Structural and optical studies on dc reactive magnetron sputtered Cu2O films

Cuprous oxide (Cu₂O) thin films were deposited by dc reactive magnetron sputtering technique onto glass substrates by sputtering of pure copper target in a mixture of argon and oxygen gases under various oxygen partial pressures in the range 8 × 10^− 3–1 × 10^− 1 Pa at a constant substrate temperature of 473 K and a sputtering pressure of 4 Pa. The dependence of cathode potential on the oxygen partial pressure was explained in terms of cathode poisoning effect. The influence of oxygen partial pressure on the structural and optical properties of Cu₂O films was systematically studied. Single phase films of Cu₂O were obtained at an oxygen partial pressure of 2 × 10^− 2 Pa. The films formed at an oxygen partial pressure of 2 × 10^− 2 Pa were polycrystalline with cubic structure and exhibited an optical band gap of 2.04 eV.     

Effects of oxygen percentage on the growth of copper oxide thin films by reactive radio frequency sputtering

Copper oxide thin films were deposited onto glass substrates by reactive radio frequency magnetron sputtering at various oxygen percentage flow rates R(O₂). X-ray diffraction analysis revealed that nanocrystallite copper oxide thin films with cubic, tetragonal, and monoclinic structure were formed at R(O₂) values of 10%, 20%, and ≥30%, respectively. Energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy were used to verify the copper oxides phases. With increased R(O₂), the root mean square surface roughness of the deposited films decreased from 4.82 nm to 1.78 nm. Moreover, both the band gap type and value changed with increased R(O₂). For R(O₂) at 20%, single phase tetragonal Cu₄O₃ thin film with a direct band gap of 2.20 eV was formed. For R(O₂) ≥ 30%, single phase monoclinic CuO thin films with an indirect band gap of 1.20 eV–1.25 eV were formed. In addition, conductive copper oxide thin films tended to form for R(O₂) < 30%, whereas insulator oxide thin films tended to form for R(O₂) ≥ 30%. Through this study, the crystallization behavior, the band gap, and the resistivity properties of the deposited copper oxide thin films as a function of the R(O₂) were obtained.     

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.     

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.     

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.     

Optical and electrical properties of ZnO films, codoped with Al and Ga deposited at room temperature by an RF sputtering method

ZnO thin films, codoped with Al and Ga, were prepared on fused quartz (FQ) and cyclo-olefin polymer (COP) substrates using a radial frequency magnetron sputtering technique at room temperature, without the introducing of oxygen. The elemental distributions of Al, Ga, Zn and O throughout the films were found and no compositional variation in working pressure was observed. A resistivity of 0.03-4.07 Ω cm in AGZ/FQ films (Fig. 2b and 0.04-5.73 Ω cm in AGZ/COP films as well as a transmittance of above 85% were obtained by appropriate control of the working pressure. Compared with the band gap energy of single crystal ZnO, the band gap energy of the AGZ/FQ thin film was somewhat higher. The band gap energy of the AGZ/FQ films showed a tendency to increase with the working pressure employed.     

Growth behavior and optical properties of N-doped Cu2O films

N-doped Cu₂O films are deposited by sputtering a CuO target in the mixture of Ar and N₂. The structures zand optical properties have been studied for the films deposited at different temperatures. It is found that N-doping can suppress the formation of CuO phase in the films. The films are highly (100) textured at low temperatures and gradually change to be highly (111) textured at the temperature of 500 °C. With the analysis of (111) and (100) grain sizes, the surface free energy and grain size of critical nuclei are suggested to dominate the film texture. The analysis of the atomic force microscopy shows that the film growth can be attributed to the surface-diffusion-dominated growth. The forbidden rule of band gap transition is found disabled in the N-doped Cu₂O films, which can be attributed to the occupation of 2p electrons of nitrogen at the top of valence band. The optical band gap energy is determined to be 2.52 ± 0.03 eV for the films deposited at different temperatures.     

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.     

Characterization of copper oxide nanolayers deposited by direct current magnetron sputtering

Direct current reactive magnetron sputtering was used to deposit the thin layers of copper oxide (Cu₂O) on glass substrates. A solid disc of pure copper as the target was sputtered in an argon gas under sputtering pressures varying from 0.133 to 4 Pa. The effects of the sputtering power and pressure on the structural and optical properties of Cu₂O thin films were systematically studied. The deposited layers were characterized using X-ray diffraction, atomic force microscopy, profilometry and spectrophotometry. The optical transmission of the films was measured in the visible region. The increase in pressure resulted in a higher growth rate than increasing sputtering power. The increase in power produced Cu₂O thin films that were detrimental to the optical transmission 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.     

Opto-electronic properties of sputter-deposited Cu2O films treated with rapid thermal annealing

Cu₂O thin films were first deposited using magnetron sputtering at 200 °C. The samples produced were then annealed by a rapid thermal annealing (RTA) system at 550 °C in a protective atmosphere with or without the addition of oxygen. After annealing, various Cu₂O and CuO films were formed. These films were characterized, as a function of oxygen concentration in RTA, using UV-VIS photometer, four-point probe, and Hall measurement system. The results show that these Cu₂O thin films annealed at 550 °C with more than 1.2% oxygen added in the protective argon atmosphere would transform into the CuO phase. Apparently, the results of RTA are sensitive to the amount of oxygen added in the protective atmosphere. The resistivity of these Cu₂O thin films decreases with the increase in the oxygen amount in the annealing atmosphere, most likely due to the increase in carrier mobility. In addition, Cu₂O/ZnO (doped with AlSc) junctions were produced at 200 °C and annealed. The rectifying effect of P-N junction disappeared after annealing, probably due to the damage of p-n interface, which directly causes current leakage at the junction.     

溅射沉积CuO薄膜的光学和电学特性

以金属铜为靶材,氧气为反应气体,保持200℃的基底温度不变,通过调节氧氩比(OFR)和反应压强,利用直流反应磁控溅射方法在玻璃衬底上制备了一系列氧化铜薄膜。利用能谱对薄膜材料的元素含量进行测量和分析,结果显示:OFR=1∶1和2∶1时,铜元素和氧元素的含量比约在0.90~0.97的范围内变动。用四探针测试仪对薄膜的电阻率进行测量,用分光光度计测量薄膜的透过率,并用外推法推导出氧化铜薄膜的禁带宽度。利用霍尔效应测试仪对薄膜的电学参数进行测量和分析。     

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.     

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.     

Properties of CuS thin films treated in air plasma

Copper sulfide thin films were grown by chemical deposition and post treated in air plasma during 20 min. Air plasma was generated by alternating current discharge at a pressure of 4 × 10² Pa. The power discharge was maintained at an output of 220 V and a current of 0.2 A. Thermal annealing at 300 °C was performed for comparison. X-ray diffraction shows that plasma treatment results in phase transformation of Cu₃₉S₂₈ (as grown) to CuS (treated by plasma). The copper lost is confirmed by X-ray fluorescence. No significant change in the optical band gap was observed due to plasma action. In addition, the electrical conductivity increases in one order of magnitude. On the other hand, the samples under plasma condition show a parallel growth to the substrate and an increase in the surface uniformity. The plasma etching removes copper due to its affinity with oxygen to form CuO, as is corroborated by optical emission spectroscopy.     

AC plasma induced modifications in β-In2S3 thin films prepared by spray pyrolysis

β-In₂S₃ thin films, deposited by spray pyrolysis, were treated in N₂ and air plasmas at 240 and 400 Pa. X-ray diffraction, SEM, and EDS analysis, and optical and electrical studies have been used to characterize the as-prepared and plasma treated thin films. The post-deposition plasma treatments affect the morphology and the optoelectronic properties of the In₂S₃ thin films. The In₂S₃ thin films treated with N₂ plasma at 240 Pa showed an optical band gap, E_g, of 2.16 eV and an electrical conductivity of 2 × 10^− 2 (Ω cm)^− 1.     

Tungsten-doped tin oxide thin films prepared by pulsed plasma deposition

Transparent conductive oxide tungsten-doped tin oxide thin films were deposited on glass substrates from ceramic targets by the pulsed plasma deposition method. The structural, electrical and optical properties have been investigated as functions of tungsten doping content and oxygen partial pressure. The lowest resistivity of 2.1 × 10^? 3 Ω?cm was reproducibly obtained, with carrier mobility of 30 cm²V^? 1s^? 1 and carrier concentration of 9.6 × 10¹⁹ cm^? 3 at the oxygen partial pressure of 1.8 Pa. The average optical transmission was in excess of 80% in the visible region from 400 to 700 nm, with the optical band gap ranging from 3.91 to 4.02 eV.     

Controlled growth and characteristics of single-phase Cu2O and CuO films by pulsed laser deposition

Copper oxide, Cu₂O and CuO, thin films have been synthesized on Si (100) substrates using pulsed laser deposition method. The influences of substrate temperature and oxygen pressure on the structural properties of copper oxide films were discussed. The X-ray diffraction results show that the structure of the films changes from Cu₂O to CuO phase with the increasing of the oxygen pressure. It is also found that the (200) and (111) preferred Cu₂O films can be modified by changing substrate temperature. The formation of Cu₂O and CuO films are further identified by Fourier transform infrared spectroscopy. For the Cu₂O films, X-ray photoelectron spectroscopic studies indicate the presence of CuO on the surface. In addition, the optical gaps of Cu₂O and CuO films have been determined by measuring the transmittance and reflectance spectra.     

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.

     

Influence of the growth conditions on the stoichiometry and on the optical properties of titanium oxide thin films prepared by reactive sputtering

Titanium oxide thin films are deposited at room temperature by reactive DC sputtering onto glass and Si (100) substrates. Different conditions of deposition were varied such as sputtering power, deposition time and oxygen partial pressure to study their influence on the titanium oxide thin films growth. The absolute amount of oxygen and the relative O/Ti composition of films have been determined by Nuclear Reaction Analysis and Rutherford Backscattering Spectroscopy, respectively. Additionally, the band-gap was determined by measuring the optical absorption and its behavior correlated with the oxygen film content. From the present study, it is possible to establish that the optical band-gap energy depends mainly on the sputtering oxygen partial pressure used at the preparation and that films prepared with a partial oxygen pressure of 4 × 10^− 2 Pa allows titanium oxide with near stoichiometric composition. Additionally, from the optical point of view, band-gap energies of 3.4 eV are obtained for near stoichiometric films and a decrease is observed for samples prepared with higher oxygen concentrations.