Optoelectronic properties of sputter-deposited Cu2O-Ag-Cu2O treated with rapid thermal annealing

Cu₂O and two types of Cu₂O-Ag-Cu₂O (CAC) multilayered thin films were deposited on glass substrates using DC-magnetron sputtering. For CAC films, the mass thickness of Ag layer was controlled at 3 nm. After deposition, some of these films were annealed using a rapid thermal annealing (RTA) system at 650 °C, in order to create embedded Ag particles. AC films were used to study the clustering effect of Ag in Ar atmosphere, as well as for forming the 2nd type of CAC film by covering another Cu₂O layer on the annealed AC structure. A UV-VIS-NIR photometer, a Hall measurement system, and a I-V measurement system were used to characterize the optical and electrical properties of these films with and without RTA. The results show that 2-dimensional Ag layer can transform into many individual particles due to its high surface tension at annealing temperature, no matter when the annealing was carried out. For CAC films, without annealing, the optical transmission and the resistivity are decreased with the inserted Ag layer. After annealing, both the transmission and resistivity are increased, possibly due to the clustering effect of Ag layer. Most importantly, it is found that the embedded Ag particles can increase the light absorption in the NIR-IR region, which can increase photo-induced current.     

Thermal evolution of morphological,optical, and photocatalytic properties of Au–Cu2O–CuO nanocomposite thin film

Plasmonic nanocomposite thin films find exciting applications in environmental remediation and photovoltaics. We report on thermal annealing driven development of morphology, structure and photocatalytic performance of Au–Cu₂O–CuO nanocomposite thin film. Nanocomposite thin film coatings of Au–Cu₂O–CuO, prepared by radio frequency (RF) magnetron co-sputtering, were annealed at different temperatures. Thermal annealing driven evolution of morphology of Au–Cu₂O–CuO nanocomposite was studied by field emission scanning electron microscopy (FESEM), which revealed significant growth in size of nanostructures from 10 nm to 69 nm upon annealing. X-ray diffraction (XRD) together with Raman studies confirmed the nanocomposite nature of Au–Cu₂O–CuO film. UV-visible diffuse reflectance spectroscopy (UV-vis-DRS) studies showed band gap variation from 2.44 eV to 1.8 eV upon annealing at 250 °C. Nanocomposite thin film annealed at 250 °C exhibited superior photocatalytic activity for organic pollutants [methylene blue (MB) and methyl orange (MO)] decomposition. The origins of thermal transformation of morphological, optical and photocatalytic behaviour of the Au–Cu₂O–CuO nanocomposite coating are discussed.

     

X-ray absorption spectroscopy study of thermally annealed Cu–Al–O thin films

Cu–Al–O thin films are deposited on (0001) sapphire substrates by radio-frequency sputtering using an Al–Cu mosaic target. The Cu/Al atomic ratio of as-deposited Cu–Al–O films is measured to be 1.1. After deposition, the Cu–Al–O films are annealed at 600, 800, and 1000 °C, respectively, for 1 h in a N₂ atmosphere. The film crystal structure, electronic structure, valence band, and electrical properties are studied. The as-deposited films are amorphous and films annealed at 600 °C contain the crystallized CuO phase; the structure becomes crystallized CuAlO₂ after annealing at 800 °C and 1000 °C. The 800 °C annealed film grows along the (00l) plane. The crystallization decreases with the growth of the (012) and (018) planes for films annealed at 1000 °C. The resistivity values of the 800 °C and 1000 °C annealed films were measured as 1.07 Ω-cm and 864.01 Ω-cm, respectively. The lower resistivity of the 800 °C annealed film is attributed to preferred (00l) growth orientation and a reduction of the energy band gap.     

Characterization of copper oxide thin films deposited by the thermal evaporation of cuprous oxide (Cu2O)

Thin films of copper oxide were deposited by thermal evaporation of cuprous oxide (Cu₂O) powder. The substrates were either unheated or heated to a temperature of 300 °C. The films were also annealed in air at a temperature of 500 °C for 3 h. The films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction and UV-visible spectrophotometry. The effects of the substrate temperature and post-deposition annealing on the chemical, structural and optical properties of the films were investigated. As-deposited films on unheated substrates consisted of mixed cupric oxide (CuO) and Cu₂O phases, with a higher concentration of the Cu₂O phase. However, the films deposited on heated substrates and the annealed films were predominantly of the CuO phase.     

Growth and characterisation of CaCu2Ox thin films by pulsed injection MOCVD

Oxides with the structure MCu₂O₂ (M = Ca, Ba, Mg and Sr) are promising materials for the development of new p-type transparent conducting oxide thin films. This paper reports preliminary results on the growth and characterisation of CaCu₂O_x thin films by pulsed injection MOCVD. By using as precursors calcium and copper tetramethylheptanedionate dissolved in meta-xylene, mixed calcium-copper films have been grown in the temperature range from 450 °C to 550 °C. At these temperatures, deposited films exhibited a high mirror reflection effect, good adherence and were reasonably uniform with the cationic composition of the films being easily controlled by adjusting the copper-calcium ratio in the precursor solution. In CaCu₂O₂, copper is in the Cu¹⁺ oxidation state and depending on the oxygen partial pressure used, the films either contained CaCu₂O₃ or a mixture of CaO, CuO and Cu₂O. Optimisation of annealing conditions increased the presence of Cu¹⁺ in the film. Films had a maximum transmittance of 50% in the visible range and were highly resistive. Appropriate annealing conditions reduced the resistivity of the films.     

Effect of oxygen on the surface morphology of CuGaS2 thin films

Since the effect of oxygen is very significant during the heat treatment of the thin films, we study the effect of this during the annealing of CuGaS₂ thin films by two different types. In this study, CuGaS₂ thin films were deposited by vacuum thermal evaporation of CuGaS₂ powder on heated glass substrates at 200 °C submitted to a thermal gradient. The films are annealed in air and under nitrogen atmosphere at 400 °C for 2 h. In order to improve our understanding of the influence of oxygen during two annealing types on device performance, we have investigated our CuGaS₂ material by X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDX) and spectrophotometry. A correlation was established between the surface roughness, growth morphology and optical properties, of the annealed CuGaS₂ thin films. It was found that annealing of CuGaS₂ film in nitrogen atmosphere leads to a decrease of the mean grain size and to an evolution of a (112) preferred film orientation. Annealing in air results in the growth of oxide phases such as CuO and modifies the films structure and their surface morphology.     

Influence of thermal annealing on microstructural,morphological, optical properties and surface electronic structure of copper oxide thin films

In this study, effect of the post-deposition thermal annealing on copper oxide thin films has been systemically investigated. The copper oxide thin films were chemically deposited on glass substrates by spin-coating. Samples were annealed in air at atmospheric pressure and at different temperatures ranging from 200 to 600°C. The microstructural, morphological, optical properties and surface electronic structure of the thin films have been studied by diagnostic techniques such as X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV–VIS) absorption spectroscopy, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The thickness of the films was about 520 nm. Crystallinity and grain size was found to improve with annealing temperature. The optical bandgap of the samples was found to be in between 1.93 and 2.08 eV. Cupric oxide (CuO), cuprous oxide (Cu₂O) and copper hydroxide (Cu(OH)₂) phases were observed on the surface of as-deposited and 600 °C annealed thin films and relative concentrations of these three phases were found to depend on annealing temperature. A complete characterization reported herein allowed us to better understand the surface properties of copper oxide thin films which could then be used as active layers in optoelectronic devices such as solar cells and photodetectors.     

Effects of oxygen flow ratios and annealing temperatures on Raman and photoluminescence of titanium oxide thin films deposited by reactive magnetron sputtering

Titanium oxide (TiO_x) thin films were deposited on the Si(100) substrates by direct-current reactive magnetron sputtering at 3-15 % oxygen flow ratios (FO₂% = FO₂/(FO₂ + FAr) × 100%), and then annealed by rapid thermal annealing (RTA) at 350-750 °C for 2 min in air. The phase, bonding and luminescence behaviors of the as-deposited and annealed TiO_x thin films were analyzed by X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) spectroscopy, respectively. The as-deposited TiO_x films were amorphous from XRD and showed weak Raman intensity. In contrast, the distinct crystalline peaks of anatase and rutile phases were detected after RTA at 550-750 °C from both XRD and Raman spectra. A mixture of anatase and rutile phases was obtained by RTA at 3 FO₂% and its amount increased with annealing temperature. Only the anatase phase was detected in the 6-15 FO₂% specimens after RTA. The PL spectra of all post-annealed TiO_x films showed a broad peak in visible light region. The PL peak of TiO_x film at 3 FO₂% at 750 °C annealing can be fitted into two Gaussian peaks at ~ 486 nm (2.55 eV) and ~ 588 nm (2.11 eV) which were attributed to deep-level emissions of oxygen vacancies in the rutile and anatase phases, respectively. The peak around 550 nm was observed at 6-15 FO₂% which is attributed to electron-hole pair recombination from oxygen vacancy state in anatase phase to valence band. The variation of intensity of PL peaks is concerned with the formation of the rutile and anatase phases at different FO₂% and annealing temperatures.     

Structural and electrical characteristics of chemical solution derived (Bi3.2La0.4Nd0.4)Ti3O12 thin films

(Bi_3.2La_0.4Nd_0.4)Ti₃O₁₂ (BLNT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by using chemical solution deposition technique, and the effects of annealing temperatures in the range of 550-750 °C on structure and electrical properties of the thin films were investigated. X-ray diffraction analysis shows that the thin films have a bismuth-layered perovskite structure with preferred (117) orientation. The surface morphology observation by field-emission scanning electron microscopy confirms that films are dense and smooth with uniformly distributed grains. The grain size of the thin films increases with increasing annealing temperature; meanwhile, the structural distortion of the thin films also increases. It was demonstrated that the thin films show good electrical properties. The dielectric constant and dielectric loss are 191 and 0.028, respectively, at 10 kHz for the thin film annealed at 600 °C, and the 2Pr value of the thin film annealed at 700 °C is 20.5 μC/cm² at an electric field of 500 kV/cm.     

Property variations of direct-current reactive magnetron sputtered copper oxide thin films deposited at different oxygen partial pressures

Cuprous oxide (Cu₂O) and cupric oxide (CuO) thin films were deposited on glass substrates at different oxygen partial pressures by direct-current reactive magnetron sputtering of pure copper target in a mixture of argon and oxygen gases. Oxygen partial pressure was found to be a crucial parameter in controlling the phases and, thus, the physical properties of the deposited copper oxide thin films. Single-phase Cu₂O thin films with cubic structure were obtained at low oxygen partial pressure between 0.147 Pa and 0.200 Pa while higher oxygen partial pressure promoted the formation of CuO thin films with base-centered monoclinic structure. Polycrystalline Cu₂O thin films deposited with oxygen partial pressure at 0.147 Pa possessed the lowest p-type resistivity of 1.76 Ω cm as well as an optical band gap of 2.01 eV. On the other hand, polycrystalline CuO thin films deposited with oxygen partial pressure at 0.320 Pa were also single phase but showed a n-type resistivity of 0.19 Ω cm along with an optical band gap of 1.58 eV.     

Crystallization and electrical resistivity of Cu2O and CuO obtained by thermal oxidation of Cu thin films on SiO2/Si substrates

In this work, we study the crystallization and electrical resistivity of the formed oxides in a Cu/SiO₂/Si thin film after thermal oxidation by ex-situ annealing at different temperatures up to 1000 °C. Upon increasing the annealing temperature, from the X ray diffractogram the phase evolution Cu → Cu + Cu₂O → Cu₂O → Cu₂O + CuO → CuO was detected. Pure Cu₂O films are obtained at 200 °C, whereas uniform CuO films without structural surface defects such as terraces, kinks, porosity or cracks are obtained in the temperature range 300-550 °C. In both oxides, crystallization improves with annealing temperature. A resistivity phase diagram, which is obtained from the current-voltage response, is presented here. The resistivity was expected to increase linearly as a function of the annealing temperature due to evolution of oxides. However, anomalous decreases are observed at different temperatures ranges, this may be related to the improvement of the crystallization and crystallite size when the temperature increases.     

Grain coarsening mechanism of Cu thin films by rapid annealing

Cu thin films have been produced by an electroplating method using nominal 9N anode and nominal 6N CuSO₄·5H₂O electrolyte. Film samples were heat-treated by two procedures: conventional isothermal annealing in hydrogen atmosphere (abbreviated as H₂ annealing) and rapid thermal annealing with an infrared lamp (abbreviated as RTA). After heat treatment, the average grain diameters and the grain orientation distributions were examined by electron backscattering pattern analysis. The RTA samples (400 °C for 5 min) have a larger average grain diameter, more uniform grain distribution and higher ratio of (111) orientation than H₂ annealed samples (400 °C for 30 min). This means that RTA can produce films with coarser and more uniformly distributed grains than H₂ annealing within a short time, i.e. only a few minutes. To clarify the grain coarsening mechanism, grain growth by RTA was simulated using the phase field method. The simulated grain diameter reaches its maximum at a heating rate which is the same order as that in the actual RTA experiment. The maximum grain diameter is larger than that obtained by H₂ annealing with the same annealing time at the isothermal stage as in RTA. The distribution of the misorientation was analyzed which led to a proposed grain growth model for the RTA method.     

Synthesis and electrochromic study of sol-gel cuprous oxide nanoparticles accumulated on silica thin film

In this study, electrochromic properties of cuprous oxide nanoparticles, self-accumulated on the surface of a sol-gel silica thin film, have been investigated by using UV-visible spectrophotometry in a lithium-based electrolyte cell. The cuprous oxide nanoparticles showed a reversible electrochromic process with a thin film transmission reduction of about 50% in a narrow wavelength range of 400-500 nm, as compared to the bleached state of the film. Using optical transmission measurement, we have found that the band gap energy of the films reduced from 2.7 eV for Cu₂O to 1.3 eV for CuO by increasing the annealing temperature from 220 to 300 °C in an N₂ environment for 1 h. Study of the band gaps of the as-deposited, colored and bleached states of the nanoparticles showed that the electrochromic process corresponded to a reversible red-ox conversion of Cu₂O to CuO on the film surface, in addition to the reversible red-ox reaction of the Cu₂O film. X-ray photoelectron spectroscopy indicated that the copper oxide nanoparticles accumulated on the film surface, after annealing the samples at 200 °C. Surface morphology of the films and particle size of the surface copper oxides have also been studied by atomic force microscopy analysis. The copper oxide nanoparticles with average size of about 100 nm increased the surface area ratio and surface roughness of the silica films from 2.2% and 0.8 nm to 51% and 21 nm, respectively.     

Degradation of thin films of YBa2Cu3O7 by annealing in air and in vacuum

The degradation of epitaxial thin films of YBa₂Cu₃O₇ has been studied as a function of annealing temperature in air and in vacuum; some samples had an evaporated overlayer of CaF₂. Degradation was monitored by the measurement of electrical properties after consecutive 30-min annealing treatments. The room-temperature resistance registered significant increases for all samples after annealing at temperatures above about 200°C; the critical current density at 77 K was degraded for annealing temperatures 400°C in air, and 200–250°C in vacuum. By annealing in oxygen at 550°C, electrical properties were restored in degraded bare YBCO samples annealed in vacuum, but not for those annealed in air.     

A new procedure to prepare semiconducting ternary compounds from binary buffer materials and vacuum-deposited copper for photovoltaic applications

This paper reports the successful deposition of CuIn₅S₈ and Cu₂SnS₃ thin films obtained on Pyrex glass substrates using annealing in a sulfur atmosphere at 550 °C during evaporation of copper on In₂S₃ and SnS₂ sprayed thin films, respectively. The CuIn₅S₈ and Cu₂SnS₃ thin films crystallize in cubic structures. Under suitable experimental conditions, this procedure, using the mobility of copper and silver elements, can be used to prepare many ternary absorber layers from binary buffer materials.     

Dye-sensitized solar cells with TiO2 nanocrystalline films prepared by conventional and rapid thermal annealing processes

Titanium oxide (TiO₂) thin films are prepared by the sol-gel method and annealed at 600 °C by conventional (CTA) and rapid thermal annealing (RTA) processes on fluorine-doped tin oxide -coated glass substrates for application as the work electrode for the dye-sensitized solar cells (DSSC). TiO₂ thin films are crystallized using a conventional furnace and the proposed RTA process at annealing rates of 5 °Cmin⁻¹ and 600 °Cmin⁻¹, respectively. The TiO₂ thin films are characterized by X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller analysis. Based on the results, the TiO₂ films crystallized by RTA show better crystallization, higher porosity and larger surface area than those of CTA. The short-circuit photocurrent and open-circuit voltage values increased from 5.2 mAcm⁻² and 0.6 V for the DSSC with the CTA-derived TiO₂ films to 8.3 mAcm⁻² and 0.68 V, respectively, for the DSSC containing RTA-derived TiO₂ films.     

Structures and properties of titania thin films annealed under different atmosphere

The effects of crystallinity, phase and oxygen vacancies on optical and photocatalytic properties of titania (TiO₂) thin films were systematically studied. The as-deposited amorphous titania films were prepared by reactive sputtering titanium metal targets in argon–oxygen plasma at 100 °C and subsequently annealed at various temperatures of 400–800 °C in air, vacuum and H₂ atmosphere. The results indicate that in general the crystallinity of the annealed films is enhanced with the increasing annealing temperature. At the same temperature, the H₂ annealed films achieve better crystallinity but containing more oxygen vacancies than the films annealed in air and in vacuum. In H₂ or in vacuum, the concentration of oxygen vacancies in the annealed films increases with increasing temperature, while in air it remains constant. Oxygen vacancies in titania film not only facilitate phase transformation but also lower the band gap of titania, and make the film visible-light responsive. Photocatalytic properties of the TiO₂ films were characterized in UV and visible light irradiation by following the Ag reduction and degradation of methylene blue. The films annealed at 600–700 °C in H₂ possess the best film crystallinity and the proper concentration of oxygen vacancies and exhibit the best photocatalytic performance under both UV and visible light.     

Cu2O thin films deposited by reactive direct current magnetron sputtering

The Cu₂O thin films were prepared on quartz substrate by reactive direct current magnetron sputtering. The influences of oxygen partial pressure and gas flow rate on the structures and properties of deposited films were investigated. Varying oxygen partial pressure leads to the synthesis of Cu₂O, Cu₄O₃ and CuO with different microstructures. At a constant oxygen partial pressure of 6.6 × 10^− 2 Pa, the single Cu₂O films can be obtained when the gas flow rate is below 80 sccm. The as-deposited Cu₂O thin films have a very high absorption in the visible region resulting in the visible-light induced photocatalytic activity.     

Role of oxygen in enhancing N-type conductivity of CuInS2 thin films

Post-growth treatments in air atmosphere were performed on CuInS₂ films prepared by the single-source thermal evaporation method. Their effect on the structural, optical and electrical properties of the films was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical reflection and transmission and resistance measurements. The films were annealed from 100 to 350 °C in air. The stability of the observed N-type conductivity after annealing depends strongly on the annealing temperature. Indeed it is shown that for annealing temperatures above 200 °C the N-type conductivity is stable. The resistance of the N-CuInS₂ thin films correlates well with the corresponding annealing temperature. The samples after annealing have direct bandgap energies of 1.45-1.50 eV.     

Investigation of Pb-Mg-Nb-O pyrochlore thin films for tunable material

PMN thin films have been investigated as a feasible material for tunable microwave applications. PMN thin films were deposited by RF magnetron sputtering from Pb₆MgNb₆O₂₂ ceramic target on platinized Si substrates. The crystallinity, thickness, surface morphology, dielectric property, and voltage tunable properties of thin films were investigated by means of X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and an impedance analyzer. The influences of sputtering substrate temperature and post-annealing on the tunable dielectric properties of thin films were investigated. Increasing the sputtering substrate temperature and annealing temperature can significantly increase tunability; however, these relations had the limitation that overly annealing temperature degraded the tunability of the thin film. A tunability of 38% at 1100 kV/cm of dc bias field under 1 MHz was achieved for a sample sputtered at 550 °C and annealed at 700 °C.