Microstructure and ferroelectric properties of Ta-doped Bi3.25La0.75Ti3O12/ZnO thin film capacitors

Ta-doped Bi_3.25La_0.75Ti₃O₁₂(BLTT)/ZnO films were fabricated on Pt(111)/Ti/SiO₂/Si substrates by a magnetron sputtering method. Firstly, ZnO crystal thin films were grown on the substrates by a reactive sputtering method. Then, BLTT thin films were deposited on the ZnO layers at room temperature and post-annealed at 600 °C. The micromorphology, ferroelectric and dielectric properties of BLTT/ZnO films were analyzed. The XRD analysis shows that ZnO buffer layer significantly reduces the crystallization temperature of BLTT thin film. The TEM results show that lamellar BLTT grains are grown on ZnO layer at a certain angle with few elements diffusion at the interface of ZnO phase and Bi₄Ti₃O₁₂ phase. The ferroelectric properties indicate that BLTT/ZnO films exhibit different remanent polarization and coercive fields under electric field with different directions. The novel mechanism of tailoring ferroelectric properties may open new possibilities for designing special ferroelectric devices.     

The evolution behavior of microstructures and optical properties of ZnO films using a Ti buffer layer

ZnO thin films without and with Ti buffer layer were prepared on Si and glass substrates by radio frequency (RF) magnetron sputtering. The effects of Ti buffer layer with different sputtering time on the microstructure and optical properties of ZnO thin films had been investigated by means of X-ray diffraction (XRD), energy dispersive spectrometer, X-fluorescence spectrophotometer and ultraviolet–visible spectrophotometer. The XRD results showed that the full-width at half-maximum (FWHM) for the ZnO (002) diffraction peak gradually decreased with the increase of sputtering time of Ti buffer layer, indicating that the crystalline quality of ZnO thin films was improved. The UV peak located at 390 nm, two blue peaks located at about 435 and 487 nm, two green peaks located at about 525 and 560 nm were observed from PL spectra. The PL spectra showed that the strongest blue light emission of ZnO films was obtained from Ti buffer layer with the sputtering time of 10 min. Meanwhile, the origins of the emission peaks were discussed through the Gaussian deconvolution. We also studied the optical band gaps.     

Influence of ZnO buffer layer on AZO film properties by radio frequency magnetron sputtering

Transparent conductive films of Al-doped zinc oxide (AZO) were deposited on glass substrates under various ZnO buffer layer deposition conditions (radio frequency (r.f.) power, sputtering pressure, thickness, and annealing) using r.f. magnetron sputtering at room temperature. This work investigates the influence of ZnO buffer layer on structural, electrical, and optical properties of AZO films. The use of grey-based Taguchi method to determine the ZnO buffer layer deposition processing parameters by considering multiple performance characteristics has been reported. Findings show that the ZnO buffer layer improves the optoelectronic performances of AZO films. The AZO films deposited on the 150-nm thick ZnO buffer layer exhibit a very smooth surface with excellent optical properties. Highly c-axis-orientated AZO/ZnO/glass films were grown. Under the optimized ZnO buffer layer deposition conditions, the AZO films show lowest electrical resistivity of 6.75 × 10⁻⁴ Ω cm, about 85% optical transmittance in the visible region, and the best surface roughness of R_a = 0.933 nm.     

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zn_i and V_o is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.     

High thermoelectric performance of high-mobility Ga-doped ZnO films via homogenous interface design

Ga-doped ZnO (GZO) thin films grown on sapphire substrates have been widely investigated as a promising transparent thermoelectric (TE) material. However, due to the large lattice mismatch and thermal expansion between the sapphire substrate and GZO film, strain-induced lattice distortion impedes the transport of electrons, leading to low carrier mobility. In this study, ZnO homo-buffer layers with different thicknesses were inserted between sapphire substrates and GZO films, and their effect on the TE properties was investigated. A thin ZnO interlayer (10 nm) effectively reduced the lattice mismatch of the GZO film and improved the carrier mobility, which contributed to the large enhancement in the electrical conductivity. Simultaneously, energy filtering occurred at the interface between GZO and ZnO, resulting in a relatively high density of states (DOS) effective mass and maintaining a high Seebeck coefficient compared to that of the unbuffered GZO films. Consequently, the GZO film with a 10 nm thick ZnO buffer layer possessed a high power factor value of 449 μW m⁻¹ K⁻² at 623 K. This study provides a facile and effective method for optimizing the TE performance of oxide thin films by synergistically improving their carrier mobility and enhancing their effective mass.     

Damp-heat durability comparison of Al-doped ZnO transparent electrodes deposited at low temperatures on glass and PI-tape/PC substrates

Durability performances are compared for Al-doped ZnO (AZO) transparent electrodes deposited on hard slide-glass and flexible polyimide-tape attached to polycarbonate (PI-tape/PC) substrates. To identify the appropriate sputtering configuration, the AZO thin films are first deposited on the glass substrates via reactive RF-magnetron sputtering under 90 sccm of argon gas and 3 sccm of oxygen gas at room temperature (RT) with 83 to 90 W of RF power for 30 min. When deposited, only the sputtering configuration with 85 W of RF power could produce the AZO films with acceptable optoelectrical properties for transparent electrodes: 80% average visual transparency and 10 Ω/□ at the thickness of 1.1 μm. The temperature at the surface of the substrates rises from RT to 88 °C due to the sputtering with 85W of RF power for 30 min, and this configuration is successfully conducted for AZO film depositions on both the glass and PI-tape/PC substrates. After exposure to a damp-heat (DH) test at 85 °C and 85% relative humidity (RH) for 25 days, the conductivity of the AZO films on the PI-tape/PC substrates is significantly degraded: many cracks are visible on the films, significantly decreasing the Hall mobility. Conversely, the films deposited on the glass substrates exhibit durable high conductivity, no cracks, and excellent stability of the Hall mobility. Despite this significant difference in Hall mobility evolution, the films on both substrates show similar patterns of a slight decrease in carrier concentrations, suggesting that chemical characteristics, extensively reported as the key for the DH degradation of AZO films, are less involved in this durability study featuring AZO films prepared via a low oxygen-to-argon gas ratio of reactive sputtering at low temperatures.     

Sandwich-structured ZnO-MnO2-ZnO thin film varistors prepared via magnetron sputtering

ZnO varistors are widely used to protect electronic circuits form transient voltages. However, it is difficult to prepare varistors with voltage less than 10 V using ZnO ceramics. Here we prepared a ZnO-MnO₂-ZnO (ZMZ) sandwich thin film via magnetron sputtering and subsequent annealing at 200-500 °C. With the increase of annealing temperature, the manganese oxide sandwich layer reacts with the upper and lower ZnO layer and becomes thinner. After annealed at 500 °C, because of ZnO grain growth, the upper and lower ZnO layers joined together. The electrical properties of ZMZ films annealed at 400 °C show strong nonlinear I-V characteristics. A ZMZ low voltage thin film varistor with planar boundary potential barrier was obtained whose nonlinear coefficient α and varistor voltage V1 mA are about 30 and 6.0 V, respectively. The stable and excellent nonlinear characteristics make it a promising candidate for overvoltage protection in low operating voltage circuits.     

Variations in cross-link density with deposition pressure in ultrathin plasma polymerized benzene and octafluorocyclobutane films

Neutron reflectometry (NR) measurements of ultrathin films from octafluorocyclobutane (OFCB) and benzene (B) precursors deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) at two pressures (0.6 and 0.05 torr) reveal that under both deposition conditions there are a 7 nm-thick surface layer and an approximately 1 nm-thick transition layer next to the substrate which have structures different than those in the middle of the film. NR measurements of films swollen with solvent reveal that the density of cross-linking next to the substrate is lower than that in the middle of the film or the region adjacent to the surface of the film for both precursors. Variations in the cross-link density with processing pressure are much stronger for PP-B films than for PP-OFCB films.     

The changes in structural and optical properties of (ZnO:AlN) thin films fabricated at different RF powers of ZnO target

AlN doped ZnO thin films were prepared on glass and Si (100) substrates by RF sputtering. The ratio of nitrogen (N₂) to Argon (Ar) used to prepare the films was 80:20. The films were deposited at different RF powers of 150 W, 175 W, 200 W, 225 W and 250 W for ZnO target and 200 W for AlN target. XRD results revealed the existence of (002) ZnO phase for RF power of ZnO target above 175 W. However, at the RF power of 150 W, the film exhibited amorphous properties. The prepared films showed transmission values above 70% in the visible range. The average calculated value of energy band gap and the refractive index were 3.43 eV and 2.29 respectively. The green and UV emission peaks were observed from PL spectra. Raman Peaks at 275.49 cm^?1 and 580.17 cm^?1 corresponding to ZnO:N and ZnO:AlN were also observed.     

Efficiency improvement of CIGS solar cells using RF sputtered TCO/Ag/TCO thin-film as prospective buffer layer

In this paper, TCO (Transparent Conductive Oxide) incorporating ultrathin Ag intermediate film is proposed as a new buffer layer to enhance the efficiency of CIGS thin-film solar cells (TFSCs). In this regard, versatile multilayer thin-films based on ZnO/Ag/ZnO and ITO/Ag/ITO structures were deposited on glass using RF magnetron sputtering technique to determine the optoelectronic parameters of the multilayer structures. The elaborated samples were then characterized using SEM, EDS, XRD, and UV–Visible absorption spectroscopy techniques to investigate the structure morphological, optical, and electronic properties. The deposited multilayer thin-films showed amorphous-like structure and exhibited a broadband absorbance over the visible and even NIR spectrum ranges, indicating its potential application as alternative buffer layers for thin-film solar cells. In this context, TCO/Ag/TCO/CIGS solar cells have been numerically investigated using the deposited multilayer optoelectronic properties. It was revealed that the estimated efficiency of the ZnO/Ag/ZnO/CIGS-based solar cell could reach 18.5% with an open circuit voltage of 0.7 V and a short-circuit current density of 34.8 mA/cm². The performances exhibited by the investigated solar cell demonstrated that ZnO/Ag/ZnO multilayer can be used as an alternative to the conventional CdS buffer layer for developing high-performance non-toxic CIGS solar cells.     

Thermochromic properties of ZnO/VO2/ZnO films on soda lime silicate glass deposited by RF magnetron sputtering

A smart window based on VO₂ is a promising thermochromic (TC) glass that can regulate heat flow through windows by solar modulation near room temperature. TC glasses with high visible-light transmittance and large difference in infrared transmittance between high- and low-temperature VO₂ phases are required to save large amounts of energy in buildings. VO₂-based multilayer films with a buffer layer and/or an anti-reflective (AR) layer are used when the films are deposited by sputtering. In this study, VO₂-based multilayer films were prepared on soda lime glass using ZnO as both the buffer and the AR layers. The structure of the multilayer film was simulated using the optical constants measured from the deposited films. The effect of buffer and AR layers on the TC properties of VO₂-based multilayer films prepared by sputtering was investigated by simulation of the multilayer structure and deposition of the films with the simulated structure. The TC properties were measured and compared with the calculated properties. Improved TC properties (luminous transmittance (T_lum) of ～50%/46% (30 °C/80 °C) and solar modulation ability (ΔT_sol) of ～14%), compared to those without the buffer and AR layer, were obtained from the ZnO/VO₂/ZnO film deposited on glass. The calculated transmittances agree better with the measured ones when the optical constants measured directly from the deposited films are used and the roughnesses of the surface/interface of the multilayer films are considered in the calculation of the optical constants.     

Effect of ZnO seed layers on the solution chemical growth of ZnO nanorod arrays

High density ZnO nanorod arrays were grown on Si substrates coated with ZnO seed layers via aqueous solution route. The ZnO seed layers were deposited on the substrate using DC reactive sputtering and RF magnetron sputtering. It was found that ZnO seed layer with (1 0 3) preferred orientation, prepared using DC reactive sputtering, did not facilitate the formation of ZnO nanorods in the solution grown process. Prior seeding of the surface by ZnO layer with (0 0 2) preferred orientation, deposited using RF magnetron sputtering, leads to nucleation sites on which ZnO nanorod arrays can grow in a highly aligned fashion. ZnO nanorods with well-defined hexagonal facets (0 0 2) were grown almost vertically over the entire substrate. The uniformity and alignment of the nanorod arrays are strongly related to the properties of underneath ZnO seed layers.     

Effect of deposition temperature and quality of free-standing diamond substrates on the properties of RF sputtering ZnO films

Highly c-axis oriented ZnO film is often deposited on diamond substrates by RF magnetron sputtering and widely used for high frequency surface acoustic wave (SAW) devices. Deposition temperature is a key factor affecting the quality of the ZnO film. Different quality polished free-standing diamond films prepared by DC Arc Plasma Jet were used as the substrates to deposit ZnO films at different temperatures. Effect of the deposition temperature and the quality of the diamond films on the properties of the ZnO films were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that highly c-axis oriented ZnO films can be much easier deposited on the optical-grade diamond films with < 111> preferred orientation than the tool-grade diamond films with < 220> preferred orientation. The optimal deposition temperature is 200 °C for highly c-axis oriented and lower roughness ZnO films. Acoustic phase velocity of more than 10,000 m/s for the SAW devices based on the ZnO/optical-grade free-standing diamond films was obtained.     

Effect of gas-timing technique on structure and optical properties of sputtered zinc oxide films

Zinc oxide thin films were prepared by the RF magnetron sputtering using a gas-timing technique whereby the flow of argon into the sputtering chamber was controlled by an on–off sequence. With this technique, polycrystalline ZnO thin films on glass substrates have been achieved without any thermal treatment of the substrate. In addition, the RF power and the gas-timing sequence can be fine-tuned to produce the hexagonal structure of ZnO thin films. X-ray diffraction (XRD) measurements confirm a (0 0 2) plane oriented wurtzite structure ZnO thin films. The optimized conditions for this hexagonal structure are an RF power of 30 W and an on–off gas-timing sequence of 50:2 s. The root mean square surface roughness of ZnO thin films measured by atomic force microscopy are in the range of 6.4–11.5 nm. The optical transmittance of ZnO thin films is over 85% in the visible range.     

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 15 nm by controlling the deposition times.     

Investigations into the impact of the substrates with ultra thin seed layers deposited by atomic layer deposition on ZnO Nanowire Arrays

ABSTRACT: The impact of various substrates and ZnO ultra thin seed layers prepared by atomic layer deposition on the geometric morphology of subsequent ZnO nanowire arrays (NWs) fabricated by the hydrothermal method was investigated. The investigated substrates included B-doped ZnO films, Indium Tin Oxide films, single crystal silicon (111), and glass sheets. Scanning electron microscopy and X-ray diffraction measurements revealed that the geometry and aligment of the NWs were controlled by surface topography of the substrates and thickness of the ZnO seed layers, respectively. According to atomic force microscopy data, we suggest that the substrate, fluctuate amplitude and fluctuate frequency of roughness on ZnO seed layers have a great impact on the alignment of the resulting NWs, wherease the influence of the seed layers' texture was negligible. Transmission Electron Microscopy and Photoluminescence spectroscopy showed that crystal defects were influenced greatly by substrates instead of seed layers.     

Effects of power and temperature on the structural,anti-icing,wettability, and optical properties of zinc oxide thin films

Zinc oxide semiconductors have received significant research attention over the past decade owing to their diverse applications. In this paper, we report the development and characterisation of ZnO thin films prepared by radio-frequency (RF) magnetron sputtering. The optical, anti-icing, wettability, and structural properties of the films were investigated at various sputtering power levels and temperatures. With an increase in the power from 175 to 250 W, the ZnO thin films showed fine (002) structures. X-ray diffraction analysis of the coated thin films revealed a considerable increase in the (002) peak intensity along the c-axis with increasing power and temperature. Increasing the sputtering power from 175 to 250 W and the deposition temperature from 150 to 300 °C led to an increase in the average size of the grains from 10.548 to 13.151 nm and from 9.97 to 13.151 nm, respectively. The water contact angle possibly depends on the RF power and temperature employed for material deposition. Within the 350–800 nm range, the prepared films achieved optical transmissions of 92%–88%, refractive indices of 1.52–1.50, and band gaps of 3.28–3.24 eV. The anti-icing properties were also improved by adjusting the sputtering power and temperature during material deposition.     

Growing continuous zinc oxide layers with reproducible nanostructures on the seeded alumina substrates using spray pyrolysis

The growth of zinc oxide thin films with controlled nanostructures on the heat resistant dielectric substrates is important for the fabrication of gas sensors, transparent electric heating elements, pyroelectric electron emitters, and many other potential electronic and optoelectronic applications. The preferred substrate for many of these applications is alumina, but the production of uniform ZnO layers on alumina is hindered by the large lattice mismatch between ZnO and Al₂O₃ hexagonal crystal structures. Here, we systematically investigate the growth process of ZnO thin films on alumina substrates using the ultrasonic spray pyrolysis (USP) of zinc chloride solutions in ethanol and, for the first time, demonstrate the deposition of uniform layers on the alumina substrates appropriately seeded using magnetron sputtering prior to USP. On the pristine substrates, random nucleation of the isolated nanocrystallites results in uneven layers, and extending the growth process leads to the hierarchical growth of facetted ZnO nanorods and pyramids with weak physical attachments to the substrate surface. In similar conditions, USP deposition on the seeded substrates reproducibly results in continuous networks of densely packed ZnO crystallites intimately attached to the substrate surface with adjustable thickness and electrical conductance. These results are compared with those obtained for SnO₂ in similar conditions. Regardless of its tetragonal crystal structure, SnO₂ reproducibly forms even layers on the pristine alumina substrates.     

Epitaxial growth of InN film on intermediate oxide buffer layer by RF-MOMBE

In this paper, we report the studies on the hetero-epitaxial growth of wurtzite indium nitride (InN) thin films on oxide buffer layer by RF metal-organic molecular beam epitaxy (RF-MOMBE) system. Oxide buffer layer was pre-sputtered using RF sputtering technique. The structural properties and surface morphology were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). We also investigated the optical properties by temperature-dependence photoluminescence (PL). Near-infrared emission peak centered at 0.75 eV was observed from PL measurement. The irregular and asymmetric PL line shape was caused by absorbed moisture and surface electron accumulation in InN films. According to the fitting results of PL spectra measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which confirm to previous reports. Our results reveal that the oxide thin film could be a suitable buffer layer for engineering the growth of InN on sapphire wafer, and it might be also applicable for other lattice-mismatched III-V hetero-epitaxial systems.     

Seed layer mediated growth of high dielectric and low leakage BaTiO3 thin film using two-step sputtering process

In this study, we demonstrated the seed layer mediated growth of high-quality BaTiO₃ (BTO) thin films using a two-step radio frequency (RF) magnetron sputtering process. Since the as-grown BTO thin films obtained by RF magnetron sputtering at the deposition temperatures of 300–500 °C were amorphous with a low dielectric constant of 20, it is necessary to develop a fabrication process for obtaining crystalline high-k BTO thin films without sacrificing other film properties such as morphology and leakage current. First, it was revealed that ex-situ post-deposition annealing (PDA) at high temperatures in the 700–800 °C range led to the crystallization of BTO films and a high dielectric constant of 121. However, the film morphology deteriorated significantly during PDA, and consequently, a high leakage current was observed due to the rough and discontinuous surface containing voids and micro-cracks. To achieve an excellent leakage current characteristic as well as a high dielectric constant for a crystalline BTO thin film, in-situ crystallization was carried out through local epitaxial growth using a crystalline seed layer. The crystalline BTO seed layer was formed by annealing a 5-nm-thick amorphous BTO film at 700 °C on which the in-situ crystallized BTO main layer was deposited at 500 °C. The in-situ crystallization method resulted in a smooth and uniform surface and a high dielectric constant of 113. In addition, the in-situ crystallized BTO film exhibited a low leakage current density of 10⁻⁶ A/cm² (at 0.8 V) displaying an improvement by a factor of 10³ compared to the ex-situ crystallized BTO film.