ZnO Buffer Research Articles

Hybrid solution processed InGaO3(ZnO)m thin films with periodic layered structures and thermoelectric properties

Short period superlattices comprising alternating InO2− and GaO+(ZnO)2 layers were fabricated by a simple hybrid solution process and reactive solid-phase epitaxy at high temperature. The epitaxial ZnO buffer layer deposited by sputtering, and an amorphous IGZO layer fabricated from a solution mixture of 1 : 1 : 1.0 (In nitrate : Ga nitrate : Zn acetate) produced pure, single-phase InGaO3(ZnO)2 films with a well-ordered layered structure and smooth surfaces, which showed intense periodic diffraction peaks. Deviation from the stoichiometric sol condition induced coexisting InGaO3(ZnO)2 and other InGaO3(ZnO)m phases and very rough surface morphologies. The solid-phase epitaxy of a single phase decreased electrical resistivity, increased the Seebeck coefficient, and significantly improved the power factor. An extremely low thermal conductivity (1.11 W m−1 K−1) was also obtained due to phonon scattering at the InO2− and GaO+(ZnO)2 interfaces by the formation of the superlattice structure. This solution-based fabrication of superlattice structures could aid the development of advanced multicomponent oxides due to its simple growth process and the adaptability of compositions.

电沉积法制备ZnO纳米柱及其机制研究

ZnO nanorods arrays were prepared by cathodic electrochemical deposition method,in which ITO-coated glass substrate were used as a cathode.The electrolyte consisted of an aqueous solution of Zn(NO3)2·6H2O with the concentration of 0.01 mol·L-1 and KCl supporting electrolyte with the concentration of 0.1 mol·L-1 were used for the depositing.The influences of depostion potential and buffer layer on the preferred orientation and density of the ZnO nanorods were investigated by scanning electron microscope(SEM) and X-ray diffraction(XRD).A significant variation of density and orientation of ZnO nanorod arrays were obtained by changing the deposition time of buffer layer.Pre-deposition ZnO buffer layer relieved the lattice mismatch between ZnO and ITO substrate,and also provided the nuclei centers for the ZnO nanorods.SEM and XRD data showed that ZnO nanorods with the highest density and best c-axis orientation can be achieved by pre-depositing a buffer layer at applied potential of-1.10 V and last for 60 s.

Effects of Cu diffusion-doping on structural, optical, and magnetic properties of ZnO nanorod arrays grown by vapor phase transport method

Well-aligned ZnO nanorods were prepared by the vapor phase transport method on Si covered with a ZnO buffer layer. After the nanostructure growth, Cu was doped into the ZnO nanorods by diffusion at three different temperatures and for different times. Undoped and Cu diffusion-doped ZnO samples are highly textured, with the c axis of the wurtzite structure along the growth direction. The incorporation of Cu caused some slight changes in the nanorod alignment, although the wurtzite crystal structure was maintained. X-ray photoelectron spectroscopy measurements revealed that Cu ions were in a divalent state and substituted for the Zn2+ ions of the ZnO matrix. Photoluminescence results at 10 K indicate that the incorporation of copper leads to a relative increase of Cu-related structured green band deep level intensity. Magnetic measurements revealed that both undoped and Cu diffusion-doped ZnO samples exhibited room temperature ferromagnetism. It was also found that bound magnetic polarons play an important role in the appearance of room temperature ferromagnetism in Cu diffusion-doped ZnO nanorods.

Morphological control of MgxZn1−xO layers grown on Ga:ZnO/glass substrates for photovoltaics

MgxZn1−xO has been used in various photovoltaic cells because its energy bandgap can be tailored by controlling the Mg composition in this ternary compound. The MgxZn1−xO layers with different surface morphologies including two-dimensional (2-D) films and one-dimensional (1-D) nanostructures are preferred for conventional p–n junction solar cells and polymer–inorganic hybrid solar cells, respectively. The MgxZn1−xO layers are sequentially grown on Ga-doped ZnO (GZO) transparent conductive electrode using metalorganic chemical vapor deposition (MOCVD). The effect of the buffer layers on MgxZn1−xO surface morphology is investigated. It is observed that MgxZn1−xO deposited at ∼500°C on a low-temperature (∼250°C) ZnO buffer layer is in the form of 2-D dense and smooth films, whereas, on a high-temperature (∼520°C) ZnO buffer layer is in the form of 1-D nanostructures. Based on the structure characterization results, a growth mechanism in terms of nucleation and texturing is proposed to explain the buffer layer effect.

Study of Morphological and Related Properties of Aligned Zinc Oxide Nanorods Grown by Vapor Phase Transport on Chemical Bath Deposited Buffer Layers

c-axis aligned ZnO nanorods were deposited by vapor phase transport on textured chemical bath deposited buffer layers. In this work, we examine the role of the buffer layer and how it influences the vapor phase transport deposition process using both scanning and transmission electron microscopes and related techniques. Vapor phase transport deposition on chemical bath deposited buffer layers is a complex growth process with many simultaneously occurring effects including (i) substantial morphological transformation at high temperature, which influences the base of the nanorods; (ii) the formation of a mixed amorphous/crystalline ZnxSi1–xOy interface during the vapor phase transport growth on silicon substrates; (iii) the overgrowth of the ZnO seed layers by the silica interface rendering them inactive for nanorod nucleation, suggesting there is a minimum critical thickness ZnO buffer layer necessary for vapor phase transport growth of ZnO nanorods on silicon substrates. We discuss the relative importance of these effects on the overall growth process and use this understanding to explain previous results in the literature.

Study on the Effect of ZnO Buffer Layer Thickness on the Properties of MgZnO Film

ZnO has recently attracted considerable attention due to its favorable properties such as the wider band gap (3.37eV) at room temperature, the large binding energy of excitons (60meV). These good photoelectric and piezoelectric properties [1-4] cause it has immensity space for developing at surface acoustic wave devices, light emitting diodes (LEDs) [5] , photodetectors [6], gas sensor and solar cells [7] etc. MgZnO has many similar properties to ZnO. Furthermore, the band gap of MgZnO is 3.3-4.0eV [9] due to the wider band gap of MgO (7.7eV [8]). In this paper, we report the characteristic of MgxZn1-xO films which were grown on c-plane sapphire with different thickness-ZnO buffer layers by MOCVD. By investigating the surface morphology, structural and optical properties, some dependences between properties of MgZnO films and the thicknesses of ZnO buffer layers can be found.

The Growth Mechanism of Vertically Aligned ZnO Nanowire Arrays on Non‐epitaxial Si(100) Substrates

AbstractIn this report, we have investigated the Au‐assisted growth of ZnO nanowires at a low temperature (550 °C) using low melting point Zn metal as a source. We supplied various Ar gas flow rate to carry Zn vapor onto Au catalysts coated Si(100) substrates to grow ZnO nanowires. When a high Ar gas flow rate was supplied, a large amount Zn vapor was carried onto the substrate and reacted with O2 to form a ZnO buffer layer covering the surface of the Au catalysts. This ZnO buffer layer has a highly c‐axis preferential orientation, which helps form vertically aligned ZnO nanowire arrays. In this condition, no Au catalysts appeared on the tips of the ZnO nanowires. When low Ar gas flow rate was supplied, Zn vapor is too low to form ZnO buffer layer on the surface of Au catalysts. From TEM analysis, we can observe that Au catalysts remain solid during growth. The detail growth mechanism of ZnO nanowires with various carrier flow rates was studied by field‐emission electron microscopy (FE‐SEM), transmission electron microscopy (TEM) and X‐ray diffraction (XRD).

Effects of Thermal Annealing in Oxygen Plasma for Buffer Layers on Properties of ZnO Thin Films

ZnO thin films with ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si(100) substrates. Before the growth of the ZnO thin films, the ZnO buffer layers were deposited on the Si substrates for 20 minutes and then annealed at the different substrate temperature ranging from 600 to 800 degrees C in oxygen plasma. The structural and optical properties of the ZnO thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and room-temperature (RT) photoluminescence (PL). A narrower full width at half maximum (FWHM) of the XRD spectra for ZnO(002) and a larger grain are observed in the samples with the thermal annealed buffer layers in oxygen plasma, compared to those of the as-grown sample. The surface morphology of the samples is changed from rugged to flat surface. In the PL spectra, near-band edge emission (NBEE) at 3.2 eV (380 nm) and deep-level emission (DLE) around 1.77 to 2.75 eV (700 to 450 nm) are observed. By increasing the annealing temperatures up to 800 degrees C, the PL intensity of the NBEE peak is higher than that of the as-grown sample. These results imply that the structural and optical properties of ZnO thin films are improved by the annealing process.

Post-fabrication annealing effects on the performance of P3HT:PCBM solar cells with/without ZnO nanoparticles

In this study, P3HT:PCBM organic photovoltaic (OPV) devices, with or without ZnO nanoparticles buffer layer between the photoactive layer (P3HT:PCBM) and the cathode (Al top electrode), were fabricated. The devices were annealed at 145°C either before or after depositing the top electrode. The objective of this study was to investigate the effects of the ZnO buffer layer and pre-/post-fabrication annealing on the general performance of these devices. The short-circuit current density (JSC), open-circuit voltage (VOC) and the external quantum efficiency (EQE) of the OPV devices were improved by the insertion of the ZnO layer and post-fabrication annealing. The post-fabrication annealed devices, with or without the ZnO layer, exhibited higher values of JSC, VOC and EQE than those of similar devices annealed before depositing the Al metal. This can be attributed to, among other things, improved charge transport across the interface between the photoactive layer and the Al top electrode as a result of post-annealing induced modification of the interface morphology.

The Effects of Using ALD-Grown ZnO Buffer Layers on the Properties of Indium Tin Oxide Grown by Chemical Solution Deposition

In comparison to ITO films prepared by chemical solution deposition on bare substrates, the use of a ZnO buffer layer and Al2O3 barrier layer has been shown to have a significant effect on morphology, measured sheet resistance and therefore resistivity. In the case of quartz substrates, ITO resistivity decreased from 9.6 x 10(-3) ohms cm to 4.3 x 10(-3) ohms cm on incorporation of a ZnO buffer layer and Al2O3 barrier layer, both grown by ALD. A change in surface morphology was observed, due to the presence of the buffer layer, however, the ZnO buffer layer was not found to influence the XRD pattern of the ITO films.

Efficiency Improvement of Solar Cell with ZnO Nanotip Array Prepared by Aqueous Solution Deposition

ZnO nanotips were synthesized on a sputtered ZnO buffer layer/ITO/glass by aqueous solution deposition with precursors of zinc nitrate and ammonia. Growth direction of ZnO nanotips can be controlled by the thickness of sputtered ZnO buffer layer. The average height of 13 μm was obtained at 70 °C for 24 hr and the diameter of ZnO nanotips was ranged from 60 nm to 100 nm.

Mechanical responses of Zn 1− xMn xO epitaxial thin films

In this study, we used nanoindentation to investigate the effect of the doping of Mn into ZnO buffer layers on the epitaxial growth of ZnO through plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. We characterized the variation of the mechanical properties of Zn 1− x Mn x O alloys as a function of the Mn content in the range ( x) from 0 to 0.16, as well as analyzing their microstructures using high-resolution transmission electron microscopy. The presence of the Mn-doped ZnO buffer layer enhanced the nanomechanical properties of the ZnO epilayers significantly. From their Berkovich indenter responses, plots of the Young's modulus ( E) and hardness ( H) of these films revealed that the value of E increased relatively steadily upon increasing the Mn composition, whereas the value of H reached its maximum when x was equal to 0.16. This discrepancy suggests that Zn 1− x Mn x O epilayers of higher Mn contents had higher shear resistances.

Drastic improvement of oxide thermoelectric performance using thermal and plasma treatments of the InGaZnO thin films grown by sputtering

Single-crystal InGaO 3(ZnO) m thin films with periodic superlattice structures suitable for transparent thermoelectric applications were fabricated using a commercially available c-plane sapphire substrate, an epitaxial ZnO buffer layer, a thermal treatment at 900 °C, and an Ar plasma treatment. The introduction of the epitaxial ZnO buffer layer led to a significant reduction in the lattice mismatch at the interface with the InGaO 3(ZnO) m films. The sandwich structure of the ZnO/InGaZnO/ZnO resulted in an increase in the ZnO content in the superlattice InGaO 3(ZnO) m thin films. With respect to thermoelectric properties, the formation of a perfect, layered structure induced an increase in the Seebeck coefficient and, at the same time, a decrease in the thermal conductivity. After complete crystallization, the Ar plasma treatment resulted in a considerable decrease in the electrical resistivity without microstructural changes and without a large decrease in the thermal conductivity. As a result, the thermoelectric properties using n-type oxide semiconductors were dramatically improved.

Effects of Annealing Treatment and Buffer Layer on Properties of ZnMgO Films Prepared on Diamond Substrates

ZnMgO films were prepared at room temperature on freestanding diamond (FSD) substrates with and without ZnO buffer layers by radio-frequency (RF) reactive magnetron sputtering method. The effects of annealing treatment and ZnO buffer layers on the structural, optical, and electrical properties of the ZnMgO films were studied by X-ray diffraction (XRD), UV-visible spectrophotometer, and electrical measurements. The experimental results suggested that the annealing treatment and buffer layer were helpful to improve the crystalline quality of ZnMgO/diamond heteroepitaxial films.

The sensitivity and dynamic response of field ionization gas sensor based on ZnO nanorods

Field ionization gas sensors based on ZnO nanorods (50–300 nm in diameter, and 3–8 μm in length) with and without a buffer layer were fabricated, and the influence of the orientation of nano-ZnO on the ionization response of devices was discussed, including the sensitivity and dynamic response of the ZnO nanorods with preferential orientation. The results indicated that ZnO nanorods as sensor anode could dramatically decrease the breakdown voltage. The XRD and SEM images illustrated that nano-ZnO with a ZnO buffer layer displayed high c-axis orientation, which helps to significantly reduce the breakdown voltage. Device A based on ZnO nanorods with a ZnO buffer layer could distinguish toluene and acetone. The dynamic responses of device A to the NOx compounds presented the sensitivity of 0.045 ± 0.007 ppm/pA and the response speed within 17–40 s, and indicated a linear relationship between NOx concentration and current response at low NOx concentrations. In addition, the dynamic responses to benzene, isopropyl alcohol, ethanol, and methanol reveals that the device has higher sensitivity to gas with larger static polarizability and lower ionization energy.

Effects of different interlayers on indium-tin oxide thin films sputtered on flexible PET substrates

Using radio-frequency magnetron sputtering, various interlayer materials (Cu, Al, Ag, and ZnO) were added to indium-tin oxide (ITO) films on a polyethylene terephthalate (PET) flexible substrate, and were compared to examples made without an interlayer. The effects of the interlayer on the ITO were investigated with X-ray diffractometry (XRD), visible wavelength photospectrometry, four-point probe electrical resistivity, and micro-scratch adhesion tests, which were all conducted at four temperatures (25°C, 35°C, 55°C, and 75°C). The ITO crystallographic texture was overwhelmingly found to be (2 2 2), for all combinations considered. The grain size was reduced in the ITO films having interlayers of Ag or Cu, and was enlarged by the ZnO. The visible light transmittance remained almost unchanged for the Al and ZnO buffers, but Ag and Cu failed to maintain the industry standard of 75–80% for a transparent film. Cu, ZnO, and especially Ag, effectively reduced, whereas, Al increased the sheet resistance of the ITO. Upon heating, the scratch resistance diminished in all cases, excluding Cu, while the sheet resistance grew with respect to temperature, for all films except those with Ag. The XRD intensity and the light transmittance were not significantly affected by temperature.

Effect of Sol-Gel Prepared ZnO Electron Selective Layer on the Performance of Inverted Organic Solar Cells

ZnO buffer layer was prepared as electron selective layers (ESL) for photo-induced electron transport and hole blocking in inverted organic solar cells (IOSCs) by sol-gel process. The effects of thickness and surface roughness of ZnO ESL on the performance of IOSCs were investigated. Power Conversion Efficiency (PCE) of the IOSC strongly varied as a function of ZnO film thickness and surface roughness, in particular, when the film thickness was <70 nm, increase in the surface roughness enhanced photovoltaic performances. It is demonstrated that optimization of the electrode thickness and roughness is essential for obtaining better performance of organic photovoltaic cells.

Effects of Metal-Organic Chemical Vapour Deposition grown seed layer on the fabrication of well aligned ZnO nanorods by Chemical Bath Deposition

Well aligned, long and uniform ZnO nanorods have been reproducibly fabricated adopting a two-steps Metal-Organic Chemical Vapour Deposition (MOCVD) and Chemical Bath Deposition (CBD) fabrication approaches. Thin (< 100 nm) ZnO buffer layers have been seeded on silicon substrates by MOCVD and ZnO layers have been subsequently grown, in form of well textured nanorods, using CBD. It has been found that the structure and thickness of the seed layer strongly influence the final morphology and the crystal texturing of ZnO nanorods as well as the CBD growth rate. There is, in addition, a strong correlation between morphologies of CBD grown ZnO nanorods and those of the seed layer underneath. Thus, nanorods deposited over low temperature MOCVD buffer layers are less homogeneous in lateral dimensions and poorly vertically oriented. On the contrary, higher temperature nano-dimensional ZnO seeds favour the CBD growth of almost mono-dimensional homologue nanorods, with an adequate control of the lateral transport of matter. The nanorod aspect ratio values decrease upon increasing the deposition temperatures of the seed layers. Moreover, the nanorods length can be tailored either by adjusting the CBD growth time or by changing concentration of the N,N,N′,N′-tetramethylethylenediamine ligand used in the CBD process. In particular, at high concentrations, the CBD process is faster with a greater global aspect ratio in agreement with a preferential one-dimensional growth of the ZnO nanostructures. Finally, these ZnO nanorod arrays possess good optical quality in accordance to the photoluminescence properties.

The effect of ZnO buffer layer on structural and optical properties of ZnO nanorods

AbstractA low‐temperature synthetic route was used to prepare oriented arrays of ZnO nanorods on ITO conducting glass substrate coated with buffer layer of ZnO seeds in an aqueous solution. The corresponding growth behavior and optical properties of ZnO nanorod arrays were studied. It was found that the nature of the buffer layer had effect on the microstructures and optical properties of the resultant ZnO nanorod arrays. X‐ray diffraction (XRD) results showed the nanorods were preferentially grown along (002) direction, but the diameter of the nanorods prepared with the buffer layer was much smaller than the without one, which can be clearly seen from the scanning electron microscopy (SEM) results. And it also found that the buffer layer was not only enhanced the density of overall coverage but also beneficial to grown the oriented arrays. Photoluminescence spectroscopy (PL) results indicated that the all the samples had the better optical behaviors. By computation, the relative PL intensity ratio of ultraviolet emission (IUV) to deep level emission (IDLE) of ZnO nanorods grown with the pure substrate was much higher than that of the sample with the buffer layer. The defects on the surface increased with the size reduction of nanorods caused by the buffer layer may be the main reason for it. And the small shift in the UV emission was caused by the rapid reduction in crystal size and compressive stress from Raman spectra results. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Mo back contact

CdS/CdTe thin film solar cells with an area of 1 cm 2 were obtained and studied in detail. A ZnO buffer layer was deposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also by using RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of the solar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched by using Br–Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Mo targets. The procedure developed in this work led us to make systematically solar cells with good efficiency. However, the series resistance has a high value for an area of 1 cm 2 (22 Ω cm 2). In order to make more detailed study, the solar cell with an area of 1 cm 2 was divided in a 3 × 3 matrix. A good homogeneity in cell properties is observed and the efficiency increases to more than 11%, fundamentally through decreasing series resistance.