ZnO Substrates Research Articles

ZnO:Al/ZnS/n-CdTe heterojunctions’ electric and photoelectric properties

The conditions for manufacturing ZnO:Al and ZnS films by RF magnetron sputtering on the surface of crystalline n-CdTe substrates of isotypic ZnO:Al/ZnS/n-CdTe heterostructures with diode properties were studied. The energy parameters of the barrier in the heterostructure are determined and the model of the energy diagram is proposed. A good agreement of the I-V-characteristics of the structure with the space-charge-limited currents theory was established. On the basis of C-V-characteristics studies of the ZnO:Al/ZnS/n-CdTe heterostructure, characteristic features of the SIS structure (semiconductor-insulator-semiconductor) with a dielectric layer made of a ZnS film and a high-resistance CdSxTe1−x solid solution were revealed. The photosensitivity of ZnO:Al/ZnS/n-CdTe heterostructures in the visible radiation range at reverse biases was investigated.

Barrier thickness dependence of the built-in electric field in pseudomorphic ZnO/Zn0.55Mg0.45O multi-quantum wells

The electric polarization of (Zn, Mg)O alloys, known as stable wide-bandgap semiconductors, is an intrinsic property that has a significant impact on the properties and performance of polar ZnO-based optoelectronic devices, such in the case of the quantum-confined Stark effect. Because Wurtzite (WZ)-ZnMgO exhibits a high potential for epitaxial growth with tunable bandgap energy, we fabricated high-quality pseudomorphic ZnMgO thin films on O-polar ZnO substrates and achieved a maximum Mg solubility of 45%. In addition, the high-quality texture and accurate design of ZnO/ZnMgO multi-quantum wells (MQWs) was confirmed. The electric-polarization-induced built-in electric field (Ein) of polar ZnO/ZnMgO MQWs was examined by using a combination of cryogenic photoluminescence measurements and self-consistent Poisson–Schrödinger method. An electric field of 1.9 MV/cm was obtained for a barrier thickness of 13.6 nm. The barrier-thickness-dependent Ein was fitted with an electrostatic model, and a maximum electric field (Einmax) of 2.4 MV/cm was obtained for a 3.4-nm-thick quantum well. This value was perfectly reproduced by using the crystal parameters of the pseudomorphic ZnMgO epilayer. These results are expected to enhance our understanding of WZ-ZnMgO crystallography and provide a method for the systematic design of polar-ZnO-based optoelectronic devices.

λ/20 surface nanostructuring of ZnO by mask-less ultrafast laser processing

Abstract Fabrication of nanostructures with a feature size much smaller than the laser wavelength is challenging due to the optical diffraction limit. It’s well known that the irradiation of polarized ultrafast laser generates periodic nanostructures, so called laser-induced periodic surface structures (LIPSS). Owing to the modulated field, the surface is periodically ablated to form specific patterns, which can be used for some photonic applications including surface-enhanced Raman scattering (SERS). In this paper, we investigate the morphologies of LIPSS on ZnO substrates by mask-less ultrafast laser processing. By adjusting the laser processing parameters, including fluence, pulse number, polarization, and pulse duration, the homogenous nanostrip array and nanopillar array are created. Furthermore, by adjusting the laser fluence, a single nanogroove with a width of ∼20 nm and a single nanocavity with a diameter of ∼24 nm are created. The gold nanoparticles are then coated on the ZnO nanopillar array for SERS application. We found that the concentration of defects in ZnO substrate is increased by the laser irradiation, which is beneficial for SERS performances to achieve an enhancement factor of SERS as high as 2.28 × 107.

Cyclic growth and tillering of layered polycrystalline zinc hydroxide sulfate and the resulting dendritic fractals

Two-dimensional dendritic fractals of layered zinc hydroxide sulfate (LZHS) were allowed to grow on a porous ZnO substrate-coated glass slide in an aqueous ZnSO4 solution through the dissolution-redeposition process. The generated LZHS seeds with polycrystalline structure gradually evolved into two-dimensional dendritic fractals with a diameter of 50∼100 μm through a plant-like growth mode involving tillering and the formation of layered tiller buds at branch ends. Such tiller mechanism is proposed to explain the evolution of LZHS fractals based on results obtained by the scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and energy dispersive X-ray detection. The synthesized polycrystalline LZHS fractals can be topologically converted into hierarchical nanostructured zinc oxide by thermal pyrolysis combined with an alkaline wash process. After the modification by octyltrimethoxysilane, the topological zinc oxide surfaces, corresponding to the immersion time of 2, 4 and 6 h, showed superhydrophobic property with a water contact angle of 153.4°, 157.6°, and 164.7°, respectively. The enhanced hydrophobicity of zinc oxide films compared with the porous ZnO substrate surface is mainly attributed to the topological fractal morphologies and hierarchical nanostructures.

Atomic and electronic structures of the interface and twin boundaries in epitaxial CuO films on a ZnO substrate

Atomic and electronic structures of the interface and twin boundaries in epitaxial CuO films on a ZnO substrate

Theoretical investigations of functionalization of graphene and ZnO monolayers with mercaptopurine at aqueous media: A dispersion-corrected DFT calculations and molecular dynamic simulations

This study presents a theoretical investigation of mercaptopurine adsorption on graphene and ZnO monolayer using first-principles simulations with dispersion-corrected density functional theory (DFT). Two different interaction orientations for interacting mercaptopurine with the surface were evaluated and the energetically most stable configuration was obtained with −1.412 eV. The ZnO surface adsorbs mercaptopurine stronger than graphene. The electronic structure analysis indicates that the adsorption of molecule reduces the band gap of both considered semiconductor substrates while their semiconducting property still remains unchanged. Furthermore, charge analyses based on the Hirshfeld method revealed that around 0.213 e was transferred from the ZnO substrate to the mercaptopurine molecule. Density functional tight binding based molecular dynamics (DFTB-MD) simulations show that mercaptopurine tends to be adsorbed on both monolayers while moving from the water aqueous media toward the surface at room temperature. The DFT-based atoms-in-molecules (AIM) analysis predicted that themercaptopurine-graphene interaction nature is typical for the noncovalent type π-π interactions while mercaptopurine chemisorbed on the ZnO surface by chemical bonding between N/S and Zn atoms. The present results provide atomic insights on the interaction properties of mercaptopurine drug with graphene and ZnO nanosheets and supply new manner and worth data to assist the experiments and potential applications in thebio and nanotechnology area.

Dependency of Ag wetting on the oxygen nonstoichiometry of oxide surfaces

Oxide substrates are frequently modified to have either an oxygen deficiency or excess to synthesize a completely continuous, ultrathin metal layer on them by strengthening the metal–oxide adhesion. However, conventional surface treatments using ionic irradiation inevitably induce detrimental structural irregularities, along with chemical modification, on the surface of oxide substrates. This study explores the dependency of Ag wetting on the oxygen deficiency and excess on the surface of a ZnO substrate. The favorable contribution of the increased oxygen deficiency and excess of ZnO surfaces in improving Ag wetting during the inceptive growth stages is evidenced by both experimental results and numerical interpretations. This improved Ag wetting is attributed to a significant reduction in free energy at the Ag–ZnO interfaces, whereas pinholes remain in thicker Ag layers on the ZnO surfaces. This finding extends earlier studies, reporting a favorable role of an increased oxygen deficiency on Ag wetting, with the point that oxygen excess beyond a threshold value is also helpful in improving Ag wetting. Our findings redefine the underlying Ag wetting mechanisms of the nonstoichiometric surface of oxide substrates. Moreover, the study provides guidelines for implementing a continuous ultrathin Ag layer while avoiding ion-induced damage to the oxide substrates.

Enhanced optoelectrical performance of ultraviolet detectors based on self-assembled porous zinc oxide nanostructures

A templated self-assembly technique was utilized in the present study to grow porous zinc oxide nanostructures. The nanostructures were formed by the electrochemical deposition of ZnO through the interstitial spaces between polymer microsphere templates. After the deposition, polymer microspheres were removed by dissolving in chloroform solvent, leaving porous ZnO nanostructures. This technique is benefited from facile controllability of the pore morphology and size by varying the diameter of microspheres. X-ray diffraction analysis showed a dominant peak corresponding to the hexagonal ZnO structure. Moreover, no significant structural strain was observed after the removal of spheres unlike the other synthesis methods of porous materials. The improved photoluminescence (PL) properties revealed an enhancement in the light capturing capability of the systems due to the multiple scattering of light in the pore walls. The porous sample showed a PL blue-shift compared to the flat one, indicating a reduction in crystallite size of ZnO nanostructures. To assess the photonic applications of synthesized porous ZnO substrates, a metal-semiconductor-metal photodetector was developed via the metallization of ZnO nanostructures, and their optoelectrical properties were tested under UV radiation. The results showed an improvement in photosensitivity and quantum efficiency of devices based on porous ZnO substrates which can be assigned to the larger exposed area and elevated rates of electron-hole generation in this sample.

Epitaxy and bonding of peelable ZnO film on graphene/ZnO substrate

High-quality flexible ZnO films are commonly used to prepare ZnO-based flexible devices. However, the existing methods for preparing flexible films have some bottlenecks, such as poor quality, high cost, and complex steps in the flexible production process. The present study presents results on peelable ZnO film grown on graphene/ZnO substrate by plasma-assisted molecular beam epitaxy (PAMBE). Double-layer graphene (DLG) can be used as a release layer to convert the contact between ZnO film and substrate from chemical bonding to van der Waals force interactions. Hence, the ZnO film grown on graphene/ZnO substrate can be peeled off by a thermal release tape. The O-plasma irradiation introduced many dangling bonds on the surface of graphene. These dangling bonds combined with Zn atoms to form COZn bonds in the nucleation of the ZnO film. This study proposes a bonding mechanism of ZnO on graphene, verified by experiments. The flexible ZnO film obtained by this method provides a simple and feasible way for potential applications in ZnO-based flexible optoelectronic devices.

Substrate dependent structural variations of biomimetic carbonated hydroxyapatite deposited on glass, Ti and sputtered ZnO thin films

The formation of hydroxyapatite films on glass, titanium (Ti), and sputtered ZnO is attempted to understand the crystal growth and its manipulation by substrates of a different kind. The substrates used are commercially available glass and polished Ti (99.9% purity), as-deposited, and annealed nanostructured ZnO thin films developed by pulsed DC magnetron sputtering, which are immersed in the calcifying solution for the nucleation and growth of the hydroxyapatite. The calcifying solution is obtained by mixing equal volumes of aqueous calcium (5 mM) and carbonate-containing phosphate (3 mM) solutions prepared at 37 °C and with a pH of ~7.4. X-ray diffraction, Raman spectroscopy, and Field Emission Scanning & Transmission Electron Microscopy (FESEM & FETEM) analyses reveal that the structure and morphology of the hydroxyapatite films are dependent on the growth duration, substrate type, and even on the post-deposition annealing of the sputtered ZnO substrates. The Ti substrate induces greater crystallinity in hydroxyapatite, whereas sputtered ZnO thin films cause a significantly reduced crystallinity. Moreover, the thicknesses of HAp are exacerbated by increasing deposition time irrespective of the substrate. FETEM analyses reveal the formation of needle-shaped hydroxyapatite nanocrystals grown on Ti and ZnO thin films. Thicker needle-shaped hydroxyapatite nanocrystals are observed when grown on Ti (~6–12 nm) than ZnO thin films (~4–6 nm). Further, the amorphous calcium phosphate phase coexists with nanocrystalline hydroxyapatite when ZnO thin films deposited at 50 and 70 W sputtering powers are used as the substrates. Both charged surfaces and the supersaturation of the calcifying solution impact the nucleation of hydroxyapatite onto substrates.

Design of Improved Acetone Gas Sensors Based on ZnO Doped rGO Nanosheets

To meet future social and environmental objectives for diagnosis of human diseases has offered to develop the proficient gas sensors devices with higher selectivity and sensitivity. For the production of ZnO doped reduced graphene oxide (ZnO doped rGO) nanocomposite, a one-pot hydrothermal approach. The morphological, structural and composition of nanocomposite were investigated to confirm ZnO nanoparticle effectively doped on rGO nanosheets. The nanocomposite has exhibited a superior acetone sensing characteristics. Furthermore, the nanocomposite has a high selectivity for acetone vapour. These findings emphasise advantageous synergistic effects among ZnO and excellent rGO sheet substrate properties.

Polarization-Induced Two-Dimensional electron gas at BeO/ZnO interface

Beryllium oxide (BeO) is a unique metal oxide with excellent thermal conductivity and dielectric strength. BeO tends to grow as wurtzite single crystals via atomic layer deposition, leading to the strong polarization of heterostructures with various substrates. We demonstrated the formation of a polarization-induced two-dimensional electron gas (2DEG) at a BeO/ZnO heterostructure interface. The polarity discontinuity induced by the c-axis-grown crystalline BeO film caused charges to accumulate on the ZnO substrate. The sheet carrier concentration and mobility of the BeO/ZnO heterostructure were 2.0 × 1014 cm−2 and 22 cm2·V−1·s−1 at room temperature, respectively, approximately 57 times and 11 times greater than those of bare ZnO, respectively. In addition, the carrier concentration was nearly constant over the temperature range of 150 K – 350 K. The 2DEG layers formed via ALD BeO films are possible channel materials for use in various electronic devices such as thin film transistors.

Application of X-ray diffraction and reflectometry methods for analysis of damaged layers on polar faces of ZnO after surface chemical-mechanical treatment

ZnO single crystals are used for the fabrication of laser targets for high-energy electron irradiated UV laser cathode-ray tubes and homoepitaxial substrates for lasers. The technology of ZnO based UV LEDs imposes strict requirements to surface quality. Chemical-mechanical polishing delivers good surface quality but it is known that polishing of ZnO polar faces may yield different results. Surface-sensitive high-resolution X-ray diffraction (HRXRD) and X-ray reflectometry (XRR) methods have been used for studying the structure of (0001) and (000–1) polar faces of ZnO after chemical-mechanical polishing. Two double-sided polished (0001) ZnO substrates have been cut out from different hydrothermally grown ingots. The damage and density depth profiles for the Zn and O faces of the specimens have been retrieved from the X-ray diffraction curves and the specular reflection curves, respectively. Intensity distributions in the vicinity of the [0002] and [0000] reciprocal lattice sites have been taken on a D8 Discover X-ray diffractometer (Bruker-AXS, Germany) in a triple-crystal setup. For separating the coherent and incoherent scattering components, the intensity profiles have been analyzed along sections perpendicular to the diffraction vector and located at different distances from the reciprocal lattice sites. The HRXRD and XRR data have been compared with atomic force microscopy (AFM) data. The HRXRD method has revealed damaged layers at both faces of the specimens, with the layer thicknesses differing for the Zn and O faces, i.e., 5–7 nm for the Zn face and 10–11 nm for the O face. The XRR method has shown that both faces are sufficiently smooth. These results have been confirmed by AFM (RMS roughness ~ 0.23 ± 0.07 nm). However, the concentration of electrons in the superficial layers has been found to change. The layer thickness proves to be greater for the O face. We have hypothesized that the phenomena observed are caused by the difference in the chemical interaction of the Zn and O faces with the polishing agents.

Application of X-ray diffraction and reflectometry methods for analysis of damaged layers on polar faces of ZnO after surface chemical-mechanical treatment

ZnO single crystals are used for the fabrication of laser targets for high-energy electron irradiated UV laser cathode-ray tubes and homoepitaxial substrates for lasers. The technology of ZnO based UV LEDs imposes strict requirements to surface quality. Chemical-mechanical polishing delivers good surface quality but it is known that polishing of ZnO polar faces may yield different results. Surface-sensitive high-resolution X-ray diffraction (HRXRD) and X-ray reflectometry (XRR) methods have been used for studying the structure of (0001) and (000–1) polar faces of ZnO after chemical-mechanical polishing. Two double-sided polished (0001) ZnO substrates have been cut out from different hydrothermally grown ingots. The damage and density depth profiles for the Zn and O faces of the specimens have been retrieved from the X-ray diffraction curves and the specular reflection curves, respectively. Intensity distributions in the vicinity of the [0002] and [0000] reciprocal lattice sites have been taken on a D8 Discover X-ray diffractometer (Bruker-AXS, Germany) in a triple-crystal setup. For separating the coherent and incoherent scattering components, the intensity profiles have been analyzed along sections perpendicular to the diffraction vector and located at different distances from the reciprocal lattice sites. The HRXRD and XRR data have been compared with atomic force microscopy (AFM) data. The HRXRD method has revealed damaged layers at both faces of the specimens, with the layer thicknesses differing for the Zn and O faces, i.e., 5–7 nm for the Zn face and 10–11 nm for the O face. The XRR method has shown that both faces are sufficiently smooth. These results have been confirmed by AFM (RMS roughness ~ 0.23 ± 0.07 nm). However, the concentration of electrons in the superficial layers has been found to change. The layer thickness proves to be greater for the O face. We have hypothesized that the phenomena observed are caused by the difference in the chemical interaction of the Zn and O faces with the polishing agents.

Investigation of optical, structural, and electrical properties of heterostructure Fe2O3 deposited by RF magnetron sputtering on ZnO layer by spray pyrolysis

In this study, firstly, ZnO thin film was produced on glass substrate by chemical spray pyrolysis method at 450 °C and then Fe2O3 layer was produced on ZnO substrate thin film by RF magnetron sputtering technique. Fe2O3/ZnO heterostructure was fabricated. The crystal structure, surface morphology and structure, chemical composition, optical and electronic properties, and electric properties of the ZnO and Fe2O3/ZnO samples were performed by X-ray diffraction (XRD), Raman spectrometer, field emission scanning electron microscope (SEM), focused ion beam (FIB) microscope, atomic force microscope (AFM), energy-dispersive X-ray (EDX), ultraviolet–visible spectrophotometer (UV–VIS), and Hall measurements, respectively. XRD and Raman measurements showed that ZnO and Fe2O3 thin films have wurtzite hexagonal and cubic crystal structures, respectively, and both are in polycrystalline form. FE-SEM and cross-sectional FIB-SEM images indicate that Fe2O3 flake-like nano-sized structures are formed on the ZnO substrate. In addition, AFM images revealed that the Fe2O3 nano-sized structure was distributed as nanosheets on the ZnO thin-film surface. UV–VIS measurements of ZnO revealed that the ZnO and Fe2O3/ZnO heterostructure band gap energies were 3.277 eV and 3.24 eV, respectively. Finally, the calculated values of electric conductivity, σ, electron density, n, and electron mobility, μ, using the data obtained from Hall measurements are 4.39 × 102 Ω−1 m−1, 6.88 × 1021 m−3, and 3.99 × 10–1 V−1m2s, respectively.

In Situ Transformation of ZIF-8 into Porous Overlayer on Ru/ZnO for Enhanced Hydrogenation Catalysis.

Supported metal catalysts play a significant role in heterogeneous catalysis in liquid phase reaction systems, but they usually suffer from a stability problem. Encapsulation of active metal species without the compromise of catalytic performance has been considered as an effective strategy. Here, we report an ultrastable Ru-based catalyst with particle size of around 1.1 nm for selective hydrogenation reaction. The highly dispersed Ru species are covered by the in situ formed porous N-C-ZnO overlayer, which is induced through the transforming of ZIF-8 shell that derives from a ZnO substrate. The resulting Ru/ZnO@N-C-ZnO catalyst can exhibit good stability in the hydrogenation of p-chloronitrobenzene after 20 cyclic runs with 100% selectivity toward p-chloroaniline. Comparatively, the naked Ru/ZnO catalyst with larger Ru particles shows serious metal leaching issue with inferior stability and poor selectivity. It is revealed that the excellent performance of Ru/ZnO@N-C-ZnO is attributed to the porous overlayer, which strengthens the bonding of Ru nanoparticles on ZnO.

Strategy for Fabricating Ultrathin Au Film Electrodes with Ultralow Optoelectrical Losses and High Stability.

A vital objective in the wetting of Au deposited on chemically heterogeneous oxides is to synthesize a completely continuous, highly crystalline, ultrathin-layered geometry with minimized electrical and optical losses. However, no effective solution has been proposed for synthesizing an ideal Au-layered structure. This study presents evidence for the effectiveness of atomic oxygen-mediated growth of such an ideal Au layer by improving Au wetting on ZnO substrates with a substantial reduction in free energy. The unexpected outcome of the atomic oxygen-mediated Au growth can be attributed to the unconventional segregation and incorporation of atomic oxygen along the outermost boundaries of Au nanostructures evolving in the clustering and layering stages. Moreover, the experimental and numerical investigations revealed the spontaneous migration of atomic oxygen from an interstitial oxygen surplus ZnO bulk to the Au-ZnO interface, as well as the segregation (float-out) of the atomic oxygen toward the top Au surfaces. Thus, the implementation of a 4-nm-thick, two-dimensional, quasi-single-crystalline Au layer with a nearly complete crystalline realignment at a mild temperature (570 K) enabled exceptional optoelectrical performance with record-low resistivity (<7.5 × 10-8 Ω·m) and minimal optical loss (∼3.5%) at a wavelength of 700 nm.

Influence of the reaction between water and ZnO on the properties of Au Schottky contacts and the photocatalytic activity of ZnO(0001) single crystals

Wet chemical treatment is a conventional surface cleaning method, and metal oxide photocatalysts are commonly used to decompose organic compounds in water. In this study, we have investigated the influence of the reaction between water and ZnO on the properties of Au Schottky contacts and the photocatalytic activity of Zn-face ZnO(0001) single crystals. The ZnO substrate was put in deionized water at temperatures between 40 °C and 90 °C (water treatment). The ZnO substrate was etched with deionized water, and the surface roughness increased with increasing water temperature. Although the water treatment had no significant influence on the surface composition of the ZnO substrate, it changed the properties of the Au contacts. Schottky contacts were formed on the as-received ZnO substrate and the ZnO substrate after the water treatment at 40 °C, whereas ohmic contacts were formed on the ZnO substrate after the water treatment at 90 °C. Photoelectron emission spectra showed that the surface Fermi level in the ZnO substrate after the water treatment at 90 °C was located just below the conduction band. However, the water treatment had no significant influence on the photocatalytic activity of the ZnO substrate. Even when the water treatment was performed at 90 °C, the surface Fermi level of the ZnO substrate in an electrolyte solution, which was estimated from photoelectrochemical measurements, was located about 0.4 eV below the conduction band.

Structural analysis of the ZnO/MgO superlattices on a-polar ZnO substrates grown by MBE

The analysis of heteroepitaxially grown ZnO/MgO superlattices on a-oriented ZnO substrates was described. The influence of unalike natural crystalline structures of ZnO and MgO on the interface’s abruptness and retaining wurtzite crystallinity was studied. Wurtzite structure was found to be retained in the MgO thin barriers up to 1.5 nm, but creation of nonuniform biaxial strains and some interdiffusion of Mg from barriers to ZnO QW’s seemed to be inevitable. The complex strain/composition heterogeneities can be explained by the difference of vapor pressure between Mg (p = 1.93 Pa) and Zn (p = 0.22 Pa) elements at growth temperatures and high mobility of Mg atoms, which are gladly incorporated into ZnO matrix. As a result, a subtle three monolayers in size ZnMgO film was created at the interfaces, which was partly responsible for retaining the wurtzite structure of coherently grown MgO layers.

Effects of Layer Thickness on the Residual Stresses of CIGS Solar Cells with Polyimide Substrate

In this paper, we investigate the effect of layer thickness on the residual stresses of copper indium gallium diselenide (CIGS) solar cells with polyimide substrate caused by CIGS layer deposition at 400?C and then cooling down to room temperature using the Finite Element Method (FEM). Moreover, we also examined the effect of layer thickness on residual stress of CIGS solar cells after cooling down to room temperature from the hotspot temperatures of 200?C, 300?C, and 400?C. Our simulated CIGS is composed of five layers: ZnO, CdS, CIGS, Mo, and PI substrate. We were able to quantify the effect of each layer’s thickness and hotspot temperature on the average stresses of each layer for the CIGS solar cells. We found that the PI substrate layer has the most significant effect on the residual stress of CIGS solar cells. Our simulation results reveal that the stress type (tensile vs. compressive) and the magnitude of stress of the CIGS layer (main absorber layer) can be controlled by changing the thickness of the PI substrate while applying a heat to CIGS solar cells. Quantitative analysis of relationship between layer thickness and thermo-mechanical stress of thin film solar cells can help solar cell manufacturers design more robust and reliable solar cells. For example, fabricating PI layer thickness less than 17 μm can improve the performance of CIGS solar cells by nullifying the compressive residual stress in the CIGS absorber layer.