Abstract
Thin films of silver-copper-oxide were deposited on glass substrates by RF magnetron sputtering of Ag80Cu20 target under various oxygen partial pressures in the range – Pa. The effect of oxygen partial pressure on the crystallographic structure and surface morphology and electrical and optical properties was systematically studied and the results were reported. The oxygen content in the films was correlated with the oxygen partial pressure maintained during the growth of the films. The films which formed at low oxygen partial pressure of Pa were mixed in phase of Ag2Cu2O3 and Ag while those deposited at Pa were grown with Ag2Cu2O3 and Ag2Cu2O4 phases. The films which formed at oxygen partial pressure of Pa showed electrical resistivity of 2.3 Ωcm and optical band gap of 1.47 eV.
1. Introduction
The silver-copper-oxygen (Ag-Cu-O) system consists of various ternary compounds Ag2Cu2O3, Ag2Cu2O4, and AgCuO2. In 1999 the first ternary silver copper oxide compound, Ag2Cu2O3 was prepared by Gomez-Romero et al. [1] in powder form by using a coprecipitation method at room temperature. Later in the second ternary silver copper oxide compound, Ag2Cu2O4 was synthesized by electrochemical oxidation of suspension of the precursor Ag2Cu2O3 [2–5] at room temperature and ozone oxidation [6]. Curda et al. [7, 8] synthesized mixed silver copper monoxide, AgCuO2, which is diamagnetic and showed mixed valence again, with the formula of O2. Most of the recent studies on silver copper oxides concentrated mainly on the determination of physical properties such as the crystal structure and thermal stability. There has been a shift towards looking at applications, with groups studying silver copper oxides as positive electrode in button cell batteries [9–12] or as new promising materials for photovoltaic applications. The silver copper oxides are p-type semiconductors, which could potentially be used as absorber material for future generation photovoltaic devices [13]. The ternary oxide of silver and copper also has novel applications in the fields of science and technology such as high Tc-super conductors [14].
Various thin films deposition techniques such as thermal oxidation of metallic films, pulsed laser deposition and sputtering were employed for the growth of metallic oxide films. Among these methods, magnetron sputtering technique is industrially practiced technique for the growth of thin films on larger area substrates. The physical properties of magnetron sputter deposited metal oxide films depend mainly on the process parameters such as oxygen partial pressure, substrate temperature, substrate bias voltage, sputter power and sputter pressure. The effect of substrate temperature on the structural, electrical, and optical properties of RF magnetron sputtered Ag-Cu-O films was reported earlier [15]. In this investigation, thin films of Ag-Cu-O were deposited on glass substrates by RF magnetron sputtering of Ag80Cu20 target at different oxygen partial pressures. The influence of oxygen partial pressure on the crystallographic structure, surface morphology, electrical and optical properties of the deposited Ag-Cu-O films was systematically studied and reported the results.
2. Experimental
Ag-Cu-O films were deposited on glass substrates by employing RF magnetron sputtering from a home-made circular planer magnetron sputtering system. The magnetron sputtering system is capable of producing base pressure of 5 × 10−4 Pa using diffusion pump and rotary pump combination. The pressure in the sputter chamber was measured with digital Pirani-Penning gauge combination. Pure Ag80Cu20 target of 50 mm diameter was used for deposition of experimental films. Pure argon was used as sputter gas and oxygen as reactive gas. Two Aalborg mass flow controllers were used to control the flow rates of sputter gas of argon and reactive gas of oxygen individually. The sputter power of 65 W was fed to the sputter target with Advanced Energy RF power generator. Ag-Cu-O films were deposited on glass substrates held at room temperature (303 K) and at different oxygen partial pressures in the range 5 × 10−3–5 × 10−2 Pa. The process parameters maintained during the growth of the films are given in Table 1.
The deposited films were characterized by studying crystallographic structure and surface morphology, electrical and optical properties. The chemical composition of the films was determined by employing X-ray energy dispersive spectroscopic analyses (EDS) attached to the scanning electron microscope (Phillips XL 30S field effect gun). The crystallographic structure of the films was determined with the glancing angle X-ray diffraction (XRD) taken on a Bruker D8 Advance Diffractometer at the glancing angle of 4° using monochromatic Cu radiation. The surface morphology of the films was analysed by employing atomic force microscope (AFM). The electrical resistivity of the films was measured at room temperature using four-probe method (Jandel multiposition wafer probe). The optical transmittance of the films was recorded using Perkin-Elmer UV-Vis-NIR double beam spectrophotometer in the wavelength range 300–2500 nm.
3. Results and Discussion
The thickness of the deposited films measured using Veeco Dektak (model 150) profilometer was in the range 210–250 nm. The deposition rate of the films was determined from the film thickness and duration of the deposition. Figure 1 shows the dependence of deposition rate on the oxygen partial pressure of the deposited films. The deposition rate of the films up to the oxygen partial pressures of 5 × 10−3 Pa was about 10.5 nm/min. The deposition rate of the films formed at oxygen partial pressure of 5 × 10−2 Pa decreased to 5.5 nm/min and at higher oxygen partial pressures it remains almost constant. The high deposition rate at low oxygen partial pressures was due to the high sputtering yield of metallic silver-copper and insufficient oxygen available in the sputter chamber to react and to form silver-copper-oxide. The decrease in the deposition rate with increase of oxygen partial pressure was due to the decrease of sputter yield in the presence of reactive gas of oxygen and formation of Ag-Cu-O films. For moderate to highly reactive chemical system, the reactive sputter process comes with the abrupt decrease in the films deposition rate when the process turned into the so-called reactive sputter mode [16]. Such a decrease in the deposition rate with the increase of oxygen partial pressure was also reported in the deposition of DC reactive magnetron sputtered silver oxide films [17] formed with silver target, cuprous oxide films [18] formed with copper target and Ag-Cu-O films with Ag50Cu50 target [19].
Figure 2 shows the representative X-ray energy dispersive spectrum of the Ag-Cu-O film formed of oxygen partial pressure of 2 × 10−2 Pa. The X-ray energy dispersive spectroscopic analysis indicated that the oxygen content in the films was correlated with the oxygen partial pressure maintained in the sputter chamber. The atomic ratio of copper to silver was nearly constant value of 0.208 ± 0.010. At low oxygen partial pressure of 5 × 10−3 Pa the oxygen content in the films was 38.6 at. %. The films deposited at oxygen partial pressure of 2 × 10−2 Pa was 49.4 at. % and at higher pressures it remains almost constant [19].
The X-ray diffraction profiles of the films deposited at different oxygen partial pressures are shown in Figure 3. The films deposited at low oxygen partial pressure of 5 × 10−3 Pa were X-ray amorphous with presence of weak diffraction peaks related to the Ag2Cu2O3 (JCPDS no. 00-004-0783) and Ag (JCPDS no. 01-073-6753). The presence of mixed phase of Ag2Cu2O3 and Ag was due to the insufficient oxygen available in the sputter chamber during the deposition of the films. When the oxygen partial pressure increased to 2 × 10−2 Pa, the films were of polycrystalline in nature. The additional peak seen at 2θ = 58° was related to the (−312) reflections of Ag2Cu2O4 (JCPDS no. 01-073-7193) and 69° connected to the (008) reflection of Ag2Cu2O3 (JCPDS no. 00-004-0783).
It revealed that the films formed at oxygen partial pressure of 2 × 10−2 Pa were mixed phase of Ag2Cu2O3 and Ag2Cu2O4 with the absence of elemental Ag. Despite further increase of oxygen partial pressure to 5 × 10−2 Pa, there was enhancement in the intensity of the (111) reflection of Ag2Cu2O4 with a reduction in the intensity of (202) reflection of Ag2Cu2O3 [19]. The crystallite size () of the films was evaluated from the full width at half maximum intensity of X-ray diffraction peaks of (202) Ag2Cu2O3 using the Debye-Scherrer’s relation: where is a constant with value of 0.89 for copper radiation and the full width at half maximum intensity of X-ray diffraction peak. The crystallite size of the films formed at oxygen partial pressure of 2 × 10−2 Pa was about 15 nm. Despite further increase of oxygen partial pressure to 5 × 10−2 Pa, the crystalline size of the films decreased to 10 nm.
Figure 4 shows atomic force micrographs of Ag-Cu-O films formed at different oxygen partial pressures. The films deposited at low oxygen partial pressure of 5 × 10−3 Pa showed irregular shape of grains with grain size of 35 nm and the root mean square roughness of 1.3 nm. It is also seen that the films were not uniform with many stick up particles, and it may be the presence of silver clusters due to low oxygen partial pressure. The films formed at oxygen partial pressure of 2 × 10−2 Pa showed fine grain structure. At oxygen partial pressure of 5 × 10−2 Pa, the formed films were of larger size grains. The grain size of the films increased from 35 to 132 nm with the increase of oxygen partial pressure from 5 × 10−3 to 5 × 10−2 Pa. The root mean square surface of the films increased from 1.3 to 4.3 nm with increase of oxygen partial pressure from 5 × 10−3 to 5 × 10−2 Pa. Figure 5 shows the variation in grain size and surface roughness of Ag-Cu-O films with the oxygen partial pressure.
The variation of electrical resistivity of Ag-Cu-O films with the oxygen partial pressure is shown in Figure 6. The electrical resistivity of the films formed at low oxygen partial pressure of 5 × 10−3 Pa was 1.8 × 10−1 Ωcm. The electrical resistivity of the films increased to 2.3 Ωcm with the increase of oxygen pressure to 2 × 10−2 Pa. Despite further increase of oxygen partial pressure to 5 × 10−2 Pa, the electrical resistivity increased to 1.2 × 102 Ωcm.
The electrical resistivity of pure silver was 1.6 × 10−6 Ωcm. The Ag2O films formed by RF magnetron sputtering at oxygen partial pressure of 2 × 10−2 Pa was 3 × 10−3 Ωcm [20]. Ravi Chandra Raju et al. [21] reported that the electrical resistivity of pulsed laser deposited AgO films was close to 2 × 105 Ωcm. The low electrical resistivity of 1.8 × 10−1 Ωcm at low oxygen partial pressure of 5 × 10−3 Pa was due to the presence of metallic silver along with Ag2Cu2O3 phase. The increase in the electrical resistivity value of 2.3 Ωcm at oxygen partial pressure of 2 × 10−2 Pa was due to the formation of mixed phase of silver copper oxide films. Despite further increase of oxygen partial pressure to 5 × 10−2 Pa, the increase of electrical resistivity may be due to the reduction in the crystallinity as shown in the XRD profile data. It is to be noted that the RF magnetron sputtered Ag2Cu2O3 films formed with Ag70Cu30 target at oxygen partial pressure showed electrical resistivity 8.2 Ωcm [20].
Figure 7 shows the wavelength dependence of optical transmittance of the films formed at different oxygen partial pressures. The films formed at low oxygen partial pressure of 5 × 10−3 Pa exhibited the low optical transmittance due to presence of metallic silver along with Ag2Cu2O3. The low optical transmittance at low oxygen partial pressure was due to the presence of metallic silver along with Ag2Cu2O3. The optical transmittance of the films increased from 10 to 60% (at wavelength 1200 nm) with the increase of oxygen partial pressure. The optical absorption edge of the films shifted towards lower wavelength side with the increase of oxygen partial pressure 5 × 10−3 to 5 × 10−2 Pa. The optical absorption coefficient () of the films was evaluated from the optical transmittance () data using the relation where is the film thickness. The optical band gap of the films was determined from the optical absorption coefficient and photon energy data assuming the direct transition takes place between the top of the valence band and the bottom of the conduction band using Tauc’s relation [22]: where is the absorption edge width parameter. Extrapolation of the linear portion of the plots of versus to resulted in the optical band gap of the films.
Figure 8 shows the plots of versus photon energy of the films formed at different oxygen partial pressures. The optical band gap of the films formed at low oxygen partial pressure of 5 × 10−3 Pa was 1.36 eV. The films formed at oxygen partial pressure of 2 × 10−2 Pa showed the optical band gap of 1.47 eV and increased to 1.50 eV at higher oxygen partial pressure to 5 × 10−2 Pa. In the literature, the reported optical band gap for Ag2O films was in the range 1.16–2.25 eV depending on the deposition methods employed and the process parameters maintained during the growth of the films [23, 24]. Rivers et al. [25] achieved a high optical band gap of 3.3 eV in Ag2O films formed by evaporation of silver in the presence of electron cyclotron resonance oxygen plasma.
4. Conclusions
RF magnetron sputtering technique was employed for deposition of Ag-Cu-O films on glass substrates by sputtering of Ag80Cu20 target at different oxygen partial pressures in the range 5 × 10−3–8 × 10−2 Pa. The influence of oxygen partial pressure on the crystallographic structure and surface morphology, electrical and optical properties was systematically investigated. X-ray diffraction studies of the films formed at oxygen partial pressure 2 × 10−2 Pa showed the mixed phase of Ag2Cu2O3 and Ag2Cu2O4 with crystallite size of 15 nm. The grain size of the films determined by AFM increased from 35 to 132 nm with the increase of oxygen partial pressure from 5 × 10−3 to 5 × 10−2 Pa. The root mean square surface roughness of the films determined from atomic force microscope increased from 1.3 to 4.3 nm with the increase of oxygen partial pressure from 5 × 10−3 to 5 × 10−2 Pa.
In conclusion, copper silver oxide films formed at oxygen partial pressure of 2 × 10−2 Pa were mixed ternary phases of Ag2Cu2O3 and Ag2Cu2O4 showed electrical resistivity of 2.3 Ωcm and optical band gap of 1.47 eV.
Acknowledgments
The authors are thankful to Professor J. F. Pierson, Institut Jean Lamour, Department CP2S, Ecole des Mines, Nancy University, Nancy, France, for providing XRD and electrical data on these films. One of the authors, A. Sreedhar, is thankful to the University Grants Commission, New Delhi, for the award of Junior Research Fellowship to carry out the present work.