Insights into Cu2O thin film absorber via pulsed laser deposition

Cuprous oxide materials are of growing interest for optoelectronic devices and were produced by several chemical and physical methods. Here, we report on the structural, optical, and electrical properties of Cu_xO thin films prepared by the pulsed laser deposition technique. The substrate temperature, as well as the oxygen partial pressure in the deposition chamber, were varied to monitor the copper to oxygen ratio within the deposited films. The growth conditions were carefully optimized to provide the highest conductivity and mobility. Thus, 100 nm thick cuprous oxide films (Cu₂O) deposited at 750 °C exhibited a resistivity of 16 Ω?cm, high mobility of 30 cm²/(V?s), and a bandgap of around 2 eV. The film deposited at the optimized deposition parameters on Nb:STO (001) substrate with Au top electrode showed a photovoltaic response with an open circuit voltage of 0.56 V. These results path the way to efficient solar cells made with Cu₂O films via the pulsed laser deposition technique.     

Influences of annealing temperature on anti-reflective performance of amorphous Ta2O5 thin films

Influences of thermal annealing on structural, optical and morphological properties of the tantalum pentoxide (Ta₂O₅) thin films were investigated and anti-reflective performances were discussed in detail. The Ta₂O₅ thin films were deposited onto Corning Glass (CG), Si, GaAs and Ge substrates by radio-frequency (RF) magnetron sputtering technique using Ta₂O₅ ceramic target. The obtained secondary ion mass spectroscopy (SIMS) analysis results showed that uniform Ta and O distribution have formed throughout depth of the films deposited on substrates. The X-Ray diffraction (XRD) results indicated that the annealed Ta₂O₅ thin films at 100, 200, 300 and 500?°C have exhibited amorphous (a-Ta₂O₅) characteristic. The increased temperature has resulted in increasing the surface roughness from 0.67 to 1.60?nm. The optical transmittance of the annealed thin films has increased from 70.85 to 80.32% with increasing temperature. Spectroscopic ellipsometer (SE) measurement results demonstrated that the increased temperature has increased the refractive index of the Ta₂O₅ thin film from 2.11 to 2.18. The Ta₂O₅ thin film has reduced the average optical reflectivity of the Si, GaAs and Ge substrates by 78, 55 and 70%, respectively. In addition, thermal annealing process has decreased the optical reflectivity of the film. The obtained experimental results showed that single-layer Ta₂O₅ thin films can be used as anti-reflective layer in optical and optoelectronic applications. The best optical transmittance and anti-reflective performance were obtained at the annealing temperature of 500?°C.     

Band structure and thermoelectric properties of inkjet printed ZnO and ZnFe2O4 thin films

The band structure and thermoelectric properties of inkjet printed ZnO and ZnFe₂O₄ thin films have been investigated. The bulk pellets were prepared by a solid-state method and thin films were deposited using an inkjet printing method. Multiple print cycles were required to fabricate homogeneous films and the composition of the thin films can be varied by varying the relative amounts of liquid deposited. It was possible to obtain high thermoelectric properties of ZnO by controlling the ratios of dopant added and the temperature of the heat treatments. XRD analysis showed that the fabricated samples have a wurtzite structure and an additional ZnAl₂O₄ phase was formed with increasing Al content and sintering temperature. It was found that the band gap of Al doped ZnO becomes smaller with increasing Al content and thus the electrical conductivity of Al_x doped ZnO (x=0.04) thin films showed the highest electrical conductivity (114.10 S/cm). The ZnFe₂O₄ samples were compared against the ZnO samples. The formation of single phase cubic spinel structure of the sintered ZnFe₂O₄ samples was found and confirmed by X-ray diffraction technique. Secondary phase Fe₂O₃ was also detected for compositions with Zn (x≤0.4). Finally, we want to report that the electrical conductivity of Zn_xFe_3−xO₄ was lower than the conductivity of the Al-doped ZnO.     

A new synergistic effect in one step sputtered ZnO/Zn2SnO4 heterojunction films for H2 sensing related to crystal structure and film compactness

In this paper, ZnO/Zn₂SnO₄ heterojunction films were one step fabricated by magnetron sputtering and the dependence of crystal structures, film compactness and H₂ sensing properties on annealing process were investigated and discussed. The results showed that three typical surface morphologies can be controlled by adjusting annealing temperatures and periods. The films annealed at the temperature of 550 °C for 6 h showed the best H₂ sensing properties. It exhibited a response (R_a/R_g) of 28.3–100 ppm H₂ at the temperature of 230 °C and the detection limit is 30.2 ppb. Meanwhile, it also showed a good selectivity and long-term stability to H₂. The H₂ sensing mechanism is attributed to the synergistic effect between ZnO (0001) signal crystal facets and ZnO/Zn₂SnO₄ heterojunction structures which enhanced the gas reactivity and resistance modulation range. On the contrary, insufficient annealing restricts the film crystallinity and the growth of hexagonal ZnO while undue annealing destroys the compactness of the films, leading to poor H₂ sensing properties.     

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.     

Low‐Temperature Deposition of Hexagonal Boron Nitride via Sequential Injection of Triethylboron and N2/H2 Plasma

Hexagonal boron nitride (hBN) thin films were deposited on silicon and quartz substrates using sequential exposures of triethylboron and N₂/H₂ plasma in a hollow‐cathode plasma‐assisted atomic layer deposition reactor at low temperatures (≤450°C). A non‐saturating film deposition rate was observed for substrate temperatures above 250°C. BN films were characterized for their chemical composition, crystallinity, surface morphology, and optical properties. X‐ray photoelectron spectroscopy (XPS) depicted the peaks of boron, nitrogen, carbon, and oxygen at the film surface. B 1s and N 1s high‐resolution XPS spectra confirmed the presence of BN with peaks located at 190.8 and 398.3 eV, respectively. As deposited films were polycrystalline, single‐phase hBN irrespective of the deposition temperature. Absorption spectra exhibited an optical band edge at ~5.25 eV and an optical transmittance greater than 90% in the visible region of the spectrum. Refractive index of the hBN film deposited at 450°C was 1.60 at 550 nm, which increased to 1.64 after postdeposition annealing at 800°C for 30 min. These results represent the first demonstration of hBN deposition using low‐temperature hollow‐cathode plasma‐assisted sequential deposition technique.     

Extremely Thin Al2O3 Surface‐Passivated Nanocrystalline ZnO Thin‐Film Transistors Designed for Low Process Temperature

Nanocrystalline ZnO (nc‐ZnO) thin‐film transistors (TFTs) exhibit inherent instability under bias/photo stresses, which originates from the oxygen molecules adsorbed on the surface of the crystal grains. The space charge region at nanocrystal surfaces that is induced by adsorbed oxygen molecules produces a high electrical potential barrier and significantly interrupts charge transport between the source and drain in nc‐ZnO TFTs. In this article, we developed high‐performance TFTs via the continuous deposition of an extremely thin Al₂O₃ layer on a nc‐ZnO channel. These devices were fabricated by atomic layer deposition at an extremely low process temperature of 150°C, including both the deposition and postannealing temperatures. The nc‐ZnO TFT with an extremely thin Al₂O₃ layer (1.8 nm) showed a significantly higher mobility (25 cm²/Vs) compared to devices without an Al₂O₃ layer (3.6 cm²/Vs). This dramatic difference was ascribed to the suppression of the chemisorption of oxygen molecules at the nanocrystal surface during thermal annealing (reducing the potential barrier width/height between adjacent nanocrystals). Furthermore, ultrathin Al₂O₃‐covered nc‐ZnO TFTs exhibited considerably enhanced electrical/photo stability due to the reduction in adsorption/desorption events of oxygen molecules on the nanocrystal surfaces (with no change in the depletion width after illumination) under gate bias or illumination stress.     

Enhanced thermoelectric properties of atomic-layer-deposited Hf:Zn16O/18O superlattice films by interface-engineering

This study examines three novel approaches for enhancing the thermoelectric (TE) properties of atomic-layer-deposited (ALD) ZnO thin films: 1) Hf-doping, which preserved the crystallinity of ZnO and provided effective phonon scattering owing to Hf's similar atomic radius to and large mass difference with Zn, leading to high power factor (PF) and low thermal conductivity (κ); 2) controlling the distribution of Hf into an alternating scattered phase/clustered phase superlattice, which balanced the high PF of the scattered phases with the low κ of the clustered phases, while providing significant energy-filtering effect to raise the Seebeck coefficient; 3) introducing ¹⁸O/¹⁶O periodicity into the Hf:ZnO films—by alternately using H₂¹⁶O and H₂¹⁸O as oxidants in the ALD processes, which further suppressed κ without compromising PF. The combination of the three approaches resulted in a maximum improvement in ZT of ~1600% over that of the undoped ZnO.     

Enhanced photocatalytic activity of TiO2-ZnO thin films deposited by dc reactive magnetron sputtering

The effect of deposition conditions on the photocatalytic activity of TiO₂-ZnO thin films was studied. By using a (Ti)₉₀-(Zn)₁₀ alloy target, the samples were deposited at room temperature on glass substrates by dc reactive magnetron sputtering and post-annealed in air at 500 °C. The dependence of the physical properties of the films on the O₂/Ar gas ratio and the deposition working pressure was investigated. XRD patterns showed mainly the formation of the anatase phase of TiO₂. Optical absorption measurements exhibited a blue shift of the band-gap energy with increasing working pressure. XPS spectra indicated the presence of the Ti⁴⁺ and Zn²⁺ oxidation states, which correspond to TiO₂ and ZnO, respectively. The chemical state of Ti was further analyzed by means of the modified Auger parameter, α’, which gave a value of ca. 873 eV. The photocatalytic property of the films was assessed by the degradation of a methylene blue aqueous solution. The maximum photocatalytic performance was observed for the samples deposited at 3.0 mTorr and O₂/Ar gas ratio of 10/90. These results are explained in terms of the structural, optical, and morphological properties of the films.     

Effect of Al doped ZnO on optical and photovoltaic properties of the p-Cu2O/n-AZO solar cells

Metal oxide thin films have fared so well in the semiconductor industry because of their superior physical, electrical, and optical properties. The applications of these materials in solar cells, biosensors, biomedicine, supercapacitors, photocatalysis, luminous materials, and laser systems are becoming increasingly popular. In this study, the influence of Al concentration on Cu₂O/AZO heterojunction thin films was examined systematically. First, arrays of n-ZnO and AZO rods were produced on an ITO substrate using a hydrothermal technique at 140 °C. Then, using an alkaline cupric lactate solution, a thin films of p-Cu₂O were electrodeposited at 60 °C onto the ZnO arrays. The structure and morphology of the produced materials and the solar cells were studied using X-ray diffraction and scanning electron microscopy. The optical measurements demonstrate a shift in the absorption edge with increasing Al content. Solar cells have been created with a device structure of ITO/ZnO/Cu₂O/Al and ITO/Al-doped ZnO/Cu₂O/Al configurations. The power conversion efficiency (?) of the inorganic solar cell with 6% Al-doped ZnO is ? = 0.282%, which is greater than the ? of the ZnO-based solar cell (? = 0.17%).     

Improving the Activity of Rh/Al2O3 Catalyst for NO Reduction by Na Addition in the Presences of H2O and O2

The effect of Na addition on the performance of Rh/Al₂O₃ catalyst for NO reduction with CO in the presence of H₂O and O₂ was investigated. The reacted catalysts were analyzed by the FTIR technique to identify the products for further investigation on the possible catalytic reaction mechanisms and the reasons behind the H₂O poisoning. Experimental results show that the removal efficiency of NO by Rh/Al₂O₃ catalyst was 63% at 250 °C but that decreased as the H₂O content increased. Adding Na to modify the Rh/Al₂O₃ catalyst significantly enhanced the conversion of NO to 99% at 250–300 °C even as the H₂O content was 1.6 vol%. The FTIR analyses results reveal that the abundant H₂O in the flue gas can compete with NO to adsorb on the surfaces of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts and further enhance the formation of NO₃ that reacts with H. The effects of H₂O on Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts can be eliminated by increasing the reaction temperature to higher than 300 °C. Rh-Na/Al₂O₃ is a feasible catalyst for NO reduction at such condition with relative high H₂O and O₂ contents.     

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.     

High‐performance varistors prepared by hot‐dipping tin oxide thin films in Sb2O3 powder: Influence of temperature

A series of SnO_x–Sb₂O₃ thin film varistors were fabricated through hot‐dipping tin oxide films deposited by radio‐frequency magnetron sputtering in Sb₂O₃ powder at varied temperatures in air. With the increase in hot‐dipping temperature (HDT) from 200°C to 600°C, the nonlinear coefficient (α) of the samples increased first and then decreased, reaching the maximum at 500°C, which was mainly determined by the completeness of high‐resistant Sb₂O₃ layer at tin oxide grain boundary and the chemical composition of tin oxide films. Correspondingly, the leakage current (I_L) decreased first and increased later. The breakdown electric field (E_100 mA) decreased constantly with increasing HDT. The SnO_x–Sb₂O₃ film varistors prepared at 500°C exhibited the optimum nonlinear properties with the maximum α of 10.88, the minimum I_L of 36.3 mA/cm², and an E_100mA of 0.0188 V/nm. The obtained nanoscaled film varistors would be promising in electrical/electronic devices working in low voltage.     

Preferential CO Oxidation in the Presence of H2, H2O and CO2 at Short Contact-times

High selectivities and conversions in the preferential oxidation of CO in the presence of large quantities of H₂, H₂O and CO₂ are demonstrated on noble metal catalysts at millisecond contact times (~10–15 ms) for temperatures between 150 and 500 °C. With a simulated water-gas shift product stream containing 0.5% CO and varying amounts of H₂, H₂O and CO₂, we are able to achieve ~90% CO conversions on a Ru catalyst at temperatures of ~300 °C using a stoichiometric amount of O₂ (0.25%). Experiments with and without O₂ and with varying H₂O reveal that significant water-gas shift occurs on Pt and Pt-ceria catalysts at temperatures between 250 and 400 °C, while significant CH₄ is formed on Ru and Rh catalysts at temperatures greater than 250 and 350 °C, respectively. The presence of H₂O blocks H₂ adsorption and allows preferential CO oxidation at higher temperatures where rates are high. We propose that a multistage preferential oxidation reactor using these catalysts can be used to bring down CO content from 5000 ppm at the reactor entrance to less than 100 ppm at very short contact-times.     

Atmospheric pressure spatial ALD of Al2O3 thin films for flexible PEALD IGZO TFT application

Atmospheric pressure spatial atomic layer deposition (AP S-ALD)-derived Al₂O₃ films were investigated on the growth temperatures (100 °C ～ 200 °C) and demonstrated as the gate insulator for IGZO thin film transistor (TFT) applications. When the growth temperature increases to 200 °C, growth per cycle (GPC) and refractive index of the Al₂O₃ films were 1.33 Å/cycle and 1.63, respectively. The film density also increased from 2.55 g/cm³ to 2.79 g/cm³ on the growth temperature, which decreasing the carbon impurities of the Al₂O₃ film (100 °C: 3.57 at%, 150 °C: 1.73 at%, 200 °C: N/A). In addition, the impurity and low growth temperature may degrade not only film surface roughness, but also electrical characteristics. As the buffer and gate insulator Al₂O₃ layers, the IGZO TFT were fabricated on a polyimide substrate. The IGZO TFTs with the Al₂O₃ layers showed excellent device performances: 52.48 cm²/V.sec of field effect mobility, ?3.09 ± 0.14 of threshold voltage, and 0.14 ± 0.01 subthreshold swing. In addition, the TFT exhibited excellent bias reliability and mechanical bending stability. This highly stability will be attributed to AP S-ALD Al₂O₃ acting as an excellent insulator.     

High temperature failure modes of In2O3 thin films and improved thermal stability using Al2O3/ZrO2 protective layers

The limiting temperature of an In₂O₃ thin film sensor is much lower than its melting point. Herein, the failure modes of In₂O₃ thin films at high temperatures, including sublimation and changes in composition, have been studied. The edge and surface layer sublimation rates increased dramatically at 1350 °C, indicating that it is the limiting temperature of no-protection In₂O₃ films. In addition, oxygen atoms will escape from In₂O₃ thin films at high temperatures, forming oxygen vacancies. As the main current carrier type in In₂O₃, the increasing number of oxygen vacancies affects the resistance of In₂O₃ thin film sensors. To solve these problems and promote the high temperature performance of In₂O₃ thin films, protection methods based on Al₂O₃ and ZrO₂ layers have been investigated. The ZrO₂ protective layer alleviated the serious considerable sublimation of In₂O₃ thin films at high temperatures, and the Al₂O₃ protective layer was beneficial for reduction the escape of oxygen atoms. Finally, different protection layers were evaluated by in-situ resistivity measurements of In₂O₃ thin films at high temperatures. The resistance of the In₂O₃ thin film resistor with a protective multilayer consisting of Al₂O₃ and ZrO₂ remained stable at 1360 °C, verifying the protection method effectively increased the thermal stability of In₂O₃ thin films.     

Decomposition of H2O2 on activated carbon obtained from olive stones

Activated carbons were prepared from olive oil solid wastes by treatment in different schemes: impregnation with H₃PO₄ followed by pyrolysis at 300–700 °C, by steam pyrolysis at 600–700 °C, or by conventional steam activation at 850 °C. Porosity characteristics were determined by analysis of nitrogen adsorption isotherms, and carbons of widely different properties and surface pH values were obtained. Decomposition of H₂O₂ in dilute unbuffered solution was followed by measuring evolved oxygen volumetrically. First‐order kinetics was followed, and the catalytic rate coefficients were evaluated. The carbons tested showed appreciable activity where evolved oxygen attained ≈10% of the stoichiometric amount in 1 h. The degree of decomposition showed inverse dependence on surface area, pore volume and mean pore dimensions. The chemical nature of the surface, rather than the porosity characteristics, was the principal factor in enhancing the disproportionation of H₂O₂ on the activated carbon surface. © 2001 Society of Chemical Industry     

Solution-based fabrication and electrical properties of CaBi4Ti4O15 thin films

Ferroelectric CaBi₄Ti₄O₁₅ (CBT) thin films were prepared by spin coating technology using solution-based fabrication. The as-deposited CBT thin films were crystallized below 600 °C and the layered perovskite were crystallized at 700 °C using CFA processing in air. The enhancement of ferroelectric properties in CBT thin films for MFIS structures were investigated and discussed. Compared the Bi₄Ti₃O₁₂ (BIT), the CBT showed the better physical and electrical characteristics. The 700 °C annealed CBT thin films on SiO₂/Si substrate showed random orientation and exhibited large memory window curves. The maximum capacitance, memory window and leakage current density were about 250 pF, 2 V, and 10^?5 A/cm², respectively.     

Recrystallization behavior,oxygen vacancy and photoluminescence performance of sputter-deposited Ga2O3 films via high-vacuum in situ annealing

Ga₂O₃ films were deposited on Si substrates through radio-frequency magnetron sputtering at room temperature and were annealed in situ in a high-vacuum environment. The as-deposited Ga₂O₃ film exhibited an island-like surface morphology and had an amorphous microstructure, with a few nanocrystalline grains embedded in it. After high-temperature in situ annealing, the films recrystallized and exhibited coalesced surfaces. Because of the thermally driven diffusion of Ga, the interfacial layer between Si and Ga₂O₃ was composed of SiGaO_x. Compared with ex situ annealing in air, in situ annealing in high vacuum is more advantageous because it enhances surface mobility and improves the crystallinity of the Ga₂O₃ films. The higher oxygen vacancy concentration of in situ annealed films revealed that oxygen atoms were easily released from the Ga₂O₃ lattice during high-vacuum annealing. Photoluminescence (PL) spectra exhibited four emission peaks centered in ultraviolet, blue, and green regions, and the peak intensities were significantly enhanced by thermal annealing at >600 °C. This work elucidates the effect of the in situ annealing treatment on the recrystallization behavior, interfacial microstructure, oxygen vacancy concentration, and PL performance of the Ga₂O₃ films, making it significant and instructional for the further development of Ga₂O₃-based devices.     

Characterization of non-stoichiometric Ga2O3-x thin films grown by radio-frequency powder sputtering

We report the synthesis and characterization of non-stoichiometric Ga₂O_3-x thin films deposited on sapphire (0001) substrates by radio-frequency powder sputtering. The chemical and electronic states of the non-stoichiometric Ga₂O_3-x thin films were investigated. By sputtering in an Ar atmosphere, the as-grown thin films become non-stoichiometric Ga₂O_2.7, due to the difference in sputtering yield between Ga and O species of the Ga₂O₃ target. The electronic states of the thin films consist of ~85% Ga³⁺ and ~15% Ga¹⁺, corresponding to Ga₂O₃ and Ga₂O, respectively. The films have the electrical characteristics of a semiconductor, with electrical conductivity of approximately 5.0 × 10^-4 S cm^-1 and a carrier concentration of 4.5 × 10¹⁴ cm^-3 at 300 K.