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The effect of oxygen partial pressure on the properties of CuFeO2 thin films prepared by RF sputtering
Vacuum 115 (2015) 1e5
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
The effect of oxygen partial pressure on the properties of CuFeO2 thin films prepared by RF sputtering Zanhong Deng a, b, Xiaodong Fang a, b, *, Suzhen Wu a, b, Shimao Wang a, b, Weiwei Dong a, b, Jingzhen Shao a, b, Ruhua Tao a, b a
Anhui Provincial Key Lab of Photonics Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, PR China Key Lab of New Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 January 2015 Received in revised form 23 January 2015 Accepted 25 January 2015 Available online 3 February 2015
Single phase CuFeO2 thin films with c-axis orientation were prepared by RF sputtering method at room temperature under oxygen partial pressure PO from 5% to 15% and then post annealed at 900 C for 4 h in flowing N2 atmosphere. The grains of the films show layer-by-layer structure and are closely gathered and densely arranged. The Eg at near-UV region shows a redshift with increment of PO which may due to the impurity levels induced by oxygen interstitials. The resistivity of the films first decreased and then increased with increasing PO. The minimum resistivity of 0.32 Ucm at room temperature for the sample 10%-PA is nearly two-order magnitude smaller than that (18 Ucm) for 5%-PA. Oxygen interstitial doping can effectively enhance the conductivity of CuFeO2 thin films. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thin films RF sputtering Oxygen partial pressure Electrical transport
Delafossite oxides CuMO2 (M is trivalent cation, such as Al, Cr, Fe, …) have been studied intensively due to their applications in catalysts [1e3], ozone sensors [4,5], diluted magnetic semiconductors [6e9], light-emitting diodes [10] and transparent ptype conducting oxides (p-TCOs) [11e13]. The delafossite structure of CuFeO2 can be described as sheets of edge-shared FeO6 octahedra alternating stacked with close-packed Cu-ions layers, and the rhombohedral 3R (R3m) or hexagonal 2H (P63/mmc) structures can be formed depending on the stacking of the layers [14]. CuFeO2 is known to be the material that exhibits multiferroism and spintronics properties at the same time. The magnetic and magnetoelectric properties of CuFeO2 have been intensively studied [15e18]. Additionally, CuFeO2 is a well-known p-type semiconductor with the largest conductivity at room temperature (sRT ¼ 1.53 U1 cm1) among the delafossites when an offstoichiometric CuFeO2þd phase is formed [19]. In CuMO2þd, excess oxygen leads to a decrease in resistivity by the formation of Cuþ/
* Corresponding author. Anhui Provincial Key Lab of Photonics Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, PR China. Tel.: þ86 551 5593661; fax: þ86 551 5593665. E-mail address: [email protected] (X. Fang). http://dx.doi.org/10.1016/j.vacuum.2015.01.025 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Cu2þ pairs, which favors electronic hopping in the copper planes [20]. CuFeO2 thin films have been prepared by pulsed laser deposition, electrodeposition, radio-frequency (RF) sputtering and solegel method [21e28]. Highly c-axis oriented single phase CuFeO2 thin films prepared by pulsed laser deposition at the substrate temperature of 750 C showed insulation properties at room temperature (RT) [21] and the films prepared by RF sputtering showed a RT conductivity of 1 mU1 cm1 when the films were annealed at 450 C for 6 h [23]. The CuFeO2 films prepared by solegel method with an anneal process of 700 C in N2 for 2 h showed a relatively high RT conductivity of 0.358 U1 cm1 with an optical transmission of 20e50% in the visible range [24]. Epitaxial CuFeO2 thin films have been successfully deposited by solegel method with RT conductivity of 1.7 U1 cm1 [25], but the optical properties have not been reported. Recently, CuFeO2 films prepared by atmospheric pressure plasma annealing showed a RT conductivity of 0.67 U1 cm1 [29]. In our previous studies, CuFeO2 thin films have been synthesized by a simple solegel method and the effect of Mgdoping on the microstructure, optical and electrical properties of CuFe1xMgxO2 (0 x 0.05) thin films have been studied [30]. In this paper, CuFeO2 thin films on Al2O3 (001) substrates have been prepared by RF sputtering followed by ex-situ annealing and the
2
Z. Deng et al. / Vacuum 115 (2015) 1e5
Fig. 1. XRD pattern of the CuFeO2 target for RF sputtering.
Fig. 2. XRD patterns of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
effect of oxygen partial pressure on the optical and electrical properties of films are studied. Polycrystalline CuFeO2 target for RF sputtering was prepared by conventional solid state reaction. Stoichiometric Cu2O (99%) and Fe2O3 (99%) were well mixed by ball-milling and then calcinated at 900 C in flowing N2 atmosphere for 10 h. The obtained powders were pelleted and finally sintered at 1000 C in flowing N2 atmosphere for 4 h. The XRD pattern of the CuFeO2 target was shown in Fig. 1. All the diffraction peaks are identified as the rhombohedral 3R (R3m) delafossite structure (PDF 75-2146).
CuFeO2 thin films were deposited on (00l) sapphire substrates at room temperature under deposition oxygen pressure O2/(O2þAr) from 5% to 15% with total gas pressure of 3 Pa. The target-tosubstrate distance is 8 cm and the sputtering power is 4 W/cm2. The as-deposited films were then post annealed in a tube furnace at 900 C for 4 h in flowing N2 atmosphere. For the sake of description, the post annealed films deposited with PO ¼ 5%, 10% and 15% are defined as sample 5%-PA, 10%-PA and 15%-PA, respectively. The XRD spectra of the films plotted in log scale are shown in Fig. 2. All the films are crystallized to delafossite-CuFeO2 phase (JCPDS #752146). Diffraction peaks of (003), (006), (012), (104), (009) and
Fig. 3. SEM images of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
Z. Deng et al. / Vacuum 115 (2015) 1e5
3
(a)
(b)
(c)
Fig. 4. (a) Optical transmittance spectra, (b) (ahn)2 versus hn plots and (c) (ahn)1/2 versus hn plots of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
(0012) are observed. The full width at half maximum (FWHM) of peak (003), (006), (009) and (0012) is significantly smaller than that of peak (012). The crystal size calculated by the FWHM of peak (006) is about 52 nm for all the samples. However, the crystal size calculated by FWHM of peak (012) is around 13 nm. This suggests preferential growth along (00l) lattice planes. The surface morphologies of the films were examined by a SIGMA HD/VP (Carl Zeiss, Germany) field-emission scanning electron microscope (FE-SEM), as shown in Fig. 3. The films are
Fig. 5. X-ray photoelectron spectra for (a) Cu-2p, (b) Fe-2p, and (c) O-1s of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
Table 1 Room temperature resistivity and atomic concentration of the samples 5%-PA, 10%PA and 15%-PA, respectively. Samples
5%-PA 10%-PA 15%-PA
Resistivities (U cm)
Stoichiometric ratio (%) Cu
Fe
O
25.2 25.1 24.4
25.0 24.5 24.2
49.8 50.4 51.4
18 0.32 72
4
Z. Deng et al. / Vacuum 115 (2015) 1e5
composed of well crystallized grains. The grains are closely gathered and densely arranged. The layer-by-layer structure of the grains is in accordance with the nature of delafossite oxides. In addition, the surface pattern is gradually changed with increasing PO. The grains become more regular and the grain boundaries become increasingly distinct with increasing PO. The film thickness is hard to be measured because of the rough surface. The optical transmission characteristics in the range of 200e1200 nm of the films are shown in Fig. 4. There are 3 absorb edges in the 300e400 nm, 500e600 nm and 700e880 nm range for all the samples, suggesting 3 optical transitions existed in the test wavelength range. This result was in accordance with the reports by Benko et al. [19] and Ong et al. [31]. The optical bandgap, EOPT is deduced by Tauc's relation (ahn)n ¼ A(hn EOPT), where a denotes the absorption coefficient obtained by the relation, a ¼ ln(T)/d. The nature of the bandgap is identified by the exponent n and an intercept of the plot (ahn)n with photon energy, hn yields the optical bandgap energy. The optical bandgaps of the thin films were obtained using the (ahn)n versus hn plots, with n ¼ 2 for direct and n ¼ 1/2 for indirect bandgap transitions. By extrapolating the straight portion of the curve, the direct bandgap at near-UV of the samples 5%-PA, 10%-PA and 15%-PA was estimated to be 3.20 eV, 3.18 eV and 3.14 eV, respectively, which presents a redshift trend with increment of PO. It indicates that the impurity energy levels were induced by O intercalation. All the films show gradual absorption variation in absorb edge region of 500e600 nm. So, it's difficult to have a best-fit value of the bandgap in the visible region. For this reason, the absorb edge in 500e600 nm is not fitted. The indirect optical bandgap was estimated to be 1.0 eV, 1.0 eV and 1.07 eV for sample 5%-PA, 10%-PA and 15%-PA, respectively. The transmittances are 32%, 27% and 23% in the wavelength of 600 nm for the samples 5%-PA, 10%-PA and 15%-PA, respectively. XPS was conducted to analyze the chemical state of samples 5%PA, 10%-PA and 15%-PA. The core-level spectra of Cu-2p, Fe-2p and O-1s of the films are shown in Fig. 5. The Cu-2p spectra of the films have only two distinct and intense peaks. The binding energy is in good agreement with the literature reports for Cuþ in CFO [23,24]. However, there are no satellites which are corresponding to Cu2þ. In order to investigate the chemical composition of the films, atomic concentration of Cu, Fe and O were calculated from the integrated area of each spectrum, as shown in Table 1. The calculated atomic concentration is close to the stoichiometry of CFO. The O concentration of sample 5%-PA is 49.8% which is slightly deficient to the stoichiometry of CFO. With increase of PO to 15%, O concentration in the films also increases to 51.4%. The changes in resistivity of the films with PO were measured by the standard four-probe d.c. method at room temperature. Because the film thickness of the post annealed samples is hard to be determined, film thickness of the as-deposited samples is adopted to calculate the resistivity. The thickness of the as-deposited films measured by profilometer is around 100 nm with roughness of 4e5 nm. As shown in Table 1, the resistivity is first decreased and then increased with PO. The lowest resistivity 0.32 Ucm is obtained with deposition oxygen partial pressure of 10%. In delafossite CuMO2, excess oxygen is reported to be an effective acceptor which leads to the decrease of resistivity [32]. In this study, the oxygen composition of the films increases with the increase of PO, which acts as acceptor doping of the films. This may be the explanation for the decrease of resistivity when PO 10%. However, further increase of oxygen concentration may lead to a shift of the Fermi level from the conduction band minimum towards the valence band maximum and thereby facilitate the formation of donor-like defects to compensate the intended acceptor doping. This may contribute to the increase of the resistivity at PO above 10%. Moreover, the grain boundaries become increasingly distinct with
increasing PO, this may also enhance the carrier scattering and contribute to the increase of resistivity. In conclusion, single phase delafossite CuFeO2 thin films were prepared by RF sputtering with post annealing at 900 C in flowing N2 atmosphere and the influence of deposition oxygen partial pressure PO on the structural and optoelectronic properties of the films were studied. The direct bandgap at near-UV of the samples presents a redshift trend with increment of PO which indicates increased impurity energy levels induced by O intercalation. The resistivity of the films first decreased and then increased with increasing PO. Oxygen interstitial introduced holes may contribute to the enhancement of conductivity, but they also facilitate the formation of donor-like defects to compensate the intended acceptor doping. The competition of these two effects makes an optimum PO and in this experiment the optimum PO is around 10%. Acknowledgments Financial support from the National Natural Science Foundation of Project No. 51172237 and No. 61306083 is gratefully acknowledged.
References [1] Kato S, Fujimaki R, Ogasawara M, Wakabayashi T, Nakahara Y, Nakata S. Oxygen storage capacity of CuMO2 (M ¼ Al, Fe, Mn, Ga) with a delafossite-type structure. Appl Catal B Environ 2009;89:183e8. [2] Ketir W, Bouguelia A, Trari M. NO3 removal with a new delafossite CuCrO2 photocatalyst. Desalination 2009;244(2009):144e52. [3] Bassaid S, Chaib M, Omeiri S, Bouguelia A, Trari M. Photocatalytic reduction of cadmium over CuFeO2 synthesized by solegel. J Photoch Photobio A 2009;201:62e8. [4] Zhou S, Fang XD, Deng ZH, Li D, Dong WW, Tao RH, et al. Room temperature ozone sensing properties of p-type CuCrO2 nanocrystals. Sens Actuat B Chem 2009;143:119e23. [5] Zheng XG, Taniguchi K, Takahashi A, Liu Y, Xu CN. Room temperature sensing of ozone by transparent p-type semiconductor CuAlO2. Appl Phys Lett 2004;85:1728e9. [6] Li D, Fang XD, Dong WW, Deng ZH, Tao RH, Zhou S, et al. Magnetic and electrical properties of p-type Mn-doped CuCrO2 semiconductors. J Phys D Appl Phys 2009;42. 055009e6. [7] Gao C, Lin FT, Zhou XS, Shi WZ, Liu AY. Fe concentration dependences of microstructure and magnetic properties for Cu(Cr1xFex)O2 ceramics. J Alloys Compd 2013;565:154e8. [8] Lin FT, Shi WZ, Liu AY. Optical bandgap modulation and magnetic characterization of Fe-doped CuCrO2 nanopowders. J Alloys Compd 2012;529:21e4. [9] Dong CJ, Yu WX, Xu M, Cao JJ, Zhang Y, Chuai YH, et al. Evidence of room temperature ferromagnetism in Co-doped transparent CuAlO2 semiconductor. J Alloys Compd 2012;512:195e8. [10] Ling B, Zhao JL, Sun XW, Tan WT, Kyaw AKK, Divayana Y, et al. Color tunable light-emitting diodes based on nanorod array heterojunctions. Appl Phys Lett 2010;97:013101e3. [11] Kawazoe H, Yasukawa M, Hyodo H, Kurita M, Yanagi H, Hosono H. P-type electrical conduction in transparent thin films of CuAlO2. Nature 1997;389: 939e42. [12] Chen HY, Chang KP, Yang CC. Characterization of transparent conductive delafossite-CuCr1xO2 films. Appl Surf Sci 2013;273:324e9. [13] Jiang HF, Lei HC, Zhu XB, Li G, Yang ZR, Song WH, et al. Effects of citric acid on properties of single phase CuAlO2 thin films derived by chemical solution deposition. J Alloys Compd 2009;487:404e8. [14] Shannon RD, Rogers DB, Prewitt CT. Chemistry of noble metal oxides. III. electrical transport properties and crystal chemistry of ABO2 compounds with the delafossite structure. Inorg Chem 1971;10:723e7. [15] Hayashi K, Fukatsu R, Nozaki T, Miyazaki Y, Kajitani T. Structural, magnetic, and ferroelectric properties of CuFe1xMnxO2. Phys Rev B 2013;87. 064418e5. [16] Haraldsen JT, Ye F, Fishman RS, Fernandez-Baca JA, Yamaguchi Y, Kimura K, et al. Multiferroic phase of doped delafossite CuFeO2 identified using inelastic neutron scattering. Phys Rev B 2010;82. 020404R-4. [17] Hayashi K, Nozaki T, Fukatsu R, Miyazaki Y, Kajitani T. Spin dynamics of triangular lattice antiferromagnet CuFeO2 crossover from spin-liquid to paramagnetic phase. Phys Rev B 2009;80:144413e5. [18] Haraldsen JT, Fishman RS, Brown G. Spin-wave dynamics for the highmagnetic-field phases of the frustrated CuFeO2 antiferromagnet predictions for inelastic neutron scattering. Phys Rev B 2012;86:024412e5. [19] Benko FA, Koffyberg FP. Opto-electronic properties of p- and n-type delafossite, CuFeO2. J Phys Chem Solids 1987;48:431e4.
Z. Deng et al. / Vacuum 115 (2015) 1e5 A, Tailhades P. Synthesis and characterization of CuFeO2þd [20] Mugnier E, Barnabe delafossite powders. Solid State Ionics 2006;177:607e12. [21] Choi DH, Moon SJ, Hong JS, An SY, Shim IB, Kim CS. Epitaxial growth of delafossite CuFeO2 thin films by pulse laser deposition. Thin Solid Films 2009;517:3987e9. [22] Li SZ, Liu J, Wang XZ, Yan BW, Li H, Liu J-M. Epitaxial growth of delafossite CuFeO2 thin films by pulse laser deposition. Phys B 2012;407:2412e5. A, Mugnier E, Presmanes L, Tailhades Ph. Preparation of delafossite [23] Barnabe CuFeO2 thin films by rf-sputtering on conventional glass substrate. Mater Lett 2006;60:3468e70. [24] Chen HY, Wu JH. Transparent conductive CuFeO2 thin films prepared by solegel processing. Appl Surf Sci 2012;258:4844e7. [25] Zhang L, Li P, Huang K, Tang Z, Liu GH, Li YB. Chemical solution deposition and transport properties of epitaxial CuFeO2 thin films. Mater Lett 2011;65: 3289e91. [26] Read CG, Park Y, Choi KS. Electrochemical synthesis of p-type CuFeO2 electrodes for use in a photoelectrochemical cell. J Phys Chem Lett 2012;3: 1872e6.
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A, Presmanes L, Tailhades P. Copper and iron based thin [27] Chapelle A, Barnabe film nanocomposites prepared by radio-frequency sputtering. Part II: elaboration and characterization of oxide/oxide thin film nanocomposites using controlled ex-situ oxidation process. J Mater Sci 2013;48:3304e14. [28] Chen HY, Fu GW. Influences of post-annealing conditions on the formation of delafossite-CuFeO2 thin films. Appl Surf Sci 2014;288:258e64. [29] Chen HY, Fu JR. DelafossiteeCuFeO2 thin films prepared by atmospheric pressure plasma annealing. Mater Lett 2014;120:47e9. [30] Deng ZH, Fang XD, Wu SZ, Zhao YP, Dong WW, Shao JZ, et al. Structure and optoelectronic properties of Mg-doped CuFeO2 thin films prepared by sol-gel method. J Alloys Compd 2014;577:658e62. [31] Ong KP, Bai K, Blaha P, Wu P. Electronic structure and optical properties of AFeO2 (A ¼ Ag, Cu) within GGA calculations. Chem Mater 2007;19:634e40. [32] Yao ZQ, He B, Zhang L, Zhuang CQ, Ng TW, Liu SL, et al. Energy band engineering and controlled p-type conductivity of CuAlO2 thin films by nonisovalent Cu-O alloying. Appl Phys Lett 2012;100:062102e4.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
The effect of oxygen partial pressure on the properties of CuFeO2 thin films prepared by RF sputtering Zanhong Deng a, b, Xiaodong Fang a, b, *, Suzhen Wu a, b, Shimao Wang a, b, Weiwei Dong a, b, Jingzhen Shao a, b, Ruhua Tao a, b a
Anhui Provincial Key Lab of Photonics Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, PR China Key Lab of New Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 January 2015 Received in revised form 23 January 2015 Accepted 25 January 2015 Available online 3 February 2015
Single phase CuFeO2 thin films with c-axis orientation were prepared by RF sputtering method at room temperature under oxygen partial pressure PO from 5% to 15% and then post annealed at 900 C for 4 h in flowing N2 atmosphere. The grains of the films show layer-by-layer structure and are closely gathered and densely arranged. The Eg at near-UV region shows a redshift with increment of PO which may due to the impurity levels induced by oxygen interstitials. The resistivity of the films first decreased and then increased with increasing PO. The minimum resistivity of 0.32 Ucm at room temperature for the sample 10%-PA is nearly two-order magnitude smaller than that (18 Ucm) for 5%-PA. Oxygen interstitial doping can effectively enhance the conductivity of CuFeO2 thin films. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thin films RF sputtering Oxygen partial pressure Electrical transport
Delafossite oxides CuMO2 (M is trivalent cation, such as Al, Cr, Fe, …) have been studied intensively due to their applications in catalysts [1e3], ozone sensors [4,5], diluted magnetic semiconductors [6e9], light-emitting diodes [10] and transparent ptype conducting oxides (p-TCOs) [11e13]. The delafossite structure of CuFeO2 can be described as sheets of edge-shared FeO6 octahedra alternating stacked with close-packed Cu-ions layers, and the rhombohedral 3R (R3m) or hexagonal 2H (P63/mmc) structures can be formed depending on the stacking of the layers [14]. CuFeO2 is known to be the material that exhibits multiferroism and spintronics properties at the same time. The magnetic and magnetoelectric properties of CuFeO2 have been intensively studied [15e18]. Additionally, CuFeO2 is a well-known p-type semiconductor with the largest conductivity at room temperature (sRT ¼ 1.53 U1 cm1) among the delafossites when an offstoichiometric CuFeO2þd phase is formed [19]. In CuMO2þd, excess oxygen leads to a decrease in resistivity by the formation of Cuþ/
* Corresponding author. Anhui Provincial Key Lab of Photonics Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, PR China. Tel.: þ86 551 5593661; fax: þ86 551 5593665. E-mail address: [email protected] (X. Fang). http://dx.doi.org/10.1016/j.vacuum.2015.01.025 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Cu2þ pairs, which favors electronic hopping in the copper planes [20]. CuFeO2 thin films have been prepared by pulsed laser deposition, electrodeposition, radio-frequency (RF) sputtering and solegel method [21e28]. Highly c-axis oriented single phase CuFeO2 thin films prepared by pulsed laser deposition at the substrate temperature of 750 C showed insulation properties at room temperature (RT) [21] and the films prepared by RF sputtering showed a RT conductivity of 1 mU1 cm1 when the films were annealed at 450 C for 6 h [23]. The CuFeO2 films prepared by solegel method with an anneal process of 700 C in N2 for 2 h showed a relatively high RT conductivity of 0.358 U1 cm1 with an optical transmission of 20e50% in the visible range [24]. Epitaxial CuFeO2 thin films have been successfully deposited by solegel method with RT conductivity of 1.7 U1 cm1 [25], but the optical properties have not been reported. Recently, CuFeO2 films prepared by atmospheric pressure plasma annealing showed a RT conductivity of 0.67 U1 cm1 [29]. In our previous studies, CuFeO2 thin films have been synthesized by a simple solegel method and the effect of Mgdoping on the microstructure, optical and electrical properties of CuFe1xMgxO2 (0 x 0.05) thin films have been studied [30]. In this paper, CuFeO2 thin films on Al2O3 (001) substrates have been prepared by RF sputtering followed by ex-situ annealing and the
2
Z. Deng et al. / Vacuum 115 (2015) 1e5
Fig. 1. XRD pattern of the CuFeO2 target for RF sputtering.
Fig. 2. XRD patterns of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
effect of oxygen partial pressure on the optical and electrical properties of films are studied. Polycrystalline CuFeO2 target for RF sputtering was prepared by conventional solid state reaction. Stoichiometric Cu2O (99%) and Fe2O3 (99%) were well mixed by ball-milling and then calcinated at 900 C in flowing N2 atmosphere for 10 h. The obtained powders were pelleted and finally sintered at 1000 C in flowing N2 atmosphere for 4 h. The XRD pattern of the CuFeO2 target was shown in Fig. 1. All the diffraction peaks are identified as the rhombohedral 3R (R3m) delafossite structure (PDF 75-2146).
CuFeO2 thin films were deposited on (00l) sapphire substrates at room temperature under deposition oxygen pressure O2/(O2þAr) from 5% to 15% with total gas pressure of 3 Pa. The target-tosubstrate distance is 8 cm and the sputtering power is 4 W/cm2. The as-deposited films were then post annealed in a tube furnace at 900 C for 4 h in flowing N2 atmosphere. For the sake of description, the post annealed films deposited with PO ¼ 5%, 10% and 15% are defined as sample 5%-PA, 10%-PA and 15%-PA, respectively. The XRD spectra of the films plotted in log scale are shown in Fig. 2. All the films are crystallized to delafossite-CuFeO2 phase (JCPDS #752146). Diffraction peaks of (003), (006), (012), (104), (009) and
Fig. 3. SEM images of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
Z. Deng et al. / Vacuum 115 (2015) 1e5
3
(a)
(b)
(c)
Fig. 4. (a) Optical transmittance spectra, (b) (ahn)2 versus hn plots and (c) (ahn)1/2 versus hn plots of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
(0012) are observed. The full width at half maximum (FWHM) of peak (003), (006), (009) and (0012) is significantly smaller than that of peak (012). The crystal size calculated by the FWHM of peak (006) is about 52 nm for all the samples. However, the crystal size calculated by FWHM of peak (012) is around 13 nm. This suggests preferential growth along (00l) lattice planes. The surface morphologies of the films were examined by a SIGMA HD/VP (Carl Zeiss, Germany) field-emission scanning electron microscope (FE-SEM), as shown in Fig. 3. The films are
Fig. 5. X-ray photoelectron spectra for (a) Cu-2p, (b) Fe-2p, and (c) O-1s of the samples 5%-PA, 10%-PA and 15%-PA, respectively.
Table 1 Room temperature resistivity and atomic concentration of the samples 5%-PA, 10%PA and 15%-PA, respectively. Samples
5%-PA 10%-PA 15%-PA
Resistivities (U cm)
Stoichiometric ratio (%) Cu
Fe
O
25.2 25.1 24.4
25.0 24.5 24.2
49.8 50.4 51.4
18 0.32 72
4
Z. Deng et al. / Vacuum 115 (2015) 1e5
composed of well crystallized grains. The grains are closely gathered and densely arranged. The layer-by-layer structure of the grains is in accordance with the nature of delafossite oxides. In addition, the surface pattern is gradually changed with increasing PO. The grains become more regular and the grain boundaries become increasingly distinct with increasing PO. The film thickness is hard to be measured because of the rough surface. The optical transmission characteristics in the range of 200e1200 nm of the films are shown in Fig. 4. There are 3 absorb edges in the 300e400 nm, 500e600 nm and 700e880 nm range for all the samples, suggesting 3 optical transitions existed in the test wavelength range. This result was in accordance with the reports by Benko et al. [19] and Ong et al. [31]. The optical bandgap, EOPT is deduced by Tauc's relation (ahn)n ¼ A(hn EOPT), where a denotes the absorption coefficient obtained by the relation, a ¼ ln(T)/d. The nature of the bandgap is identified by the exponent n and an intercept of the plot (ahn)n with photon energy, hn yields the optical bandgap energy. The optical bandgaps of the thin films were obtained using the (ahn)n versus hn plots, with n ¼ 2 for direct and n ¼ 1/2 for indirect bandgap transitions. By extrapolating the straight portion of the curve, the direct bandgap at near-UV of the samples 5%-PA, 10%-PA and 15%-PA was estimated to be 3.20 eV, 3.18 eV and 3.14 eV, respectively, which presents a redshift trend with increment of PO. It indicates that the impurity energy levels were induced by O intercalation. All the films show gradual absorption variation in absorb edge region of 500e600 nm. So, it's difficult to have a best-fit value of the bandgap in the visible region. For this reason, the absorb edge in 500e600 nm is not fitted. The indirect optical bandgap was estimated to be 1.0 eV, 1.0 eV and 1.07 eV for sample 5%-PA, 10%-PA and 15%-PA, respectively. The transmittances are 32%, 27% and 23% in the wavelength of 600 nm for the samples 5%-PA, 10%-PA and 15%-PA, respectively. XPS was conducted to analyze the chemical state of samples 5%PA, 10%-PA and 15%-PA. The core-level spectra of Cu-2p, Fe-2p and O-1s of the films are shown in Fig. 5. The Cu-2p spectra of the films have only two distinct and intense peaks. The binding energy is in good agreement with the literature reports for Cuþ in CFO [23,24]. However, there are no satellites which are corresponding to Cu2þ. In order to investigate the chemical composition of the films, atomic concentration of Cu, Fe and O were calculated from the integrated area of each spectrum, as shown in Table 1. The calculated atomic concentration is close to the stoichiometry of CFO. The O concentration of sample 5%-PA is 49.8% which is slightly deficient to the stoichiometry of CFO. With increase of PO to 15%, O concentration in the films also increases to 51.4%. The changes in resistivity of the films with PO were measured by the standard four-probe d.c. method at room temperature. Because the film thickness of the post annealed samples is hard to be determined, film thickness of the as-deposited samples is adopted to calculate the resistivity. The thickness of the as-deposited films measured by profilometer is around 100 nm with roughness of 4e5 nm. As shown in Table 1, the resistivity is first decreased and then increased with PO. The lowest resistivity 0.32 Ucm is obtained with deposition oxygen partial pressure of 10%. In delafossite CuMO2, excess oxygen is reported to be an effective acceptor which leads to the decrease of resistivity [32]. In this study, the oxygen composition of the films increases with the increase of PO, which acts as acceptor doping of the films. This may be the explanation for the decrease of resistivity when PO 10%. However, further increase of oxygen concentration may lead to a shift of the Fermi level from the conduction band minimum towards the valence band maximum and thereby facilitate the formation of donor-like defects to compensate the intended acceptor doping. This may contribute to the increase of the resistivity at PO above 10%. Moreover, the grain boundaries become increasingly distinct with
increasing PO, this may also enhance the carrier scattering and contribute to the increase of resistivity. In conclusion, single phase delafossite CuFeO2 thin films were prepared by RF sputtering with post annealing at 900 C in flowing N2 atmosphere and the influence of deposition oxygen partial pressure PO on the structural and optoelectronic properties of the films were studied. The direct bandgap at near-UV of the samples presents a redshift trend with increment of PO which indicates increased impurity energy levels induced by O intercalation. The resistivity of the films first decreased and then increased with increasing PO. Oxygen interstitial introduced holes may contribute to the enhancement of conductivity, but they also facilitate the formation of donor-like defects to compensate the intended acceptor doping. The competition of these two effects makes an optimum PO and in this experiment the optimum PO is around 10%. Acknowledgments Financial support from the National Natural Science Foundation of Project No. 51172237 and No. 61306083 is gratefully acknowledged.
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