ZnO Buffer Layer Research Articles

Thermal pretreatment of sapphire substrates prior to ZnO buffer layer growth

The properties of ZnO buffer layers grown via metal-organic chemical vapor deposition (MOCVD) on sapphire substrates after various thermal pretreatments are systematically investigated. High-temperature pretreatments lead to significant modifications of the sapphire surface, which result in enhanced growth nucleation and a consequent improvement of the surface morphology and quality of the ZnO layers. The evolution of the surface morphology as seen by atomic force microscopy indicates an obvious growth mode transition from three-dimensional to quasi-two-dimensional as the pretreatment temperature increases. A minimum surface roughness is obtained when the pretreatment temperature reaches 1150 °C, implying that a high-temperature pretreatment at 1150 °C or above may lead to a conversion of the surface polarity from O-face to Zn-face, similar to processes in GaN material growth via MOCVD. By analyzing the evolution of the film properties as a function of pretreatment temperature, the optimal condition has been determined to be at 1150 °C. This study indicates that a high-temperature pretreatment is crucial to grow high-quality ZnO on sapphire substrates by MOCVD.

Vanadium Dioxide Thin Films Deposited on ZnO Buffer Layer for Smart Thermochromic Glazing of Windows

Vanadium oxide thin films have been deposited on glass and ZnO-coated glass substrates by reactive RF-magnetron sputtering deposition at different substrate temperature. The introduction of ZnO buffer layer could increase the transmittance. The buffer layer ZnO could lead to the increase of the crystallinity quality of VO2 films and substrate temperature could be decreased to 100°C for the obtaining of polycrystalline VO2 structure. The structure of VO2/ZnO/glass is considered to be potentially applicable to smart windows of high total energy efficiency in architectures or automobiles.

Defect-mediated ferromagnetism in ZnO:Mn nanorods

In this work, the structural, chemical and magnetic properties of ZnO:Mn nanorods were investigated. Firstly, well-aligned ZnO nanorods with their long axis parallel to the crystalline c-axis were successfully grown by the vapor phase transport technique on Si substrates coated with a ZnO buffer layer. Mn metal was then diffused into these nanorods at different temperatures in vacuum. From SEM results, ZnO:Mn nanorods were observed to have diameters of ∼100 nm and lengths of 4 μm. XPS analysis showed that the Mn dopant substituted into the ZnO matrix with a valence state of +2. Magnetic measurements performed at room temperature revealed that undoped ZnO nanorods exhibit ferromagnetic behavior which may be related to oxygen vacancy defect-mediated d 0 ferromagnetism. ZnO:Mn samples were seen to show an excess room temperature ferromagnetism that is attributed to the presence of oxygen vacancy defects forming bound magnetic polarons involving Mn.

Efficiency limitations of thermally evaporated thin-film SnS solar cells

Thin-film solar cells with SnS as absorber material were prepared by thermal evaporation of SnS. The cells were built in a superstrate configuration using Al : ZnO coated glass as front contact with an intrinsic ZnO buffer layer and/or CdS window layer and a gold back contact. The IV-characteristics and external quantum efficiency of the devices were determined. The best CdS/SnS solar cell showed a conversion efficiency of 1.6%, a short circuit current density of 19 mA cm−2 and an open circuit voltage of 217 mV. Moreover, band alignments at the interfaces AZO/SnS, CdS/SnS and SnS/Au were determined with in situ x-ray photoelectron spectroscopy to correlate the open circuit voltage limitations of the investigated solar cell device structures.

Effect of ZnO Buffer Layer on the Bending Durability of ZnO:Ga Films Grown on Flexible Substrates: Investigation of Surface Energy, Electrical, Optical, and Structural Properties

ZnO:Ga (GZO) transparent conducting oxide (TCO) films are deposited on flexible polyethersulfone (PES) substrates using the radio frequency sputtering technique. The bending durability of flexible TCOs is improved by inserting 100-nm-thick ZnO buffer layers. The strain of the samples with and without buffer layers arising from bending is studied using X-ray diffraction. After the insertion of 100-nm-thick ZnO buffer layers, the strain of GZO films without ZnO after outward and inward bending for 2000 cycles decreases from 2.063 ×10-3 and 2.203 × 10-3 to 1.74 × 10-3 and 1.966 × 10-3, respectively. Such strain variation is caused by the difference in adhesive force at the surface of 100-nm-thick ZnO/PES and PES. The surface energy of PES and 100-nm-thick ZnO/PES bent outwards and inwards for 2000 cycles increased from 34.38 and 30.56 to 36.45 and 36.03 mJ/m2, respectively.

Enhanced Performance in Inverted Polymer Solar Cells with D–π–A‐Type Molecular Dye Incorporated on ZnO Buffer Layer

We report the superior characteristics of a ZnO buffer layer covered with a phenothiazine-based, π-conjugated donor-acceptor (D-π-A)-type organic dye (called "d-ZnO"). The use of this system for the performance enhancement of inverted bulk heterojunction polymer solar cells (PSCs) with the configuration of indium tin oxide/d-ZnO/polymer:PC71 BM/MoO3 /Ag (PC71 BM=[6,6]-phenyl C71 butyric acid methyl ester) is investigated. The layer of organic dyes anchored on the ZnO surface through carboxylate bonding reduces the shunt path on bare ZnO surface and provides better interfacial contacts and energy level alignments between the ZnO layer and the photoactive layer. This phenomenon consequently leads to highly enhanced photovoltaic parameters (fill factor, open-circuit voltage, and short-circuit current density) and power conversion efficiencies (PCEs). Inverted solar cells containing the d-ZnO layer not only revealed about 34% (PCE: 4.37%) and 18% (PCE: 7.11%) improvement in the PCEs of the representative poly-3(hexylthiophene) (P3HT) and low-band-gap poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-alt-[2-(2'-ethylhexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-C-T) polymer systems, respectively, but also showed 2-4 times longer device lifetimes than their counterparts without the organic dye layer. These results demonstrate that this simple approach used in inverted PSCs with a metal oxide buffer layer could become a promising procedure to fabricate highly efficient and stable PSCs.

Improvement of Resistive Switching Performances in ZnLaO Film by Embedding a Thin ZnO Buffer Layer

The Pt/ZnLaO/ZnO/Pt structure resistive switching (RS) device was fabricated by chemical solution deposition (CSD) method, in which ZnO was used as a buffer layer. Compared with the Pt/ZnLaO/Pt structure device, by embedding a thin ZnO buffer layer between the ZnLaO and the Pt bottom electrode (BE), Pt/ZnLaO/ZnO/Pt structure device exhibits much better RS performances, such as narrow dispersion of resistance state and switching voltage. The reason for improvement of the RS behaviors in the Pt/ZnLaO/ZnO/Pt structure device is that the ZnO buffer layer may provide easy path to form a fixed conducting filament in the device.

Photoluminescence characterization of photovoltaic effect in ZnO/CdS/Cu(In,Ga)Se2 heterostructure

A series of studies on photoluminescence (PL) and time-resolved PL in Cu(In,Ga)Se2 (CIGS) solar cells is presented with emphasis on the photovoltaic effect in the ZnO/CdS/CIGS heterostructure. The photovoltage created by an excitation light increases PL yield and PL decay time of the near-band-edge PL of the CIGS absorber layer in the CIGS solar cell. Under the open circuit condition, the photovoltage reduces the carrier separation field in the space charge region. This reduces the collection of photoexcited carriers, which results in the high PL yield and long PL decay time. The two-dimensional PL intensity mapping of the CIGS solar cell reflects the photovoltage distribution, and the results are compared with the light-beam-induced current image. PL characterization of the CIGS solar cell process has been studied. A chemical-bath deposition of the CdS buffer layer and the following sputter-deposition of the high-resistivity ZnO buffer layer do not change PL intensity and PL decay time of the CIGS layer. On the other hand, a metalorganic chemical vapor deposition (MOCVD) of the high-resistivity ZnO layer leads to high PL intensity and the long PL decay time. The result is discussed in terms of the thermal diffusion of Cd from CdS layer into the CIGS absorber layer during the MOCVD process at an elevated temperature. The formation of the buried pn junction in the CIGS layer due to the Cd-diffusion is discussed in terms of an electron-beam induced current measurement. The utilization of PL as a measure of the junction formation is discussed.

Improved Performance of Organic Light-Emitting Diodes Fabricated on Al-Doped ZnO Anodes Incorporating a Homogeneous Al-Doped ZnO Buffer Layer Grown by Atomic Layer Deposition

In this work, we investigated the use of a homogeneous Al-doped zinc oxide (AZO) buffer layer to improve the performance of an organic light-emitting diode (OLED) device fabricated on an AZO anode. For this, 10-nm-thick AZO buffer layers with Al doping concentrations of 3.1, 4.1, and 5.1 at % were grown on 140-nm-thick AZO anode films containing 2.1 at % Al by atomic layer deposition. The electrical resistivity of the AZO anode with a homogeneous AZO buffer layer decreased with an increase in Al doping concentration up to 4.1 at %; however, the resistivity increased at higher doping concentrations in the AZO buffer layer. On the other hand, the work functions of the AZO anode with the AZO buffer layer containing various Al doping concentrations gradually increased with an increase in Al doping concentration from 3.1 to 5.1 at %. Therefore, the best film properties were obtained for an AZO anode with an AZO buffer layer containing 4.1 at % Al, and the work function value for this film was 4.64 eV. The highest luminance and current efficiency values were optimized to be 20290 cd/m(2) and 13.4 cd/A, respectively, with the OLED device composed of a DNTPD/TAPC/Bebq2:10% doped RP-411/Bphen/LiF/Al structure on an AZO anode with an AZO buffer layer containing 4.1 at % Al.

Influence of metal organic chemical vapour deposition growth conditions on vibrational and luminescent properties of ZnO nanorods

A detailed optical characterization by means of micro Raman and cathodoluminescence spectroscopy of catalyst-free ZnO nanorods grown by atmospheric-metal organic chemical vapour deposition has been carried out. This characterization has allowed correlating the growth conditions, in particular the precursors partial-pressures and growth time, with the optical properties of nanorods. It has been shown that a high Zn supersaturation can favor the incorporation of nonradiative recombination centers, which can tentatively be associated with ZnI-related defects. Characterization of individual nanorods has evidenced that ZnI-related defects have a tendency to accumulate in the tip part of the nanorods, which present dark cathodoluminescence contrast with respect to the nanorods bottom. The effect of a ZnO buffer layer on the properties of the nanorods has been also investigated, showing that the buffer layer improves the luminescence efficiency of the ZnO nanorods, revealing a significant reduction of the concentration of nonradiative recombination centers.

Effect of ZnO buffer layer thickness on the epitaxial growth of GaN by reactive magnetron sputtering

Epitaxial GaN films were grown on sputtered ZnO buffer layers of thickness 25–200nm over c-plane sapphire by reactive sputtering of GaAs in nitrogen at 700°C. The epitaxial quality and microstructure have been studied by high resolution X-ray diffraction in phi (ϕ) and omega (ω) scan geometries. The surface morphology of epilayers was studied by atomic force microscopy and scanning electron microscopy and their crystalline quality was assessed by Raman spectroscopy. These studies have shown that ZnO buffer layers of 50–100nm facilitate growth of GaN epilayers of high crystalline quality, compared to those grown on thinner and thicker ZnO buffer layers.

Influence of ZnO buffer layer on microstructure and Raman scattering of ZnO:Ag film on Si substrate

Pure ZnO and Ag-doped ZnO (ZnO:Ag) films were deposited on ZnO buffer layer and p-Si (100) substrate by radio frequency reactive magnetron sputtering. ZnO buffer layer was deposited at lower power. The microstructures of the films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis techniques. The results indicated that ZnO:Ag film had a stronger preferred orientation toward the c-axis, a more uniform grain size and smoother surface after using ZnO buffer layer, and the residual strain was reduced. In addition, stronger Raman scattering was observed from ZnO:Ag film on ZnO buffer layer than that on Si substrate. It was because the ZnO buffer layer effectively relaxed the partial stress induced by the large lattice mismatch between the film and Si. Two additional local vibrational modes (LVMs) at 245 and 495.8cm−1 induced by Ag dopant in ZnO:Ag films were observed by Raman spectra, corresponding to Ag atoms located at O sites (LVM of Zn–Ag) and Ag atoms in the films. It meant that the LVMs could act as an indication of Ag incorporation into ZnO:Ag film.

Interface Recombination in Depleted Heterojunction Photovoltaics based on Colloidal Quantum Dots

AbstractInterface recombination was studied in colloidal quantum dot photovoltaics. Optimization of the TiO2‐PbS interface culminated in the introduction of a thin ZnO buffer layer deposited with atomic layer deposition. Transient photovoltage measurements indicated a nearly two‐fold decrease in the recombination rate around 1 sun operating conditions. Improvement to the recombination rate led to a device architecture with superior open circuit voltage (VOC) and photocurrent extraction. Overall a 10% improvement in device efficiency was achieved with Voc enhancements up to 50 mV being realized.

Effects of particle morphology of ZnO buffer layer on the performance of organic solar cell devices

The performance of poly(3-hexyltheopene):[6,6]-phenyl C61-butyric acid methyl ester or P3HT:PCBM based organic solar cell (OSC) devices can be improved by adding an electron extraction layer of a wide band gap semiconducting material such as ZnO or TiO2 that facilitates the electron transport from the photo-active layer (P3HT:PCBM blend) to the top metal electrode (e.g. Al) and, at the same time, blocks holes from reaching the top electrode. Other factors that determine performance of the OSC devices include morphology, thickness and donor–acceptor ratio. In this study we investigated the effects of concentration and particle morphology (nanoparticle versus nanoflake) of ZnO electron extraction layer on the performance of the OSC devices with configuration ITO/PEDOT:PSS/P3HT:PCBM/ ZnO/Al. The concentration of ZnO nanoparticle or nanoflake solutions was varied from 0.5 to 20mg/ml. A power conversion efficiency (PCE) of 3.08% was recorded from devices incorporating ZnO nanoflake electron extraction layer, whereas PCE of 2.37% was recorded from devices with ZnO nanoparticles as the electron extraction layer. The maximum PCE was obtained from a concentration of 0.5mg/ml ZnO for both devices. The influence of the particle morphology and the concentration of the ZnO electron extraction layer on the general performance of the OSC devices is discussed in detail.

Growth of highly near-c-axis oriented ferroelectric LiNbO3 thin films on Si with a ZnO buffer layer

Ferroelectric LiNbO3 thin film of high near-c-axis orientation was grown on Si by pulsed laser deposition using a thin ZnO buffer layer of high c-axis orientation. Both the LiNbO3 film and the ZnO layer consist of columnar grains with random in-plane twist relative to each other. The c axes of LiNbO3 grains tilt small angles with respect to the film growth direction and the deviation angles follow nearly Gaussian probability distribution within 5°. The out-of-plane tilt nucleation of LiNbO3 lattices on the waved surfaces of the ZnO layer leads to the near-c-axis oriented growth of the LiNbO3 film.

Synthesis and characterization of ZnO/ZnMgO multiple quantum wells by molecular beam epitaxy

The growth of single and multiple (three) ZnO/ZnMgO quantum well samples on sapphire substrates, through a two-step temperature variation growth of ZnO buffer layers by molecular beam epitaxy (MBE), were investigated. For single quantum well (QW) growth, the thicker first ZnMgO barrier layer about 220 nm on the high-temperature growth ZnO (HT-ZnO) buffer layer, accumulated larger compressive stress, to achieve higher quality ZnO/ZnMgO QW growth. In the temperature-dependent photoluminescence (PL) results, the obvious S-shape variation of emission peak positions presented the stronger exciton confinement ability of QW in the higher magnesium concentrations of ZnMgO barrier layer growth. Compared to the control sample, the quantum confinement resulted in blueshift PL peaks of QW samples at low temperature. The multiple quantum well (MQWs) structure increased the exciton confinement ability to enhance the light emission efficiency of the sample. The three ZnO/ZnMgO MQWs structures were found clearly by high-resolution transmission electron microscopy.

All‐Solution‐Processed InGaO3(ZnO)m Thin Films with Layered Structure

We fabricated the crystallized InGaZnO thin films by sol‐gel process and high‐temperature annealing at 900°C. Prior to the deposition of the InGaZnO, ZnO buffer layers were also coated by sol‐gel process, which was followed by thermal annealing. After the synthesis and annealing of the InGaZnO, the InGaZnO thin film on the ZnO buffer layer with preferred orientation showed periodic diffraction patterns in the X‐ray diffraction, resulting in a superlattice structure. This film consisted of nanosized grains with two phases of InGaO3(ZnO)1 and InGaO3(ZnO)2 in InGaZnO polycrystal. On the other hand, the use of no ZnO buffer layer and randomly oriented ZnO buffer induced the absence of the InGaZnO crystal related patterns. This indicated that the ZnO buffer with high c‐axis preferred orientation reduced the critical temperature for the crystallization of the layered InGaZnO. The InGaZnO thin films formed with nanosized grains of two‐phase InGaO3(ZnO)m superlattice showed considerably low thermal conductivity (1.14 Wm−1 K−1 at 325 K) due to the phonon scattering from grain boundaries as well as interfaces in the superlattice grain.

Effects of ZnO Buffer Layer on Characteristics of ZnO:Ga Films Grown on Flexible Substrates: Investigation of Surface Energy, Electrical, Optical, and Structural Properties

Ga-doped ZnO (GZO) transparent conductive oxide thin films were deposited on flexible polyethersulphone (PES) substrates by radio-frequency sputtering at room temperature. With the addition of an optimized 100-nm-thick ZnO buffer layer, the transmittance, carrier concentration, Hall mobility, and resistivity of GZO films improved from 88.3 to 94.45%, −2.89 × 1021 to −3.39 × 1021 cm−3, 1.76 to 7.97 cm2/V-s, and 1.32 × 10−3 to 2.201 × 10−4 Ω-cm, respectively. The higher surface energy (49.85 mJ/m2) of the 100-nm-thick ZnO layer deposited on the PES substrate compared to those of a bare PES substrate and ZnO layers of other thicknesses deposited on PES substrates is likely responsible for the superior GZO quality obtained with the GZO/100-nm-thick ZnO buffer/PES structure. Moreover, the best adhesion at the GZO surface was observed for the GZO/100-nm-thick ZnO buffer/PES structure based on it having the highest surface energy (67.33 mJ/m2). The GZO/100-nm-thick ZnO buffer/PES structure thus has potential to replace the GZO/PES structure for use in flexible transparent optoelectronic devices.

Understanding the Factors That Control the Formation and Morphology of Zn5(OH)8(NO3)2·2H2O through Hydrothermal Route

The influence of the choice of ethanol‐water volume ratio, concentration of zinc salt, and ZnO buffer layer on the formation and morphology of grown from the hydrothermal route was systematically discussed. Experimental results suggested that rectangle sheets and upright‐standing plates were obtained by limiting ethanol‐water volume ratio. The concentration of zinc salt was crucial for getting phase‐pure . The presence of ZnO buffer layer could lead to the that chemical composition of product grown on the substrate was totally different from the product grown in the solution. Possible formation mechanism of was also studied. Raman spectrum of displays a complex behavior with four modes, which can be assigned to the vibrational modes of Zn–H–O, Zn–O, H2O‐nitrate, and nitrate. Porously ZnO rectangle sheets were obtained by thermal treatment of rectangle sheets.

An energy-harvesting scheme employing CuGaSe2 quantum dot-modified ZnO buffer layers for drastic conversion efficiency enhancement in inorganic–organic hybrid solar cells

We demonstrated a promising route to enhance the performance of inverted organic photovoltaic (OPV) devices by the incorporation of CuGaSe2 (CGS) quantum dots (QDs) into the ZnO buffer layer of P3HT:PCBM-based devices. The modification of QDs provides better band alignment between the organic/cathode interface, improves ZnO crystal quality, and increases photon absorption, leading to more effective carrier transport/collection. By employing this energy-harvesting scheme, short-circuit current density, open-circuit voltage, and fill factor of the OPV device after CGS QD modification are improved by 9.43%, 7.02% and 6.31%, respectively, giving rise to a 23.8% enhancement in the power conversion efficiency.