ZnO Substrates Research Articles

The electric-field and strain inducing electronic and optical properties of the blue phosphorene/ZnO heterostructures

Abstract The effect of ZnO substrate on the structural and electronic properties of blue phosphorene were systematically investigated by first-principles calculations. The results show the band gaps of heterostructures are narrow than pure states and can be tuned sensitive by electric field. Zn pattern heterostructure is dynamically stable. In addition, EΓ and band gap can be tuned flexible which is reducing as strain increase and the structures turn from semiconductor to metal by compressive strain increasing, the optical properties indicate the optical absorption peak of the material is in the visible and ultraviolet region.

Long‐Range Ordered Crystals of 3D Inorganic–Organic Heterojunctions via Colloidal Lithography

AbstractColloidal lithography (CL) has evolved as an alternative to conventional photo‐ and electron‐beam lithography to pattern surfaces with nanometer range resolution. As CL offers substrate‐independent precise positioning and patterning of nanomaterials as long‐range ordered crystals, this has seen new opportunities in optoelectronics. Herein, the scope of CL is expanded to fabricate for the first time, 3D organic–inorganic heterojunction photocatalysts with well‐controlled spacing and coverage density. To achieve this, monodisperse polystyrene (PS) beads of different sizes are used as colloidal masks on a ZnO substrate. Electron beam assisted silver deposition onto these PS masks, and subsequent removal of PS lead to the formation of patterns of silver nanostars on the ZnO thin film. The solid–vapor reaction of silver nanostars with a metal‐coordinating charge‐transfer complex of 7,7,8,8‐tetracyanoquinodimethane (TCNQ) allows spontaneous conversion of Ag nanostars to the large aspect ratio nanowires of metal–organic AgTCNQ semiconductors. This strategy, combining the strengths of CL with high electron affinity of TCNQ molecules allows facile fabrication of long‐range patterns of the heterojunctions of organic (AgTCNQ) and inorganic (ZnO) semiconductors. These surface‐supported 3D heterojunctions act as outstanding photocatalysts through their ability to efficiently separate the electron–hole pairs and thus increasing the electron–hole life times.

Ultraviolet luminescence enhancement of planar wide bandgap semiconductor film by a hybrid microsphere cavity/dual metallic nanoparticles sandwich structure.

Here we report a novel hybrid structure composing of microsphere array (MA), Al nanoparticles (Al-NPs), ZnO thin film (luminescence layer), Au nanoparticles (Au-NPs), and substrate (sapphire) for ultraviolet (UV) luminescence enhancement of planar wide bandgap semiconductor film. The plasmonic sandwich structure of Al-NPs/ZnO/Au-NPs boosts the hot electron state density in the conduction band by electron trapping from deep-defect level of ZnO and localized surface plasmon resonances (LSPRs) coupling around dual metallic NPs, enhancing UV emission and suppressing visible emission efficiently. The dielectric microsphere array capping on the plasmonic sandwich structure further increases UV emission intensity via photonic nanojets, optical whispering-gallery modes (WGMs), and directional antenna effect, by which the interaction between photon and exciton is strengthened. The contribution of microsphere cavity coupling with LSPRs to UV luminescence enhancement is therefore revealed. The maximum enhancement ratio of up to two orders of magnitude (~250-fold) is achieved by the optimized 5.06-μm-diameter-MA/Al-NPs/ZnO/Au-NPs/sapphire structure and the UV emission is highly directional with a divergent angle of ~5°. The present work provides a simple and easily-prepared structure incorporating optical WGMs and LSPRs to manipulate UV luminescence of planar wide-bandgap semiconductors for potential optoelectronic applications.

Remote Epitaxy of ZnO Microrods on Graphene-Coated Substrates

Two-dimensional nanomaterials are emerging for use as seed substrates toward transferrable, transparent, or flexible device applications.[1,2] In the van der Waals heteroepitaxy, graphene acted as seed for epitaxial overlayers of III–V or II–VI nanostructures or thin films.[3-5] Because the hydrophobic surface of graphene, given from lack of dangling bonds, substantially hinders the high-density nucleation in heteroepitaxy,[3] the use of graphene substrates was precluded for practical use as a substrate in the semiconductor epitaxy. Such surface properties of graphene have been known to be changeable by contacting other substances underneath graphene layer, because of the wetting transparency of graphene. More recently, the chemical property transparency was applied to the epitaxy technique to obtain single crystalline thin film overlayer whose crystal structure and orientation can be copied from underlying substrate across graphene. This is the so-called remote epitaxy, which was firstly demonstrated by Kim et al.[6,7] Here, we report on remote homoepitaxy of ZnO microrods on graphene-coated ZnO substrates with different a- and c-planes. The remote epitaxy was carried out via a hydrothermal growth using a nutrient solution with zinc nitrate hexahydrate and hexamethylenetetramine at 95 °C. For preparation of a- and c-plane ZnO substrate, ZnO thin films were heteroepitaxially coated on r-plane Al2O3 and c-plane GaN, respectively. Then, monolayer graphene, which was synthesized via a chemical vapor deposition (CVD) on copper foil, was transferred onto these substrates using a poly(methyl-methacrylate)-supported etching-and-transfer technique. ZnO epitaxial overlayers grew to be horizontally and vertically aligned microrod arrays on a-plane and c-plane ZnO substrates, respectively, across graphene layers, which were also found to show a well-defined long-range in-plane orientation over the entire surface of graphene-coated ZnO substrate, despite the presence of CVD-grown graphene with typical domain size of several micrometers. Cross-sectional transmission electron microscopic and selected area electron diffraction analyses revealed the homoepitaxial relationship with a remote gap of graphene. We further studied the successful remote homoepitaxy of ZnO across bilayer- and trilayer graphene, on which lower density of horizontal- and vertical ZnO microrods were obtained in comparison with that on monolayer graphene-coated substrate. This result strongly suggests that ZnO substrates played a crucial role in determining the crystallographic orientation and lattice arrangement of ZnO microrod overlayer across graphene layer, but as increasing the thickness of graphene, the impact of substrate is lesser for nucleation and growth of ZnO overlayer in the remote homoepitaxy. Such finding that electric field given from the substrate can be penetrated across graphene was simulated via density functional theory (DFT) calculations. The DFT calculations also indicated that the field penetration strength decreases with the increase of graphene thickness. The ability of graphene, which can be released from the host substrate without covalent bonds, was exploited to transfer the microrod arrays via a thermal release tape exfoliation technique. After the exfoliation, the host substrate was reused for repeating the remote epitaxy over again. This study opens a way for producing well aligned, transferrable epitaxial semiconductor microrod arrays while regenerating the substrate for cost-saving device manufacturing process. [1] K. Chung, C. H. Lee and G. C. Yi, Science, 2010, 330, 655–657. [2] C. H. Lee, Y. J. Kim, Y. J. Hong, S. R. Jeon, S. Bae, B. H. Hong, G. C. Yi, Adv. Mater., 2011, 23, 4614–4619. [3] Y. J. Hong, T. Fukui, ACS Nano, 2011, 5, 7576–7584. [4] A. M. Munshi, D. L. Dheeraj, V. T. Fauske, D. C. Kim, A. T. J. van Helvoort, B. O. Fimland, H. Weman, Nano Lett., 2012, 12, 4570–4576. [5] Y. J. Hong, J. W. Yang, W. H. Lee, R. S. Ruoff, K. S. Kim, T. Fukui, Adv. Mater., 2013, 25, 6847–6853. [6] Y. Kim, S. S. Cruz, K. Lee et al., Nature, 2017, 544, 340–343. [7] W. Kong, H. S. Li, K. Qiao et al., Nat. Mater., 2018, 17, 999–1004.

Growth mechanism of Sb2Se3 thin films for photovoltaic application by vapor transport deposition

Sb2Se3 is a rapidly advancing sustainable thin film solar cell technology demonstrating 9.2% conversion efficiency recently. Because of its unique quasi-1D crystal structure, the orientation of absorbers plays a key role in charged carrier transport and therefore in solar cell performance. However, deeper understanding how deposition conditions affect film orientation and ability to control it are missing. Herein using vapor transport deposition (VTD) method we studied growth process of Sb2Se3 on Mo, MoSe2, CdS, ZnO and TiO2 substrates. We varied the temperature of substrate and source to imitate various growth conditions. We have found three distinct growth regimes at which films exhibit characteristic features. Compact and large grained morphology with explicit (hkl, l≠0) orientation was achieved when high growth rate regime governed by evolution selection principle was induced on all substrates. Proposed growth model takes into account the influence of substrate and provides directions to obtain Sb2Se3 films with desired morphology and orientation. In conclusion, we have compared morphological properties of Sb2Se3 films synthesized by various evaporation based methods in literature with the properties predicted by the model. A fair agreement was found, proposing that this model can serve as a guideline for Sb2Se3 thin film optimization using evaporation based methods.

ZnO/(Zn)MgO polar and nonpolar superlattices

The bandgaps of short period ZnO/(Zn)MgO superlattices deposited on c-, m-, and a-ZnO substrates were examined both theoretically and experimentally. Ab initio calculations showed that the bandgaps of c-oriented polar superlattices are smaller than those of nonpolar ones; however, this is mainly due to different geometric configurations, because the influence of internal electric fields existing in polar superlattices is not very significant. The calculations revealed that for 5–6 MgO monolayers in the barriers, the bandgap values become independent of the barrier thickness, which suggests that such superlattices can be treated as sets of isolated ZnO wells. In the experimental part of this work, it is demonstrated that short period ZnO/MgO and ZnO/ZnMgO superlattices can be grown successfully on differently oriented crystalline bulk ZnO substrates using molecular beam epitaxy. The bandgaps of the superlattices were determined from low temperature photoluminescence measurements. It is shown that they agree well with the theoretical results.

Shape-controlled synthesis of bare ZnO nanomaterials with improved photoactivity and photostability

ABSTRACTUnderstanding the growth mechanisms of nanomaterials usually leads to the successful preparation of targeted micro-structures with excellent properties. In this paper, flower-like and roughly spherical ZnO nanocrystals have been successfully synthesised through facile wet chemical methods, i.e. hydrothermal pathways and homogeneous precipitation method. The photocatalytic activities of the prepared photocatalysts were evaluated by degradation of rhodamine B (RhB) and phenol under ultraviolet (UV) irradiation. Remarkably, the ZnO powder via homogeneous precipitation (urea/Zn2+ = 2.0) exhibited better photocatalytic performance and photostability than those of flower-like ZnO substrate. The enhanced photocatalytic properties could be attributed to more active catalytic sites and effective separation of carriers, which were confirmed by low-temperature nitrogen adsorption, photocurrent responses, and photoluminescence spectral analysis. A possible mechanism for the excellent photocatalytic activity of ZnO substrates was proposed.

Determination on Origination of Surface Enhanced Raman Scattering Effect on Nano ZnO Substrate

Determination on Origination of Surface Enhanced Raman Scattering Effect on Nano ZnO Substrate

Epitaxial growth of InN thin films by plasma-enhanced atomic layer deposition

In this study, we report on the growth of crystalline InN thin films by plasma-enhanced atomic layer deposition (PE-ALD). By systematically investigating the growth parameters, we determined the process window for crystalline InN films growth by PE-ALD. Under the optimal conditions, we compared Si (100), Al2O3 (0001), and ZnO (0001) substrates with different lattice mismatches to InN. High resolution X-ray diffraction (HR-XRD) measurements showed that we obtained the epitaxial InN thin film on the ZnO (0001) substrate, which has the smallest lattice mismatch. The crystal orientation relationship is found to be InN[0001]∥ZnO[0001] and InN[10−10]∥ZnO[10−10]. HR-XRD and high resolution transmission electron microscopy measurements revealed that the InN films are fully relaxed, and no voids or interlayer is observed at the interface. Our results show that the epitaxial growth of the InN film can be obtained by PE-ALD at low temperature.

Short infrared wavelength quantum cascade detectors based on m-plane ZnO/ZnMgO quantum wells

This paper reports on the demonstration of quantum cascade detectors (QCDs) based on ZnO/ZnMgO quantum wells (QWs) grown by molecular beam epitaxy on an m-plane ZnO substrate. The TM-polarized intersubband absorption is peaked at a 3 μm wavelength. The sample has been processed in the form of square mesas with sizes ranging from 10 × 10 μm2 up to 100 × 100 μm2. The I-V characteristics reveal that 86% of the 260 devices are operational and that the surface leakage current is negligible at room temperature, which is not the case at 77 K. The photocurrent spectroscopy of 100 × 100 μm2 QCDs reveals a photocurrent resonance at a 2.8 μm wavelength, i.e., slightly blue-shifted with respect to the intersubband absorption peak. The photocurrent persists up to room temperature. The calibrated peak responsivity amounts to 0.15 mA/W under irradiation at Brewster's angle of incidence. This value allows us to estimate the transfer efficiency (1.15%) of the photoexcited electrons into the active QW of the next period.

Optically rough and physically flat TCO substrate formed by coating ZnO thin film on pyramid-patterned glass substrate

An optically rough and physically flat (OR-PF) TCO substrate was developed by spin-coating Al-doped ZnO thin film on glass substrate patterned with random pyramidal morphology to enhance the light absorption in superstrate-type thin-film silicon solar cells without deteriorating the growth of photoactive layers. This substrate possesses flat surface with a root mean square roughness of 10.9 nm, high visible light transmittance of over 86%, a sheet resistance of 29.5 Ω/sq, and moderate light-scattering ability. A surface morphology of random pyramidal morphology was formed on the glass substrate by replicating the pyramidal surface texture of a ZnO thin film with the room-temperature nanoimprint technique. Employed as the front electrode of hydrogenated amorphous Si single-junction solar cells, this OR-PF substrate enabled a Voc of 0.91 eV, an FF of 0.71, a Jsc gain of approximately 8%, and an efficiency gain of approximately 20% with respect to the AZO-coated flat glass substrate. The Jsc gain is attributed to the enhancement in the light absorption of a-Si:H solar cells triggered by the diffraction behaviour of the interfacial pyramidal morphology, while the achievement of large Voc and FF is ascribed to the improvement in the growth quality of the photoactive layers. Based on the calculation results of the bidirectional scattering distribution function, this substrate empowers comparable intensity of the diffraction peaks for 600–700 nm wavelength light but scatters less light away from the specular direction with respect to the conventional pyramid-textured ZnO substrate. This OR-PF substrate would enable efficient photon management and the growth of high-quality photoactive materials simultaneously for thin-film silicon solar cells.

Domain epitaxy of crystalline BeO films on GaN and ZnO substrates

AbstractWe demonstrated the growth of wurtzite‐crystalline beryllium oxide (BeO) thin films on GaN and ZnO substrates using atomic layer deposition (ALD). Single‐crystalline BeO were epitaxially grown on GaN. Despite the inherently large lattice mismatch of BeO and GaN atoms, the 6/5 and 7/6 domain‐matched structures dramatically reduced the residual strain in BeO thin films. On the other hand, the lattice mismatch of BeO and ZnO was not effectively accommodated in the mixed domains. X‐ray diffraction (XRD) confirmed the in‐plane crystallization of BeO‐on‐substrates in the (002){102}BeO||(002){102}Sub orientation and relaxation degrees of 20.8% (GaN), 100% (ZnO). The theoretical critical thicknesses of BeO for strain relaxation were 2.2 μm (GaN) and 1.6 nm (ZnO), calculated using a total film energy model. Transmission electron microscopy (TEM) and Fourier‐filtered imaging supported the bonding configuration and crystallinity of wurtzite BeO thin films on GaN and ZnO substrates.

Preparation and temperature-dependent photoelectrical properties of VO2/AZO heterojunctions

Abstract The preparation of n-N homotypic heterojunction by growing n-type VO2 film on N-type Al-doped ZnO substrate using DC magnetron sputtering and post-annealing was reported in this paper. The composition and microstructure of the VO2/AZO heterojunction were measured by XRD and SEM. The photoelectrical properties and the phase transition properties were tested by adjusting voltage at different temperature. The forward current was perfectly fitted on the thermionic emission model and the breakdown voltage was about 5 V at 20 °C. Meanwhile, the threshold voltage of current mutation was 3 V at 20 °C and decreased with increasing temperature until it completely disappeared at 60 °C. The voltage dependence transmittance of VO2/AZO heterojunction at 1310 nm was obtained. It indicated that the decrease of transmittance is the double effect of thermally induced phase transition and electrically induced phase transition.

Characterization of Graphite/ZnO Schottky Barriers Formed on Polar and Nonpolar ZnO Surfaces

The authors demonstrate that the electrical properties of Schottky junctions fabricated by the deposition of colloidal graphite strongly depend on the crystallographic orientation of the ZnO substrate. The current‐voltage, capacitance‐voltage, and impedance measurements indicate that near‐ideal Schottky junctions form on c‐plane, while on a‐ and m‐plane the junctions are laterally inhomogeneous. This behavior is assigned to higher concentration of native point defects in the near‐surface region of nonpolar ZnO substrates. The authors further present an extended equivalent circuit model, which corresponds to actual structure of the junctions, and sheds light on their electrical transport properties.

Epitaxially grown semi-vertical and aligned GaTe two dimensional sheets on ZnO substrate for energy harvesting applications

Two-dimensional (2D) materials are particularly interesting for increasing the efficiency of optoelectronic devices. Here, we report a solar cell structure based on 2D GaTe nanosheets on m-plane ZnO substrate. Specifically, the 2D GaTe nanosheets form a semi-vertical/tilted array to combine sufficient solar light absorption and efficient carrier conduction. The formed GaTe layer of about 70 nm and the basal plane have a tilted angle of 53 degrees with respect to the substrate. Such an alignment is found to be induced by the interface lattice mismatch between GaTe and ZnO. The heterojunction photovoltaic device achieves a power conversion efficiency (PCE) of 6.64% demonstrating that the controlled integration of 2D materials in 3D configurations is promising for increasing the efficiency in various kinds of energy devices.

Tunable Localized Surface Plasmon Resonance and Broadband Visible Photoresponse of Cu Nanoparticles/ZnO Surfaces.

Plasmonic Cu nanoparticles (NP) were successfully deposited on ZnO substrates by atomic layer deposition (ALD) owing to the Volmer-Weber island growth mode. An evolution from Cu NP to continuous Cu films was observed with an increasing number of ALD cycles. Real and imaginary parts of the NP dielectric functions, determined by spectroscopic ellipsometry using an effective medium approach, evidence a localized surface plasmon resonance that can be tuned between the visible and near-infrared ranges by controlling the interparticle spacing and size of the NP. The resulting Cu NP/ZnO device shows an enhanced photoresponse under white light illumination with good responsivity values, fast response times, and stability under dark/light cycles. The significant photocurrent detected for this device is related to the hot-electron generation at the NP surface and injection into the conduction band of ZnO. The possibility of tuning the plasmon resonance together with the photoresponsivity of the device is promising in many applications related to photodetection, photonics, and photovoltaics.

Local Structure and Point-Defect-Dependent Area-Selective Atomic Layer Deposition Approach for Facile Synthesis of p-Cu2O/n-ZnO Segmented Nanojunctions

Area-selective atomic layer deposition (AS-ALD) has attracted much attention in recent years due to the possibility of achieving accurate patterns in nanoscale features, which render this technique compatible with the continuous downscaling in nanoelectronic devices. The growth selectivity is achieved by starting from different materials and results (ideally) in localized growth of a single material. We propose here a new concept, more subtle and general, in which a property of the substrate is modulated to achieve localized growth of different materials. This concept is demonstrated by selective growth of high-quality metallic Cu and semiconducting Cu2O thin films, achieved by changing the type of majority point defects in the ZnO underneath film exposed to the reactive species using a patterned bilayer structure composed of highly conductive and highly resistive areas, as confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The selective growth of these materials in a patterned ZnO/Al-doped ZnO substrate allows the fabrication of p-Cu2O/n-ZnO nanojunctions showing a nonlinear rectifying behavior typical of a p-n junction, as confirmed by conductive atomic force microscopy (C-AFM). This process expands the spectra of materials that can be grown in a selective manner by ALD and opens up the possibility of fabricating different architectures, taking advantage of the area-selective deposition. This offers a variety of opportunities in the field of transparent electronics, catalysis, and photovoltaics.

Investigation of high indium-composition InGaN/GaN heterostructures on ZnO grown by metallic organic chemical vapor deposition

The optical properties and film quality for a series of high-In composition InGaN films grown on ZnO substrate by metal-organic chemical vapor deposition (MOCVD) are characterized by using high resolution X-ray diffraction (HRXRD), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman scattering spectroscopy (RSS). The In composition is evaluated by analyzing the RBS and PL emission spectra. The XPS measurements revealed the diffusion of Zn atoms from the substrate into InGaN films. All the analyses of experimental measurements have shown that the growth temperature played an important role in indium composition as well as of film quality. An optimum growth temperature is a necessary condition for obtaining high-quality films.

Effects of Cu contents on defects formation in molecular dynamics simulations of ZnO:Cu films deposition

The development of p-type ZnO film is related to the acceptor doping and defects, which are difficult to assess. Copper doped ZnO (ZnO:Cu) film with varying Cu content on a ZnO(0 0 1) substrate were simulated using molecular dynamics simulations. The COMB3 potential was used to describe the interaction between the depositing and substrate atoms. The growth of the ZnO:Cu films on the ZnO(0 0 1) substrate was a 3D island mode. Numerous oxygen vacancies (VO), zinc vacancies (VZn) and dislocations existed in the deposition layers. Cu-Zn alloy clusters combined easily with Cu and Zn atoms, increasing the mass of the deposit. This resulted in the content of acceptor defect VZn in ZnO layers increasing with the increase of Cu atoms. This could explain the p-type characteristics of ZnO:Cu film.

Durable superhydrophobic and underwater superoleophobic cotton fabrics growing zinc oxide nanoarrays for application in separation of heavy/light oil and water mixtures as need

Based on controlling ZnO nanoarrays morphology and employing fluorine-free modification agent, superhydrophobicity in air and superoleophobicity under water were successfully achieved on the prepared cotton fabric surfaces via a two-step hydrothermal reaction. To increase adhesion between ZnO crystal layer and substrate surface, two kinds of silane coupling agents (hydrophobic TTOP-12 and hydrophilic KH550) were added selectively in the pretreatment process. The as-prepared fabric exhibited great tolerance and durableness towards external harsh environments, such as mechanical abrasion, UV radiation, immersion of acid or alkali, and high temperature. In comparison, superhydrophobic fabrics showed superior separation effect for heavy oil and water mixture, while underwater superoleophobic fabric can also show efficient separation of water and light oil with high separation efficiency and flux. In a word, heavy/light oil and water mixtures can be separated efficiently as need by choosing cotton fabrics with various wettabilities.