Quality Of ZnO Films Research Articles

Topology and texture controlled ZnO thin film electrodeposition for superior solar cell efficiency

It is known that the properly textured ZnO film can increase light-scattering and therefore improves light-trapping inside the solar cells, resulting in increased Jsc value and consequently solar cell efficiency. Using a low cost solution-based electrochemical deposition, we have developed a ZnO thin film deposition process that offers fine-control of crystal orientation, surface morphology and texture by tuning an additive concentration. Using the optimized textured ZnO films, we have achieved superior light trapping performance especially in long wavelength spectrum (λ>610nm) comparing to the vacuum sputtered ZnO. Using single junction a-SiGe:H and multi-junction a-Si:H/a-SiGe:H/a-SiGe:H thin film solar cells, high quantum efficiency (current density) and solar cell efficiencies are attained. It demonstrates that the low cost electrochemical ZnO deposition is an effective method to prepare high quality ZnO film for solar cells.

Improved photoelectrical properties of n-ZnO/p-Si heterojunction by inserting an optimized thin Al₂O₃ buffer layer.

The n-ZnO/p-Si heterojunction with an ultrathin Al₂O₃ buffer layer was prepared by atomic layer deposition. X-ray diffraction revealed that the crystalline quality of (100)-oriented ZnO films was improved with an Al₂O₃ buffer layer. The n-ZnO/p-Si heterojunction with 5 nm inserted Al₂O₃ layer shows the best electrical characteristics, with a dark current of 0.5 μA at a reverse bias of -2 V and increasing the photo-to-dark current ratio effectively by 8 times. These results demonstrated that Al₂O₃ buffer layer with optimized thickness exhibits significant advantages in enhancing the crystal quality of ZnO film and improving the photoelectrical properties of n-ZnO/p-Si photodetectors.

ZnO thin films prepared on titanium substrate by PLD technique at different substrate temperatures

ZnO thin films were grown by pulsed laser deposition on titanium substrates at different substrate temperatures ranging from 300 to 700 °C. X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS),photoluminescence, and Raman spectroscopy are employed to investigate the change of properties. XRD, XPS, and Raman data showed that the films consisted of TiO2 at high substrate temperature, which will deteriorate the crystallization quality of ZnO films. The optimum temperature for the growth of ZnO films on the Ti substrate is about 500 °C in this paper. The ZnO films grown on titanium substrate can be used in direct current, microwave, and medical applications. Copyright © 2014 John Wiley & Sons, Ltd.

ZnO deposited on Si (111) with Al2O3 buffer layer by atomic layer deposition

ZnO films were grown on Si (111) substrates by atomic layer deposition (ALD) at a low-temperature of 200 °C, and the structural and optical properties of the films were investigated. By inserting an ultrathin Al2O3 buffer layer between ZnO and Si, the crystalline quality of ZnO films was improved, resulting in the increase of the intensity of the photoluminescence.

Emission and Structure Varieties in ZnO:Ag Nanorods Obtained by Ultrasonic Spray Pyrolysis

Scanning electronic microscopy (SEM), X ray diffraction (XRD) and photoluminescence (PL) have been applied to the study of the structural and optical properties of ZnO nanocrystals prepared by the ultrasonic spray pyrolysis (USP) at different temperatures. The variation of temperatures and times at the growth of ZnO films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO nanocrystals (NCs), as well as to vary their photoluminescence spectra. The study has revealed three types of PL bands in ZnO NCs: defect related emission, the near-band-edge (NBE) PL, related to the LO phonon replica of free exciton (FE) recombination, and FE second-order diffraction peaks. The PL bands related to the LO phonon replica of FE in PL spectra measured at room temperature testify on the high quality of ZnO films prepared by the USP technology.

Investigate the interface structure and growth mechanism of high quality ZnO films grown on multilayer graphene layers

The mechanically stable ZnO films with excellent optical properties were directly grown on the graphene layers without any additional treatments by using pulsed laser deposited (PLD). XRD, Raman spectrum and temperature-dependent photoluminescence (TDPL) are employed to study the structure and optical properties of ZnO films grown on the graphene layers. By adopting a plane wave ultrasoft pseudopotential method based on density functional theory, the adsorption binding energy of ZnO molecules deposited on graphene surface is calculated. It is suggested that formation of C–Zn bond rather than the C–O bond during initial stages of adsorption. What are more; the binding energies and interfacial adsorption energy were calculated. It can be concluded that the interface structure model is tend to be graphene–Zn–O–Zn–O in ZnO thin films grown on graphene. The ZnO-graphene nano-film is the promising candidate for potential application in flexible optoelectronic devices.

Emission Diversity of ZnO Nanocrystals with Different Growth Temperatures

ABSTRACTScanning electronic microscopy (SEM), X ray diffraction (XRD) and photoluminescence (PL) have been applied to the study of structural and optical properties of ZnO nanocrystals prepared by the ultrasonic spray pyrolysis (USP) at different temperatures. The variation of temperatures and times at the growth of ZnO films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO nanocrystals (NCs), as well as to vary their photoluminescence spectra. The study has revealed three types of PL bands in ZnO NCs: defect related emission, the near-band-edge (NBE) PL, related to the LO phonon replica of free exciton (FE) recombination, and its second-order diffraction peaks. The PL bands, related to the LO phonon replica of FE, and its second-order diffraction in the room temperature Pl spectrum testify on the high quality of ZnO films prepared by the USP technology.

Sputter deposition of high quality ZnO films

Sputter deposition of high quality ZnO films

Microstructure Properties of ZnO/ Indium Tin Oxide and ZnO/CuInSe2 grown by Electrochemical Deposition

In this research, we grew a ZnO film as a buffer layer on indium tin oxide (ITO) substrates and CuInSe2/ITO (CIS/ITO) substrates by electrochemical deposition. To examine material quality of ZnO films, multiple material and optical analyses including scanning electron microscope images, electron probe microanalyzer images, UV-VIS, and x-ray diffraction were conducted. The results indicate that an appropriate amount of H2O2 might enhance the growth of ZnO films on the ITO substrate. Furthermore, triethanolamine addition might suppress the reaction between KCl and the CIS substrate to enhance the growth of the ZnO film on the CIS substrate. The ZnO/CIS structure may replace the toxic CdS/CIS structure for the further development for the CIS solar cell.

Growth of high quality ZnO thin films with a homonucleation on sapphire

ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373 nm for 1 μm × 1 μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking curves were measured to be 13 and 26 arc/s, respectively. This FWHM value is lower than any reported to date in the literature, with ω and ω-2θ values indicating excellent coherence of the epitaxial layer along the interface and the growth direction, accordingly. These x-ray diffraction and surface roughness values are lower than those obtained using common nucleation layers such as MgO, indicating that growth with ZnO nucleation layers on sapphire may lead to higher quality electrical and optical devices.

Temperature-dependent luminescence dynamics for ZnO thin films

The paper presents the photoluminescence investigation of zinc oxide thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz wafers. The films with different thickness (number of layers) were annealed at different temperatures after the preparation process. It was found that high quality, transparent ZnO thin films could be produced on quartz substrates at relatively low annealing temperature (450–550 \(^{\circ }\mathrm{C}\)). The dependence of the ZnO thin film quality was studied by X-ray diffraction and atomic force microscopy techniques. Optical properties were investigated by classic and time-resolved photoluminescence (TRPL) measurements. Photoluminescence spectra allowed us to estimate energy of the free excitons, bond excitons and their longitudinal optical (LO) phonon replicas as a function of the annealing temperature. An innovative TRPL technique let us precisely measure the decay time of the free- and bond excitons’ in the real time. TRPL measurements as a function of temperature reveal a biexponential decay behavior with typical free/bound exciton decay constants of 970/5310 ps for the as-grown sample and 1380/5980 ps after annealing process. Presented spectra confirm high structural and optical quality of investigated films. We proved that the thermal treatment improve both optical and structural quality and extend the photoluminescence’s lifetimes. The obtained experimental results are important for identification of exciton’s peaks and their LO phonon replicas for the investigated ZnO films.

Sol–gel derived nanostructure undoped and cobalt doped ZnO: Structural, optical and electrical studies

Nanostructure ZnO and ZnO:Co films were deposited onto borofloat glass substrates by the sol–gel method using spin coating technique. X-ray diffraction (XRD) study was used to investigate the effects of cobalt dopant on the crystalline structure and orientation of the films. XRD observations showed that the crystalline quality of ZnO film decreased with increasing cobalt content. The effect of Co incorporation on the surface morphology was clearly observed from scanning electron microscopy (SEM) images. The chemical composition of the 3% Co doped nanostructure ZnO film was confirmed by X-ray photoelectron spectroscopy (XPS). The doping of ZnO with Co causes a decrease in particle size and crystallinity while it leads to an increase in the band gap, which changes in accordance with the Burstein–Moss effect. The optical constants of the films such as refractive index and extinction coefficient were determined using transmittance and reflectance spectra. The sheet resistance of the films, which was calculated using Van der Pauw method, first increased by about two orders of magnitude and then tended to decrease.

Effect of oxygen content on the structural and optical properties of ZnO films grown by atmospheric pressure MOCVD

Atmospheric pressure MOCVD was used to deposit ZnO layers on sapphire and homoepitaxial template under different oxygen flow rates. Oxygen content affects the lattice constant value and texture coefficient of the films as evidenced by the θ–2θ peaks position and their intensity. Films deposited at lower oxygen flow rate possess higher value of strain and stresses. ZnO films deposited at high oxygen flow rates show intense UV emissions while samples prepared under oxygen deficient conditions exhibited defect related emission along with UV luminescence. The results are compared to the ZnO films deposited homoepitaxially on annealed ZnO samples. The data obtained suggest that ZnO stoichiometry is responsible for the structural and optical quality of ZnO films.

Growth of ZnO Films on Si(111) by Metalorganic Chemical Vapor Deposition with AlN and Low-Temperature ZnO Double Buffers

To enhance the quality of ZnO films on Si(111) substrate, single layers of low-temperature ZnO (LT-ZnO) and AlN, as well as a combination of AlN and LT-ZnO layer, were used as intermediate layers by atmospheric pressure metalorganic chemical vapor deposition system. Only polycrystalline ZnO film was formed when a LT-ZnO single buffer was used. Crystal quality was enhanced when LT-ZnO was replaced by 20 nm AlN as the single buffer. The full width at half maximum (FWHM) of ZnO(0002) x-ray diffraction ω-rocking curve was 642 arcsec. Cracks began to appear on the film surface as crystallinity was enhanced. A ZnO mosaic single-crystal film with a mirror-like surface was successfully fabricated when a combined AlN and LT-ZnO served as buffer, and its FWHM of ZnO(0002) ω-rocking curve peak was only 460 arcsec. The film surface was smoother but cracks were still evident on the film. Contrary to the three-dimensional growth mode of samples with a single buffer, a quasi-two-dimensional growth mode was realized for the double-buffered high-temperature ZnO layer. Calculated film thickness was 2.14 μm, and the growth rate reached 4.3 μm/h based on the laser in situ laser reflectance trace.

MgO(111)上ZnO薄膜的外延生长及其结构和光学特性

由于ZnO在光电器件的应用前景，其高质量薄膜的制备是研究热点之一。本文通过分子束外延生长法在MgO(111)单晶衬底上生长ZnO薄膜，表征了其结构和特性，探讨了不同生长条件对薄膜质量的影响。结果表明，先低温生长ZnO缓冲层，再高温生长ZnO薄膜，有望提高ZnO薄膜的质量。原位反射高能电子束衍射(RHEED)和异位的X射线衍射(XRD)分别测量出薄膜的面内结构和沿[0001]的单晶域高取向结构，并确定薄膜和衬底的外延关系为ZnO[01-10]//MgO[1-10]和ZnO[2-1-10]//MgO[11-2]。透射谱显示了ZnO的特征光学带隙。 The growth of high quality ZnO films is highly desirable due to the promising applications of ZnO in optoelectronics. In this paper, ZnO films were grown on the MgO(111) substrates via the growth technique of molecular- beam epitaxy and their structural and optoelectronic properties were characterized. In particular, the influence of growth condition on the film qualify was investigated. The results show that, inducing a low temperature ZnO buffer layer before the high temperature growth of ZnO films will help to improve the film quality. In situ reflection high-energy electron diffraction (RHEED) and ex situ X-ray Diffraction (XRD) measurements indicate that the ZnO film and the MgO substrate follow the epitaxial relationship: ZnO[01-10]//MgO[1-10] and ZnO[2-1-10]//MgO[11-2]. Transmission Spectra show the characteristic optical bandgap of ZnO.

Heteroepitaxial growth and characterization of ZnO films on Gd3Ga5O12 garnet substrates

ZnO (0001) films have been grown epitaxially on (111) and (001) planes of Gd3Ga5O12 (GGG) substrates, which have different orientations and symmetries. The X-ray diffraction revealed that the out-of-plane and in-plane epitaxial relations of ZnO films on the GGG (111) substrates [ZnO/GGG (111)] were the [0001]ZnO || [001]GGG and [10 − 10]ZnO || [1 − 12]GGG ± 21°, while ZnO films on the GGG (001) substrates [ZnO/GGG (001)] showed the out-of-plane and in-plane epitaxial relations with the [0001]ZnO || [001]GGG and [10 − 10]ZnO || [100]GGG, [010]GGG. The mosaic spread of ZnO/GGG (001) along the in-plane and out-of-plane directions was larger than that of ZnO/GGG (111), which was closely related to a number of dislocations generated in the ZnO lattices. These mosaic structures mainly determined crystal quality of ZnO films, as also confirmed by transmittance electron microscopy (TEM). Moreover, high-resolution TEM showed that the lattice of ZnO along the [0001] direction was connected with lattices of GGG along the [111] and [001] directions without interface buffer layers. Finally, crystalline quality of ZnO films was also clarified by temperature-dependent photoluminescent spectroscopy. This study contributes to provide a new device concept on oxide semiconductor/garnet interfaces.

P-type conductivity control of heteroepitaxially grown ZnO films by N and Te codoping and thermal annealing

The authors report on the p-type conductivity in the ZnO films, grown on c-Al2O3 substrates by molecular-beam epitaxy, using N and Te codoping and thermal annealing. In the electrical properties, the N and Te codoping effectively suppresses the background electron concentration of ZnO films, and the thermal annealing causes the conductivity conversion from n-type to p-type. In the structural properties, the N and Te codoping deteriorates the crystalline quality of ZnO films, even if Te doping contributes to improve the crystallinity, and the thermal annealing recovers the degraded crystalline quality again. In the optical properties, the N and Te codoping simultaneously increases the donor-related emission, the acceptor-related emission, and the nonradiative recombination in ZnO films, while the thermal annealing relatively enhances the acceptor-related emission. It is proposed that (i) the N and Te codoping simultaneously induces N-related defects and donor-type defects along with free acceptors in the ZnO films, but the amount the N-related defects is larger than the donor-type defects and the free acceptors; and (ii) the thermal annealing relatively activates the N-related defects and suppresses the donor-type defects.

The effect of surfactant on the structure and properties of ZnO films prepared by electrodeposition

► Surfactant op-10 as additive. ► Adding surfactant plays an important role in improvement crystal quality of ZnO films. ► Deposition rate of the films is increased about two times after adding surfactant. ► The quality of deposited ZnO films was improved for an optimized surfactant ratio 0.5%. The effect of surfactant on the formation, structure and properties of ZnO films synthesized by electrodeposition was studied in this work. It was carried out in an aqueous Zn(NO 3 ) 2 solution containing surfactant op-10 using cathodic galvanostatic method. The results showed that the additive surfactant effectively inhibited hydrogen evolution reaction on cathode surface, maintained stability of the solution pH and improved deposition rate of the films to two times. Grown ZnO films with uniform grain and smooth surface were observed by using atomic force microscopy. Optical characterizations indicated that average optical transmittance of such films was more than 80% in the visible wavelength range, and its optical band gap was near 3.21 eV.

Carrier localization in ZnO quantum wires

ZnO quantum wires are grown on patterned silicon substrates using plasma-assisted molecular beam epitaxy. A crystallographic etch process exposes the Si(111) planes from Si(001) substrates so that V-grooves are formed. The large misfit between the Si(111) and ZnO(0001) lattice constants leads to low ZnO film quality after growth. Subsequent thermal annealing improves the electronic and structural quality of the film substantially. In the center region of the wire we find a strong photoluminescence intensity enhancement, which arises from carrier localization in the ZnO quantum wire. Two-dimensional self-consistent calculations of the electronic structure show excellent agreement with the experimental results.

High optical quality ZnO films grown on graphite substrate for transferable optoelectronics devices by ultrasonic spray pyrolysis

ZnO thin films were deposited on graphite substrates by ultrasonic spray pyrolysis method. The effects of substrate temperature and film thickness on the crystalline structure, morphology, and optical properties of the as-grown ZnO films were investigated systemically. Results illustrated that dense ZnO films with hexagonal wurtzite structure were uniformly distributed on the substrate. Strong near-band edge ultraviolet (UV) emission peaks were observed in room temperature photoluminescence (PL) spectra for the samples prepared under optimized parameters, yet the usually observed defect related deep level emissions were nearly undetectable, indicating high optical quality ZnO films could be achieved via this easy process under optimal conditions. The successful growth of polycrystalline ZnO films on graphite offers the significant opportunity to be readily transferred onto any rigid or flexible foreign substrates, since the graphite substrates consist of weakly bonded layer structure.