Crystallinity Of ZnO Films Research Articles

Electrodeposition of Simonkolleite as a Low-Temperature Route to Crystalline ZnO Films for Dye-Sensitized Solar Cells

A pulsed electrodeposition procedure has been developed to prepare macroporous films of the semiconducting mineral simonkolleite, Zn5(OH)8Cl2·H2O, on glass/FTO substrates. The morphology is characterized by the presence of smooth, micron-sized hexagonal platelets, and relatively thick films of 6 μm can be prepared in about 15 min. Simonkolleite can be transformed into crystalline, mesoporous ZnO using a relatively low temperature thermal treatment (250 °C). While the macrostructure of hexagonal platelets is maintained, they become mesoporous due to the removal of water and Cl−-containing reaction products. Dye-sensitized solar cells were fabricated with both simonkolleite and ZnO films, using the recently reported fully organic benzothiadiazole-based photosensitizer MG-207, and an electrolyte solution based on either the I−/I3 − or the Co(bpy)3 2+/3+ redox couple. An efficiency of 1.74% has been obtained for ZnO/MG-207/Co(bpy)3 2+/3+ solar cells, illustrating the promise of crystalline, mesoporous ZnO obtained via a low-temperature simonkolleite route for portable and lightweight devices based on flexible substrates.

Oxygen Vacancy Dynamics in Highly Crystalline Zinc Oxide Film Investigated by PIERS Effect.

Surface-enhanced Raman spectroscopy (SERS) is commonly employed as an analysis or detection tool of biological and chemical molecules. Recently, an alternative section of the SERS field has appeared, called photo-induced enhanced Raman spectroscopy (PIERS). This PIERS effect is based on the production of the oxygen vacancies (V) in metal-oxide semiconductor thin-film (or other structures) by irradiation with UV light, thus enabling a Raman signal enhancement of chemical molecules through charge transfer processes between this photo-irradiated semiconductor film (or other structures) and these chemical molecules via metallic nanoparticles deposited on this photo-irradiated substrate. The PIERS technique can enable studying the dynamics of the oxygen vacancies under ambient and operando conditions compared to conventional tools of analysis. In this paper, we present the results obtained on the formation and healing rates of surface oxygen vacancies (V) in a highly crystalline ZnO film investigated by the PIERS effect, and we compare these results to the literature in order to study the effect of the crystallinity on these formation and healing rates of V in a ZnO film.

Building on soft hybrid perovskites: highly oriented metal oxides as electron transport and moisture resistant layers

Poor stability under humid environment is one of the key issues of inorganic–organic hybrid perovskite materials, which limits the transformation of perovskite optoelectronic devices from laboratory to industry. In this paper, a dense ZnO electron transport layer is fabricated directly on top of perovskite films by pulsed laser deposition (PLD) as the first line of defence against the penetration of H2O into perovskite, which shows excellent moisture resistance when compared with conventional organic and oxide nanoparticle counterparts. Smooth, dense and highly oriented crystalline ZnO films are obtained by deposition at near-room-temperature. And importantly, in the meantime, the structure and morphology of underlying perovskite layer are preserved.

Atomic Layer Deposition of ZnO on InP Quantum Dot Films for Charge Separation, Stabilization, and Solar Cell Formation

AbstractTo improve the stability and carrier mobility of quantum dot (QD) optoelectronic devices, encapsulation or pore infilling processes are advantageous. Atomic layer deposition (ALD) is an ideal technique to infill and overcoat QD films, as it provides excellent control over film growth at the sub‐nanometer scale and results in conformal coatings with mild processing conditions. Different thicknesses of crystalline ZnO films deposited on InP QD films are studied with spectrophotometry and time‐resolved microwave conductivity measurements. High carrier mobilities of 4 cm2 (V s)−1 and charge separation between the QDs and ZnO are observed. Furthermore, the results confirm that the stability of QD thin films is strongly improved when the inorganic ALD coating is applied. Finally, proof‐of‐concept photovoltaic devices of InP QD films are demonstrated with an ALD‐grown ZnO electron extraction layer.

Modification of near band edge emission and structure with Ga-related clusters in Ga-doped ZnO nanocrystal films

Transparent conducting Ga-doped ZnO films were grown by ultrasonic spray pyrolysis with the different Ga contents of 1.0–6.5 at. %. The Ga impact on the morphology, crystal structure, photoluminescence (PL), Ga cluster formation, and electrical resistivity of ZnO nanocrystal films has been investigated. ZnO-Ga films are characterized by the hexagonal wurtzite structure with the (002) preferential orientation. It was shown that the PL intensity of near band edge (NBE) emission band A (3.18 eV) enlarges at a Ga doping of 1.0–3.0 at. % together with the decrease in electrical resistivity. Simultaneously, the XRD peaks shift to high values due to the decrease in interplanar distances, and ZnO crystallinity improves. New NBE emission band B (3.08 eV) was detected in the PL spectra of films with the Ga content ≥3.0 at. %. The PL band B was assigned to the optical transitions via Ga-related clusters formed by Ga atoms at higher Ga concentrations. Simultaneously, the 2Θ positions of XRD peaks decrease, owing to the increase in the ZnO crystal lattice parameter, as well as the fall down in the NBE emission intensity and ZnO film crystallinity. To study the Ga ion charge states and Ga cluster formation in the ZnO:Ga films, x-ray photoelectron spectra have been investigated. The optimal Ga concentration in the ZnO films has been estimated.

Highly crystalline ZnO film decorated with gold nanospheres for PIERS chemical sensing.

In this paper, we report on the study of a novel type of substrate based on a highly crystalline ZnO film photo-irradiated using UV for enhancing the Raman signal. This effect is called photo-induced enhanced Raman spectroscopy (PIERS). This PIERS substrate is composed of a photo-irradiated thin ZnO film on which gold nanoparticles are deposited and allows large photo-induced SERS enhancement to be obtained for the chemical detection of small molecules compared to normal SERS signals. This photo-induced SERS enhancement is due to increasing electron density of the gold nanoparticles and charge transfer mechanisms. Here, we achieve a high quality PIERS substrate, the signal of which exhibits weaker fluctuations and a similar or greater gain (up to 7.52) than those reported in the current literature. Henceforth, these PIERS substrates can be of great potential for industrial applications.

In-related complex defects and emission of in-doped ZnO nanocrystal films

ZnO nanocrystal (NC) films grown by ultrasonic spray pyrolysis, doped with the In atoms in different concentrations and annealed at 400 °C for 4 h in nitrogen flow have been studied. The scanning electron microscopy (SEM), X ray diffraction (XRD), photoluminescence (PL) and X-ray photo electronic spectroscopy (XPS) have been used. It was shown that the PL intensity of near band edge (NBE) emission enlarges significantly and the PL intensity of green PL band suppresses at small In contents ≤ 2.0 at%. The later confirms the occupation of zinc vacancies by In ions with the formation of the substitutional InZn donor defects and ZnO crystal quality improving.At higher In contents the new NBE emission band B (3.034eV) appears in PL spectra and its peak shifts to lower energy with In content increasing. The joint analysis of PL and XRD results permits to suppose that the defects responsible for the PL band B are some complexes, which were formed by the coagulation of the substitutional and interstitial In atoms. Simultaneously, the PL intensity and ZnO film crystallinity falling down and the ZnO crystal lattice parameters increase. To study a nature of the In related complex defects, XPS spectra have been investigated for the films with the different In contents. XPS spectrum varying versus In contents has confirmed the complex In defect nature. The dependence of Ini complex defect formation versus In contents in ZnO NC films is analyzed and discussed. The optimal In concentration range to fabricate the ZnO films with high optical parameters has been estimated.

Band-edge emission, defects, morphology and structure of in-doped ZnO nanocrystal films

In-doped ZnO films grown by ultrasonic spray pyrolysis have been studied by means of the scanning electron microscopy (SEM), energy dispersive X ray spectroscopy (EDS) and X ray diffraction (XRD) methods. The photoluminescence (PL), transmittance and absorbance have been controlled as well. It was shown that the ZnO optical band gap demonstrates the blue high energy shift to 3.31 eV at 300 K and the PL intensity of near band edge (NBE) emission enlarges at In doping 0.5–2.5 at%. Simultaneously, the positions of XRD peaks and their intensities vary insignificantly owing to the difference in the In3+and Zn2+ ionic radii. Meanwhile, intensity decreasing the green PL band confirms the occupation of the zinc vacancies by In ions with the formation of substitutional InZn defects and ZnO crystal quality improving.At higher In contents the new PL band (3.034eV) appears in PL spectra and its peak shifts to lower energy with In content increasing. This PL band was attributed to the emission via the complex defects, formed by Ini interstitial atoms. Simultaneously, the PL intensity and ZnO film crystallinity falling down, the ZnO crystal lattice parameters increase and the ZnO optical band gap demonstrates the red low energy shift. To reveal a nature of the optical transition responsible for the new PL band, PL spectra have been studied in the temperature range 11–290 K. The dependence of the Ini complex defect formation versus In contents in ZnO NC films is analyzed and discussed. The optimal In concentration range to fabricate the ZnO films with high optical parameters has been estimated.

Near room temperature atomic layer deposition of ZnO thin films on poly (methyl methacrylate) (PMMA) templates: A study of structure, morphology and photoluminescence of ZnO as an effect of template confinement

ZnO thin films were grown on confined Poly (methyl methacrylate) (PMMA) templates, with high coverage and sharp interface, by atomic layer deposition (ALD) at near room temperature (at 35 °C). Crystalline ZnO films, with c-axis orientation along surface normal, were formed on PMMA templates. With decreasing PMMA template thickness, from 80 nm (5Rg, Rg ∼ 16 nm, the radius of gyration of PMMA) to 32 nm (2Rg) and below (5 nm, 0.3Rg), the photoluminescence (PL) of ALD grown ZnO films changes significantly. Pronounced deep level emission (DLE), corresponding to the oxygen vacancy related defect level (VO++) of ZnO grown on 5Rg PMMA, reduced noticeably for ZnO on 2Rg PMMA and below. Furthermore, in the near-band-edge emission (NBE) of ZnO, the contribution of free-exciton (FX) with respect to its phonon replica (FX-2LO) increased with reduction of PMMA template thickness from 5Rg to 2Rg or below. The observed trend of the photoluminescence spectra indicates significant reduction of surface induced defects in the ZnO nanostructures, formed on the thinner PMMA templates (2Rg and 0.3Rg) and can be ascribed to the formation of smaller ZnO nanoparticles on 5Rg PMMA, which is consistent with the results obtained from structure and surface morphology study.

Preparation of highly visible transparent ZnO/cellophane UV-shielding film by RF magnetron sputtering

A novel UV-shielding film created by sputtering ZnO film onto a renewable and biodegradable cellophane film is proposed. The dependence of the optical properties of the ZnO/cellophane hybrid film on the sputtering power, pressure, and film thickness was studied systematically. A high crystalline quality ZnO film with preferred [002] orientation was prepared on a cellophane substrate for the first time. Unlike conventional UV-shielding films consisting of nanostructures and synthetic polymers, ZnO/cellophane hybrid film can possess high visible light transparency and excellent UV-shielding capacity at the same time. The improved performance can be attributed to the high crystalline quality and the antireflection effect of the ZnO film. A typical hybrid film exhibits a UV-shielding ratio of 96.5% while exhibiting a transmittance of visible light of over 80%. ZnO/cellophane hybrid films can be applied in a vast range of UV-shielding applications, such as sunburn protection and short-wave radiation protection in the laboratory.

Influence of ZnO-Based Sub-Layers on the Growth of Hydrogen Doped Indium Oxide

Because of its high mobility, hydrogen-doped indium oxide (In2O3:H) has a high potential as front contact in thin film or Si heterojunction solar cells. Certain growth conditions are required to process high-mobility In2O3:H. In Cu(In,Ga)Se2 (CIGS) devices, the growth of In2O3:H is influenced by the existing sublayers. Therefore, the intention of the present study is to investigate these influences on In2O3:H, deposited by pulsed direct current magnetron sputtering onto bare glass substrates or Zn(O,S) and ZnO layers, as they are commonly used in CIGS solar cells. The amorphous Zn(O,S) and the crystalline ZnO films were deposited by radio frequency sputtering onto planar glass as well as rough CIGS samples. On the basis of X-ray diffraction and transmission electron microscopy measurements, the structure of the as-grown In2O3:H films was evaluated. X-ray diffraction patterns show amorphous growth on glass and Zn(O,S) and a higher crystallinity of In2O3:H films grown on ZnO layers. The preferred orientatio...

Rhodamine (B) photocatalysis under solar light on high crystalline ZnO films grown by home-made DC sputtering

ZnO thin films were deposited by home-made DC sputtering of zinc target under mixed gases (Argon, Oxygen) plasma on glass substrates. Films were deposited by varying oxygen partial pressure (PO2) from 0.09 to 1.3 mbar in the deposition chamber, at a fixed substrate temperature of 100 °C. The samples were characterized by photoluminescence (PL), X-ray diffraction (XRD), optical transmissions (UV–vis), scanning electron microscopy (SEM) and electrical (Hall effect) measurements. The results indicate that by varying the oxygen pressure in the deposition chamber, the films show a precise and well defined photoluminescence emissions for each range of pressure covering almost the entire visible domain (UV, UV-Violet, Violet, Blue, and Red) with high intensities. Moreover, the deposited films have different defects levels. The XRD analysis indicates that the films are well grown along the c-axis peak, but with different crystalline quality. Optical measurements reveal a high transmission, up to 90%, in the spectral region between 400 and 2500 nm and a large variation of the optical band gap (3.16–4.34 eV). As an application of the deposited ZnO films, the photo-catalytic degradation of a synthetic solution of Rhodamine B (RhB) poured on a ZnO thin film was successfully achieved and an elimination rate of 38% was obtained after exposing the film to solar light for 3 h.

Tuning Excitonic Properties of Pure and Mixed Halide Perovskite Thin Films via Interfacial Engineering

AbstractThe authors explore the potential of ZnO layers of different morphologies, including single crystalline, micro‐structured, and nano‐structured substrates, for tuning exciton binding energy and influencing charge extraction when interfaced with pure (CH3NH3PbI3) and mixed (CH3NH3PbI3−xClx) halide hybrid perovskite (PVSK) thin films. Electron microscopy characterization of the PVSK/ZnO interfaces are correlated with charge transfer properties, probed by means of temperature, power, and time‐resolved photoluminescence (PL) spectroscopy. The results show that at room temperature, the single crystalline ZnO film promotes PL quenching, and reduces recombination lifetime along with exciton density in the PVSK films, all indicative of efficient electron extraction. Nevertheless, the micro‐structured ZnO layers exhibit a mild increase of the PVSK PL at room temperature, and the nano‐structured ZnO enhances PL by up to several thousand‐fold, while simultaneously enhancing recombination rates by 50%. These trends are temperature dependent, and the findings highlight two opposing aspects of how excitonic dissociation in PVSK thin films is affected by the morphology of the underlying ZnO layers. While the single crystalline ZnO can be leveraged as an efficient electron extraction layer for application in photovoltaic devices, the micro‐ and nano‐structured ones offer potential new opportunities for utilization of high quantum yield hybrid perovskites in opto‐electronic platforms.

Fabrication of thin ZnO films with wide-range tuned optical properties by reactive magnetron sputtering

We report the manufacturing of thin zinc oxide films by reactive magnetron sputtering at room temperature, and examine their structural and optical properties. We show that the partial oxygen pressure in DC mode can have dramatic effect on absorption and refractive index (RI) of the films in a broad spectral range. In particular, the change of the oxygen pressure from 7% to 5% can lead to either conventional crystalline ZnO films having low absorption and characteristic descending dependence of RI from 2.4–2.7 RIU in the visible to 1.8–2 RIU in the near-infrared (1600 nm) range, or to untypical films, composed of ZnO nano-crystals embedded into amorphous matrix, exhibiting unexpectedly high absorption in the visible–infrared region and ascending dependence of RI with values varying from 1.5 RIU in the visible to 4 RIU in the IR (1600 nm), respectively. Untypical optical characteristics in the second case are explained by defects in ZnO structure arising due to under-oxidation of ZnO crystals. We also show that the observed defect-related film structure remains stable even after annealing of films under relatively high temperatures (30 min under 450 °C). We assume that both types of films can be of importance for photovoltaic (as contact or active layers, respectively), as well as for chemical or biological sensing, optoelectronics etc.

Fabrication of ZnO-Based Thermoelectric Micro-Devices by Electrodeposition

In order to fabricate micro-thermoelectric devices, ZnO films were prepared via electrodeposition, and the effect of deposition condition on the films was investigated. It was found that morphology and crystallinity of ZnO films were strongly affected by applied potential, bath composition and bath temperature, and using the optimized condition, ZnO-based micro-thermoelectric devices were fabricated and device performance was evaluated. For electrodeposition, acetate ion was added into the bath to prevent several drawbacks such as formation of pits and cracks. From the solution containing zinc nitrate and sodium acetate as a source of acetate ion, ZnO films were electrodeposited. The films with smooth morphology and sufficient adhesion strength were obtained at −0.9 V vs. Ag/AgCl with the addition of 10 mmolL−1 sodium acetate. The films with higher crystallinity and thermoelectric properties were obtained from the bath with higher temperature of 80°C. The micro-thermoelectric device, which consisted of four arrays and each array had 55 pairs of thermoelectric elements, was fabricated and it generated 0.73 nW as maximum power. The power density was approximately 40 times larger than that of the ZnO thin films, which demonstrated that electrodeposited ZnO could be applied to thermoelectric micro-devices.

P-type single-crystalline ZnO films obtained by (Na,N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown by plasma-assisted molecular beam epitaxy technique on c-plane sapphire substrates. The films have been implanted with fixed fluence of 130 keV Na and 90 keV N ions at 460 °C. It is observed that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 1.24 × 1016–1.34 × 1017 cm−3, hole mobilities between 0.65 and 8.37 cm2 V−1 s−1, and resistivities in the range of 53.3–80.7 Ω cm by Hall-effect measurements. There are no other secondary phase appearing, with (0 0 2) (c-plane) orientation after ion implantation as identified by the X-ray diffraction pattern. It is obtained that Na and N ions were successfully implanted and activated as acceptors measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Na and N ions make p-type characteristics excellent as others deposited by traditional techniques. It is concluded that Na and N ion implantation and dynamic annealing are essential in forming p-type single-crystalline ZnO films.

Facilitating epitaxial growth of ZnO films on patterned GaN layers: A solution-concentration-induced successive lateral growth mechanism

The hydrothermal epitaxy of ZnO films on a patterned GaN layer with a honeycomb etching hole array is demonstrated. Through m-planes of the GaN layer exposed on the vertical walls of the etching holes, highly crystalline ZnO films via multiple lateral growth stages can be realized. It is found that higher concentrations of zinc nitrate hexahydrate (ZNH) and hexamethylenetetramine (HMT) in hydrothermal solution yield a larger number of ZnO molecules to speed up ZnO growth during the initial stage of hydrothermal growth, also create secondary crystals and initialize further lateral growth stages to bridge neighboring ZnO prisms after smooth surfaces formed on the m-plane of a ZnO prism. A successive lateral growth mechanism that strongly depends on ZNH and HMT concentrations in the hydrothermal solution is proposed and discussed.

Influence of Silver and Gold Nanoparticles and Thin Layers on Charge Carrier Generation in InGaN/GaN Multiple Quantum Well Structures and Crystalline Zinc Oxide Films

It has been shown that Ag and Au nanoparticles and thin layers influence charge carrier generation in InGaN/GaN multiple quantum well structures and crystalline ZnO films owing to the surface morphology heterogeneity of the semiconductors. When nanoparticles 10 < d < 20 nm in size are applied on InGaN/GaN multiple quantum well structures with surface morphology less nonuniform than that of ZnO films, the radiation intensity has turned out to grow considerably because of a plasmon resonance with the participation of localized plasmons. The application of Ag or Au layers on the surface of the structures strongly attenuates the radiation. When Ag and Au nanoparticles are applied on crystalline ZnO films obtained by rf magnetron sputtering, the radiation intensity in the short-wavelength part of the spectrum increases insignificantly because of their highly heterogeneous surface morphology.

P-type single-crystalline ZnO films obtained by (Li, N) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 30 keV Li and 80 keV N ions at 460 °C. It is shown that dually-implanted single crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.5 × 1015–2.3 × 1016 cm−3, hole mobilities between 0.8 and 1.04 cm2 V−1 s−1, and resistivities in the range of 328.9–2081 Ω cm by Hall-effect measurements. The ZnO film exhibit (0002) (c-plane) orientation with no secondary phase appearing after implantation as identified by the X-ray diffraction pattern. It is confirmed that Li and N ions were effectively implanted and successfully behave as acceptor measured by XPS and SIMS results. Also compared to other similar studies, lower amount of Li and N ions make p-type characteristics excellent. It is concluded that Li and N ion implantation and dynamic annealing play a critical role in forming p-type single-crystalline ZnO films.

Growth of Ga-doped ZnO films by thermal oxidation with gallium and their optical properties

Metal gallium was evaporated onto ZnS films by physical vapor deposition method and then the thermal oxidation in the air was subsequently performed for the growth of Ga-doped ZnO films. The microstructures, photoluminescence (PL) and optical absorption properties of the Ga-doped ZnO films prepared under different deposition and oxidation conditions were investigated. The results showed that certain Ga doping can decrease the defects level, improve the crystallinity of ZnO films, and it became more effective with the extension of oxidation time. As the oxidation time increased, the Ga-doped ZnO films became more compact and uniform, displaying higher crystallinity. In addition, the optical band gaps of the ZnO films increased, the PL intensity of the visible emission decreased, and the luminescent center of the visible emission changed. Among them, the 505 nm emission resulted from oxygen vacancy, while the 539 nm emission was associated with oxygen interstitial.