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.     

Preparation of nanocrystalline Cu<Subscript>2</Subscript>O thin film by pulsed laser deposition

In this paper, the synthesis of nanocrystalline copper oxides Cu₂O and CuO thin films on glass substrates using a pulsed 532 nm Nd:YAG laser is presented. Deposition of films is achieved at two different substrate temperatures. The influence of substrate temperature on the structural and optical properties of copper oxide films are discussed and analyzed. The X-ray diffraction (XRD) results show that the deposited films are crystalline in nature. Films prepared at 300 °C substrate temperature were Cu₂O and has (111) and (200) diffracted peaks, while films grown at 500 °C were CuO and has (111) and (020) planes. The morphology of deposited films were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM). The optical energy gap of Cu₂O and CuO films have been determined and found to be 2.04 and 1.35 eV respectively.     

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.     

Copper(I) oxide micro-cubical structures formation by metal organic chemical vapor deposition from copper(II) acetylacetonate

The copper oxide films were deposited by decomposition of copper(II) acetylacetonate precursor in a hot wall horizontal flow tubular furnace by chemical vapor deposition method. The obtained data revealed change in phase of the deposits from crystalline copper(I) oxide (Cu₂O) to copper(II) oxide (CuO) structures on increasing the substrate temperature from 195 to 430 °C. The scanning electron microscope images showed the formation of mono-dispersed micron-sized Cu₂O cubical structures. The Cu₂O micro-cubes were observed to have a comparatively higher optical transmittance than CuO film in the visible spectral region. The Cu₂O micro-cubical films are expected to underlie the potential use as cost effective transparent conducting oxide electrode in solar cell applications.     

Low temperature (< 100 °C) deposited P-type cuprous oxide thin films: Importance of controlled oxygen and deposition energy

With the emergence of transparent electronics, there has been considerable advancement in n-type transparent semiconducting oxide (TSO) materials, such as ZnO, InGaZnO, and InSnO. Comparatively, the availability of p-type TSO materials is more scarce and the available materials are less mature. The development of p-type semiconductors is one of the key technologies needed to push transparent electronics and systems to the next frontier, particularly for implementing p-n junctions for solar cells and p-type transistors for complementary logic/circuits applications. Cuprous oxide (Cu₂O) is one of the most promising candidates for p-type TSO materials. This paper reports the deposition of Cu₂O thin films without substrate heating using a high deposition rate reactive sputtering technique, called high target utilisation sputtering (HiTUS). This technique allows independent control of the remote plasma density and the ion energy, thus providing finer control of the film properties and microstructure as well as reducing film stress. The effect of deposition parameters, including oxygen flow rate, plasma power and target power, on the properties of Cu₂O films are reported. It is known from previously published work that the formation of pure Cu₂O film is often difficult, due to the more ready formation or co-formation of cupric oxide (CuO). From our investigation, we established two key concurrent criteria needed for attaining Cu₂O thin films (as opposed to CuO or mixed phase CuO/Cu₂O films). First, the oxygen flow rate must be kept low to avoid over-oxidation of Cu₂O to CuO and to ensure a non-oxidised/non-poisoned metallic copper target in the reactive sputtering environment. Secondly, the energy of the sputtered copper species must be kept low as higher reaction energy tends to favour the formation of CuO. The unique design of the HiTUS system enables the provision of a high density of low energy sputtered copper radicals/ions, and when combined with a controlled amount of oxygen, can produce good quality p-type transparent Cu₂O films with electrical resistivity ranging from 10² to 10⁴ Ω-cm, hole mobility of 1-10 cm²/V-s, and optical band-gap of 2.0-2.6 eV. These material properties make this low temperature deposited HiTUS Cu₂O film suitable for fabrication of p-type metal oxide thin film transistors. Furthermore, the capability to deposit Cu₂O films with low film stress at low temperatures on plastic substrates renders this approach favourable for fabrication of flexible p-n junction solar cells.     

Mechanisms of carrier generation and transport in Ni-doped Cu2O

The density and mobility of hole carriers in Ni-doped and undoped cuprous oxide (Cu₂O) films prepared by pulsed laser deposition (PLD) from Ni-doped and undoped CuO targets, respectively, were measured in order to examine the mechanisms of carrier generation and transport in doped films. The temperature dependence of the carrier density of the films revealed that regardless of the Ni content, the activation energies of the acceptor level of the films are 0.22-0.25 eV. The temperature dependence of the mobility of the films changed from −0.58 to ∼0 by doping with Ni. These results evidenced that hole carriers in Ni-doped Cu₂O as well as in undoped Cu₂O were generated by Cu vacancies and were primarily scattered by neutral impurity scattering centers. X-ray diffraction (XRD) measurements of the films showed that the mass fraction of Cu₂O in the films decreased with increasing Ni content, while that of CuO increased. It was also found that the reduction process of CuO to Cu₂O was suppressed by the Ni doping.     

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.     

Dependence of solar selectivity of copper-black films on their structure,chemical composition and morphology

The composition and morphology of anodically grown solar-selective copper-black films have been investigated using X-ray photoelectron spectroscopy with depth profiling, scanning electron microscopy and X-ray diffraction. It has been found that the films showing the best selectivity have their top layer consisting predominantly of crystalline cupric oxide (CuO). In the underlying region the dominating phase is cuprous oxide (Cu₂O) with Cu/O ratio increasing systematically towards the copper substrate. It is shown that the enhanced absorption in the solar region of the spectrum is due to the stacking of layers with graded composition and also due to the surface morphology of the film.     

The influence of oxygen pressure on the growth of CuO nanostructures prepared by RF reactive magnetron sputtering

Copper oxide films were prepared by RF reactive magnetron sputtering at different percentages of oxygen pressure in a Ar:O₂ reactive gas mixture at room temperature. The structural and optical properties of CuO films were investigated by a field emission scanning electron microscope, Raman spectroscopy, X-ray diffraction and UV–Visible spectrophotometer. The structure of the deposited film changed from a mixture of Cu₂O + CuO phases to a pure CuO phase with an increase in oxygen percentage. In addition the crystallite size increased from 12 to 24 nm as the oxygen pressure percentage increased. The optical transmittance significantly increased with the increase of the oxygen pressure percentage and the optical band gap of the film increased from 1.33 to 1.41 eV. The film prepared with 30 and 40 % oxygen pressure showed (002) crystallographic orientation. The I–V characteristic of p-CuO/n-Si heterojunction diode was also found to be dependent on the oxygen pressure percentage.     

Effect of solution concentration on surface morphology,chemical composition and photoresponse of CuO/Cu2O composite thin films grown by hydrothermal synthesis

Crystal structure, surface morphology, surface chemical composition and photocurrent response curves of the thin films were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical workstation. The results show that the composite thin films are composed of CuO and Cu₂O phase. The relative content of copper (I) oxide on the composite thin films increases with the increase of solution concentration. Leaf-like CuO particles have been observed on the thin film grown in the 0.5 mol/L solution. Some regular octahedral and truncated octahedral Cu₂O particles have been observed on the surface of thin films grown in the 0.6 and 0.8 mol/L solution, respectively. The best photocurrent response and the fastest growth and decay have been observed in the thin film grown in 0.6 mol/L solution.     

Green synthesis of bi-component copper oxide composites and enhanced photocatalytic performance

A facile and green route was proposed for the synthesis of bi-component copper oxide composite without any templates and additives. The effects of D-(+)-glucose amount, reaction temperature, and reaction time on the morphology and constitution of the products were investigated by SEM, X-ray diffraction, and UV-vis DRS in detail. The results indicate that a series of Cu₂O–CuO bi-component copper oxide composites with various morphologies can be easily obtained. Structure characterisation and photocatalytic tests show that the bi-component Cu₂O–CuO composites exhibited better photodecolouration of methylene blue than those of the Cu₂O, CuO, and Cu–CuO compounds owing to the existence of the synergistic effect between the CuO and Cu₂O.     

One step room temperature photodeposition of Cu/TiO2 composite films and its conversion to CuO/TiO2

Cu/TiO₂ composite films were prepared at low temperature on glass substrates by a photodeposition method. Films were deposited by irradiating the substrate while in contact with an aqueous TiO₂ suspension containing copper(II) nitrate and ethanol. Cu/TiO₂ composite films of 500 nm in thickness were deposited at room temperature after a short irradiation time (15 min) with a 125 W mercury vapour lamp. According to scanning electron microscopy observations, the obtained films were homogeneous and porous. Energy dispersive X-ray spectroscopy analysis revealed a 3:1 Cu:Ti atomic ratio. Grazing angle X-ray diffraction analysis showed that the films contained Cu and TiO₂ as major components and Cu₂O as a minor component. Heat treatment at 400 °C in air for a period of 3 h transformed the initial material into a CuO/TiO₂ composite, improved the adhesion to the substrate and favoured a more regular distribution of copper oxide according to backscattering micrographs.     

Photoactive TiO2/CuxO composite films for photocatalytic degradation of methylene blue pollutant molecules

Applying photocatalysts for utilizing solar energy is attractive because of its clean and limitless characteristics, but they have giant obstacles such as wide bandgaps, photocorrosion, and rapid charge recombinations to overcome for applications. In this study, the TiO₂/Cu_xO composite films are investigated as photoactive materials for the photodegradation ofmethylene blue (MB) pollutant molecules under standard sun light illumination. The TiO₂ layer is firstly fabricated on the FTO substrate by a doctor blade method, and then the second copper component is introduced by a spin-coating. Then, the composite films are thermally sintered to form cuprous and cupric oxides. This additional copper oxide phases plays a crucial role in achieving high photodegradation performance of MB under light irradiation. The best MB photodegradability from the TiO₂/Cu_xO composite films was achieved with the 3.6 wt% Cu_xO content case, and it is probably due to the effective charge separations with reduced recombinations in the copper oxide phases by the Z-scheme band arrangements between Cu₂O and CuO components. The composite films were characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy for the film surface and cross-sectional morphology, crystalline structure, atomic binding energy, and light absorbance, respectively.     

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.     

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.     

Preferentially oriented thin-film growth of CuO(1 1 1) and Cu2O(0 0 1) on MgO(0 0 1) substrate by reactive dc-magnetron sputtering

Copper oxide films deposited on MgO(0 0 1) substrates by reactive magnetron sputtering under the metal-mode condition were studied by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) analyses for structural analysis, and X-ray-excited Auger electron spectroscopy (XAES) for chemical bonding analysis. CuO(1 1 1) thin films grew from their initial growth stage maintaining the same crystallinity on MgO(0 0 1) substrates at 400°C. When the substrate temperature was increased to 600 °C, the as-sputtered films comprise Cu(0 0 1), amorphous Cu₂O phase, and Cu₂O(0 0 1) phase. The Cu(0 0 1) phase was observed at initial growth stage. This is probably because O₂ gas molecules could not sufficiently stick to the MgO substrate at 600 °C. Single phase of Cu₂O(0 0 1) was obtained by the cooling of the as-sputtered films in O₂ atmosphere. The growth of single phase Cu₂O(0 0 1) is considered as a solid-phase heteroepitaxial growth on MgO(0 0 1) surface, which was caused by incorporating O₂ gas into the as-sputtered films.     

Fabrication and mechanism study of CuO layers on double surfaces of polyimide substrate using surface modification

Formation process and mechanism of continuous CuO layers on double surfaces of polyimide films were studied. The composite films were prepared using the facile surface modification and ion exchange technique. By alkaline-induced chemical modification and ion-exchange reaction, Cu²⁺ ions were incorporated into the surface of polyimide substrate. Thermal treatment in ambient atmosphere resulted in the formation of CuO particles that further agglomerated on the film surface and produced well-defined CuO thin layers on the double surfaces of polyimide films. The changes in the chemical structure, surface morphology, crystalline state and the surface roughness with the increase of ambient temperature were investigated. It was interesting to find that the conversion of metallic copper and low valence sub-oxide Cu₂O to high valence oxide CuO was observed in the thermal treatment process. The agglomeration mechanism for the CuO particles was proposed and proved by three steps, which illustrated that copper-catalyzed and oxygen-assisted decomposition of the polyimide overlayer resulted in the agglomeration of CuO particles. The final composite films retained the thermal stability of the pure polyimide.     

Observation of Room Temperature Ferromagnetism in Conducting and Insulating Cu doped ZnO Thin Films

With pulsed laser deposition, the Cu_0.04Zn_0.96O thin films are grown at 600 °C under three different oxygen pressures, namely PO₂ = 0.00, 0.02, and 1.00 Pa. X-ray diffraction shows single-phase material for the samples grown under PO₂ = 0.00 and 1.00 Pa and CuO secondary phase for the PO₂ = 0.02 Pa grown sample. The observation of satellite structures in the Cu 2p core level X-ray photoelectron spectroscopy (XPS) spectra suggest the presence of Cu²⁺ and CuO secondary phases in the samples grown at PO₂ = 0.02 and 1.00 Pa. The sample grown under vacuum (PO₂ = 0.00 Pa) shows mixed Cu oxidation state of 1 + or 2 + . The sample grown without oxygen is n-type and those grown with oxygen are highly insulating. The insulating sample grown at PO₂ = 0.02 Pa shows highest magnetization due to possible collective behavior of Cu²⁺ – O _v – Cu²⁺ network in the form of bound magnetic polaron (BMP) and ferromagnetic superexchange interaction coming from uncompensated surface spins of the Cu ions in the CuO secondary phase. Both delocalized electrons (～3.32 × 10¹⁸) due to oxygen deficient defects and reduced amount of effective Cu²⁺ ions discredit the BMP model for this vacuum grown sample, and magnetism is suggested due to O _v and presence of possible CuO secondary 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.     

Photoelectrochemical solar cell studies on electroplated cuprous oxide thin films

Cuprous oxide (Cu₂O) is an interesting p-type semiconductor with a band gap of 2 eV suitable for solar cell applications. Deposition of Cu₂O thin films by electrodeposition from aqueous solutions is a low temperature and inexpensive technique. in the present work, Cu₂O thin films were cathodically deposited on Cu and tin oxide coated glass substrates by the cathodic reduction of copper (II) lactate solution. The optimized deposition conditions to synthesize cuprous oxide thin films were experimentally identified as; Deposition potential: −0.555 V versus SCE, pH: 9.0 ± 0.1, Bath temperature: 70^∘C. X-ray diffraction studies revealed the formation of single phase cubic Cu₂O films. The effect of annealing on the structure and morphology of Cu₂O thin films are studied. The dielectric susceptibility, optical conductivity and packing density are evaluated. Photoelectrochemical solar cells based on p-Cu₂O films are constructed. Spectral response studies indicate a peak in photo current density around 600 nm corresponding to the band gap of Cu₂O thin films. The effects of annealing, chemical etching and photo etching on the solar cell parameters are studied.