activated ZnO Thick Films

Gas Sensing Performance of Al₂O₃ Activated ZnO Thick Films

Love-mode surface acoustic wave devices based on multilayers of TeO2/ZnO[formula omitted]/Si(1 0 0) with high sensitivity and temperature stability

A multilayer structure of TeO2/interdigital transducers (IDTs)/ZnO(112¯0)/Si(100) was proposed and investigated to achieve both high sensitivity and temperature-stability for bio-sensing applications. Dispersions of phase velocities, electromechanical coupling coefficients K2, temperature coefficient of delay (TCD) and sensitivity in the multilayer structures were simulated as functions of normalized thicknesses of ZnO (hZnO/λ) and TeO2 (hTeO2/λ) films. The fundamental mode of Love mode (LM) - surface acoustic wave (SAW) shows a larger value of K2 and higher sensitivity compared with those of the first mode. TeO2 film with a positive TCD not only compensates the temperature effect induced due to the negative TCD of ZnO(112¯0)/Si(100), but also enhances the sensitivity of the love mode device. The optimal normalized thickness ratios were identified to be hTeO2/λ=0.021 and hZnO/λ=0.304, and the devices with such structures can which generate a normalized sensitivity of −1.04×10−3m3/kg, a TCD of 0.009ppm/°C, and a K2 value of 2.76%.