Cuprous Oxide Films Research Articles

Nitrogen doping of cuprous oxide films: A surface science perspective

Nitrogen doping of Cu2O films grown on Au(111) and Pt(111) supports was explored by a variety of surface-science techniques, including electron-diffraction, X-ray photoelectron-spectroscopy (XPS), scanning tunneling microscopy and photoluminescence spectroscopy (PL). The films were prepared by Cu vapor deposition and high-pressure oxidation at 50 mbar O2. Nitrogen was inserted by adding N2 to the reactive gas or via sputter doping. Only the latter resulted in a clear N1s signal in XPS, compatible with the insertion of N-atoms at O substitutional sites. The N-doping caused an overall degradation of the oxide lattice and suppressed the formation of the (√3×√3)R30° surface reconstruction observed on pristine Cu2O(111). Moreover, the oxide Fermi level shifted from the valence-band top into the band gap, indicative for a reduced p-type conductivity of the sample upon doping. The N-dopants featured low thermal stability and largely desorbed at around 500 K, leaving behind a pronounced 850 nm PL peak due to O vacancy emission. Our findings indicate that the N-atoms initially occupy O substitutional sites but get removed easily at moderate temperature, casting doubts whether N-doping is a suitable pathway to improve the conductance and luminescence behavior of Cu2O.

DFT electronic structure investigation of chromium ion-implanted cupric oxide thin films dedicated for photovoltaic absorber layers

This study explores the enhancement of cupric oxide (CuO) thin films for photovoltaic applications through chromium doping and subsequent annealing. Thin films of CuO were deposited on silicon and glass substrates using reactive magnetron sputtering. Chromium was introduced via ion implantation, and samples were annealed to restore the crystal structure. The optical and structural properties of the films were characterized using X-ray diffraction, spectrophotometry, and spectroscopic ellipsometry. Results indicated that implantation reduced the absorbance and conductivity of the films, while annealing effectively restored these properties. Sample implanted with 10 keV energy and 1 × 1014 cm−2 dose of Cr ions, after annealing had sheet resistance of 1.1 × 106 Ω/sq compared to 1.7 × 106 Ω/sq for non implanted and annealed CuO. Study of crystalline structure confirmed the importance of annealing as it reduced the stress present in the material after deposition and implantation. Density Functional Theory (DFT) calculations were performed to investigate the electronic structure and optical properties of CuO with varying levels of chromium doping. Calculations revealed an energy gap of 1.8 eV for undoped CuO, with significant changes in optical absorption for doped samples. Energy band gap determined using absorbance measurement and Tauc plot method had value of 1.10 eV for as deposited CuO. Samples after implantation and annealing had energy band gap value increased to about 1.20 eV. The study demonstrates that chromium doping and subsequent annealing can enhance the optical and electronic properties of CuO thin films, making them more efficient for photovoltaic applications.

Author correction to “Enhanced property of thin cuprous oxide film prepared through green synthetic route”

This correction adds some information to our publication [Chin. J. Chem. Phys. 32, 365–372 (2019)] that we previously missed to include.

Effects of Postdeposition Annealing on the Electrical Properties of Cu2O/4H–SiC PiN Diodes

Copper oxide (Cu2O) is a promising p‐type material owing to its high absorption coefficient, suitable bandgap width, chemical stability, nontoxicity, and abundance. In this study, the effect of postdeposition annealing (PDA) on the electrical properties of Cu2O thin films deposited on silicon carbide (SiC) substrates is investigated. The films are subjected to PDA using radio‐frequency sputtering at various temperatures in air. During PDA, the Cu2O films are maintained at <300 °C and transitioned to cupric oxide (CuO) at 400 °C. The as‐deposited Cu2O film exhibits a low oxidation peak (Cu2+), whereas the phase‐transformed CuO films exhibit a higher binding energy with the emergence of satellite peaks. The rectification ratios of the Cu2O device annealed at 300 °C and CuO device annealed at 400 °C are determined as 2.02 × 107 and 1.01 × 105, respectively, denoting the substantial enhancement of approximately 55.04 for the Cu2O/SiC device and degradation of approximately 0.28 for the CuO/SiC device relative to their non‐annealed counterparts. Therefore, in this study, the significant improvement in the performance of Cu2O thin films with the meticulous deposition control of potential p‐type materials, such as Cu2O and CuO, for high‐throughput and low‐cost electronic applications is demonstrated.

Photo-electric properties of ultra-thin cuprous oxide films prepared by electrodeposition

Photo-electric properties of ultra-thin cuprous oxide films prepared by electrodeposition

Electrical and Optical Properties of a Cu2O/β‐Ga2O3 pn‐Junction

A pn‐heterojunction is fabricated by depositing an n‐type β‐Ga2O3 film by pulsed laser deposition (PLD) on c‐cut Al2O3. P‐type cuprous oxide films, Cu2O, are then deposited by PLD, as well as by radio frequency (RF) magnetron sputtering. It is concluded that hole injection is prohibited by a 3.26 eV valence band barrier, as measured by X‐ray photo‐electron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Heterojunction diode structures are prepared on the front side and electrical measurements demonstrate a rectification ration of 8 orders of magnitude and an ideality factor close to 2, indicating interface recombination‐controlled forward current. The junction is also optically active and shows a very fast photo‐response to 275 nm UV light.

Investigation of plasma process in deposition of cupric oxide film produced by radio frequency magnetron sputtering

The plasma process in the deposition of cupric oxide (CuO) film was investigated. The films were synthesized by balanced radio frequency magnetron sputtering. The Effect of substrate temperature, working pressure, and bias voltage on the film properties and their relations to the plasma factors was studied. These parameters affect the kinetic energy and flux of different plasma species, especially ions in the plasma sheath near the substrate. The results showed that all the films were grown with CuO phase composition, however the crystallite size is mainly determined by the substrate temperature than the working pressure or bias voltage. The results also show that the films growth rate and chemical composition are more influenced by the working pressure, so that increasing the working pressure increased the growth rate as well as the oxygen concentration in the film composition. Furthermore it was shown that with increasing working pressure and substrate temperature, the concentration and mobility of the carriers increase. Finally, both optical band gap energy and resistivity of the films decreases by increasing substrate temperature and working pressure. But changing the bias voltage did not change these characteristics.

Crystal phase control of copper oxide thin films by process pressure during high power impulse magnetron sputtering

High-performance P-type cuprous oxide (Cu2O) film was prepared at room temperature by high power impulse magnetron sputtering. Optical emission spectra revealed that the ratio of Cu radicals/ions in the plasma significantly decreased with increasing process pressure due to the reduction of sputtering yield as the Cu target surface was oxidized by the increased oxygen radicals/ions. In addition, the increase of self-sputtering yield for Cu cations with increasing process pressure reduced the arrival ratio of Cu species at the substrate surface. As a result, the film crystal phase transformed from Cu2O to Cu4O3 and CuO with increasing process pressure. X-ray photoelectron spectra and Hall effect test results revealed that the oxygen vacancy defects in the films were passivated by the increased oxygen species at higher process pressure, leading to the inhibition of electron background and the emergence of net hole concentration. A mobility of 37.3 cm2/V·s was achieved, which is very high for room-temperature-deposited Cu2O film and comparable to high-temperature (300–600 °C) deposited/post-annealed Cu2O film. Finally, Cu2O thin film transistors (TFTs) exhibited reasonable switching characteristics with a low off-current of 0.3 nA without post-annealing treatment, showing an advantage in low-temperature preparation.

Research on pH Sensor Based on Cuprous Oxide Film Modified with Tungsten Trioxide Nanoparticles

Research on pH Sensor Based on Cuprous Oxide Film Modified with Tungsten Trioxide Nanoparticles

Grain boundary passivation in cuprous oxide thin films via nitrogen annealing

In this work, polycrystalline cuprous oxide (Cu2O) films deposited by magnetron sputtering were annealed in situ in N2 atmospheres. Room-temperature and temperature-dependent Hall-effect measurements were performed on as-deposited and N2-annealed Cu2O films to probe the electrical transport mechanisms. The results show that the hole mobility of 0.8 Pa N2-annealed Cu2O film is enhanced compared with its as-deposited counterpart. It is shown that at low temperatures, the hole mobilities of Cu2O films appear to be limited by grain boundary scattering, while acoustic-phonon scattering comes into play at elevated temperatures. Moreover, the grain boundary potential barrier height which inhibits the hole transport is found to be reduced by N2-annealing at 0.8 Pa, thereby leading to the observed improvement in the hole mobility. It is thought that N2 is mainly concentrated at the grain boundaries in polycrystalline Cu2O films, thus passivating the grain boundary defect trap states. The results reported in this work suggest that N2-annealing induced grain boundary passivation could be an effective method to improve the photoelectric properties of polycrystalline Cu2O films.

Understanding atomic interaction between cuprous oxide film and aggressive chloride solution

Considering the structural complexity of actual copper (Cu) passive films consisting of Cu2O, the multiple roles of aggressive chloride anion (Cl–) on Cu2O are revealed through first-principles calculations. Despite different Cl– adsorption susceptibilities of various Cu2O surfaces, similar surface O vacancy formation mechanisms promoted by Cl– adsorption are elucidated, accompanying with further Cu vacancy propagation. Increasing surface Cu vacancy concentration and unchanged Cu vacancy diffusion coefficient induced by Cl– adsorption lead to increasing diffusion flux. These results reveal the atomic interaction between Cu2O film and Cl ion, which is helpful to understand the depassivation mechanism of Cu.

Novel application of electrochemical test for the controllable electrodeposition of Cu2O and metallic Cu film

As important industrial materials, the preparation of metallic copper and cuprous oxide (Cu2O) film is a hotspot in related fields. In this work, Cu2O and metallic copper films were controllably fabricated on the conductive substrate in an electrolyte containing 0.02 M cupric acetate and 0.1 M sodium acetate at different voltages. Linear sweep voltammetry test, as an effective way, was used to determine the selection of electrodeposition voltage. Pure Cu2O film can be prepared at a lower deposition voltage, and metallic copper film can be obtained at a higher deposition voltage and a longer deposition time. The obtained Cu2O film has complex surface morphology and exhibited n-type conductivity. The composition and morphology of the films were related to the deposition voltage. These results provide a new strategy and sight for the controllable preparation of cuprous oxide and metallic copper in the field of solar cells and integrated circuits.

Electrical properties of vertical Cu2O/β-Ga2O3 (001) p–n diodes

In this work, p-type cuprous oxide (Cu2O) films grown on beta gallium oxide (β-Ga2O3) substrates by magnetron sputtering were reported. The resulting vertical Cu2O/β-Ga2O3 heterojunction p–n diodes demonstrated superior performance compared to devices fabricated with polycrystalline Cu2O thin films. Meanwhile, analysis of the discrepancies between the built-in potential and turn-on voltage revealed diverse carrier transport mechanisms in the fabricated devices. Numerical fitting of the forward J–V characteristics further discerned that distinct carrier transport mechanisms dominated under various bias voltages or temperature conditions. At 300 K, trap-assisted tunneling dominates the regime because of the presence of defects in β-Ga2O3 or Cu2O. While the bias voltage is low, the polycrystalline nature of the films formed at room temperature leads to the prevalence of grain boundaries as the primary source of interface-type defects at the Cu2O/β-Ga2O3 interface. Consequently, the dominant mechanism governing carrier transport is interface recombination. As the temperature increases, however, thermionic emission becomes more important. This study presents an opportunity for further investigation into the epitaxial growth of Cu2O and provides insights into the carrier transport mechanism of β-Ga2O3-based heterojunctions.

Influence of Al Doping on the Physical Properties of CuO Thin Films

The synthesis of cupric oxide (CuO) films on cost-efficient, optical grade borosilicate-crown glass substrates (BK7) via chemical spray pyrolysis (CSP), either in pure form or with a low concentration of Al doping (below 1%), is presented and discussed. As a non-toxic p-type semiconductor, exhibiting monoclinic crystal structure and widely tuneable band gap (Eg), it is used in various applications. The optical properties, morphology and crystalline phases of CuO films are influenced by substrate temperature during thin film growth (annealing) and also by chemical doping very often introduced to modify grain boundary energy. The importance of our research subject is therefore perfectly justified and is essentially based on the fact that the potential fields of application are wide. Thus, herein we emphasize impact of the annealing stage and Al doping upon the structural, optical and electrical properties of the resulting product. Raman spectroscopy analysis confirms the presence of vibrational bands characteristic of a CuO phase, while X-ray diffraction (XRD) confirms the polycrystalline nature of the pure films. The thickness of the CuO films grown at 350 °C over three annealing intervals is proportional to the annealing time, while the crystallite phase in the films is proportional with the annealing temperature. Furthermore, XRD analysis of the Al:CuO films indicates the formation of a monoclinic-type structure (CuO phase) exhibiting a preferred orientation along the (002) plane, together with a significant grain size reduction from ~88 to ~45 nm as Al content increases. The transmittance spectra (between 400 and 800 nm) reveal a decrease in the transmittance from 48% to 15% with as the Al doping ratio increases. Additionally, the bandgap energy of the films is measured, modelled and discussed, using data from an ultraviolet–visible (UV-Vis) spectrophotometer. The calculated Eg is approximately 3.5 eV, which decreases with respect to the increasing annealing temperature, while the electrical resistivity varies from ~19 to ~4.6 kOhm.cm. Finally, perspectives and applications of CuO films are suggested, since the films are found to have a remarkable improvement in their structure and optical properties when doped with Al.

Understanding the role of oxygen-vacancy defects in Cu2O(111) from first-principle calculations

The presence of defects, such as copper and oxygen vacancies, in cuprous oxide films determines their characteristic carrier conductivity and consequently their application as semiconducting systems. There are still open questions on the induced electronic re-distribution, including the formation of polarons. Indeed, to accurately reproduce the structural and electronic properties at the cuprous oxide surface, very large slab models and theoretical approaches that go beyond the standard generalized gradient corrected density functional theory are needed. In this work we investigate oxygen vacancies formed in proximity of a reconstructed Cu2O(111) surface, where the outermost unsaturated copper atoms are removed, thus forming non-stoichiometric surface layers with copper vacancies. We address simultaneously surface and bulk properties by modelling a thick and symmetric slab, to find that hybrid exchange-correlation functionals are needed to describe the oxygen vacancy in this system. Our simulations show that the formation of oxygen vacancies is favoured in the sub-surface layer. Moreover, the oxygen vacancy leads to a splitting and left-shift of the shallow hole states in the gap, which are associated with the deficiency of copper at the surface. These findings suggest that surface electronic structure and reactivity are sensitive to the presence of oxygen vacancies, also when the latter are formed deeper within the film.

Highly Stable and Enhanced Performance of p-i-n Perovskite Solar Cells via Cuprous Oxide Hole-Transport Layers.

Solar light is a renewable source of energy that can be used and transformed into electricity using clean energy technology. In this study, we used direct current magnetron sputtering (DCMS) to sputter p-type cuprous oxide (Cu2O) films with different oxygen flow rates (fO2) as hole-transport layers (HTLs) for perovskite solar cells (PSCs). The PSC device with the structure of ITO/Cu2O/perovskite/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/bathocuproine (BCP)/Ag showed a power conversion efficiency (PCE) of 7.91%. Subsequently, a high-power impulse magnetron sputtering (HiPIMS) Cu2O film was embedded and promoted the device performance to 10.29%. As HiPIMS has a high ionization rate, it can create higher density films with low surface roughness, which passivates surface/interface defects and reduces the leakage current of PSCs. We further applied the superimposed high-power impulse magnetron sputtering (superimposed HiPIMS) derived Cu2O as the HTL, and we observed PCEs of 15.20% under one sun (AM1.5G, 1000 Wm-2) and 25.09% under indoor illumination (TL-84, 1000 lux). In addition, this PSC device outperformed by demonstrating remarkable long-term stability via retaining 97.6% (dark, Ar) of its performance for over 2000 h.

Protected ultrathin cuprous oxide film for photocatalysis: Excitation and relaxation dynamics

In the search for cost-effective ways to produce green hydrogen, harnessing the most abundant renewable energy source---sunlight---directly through photoelectrochemical water splitting is highly desirable. Cuprous oxide (${\mathrm{Cu}}_{2}\mathrm{O}$) has proven itself as a material with great potential for the hydrogen evolution reaction (HER) and stands out by its Earth abundance and low processing cost. In this photoelectron spectroscopy study we investigated the electron dynamics in a heterostructure comprised of a ${\mathrm{Cu}}_{2}\mathrm{O}$ surface oxide grown on Cu(111) underneath a hexagonal boron nitride ($h$-BN) film, which replaces a conventional capping layer for corrosion protection. Our results show that it can be a viable cuprous oxide-based photoelectrode material. The $h$-BN film stays intact during the oxidation, and the oxide layer forms an ordered structure with defined valence and conduction bands. The conduction band is at a suitable energy to drive the HER for water splitting, but the photoexcited electrons display short lifetimes. Electrons are mainly excited in the copper substrate and are then captured in long-lived defect states in the ${\mathrm{Cu}}_{2}\mathrm{O}$ layer. The water splitting efficiency of this photocathode may still be improved by reducing the defect density.

Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films

Cuprous oxide (Cu2O) crystals and films of 10–65 nm thickness are investigated via electron diffraction, scanning tunneling microscopy (STM), and photoluminescence (PL) spectroscopy at temperature between 100 to 300 K. While the chemical composition and surface morphology of both systems are identical, large differences are found in the optical response. Bulk Cu2O shows pronounced PL peaks at 620, 730, and 920 nm, compatible with the radiative decay of free and bound excitons, whereas broad and asymmetric peaks at 775 and 850 nm are found for Cu2O films grown on Au(111) and Pt(111) supports. The latter represent the PL signature of VO2+ and VO+ defects, being inserted with substrate‐dependent concentrations due to the oxygen‐poor preparation of the Cu2O films. Despite the strong VO signature in PL, all Cu2O samples show p‐type conductance behavior in STM spectroscopy, indicating an abundance of Cu defects in the lattice. The fact that O vacancies still govern the thin‐film PL is explained by a more efficient recombination via VO‐ than Vcu‐emission channels, as the latter requires exciton formation first. Herein, the high sensitivity of low‐temperature PL to probe the defect landscape of dielectrics, being neither reached by photoelectron spectroscopy nor scanning probe techniques is demonstrated.

Platinum (Pt), gold (Au), and silver (Ag) ohmic contacts to cupric oxide (CuO) films deposited by air-based sputtering and thermal annealing

The present work reports the fabrication of three different ohmic contacts (platinum (Pt)/CuO, gold (Au)/CuO, and silver (Ag)/CuO). Structural properties of the films measured by X-ray diffraction and Raman techniques confirmed that the Cu2O films prepared by DC sputtering using air as a reactive gas, transformed into pure CuO films after thermal annealing. The band gap of the CuO films of 1.33 eV was estimated by reflectance measurements. Hall measurements confirmed the p-type conductivity of the CuO films with an acceptor concentration of 2.04x1016 cm−3.The linear behavior in both directions of polarization observed in the current–voltage measurements confirmed the formation of ohmic contacts between the characterized three metals and CuO. Based on the slope of the current–voltage curve, the best ohmic contact was that fabricated between Au and CuO. The mechanism governing the transport through the interface between the metal/CuO ohmic contact was thermionic emission.

CuO:Se composite films and its photovoltaic application

Cupric oxide (CuO) and selenium (Se) are old photoelectric materials, and their photovoltaic properties have always received attention. However, CuO has a high melting point (1026℃) and will decompose near its melting point, so it is difficult to prepare high crystallinity CuO films. Selenium (Se) can not efficiently absorb sunlight because of its large optical band gap (1.9 eV). Here, a CuO:Se composite film was prepared by radio frequency magnetron co-sputtering of CuO and Se and low temperature annealing (200℃). The experimental results show that this composite material film has the advantages of CuO and Se, and overcome their disadvantages. The material has strong absorption of light in the entire solar spectral range (250-2500 nm, average absorptance ~80%). A prototype solar cell based on this composite film was fabricated and its power conversion efficiency (PCE) is up to 2.05%, which is much higher than that of the pure CuO (0.007%) or pure Se (1.05%) film solar cells. This material should be promising for photovoltaic and other photo-electrical application.