ZnO Buffer Research Articles

Epitaxial growth of non-polar m-plane AlN film on bare and ZnO buffered m-sapphire

Non-polar m-plane AlN films were grown epitaxially on bare and ZnO buffered m-plane sapphire by laser molecular beam epitaxy. Based on X-ray θ−2θ and Φ scanning results, the in-plane epitaxial relationships of AlN/sapphire and AlN/ZnO/sapphire are [0001]AlN∥[112¯0]sapphire and [0001]AlN∥[0001]ZnO∥[112¯0]sapphire, respectively. A direct epitaxy without in-plane rotation is observed between AlN and ZnO, due to a small lattice mismatch, which results in a higher bond areal density of AlN/ZnO/sapphire than that of AlN/sapphire. ZnO buffer layer transforms the in-plane residual strain from compressive strain to tensile strain along the direction of [0001]AlN, owing to the change in the in-plane lattice mismatch. X-ray rocking curve and atomic force microscope reveal that ZnO buffer layer can change the mosaicity spread along the direction of [0001]AlN and [112¯0]AlN, leading to the opposite striated anisotropic morphology of AlN/sapphire and AlN/ZnO/sapphire. The decrease in anisotropic characteristic of AlN/ZnO/sapphire is consistent with the result of in-plane lattice mismatch.

Comparison of physical and electrical properties of GZO/ZnO buffer layer and GZO as source and drain electrodes of α-IGZO thin-film transistors

In this research, top-gate bottom-contact thin-film transistors (TFTs) made with amorphous indium gallium zinc oxide (α-IGZO) active layers were grown using the radio-frequency sputtering technique. Two kinds of source and drain (S/D) electrodes, namely bi-layer GZO/100-nm ZnO buffer layer/Corning 1737 and single-layer GZO/Corning 1737, used in the TFT devices and the electric characteristics of the devices were compared. To explain the differences in the TFT performances with these different S/D electrodes, X-ray reflectivity (XRR) and contact angles were measured. The α-IGZO TFT with the bi-layer GZO/100-nm ZnO buffer layer structure as S/D electrodes exhibited superior device performance compared to that of the TFT with a single-layer GZO structure, with a higher thin film density (5.94g/cm3), lower surface roughness (0.817nm), and larger surface energy (62.07mJ/m2) and better adhesion properties of neighboring α-IGZO films. In addition, the mechanisms responsible for the GZO/100-nm ZnO buffer layer/Corning 1737 structure S/D electrodes improving the device characteristics were systematically investigated. The α-IGZO TFT saturation mobility, subthreshold voltage, on/off current ratio, and the trap density of the GZO/100-nm ZnO buffer layer/Corning 1737 S/D electrodes were 13.5cm2V−1S−1, 0.43V/decade, 3.56×107, and 5.65×1012eV−1cm−2, respectively, indicating the potential of this bi-layer structure to be applied to large-area flat-panel displays.

Modifying hydrogen bonding interaction in solvent and dispersion of ZnO nanoparticles: impact on the photovoltaic performance of inverted organic solar cells

We demonstrate that the morphology of ZnO buffer layers for the inverted organic solar cells (IOSC) was improved by controlling the molecular interaction in solvents to disperse ZnO nanoparticles (NPs). Dispersion of ZnO NPs in non-polar solvent usually resulted in turbid or semi-transparent solution. In this study, uniform dispersion of ZnO NPs was achieved through applying mixed solvents composed of both non-polar and polar solvents without the assistance of an organic surfactant. From the analysis based on Hansen solubility parameter theory, the improvement was mainly associated with hydrogen bonding interactions between the two component of solvents. Smooth ZnO buffer layers were deposited using the colloidal solution based on the binary solvent mixture and the photovoltaic performance of the IOSC was highly improved by optimizing solvent mixtures.

High-performance inverted solar cells with a controlled ZnO buffer layer

ZnO is a versatile cathode buffer layer for organic photovoltaics (OPV) due to its appealing optical and electronic properties. Using the sol–gel method, we find that the processing temperature of ZnO cathode buffer layers significantly influences the device performance of inverted polymer OPVs composed of blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In particular, ZnO processed at relatively low temperatures results in better device performance than those processed at higher temperatures despite the improved crystallinity and electron mobility of the latter. We attribute this finding to the tuning of the ZnO work function with the annealing temperature, which determines the interface energetics at the cathode and thus influences the open circuit voltage, series resistance and fill factor.

Patterned growth of ZnO nanowalls by nanoparticle-assisted pulsed laser deposition

We have succeeded in controlling the growth density of ZnO nanowires synthesized by nanoparticle-assisted pulsed laser deposition (NAPLD) using a ZnO buffer layer and laser irradiation. The density of the nanowires decreased with increasing thickness of the buffer layer. In addition, the nanowire density also decreased by laser irradiation to the buffer layer. Patterned growth of ZnO nanowalls using NAPLD was successfully achieved with the help of the buffer layer patterned by interfering laser beams.

Effect of substrate temperature on the structural, electrical, and optical properties of GZO/ZnO films deposited by radio frequency magnetron sputtering

Ga-doped ZnO (GZO)/ZnO bi-layered films were deposited on glass substrates by radio frequency magnetron sputtering at different substrate temperatures of 100, 200 and 300°C to investigate the effects of substrate temperature on the structural, electrical, and optical properties of the films. Thicknesses of the GZO and ZnO buffer layer were kept constant at 85 and 15nm by controlling the deposition times.The films deposited at room temperature had a relatively low optical transmittance of 80.5%, while films deposited at substrate temperature of 300°C showed a higher transmittance of 84.5% compared to the other films. Electrical resistivity of the films was also influenced by substrate temperature and the lowest resistivity of 2.7×10−3Ωcm was observed for films deposited at 300°C. The observed result means that increasing the substrate temperature enhanced the optical transmittance and electrical conductivity of GZO/ZnO bi-layered films, simultaneously.

Growth Control of ZnO Nano-Crystals by Multi-Beam Interference Patterning

We have succeeded in controlling growth density of the ZnO nanowires by introduction of a ZnO buffer layer. Low-density hexagonal cone-shape ZnO cores are formed on the buffer layer, and vertically-aligned ZnO nanowires are grown on the cores. The density of the nanowires was decreased by laser irradiation to the buffer layer before growth of the ZnO nanowires. In addition, periodic growth of ZnO nano-crystals was demonstrated using four beam interference patterning.

Effects of Ga- and Al-Codoped ZnO Buffer Layer on the Performance of Inverted Polymer Solar Cells

A Ga, Al-doped zinc oxide (GAZO) buffer layer was applied to inverted polymer solar cells (PSCs) based on P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C61-butyric acid methyl ester] blend films. The work function of the GAZO layer on indium-tin oxide (ITO) was measured to be 4.45 eV. The insertion of the GAZO layer between the ITO electrode and the P3HT:PCBM blend film in the inverted PSC led to an improved short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF) compared to those of the reference cell without GAZO layer. The Jsc enhancement in the inverted PSC with the GAZO layer was attributed to both the effective electron extraction and the increased crystallinity of P3HT, and the work function difference between Ag and GAZO layer induced the increase in Voc. The improved FF value was due to the lowered series resistance and elevated shunt resistance. Thus, the power conversion efficiency of the device with the GAZO layer was improved by more than 200% relative to the reference cell.

Characteristics of a ZnTiO and ITO cathode-integrated electrodes for inverted organic solar cells

We investigated the characteristics of TiO2 and ZnO co-sputtered ZnTiO (ZTO) buffer layer for use in inverted organic solar cell (IOSCs). The 40 nm-thick ZTO buffer layer deposited on a crystalline ITO cathode showed different device performances depending on the RF and DC powers applied on the TiO2 and ZnO targets, respectively. Compared to IOSCs with a TiO2-rich ZTO buffer layer, the IOSCs with a ZnO-rich ZTO buffer layer showed better performances due to their higher optical transmittances and well-matched work function. At the optimized power ratio of ZnO:TiO2 (70:30%), the ZTO buffer layer exhibited an optical transmittance of 78% at a wavelength of 550 nm. The IOSCs with the optimized ZTO buffer layer exhibited a good cell-performance with a fill factor of 52.05%, a short circuit current of 8.81 mA/cm2, an open circuit voltage of 0.66 V, and a power conversion efficiency of 3.03%, which are comparable to IOSCs containing a conventional sol–gel processed ZnO buffer layer. Furthermore, the microstructure and interface structure for the IOSC with the ZTO buffer layer and ITO cathode-integrated electrode was investigated in detail using high resolution transmission electron microscopy.

Effective light trapping enhanced near-UV/blue light absorption in inverted polymer solar cells via sol–gel textured Al-doped ZnO buffer layer

The effects of sol–gel textured Al-doped ZnO (AZO) cathode buffer layer on the performance of the inverted polymer solar cells have been investigated in this work. Textured AZO, together with a reflective back electrode provides efficient light-trapping, inducing further light absorption in the active layer. The AZO films were characterized with respect to their morphology, optical and electrical properties. Due to the enhanced absorption at near-UV/blue light region (300–500nm), the short current density of the textured-AZO device is increased from 9.52 to 11.30mA/cm2 compared with the reference cell based on the flat-AZO. Power conversion efficiency is increased substantially from 2.44% to 2.87% since without loss in the fill factor and open circuit voltage.

Low temperature, solution-processed alumina for organic solar cells

In this work, we have reported for the first time a facile route for developing solution-processed Al2O3 film at a greatly reduced processing temperature and studied its applications in producing inverted polymer solar cells (PSCs). These PSCs using Al2O3 thin film as the electron-extraction layer demonstrated improved diode characteristics and achieved a 20% higher power conversion efficiency than devices using the conventional ZnO buffer layer. Furthermore, the low temperature processing of the Al2O3 film makes it compatible with fabrication of flexible organic electronic devices based on plastic substrates.

Structural and optical properties of ZnO nanorods grown chemically on sputtered GaN buffer layers

ZnO nanorods were grown on 200nm thick sputtered ZnO and GaN buffer layers on quartz substrates by chemical bath deposition. Field emission scanning electron microscopy and X-ray diffraction studies show that the ZnO nanorods on GaN buffer layer possess larger diameter and smaller lengths and are vertically misaligned, compared to those grown on ZnO buffer layer. These differences are attributed to lack of complete c-axis orientation of crystallites in GaN buffer layer, its lattice mismatch with that of ZnO and a hindered nucleation process of ZnO on GaN surface, owing to a finite nucleation barrier and limited surface diffusion. Photoluminescence spectrum of ZnO nanorods on GaN buffer layer, however, exhibits a much stronger near-band-edge luminescence and drastically suppressed defect luminescence compared to the luminescence spectrum of the nanorods grown on ZnO buffer layer. Deconvolution of the photoluminescence peaks and Raman studies indicate significant reduction of oxygen vacancies and gallium incorporation in the ZnO nanorods grown on GaN buffer layer. These observations suggest the possibility of exchange reaction mediated by the aqueous medium, particularly during the initial stages of growth.

Effects of Nitrogen on Crystal Growth of Sputter-Deposited ZnO Films for Transparent Conducting Oxide

We have studied the effects of the N2 gas flow rate on the surface morphology of ZnO films deposited by the sputtering of a ZnO target using Ar/N2. Height-height correlation function (HHCF) analysis indicates that introducing a small amount of N2 (<5 sccm) to the sputtering atmosphere enhances adatom migration, leading to a larger grain size in the ZnO films associated with an increase in the lateral correlation length. The HHCF analysis also reveals that films deposited with and without N2 exhibit a self-affine fractal surface structure. We demonstrate that utilizing such ZnO films deposited using Ar/N2 as buffer layers, the crystallinity of ZnO:Al (AZO) films on the buffer layers can be greatly improved. The electrical resistivity of 100-nm-thick AZO films decreases from 1.8×10-3 to 4.0×10-4 Ω·cm by utilizing a ZnO buffer layers prepared at N2 flow rate of 5 sccm.

Domain matched epitaxial growth of Bi1.5Zn1Nb1.5O7 thin films by pulsed laser deposition

Abstract Bi 1.5 Zn 1 Nb 1.5 O 7 (BZN) epitaxial thin films were grown by pulsed laser deposition on Al 2 O 3 with a double ZnO buffer layer through domain matching epitaxy (DME) mechanism. The pole figure analysis and reciprocal space mapping revealed the single crystalline nature of the thin film. The pole figure analysis also shows a 60° twinning for the (2 2 2) oriented crystals. Sharp intense spots in the SAED pattern also indicate the high crystalline nature of BZN thin film. The Fourier filtered HRTEM images of the BZN–ZnO interface confirms the domain matched epitaxy of BZN with ZnO buffer. An electric field dependent dielectric tunability of 68% was obtained for the BZN thin films with inter digital capacitors patterned over the film.

Effect of Oxide Buffer Layer on the Thermochromic Properties of VO2 Thin Films

VO2 thin films were deposited on soda lime glass substrates with ZnO, TiO2, SnO2, and CeO2 thin films applied as buffer layers between the VO2 films and the substrates in order to investigate the effect of buffer layer on the formation and the thermochromic properties of VO2 film. Buffer layers with thicknesses over 50 nm were found to affect the formation of VO2 film, which was confirmed by XRD spectra. By using ZnO, TiO2, and SnO2 buffer layers, monoclinic VO2 (VO2(M)) film was successfully fabricated on soda lime glass at 370 °C. On the contrary, films of VO2(B), which is known to have no phase transition near room temperature, were formed rather than VO2(M) when the film was deposited on CeO2 buffer layer at the same film deposition temperature. The excellent thermochromic properties of the films deposited on ZnO, TiO2, and SnO2 buffer layers were confirmed from the temperature dependence of electrical resistivity from room temperature to 80 °C. Especially, due to the tendency of ZnO thin film to grow with a high degree of preferred orientation on soda lime glass at low temperature, the VO2 film deposited on ZnO buffer layer exhibits the best thermochromic properties compared to those on other buffer layer materials used in this study. These results suggest that deposition of VO2 films on soda lime glass at low temperature with excellent thermochromic properties can be achieved by considering the buffer layer material having structural similarity with VO2. Moreover, the degree of crystallization of buffer layer is also related with that of VO2 film, and thus ZnO can be one of the most effective buffer layer materials.

Role of reaction atmosphere in preparation of potassium tantalate through sol–gel method

Potassium tantalate (KT) thin films and powders of both K2Ta2O6 (KT pyrochlore) and KTaO3 (KT perovskite) structures were prepared by means of chemical solution deposition method using Si(111) with ZnO and MgO buffer layers as a substrate. The influence of reaction atmosphere on reaction pathway and phase composition for both KT powders, and KT thin films has been studied mainly by means of powder diffraction and infrared spectroscopy. When an oxygen flow instead of static air atmosphere has been used the process of pyrolysis in oxygen runs over much narrower temperature interval (200–300 °C), relatively to air atmosphere (200–600 °C) and almost no (in case of powders), or no (in case of thin films) pyrochlore intermediate phase has been detected in comparison with treatment in air, where the pyrochlore phase is stable at temperatures 500–600 °C (powders). KT perovskite phase starts to crystallize at temperatures 50° and 150 °C lower compared to air atmosphere in case of powders and thin films, respectively. Microstructure formed by near-columnar grains and small grains of equiaxed shape was observed in films treated in oxygen and air atmosphere, respectively.

Fabrication and photoresponse of ZnO nanowires/CuO coaxial heterojunction

The fabrication and properties of n-ZnO nanowires/p-CuO coaxial heterojunction (CH) with a photoresist (PR) blocking layer are reported. In our study, c-plane wurtzite ZnO nanowires were grown by aqueous chemical method, and monoclinic CuO (111) was then coated on the ZnO nanowires by electrochemical deposition to form CH. To improve the device performance, a PR layer was inserted between the ZnO buffer layer and the CuO film to serve as a blocking layer to block the leakage current. Structural investigations of the CH indicate that the sample has good crystalline quality. It was found that our refined structure possesses a better rectifying ratio and smaller reverse leakage current. As there is a large on/off ratio between light on and off and the major light response is centered at around 424 nm, the experimental results suggest that the PR-inserted ZnO/CuO CH can be used as a good narrow-band blue light detector.

Dielectric back scattering patterns for light trapping in thin-film Si solar cells

We experimentally compare the light trapping efficiency of dielectric and metallic backscattering patterns in thin-film a-Si:H solar cells. We compare devices with randomly patterned Ag back contacts that are covered with either flat or patterned aluminum-doped ZnO (AZO) buffer layers and find the nanostructure at the AZO/a-Si:H interface is key to achieve efficient light trapping. Simulations show that purely dielectric scattering patterns with flat Ag and a patterned AZO/a-Si:H interface can outperform geometries in which the Ag is also patterned. The scattering from the dielectric patterns is due to geometrical Mie resonances in the AZO nanostructures. The optimized dielectric geometries avoid parasitic Ohmic losses due to plasmon resonances in the Ag, and open the way to a large number of new light trapping designs based on purely dielectric resonant light scattering.

GaAs nanowires grown on Al-doped ZnO buffer layer

We report a pathway to grow GaAs nanowires on a variety of substrates using a combination of atomic layer deposition and metallo-organic vapor phase epitaxy (MOVPE). GaAs nanowires were grown via MOVPE at 430–540 °C on an atomic-layer-deposited Al:ZnO buffer layer. The resulting nanowires were affected only by the properties of the buffer layer, allowing nanowire growth on a number of substrates that withstand ∼400 °C. The growth occurred in two phases: initial in-plane growth and subsequent out-plane growth. The nanowires grown exhibited a strong photoluminescence signal both at room temperature and at 12 K. The 12 K photoluminescence peak was at 1.47 eV, which was attributed to Zn autodoping from the buffer layer. The crystal structure was zincblende plagued with either twin planes or diagonal defect planes, which were related to perturbations in the seed particle during the growth. The used method combines substrates with variable properties to nanowire growth on a transparent and conductive Al:ZnO buffer layer.

Growth mechanism studies of ZnO nanowire arrays via hydrothermal method

Well-controlled ZnO nanowire arrays have been synthesized using the hydrothermal method, a low temperature and low cost synthesis method. The process consists of two steps: the ZnO buffer layer deposition on the substrate by spin-coating and the growth of the ZnO nanowire array on the seed layer. We demonstrated that the microstructure and the morphology of the ZnO nanowire arrays can be significantly influenced by the main parameters of the hydrothermal method, such as pH value of the aqueous solution, growth time, and solution temperature during the ZnO nanowire growth. Scanning electron microscopy observations showed that the well oriented and homogeneous ZnO nanowire arrays can be obtained with the optimized synthesis parameters. Both x-ray diffraction spectra and high-resolution transmission electron microscopy (HRTEM) observations revealed a preferred orientation of ZnO nanowires toward the c-axis of the hexagonal Wurtzite structure, and HRTEM images also showed an excellent monocrystallinity of the as-grown ZnO nanowires. For a deposition temperature of 90 °C, two growth stages have been identified during the growth process with the rates of 10 and 3 nm/min, respectively, at the beginning and the end of the nanowire growth. The ZnO nanowires obtained with the optimized growth parameters owning a high aspect ratio about 20. We noticed that the starting temperature of seed layer can seriously influence the nanowire growth morphology; two possible growth mechanisms have been proposed for the seed layer dipped in the solution at room temperature and at a high temperature, respectively.