Band Gap Of ZnO Films Research Articles

Structural and optical properties of copper-doped ZnO thin films at different weight percentage

ZnO thin films have been synthesized by chemical vapor deposition technique at substrate temperature 400˚C and doped with Cu at (1,3,5 wt.%). The morphologies of pure and doped ZnO films were investigated by SEM and the grain size was become larger by increasing the amount of Cu doped. From X-ray diffraction measurements, the maximum intensity peak was (002) for ZnO:Cu films at diffraction angle 34.4˚ and crystallized of hexagonal phase. Based on XPS Measurements on the surface of ZnO:Cu films, it has been found that the peak was shift to higher diffraction angle and full width half maximum of the films were become wider with increasing Cu doping. The band gap of ZnO film was 3.3 eV and it was decreased as an increasing of Cu dopant.

Synthesis, characterization and fuzzy simulation of ZnO thin film on flexible substrate

ZnO is promising material for the electronic and optoelectronic devices. In present work we have fabricated the ZnO film by DC reactive magnetron sputtering. The variations of reactive and sputtering gases affect the crystallite size and band gap of ZnO film. In present work the ZnO film is prepared at 50 watt power by DC reactive spurting method. The fuzzy simulation has been performed to estimate the best argon oxygen gas ratio which gives the better crystallinity and band-gap. The structural analysis shows that the ZnO film has hexagonal wurtzite structure. The UV-vis spectroscopy has been employed to find the band gap.the measured band gap value of ZnO is 3.21 eV. The fuzzy rule based system and characterization results are in accordance with each other with a minimal difference of less than 1%.

Mg substitution effect on the electron affinity of ZnO films

We investigated the effect of Mg doping on the electron affinity of ZnO thin films. MgxZn1−xO (x = 0–0.29) composition-gradient films were deposited on an α-Al2O3(001) substrate using combinatorial pulsed laser deposition. The combinatorial high-throughput analysis of the optical transmittance systematically revealed that the bandgap of ZnO films was tunable between 3.3 and 4.0 eV by doping the ZnO thin films with Mg2+ ions. To investigate the electronic structure, photoelectron yield spectroscopy measurements were performed on the MgxZn1−xO composition-gradient films. The ionization potential, which denotes the distance between the valence band maximum and vacuum level, was independent of the Mg content in the ZnO films. By comparing with the optical bandgap results, the electron affinity was tunable from 4.1 to 3.5 eV by the Mg content in the MgxZn1−xO films.

Effects of electron doping on the d0 magnetism in N-implanted ZnO and ZnAlO films

In this paper, N ions are implanted into ZnO and ZnAlO films with different carrier concentrations prepared by radio frequency reactive magnetron sputtering on sapphire substrates. The structural, electrical, optical and magnetic properties of N-doped and (Al, N) co-doped ZnO films is investigated, and the particular emphasis is placed on the effects of carrier concentration on the defects induced magnetism in the films. Our results show that all the doped ZnO films are ferromagnetic at room temperature. A trace amount of additional Al doping has a significant effect on the improvement of ferromagnetic properties, and a maximum saturation magnetization of 73 emu/cm3 is obtained for (Al, N) co-doped ZnO films. The optical band gaps of ZnO films increase with the increasing Al doping content, which is owing to the combined effect of high carrier concentration and the Burstein Moss effect. The average transmittance of all ZnO films exceeds 80%. Our results confirm that appropriate electron doping can effectively enhance the magnetic moment in N implanted ZnO which may also apply to other ZnO systems with d0 magnetism.

Circularly Polarized Light Detection by Chiral Photonic Cellulose Nanocrystal with ZnO Photoconductive Layer in Ultraviolet Region.

Circularly polarized light (CPL) detection and polarization state recognition are required for a wide range of applications. Conventional polarization detection with optical components causes difficulties for miniaturization and integration. An effective design strategy is proposed for direct CPL detection with chiral material. Here, we realized direct CPL detection based on the combination of chiral photonic cellulose nanocrystal (CNC) and ultraviolet-sensitive ZnO photoconductive material. The CNC layer deposited by evaporation-induced self-assembly established the left-handed chiral nematic structure with a photonic bandgap (PBG) to recognize left-handed CPL (LCPL) and right-handed CPL (RCPL) at specific wavelengths. The PBG of CNC layer has been modulated by the adjustment of chiral nematic pitch to match the semiconductor bandgap of ZnO film in ultraviolet region. The photocurrents under RCPL and LCPL are 2.23 × 10−6 A and 1.77 × 10−6 A respectively and the anisotropy factor Δgpc of 0.23 is acquired for the CPL detection based on the chiral photonic CNC. This design provides a new approach to the detection of CPL polarization state with competitive performance.

Morphological evolution and its correlation with optical and field emission properties in pulsed laser deposited ZnO nanostructures

The present report illustrates the effect of growth temperature on the morphological features, optical, and field emission properties (FE) of ZnO thin films deposited on Si substrate (100) through the pulsed laser deposition (PLD) technique. The as-deposited films have been characterized by employing various characterizations tools particularly X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), High- resolution TEM, etc., and confirms the successful deposition of the films. It demonstrates that as the growth temperature increases from ∼ 5000C to ∼ 8000C during 30 min, the spherical particle-like nanostructures transform into the truncated nanocones. Further, at a fixed deposition temperature of ∼ 8000C, and with enhancement in the growth time from ∼10 to 30 min, the nanoparticles get transformed into nanocones. Henceforth, the substrate temperature and growth time have been identified as determinant factors for the formation of these ZnO nanocones. XRD and TEM analysis suggest that the majority of nanocones have been aligned epitaxially with a preferred c-axis orientation on the Si substrates. Optical measurements reveal that the optical band gap of ZnO films exhibits a decreasing tendency from ∼3.26 to 3.22 eV with an increase in substrate temperature from ∼ 500 to 8000C. Subsequently, the field emission (FE) measurement of these aligned ZnO nanocones signifies a stable field emission with a low threshold field (2.40 V/μm) and a high field enhancement factor (∼2786). Their tapered morphologies and small tip diameters suggest as-deposited samples to be highly reproducible and henceforth as-deposited ZnO nanocones bearing excellent FE properties, through high-pressure PLD growth method without using any catalyst offer usage in various futuristic electronic applications.

Synthesis and optical analysis of nanostructured F-doped ZnO thin films by spray pyrolysis: Transparent electrode for photocatalytic applications

In this paper, spray pyrolysis as a valuable cost-effective deposition technique was used to obtain nanostructured thin films of ZnO and ZnO: F on soda-lime glass substrates at 400 °C. The crystal structure and surface morphology of the as-deposited thin films have been characterized by the X-ray diffraction (XRD) technique and high-resolution atomic force microscope (AFM). XRD patterns indicated the wurtzite hexagonal structure of the deposited films with mean grain size in nanoscale. AFM and XRD confirmed that the mean grain size of ZnO film was increased with the F doping level. The optical parameters like refractive index, absorption coefficient, and optical bandgap have been obtained through Swanepoel's model depending on the transmission spectra of the deposited films. The direct optical band gap of ZnO film was slightly decreased with the F doping level. The single oscillator model was applied to determine the dispersion parameters of the films under study. The photocatalytic activity of the ZnO and ZnO: F thin films for the phenol photodegradation under UV-irradiation also has been evaluated to purify water from phenol as colorless organic waste. 3%F doped ZnO film has the highest constant rate of phenol photodegradation with high stability in reusing and recycling, confirming the high efficiency of F doped ZnO films as environmentally friendly photocatalysts for the removal of phenol from water.

Crystallographic disorders depending on monovalent cations addition and their effects on ZnO's characteristics

Pure and lithium (Li) doped ZnO films have been grown on soda lime glass substrates by chemical spray pyrolysis technique. Zinc chloride (ZnCl2) and lithium chloride (LiCl), resoluble in deionized water, were used in the process. The growth mechanism of the films as a function of Li content, resulting texture and topography were discussed in detail by X-ray diffraction method (XRD) and scanning electron microscopy (SEM). Obtained films exhibited single phase with polycyrstalline nature. At the same time, changes in crystal quality due to lithium addition were observed and these changes were explained as the presence of lithium atoms in the ZnO's crystal structure as interstitial atoms and the replacement of Li+ and Zn2+ions. Topographical features of the films were confirmed by SEM images. In SEM photographs, it was observed that the crystallite size of the films varied as a result of an increase in the lithium doping concentration. Thus, a significant correlation was found between the structural and morphological properties of the films due to Li contribution. Optical properties of the films were evaluated by UV-VIS-NIR spectroscopy and found that the optical band gap of ZnO films exhibit increasing tendency up to 4 at.%Li content and decrease for further Li content. Elemental composition and presence of Li were approved by X-ray photoelectron spectroscopy (XPS) and Li 1s peak were observed in 55.6 eV in 5 at.% Li doped ZnO films.

Influence of Ni doping on the nitric oxide gas sensing properties of Zn1−xNixO thin films synthesized by silar method

Zn1−xNixO thin films (x = 0, 0.05, 0.10, 0.15 and 0.20) were systematically synthesized by Successive Ionic Layer Adsorption And Reaction (SILAR) method with 40 SILAR cycle and then were annealed at 300 °C. Influence of nickel doping and annealing treatment on the structural, morphological, optical and gas sensing properties of Zn1−xNixO thin films were investigated. X-ray diffraction (XRD), scanning electron microscope (SEM) and optical absorption measurements were used for characterization of the films. The gas sensing properties of the Zn1−xNixO sensors for low nitric oxide (NO) gas concentrations were investigated as a function of operating temperature and doping concentration. The gas sensing results showed that the doping process strongly influenced the response of ZnO sensors. The optimal operating temperature of the sensors was found of 95 °C, the response of the annealed Zn00.90Ni0.10O sensor for 25 ppm NO reached a maximum of 63%. The Zn0.90Ni0.10O sensor exhibited higher response and better response/recovery times than the others. It was concluded that Ni doping and annealing treatment enhanced the gas sensing properties of sensors by changing microstructure, morphology, bandgap of ZnO films.

Photoluminescence and optical response of ZnO films deposited on silicon and glass substrates

ZnO films are synthesized by RF magnetron sputtering on silicon and glass substrates. Dry plasma is used to etch silicon at 5 min as well as 30 min for the purpose of surface modifications before depositing the films. ZnO films are compared with non etched substrate ones. Surface morphology characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Photoluminescence (PL) of ZnO films increased with increasing etching substrate time. The Photoresponse using Deuterium lamp, Ar laser and He-Cd laser was increased for ZnO films deposited on etched silicon substrate from 0 min to 30 min. No significant change was observed for optical band gap of ZnO films deposited on glass substrate. Films deposited on non etched Si (100) and glass substrates exhibit (002) preferred orientation, but the films with etching time for 5 and 30 min to silicon substrates become polycrystalline with accompanied (100) and (101) orientations as analyzed using x-ray diffraction (XRD). Electrical resistivity increases from 31 × 10−4 Ohm.cm to 218 × 10−4 Ohm.cm with increasing etching time of Si substrate.

Effects of yttrium doping on the electrical performances and stability of ZnO thin-film transistors

In this paper, we have investigated the effects of yttrium (Y) doping on the electrical performance and stability of ZnO thin film transistors (TFTs). Here, Y-doped ZnO TFTs were fabricated by using radio frequency magnetron sputtering at 150 °C. As a result, the 1% Y-doped ZnO TFT exhibits a small threshold voltage shifts of 2.5 V under positive bias stress and −2.8 V under negative bias stress as well as desirable device performance with field effect mobility of 9.8 cm2/V s, a subthreshold swing of 320 mV/decade and on/off current ratio of 107, respectively. Based on the XPS analysis and electrical characterizations, the improvement in stability and electrical properties of ZnO TFTs were attributed to the appropriate Y doping concentration, which not only could control the carrier concentration and broaden the band gap of ZnO film, but also suppress the oxygen vacancy defects and passivate the trap density at the SiO2/ZnO interfaces. Consequently, the high stability and excellent electrical performances of Y-doped ZnO TFTs show great potential for use in flat panel displays.

Effect of deposition time on sputtered ZnO thin films and their gas sensing application

Nowadays, advanced industrialization and population growth have led to increasing the environmental related issues. This paper reports the effect of deposition time on ZnO films deposited on to the glass substrate by using rf magnetron sputtering technique and their further use for gas sensing applications. Herein, deposition time is considered to be changed from 300 s, 800 s (S1, S2). The thickness of deposited films lies in the range of 130–180 nm. The synthesized films were characterized by various techniques in terms of structural, morphological, optical and gas sensing properties. The typical crystal size of ZnO films was found to be in the range of 15–27 nm. FESEM analysis revealed the growth of nanospheres was lies in the range of 80–120 nm. Fourier transform infrared spectroscopy confirmed the ZnO bonding located at a wavelength of 430 cm−1. The average optical transmittance of the film was about 90–95% in the visible range. The optical band gap of ZnO films was decreased from 3.31 to 3.29 eV. The detailed characterization study showed 800 s is an optimum deposition time for good optoelectronic properties. For gas sensing application, highest sensitivity was obtained at operating temperature of 205 °C. Prepared films have a quick response and fast recovery time in the range of 128 s and 163 s respectively. These response and recovery time characteristics were explained by valence ion mechanism.

Optical, Structural, and Resistive Switching Characteristics of Atomic-Layer-Deposited ZnO Films with Their Thickness Variation

Resistive random access memory (RRAM) which has simple structure of metal-insulator-metal (MIM) resistor has been focused as a promising device to be applied for memory device due to its non-volatile characteristics and low power consumption. Especially, ZnO films have emerged for applications to transparent or semi-transparent devices in the range of visible light, owing to their wide band gap (3.3 eV) and high transmittance. The resistor with ZnO insulator shows the low-operation reset or set voltage below ±2 V. However, the crystallinity, conduction path, refractive index, energy band gap, and oxygen vacancy concentration of ZnO films vary with the film thickness. That is, as the optical, structural, and electrical properties of ZnO film depend on the thickness of the deposited film, the resistive switching characteristics of resistor made with ZnO such as operation voltage and on/off ratio also the function of its thickness. In this study, we fabricated the ZnO insulators at lower temperature with atomic layer deposition (ALD) method and investigated their optical, structural, and electrical properties. The MIM resistor devices were also made with ZnO insulator with different thickness and their resistive switching characteristics were evaluated. The switching parameters were analyzed with the relation of material properties. The MIM resistors with different thickness of ZnO films were fabricated on p-type Si substrates. First, Ru films of 100 nm were deposited by DC sputtering on the Si substrates at room temperature. Afterward, 20, 40, 60, 80, and 100 nm-thick ZnO insulators were deposited on Ru films by ALD technique at 150 °C with the use of diethylzinc and water. Top 100 nm-thick Al electrodes were formed on the insulator by DC sputtering and the resistor cells with 100 µm × 100 µm were patterned by lift-off lithography. The thickness and band gap of each ZnO film were confirmed by spectroscopic ellipsometry. Band gap of ZnO film was increased from 3.0 eV to 3.15 eV with the increase of thickness from 20 nm to 60 nm. The crystallinity were also increased with the higher thickness of ZnO insulators, determined by XRD. The switching characteristics of Al/ZnO/Ru MIM resistors were obtained from the current-voltage (J-V) curves measured by a semiconductor parameter analyzer. Furthermore, as the ZnO film thicken, the switching parameters also changed to the higher values, that is, the higher reset and set voltages, and even higher on/off current ratio. From the above results, it is concluded that the optical band gap of ZnO and resistive switching factors of the resistor with ZnO insulator are related to the thickness of a ZnO film. However, as the on/off current ratio and operation voltages are trade-off, good signal identification also causes the huge power consumption. Therefore, the optimized thickness of ZnO insulator is needed to operate good performance resistive switching memory. Figure 1

Evolution of surface topography and optical band gap of ZnO film deposited on NiO/Si(100)

Evolution of surface features and optical band gap of ZnO thin films deposited on different NiO/Si(100) are reported. In order to create different initial microstructure, we first deposited NiO film on Si(100) at 3 different temperatures (400°C, 650°C, and 700°C) by pulsed laser deposition. These NiO/Si(100) films are used as substrate for the deposition of ZnO films. Combining the results obtained from grazing incidence X‐ray diffraction, atomic force microscope, and UV‐Visible characterization, our study indicated that the microstructure of the substrate takes the important role in dictating properties of the film. Our study also indicated that one needs to choose appropriate synthesis condition to achieve good quality ZnO films.

Preparation and characterization of Cd-doped ZnO thin films by spin coating method

ABSTRACTCd-doped ZnO thin films were deposited on a glass substrate via spin coating techniques. The structural properties of the thin film were studied by X-ray diffractometry. X-ray diffraction data showed that the grown films were oriented in (100) direction with hexagonal wurtzite phase. The average calculated grain size ranged from 10 to 12 nm. The lattice constants of the as-prepared ZnO films were calculated using standard formulae, and the values are arrived as a = 3.25 Å and c = 5.20 Å, which confirm the hexagonal of the materials. The Scanning Electron Microscope (SEM) images reveal the polycrystalline structure of pure ZnO film. The minimum thickness is found to be 0.7 µm and the maximum is 1.4 µm. The FT-IR spectroscopy shows some absorption bands at 1427.32 cm−1 corresponding to the stretching vibration of C–O band. Various absorption band peaks in the range of 3350–3450 cm−1 correspond to O–H species. From the UV spectral analysis, the band gap of ZnO film coated over a glass substrate is found to be 3.4 eV, the band gap of pure ZnO film is calculated as 3.44 eV, and that of Cd-doped ZnO film for 2, 4, and 6 wt% was 3.43, 3.429, and 3.423 eV respectively.

Role of evaporation time on the structural and optical properties of ZnO films deposited by thermal evaporator

Zinc oxide films are deposited on Si substrates by thermal evaporator for different evaporation times (ET). XRD pattern shows the development of different diffraction peaks related to Zn, ZnO and Zn 2 SiO 4 phases which confirms the deposition of composite film. The orientation transformation is observed with increasing ET. The maximum peak intensity of ZnO (1 0 1) plane is observed at 3 h ET. The dislocation density observed in ZnO (1 0 1) plane varies from 1.53 × 10 -3 nm -2 to 8.94 × 10 -3 nm -2 . The lattice parameters of ZnO are found to be a = 3.243 A and c = 5.197 A. FTIR analysis confirms the formation of ZnO films. SEM microstructures exhibit the formation nano-wires, nano-bars, nano-strips and nano-needles. The optical energy band gap of ZnO films deposited for various ET varies from 3.98 eV to 4.06 eV. Results show that the peak intensity of ZnO (1 0 1) plane, orientation transformation and the presence of Si content are responsible to increase the energy band gap of ZnO films.

Effect of swift heavy ion on structural and optical properties of highly transparent zinc oxide films

Swift heavy ion (SHI)-induced modification in structural and optical properties of ZnO thin films deposited by sol–gel method is investigated. These thin films were irradiated by 120 MeV ions of Ag+9 at various ion fluencies. The structural properties were studied using grazing incidence X-ray diffraction, and it shows that the average crystallite size of ZnO films is observed to decrease by the irradiation. The band gap of ZnO films is varied little without any appreciable change in the shape of near-band-edge absorption by SHI irradiation. It is also observed that the average transmittance in the visible region is reduced from ~88 to ~45 %. It can also be mentioned that the interference fringes from the transmittance spectra disappear on exposure to irradiation. The modifications of the observed structural and optical properties are explained in terms of defects induced by energy deposited in the lattice by SHI irradiation.

Morphological and Optical Properties of ZnO Thin Films Prepared by Spray Pyrolysis on Glass Substrates at Various Temperatures for Integration in Solar Cell

ZnO thin films are widely used as antireflection layer in solar cells and can be grown by various techniques implemented at various temperatures. In this contribution, we present the morphological and optical properties of ZnO thin films prepared by spray pyrolysis on glass substrates at various growth temperatures from 350 to 550°C. The surface morphology of the films, analysed by scanning electron microscopy SEM, is modified with substrate temperature. Optical characterizations of ZnO films as a function of temperature were carried out by transmittance and photoluminescence (PL) measurements. Main optical properties are as follow: All the films are highly transparent in the visible region of the electromagnetic spectrum. The average transmission in the visible range (400–800nm) was greater than 80%. The band gap of ZnO films increases with increasing substrate temperatures attributed to the increase of the grain size of thesample.PL spectrum of ZnO films can be divided into the UV emission and the visible broadband emission attributed to the near band edge emission (NBE) and to the deep level emissions (DLE), respectively. The biggest ratio of UV emission on visible emission of the PL intensity is observed for the ZnO thin film deposited at 550°C by the spray pyrolysis technique. Therefore, we suggest that this original result indicate the most suitable growing conditions for obtaining high quality sprayed ZnO thin films with higher luminescence performances.

Optical properties of Zn1−x Al x O:NiO transparent metal oxide composite thin films prepared by sol–gel method

The films of Zn1−x Al x O:NiO (AZO; Al/Zn = 1.5 at.%, x = 0.5–2.0) were synthesized on glass substrates by sol–gel method. The morphological properties of the films were studied by atomic force microscopy. The surface morphology of the films is found to depend on the concentration of NiO. The optical band gap, Urbach energy and optical constants such as refractive index, extinction coefficient and real and imaginary parts of the dielectric constant of the films were determined. The refractive index dispersion of the films obeys the single-oscillator model, and the single-oscillator parameters were determined. The optical band gap of ZnO film was found to be 3.76 eV, and the optical band gap of the films increases with NiO doping. The obtained results suggest that Zn1−x Al x O:NiO films can be used as optical material in optical communication applications.

Growth of residual stress-free ZnO films on SiO2/Si substrate at room temperature for MEMS devices

ZnO thick Stress relaxed films were deposited by reactive magnetron sputtering on 2”-wafer of SiO2/Si at room temperature. The residual stress of ZnO films was measured by measuring the curvature of wafer using laser scanning method and found in the range of 0.18 x 109 to 11.28 x 109 dyne/cm2 with compressive in nature. Sputter pressure changes the deposition rates, which strongly affects the residual stress and surface morphologies of ZnO films. The crystalline wurtzite structure of ZnO films were confirmed by X-ray diffraction and a shift in (0002) diffraction peak of ZnO towards lower 2θ angle was observed with increasing the compressive stress in the films. The band gap of ZnO films shows a red shift from ∼3.275 eV to ∼3.23 eV as compressive stress is increased, unlike the stress for III-nitride materials. A relationship between stress and band gap of ZnO was derived and proposed. The stress-free growth of piezoelectric films is very important for functional devices applications.