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Structural and optical properties of ZnO thin films with heavy Cu-doping prepared by magnetron co-sputtering
Materials Letters 143 (2015) 319–321
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structural and optical properties of ZnO thin films with heavy Cu-doping prepared by magnetron co-sputtering Yong Liu a,b, Haonan Liu a,b, Yang Yu a,b, Qing Wang a,b, Yinglan Li a,b, Zhi Wang a,b,n a b
Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Institute of Technology, People's Republic of China School of Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2014 Accepted 27 December 2014 Available online 5 January 2015
Cu-doping can modulate the properties of ZnO thin films. Most previous researchers studied ZnO:Cu films with moderate doping level, while this work was focused on heavy Cu-doping (13–23.8%) effect on ZnO thin films prepared by magnetron co-sputtering. The microstructure and optical properties of the films were systematically investigated. The results show that moderate Cu-doping enhances the crystal grain size of the films, whereas heavy doping reduces it. The fluorescence quenching effect, the red shift phenomenon and the band gap narrowing effect in emission and absorption characteristics resulting from Cu-doping are more significant at heavy doping level. A promising band gap reduction of 0.64 eV was achieved at 23.8% doping level. & 2015 Elsevier B.V. All rights reserved.
Keywords: Sputtering Thin films ZnO Cu-doping Optical materials and properties
1. Introduction In recent years, ZnO has been investigated extensively because of its potential values for application in many fields [1]. ZnO can be doped by many elements, such as Mg [2], Ti [3], Al [4], In [5], and Cu [6], to meet the demands of different application. Among various dopants, Cu can be easily doped in the lattice of ZnO for its similar radius and electronic shell to Zn atom. Many properties of the ZnO thus can be modulated by Cu content [6–8]. For example, Cu doping can reduce the band gap and enhance the absorption coefficient [8], which is critical to the application in visible light region. In most previous study on the Cu-doping effects, Cu content lies in moderate doping region (below 12.5%) [9,8,10], while this study is focused on heavy doping region (13– 23.8%). The microstructure and optical properties of the films with different doping composition were systematically investigated. The effects of heavy Cu-doping, some of which are different from those of moderate doping, were found and will be discussed. Another difference between this work and other works is the deposition method. Most previous studies used Zn target with Cu chips [7,8] in magnetron sputtering. We prepared ZnO:Cu films by magnetron co-sputtering of a ZnO and a Cu target, and the doping content in films is easily modulated by the sputtering power of Cu target in this method. 2. Material and methods The films were fabricated by magnetron co-sputtering of two targets. A ZnO target was sputtered by a RF power supply, and a n
Corresponding author. E-mail address: [email protected] (Z. Wang).
http://dx.doi.org/10.1016/j.matlet.2014.12.133 0167-577X/& 2015 Elsevier B.V. All rights reserved.
metal Cu target was sputtered by a DC power supply. The sputtered atoms off-normally deposited on glass substrates for 1 h. The purity of targets was 99.99% and the distances between the targets and the substrates were 130 mm. To avoid excessive doping, a shutter was pulsedly used in front of the Cu target. The period of the shutter was 55 s, and deposition time was 5 s in each period of the shutter. The base vacuum was 4.5 10 4 Pa, and the working pressure was controlled at 0.5 Pa. The flow rates of Ar and O2 were set as 25 and 5 sccm respectively. The sputtering power of ZnO was kept at 100 W, and the sputtering power of Cu was set to 0, 8, 10, 15, and 20 W to obtain different doping content. The substrate temperature was optimized at 500 1C. The alignment and microstructure were investigated by an X-ray diffraction (XRD) spectroscopy system (D8 ADVANCE) with a Cu-Kα1 source. The photoluminescence (PL) measurement was carried out by the 325 nm line from a He–Cd laser, and the signals were recorded by an FLS920 spectrophotometer. The optical absorption characteristics were measured by an ultraviolet visible spectrophotometer (UV-Vis, HITACHI U-3310).
3. Results and discussion Fig. 1(a) shows XRD patterns of all the films. The Cu-doping content was calculated by the ratio of Cu%/(Cu% þZn%) based on the results of an energy dispersive spectroscopy, and is indicated above the corresponding curves. We can see that the pure ZnO films exhibit a single phase with (002)-oriented hexagonal wurtzite structure. For the samples of the Cu contents below 20.8%, only (002) peak of ZnO, and no diffraction peaks of copper and cuprate emerges. It can be concluded that doping Cu atoms substituted Zn atoms at lattice. Fig. 1(b) shows that the (002) peak
320
Y. Liu et al. / Materials Letters 143 (2015) 319–321
(100)
20 W, 23.8%
(002) 15 W, 20.8%
2 of (002) peak ( )
35.0
Cu sputtering power and Cu content:
34.8
34.6
intensity (
34.4 0
10
15
20
0 W, 0% 30
40
50
60
70
80
2 ()
20
0.52
0.48
18
0.44
Grain size (nm)
FWHM of (002) peak ( )
10 W, 16.4%
8 W, 13%
20
5
Sputtering power (W)
16 0
5
10
15
20
Sputtering power (W)
Fig. 1. XRD results of the films with different Cu content. (a) θ–2θ scan pattern; (b) 2θ value of (002) peak; (c): FWHM of (002) peak and the crystallite grain size.
Fig. 2. PL spectra of films with different Cu content; the inset shows the details of the lowest two curves.
shifts gradually to high angle side when the Cu content increases. According to Bragg equation, 2d sin θ ¼ nλ, the crystalline plane distance d002 and the lattice constant c become smaller with Cu content increasing. The radius of Cu þ , Cu2 þ , and Zn2 þ is 0.096 nm, 0.072 nm, and 0.074 nm respectively. Thus the decrease of c indicates that substitutional Cu2 þ is the dominant doping form in the ZnO:Cu films, because only the Cu2 þ can lead to the reduction of the lattice constant. The intensity of the (002) peak in Fig. 1(a) firstly increases, and then decreases with increasing Cu content. When the sputtering power of the Cu is 20 W (for the 23.8% doped sample), a weak (100) diffraction peak of ZnO appears. Fig. 1(c) shows the full width of half maximum (FWHM) value β of (002) peak and the crystallite grain size L as a function of the sputtering power. L was calculated according to the Scherrer equation, L ¼ 0:94λ=β cos θ. We can see that as the sputtering power increases, the FWHM firstly becomes smaller and then becomes larger. Consequently, the crystallite size L firstly increases and then decreases. These XRD results illustrate that moderate Cu-doping can promote the preferential (002) orientation, while heavy doping can inhibit it. The increase in the extent of c-orientation may be due to the
fact that a moderate quantity of Cu atoms share the oxygen with Zn atoms [7]. The ZnO (100) peak in the sample with 23.8% Cu-doping reflects that the crystal planes perpendicular and parallel to the substrates were mixed together [10] at heavy doping level. Fig. 2 shows the PL emission spectra of the films. It is obvious that the increasing Cu percent significantly diminishes the PL intensity. This fluorescence quenching effect is attributed to the enhancement of non-radiative transitions resulting from the Cu incorporation [11]. A UV emission peak is observed at 384 nm (3.223 eV), which arises from the Auger recombination corresponding to near-band edge (NBE) transition. In Auger recombination processes, the energy released by an electron recombination is immediately absorbed by another electron, and then this energy is dissipated by phonons. In wide band gap materials, Auger process depends on the concentration of doping atom and defects in the lattice. Cu-doping enhances the formation of deep in-gap states of high density [12]. The Cu centers in ZnO act as traps to the excited electrons, thus diminish the PL efficiency [11]. With the Cu dopant increasing, NBE emission intensity is reduced quickly, as shown in Fig. 3. The origin of the emission peaks was analyzed according to the band structure and energy level of the defects in ZnO [1]. The emission peaks observed at 416 nm and 438 nm correspond to the transition from the conduction band bottom to Zinc vacancy (VZn) and the transition from the Zinc interstitial (Zni) to the valence band top, respectively [13]. The Cu2þ incorporation reduces the concentration of the VZn and Zni, thus the intensity of the blue emission peaks decreases. The weak green emission peaks observed at 501 nm (2.478 eV) possibly originate from the transition from conduction band bottom to Oxygen interstitial (Oi) [14]. The weak blue emission peaks found at 479 nm and 492 nm are attributed to the transition from antisite Zinc to VZn and Oi respectively. The reduction of these peak reflects the concentration of VZn and Oi was reduced by Cu incorporation. The UV-Vis patterns of films are shown in Fig. 3(a). The optical transmission of the pure ZnO films is very high in the visible range (370–800 nm). Because the band gap Eg of ZnO at room temperature (3.37 eV) is larger than the maximum energy of visible photon (3.1 eV), the irradiation of visible light cannot cause intrinsic excitation. Cu dopant significantly changes the absorption characteristics of films. The Cu dopant enhances the concentration of free carrier and the conductivity of the film, and enhances the absorption consequently. So
Y. Liu et al. / Materials Letters 143 (2015) 319–321
321
Eg (eV)
3.2
Cu sputtering power and Cu content:
absorbance
20 W, 23.8%
3.0 2.8 2.6 0
15 W, 20.8%
5
10
15
20
25
Cu composition (%) 0.64 eV
0.6
Eg reduction (eV)
10 W, 16.4% 8 W, 13.0% 0 W, 0%
0.39 eV
0.4
0.27 eV
0.20 eV
0.2 0.0
200
300
400
500
600
700
800
(nm)
0
5
10
15
20
25
Cu composition (%)
Fig. 3. UV-Vis absorption patterns of films with different Cu content (a), and Cu content dependence of the optical band gap Eg (b) and Eg reduction (c).
the intensity of absorption peak increases gradually with increasing Cu-content as shown in Fig. 3. Additionally, the absorption edge of long wavelength λa, which is determined by extrapolating the linear portion of each curve in Fig. 3 (a), shifts towards long wavelength. Chakraborti et al. [15], explained this red-shift as a result of a uniform substitution of Cu for Zn in the lattice. This substitution was achieved in our samples, as described above. The optical absorption characteristics reflect change of band structure. The optical band gap Eg can be obtained from λa by Eg ¼ 1240=λa . The dependence of the Eg on Cu composition is shown in Fig. 3 (b), in which Eg decreases monotonously with increasing Cu content. This band-gap narrowing effect is consistent with the results of Ahn et al. for p-type ZnO:Cu films [8]. They stated that Eg narrowing was caused by the moving up of the valance band and impurity band. Furthermore, because the lattice constant c become smaller with Cu content increasing (in Fig. 1), according to tight binding approximation, the energy bands become wider, and band gap becomes narrower consequently. The Eg reduction, calculated by the difference between the Eg of ZnO:Cu and pure ZnO films (EgZnO:Cu EZnO g ), is shown in Fig. 3(c). The value increases quickly with increasing Cu composition. In the moderate doping region, the reduction values in our work are a little larger than that in Ref. [8] (0.22 eV at 10%Cu) and ref [16] (0.27 eV at 12.5%Cu). At heavy doping level, the Eg reduction is 0.4 eV at 20.8% Cu and 0.64 eV at 23.8%Cu. The values and the gradient of the curve indicate that band gap reduction is more sensitive to doping content in heavily doping region. 4. Conclusions Different doping level of ZnO:Cu films can be easily achieved by magnetron co-sputtering. At heavy doping level, substitutional doping can also be obtained and some doping effects of moderate doping, such as reduction of lattice constant, PL quenching, and gap narrowing, are more significant. Cu-doping can reduce the lattice constant c. The moderate Cu doping can promote the preferential (002) orientation, but excessive doping can depress it. All the PL peaks in ZnO films
appear in heavy doping region. Cu dopant significantly diminishes the intensity of all the PL peaks. The intensity of UV-Vis absorption peak increases gradually with Cu content for the increase of free carrier concentration. The red-shift and Eg narrowing phenomenon appear. A promising Eg reduction value of 0.64 eV was achieved at heavy doping level. This large Eg reduction gives a splendid future to enhance the absorption efficiency of visible light.
Acknowledgments The research is financially supported by National Natural Science Foundation of China (Project 51002010). The authors thank Dr. Chenglin Heng, and Dr. Shixiang Lu, for PL and UV-Vis measurements respectively.
References [1] Özgür Ü, Alivov YI, Liu C, Teke A, Reshchikov MA, Dogan S, et al. J Appl Phys 2005;98:041301. [2] Wang YS, Thomas PJ, OBrien P. J Phys Chem B 2006;110:21412–5. [3] Zhong ZY, Zhang T. Mater Lett 2013;96:237–9. [4] Lee S, Kim SH, Kim K, Kingon AI, Paine DC, No K. Mater Lett 2012;85:88–90. [5] Ooi PK, Ng SS, Abdullah MJ, Hassan Z. Mater Lett 2014;116:396–8. [6] Xing GZ, Yi JB, Tao JG, Liu T, Wong LM, Zhang Z, et al. Adv Mater 2008;20:3521–7. [7] Xue H, Chen Y, Xu XL, Zhang GH, Zhang H, Ma SY. Phys E 2009;41:788–91. [8] Ahn K, Deutsch T, Yan Y, Jiang C, Perkins CL, Turner J, et al. J Appl Phys 2007;102:023517. [9] Drmosh QA, Rao SG, Yamania ZH, Gondal MA. Appl Surf Sci 2013;270:104–7. [10] Wang YP, Lu JG, Bie X, Ye ZZ, Li X, Song D, et al. Appl Surf Sci 2011;257:5966–71. [11] Huso J, Morrison JL, Mitchell J, Casey E, Hoeck H, Walker C, et al. Appl Phys Lett 2009;94:061919. [12] Yan Y, Al-Jassim MM, Wei SH. Appl Phys Lett 2006;89:181912. [13] Mahroug A, Boudjadar S, Hamrit S, Guerbous L. Mater Lett 2014;134:248–51. [14] Vanheusden K, Warren WL, Seager CH, Tallant DR, Voigt JA, Gnade BE. J Appl Phys 1996;79:7983–90. [15] Chakraborti D, Narayan J, Prater JT. Appl Phys Lett 2004;90:062504. [16] Ferhat M, Zaoui A, Ahuja R. Appl Phys Lett 2009;94:142502.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structural and optical properties of ZnO thin films with heavy Cu-doping prepared by magnetron co-sputtering Yong Liu a,b, Haonan Liu a,b, Yang Yu a,b, Qing Wang a,b, Yinglan Li a,b, Zhi Wang a,b,n a b
Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Institute of Technology, People's Republic of China School of Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 November 2014 Accepted 27 December 2014 Available online 5 January 2015
Cu-doping can modulate the properties of ZnO thin films. Most previous researchers studied ZnO:Cu films with moderate doping level, while this work was focused on heavy Cu-doping (13–23.8%) effect on ZnO thin films prepared by magnetron co-sputtering. The microstructure and optical properties of the films were systematically investigated. The results show that moderate Cu-doping enhances the crystal grain size of the films, whereas heavy doping reduces it. The fluorescence quenching effect, the red shift phenomenon and the band gap narrowing effect in emission and absorption characteristics resulting from Cu-doping are more significant at heavy doping level. A promising band gap reduction of 0.64 eV was achieved at 23.8% doping level. & 2015 Elsevier B.V. All rights reserved.
Keywords: Sputtering Thin films ZnO Cu-doping Optical materials and properties
1. Introduction In recent years, ZnO has been investigated extensively because of its potential values for application in many fields [1]. ZnO can be doped by many elements, such as Mg [2], Ti [3], Al [4], In [5], and Cu [6], to meet the demands of different application. Among various dopants, Cu can be easily doped in the lattice of ZnO for its similar radius and electronic shell to Zn atom. Many properties of the ZnO thus can be modulated by Cu content [6–8]. For example, Cu doping can reduce the band gap and enhance the absorption coefficient [8], which is critical to the application in visible light region. In most previous study on the Cu-doping effects, Cu content lies in moderate doping region (below 12.5%) [9,8,10], while this study is focused on heavy doping region (13– 23.8%). The microstructure and optical properties of the films with different doping composition were systematically investigated. The effects of heavy Cu-doping, some of which are different from those of moderate doping, were found and will be discussed. Another difference between this work and other works is the deposition method. Most previous studies used Zn target with Cu chips [7,8] in magnetron sputtering. We prepared ZnO:Cu films by magnetron co-sputtering of a ZnO and a Cu target, and the doping content in films is easily modulated by the sputtering power of Cu target in this method. 2. Material and methods The films were fabricated by magnetron co-sputtering of two targets. A ZnO target was sputtered by a RF power supply, and a n
Corresponding author. E-mail address: [email protected] (Z. Wang).
http://dx.doi.org/10.1016/j.matlet.2014.12.133 0167-577X/& 2015 Elsevier B.V. All rights reserved.
metal Cu target was sputtered by a DC power supply. The sputtered atoms off-normally deposited on glass substrates for 1 h. The purity of targets was 99.99% and the distances between the targets and the substrates were 130 mm. To avoid excessive doping, a shutter was pulsedly used in front of the Cu target. The period of the shutter was 55 s, and deposition time was 5 s in each period of the shutter. The base vacuum was 4.5 10 4 Pa, and the working pressure was controlled at 0.5 Pa. The flow rates of Ar and O2 were set as 25 and 5 sccm respectively. The sputtering power of ZnO was kept at 100 W, and the sputtering power of Cu was set to 0, 8, 10, 15, and 20 W to obtain different doping content. The substrate temperature was optimized at 500 1C. The alignment and microstructure were investigated by an X-ray diffraction (XRD) spectroscopy system (D8 ADVANCE) with a Cu-Kα1 source. The photoluminescence (PL) measurement was carried out by the 325 nm line from a He–Cd laser, and the signals were recorded by an FLS920 spectrophotometer. The optical absorption characteristics were measured by an ultraviolet visible spectrophotometer (UV-Vis, HITACHI U-3310).
3. Results and discussion Fig. 1(a) shows XRD patterns of all the films. The Cu-doping content was calculated by the ratio of Cu%/(Cu% þZn%) based on the results of an energy dispersive spectroscopy, and is indicated above the corresponding curves. We can see that the pure ZnO films exhibit a single phase with (002)-oriented hexagonal wurtzite structure. For the samples of the Cu contents below 20.8%, only (002) peak of ZnO, and no diffraction peaks of copper and cuprate emerges. It can be concluded that doping Cu atoms substituted Zn atoms at lattice. Fig. 1(b) shows that the (002) peak
320
Y. Liu et al. / Materials Letters 143 (2015) 319–321
(100)
20 W, 23.8%
(002) 15 W, 20.8%
2 of (002) peak ( )
35.0
Cu sputtering power and Cu content:
34.8
34.6
intensity (
34.4 0
10
15
20
0 W, 0% 30
40
50
60
70
80
2 ()
20
0.52
0.48
18
0.44
Grain size (nm)
FWHM of (002) peak ( )
10 W, 16.4%
8 W, 13%
20
5
Sputtering power (W)
16 0
5
10
15
20
Sputtering power (W)
Fig. 1. XRD results of the films with different Cu content. (a) θ–2θ scan pattern; (b) 2θ value of (002) peak; (c): FWHM of (002) peak and the crystallite grain size.
Fig. 2. PL spectra of films with different Cu content; the inset shows the details of the lowest two curves.
shifts gradually to high angle side when the Cu content increases. According to Bragg equation, 2d sin θ ¼ nλ, the crystalline plane distance d002 and the lattice constant c become smaller with Cu content increasing. The radius of Cu þ , Cu2 þ , and Zn2 þ is 0.096 nm, 0.072 nm, and 0.074 nm respectively. Thus the decrease of c indicates that substitutional Cu2 þ is the dominant doping form in the ZnO:Cu films, because only the Cu2 þ can lead to the reduction of the lattice constant. The intensity of the (002) peak in Fig. 1(a) firstly increases, and then decreases with increasing Cu content. When the sputtering power of the Cu is 20 W (for the 23.8% doped sample), a weak (100) diffraction peak of ZnO appears. Fig. 1(c) shows the full width of half maximum (FWHM) value β of (002) peak and the crystallite grain size L as a function of the sputtering power. L was calculated according to the Scherrer equation, L ¼ 0:94λ=β cos θ. We can see that as the sputtering power increases, the FWHM firstly becomes smaller and then becomes larger. Consequently, the crystallite size L firstly increases and then decreases. These XRD results illustrate that moderate Cu-doping can promote the preferential (002) orientation, while heavy doping can inhibit it. The increase in the extent of c-orientation may be due to the
fact that a moderate quantity of Cu atoms share the oxygen with Zn atoms [7]. The ZnO (100) peak in the sample with 23.8% Cu-doping reflects that the crystal planes perpendicular and parallel to the substrates were mixed together [10] at heavy doping level. Fig. 2 shows the PL emission spectra of the films. It is obvious that the increasing Cu percent significantly diminishes the PL intensity. This fluorescence quenching effect is attributed to the enhancement of non-radiative transitions resulting from the Cu incorporation [11]. A UV emission peak is observed at 384 nm (3.223 eV), which arises from the Auger recombination corresponding to near-band edge (NBE) transition. In Auger recombination processes, the energy released by an electron recombination is immediately absorbed by another electron, and then this energy is dissipated by phonons. In wide band gap materials, Auger process depends on the concentration of doping atom and defects in the lattice. Cu-doping enhances the formation of deep in-gap states of high density [12]. The Cu centers in ZnO act as traps to the excited electrons, thus diminish the PL efficiency [11]. With the Cu dopant increasing, NBE emission intensity is reduced quickly, as shown in Fig. 3. The origin of the emission peaks was analyzed according to the band structure and energy level of the defects in ZnO [1]. The emission peaks observed at 416 nm and 438 nm correspond to the transition from the conduction band bottom to Zinc vacancy (VZn) and the transition from the Zinc interstitial (Zni) to the valence band top, respectively [13]. The Cu2þ incorporation reduces the concentration of the VZn and Zni, thus the intensity of the blue emission peaks decreases. The weak green emission peaks observed at 501 nm (2.478 eV) possibly originate from the transition from conduction band bottom to Oxygen interstitial (Oi) [14]. The weak blue emission peaks found at 479 nm and 492 nm are attributed to the transition from antisite Zinc to VZn and Oi respectively. The reduction of these peak reflects the concentration of VZn and Oi was reduced by Cu incorporation. The UV-Vis patterns of films are shown in Fig. 3(a). The optical transmission of the pure ZnO films is very high in the visible range (370–800 nm). Because the band gap Eg of ZnO at room temperature (3.37 eV) is larger than the maximum energy of visible photon (3.1 eV), the irradiation of visible light cannot cause intrinsic excitation. Cu dopant significantly changes the absorption characteristics of films. The Cu dopant enhances the concentration of free carrier and the conductivity of the film, and enhances the absorption consequently. So
Y. Liu et al. / Materials Letters 143 (2015) 319–321
321
Eg (eV)
3.2
Cu sputtering power and Cu content:
absorbance
20 W, 23.8%
3.0 2.8 2.6 0
15 W, 20.8%
5
10
15
20
25
Cu composition (%) 0.64 eV
0.6
Eg reduction (eV)
10 W, 16.4% 8 W, 13.0% 0 W, 0%
0.39 eV
0.4
0.27 eV
0.20 eV
0.2 0.0
200
300
400
500
600
700
800
(nm)
0
5
10
15
20
25
Cu composition (%)
Fig. 3. UV-Vis absorption patterns of films with different Cu content (a), and Cu content dependence of the optical band gap Eg (b) and Eg reduction (c).
the intensity of absorption peak increases gradually with increasing Cu-content as shown in Fig. 3. Additionally, the absorption edge of long wavelength λa, which is determined by extrapolating the linear portion of each curve in Fig. 3 (a), shifts towards long wavelength. Chakraborti et al. [15], explained this red-shift as a result of a uniform substitution of Cu for Zn in the lattice. This substitution was achieved in our samples, as described above. The optical absorption characteristics reflect change of band structure. The optical band gap Eg can be obtained from λa by Eg ¼ 1240=λa . The dependence of the Eg on Cu composition is shown in Fig. 3 (b), in which Eg decreases monotonously with increasing Cu content. This band-gap narrowing effect is consistent with the results of Ahn et al. for p-type ZnO:Cu films [8]. They stated that Eg narrowing was caused by the moving up of the valance band and impurity band. Furthermore, because the lattice constant c become smaller with Cu content increasing (in Fig. 1), according to tight binding approximation, the energy bands become wider, and band gap becomes narrower consequently. The Eg reduction, calculated by the difference between the Eg of ZnO:Cu and pure ZnO films (EgZnO:Cu EZnO g ), is shown in Fig. 3(c). The value increases quickly with increasing Cu composition. In the moderate doping region, the reduction values in our work are a little larger than that in Ref. [8] (0.22 eV at 10%Cu) and ref [16] (0.27 eV at 12.5%Cu). At heavy doping level, the Eg reduction is 0.4 eV at 20.8% Cu and 0.64 eV at 23.8%Cu. The values and the gradient of the curve indicate that band gap reduction is more sensitive to doping content in heavily doping region. 4. Conclusions Different doping level of ZnO:Cu films can be easily achieved by magnetron co-sputtering. At heavy doping level, substitutional doping can also be obtained and some doping effects of moderate doping, such as reduction of lattice constant, PL quenching, and gap narrowing, are more significant. Cu-doping can reduce the lattice constant c. The moderate Cu doping can promote the preferential (002) orientation, but excessive doping can depress it. All the PL peaks in ZnO films
appear in heavy doping region. Cu dopant significantly diminishes the intensity of all the PL peaks. The intensity of UV-Vis absorption peak increases gradually with Cu content for the increase of free carrier concentration. The red-shift and Eg narrowing phenomenon appear. A promising Eg reduction value of 0.64 eV was achieved at heavy doping level. This large Eg reduction gives a splendid future to enhance the absorption efficiency of visible light.
Acknowledgments The research is financially supported by National Natural Science Foundation of China (Project 51002010). The authors thank Dr. Chenglin Heng, and Dr. Shixiang Lu, for PL and UV-Vis measurements respectively.
References [1] Özgür Ü, Alivov YI, Liu C, Teke A, Reshchikov MA, Dogan S, et al. J Appl Phys 2005;98:041301. [2] Wang YS, Thomas PJ, OBrien P. J Phys Chem B 2006;110:21412–5. [3] Zhong ZY, Zhang T. Mater Lett 2013;96:237–9. [4] Lee S, Kim SH, Kim K, Kingon AI, Paine DC, No K. Mater Lett 2012;85:88–90. [5] Ooi PK, Ng SS, Abdullah MJ, Hassan Z. Mater Lett 2014;116:396–8. [6] Xing GZ, Yi JB, Tao JG, Liu T, Wong LM, Zhang Z, et al. Adv Mater 2008;20:3521–7. [7] Xue H, Chen Y, Xu XL, Zhang GH, Zhang H, Ma SY. Phys E 2009;41:788–91. [8] Ahn K, Deutsch T, Yan Y, Jiang C, Perkins CL, Turner J, et al. J Appl Phys 2007;102:023517. [9] Drmosh QA, Rao SG, Yamania ZH, Gondal MA. Appl Surf Sci 2013;270:104–7. [10] Wang YP, Lu JG, Bie X, Ye ZZ, Li X, Song D, et al. Appl Surf Sci 2011;257:5966–71. [11] Huso J, Morrison JL, Mitchell J, Casey E, Hoeck H, Walker C, et al. Appl Phys Lett 2009;94:061919. [12] Yan Y, Al-Jassim MM, Wei SH. Appl Phys Lett 2006;89:181912. [13] Mahroug A, Boudjadar S, Hamrit S, Guerbous L. Mater Lett 2014;134:248–51. [14] Vanheusden K, Warren WL, Seager CH, Tallant DR, Voigt JA, Gnade BE. J Appl Phys 1996;79:7983–90. [15] Chakraborti D, Narayan J, Prater JT. Appl Phys Lett 2004;90:062504. [16] Ferhat M, Zaoui A, Ahuja R. Appl Phys Lett 2009;94:142502.