ZnO Buffer Layer Research Articles

Effect of Buffer Layer Thickness on the Growth Properties of Hydrothermally Grown ZnO Nanorods

We investigated the effect of ZnO buffer layer thickness on the growth of hydrothermally grown ZnO nanorods. A series of ZnO buffer layers with different thicknesses was deposited on a p-Si (111) substrate using a co-sputtering system. After annealing the ZnO buffer layer, ZnO nanorods grown were grown hydrothermally at 95 degrees C. Unlike ZnO nanorods grown on as-deposited ZnO buffer layer, the diameter and length of ZnO nanorods grown on annealed ZnO buffer layers can be controlled. The structural and optical properties of ZnO nanorods grown on annealed ZnO buffer layers were analyzed by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence. The influence of ZnO buffer layer thickness on ZnO nanorods growth is discussed.

Epitaxial growth of non-polar a-plane AlN films by low temperature sputtering using ZnO buffer layers

This article presents an investigation on the epitaxial growth of non-polar a-plane AlN thin films by low temperature sputtering using ZnO buffer layers. Prior to the deposition of the AlN films, epitaxial growth of a-plane ZnO thin films on r-plane sapphire substrates was performed by a metalorganic chemical vapor deposition (MOCVD). The effect of MOCVD growth conditions on surface morphology and crystallinity of ZnO epi-layer was examined to optimize the growth process of buffer layer. The resulting ZnO epi-layers were used as buffer layers to grow non-polar AlN by low temperature sputtering. The measurements of XRD 2theta/omega- and phi-scans indicate that the epitaxial relationship among AlN, ZnO and sapphire substrate is 11 2 ¯ 0 AlN // 11 2 ¯ 0 ZnO // 1 1 ¯ 02 A l 2 O 3 and 1 1 ¯ 00 AlN // 1 1 ¯ 00 ZnO // 11 2 ¯ 0 A l 2 O 3 . Cross-sectional transmission electron microscopy (XTEM) revealed that no reactive intermediate layers formed at the interfacial region between AlN film and ZnO buffer layer. The a-plane ZnO layers can be used as lattice matched templates for epitaxial growth of non-polar AlN by low temperature sputtering.

MgZnO/ZnO quantum well nanowire heterostructures with large confinement energies

Mg 0.25 Zn 0.75 O / ZnO -quantum well nanowire heterostructures were grown with a three-step pulsed laser deposition process. To avoid shadowing effects during the coating, the ZnO nanowires were grown with a low area density on a ZnO buffer layer deposited on an a-plane sapphire substrate. By using spatially resolved cathodoluminescence measurements, the luminescence of axial and radial quantum wells were clearly distinguished. The large bandgap energy of the Mg0.25Zn0.75O barrier material (≈3.85 eV) made it possible to tune the energy of quantum wells from 3.4 to 3.7 eV. The homogeneity of the radial quantum well along the wire axis was probed, revealing that only small fluctuations of about 4 meV are found in the main part of the nanowire. Near the tip of the nanowire, the energy of the radial quantum well increases due to locally modified growth conditions reducing the growth rate by up to 15%. Furthermore, the growth rates of the axial and radial quantum wells were determined, showing that the one in axial direction is a factor of about 2 larger than the one in radial direction.

Highly Sensible ZnO Nanowire Ultraviolet Photodetectors Based on Mechanical Schottky Contact

Highly sensible ultraviolet (UV) photodetectors were fabricated using vertically aligned dense ZnO nanowires based on mechanical Schottky contact with Pt coated polyimides. The effective mechanical contact was achieved by depositing transparent Al-doped ZnO (AZO) buffer layers prior to the growth of the nanowires, resulting in highly uniform nanowires with similar lengths. The AZO buffer and ZnO nanowires were prepared by atomic layer deposition (ALD) and metalorganic chemical vapor deposition, respectively. The electrical conductivity of the nanowires was varied by controlling the density of oxygen vacancies by using a controlled oxygen flow rate. The high conducting nanowire-based photodetector concurrently achieved a high photocurrent signal, fast response, and fast recovery, compared with high conducting film and low conducting nanowire photodetectors. A detailed discussion regarding the developed UV photo-detecting mechanism by the different conductivities and Schottky contact is presented in this work.

Structural and Photoluminescence Properties of ZnO Films Grown on 6H-SiC Substrates by Low-Temperature Atomic Layer Deposition

We have applied atomic layer deposition (ALD) technique to grow high-quality ZnO films on 6H-SiC substrates at low temperature. The columnar growth with c axis normal to the substrate surface occurred in the ZnO films grown at temperatures above 270°C. The crystalline quality was improved by post-deposition annealing at 650°C. However, no preferred orientation growth was observed in the ZnO films deposited at a low temperature of 180°C, even after the post-annealing treatment. A two-step approach was developed to prepare high-quality and highly orientated ZnO films at a low deposition temperature of 180°C, by introducing ZnO buffer layer grown at a high temperature of 300°C. Optically pumped stimulated emission in the ZnO films with a high threshold intensity of 750 kW/cm2 was observed at room temperature, indicating that both the carrier and optical confinement are important to achieve the low-threshold stimulated emission in ZnO films.

Applications of ZnO in organic and hybrid solar cells

As an n-type inorganic semiconductor, ZnO has been widely used in organic solar cells (OSCs) and hybrid solar cells (HSCs) due to its salient characteristics such as low cost, easy synthesis, non-toxicity, high stability, and good optoelectronic properties. This article reviews the applications of ZnO in solar cells, including ZnO/organic HSCs, and OSCs with ZnO acting as electrode buffer layers or transparent electrodes. For ZnO/organic HSCs, ZnO serves as the electron acceptor material, while organic semiconductors act as electron donor materials. For the buffer layers or electrode applications, ZnO is used as an electron collection and hole blocking material where its structure plays an important role in the determination of the device performance (e.g., power conversion efficiency, lifetime, stability, etc.). Special emphasis goes to the device performance of OSCs and HSCs, which depends not only on the active materials and the device configurations, but also on the structural characteristics of the ZnO buffer layer. Finally, we briefly give an analysis on the opportunities and challenges for this promising semiconductor in OSCs and HSCs.

Solution processed inverted tandem polymer solar cells with self-assembled monolayer modified interfacial layers

Inverted tandem bulk-heterojunction solar cells with comparable efficiency to single layer devices have been demonstrated by utilizing two thin layers (∼50 nm) of poly-(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester as the active material and a fullerene self-assembled monolayer (C60-SAM) to modify the interfaces between the ZnO buffer layer and the active layer. Single and tandem solar cells without the SAM modification have much lower efficiencies than the ones with modification. The successful demonstrations of inverted tandem devices with SAM modification give promise of further device improvements if active materials with complementary absorption can be used.

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150nm-thick RuO2-coated SiO2/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO2/SiO2/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5nm-thick ZnO buffer layer on the RuO2/SiO2/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300°C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5nm-thick ZnO buffered RuO2/SiO2/Si substrates grown in 0.02M electrolyte at −1.5mA with a subsequent annealing in vacuum at 300°C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.

Structure and properties of GZO thin films grown on ZnO buffer layers

Thin gallium-doped zinc oxide (in GZO the Ga 2O 3 contents are approximately 3 wt%) films having different ZnO buffer layers were deposited using radio frequency (rf) magnetron sputtering. The use of a grey-based Taguchi method to determine the processing parameters of ZnO buffer layer deposition has been studied by considering multiple performance characteristics. A Taguchi method with an L 9 orthogonal array, signal-to-noise (S/N) ratio, and analysis of variance (ANOVA) is employed to investigate the performance characteristics in the deposition operations. The effect and optimization of ZnO buffer deposition parameters (rf power, sputtering pressure, thickness, and annealing) on the structure, morphology, electrical resistivity, and optical transmittance of the GZO films are studied. Annealing treatment and reduction in thickness resulted in a decrease in root-mean-square (RMS) surface roughness of the ZnO buffer layer. Using the optimal ZnO buffer layer obtained by the application of the grey-based Taguchi method, the electrical resistivity of GZO films was decreased from 2.94×10 −3 to 9.44×10 −4 Ω cm and the optical transmittance in the visible region was slightly increased from 84.81% to 85.82%.

ZnO:Ag film growth on Si substrate with ZnO buffer layer by rf sputtering

ZnO buffer layers were deposited on n-Si (100) substrate by rf magnetron sputtering at a lower power of 40W. Then Ag-doped ZnO (SZO) films were deposited on buffered and non-buffered Si at a higher sputtering power of 100W. The effects of buffer layer on the structural, electrical and optical properties of SZO films were investigated. The three-dimensional island growth process of ZnO buffer layer was discussed. The energy band diagram of p-SZO/n-Si heterojunction was constructed based on Anderson's model. Results show the ZnO buffer layer leads to better properties of SZO film, including larger grain size, smoother surface, higher carrier mobility, better rectifying behavior, lower interface state density, and weaker deep-level emission. It is because the ZnO buffer layer effectively relaxes the partial stress induced by the large lattice mismatch between SZO and Si.

Effect of Buffer Layer on the Properties of Laser Ablated PZT Thin Films

Lead zirconate titanate (PbZr x Ti1−x O3) or PZT ceramics are a class of piezoelectric materials that are currently experiencing widespread use in industry as electromechanical devices. PtSi/ZnO/PZT thin films were deposited by pulsed laser deposition at relatively low substrate temperature. The PZT thin films on PtSi substrates and on ZnO buffer layer were deposited at substrate temperature 300°C. The composition analysis shows that the film deposited at low temperature is stoichiometric. The films exhibit ferroelectric nature with coercive field of 19.6 kV/cm for 800 nm thick film. The leakage current density of the films shows a good insulating behavior.

Improved Inverted Organic Solar Cells With a Sol–Gel Derived Indium-Doped Zinc Oxide Buffer Layer

We studied sol-gel derived indium-doped zinc oxide (IZO) with various indium contents as a functional buffer layer in inverted polymer:fullerene bulk-heterojunction solar cell. The short-circuit current density was observed to increase by doping indium in pure ZnO buffer layer. The maximum current density was obtained with a 1 at.% indium doping. Although the open-circuit voltage and fill factor reduced slightly, the inverted organic solar cell with 1 at.% IZO buffer layer showed a power conversion efficiency of 3.3%, which is higher compared to that (2.94%) of the device with undoped ZnO buffer layer under illumination of AM1.5G. The better performance is due to combined effects of improvement in charge collection and higher optical transmittance of electrode/buffer layer stack.

Effect of GaN interlayer on polarity control of epitaxial ZnO thin films grown by molecular beam epitaxy

Epitaxial ZnO thin films were grown on nitrided (0001) sapphire substrates with an intervening GaN layer by rf-plasma-assisted molecular beam epitaxy. It was found that polarity of the ZnO epilayer could be controlled by modifying the GaN interlayer. ZnO grown on a distorted 3-nm-thick GaN interlayer has Zn-polarity while ZnO on a 20-nm-thick GaN interlayer with a high structural quality has O-polarity. High resolution transmission electron microscopy analysis indicates that the polarity of ZnO epilayer is controlled by the atomic structure of the interface between the ZnO buffer layer and the intervening GaN layer.

Temperature Dependent Optical Properties of c axis Oriented LiNbO$_{3}$ Thin Film Using Surface Plasmon Resonance

A prism coupling based Surface Plasmon Resonance (SPR) technique, assembled in the laboratory, has been employed to study the optical properties of highly (006) oriented LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film in Kretschmann-Reather configuration. A (006) oriented LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> film with c axis normal to the substrate surface is deposited at a substrate temperature of 500°C on the face of BK7 glass prism using (002) ZnO buffer layer. The SPR data on oriented LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film is measured over a wide range of temperature (300 to 573 K) at 633 nm wavelength. No report is available in the literature on the temperature dependent optical and dielectric properties of c axis oriented LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film at optical frequency. Complex dielectric constant of the film is obtained in the present study by fitting the experimental SPR data with theory. The linear change in the refractive index and dielectric constant of the as grown (006) LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film with temperature (300-573 K) indicates the importance of SPR technique in probing temperature dependent optical properties of a thin film and shows the promising application of prepared multi-layered structure on prism as effective temperature sensor.

Composition Controlled Superlattice InGaO3(ZnO)m Thin Films by Thickness of ZnO Buffer Layers and Thermal Treatment

Single crystal InGaO3(ZnO)m compound films with a superlattice structure for thermoelectric applications were fabricated on sapphire substrates with epitaxial ZnO buffer layers. Using a buffer layer with an appropriate thickness allows us to form InGaO3(ZnO)m films with complete superlattice structures along the c-axis at annealing temperatures below 1000 °C. The crystal phases of the InGaO3(ZnO)m films annealed at 900 °C were assigned to InGaO3(ZnO)1 and InGaO3(ZnO)2, depending on the thickness of the buffer layer and annealing time. When compared to the as-grown films, these InGaO3(ZnO)m films with superlattice structures exhibited both a lower electrical resistivity and higher Seebeck coefficient, due to the quantum confinement effect, resulting in improved thermoelectric properties.

Growth and characterization of nonpolar a-plane ZnO films on perovskite oxides with thin homointerlayer

Nonpolar a-plane ZnO thin films were deposited on the single-crystal perovskite SrTiO 3 (1 0 0) substrates at 750 °C by radio-frequency magnetron sputtering. The effects of ultrathin 30 nm-thick ZnO buffer layer grown at 300–600 °C on the physical properties of ZnO/SrTiO 3(STO) thin films are investigated. A high growth temperature of ZnO buffer layer enhances not only the (1 1 0)-texture of ZnO/STO thin films but also the crystalline quality of the film. However, the ZnO/STO thin film without a ZnO buffer layer has a poor crystalline quality comparing to those with the ZnO buffer layer. Atomic force microscopy morphology studies reveal that the ZnO buffer layer largely decreases the surface roughness of the ZnO/STO thin films. This may be because of the thin ZnO buffer layer effectively decreases the stress between the ZnO and STO. The results of X-ray diffraction, high-resolution transmission electron microscopy, and photoluminescence spectra show that a high-quality epitaxial ZnO (1 1 0)/STO (1 0 0) thin film that emits UV light at room temperature can be formed with a thin ZnO buffer layer grown at 600 °C.

Role of twin boundaries in semiconductor to metal transition characteristics of VO2 films

Epitaxial (002) VO2 films were grown on c-sapphire with Ga:ZnO and ZnO buffer layers. We investigated the influence of twin boundaries on the semiconductor-to-metal transition (SMT) characteristics of VO2, when current flows parallel and perpendicular to the twin boundaries. The structure of the twin boundary was kept the same for these two configurations. The hysteresis in SMT characteristics is considerably reduced when current flows parallel to the boundaries compared to that in the normal direction of the boundaries. We present a model to explain these observations and discuss the role of these boundaries on the SMT characteristics, and the importance of grain boundary engineering in the design of VO2 based devices.

ZnO as a buffer layer for growth of BiFeO3 thin films

Multiferroic BiFeO3 thin film was grown on the ZnO-buffered Pt/TiO2/SiO2/Si(100) substrate by off-axis radio frequency magnetron sputtering, where the ZnO buffer layer gave rise to a strong (110) texture for the BiFeO3 thin film. The resulting BiFeO3/ZnO thin film exhibits diode-like and resistive hysteresis behavior, in which the resistive hysteresis and rectifying ratio are dependent on the applied voltage and temperature. The resistive switching behavior of the BiFeO3/ZnO thin film is shown to relate to the trap-controlled space charge limited conduction and interface-limited Fowler–Nordheim tunneling, while the polarization reversal takes place in the BiFeO3 layer of the heterostructure. The BiFeO3/ZnO thin film is also demonstrated with a higher remanent polarization (2Pr∼153.6 μC/cm2), a much lower dielectric loss (tan δ∼0.012), and a better fatigue endurance as compared to those of the BiFeO3 thin film without a ZnO buffer layer, where the much reduced leakage is largely responsible for the enhanced ferroelectric behavior. The ZnO as a buffer layer for BiFeO3 significantly changes the dielectric relaxation and conduction mechanisms, when the dielectric relaxation and electrical conduction are governed by the thermal excitation of carriers from the second-ionization and short-range motion of oxygen vacancies, respectively, while the relaxation process remains the same over the entire temperature range of 20 to 200 °C investigated in the present study.

Inverted Organic Solar Cells with ZnO Thin Films Prepared by Sol–Gel Method

The influences of buffer layers on the performance of hybrid solar cells that are fabricated with an active layer blend of poly (3-hexylthiophene) (P3HT) and [6, 6]-phenyl-C61-buytyric acid methyl ester (PCBM), were investigated. The ZnO thin films were used as an electron transport layer in solar cells to improve the photovoltaic performances. To explore the effects of ZnO film thickness, ZnO thin films were deposited as an electron transport layer by spin coating them one to four times at 4000 rpm using zinc acetate dehydrate that was dissolved in a mixed solution of 2-methoxyethanol and ethanolamine. We fabricated IOSCs on PES substrate, consist of Au/MoO 3 /PCBM:P3HT/ZnO/ITO. The optical property of the ZnO buffer layers was investigated. Furthermore, we found the optimum ZnO buffer layer by using the sol―gel method to enhance the performance of the solar cells.

Effects of Electrodeposited Ga-doped ZnO Buffer Layer on the Performance of Inverted Organic Solar Cells

ZnO and Ga-doped ZnO thin films were electrodeposited as buffer layers between an indium tin oxide electrode and a photoactive layer for photo-induced electron transport and photo-induced hole blocking in inverted organic solar cells. Ga-doped ZnO thin films showed improved optical and electrical properties than the bare ZnO thin films, which also affected the performance of OCSs. The device with the Ga-doped ZnO buffer layer presented a higher Jsc value than that with bare ZnO which was caused by the higher transparency and carrier mobility of the deposited buffer layer. However, the Jsc value decreased when the gallium concentration in the films was increased, which induced the larger grain boundary and higher surface roughness and lead to carrier scattering and poor wettability between the buffer layers.