Nanocrystalline ZnO Thin Films Research Articles

Distinct band UV–Visible photo sensing property of ZnO-Porous silicon (PS):p-Si hybrid MSM heterostructure

Present article deals with fabrication of UV–Visible radiation sensing photodetector (PD) by architecting planar and sandwiched (PD1 and PD2) electrode configurations on ZnO-PS:p-Si heterostructure. PS:p-Si structure is prepared by electrochemical anodization method. Thin ZnO layer of ~162 nm is deposited on PS:p-Si substrate using pulsed DC magnetron sputtering technique. Gold (Au) is combined with Aluminium (Al) to provide rectifying Schottky contacts to the devices. Prior to device testing it is tested for various conventional characterizations. FESEM studies confirm uniform deposition of nanocrystalline ZnO thin film with mean particle size of ~ 35 nm, whereas its cross section shows uniform interface between ZnO and PS with combined thickness of ~700 nm. Cross sectional EDX analysis confirms diffusion of ZnO particles into PS layer. UV–vis absorption spectrum gives bandgap of ZnO as ~3.3 eV. Reflectance measurement of the heterostructure shows decrease in reflectivity compared to bulk c-Si with ZnO-PS:p-Si showing multiple interference pattern compared to bare ZnO coated on glass within the spectral range 200 nm-800 nm. The pattern is remarkably increased compared to that of PS:p-Si. Photoluminescence study reveals various luminescence bands peaked at ~378 nm, 388 nm, 398 nm, 410 nm, 420 nm, 433 nm, 450 nm, 467 nm, 482 nm 492 nm and 597 nm. Current- Voltage (I–V) measurement for dark current (Id) gives low leakage current of the devices PD1 and PD2 respectively as 6.6×10−8A and 7.5×10−7A at -5V, which is however an order of magnitude more than the ZnO coated one on glass. Both the devices PD1 and PD2 show UV–Visible photoresponse, whereas the ZnO device on glass shows response to UV (375 nm) only. Both the devices show response time ~1 s in visible range which is nearly half the value in UV range.

Anomalous photoconductivity in chemical spray pyrolysis deposited nano-crystalline ZnO thin films

Nano-crystalline ZnO thin films were deposited by chemical spray pyrolysis technique by maintaining the substrate at a temperature of 493 K. Structural characterization using XRD analysis identified the preferential orientation of the deposited films to be along the (002) plane. Photoluminescence studies showed emissions at 381 nm, 395 nm and 469 nm from the samples. The samples under halogen illumination in ambient condition exhibited a photosensitivity of ∼ 106. Under the same illumination pulse width and intensity in vacuum condition of 5x 10−5 T, the photosensitivity was an order higher ∼ 107. The photo response of the films to UV illumination under vacuum condition showed persistent photoconductivity, whereas under air ambient showed an anomalous behaviour of decrease of photocurrent under continuous illumination. We model the anomalous behaviour based on the mechanism of oxygen absorption and desorption from the surface of the nano-crystalline ZnO thin film. The dual photoconduction exhibited by our ZnO thin films can help multi-functional applications in artificial intelligence.

Structural, optical, and magnetic properties of Co-doped ZnO nanocrystalline thin films for spintronic devices

Different compositions of Zn1−xCoxO (0 ≤ x ≤ 0.15) nanocrystalline compounds were produced in terms of ball milling method, thin films of these compounds were prepared by electron beam evaporation method. XRD pattern was utilized to study the structural properties of these films. Hexagonal wurtzite-type structure was displayed for all of the films. Using XRD pattern, the crystallite and lattice strain were calculated. The crystallite size decreases but the lattice strain's value rises up with accumulating dopants of Co. Increasing in lattice strain may be related to the increase in the concentration of lattice imperfections. The optical constants, n and k of the Zn1−xCoxO nanocrystalline films were calculated in terms of spectroscopic ellipsometry measurements in a range of 300–1100 nm. With more doping of Co, the refractive index showed an increase, as well. According to Tauc relation, the optical energy gap of the Zn1−xCoxO films was calculated and proved to be direct transition. The energy gap found to be decreased with the increasing of the Co concentration. In addition, studying the magnetic properties of Zn1−xCoxO films reveal room temperature ferromagnetism.

Elucidation of structural, morphological, optical and photoluminescence properties of single and (In, Ga) co-doped ZnO nanocrystalline thin films

Single and co-doped ZnO thin films are currently under intense investigation and development for optoelectronic applications. Here in this study, pristine, indium-doped (IZO), gallium-doped (GZO) and co-doped (IGZO) ZnO thin films were deposited on a glass substrate using radio frequency magnetron sputtering. A comparative study of all the films was carried out on the basis of their various properties. The effect of single and co-doping on the structural (X-ray diffraction (XRD) studies and Raman studies), morphological (field emission scanning electron microscopy and energy dispersive X-ray spectroscopy studies) and optical properties (ultraviolet–visible (UV–Vis) and photoluminescence (PL)) of the deposited films was investigated. X-ray photoelectron spectroscopy (XPS) characterization was employed to analyse the surface chemical composition and bonding of the deposited film. From the XRD patterns, it was found that the films were highly crystalline in nature and preferentially oriented along the (002) direction with a hexagonal wurtzite structure, consistent with Raman analysis. IGZO films displayed a dramatic improvement in the surface morphology as compared with the single dopant films due to the compensation effect of gallium and indium doping which reduced the lattice strain. The XPS analysis confirmed the presence of the oxidized dopants in each film. All thin films have shown excellent optical properties with more than 90% transmission in the visible range of light. The blue-shift of the absorption edge accompanied by the increase of the optical band gap confirmed the Burstein–Moss effect. The UV PL peak originated from the near band edge emission of crystalline ZnO, while the visible PL was associated with the radiative transition related to oxygen interstitial (Oi) defects in the ZnO structure.

Enhanced near-band edge emission in pulsed laser deposited ZnO/c-sapphire nanocrystalline thin films

We report on the enhanced near-band edge emission with negligible defect-assisted emission in ZnO nanocrystalline thin films of varying thickness on c-sapphire substrates. X-ray diffraction studies reveal wurtzite structure of thin films grown preferentially along (002) plane. The crystallinity and grain size of the deposited films have been increased with increase in film thickness. The band gap of the films was calculated by Tauc’s plot which shows negligible change in band gap with increase in thickness of ZnO film. The various Raman modes observed in ZnO thin films of different thicknesses support the formation of hexagonal wurtzite structure. The room temperature photoluminescence (PL) spectrum shows the strong near-band edge emission in the wavelength range 378–381 nm which further enhances with film thickness. The enhanced PL emission in near ultraviolet visible (UV) region with negligible defect-assisted emission may find potential applications in ZnO-based UV-excited light-emitting diodes.

Role of co-doping on structural, optical and magnetic properties of nano-crystalline ZnO thin films

This paper refers to investigation of structure, surface morphology, optical parameters and magnetic characteristics of the prepared ZnO thin films when Co concentration varies from 1 to 5 at wt%. X-ray diffraction study revealed that deposited films were poly-crystalline with hexagonal wurtzite structure. The band gap energy was calculated by using the envelope method. It’s value was decreased from 3.334 to 3.22 eV with increasing Co concentration up to 4 at wt% and then increased to 3.335 eV for 5 at wt% Co. The magnetic study indicated a ferromagnetic trend for all thin films. An oxygen vacancies–related F-center is a possible physical mechanism for generation of ferromagnetism. Scanning Electron Microscopy exhibited rough and porous morphology.

Microstructural evolution and optical dispersion of sputtered ZnO thin films at low annealing temperature

In an attempt to understand microstructural evolution and its effect on optical properties, nanocrystalline ZnO thin films are deposited by pulsed DC magnetron sputtering and then heat treated at a low temperature for several hours. X-ray diffraction experiments confirm that texture and residual stress are sensitive to morphological changes from globular to plate-like grains. Nanocrystalline ZnO thin films are highly transparent to visible light but opaque to UV light below 350 nm. The transparency in the UV region is limited by its fundamental absorption below 400 nm. The spectral dependence of optical constants demonstrates normal dispersion and the films deposited at 70 W of sputtering power and then heat treated have higher values of refractive indices and significantly high extinction. The dispersion parameters evaluated under the purview of a single oscillator model propose vital linkages of optical properties with the microstructure of ZnO thin films. Dispersion parameters viz. plasma frequency can be attuned by controlling the microstructure of ZnO thin films via simply varying process parameters and post-deposition heat treatment at low temperature. This study confirms that high tensile residual stress and texture are favorable for the development of novel nanocrystalline ZnO thin films.

Changes in the growth orientation, morphological and optical properties of sol-gel nanocrystalline ZnO thin films coated with different thickness

ABSTRACTNanocrystalline ZnO thin films with the varied thickness were deposited on glass substrate using sol-gel spin coating method, and the different thickness of samples was achieved via changing the number of layers. X-ray diffraction patterns of samples showed the preferential orientation at (002) planeupon coating with 2 layers and the gradual increase in random orientation with coating up to 6 layers. Scanning electron microscopeimages appeared a few layer interfaces for 4 and 6 layers sample, and atomic force microscope images revealed the larger particle size and rougher surfacewith increasing thickness, which showed the obvious effects on optical properties of samples too. The increased absorption and reduced energy gap were found through deducing transmittance curves of samples. Further, photoluminescence characteristic was measured and revealed that there are the best intrinsic emission in ultraviolet region and lowest defect emissions in visible region for the sample coated with 4 layers.

Investigation on Structural, Morphological, Optical and Ammonia Sensing Properties Indium Doped Nano Crystalline ZnO Thin Films Synthesized by Spray Pyrolysis Technique

In the present work, nanostructured In doped ZnO thin films were synthesized using spray pyrolysis technique with different molarity concentrations (0.001–0.004). X-ray diffraction patterns confirms the polycrystalline nature of the films with hexagonal structure. The crystallite size is going to be increases with increase in dopant concentration. The field-emission scanning electron micrograph of undoped ZnO exhibits spherical shaped particles with intergrain pores and the intergrain pores decreases with increases in indium concentration. The transmittance and band gap is going to be decreases with increase in indium concentration. The ammonia (NH3) sensing properties of the undoped ZnO and In doped ZnO thin films were carried out at room temperature and the sensing responses of the samples towards NH3 concentrations were reported.

On the question of structure of ZnO thin films formed by IBAD and subsequently implanted with silver ions

Nanocrystalline ZnO thin films with a thickness of ∼ 235 nm were synthesized by ion beam-assisted deposition (IBAD) technique using a metal target of zinc and oxygen (O2) as a reactive gas. The near-surface region of the synthesized films was subsequently implanted with 30 keV Ag+ ions in the fluence range of (0.25-1)×1017 ion/cm2 at high ion current density of 12 μA/cm2. The structure parameters and morphology of as-deposited and subsequently implanted with silver ions ZnO films were investigated by X-ray diffraction and scanning electron microscopy techniques. It was found that the as-deposited ZnO films have inhomogeneous structure, which consists of nanocrystallites and disordered amorphous phase. The nanocrystallites of the obtained ZnO thin films have values of lattice parameters higher than for a bulk ZnO. Subsequent implantation with silver ions leads to a significant radiation heating and microstress relaxation of the film as well as to an increase in the size of nanocrystallites due to the amorphous phase.

Understanding ammonia adsorption and charge transfer process on ZnO using experimental and DFT approach

Nanocrystalline ZnO thin film was deposited using spray pyrolysis technique on a glass substrate. The morphological, structural, chemical state, sensing response, as well as the bond formation of ammonia exposed ZnO were studied. The ammonia adsorption mechanism was examined for the ZnO thin films at ambient temperature (32 °C) through experimental and DFT studies. It revealed that ammonia was adsorbed over ZnO via N-Zn and O-H and transferred charge to ZnO. The electronic structure parameters such as charge transfer, HOMO/LUMO gap, and DOS for (ZnO)12 cluster were analysed and reported. Nitrogen formation during the desorption of ammonia over ZnO was verified through GC-MS.

The Sputter Time Duration Effect on the Structural and Optical Properties of Zinc Oxide by RF Magnetron Sputtering

Nanocrystalline ZnO thin films have been fabricated by an RF magnetron sputtering method for the different time duration. The effect of process parameters on the structural, morphological and Optical properties of ZnO were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Photoluminescence and UV-Vis. Spectrometer. All of the films were polycrystalline, and the preferred crystal plane orientation was (101) corresponding to 2𝜃 = 36.57∘, the crystallite sizes were increased from 12 nm to 18 nm as in XRD calculations and increase from 24nm to 33 nm as in SEM image. Optical constants, such as refractive index n, were determined from the transmittance spectrum in the ultraviolet -visible. The refractive index varies in the range of 1.664 to 8.706 with an increase in deposition time. Optical energy gap estimated from the transmittance (3.304 eV) and the PL (3.44 eV) shows good agreement and increased as the time duration was increased.

Effect of Annealing Temperature on Structural and Optical Properties of Sol–Gel-Derived ZnO Thin Films

Nanocrystalline ZnO thin films were deposited on glass substrate via sol–gel dip-coating technique then annealed at 300°C, 400°C, and 500°C for 1 h. Their optical, structural, and morphological properties were studied using ultraviolet–visible (UV–Vis) spectrophotometry, x-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM). XRD diffraction revealed that the crystalline nature of the thin films increased with increasing annealing temperature. The c-axis orientation improved, and the grain size increased, as indicated by increased intensity of the (002) plane peak at 2θ = 34.42° corresponding to hexagonal ZnO crystal. The average crystallite size of the thin films ranged from 13 nm to 23 nm. Increasing the annealing temperature resulted in larger crystallite size and higher crystallinity with increased surface roughness. The grain size according to SEM analysis was in good agreement with the x-ray diffraction data. The optical bandgap of the thin films narrowed with increasing annealing temperature, lying in the range of 3.14 eV to 3.02 eV. The transmission of the thin films was as high as 94% within the visible region. The thickness of the thin films was 400 nm, as measured by ellipsometry, after annealing at the different temperatures of 300°C, 400°C, and 500°C.

Surface modification and chemical sensitivity of sol gel deposited nanocrystalline ZnO films

Sol gel prepared nanocrystalline ZnO thin films are modified by combined usage of complexing agent (diethanolamine or monoethanolamine) and polymer (hydroxypropyl cellulose or ethylcellulose) in the initial sol. X-ray diffraction, Scanning electron microscopy and Atomic force microscopy are applied to get information about the crystallographic structure, microstructure, surface roughness and morphology of the films. It is shown that the unmodified films are smooth (root mean square roughness of 1.6–3.5 nm) with no surface cracks and consist of densely packed nanocrystals. Peaks, pits and aggregates of various density and size are observed on the surface of the modified films and the surface roughness increases up to 39 nm. It is found that the modification effect is determined by the specific combination ‘polymer - complexing agent’ used during the film preparation. The influence of the surface modification on the films' sensitivity to ethanol vapors at room temperature is studied by carrying out resistive measurements. A sensitivity increase of up to ten times is reached when combinations ‘diethanolamine - ethylcellulose’ and ‘monoethanolamine - hydroxypropyl cellulose’ are used causing significant increase of the total contact surface.

Optical and sensing properties of Al doped ZnO nanocrystalline thin films prepared by spray pyrolysis

Nanocrystalline undoped and aluminum doped ZnO films are deposited by spray pyrolysis method. All film samples show polycrystalline nature and hexagonal wurtzite structure of ZnO. Crystallite size decreases with increase in dopant concentration. The lattice constant-a of undoped ZnO thin film is greater than doped samples whereas c-lattice constant is maximum for 3at.% Al doping. Bond length decreases whereas dislocation density increases in doped samples. Doping decreases the transmission of the films and the optical band gap of ZnO is also reduced from 3.28 to 3.18 eV. SEM shows porous spot-like structures which change to web of needle-like structures when dopant is introduced. Surface morphology of the films further evolves to open ends of tubular structures as the doping is increased further. Undoped ZnO shows maximum sensitivity at 400°C for higher concentration of CO2 whereas aluminium doped samples show maximum sensitivity either at 350°C or below for all CO2 concentrations.

Tuning NBE emission and optical band gap of nanocrystalline ZnO thin films using Fe dopant

Tuning of 100meV in optical band gap of nanocrystalline Fe doped ZnO thin films deposited by sol gel spin coating is obtained. Band gap increases monotonically from 3.16 to 3.26eV as dopant is increased. Polycrystalline nature and hexagonal wurtzite structure of undoped and Fe doped films is revealed from XRD. The crystallite size lies between 8 to 15nm. Films are highly transparent and doping increases transmission monotonically. NBE emission occurs at 399 nm which shifts to higher energy with increasing dopant concentration. Defect emission occurs at 486nm. Intensity of the NBE emission is higher for the doped samples and is highest for 2at.% doping. Similar variation in band gap energy and in the NBE emission energy with dopant concentration is obtained.

Enhancement in NBE emission and optical band gap by Al doping in nanocrystalline ZnO thin films

Transparent Al doped ZnO nanocrystalline films with a crystallite size less than 19 nm are obtained by spray pyrolysis. Band gap increases monotonically from 3.16 to 3.31 eV with increasing aluminum dopant up to 1.56 at.% facilitating increasing width of a transmission window in addition to the band gap tuning of 4.74% which compares favorably well with literature. UV emission with continuously increasing intensity is obtained which reflects on the good crystalline quality of the films. Also the defect emissions are suppressed remarkably as the dopant Al concentration increases in ZnO. The band gap tuning by quite small increment in dopant amount makes the present films, much attractive for the fabrication of light emitting devices with a much sought-for benefit of large area fabrication. FESEM shows the surface is granular with grain size lying in the range of 20–35 nm and EDX confirms the presence of Al in the doped samples

High-quality solution-based Co and Cu co-doped ZnO nanocrystalline thin films: Comparison of the effects of air and argon annealing environments

Solution-based high-quality Co and Cu co-doped ZnO thin films that were annealed in air and argon were investigated in terms of their structural, optical and magnetic properties. Compared with argon-annealed thin films, the air-annealed thin films exhibited a single phase ZnO-like wurtzite structure without any secondary phases and higher crystalline quality. Surface analyses revealed denser, smoother and much more homogeneous film surfaces with larger grain size in samples that were sintered in air as compared to those sintered in argon. The incorporation of Co2+ and Cu2+ ions into the ZnO lattice at the Zn2+ site and the presence of Zn, Co, Cu and O atoms were confirmed for both sintering atmospheres. Optical measurements indicated that increased Cu doping levels decreased the band gap energy of the Co-doped ZnO thin films for both the annealing environments. Ultra-violet emission bands at approximately 389–398 nm (i), violet emission bands at approximately 417–424 nm (ii), blue emission bands that are centred at 456 nm (iii) and green emission bands that are centred at 523 nm (iv) were observed. In addition, sharply decreased UV emission and upsurged green emission were detected with increased Cu doping level in Co-doped ZnO, as well as with shifting annealing atmosphere from air to argon. Magnetic measurements indicated that the ferromagnetic response of the Co and Cu co-doped ZnO thin films was stronger in samples that were sintered in argon those than in air due to the enhanced oxygen vacancies and increased Cu doping level of up to 3%.

DMMP Sensing Performance of Undoped and Al Doped Nanocrystalline ZnO Thin Films Prepared by Ultrasonic Atomization and Pyrolysis Method

Highly textured undoped (pure) and Al doped ZnO nanocrystalline thin films prepared by ultrasonic atomization and pyrolysis method are reported in this paper. ZnCl2 water solution was converted into fine mist by ultrasonic atomizer (Gapusol 9001 RBI Meylan, France). The mist was pyrolyzed on the glass substrates in horizontal quartz reactor placed in furnace. The Structural and microstructural properties of the films were characterized by X-Ray Diffractometer (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). FESEM and TEM analysis revealed that the ZnO thin films prepared were porous monodisperse and nanocrystalline in nature, with average particle size of 15 nm. The optical properties of thin films were characterized by UV-Visible and Photoluminescence (PL) spectroscopy. The films were cut in typical size and electrical contactswere made using silver paste and copper wires. The thin film based sensors so prepared were tested on exposing the simulants of chemical warfare agent (CWA) like: DMMP, CEES and CEPS. Sensors showed better response to DMMP (a simulant of sarin gas) as compared with its responses to CEES and CEPS. Al (1 at%) doped ZnO thin film based sensor showed highest response to DMMP (2 ppm). The simulant response, selectivity and response-recovery time of the sensors were measured and presented. The role of Al dopant in ZnO to enhance DMMP response is discussed.

Performance analyses of Schottky diodes with Au/Pd contacts on n-ZnO thin films as UV detectors

In this paper, we report fabrication and performance analyses of UV detectors based on ZnO thin film Schottky diodes with Au and Pd contacts. RF magnetron sputtering technique has been used to deposit the nano-crystalline ZnO thin film, at room temperature. Characterization techniques such as XRD, AFM and SEM provided valuable information related to the micro-structural & optical properties of the thin film. The results show that the prepared thin film has good crystalline orientation and minimal surface roughness, with an optical bandgap of 3.1 eV. I-V and C-V characteristics were evaluated that indicate non-linear behaviour of the diodes with rectification ratios (IF/IR) of 19 and 427, at ± 4 V, for Au/ZnO and Pd/ZnO Schottky diodes, respectively. The fabricated Schottky diodes when exposed to a UV light of 365 nm wavelength, at an applied bias of −2 V, exhibited responsivity of 10.16 and 22.7 A/W, for Au and Pd Schottky contacts, respectively. The Pd based Schottky photo-detectors were found to exhibit better performance with superior values of detectivity and photoconductive gain of 1.95 × 1010 cm Hz0.5/W & 77.18, over those obtained for the Au based detectors which were observed to be 1.23 × 1010 cm Hz0.5/W & 34.5, respectively.