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Characteristics of copper oxide films deposited by PBII&D
Surface & Coatings Technology 201 (2007) 6712 – 6714 www.elsevier.com/locate/surfcoat
Characteristics of copper oxide films deposited by PBII&D Xinxin Ma a,⁎, Gang Wang a , Ken Yukimura b , Mingren Sun a a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China b Department of Electrical Engineering, Faculty of Engineering, Doshisha University, 1-3 Tatara-Miyakodani, Kyotanabe, Kyoto 610-0321, Japan Available online 27 October 2006
Abstract Copper oxide films were deposited by plasma based ion implantation and deposition using a copper antenna as rf sputtering ion source. A gas mixture of Ar + O2 was used as working gas. During the process, copper that was sputtered from the rf antenna reacted with oxygen and was deposited on a silicon substrate. The composition and the chemical state of the deposited films were analyzed by XPS. The structure of the films was detected by XRD. It is observed that Cu2O film has been prepared on the Si substrate. It is found that the microstructure of the deposited film is amorphous for the applied voltage of −5 kV. The surface layer of the deposited films is CuO. This is because the surface layer absorbs the oxygen from ambient air after the treated sample was removed from the vacuum chamber. An appropriate applied voltage, 2 kV under the present conditions, brings the lowest resistance. It is also seen that the maximum absorbance of the deposited films moves to a lower wavelength with increased applied voltage. © 2006 Elsevier B.V. All rights reserved. PACS: 81.15. -z; 68.55.-a; 73.90.+f; 78.20.-e Keywords: PBII; Absorption spectrum; Copper oxide film; Film; Resistance
1. Introduction Copper oxide (Cu2O) films are very useful material in the construction of photovoltaic solar cells and photoelectric cells because of the favourable band gap of Cu2O, low fabrication cost and abundant production rate [1]. Various deposition techniques such as thermal oxidation, reactive sputtering, electro-deposition, and plasma evaporation have been used in the preparation of Cu2O films [2–7]. Plasma based ion implantation and deposition (PBII&D) has an advantage of flexibility to set the deposition parameters for three dimensional components. Concerning the sputtering deposition combined with PBII&D, Malik et al. [8] prepared TaN thin films by sputtering tantalum in a mixed gas of argon and nitrogen. Nakamura and Suzuki [9] proposed that the sputtered copper species were fully ionized at relatively high gas pressures (∼50–100 mTorr), and metal droplets were hardly generated in this sputtering system. Thus, in the case of copper, the sputtering method may act as an ion source and a supply source of deposited material as well. The present research is a feasibility test to deposit copper oxide films by reactive deposition with PBII&D using the copper sputtering method proposed by ⁎ Corresponding author. Tel.: +86 451 86418835; fax: +86 451 86413922. E-mail address: [email protected] (X. Ma). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.033
Nakamura and Suzuki. The sputtering rate of copper is estimated to be approximately 50 nm/min [9]. In this paper, a copper antenna was used as a sputtering ion source and a series of copper oxide films were deposited by varying the applied pulse voltage to the sample. The composition, structure, resistivity and absorbance spectrum of the deposited films were investigated. 2. Experimental Copper oxide films were deposited by plasma based ion implantation and deposition (PBII&D). The facility used in this experiment is shown in Fig. 1. In the vacuum chamber, a copper (Cu) antenna was floated from the ground, and set as the rf sputtering source with a frequency of 13.56 MHz and a power of 100 W. Silicon samples were located at 80 mm below the antenna. A series of negative high voltage pulses of 0–10 kV with a pulse width of 20 μs and with a frequency of 400 Hz were applied to the sample. A gas mixture of argon and oxygen in a proportion of 4:1 was introduced into the vacuum chamber, where a base pressure was approximately 9 × 10− 4 Pa. The working gas pressure of the experiment was approximately 10− 1 Pa set by the gas flow rate. During the process, copper particles were sputtered from the rf antenna by argon ion bombardment, and reacted with oxygen gas
X. Ma et al. / Surface & Coatings Technology 201 (2007) 6712–6714
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Fig. 3. XRD patterns from films deposited with various pulsed voltages.
Fig. 1. Schematic of the experimental facility.
to make a compound film on a silicon substrate. In this process, copper and oxygen ions were also generated, and implanted into the sample surface. The process time was 2 h. A PHI 5700 X-ray photoelectron spectroscopy (XPS) instrument was used to measure the chemical composition and chemical state of the deposited films. Argon ions with an energy of 3 keV were used to etch the samples at an etching rate of 2 nm/min. A Philips X'pert X-ray diffraction (XRD) system was used to analyze the phase structure of the deposited films with a glancing angle of 5° and with a step of 0.02°. A fourpoint probe system was used to measure the resistance of the deposited films. A Lambda 950 UV/VIS spectral photometer was used to measure the light absorbance of the films in the wavelength range from 300 to 700 nm. 3. Results and discussion 3.1. Composition of deposited films The composition of the deposited films was measured by XPS. In the survey mode, it is found that there are only oxygen
Fig. 2. Binding energy of Cu2p from the surface and subsurface of deposited film by XPS.
and copper existing in the films. By using the intensity integration at peaks of Cu2p and O1s, it is found that the contents of Cu and O are about 65 at.% and 35 at.%, respectively. No obvious differences in the element composition and the atomic ratio of the films by the applied voltage are found. The atomic ratio of Cu to O is about 1.85:1. 3.2. Phase structure of deposited films The phase structure of the films is determined by the XPS and XRD results. From the Cu2p-binding energy from XPS, the chemical state of Cu in the films is confirmed. Fig. 2 shows the XPS result of the film which was deposited at 0.3 kV, at the surface and near surface areas at an argon ion sputtering time of 2 min. It is found that the characteristic peaks of Cu2p3/2 are seen at binding energies of 933.9 eV and 932.5 eV, which correspond to CuO and Cu2O, respectively. This result suggests that the structure of the deposited film is Cu2O, while CuO at the surface is due to the storage of the sample in ambient air. The same result was obtained for the films deposited with other applied bias voltages. Fig. 3 shows the XRD patterns at a glancing angle of 5° from films deposited at applied voltages of 0.3, 1 and 5 kV with a negative polarity. It is seen that the patterns show the dependence of the applied voltage. It is found that patterns based on CuO and Cu2O are seen at applied voltages as low as
Fig. 4. Relationship of film resistance with pulsed voltage.
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X. Ma et al. / Surface & Coatings Technology 201 (2007) 6712–6714
Fig. 5. Absorption spectrum of film deposited at various voltages.
Fig. 5 shows the normalized light absorption of the films deposited at various applied voltages. It is found that the absorbance tends to decrease with wavelength for applied voltages lower than 2 kV, and the wavelength showing the lowest absorbance is near a wavelength of 600 nm, while the lowest absorption is shifted to a lower wavelength at a higher applied voltage. The lowest absorbance is seen at a wavelength from 400 to 550 nm for applied voltages as high as 2 and 5 kV. Such a change of the light absorbance may be caused by the structure change in the films. For preparing films on a large size substrate with three dimensional shapes, the uniformity of the prepared films becomes important. In order to realize this, a modified type of antenna such as spiral shape type should be applied. 4. Conclusion
0.3 kV and 1 kV, while no diffraction peaks appear at an applied voltage of 5 kV. This result may show that high energy implantation causes a non-crystalline film structure during the deposition. 3.3. Properties of the deposited film with applied voltage Fig. 4 shows the film resistance as a function of the applied voltage. It is found that the minimum resistance is observed at 2 kV. The resistance is approximately 5 kΩ at an applied voltage of 0.3 kV, and decreases with increasing bias voltage, followed by a minimum resistance at an applied voltage of 2 kV. At an applied voltage higher than 2 kV, the resistance increases. Film thickness, film structure and surface morphology are main factors that affect the film resistance. It is seen that the film thickness decreases with increasing applied voltage, and the film structure changes from crystalline to non-crystalline. Generally, smaller thickness causes lower resistance, and noncrystalline structure with a higher defect density results in a higher resistance. Thus, the resistance of the deposited films decreases, and then increases with applied voltage as shown in Fig. 4. The roughnesses of the deposited film of Ra measured by AFM are approximately 2.01, 2.21, 1.86 and 3.30 nm for films deposited at biases of 0, 0.4, 1 and 5 kV, respectively, although it is commonly found that the surface morphology of the films is uniform. Thus, an increase in the surface roughness is seen with increasing applied voltage, and also results in an increase in the resistance.
Cu2O films are prepared by rf sputtering of the antenna material of copper without changing the element composition with a constant atomic ratio. The binding of copper and oxygen is confirmed. The contents of Cu and O in the deposited films are about 65 at.% and 35 at.%, respectively. It is shown from the XRD pattern that applied voltage plays a very important role in forming the film structure. With increasing applied voltage, the structure of the deposited films tends to be non-crystalline. The film resistance also shows the dependence of the applied voltage. A low film resistance is present at applied pulsed voltage of 2 kV in the present work. Thus, this method using floating antenna sputtering is promising as an ion source with a deposition of metal ion species. References [1] C. Jayewardena, K.P. Hewaparakrama, D.L.A. Wijewardena, H. Guruge, Sol. Energy Mater. Sol. Cells 56 (1998) 29. [2] A. Scherer, D.T. Inal, A.J. Singh, Sol. Energy Mater. 7 (1983) 467. [3] A. Roos, B. Karlson, Sol. Energy Mater. 9 (1983) 139. [4] A. Fujinaka, A. Berezin, J. Appl. Phys. 54 (1983) 3582. [5] T. Mahalingam, J.S.P. Chitra, J.P. Chu, P.J. Sebastian, Mater. Lett. 58 (2004) 1802. [6] A.K. Mukhopadhyay, A.K. Chakraborty, A.P. Chatterjee, S.K. Lahiri, Thin Solid Films 209 (1992) 42. [7] B. Millet, C. Fiaud, C. Hinnen, E.M.M. Sutter, Corros. Sci. 37 (1995) 1903. [8] S.M. Malik, K. Sridharan, R.P. Fetherston, A. Chen, J.R. Conrad, J. Vac. Sci. Technol., B 12 (1994) 843. [9] K. Nakamura, H. Suzuki, Surf. Coat. Technol. 196 (2005) 180.
Characteristics of copper oxide films deposited by PBII&D Xinxin Ma a,⁎, Gang Wang a , Ken Yukimura b , Mingren Sun a a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China b Department of Electrical Engineering, Faculty of Engineering, Doshisha University, 1-3 Tatara-Miyakodani, Kyotanabe, Kyoto 610-0321, Japan Available online 27 October 2006
Abstract Copper oxide films were deposited by plasma based ion implantation and deposition using a copper antenna as rf sputtering ion source. A gas mixture of Ar + O2 was used as working gas. During the process, copper that was sputtered from the rf antenna reacted with oxygen and was deposited on a silicon substrate. The composition and the chemical state of the deposited films were analyzed by XPS. The structure of the films was detected by XRD. It is observed that Cu2O film has been prepared on the Si substrate. It is found that the microstructure of the deposited film is amorphous for the applied voltage of −5 kV. The surface layer of the deposited films is CuO. This is because the surface layer absorbs the oxygen from ambient air after the treated sample was removed from the vacuum chamber. An appropriate applied voltage, 2 kV under the present conditions, brings the lowest resistance. It is also seen that the maximum absorbance of the deposited films moves to a lower wavelength with increased applied voltage. © 2006 Elsevier B.V. All rights reserved. PACS: 81.15. -z; 68.55.-a; 73.90.+f; 78.20.-e Keywords: PBII; Absorption spectrum; Copper oxide film; Film; Resistance
1. Introduction Copper oxide (Cu2O) films are very useful material in the construction of photovoltaic solar cells and photoelectric cells because of the favourable band gap of Cu2O, low fabrication cost and abundant production rate [1]. Various deposition techniques such as thermal oxidation, reactive sputtering, electro-deposition, and plasma evaporation have been used in the preparation of Cu2O films [2–7]. Plasma based ion implantation and deposition (PBII&D) has an advantage of flexibility to set the deposition parameters for three dimensional components. Concerning the sputtering deposition combined with PBII&D, Malik et al. [8] prepared TaN thin films by sputtering tantalum in a mixed gas of argon and nitrogen. Nakamura and Suzuki [9] proposed that the sputtered copper species were fully ionized at relatively high gas pressures (∼50–100 mTorr), and metal droplets were hardly generated in this sputtering system. Thus, in the case of copper, the sputtering method may act as an ion source and a supply source of deposited material as well. The present research is a feasibility test to deposit copper oxide films by reactive deposition with PBII&D using the copper sputtering method proposed by ⁎ Corresponding author. Tel.: +86 451 86418835; fax: +86 451 86413922. E-mail address: [email protected] (X. Ma). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.033
Nakamura and Suzuki. The sputtering rate of copper is estimated to be approximately 50 nm/min [9]. In this paper, a copper antenna was used as a sputtering ion source and a series of copper oxide films were deposited by varying the applied pulse voltage to the sample. The composition, structure, resistivity and absorbance spectrum of the deposited films were investigated. 2. Experimental Copper oxide films were deposited by plasma based ion implantation and deposition (PBII&D). The facility used in this experiment is shown in Fig. 1. In the vacuum chamber, a copper (Cu) antenna was floated from the ground, and set as the rf sputtering source with a frequency of 13.56 MHz and a power of 100 W. Silicon samples were located at 80 mm below the antenna. A series of negative high voltage pulses of 0–10 kV with a pulse width of 20 μs and with a frequency of 400 Hz were applied to the sample. A gas mixture of argon and oxygen in a proportion of 4:1 was introduced into the vacuum chamber, where a base pressure was approximately 9 × 10− 4 Pa. The working gas pressure of the experiment was approximately 10− 1 Pa set by the gas flow rate. During the process, copper particles were sputtered from the rf antenna by argon ion bombardment, and reacted with oxygen gas
X. Ma et al. / Surface & Coatings Technology 201 (2007) 6712–6714
6713
Fig. 3. XRD patterns from films deposited with various pulsed voltages.
Fig. 1. Schematic of the experimental facility.
to make a compound film on a silicon substrate. In this process, copper and oxygen ions were also generated, and implanted into the sample surface. The process time was 2 h. A PHI 5700 X-ray photoelectron spectroscopy (XPS) instrument was used to measure the chemical composition and chemical state of the deposited films. Argon ions with an energy of 3 keV were used to etch the samples at an etching rate of 2 nm/min. A Philips X'pert X-ray diffraction (XRD) system was used to analyze the phase structure of the deposited films with a glancing angle of 5° and with a step of 0.02°. A fourpoint probe system was used to measure the resistance of the deposited films. A Lambda 950 UV/VIS spectral photometer was used to measure the light absorbance of the films in the wavelength range from 300 to 700 nm. 3. Results and discussion 3.1. Composition of deposited films The composition of the deposited films was measured by XPS. In the survey mode, it is found that there are only oxygen
Fig. 2. Binding energy of Cu2p from the surface and subsurface of deposited film by XPS.
and copper existing in the films. By using the intensity integration at peaks of Cu2p and O1s, it is found that the contents of Cu and O are about 65 at.% and 35 at.%, respectively. No obvious differences in the element composition and the atomic ratio of the films by the applied voltage are found. The atomic ratio of Cu to O is about 1.85:1. 3.2. Phase structure of deposited films The phase structure of the films is determined by the XPS and XRD results. From the Cu2p-binding energy from XPS, the chemical state of Cu in the films is confirmed. Fig. 2 shows the XPS result of the film which was deposited at 0.3 kV, at the surface and near surface areas at an argon ion sputtering time of 2 min. It is found that the characteristic peaks of Cu2p3/2 are seen at binding energies of 933.9 eV and 932.5 eV, which correspond to CuO and Cu2O, respectively. This result suggests that the structure of the deposited film is Cu2O, while CuO at the surface is due to the storage of the sample in ambient air. The same result was obtained for the films deposited with other applied bias voltages. Fig. 3 shows the XRD patterns at a glancing angle of 5° from films deposited at applied voltages of 0.3, 1 and 5 kV with a negative polarity. It is seen that the patterns show the dependence of the applied voltage. It is found that patterns based on CuO and Cu2O are seen at applied voltages as low as
Fig. 4. Relationship of film resistance with pulsed voltage.
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X. Ma et al. / Surface & Coatings Technology 201 (2007) 6712–6714
Fig. 5. Absorption spectrum of film deposited at various voltages.
Fig. 5 shows the normalized light absorption of the films deposited at various applied voltages. It is found that the absorbance tends to decrease with wavelength for applied voltages lower than 2 kV, and the wavelength showing the lowest absorbance is near a wavelength of 600 nm, while the lowest absorption is shifted to a lower wavelength at a higher applied voltage. The lowest absorbance is seen at a wavelength from 400 to 550 nm for applied voltages as high as 2 and 5 kV. Such a change of the light absorbance may be caused by the structure change in the films. For preparing films on a large size substrate with three dimensional shapes, the uniformity of the prepared films becomes important. In order to realize this, a modified type of antenna such as spiral shape type should be applied. 4. Conclusion
0.3 kV and 1 kV, while no diffraction peaks appear at an applied voltage of 5 kV. This result may show that high energy implantation causes a non-crystalline film structure during the deposition. 3.3. Properties of the deposited film with applied voltage Fig. 4 shows the film resistance as a function of the applied voltage. It is found that the minimum resistance is observed at 2 kV. The resistance is approximately 5 kΩ at an applied voltage of 0.3 kV, and decreases with increasing bias voltage, followed by a minimum resistance at an applied voltage of 2 kV. At an applied voltage higher than 2 kV, the resistance increases. Film thickness, film structure and surface morphology are main factors that affect the film resistance. It is seen that the film thickness decreases with increasing applied voltage, and the film structure changes from crystalline to non-crystalline. Generally, smaller thickness causes lower resistance, and noncrystalline structure with a higher defect density results in a higher resistance. Thus, the resistance of the deposited films decreases, and then increases with applied voltage as shown in Fig. 4. The roughnesses of the deposited film of Ra measured by AFM are approximately 2.01, 2.21, 1.86 and 3.30 nm for films deposited at biases of 0, 0.4, 1 and 5 kV, respectively, although it is commonly found that the surface morphology of the films is uniform. Thus, an increase in the surface roughness is seen with increasing applied voltage, and also results in an increase in the resistance.
Cu2O films are prepared by rf sputtering of the antenna material of copper without changing the element composition with a constant atomic ratio. The binding of copper and oxygen is confirmed. The contents of Cu and O in the deposited films are about 65 at.% and 35 at.%, respectively. It is shown from the XRD pattern that applied voltage plays a very important role in forming the film structure. With increasing applied voltage, the structure of the deposited films tends to be non-crystalline. The film resistance also shows the dependence of the applied voltage. A low film resistance is present at applied pulsed voltage of 2 kV in the present work. Thus, this method using floating antenna sputtering is promising as an ion source with a deposition of metal ion species. References [1] C. Jayewardena, K.P. Hewaparakrama, D.L.A. Wijewardena, H. Guruge, Sol. Energy Mater. Sol. Cells 56 (1998) 29. [2] A. Scherer, D.T. Inal, A.J. Singh, Sol. Energy Mater. 7 (1983) 467. [3] A. Roos, B. Karlson, Sol. Energy Mater. 9 (1983) 139. [4] A. Fujinaka, A. Berezin, J. Appl. Phys. 54 (1983) 3582. [5] T. Mahalingam, J.S.P. Chitra, J.P. Chu, P.J. Sebastian, Mater. Lett. 58 (2004) 1802. [6] A.K. Mukhopadhyay, A.K. Chakraborty, A.P. Chatterjee, S.K. Lahiri, Thin Solid Films 209 (1992) 42. [7] B. Millet, C. Fiaud, C. Hinnen, E.M.M. Sutter, Corros. Sci. 37 (1995) 1903. [8] S.M. Malik, K. Sridharan, R.P. Fetherston, A. Chen, J.R. Conrad, J. Vac. Sci. Technol., B 12 (1994) 843. [9] K. Nakamura, H. Suzuki, Surf. Coat. Technol. 196 (2005) 180.