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Delafossite–CuFeO2 thin films prepared by atmospheric pressure plasma annealing
Materials Letters 120 (2014) 47–49
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Delafossite–CuFeO2 thin films prepared by atmospheric pressure plasma annealing Hong-Ying Chen n, Jun-Rong Fu Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chiken Kuang Road, Kaohsiung 807, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2013 Accepted 7 January 2014 Available online 15 January 2014
This study reports the use of atmospheric pressure plasma annealing to prepare the delafossite–CuFeO2 thin films. The pure delafossite–CuFeO2 phase was formed using atmospheric pressure plasma annealing at 5% O2, which had a large agglomerate morphology. The Cu-2p3/2 and the Fe-2p3/2 photoelectron peaks in the delafossite–CuFeO2 thin films were centered at 932.8 eV and 710.2 eV, indicating the chemical state of Cu and Fe were þ1 and þ3. The delafossite–CuFeO2 thin films had an optical bandgap of 3.1 eV. The electrical conductivity of the thin films was 0.677 0.02 S cm 1 with the carrier concentration of (2.56 71.24) 1018 cm 3. Hence, an atmospheric pressure plasma annealing offers an effective tool and a feasible method for preparing the delafossite–CuFeO2 thin films. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ceramics Thin films Delafossite CuFeO2 Atmospheric pressure plasma
1. Introduction P-type ternary Cu-based wide-bandgap oxide semiconductors have become attractive in recent years because they combine electrical conductivity and optical transparency in the visible region in a single material [1]. This unique optoelectronic property offering p-type Cu-based wide-bandgap materials are being one of the candidates of transparent conductive oxides (TCOs). Therefore, p-type TCOs with delafossite structure have numerous applications, including solar cells, flat panel displays, electromagnetic shielding devices, light-emitting diodes, and transparent heat sources [2]. Among these Cu-based delafossites, CuFeO2 has a higher electrical conductivity than the others [1]. To date, delafossite– CuFeO2 thin films can be prepared using techniques such as pulsed laser deposition [3–5], sputtering [6,7], electrochemical deposition [8], and chemical solution methods [9–12]. Among these deposition techniques, chemical solution-based methods have recently become a potential alternative technique for preparing Cu-based delafossite films [13,14]. This method generally requires an annealing process to form the desirable Cu-delafossites [8–12,14]. However, such annealing process generally performs using a muffle furnace, which is time-consuming. Therefore, looking for a fast and economical annealing process is necessary. Our previous study reported the atmospheric pressure plasma annealing to prepare the Cu2O thin films [15]. In this letter, we attempt to
n
Corresponding author. Tel.: þ 886 7 3814526x5130; fax: þ 886 7 3830674. E-mail address: [email protected] (H.-Y. Chen).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.017
prepare the delafossite–CuFeO2 thin films using an atmospheric pressure plasma annealing. 2. Experimental details Copper(II) acetate monohydrate (0.02 mol, purity 98%, SHOWA) and iron(III) nitrate nonahydrate (0.02 mol, purity 99%, SHOWA) were dissolved in 70 mL ethanol, and triethanolamine (0.03 mol, purity 95%þ, TEDIA, USA) was then added to the solution. This precursor was then spin coated onto quartz substrates at 2500 rpm for 15 s. Afterwards, the specimens were annealed at 500 1C in air for 1 h at the ramp rate of 5 1C/min and were annealed using atmospheric pressure plasma with different oxygen contents. An atmospheric microwave plasma system has been described elsewhere [15]. In the present study, the flow rate of the swirl N2 was fixed at 9 lpm and the flow rate of the axial gas was fixed at 1 lpm using N2 with 0–10% O2, which accomplished that the total flow rate of the inject gas was 10 lpm. The purities of N2 and O2 gases were 99.9% and 99.97%, respectively. The annealing temperature by the atmospheric pressure plasma was 650 1C and the time from plasma ignition to turn-off was 20 min. The crystal structure of the thin films was examined by an X-ray diffractometer with the Cu-Kα radiation. A ULVAC-PHI PHI 5000 VersaProbe X-ray photoelectron spectrometer with Al Kα (hν ¼1486.6 eV) was used to determine the chemical states of the delafossite–CuFeO2 thin films. The spectra were acquired after sputter cleaning with an Ar ion gun at 3 keV for 2 min and calibrated with respect to the C-1s peak at 284.8 eV, which were fitted using a nonlinear least squares fit with a Gaussian/Lorentzian peak shape (G/L mixing ratio ¼ 0.3). The surface morphology of the
48
H.-Y. Chen, J.-R. Fu / Materials Letters 120 (2014) 47–49
films was analyzed by a field emission scanning electron microscope. A UV–vis spectrometer was employed to determine the optical properties of the thin films. The electrical properties of the
thin films were measured using the van der Pauw configuration in a standard Hall-effect analyzer.
3. Results and discussion
Fig. 1. (a) Grazing incidence X-ray diffraction pattern of the sol–gel derived thin films annealed in air at 500 1C and annealed using atmospheric pressure plasma with different oxygen contents. (◇:Cu, △:CuO, ○:CuFe2O4, ●:CuFeO2) and (b) the surface morphology of the delafossite–CuFeO2 thin films.
Fig. 1(a) shows the X-ray diffraction pattern of the sol–gelderived thin films annealed using atmospheric pressure plasma with different oxygen contents. Cu (JCPDS #89-2530) and CuFe2O4 (JCPDS #77-0010) were predominant phases below 3% O2. Pure delafossite–CuFeO2 (JCPDS #75-2146) was formed at 5% O2. The obtained delafossite-CuFeO2 thin films have a similar ratio in the relative intensity respect to the JCDPS #75-2126. This might be due to the atmospheric pressure provides enough energy for the growth of the delafossite-CuFeO2 thin films under equilibrium condition owing to the atmospheric pressure is an thermal plasma. CuO (JCPDS #89-5899) and delafossite–CuFeO2 phases were found at 10% O2. Hence, the lattice parameters of the delafossite–CuFeO2 thin films were a¼ 0.3030 nm and c ¼1.7135 nm. Fig. 1(b) shows the surface morphology of the delafossite–CuFeO2 thin films, in which a compact with large agglomerate feature was observed. The thickness of the delafossite–CuFeO2 thin film was approximately 80 nm by the cross-sectional view using FE-SEM. Fig. 2 (a) shows the Cu-2p spectrum of the delafossite–CuFeO2 thin films, which has two intense peaks at Cu-2p3/2 ¼932.8 70.2 eV and Cu-2p1/2 ¼952.7 70.2 eV. This binding energy is in good agreement with the literature reports for Cu þ [11,12,16]. Fig. 2 (b) shows the Fe-2p spectrum of the thin films, which has the binding energies of Fe-2p3/2 ¼710.270.2 eV and Fe-2p1/2 ¼723.27 0.2 eV indicating the Fe3 þ oxidation state [11,12,16]. Fig. 3 (a) displays the transmittance spectrum of the delafossite–CuFeO2 thin film, where the observed absorption edge occurred at approximately between 300 nm and 400 nm and the visible transmittance ranged from 40% to 55%. Fig. 3(b) displays several absorption peaks of the thin films, which mainly arise from the excitation of the electrons from the valence band to the conduction band (at 4.5 eV) and the charge-transfer excitation from the valence band to Fe-eg and Fe-t2g orbitals (at 3.5 eV) [4]. Moreover, the optical bandgap of the thin films was 3.1 eV, which is consistent with those literature reports [11,12]. The hot-probe measurement confirmed the p-type characteristics of the delafossite–CuFeO2 thin film. The delafossite–CuFeO2 thin films had the electrical conductivity of 0.6770.02 S cm 1 and the carrier concentration of (2.56 71.24) 1018 cm 3.
Fig. 2. XPS spectra of the delafossite–CuFeO2 thin films: (a) Cu-2p and (b) Fe-2p. (●: experimental data, : fitting result).
H.-Y. Chen, J.-R. Fu / Materials Letters 120 (2014) 47–49
49
Fig. 3. The optical properties of the delafossite–CuFeO2 thin films: (a) transmittance spectrum and (b) optical bandgap (inset: the absorption coefficient).
4. Conclusions In this letter, single delafossite–CuFeO2 thin films can be obtained using atmospheric pressure plasma annealing at 5% O2. The obtained delafossite–CuFeO2 thin films exhibited a large agglomerate morphology and the chemical states in the films were Cu þ and Fe3 þ . The delafossite–CuFeO2 thin films had an optical bandgap of 3.1 eV. The electrical conductivity of the thin films was 0.6770.02 S cm 1 with the carrier concentration of (2.5671.24) 1018 cm 3. Hence, atmospheric pressure plasma annealing provides a short annealing time and a feasible preparation method for the delafossite–CuFeO2 thin films. Acknowledgments We thank the National Science Council of the ROC for financial assistance under Grant number NSC 101-2221-E-151-029. References [1] Marquardt MA, Ashmore NA, Cann DP. Thin Solid Films 2006;496:146–56. [2] Banerjee AN, Chattopadhyay KK. Prog. Cryst. Growth Charact. Mater. 2005;50:52–105.
[3] Li SZ, Liu J, Wang XZ, Yan BW, Li H, Liu JM. Phys. B: Condens. Matter 2012;407:2412–5. [4] Hiraga H, Makino T, Fukumura T, Weng H, Kawasaki M. Phys. Rev. B. 2011;84: 041411 (R). [5] Choi DH, Moon SJ, Hong JS, An SY, Shim I-B, Kim CS. Thin Solid Films 2009;517: 3987–9. [6] Mugnier E, Barnabé A, Presmanes L, Tailhades P. Thin Solid Films 2008;516:1453–6. [7] Barnabé A, Mugnier E, Presmanes L, Tailhades P. Mater. Lett. 2006;60: 3468–70. [8] Read CG, Park Y, Choi K-S. J. Phys. Chem. Lett. 2012;3:1872–6. [9] Deng Z, Fang X, Wu S, Zhao Y, Dong W, Shao J, et al. J. Alloys Compd. 2013;577:658–62. [10] Zhang L, Li P, Huang K, Tang Z, Liu G, Li Y. Mater. Lett. 2011;65:3289–91. [11] Chen H-Y, Wu J-H. Appl. Surf. Sci. 2012;258:4844–7. [12] Chen H-Y, Wu J-H. Thin Solid Films 2012;520:5029–35. [13] Götzendörfer S, Bywalez R, Löbmann P. J. Sol–Gel Sci. Technol. 2009;52:113–9. [14] Götzendörfer S, Polenzky C, Ulrich S, Löbmann P. Thin Solid Films 2009;518: 1153–6. [15] Han S, Chen H-Y, Kuo L-T, Tsai C-H. Thin Solid Films 2008;517:1195–9. [16] Pavunny SP, Kumar A, Katiyar RS. J. Appl. Phys. 2010;107:013522.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Delafossite–CuFeO2 thin films prepared by atmospheric pressure plasma annealing Hong-Ying Chen n, Jun-Rong Fu Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chiken Kuang Road, Kaohsiung 807, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2013 Accepted 7 January 2014 Available online 15 January 2014
This study reports the use of atmospheric pressure plasma annealing to prepare the delafossite–CuFeO2 thin films. The pure delafossite–CuFeO2 phase was formed using atmospheric pressure plasma annealing at 5% O2, which had a large agglomerate morphology. The Cu-2p3/2 and the Fe-2p3/2 photoelectron peaks in the delafossite–CuFeO2 thin films were centered at 932.8 eV and 710.2 eV, indicating the chemical state of Cu and Fe were þ1 and þ3. The delafossite–CuFeO2 thin films had an optical bandgap of 3.1 eV. The electrical conductivity of the thin films was 0.677 0.02 S cm 1 with the carrier concentration of (2.56 71.24) 1018 cm 3. Hence, an atmospheric pressure plasma annealing offers an effective tool and a feasible method for preparing the delafossite–CuFeO2 thin films. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ceramics Thin films Delafossite CuFeO2 Atmospheric pressure plasma
1. Introduction P-type ternary Cu-based wide-bandgap oxide semiconductors have become attractive in recent years because they combine electrical conductivity and optical transparency in the visible region in a single material [1]. This unique optoelectronic property offering p-type Cu-based wide-bandgap materials are being one of the candidates of transparent conductive oxides (TCOs). Therefore, p-type TCOs with delafossite structure have numerous applications, including solar cells, flat panel displays, electromagnetic shielding devices, light-emitting diodes, and transparent heat sources [2]. Among these Cu-based delafossites, CuFeO2 has a higher electrical conductivity than the others [1]. To date, delafossite– CuFeO2 thin films can be prepared using techniques such as pulsed laser deposition [3–5], sputtering [6,7], electrochemical deposition [8], and chemical solution methods [9–12]. Among these deposition techniques, chemical solution-based methods have recently become a potential alternative technique for preparing Cu-based delafossite films [13,14]. This method generally requires an annealing process to form the desirable Cu-delafossites [8–12,14]. However, such annealing process generally performs using a muffle furnace, which is time-consuming. Therefore, looking for a fast and economical annealing process is necessary. Our previous study reported the atmospheric pressure plasma annealing to prepare the Cu2O thin films [15]. In this letter, we attempt to
n
Corresponding author. Tel.: þ 886 7 3814526x5130; fax: þ 886 7 3830674. E-mail address: [email protected] (H.-Y. Chen).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.017
prepare the delafossite–CuFeO2 thin films using an atmospheric pressure plasma annealing. 2. Experimental details Copper(II) acetate monohydrate (0.02 mol, purity 98%, SHOWA) and iron(III) nitrate nonahydrate (0.02 mol, purity 99%, SHOWA) were dissolved in 70 mL ethanol, and triethanolamine (0.03 mol, purity 95%þ, TEDIA, USA) was then added to the solution. This precursor was then spin coated onto quartz substrates at 2500 rpm for 15 s. Afterwards, the specimens were annealed at 500 1C in air for 1 h at the ramp rate of 5 1C/min and were annealed using atmospheric pressure plasma with different oxygen contents. An atmospheric microwave plasma system has been described elsewhere [15]. In the present study, the flow rate of the swirl N2 was fixed at 9 lpm and the flow rate of the axial gas was fixed at 1 lpm using N2 with 0–10% O2, which accomplished that the total flow rate of the inject gas was 10 lpm. The purities of N2 and O2 gases were 99.9% and 99.97%, respectively. The annealing temperature by the atmospheric pressure plasma was 650 1C and the time from plasma ignition to turn-off was 20 min. The crystal structure of the thin films was examined by an X-ray diffractometer with the Cu-Kα radiation. A ULVAC-PHI PHI 5000 VersaProbe X-ray photoelectron spectrometer with Al Kα (hν ¼1486.6 eV) was used to determine the chemical states of the delafossite–CuFeO2 thin films. The spectra were acquired after sputter cleaning with an Ar ion gun at 3 keV for 2 min and calibrated with respect to the C-1s peak at 284.8 eV, which were fitted using a nonlinear least squares fit with a Gaussian/Lorentzian peak shape (G/L mixing ratio ¼ 0.3). The surface morphology of the
48
H.-Y. Chen, J.-R. Fu / Materials Letters 120 (2014) 47–49
films was analyzed by a field emission scanning electron microscope. A UV–vis spectrometer was employed to determine the optical properties of the thin films. The electrical properties of the
thin films were measured using the van der Pauw configuration in a standard Hall-effect analyzer.
3. Results and discussion
Fig. 1. (a) Grazing incidence X-ray diffraction pattern of the sol–gel derived thin films annealed in air at 500 1C and annealed using atmospheric pressure plasma with different oxygen contents. (◇:Cu, △:CuO, ○:CuFe2O4, ●:CuFeO2) and (b) the surface morphology of the delafossite–CuFeO2 thin films.
Fig. 1(a) shows the X-ray diffraction pattern of the sol–gelderived thin films annealed using atmospheric pressure plasma with different oxygen contents. Cu (JCPDS #89-2530) and CuFe2O4 (JCPDS #77-0010) were predominant phases below 3% O2. Pure delafossite–CuFeO2 (JCPDS #75-2146) was formed at 5% O2. The obtained delafossite-CuFeO2 thin films have a similar ratio in the relative intensity respect to the JCDPS #75-2126. This might be due to the atmospheric pressure provides enough energy for the growth of the delafossite-CuFeO2 thin films under equilibrium condition owing to the atmospheric pressure is an thermal plasma. CuO (JCPDS #89-5899) and delafossite–CuFeO2 phases were found at 10% O2. Hence, the lattice parameters of the delafossite–CuFeO2 thin films were a¼ 0.3030 nm and c ¼1.7135 nm. Fig. 1(b) shows the surface morphology of the delafossite–CuFeO2 thin films, in which a compact with large agglomerate feature was observed. The thickness of the delafossite–CuFeO2 thin film was approximately 80 nm by the cross-sectional view using FE-SEM. Fig. 2 (a) shows the Cu-2p spectrum of the delafossite–CuFeO2 thin films, which has two intense peaks at Cu-2p3/2 ¼932.8 70.2 eV and Cu-2p1/2 ¼952.7 70.2 eV. This binding energy is in good agreement with the literature reports for Cu þ [11,12,16]. Fig. 2 (b) shows the Fe-2p spectrum of the thin films, which has the binding energies of Fe-2p3/2 ¼710.270.2 eV and Fe-2p1/2 ¼723.27 0.2 eV indicating the Fe3 þ oxidation state [11,12,16]. Fig. 3 (a) displays the transmittance spectrum of the delafossite–CuFeO2 thin film, where the observed absorption edge occurred at approximately between 300 nm and 400 nm and the visible transmittance ranged from 40% to 55%. Fig. 3(b) displays several absorption peaks of the thin films, which mainly arise from the excitation of the electrons from the valence band to the conduction band (at 4.5 eV) and the charge-transfer excitation from the valence band to Fe-eg and Fe-t2g orbitals (at 3.5 eV) [4]. Moreover, the optical bandgap of the thin films was 3.1 eV, which is consistent with those literature reports [11,12]. The hot-probe measurement confirmed the p-type characteristics of the delafossite–CuFeO2 thin film. The delafossite–CuFeO2 thin films had the electrical conductivity of 0.6770.02 S cm 1 and the carrier concentration of (2.56 71.24) 1018 cm 3.
Fig. 2. XPS spectra of the delafossite–CuFeO2 thin films: (a) Cu-2p and (b) Fe-2p. (●: experimental data, : fitting result).
H.-Y. Chen, J.-R. Fu / Materials Letters 120 (2014) 47–49
49
Fig. 3. The optical properties of the delafossite–CuFeO2 thin films: (a) transmittance spectrum and (b) optical bandgap (inset: the absorption coefficient).
4. Conclusions In this letter, single delafossite–CuFeO2 thin films can be obtained using atmospheric pressure plasma annealing at 5% O2. The obtained delafossite–CuFeO2 thin films exhibited a large agglomerate morphology and the chemical states in the films were Cu þ and Fe3 þ . The delafossite–CuFeO2 thin films had an optical bandgap of 3.1 eV. The electrical conductivity of the thin films was 0.6770.02 S cm 1 with the carrier concentration of (2.5671.24) 1018 cm 3. Hence, atmospheric pressure plasma annealing provides a short annealing time and a feasible preparation method for the delafossite–CuFeO2 thin films. Acknowledgments We thank the National Science Council of the ROC for financial assistance under Grant number NSC 101-2221-E-151-029. References [1] Marquardt MA, Ashmore NA, Cann DP. Thin Solid Films 2006;496:146–56. [2] Banerjee AN, Chattopadhyay KK. Prog. Cryst. Growth Charact. Mater. 2005;50:52–105.
[3] Li SZ, Liu J, Wang XZ, Yan BW, Li H, Liu JM. Phys. B: Condens. Matter 2012;407:2412–5. [4] Hiraga H, Makino T, Fukumura T, Weng H, Kawasaki M. Phys. Rev. B. 2011;84: 041411 (R). [5] Choi DH, Moon SJ, Hong JS, An SY, Shim I-B, Kim CS. Thin Solid Films 2009;517: 3987–9. [6] Mugnier E, Barnabé A, Presmanes L, Tailhades P. Thin Solid Films 2008;516:1453–6. [7] Barnabé A, Mugnier E, Presmanes L, Tailhades P. Mater. Lett. 2006;60: 3468–70. [8] Read CG, Park Y, Choi K-S. J. Phys. Chem. Lett. 2012;3:1872–6. [9] Deng Z, Fang X, Wu S, Zhao Y, Dong W, Shao J, et al. J. Alloys Compd. 2013;577:658–62. [10] Zhang L, Li P, Huang K, Tang Z, Liu G, Li Y. Mater. Lett. 2011;65:3289–91. [11] Chen H-Y, Wu J-H. Appl. Surf. Sci. 2012;258:4844–7. [12] Chen H-Y, Wu J-H. Thin Solid Films 2012;520:5029–35. [13] Götzendörfer S, Bywalez R, Löbmann P. J. Sol–Gel Sci. Technol. 2009;52:113–9. [14] Götzendörfer S, Polenzky C, Ulrich S, Löbmann P. Thin Solid Films 2009;518: 1153–6. [15] Han S, Chen H-Y, Kuo L-T, Tsai C-H. Thin Solid Films 2008;517:1195–9. [16] Pavunny SP, Kumar A, Katiyar RS. J. Appl. Phys. 2010;107:013522.