Characterization and optoelectronic properties of sol–gel-derived CuFeO2 thin films

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Characterization and optoelectronic properties of sol

–gel-derived CuFeO

2

thin

ﬁlms

Hong-Ying Chen

⁎

, Jia-Hao Wu

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Kaohsiung 807, Taiwan

a b s t r a c t

a r t i c l e i n f o

Article history: Received 3 August 2011

Received in revised form 23 February 2012 Accepted 11 March 2012

Available online 20 March 2012 Keywords:

Transparent conductive oxide Sol–gel deposition Thinﬁlms Delafossite Copper iron oxide Annealing X-ray diffraction

In this study, CuFeO2thinﬁlms were deposited onto quartz substrates using a sol–gel and a two-step

anneal-ing process. The sol–gel-derived ﬁlms were annealed at 500 °C for 1 h in air and then annealed at 600 to 800 °C for 2 h in N2. X-ray diffraction patterns showed that the annealed sol–gel-derived ﬁlms were CuO

and CuFe2O4phases in air annealing. When theﬁlms were annealed at 600 °C in N2, an additional CuFeO2

phase was detected. As the annealing temperature increased above 650 °C in N2, a single CuFeO2phase

was obtained. The binding energies of Cu-2p3/2, Fe-2p3/2, and O-1s were 932.5 ± 0.1 eV, 710.3 ± 0.2 eV and

530.0 ± 0.1 eV for CuFeO2thinﬁlms. The chemical composition of CuFeO2thinﬁlms was close to its

stoichi-ometry, which was determined by X-ray photoelectron spectroscopy. Thermodynamic calculations can ex-plain the formation of the CuFeO2phase in this study. The optical bandgap of the CuFeO2thinﬁlms was

3.05 eV, which is invariant with the annealing temperature in N2. The p-type characteristics of CuFeO2thin

films were confirmed by positive Hall coefficients and Seebeck coefficients. The electrical conductivities of CuFeO2thinfilms were 0.28 S cm− 1and 0.36 S cm− 1during annealing at 650 °C and 700 °C, respectively,

in N2. The corresponding carrier concentrations were 1.2 × 1018cm− 3 (650 °C) and 5.3 × 1018cm− 3

(700 °C). The activation energies for hole conduction were 140 meV (650 °C) and 110 meV (700 °C). These results demonstrate that sol–gel processing is a feasible preparation method for delafossite CuFeO2thinﬁlms.

1. Introduction

Transparent conducting oxides (TCOs) are well known and have been widely used by the optoelectronic industry in various applica-tions, such asflat panel displays, touch panels, and solar cells, and by the research community. Currently, the most popular TCOs have n-type characteristics. By contrast, p-type TCOs have not been exam-ined thoroughly. A series of p-type TCOs with a delafossite structure was recently prepared using various techniques[1–3]. P-type thin films with a delafossite structure have attracted significant research interest because of their optoelectronic properties, which have tre-mendous potential for practical applications. One of the main pro-posed applications of these _{films is a transparent p–n junction} semiconductor device, which is a structure that combines the two types of TCO materials into a p–n junction. This device can absorb ul-traviolet light and transmit visible light; thus, it can be used as a transparent functional window in microelectronic devices[1–3].

CuFeO2belongs to one of the group of delafossite; it has relatively higher conductivity compared to other delafossites, excluding CuCrO2

[2]. To date, few researchers have attempted to prepare CuFeO2thin ﬁlms by using techniques such as pulsed laser deposition[4] and

radio-frequency (RF) sputtering[5,6]. Choi et al.[4]used the pulsed laser deposition method to deposit CuFeO2thinfilms on amorphous glass substrates at a growth temperature of 750 °C. Their results indi-cated that CuFeO2film has a conductivity of 2.21×10− 5S cm− 1with a Hall coefficient of 1.84×106_m2_{/C, which suggests that it is an} insu-lator. Barnabé et al.[5,6]used RF sputtering to deposit CuFeO2thin films on glass substrates. The as-deposited films in their study were electrical insulators with a direct optical bandgap of 2 eV. After annealing under air at 450 °C for 6 h, the conductivity of thefilms reached 1.03 × 10− 3S cm− 1. Barnabé et al.[5]examined the p-type semiconducting properties by using thermopower measurements be-cause of the failure of the Hall measurement.

However, these vacuum-based processes are complex and time consuming. By contrast, the chemical solution method of preparing TCO films provides numerous benefits, including low costs, easy setup, large area coatings, and mass production. Previous studies have demonstrated that the sol–gel method is a powerful technique for growing delafossite thinfilms[7–19]. This study reports the depo-sition of p-type CuFeO2thinfilms on quartz substrates by using sol– gel processing. The microstructure and optoelectronic properties of sol–gel-derived CuFeO2thinfilms are investigate using X-ray diffrac-tion, X-ray photoelectron spectroscopy (XPS),field emission scanning electron microscopy (FE-SEM), ultraviolet_{–visible (UV–VIS)} spectros-copy, and the Hall-effect measurement. In addition, the phase changes of CuFeO2 thin films are examined using thermodynamic analysis.

Thin Solid Films 520 (2012) 5029–5035

⁎ Corresponding author. Tel.: +886 7 3814526x5130; fax: +886 7 3830674. E-mail address:[email protected](H.-Y. Chen).

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2. Experimental details

Delafossite CuFeO2thinfilms on quartz substrates were prepared by single spin-coating and sequential annealing. Specifically, 0.02 mol Cu(CH3COOH)2·H2O and 0.02 mol Fe(NO3)3·9H2O were dissolved in 60 mL of ethanol before 4.5 g of triethanolamine was added to the solution. This precursor, with the desired stoichiometric ratio, was then spin-coated onto quartz substrates at 2500 rpm for 15 s. Next, these specimens were annealed at 500 °C in air for 1 h at a ramp rate of 5 °C/min. To obtain the single CuFeO2phase, the spec-imens were subsequently annealed at 600 to 800 °C inflowing nitro-gen gas (99.995%) for 2 h at a ramp rate of 5 °C/min.

After annealing, the crystal structures of theﬁlms were examined using a grazing incidence X-ray diffraction (GIXD, Bruker D8 Discov-er) with a Cu Kα (λ=0.154 nm) excitation source by an incidence angle of 1°. The operating voltage and current were 40 kV and 40 mA, respectively. The scan rate was 4°/min, and the collected in-terval was 0.01° (2θ). XPS was performed using a Physical Electronics ESCA PHI 1600 spectrometer equipped with an Omni Focus III lens. The exciting X-ray source for XPS was Mg Kα (hν=1253.6 eV). Prior to the measurement, the surface was sputter-cleaned using an

Fig. 1. GIXD pattern of CuFeO2thinﬁlms prepared using sol–gel processing (●: CuFeO2

(R3m),△: CuO, ○: CuFe2O4). The broadening peak observed at 2θ~20° is attributed to

the quartz substrate.

Fig. 2. (pO2)equil.values as a function of the annealing temperature.

Fig. 3. X-ray photoelectron spectra for (a) Cu-2p, (b) Fe-2p, and (c) O-1s of sol–gel de-rivedﬁlms annealed at 650 °C to 800 °C in N2.

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[4] D.H. Choi, S.J. Moon, J.S. Hong, S.Y. An, I.-B. Shim, C.S. Kim, Thin Solid Films 517 (2009) 3987.

[5] A. Barnabé, E. Mugnier, L. Presmanes, P. Tailhades, Mater. Lett. 60 (2006) 3468. [6] E. Mugnier, A. Barnabé, L. Presmanes, P. Tailhades, Thin Solid Films 516 (2008)

1453.

[7] S. Götzendörfer, C. Polenzky, S. Ulrich, P. Löbmann, Thin Solid Films 518 (2009) 1153.

[8] S. Götzendörfer, R. Bywalez, P. Löbmann, J. Sol-Gel Sci. Technol. 52 (2009) 113. [9] J. Ding, Y. Sui, W. Fu, H. Yang, S. Liu, Y. Zeng, W. Zhao, P. Sun, J. Guo, H. Chen, M. Li,

Appl. Surf. Sci. 256 (2010) 6441.

[10] R. Bywalez, S. Götzendörfer, P. Löbmann, J. Mater. Chem. 20 (2010) 6562. [11] G. Li, X. Zhu, H. Lei, H. Jiang, W. Song, Z. Yang, J. Dai, Y. Sun, X. Pan, S. Dai, J. Sol-Gel

Sci. Technol. 53 (2010) 641.

[12] B. Szyszka, P. Loebmann, A. Georg, C. May, C. Elsaesser, Thin Solid Films 518 (2010) 3109.

[13] J. Wang, P. Zheng, D. Li, Z. Deng, W. Dong, R. Tao, X. Fang, J. Alloys Compd. 509 (2011) 5715.

[14] Y. Wang, Y. Gu, T. Wang, W. Shi, J. Alloys Compd. 509 (2011) 5897.

[15] H.F. Jiang, X.B. Zhu, H.C. Lei, G. Li, Z.R. Yang, W.H. Song, J.M. Dai, Y.P. Sun, Y.K. Fu, J. Alloys Compd. 509 (2011) 1768.

[16] Y.F. Wang, Y.J. Gu, T. Wang, W.Z. Shi, J. Sol-Gel Sci. Technol. 59 (2011) 222. [17] H.F. Jiang, X.B. Zhu, H.C. Lei, G. Li, Z.R. Yang, W.H. Song, J.M. Dai, Y.P. Sun, Y.K. Fu, J.

Sol-Gel Sci. Technol. 58 (2011) 12.

[18] L. Zhang, P. Li, K. Huang, Z. Tang, G. Liu, Y. Li, Mater. Lett. 65 (2011) 3289. [19] A. Banerjee, K.K. Chattopadhyay, S.W. Joo, Phys. B 406 (2011) 220.

[20] I. Barin, G. Platzki, Thermochemical Data of Pure Substances, VCH, New York, 1995, p. 620.

[21] S.P. Pavunny, A. Kumar, R.S. Katiyar, J. Appl. Phys. 107 (2010) 013522. [22] J. Ghijsen, L.H. Tjeng, J. Van Elp, H. Eskes, J. Westerink, G.A. Sawatzky, M.T. Czyzyk,

Phys. Rev. B 38 (1988) 11322.

[23] H. Hiraga, T. Makino, T. Fukumura, H. Weng, M. Kawasaki, Phys. Rev. B 84 (2011) 041411 (R).

[24] F.A. Benko, F.P. Koffyberg, J. Phys. Chem. Solids 48 (1987) 431. [25] K.P. Ong, K. Bai, P. Blaha, P. Wu, Chem. Mater. 19 (2007) 634.

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