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Investigation of the blue–green emission and UV photosensitivity of Cu-doped ZnO films
Materials Science in Semiconductor Processing 16 (2013) 1079–1085
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Investigation of the blue–green emission and UV photosensitivity of Cu-doped ZnO films F.M. Li, C.T. Zhu, S.Y. Ma n, A.M. Sun, H.S. Song, X.B. Li, X. Wang College of Physics and Electronic Engineering, Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, Northwest Normal University, Lanzhou, Gansu 730070, China
a r t i c l e i n f o
abstract
Available online 3 May 2013
Cu-doped zinc oxide (ZnO:Cu) films were deposited on p-Si (100) substrates using radiofrequency reactive magnetron sputtering. The structure and optical properties of the films were characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and fluorescence spectroscopy. XRD and SEM results revealed that ZnO:Cu film had a better preferential orientation along the c-axis compared with pure ZnO film. The chemical state of copper and oxygen in ZnO:Cu films was investigated by XPS. The results suggest that the Cu ion has a mixed univalent and bivalent state. The integrated Cu2+/Cu+ intensity ratio increased with the O2 partial pressure. Photoluminescence measurements at room temperature revealed a double peak in the blue regions and a green emission peak. The close relationship between the valence state of Cu ions and the blue–green emission is discussed in detail. A higher photocurrent was observed for ZnO:Cu films under UV illumination. UV photodetectors based on ZnO: Cu films have high sensitivity and fast response and recovery times. Under periodic UV illumination at 380 nm the ZnO:Cu films showed stable photocurrent growth and decay, so the films are potential candidate materials for UV photodetectors. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: ZnO:Cu film Blue–green emission I–V characteristics UV photosensitivity
1. Introduction Owing to its wide direct bandgap (3.37 eV) and high exciton binding energy (60 meV), ZnO has attracted considerable attention for the fabrication of short-wavelength optoelectronic devices, such as light-emitting and laser diodes [1], blue and UV light emitters [2], and UV photodetectors [3–7]. Doping and surface modification are regarded as effective approaches for optimizing the optical and electrical properties of ZnO films. Surface modifications include capping with various metals or oxides (e.g., Ti, TiO2) [8,9] and embedding of metal nanoparticles (e.g., Ag, Au, Pt) [10,11] to enhance the ZnO UV emission via surface passivation and surface plasmon coupling, respectively. ZnO films have been doped with various ions to obtain different luminescent materials.
n
Corresponding author. Tel.: +86 15293153606; fax: +86 9317971503. E-mail address: [email protected] (S.Y. Ma).
El Jouad et al. observed emission (650–750 nm) for ZnO:Ce films [12]. In general, white light is obtained by combining red, green, and blue light emitted by different molecules [13]. Therefore, if emission in the red region from ZnO:Ce can be combined with blue and green emission from ZnO:Cu films, it would be possible to obtain white light using a bi-layer emitting structure, which is significant for display applications. To obtain good white light emission, we first evaluated the effect of Cu ions on blue and green emission by ZnO:Cu films. UV sensors are used in many industries, and high photosensitivity and low response and recovery times are important parameters for these sensors. ZnO:Cu films with a wide bandgap could be used as UV sensors. ZnO:Cu films were deposited on p-Si (100) substrates under various oxygen partial pressures. The mechanism of the blue and green emission by ZnO:Cu films was analyzed. In particular, the intensity of the blue–green emission in relation to the chemical state of the Cu ion was studied in detail. The ZnO:Cu films showed UV sensitivity
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similar to that of undoped ZnO films. The UV sensitivity of ZnO:Cu films is discussed. 2. Experimental 2.1. Preparation of ZnO:Cu films A high-purity Zn target ( 99.9%, 60 mm in diameter) and p-Si (100) substrates were used to prepare the films. Cu foils were pasted on the Zn target to yield a Cu coverage area of 2% of the effective sputtering area of the Zn target. The films were deposited at 200 1C under RF power of 100 W during sputtering for 1 h. The atmosphere for film growth was changed by varying the O2/Ar ratio (8:10, 12:10, 16:10, and 20:10 sccm) at a constant working pressure of 1.0 Pa. 2.2. Instrumentation The Cu doping concentration in ZnO:Cu films was 1.80 70.2 at%, as determined on an energy-dispersive X-ray spectrometer (S-4800). The crystal structures were studied by X-ray diffraction (XRD, D/Max-2400) using Cu Kα radiation (λ ¼0.15406 nm). The surface morphology was characterized by scanning electronic microscopy (SEM; S-4800). The chemical state of Cu in the ZnO:Cu films was investigated by X-ray photoelectron spectroscopy (XPS; ESCA LAB 220-XL). Photoluminescence (PL) measurements were carried out using a xenon lamp (RF-5301, wavelength 250 nm) for excitation. Photocurrent was measured on a CHI 600D electrochemical workstation using a 150-W xenon lamp as the illumination source and a grating monochromator (1200 grooves/mm) to provide monochromatic light at wavelengths of 300–800 nm. Current–voltage measurements were carried out using a dual-channel digital power source (Keithley 2612). Two circular gold contacts (diameter 2 mm) were sputtered on samples.
It is evident that the grain size on the surface of pure ZnO film was not uniform. However, the ZnO:Cu film exhibited a uniform grain size and a smooth surface, indicating good crystal quality. Fig. 3a shows the (002) diffraction peak for ZnO:Cu films fabricated at different O2 partial pressures. As the O2/Ar ratio increased, the intensity of the (002) diffraction peak first increased and then decreased, reaching a maximum at 16:10 sccm. This indicates that the (002) orientation of ZnO:Cu films was promoted by an appropriate O2 partial pressure. Moreover, the (002) peak shifted to lower diffraction angles with increasing O2/Ar ratio and reached a minimum of 33.981 at 12:10 sccm. According to the Bragg formula λ ¼2d sin θ, the decrease in diffraction angle corresponds to an increase in the interplanar spacing (d002). In our experiment, the valence of Cu could be assumed to be +1 or/and +2 in the ZnO:Cu films. The atomic radius of Cu+, Cu2+, and Zn2+ is 0.096, 0.072, and 0.074 nm, respectively. Thus, Cu+ substitution for Zn2+ ions increased as the O2/Ar ratio increased from 8:10 to 12:10 sccm. However, a further increase in the O2/Ar ratio shifted the (002) peak to higher diffraction angles, reaching 34.441 at 20:10 sccm. This represents a slight shift to higher angle compared with the standard XRD spectrum for ZnO powder. It is possible that Cu+ ions decrease and Cu2+ ions increase when the O2/Ar ratio exceeds 12:10 sccm, and Cu2+ substitution for Zn2+ is predominant in the ZnO:Cu film prepared at 20:10 sccm. Therefore, we can hypothesize that more Cu+ (Cu2+) substitution could occur in ZnO:Cu films grown in an oxygen-poor (-enriched) environment. Fig. 3b shows the full width at half-maximum (FWHM), strain, and grain size from XRD patterns of the films as a function of the O2 partial pressures. FWHM first decreased, reaching a minimum of 0.351 for 16:10 sccm, and then increased (Fig. 3b). This indicates that the ZnO:Cu film fabricated at 16:10 sccm had the best crystal quality among all the ZnO:Cu films. The crystallite size of the samples was 16–24 nm, as calculated using the Scherrer equation [15]
3. Results and discussion D¼ 3.1. Structure and optical properties Fig. 1 shows XRD patterns for pure ZnO and ZnO:Cu films prepared at an O2/Ar ratio of 8:10. Both samples exhibit (100), (002), and (101) diffraction peaks, indicating that the films were polycrystalline with a hexagonal wurtzite ZnO structure. The intensity of the (002) diffraction peak decreased and the other diffraction peaks almost disappeared after Cu doping, indicating that the ZnO:Cu film had a better preferential orientation along the c-axis compared with pure ZnO film. It is clear from the inset in Fig. 1 that the (002) diffraction peak of ZnO:Cu shifted to a lower angle compared with pure ZnO, which implies that the lattice constant changed after Cu doping. Cu+ substitution for Zn2+ ions in the lattice results in an increase in the lattice constant and the (002) crystalline plane distance, which would lead to a decrease in diffraction angle compared with undoped ZnO [14]. The surface texture of pure ZnO and ZnO:Cu films prepared at 8:10 sccm were observed by SEM (Fig. 2).
0:9λ ; β cosθ
where λ is the X-ray wavelength, θ is the diffraction angle and β is the FWHM for the (002) peak. The plane stress of ZnO:Cu films was calculated according to [16] s ¼ −233
c−c0 ; c0
where c0 is the lattice constant of ZnO film without defects (c0 ¼0.5205 nm) and c is the lattice constant for our ZnO: Cu films. Samples grown in an oxygen-enriched environment (e.g., O2/Ar¼20:10) had the lowest strain (Fig. 3b). Narrow-scan XPS spectra and simulated lines for Cu 2p3/2 and O 1s for ZnO:Cu films prepared at 8:10 and 20:10 sccm are shown in Fig. 4. It is evident that the Cu 2p3/2 spectrum can be Gaussian fitted to Cu+ (932.7 eV) and Cu2+ (933.8 eV) components (Fig. 4a,b) [17]. Thus, the results reveal that Cu ions exist predominantly in a univalent and bivalent state in films prepared at 8:10 and 20:10 sccm, respectively. This conclusion is consistent with the XRD results. The XPS spectrum for O 1s can be
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Fig. 1. XRD patterns for pure ZnO and ZnO:Cu films prepared at an O2/Ar ratio of 8:10 sccm. The inset shows an enlargement of the (002) diffraction peak.
Fig. 2. SEM images of (a) pure ZnO and (b) ZnO:Cu films prepared at an O2/Ar ratio of 8:10 sccm.
Fig. 3. (a) Enlargement of the (002) diffraction peak for ZnO:Cu films prepared at various O2/Ar ratios. (b) Plot of FWHM, strain and grain size from XRD patterns for the films as a function of the O2/Ar ratio.
divided into three peaks centered at 529.8, 531.6, and 532.4 eV (Fig. 4c,d). According to Islam et al., the peaks at low and intermediate binging energy can be attributed to O2 − ions at intrinsic sites (Oi) and O2− ions in oxygen-deficient (Vo) regions, respectively[18]. The peak at 532.4 eV is usually attributed to loosely bound oxygen (So) on the film surface.
Therefore, major oxygen vacancies and interstitial deficiencies may be present in the ZnO:Cu films prepared at 8:10 and 20:10 sccm, respectively. The PL spectrum of pure ZnO film in Fig. 5a shows a strong emission peak at 380 nm due to exciton transition and a weak broad band in the range 410–480 nm. Fig. 5b
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Fig. 4. High-resolution XPS spectra (open squares) and simulated lines for Cu 2p3/2 and O 1s regions on the surface of ZnO:Cu films prepared at O2/Ar ratios of 8:10 and 20:10 sccm.
Fig. 5. PL spectra for (a) pure ZnO film prepared at an O2/Ar ratio of 8:10 sccm and (b) ZnO:Cu films prepared at various O2/Ar ratios.
shows PL spectra for ZnO:Cu films prepared at various O2/Ar ratios. A double peak at approximately 440 nm (2.82 eV) and 480 nm (2.57 eV) is evident in the blue region. However, when the O2/Ar ratio exceeds 16:10 sccm, the peak at 440 nm shifts to longer wavelength and that at 480 nm disappeared. A peak located at approximately 530 nm in the green region is also evident in Fig. 5b. According to the XRD and XPS results, Cu+ and Cu2+ substitutions and interstitial Cu2+ ions may be the main
impurities in the ZnO:Cu films. Cu+ ions increased monotonically as the O2/Ar ratio increased from 8:10 to 12:10 sccm. On the one hand, interstitial Zn might increase with Cu+ substitution. On the other hand, Zn vacancies might increase with interstitial Cu2+ because ZnO is a self-assembling oxide. According to results calculated by Xu et al., the double peak at 440 nm (2.82 eV) and 480 nm (2.57 eV) can be attributed to transition of Zn vacancies to the bottom of the conduction band and to interstitial
F.M. Li et al. / Materials Science in Semiconductor Processing 16 (2013) 1079–1085
Zn and Zn vacancies, respectively [19]. Therefore, this double-peak emission became strong. It is evident that the ZnO:Cu film prepared at 16:10 sccm had the best crystal quality, which resulted in weakened blue–green emission due to a decrease in point defects. As the O2/Ar ratio increased, Cu2+ ions became predominant in the ZnO: Cu films, especially at 20:10 sccm. It is possible that greater Cu2+ substitution in the ZnO:Cu films might increase interstitial Zn defects and decrease Zn vacancies for films grown in an oxygen-enriched environment. The shift of the blue emission peak to 460 nm (Fig. 5b) can be attributed to electron transition from interstitial Zn levels to the top of the valence band. In addition, the green emission by ZnO might due to electron transition from deep oxygen vacancy levels to the top of the valance band [20]. In our films, Cu2+ ions exist as interstitials that share oxygen with Zn atoms; this increases O vacancies and interstitial defects, which is the main reason for the green emission. 3.2. UV photosensitivity of ZnO:Cu films Fig. 6a shows I–V curves for the ZnO:Cu film prepared at 16:10 sccm in the dark and on illumination with light of various wavelengths. The inset shows the set-up for I–V measurements. As soon as the film was illuminated, the magnitude of the current increased compared to the film in dark in a wavelength-dependent manner. As the illuminating wavelength was blue-shifted, the current increased for the same voltage. The higher current observed for the films under UV illumination reflects significant UV photosensitivity. As shown in Fig. 6a, the absolute value of the current increased slowly at first and then rapidly increased when the voltage exceeded 71 V. The main reason may be that a potential barrier of 1 eV exists between the Fermi level of the Au electrode and the conduction band of the ZnO film, as shown in Fig. 7. The work function (W) of Au is 5.1 eV and the electron affinity energy (X) of ZnO film is approximately 4.1 eV. Therefore, the current is lower for absolute voltage magnitudes o1 V. Fig. 6b shows log I–V curves for
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the ZnO:Cu films. The behavior of the current is almost symmetric for positive and negative voltages and the higher dark current is primarily due to the increase in free carrier concentration caused by Cu doping in the ZnO lattice [21]. ZnO is a wide-bandgap semiconductor and can be excited by UV irradiation for photocurrent generation. Changes in photocurrent for ZnO films are primarily caused by adsorption and desorption of oxygen molecules on the film surface [6,22,23]. In air, oxygen molecules adsorb on the surface of ZnO films. Adsorbed oxygen molecules capture electrons from the conduction band and change to O2− or O− (O2− are believed to predominate). Under UV illumination, electron–hole pairs generated by light absorption take part in reducing O2− or O− and release O2 gas and free electrons [24], resulting in an increase in film photocurrent. Photocurrent results under illumination at 380 nm for undoped ZnO and ZnO:Cu films prepared at various O2/Ar ratios are shown in Fig. 8. The photocurrent rapidly increased at first and then slowly increased asymptotically over time. When the UV light was turned off, the photocurrent rapidly decreased, followed by a slower decrease. The film prepared at 16:10 sccm had the strongest photocurrent response (Fig. 8). According the O 1s XPS analysis, the film prepared at 16:10 sccm had more O2− ions on the surface. The XRD analysis suggests that the ZnO:Cu film prepared at 16:10 sccm had the best crystal quality.
Fig. 7. Schematic of the band alignment for ZnO and the Au electrode.
Fig. 6. (a) Linear and (b) log I–V curves for ZnO:Cu films prepared at an O2/Ar ratio of 16:10 sccm in the dark and illuminated by light of various wavelengths. Inset: set-up for I–V measurements.
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Therefore, electrons from the valence band transfer to the conduction band and then the holes participate in reducing O2− or O−, leading to the release of more free electrons, under illumination at 380 nm. The lower photocurrent for films prepared at 8:10, 12:10 and 20:10 sccm can be explained as follows. O2− vacancies and interstitial deficiencies recombine with holes in the valence band under UV illumination, whereas O2− ions absorbed on the film surface are unavailable for recombination with holes and release of free electron. Moreover, empty lowerenergy Cu 3d states and [CuZn+Zni]x complex defects could capture free electrons [25], resulting in a decrease in photocurrent. For the ZnO film, the electron concentration would increase after Cu doping, so the photocurrent is higher for ZnO:Cu film prepared at 16:10 than for pure ZnO film. Fig. 9 shows the photocurrent for ZnO:Cu film (16:10 sccm) under periodic UV illumination at 380 nm. The photocurrent shows stable increases and decreases. Therefore, our ZnO:Cu films are promising candidates for UV sensors.
4. Conclusion Pure ZnO and ZnO:Cu films were deposited on p-Si (100) substrates under various oxygen partial pressures. XRD and SEM results revealed that ZnO:Cu films had a better preferential orientation along the c-axis compared with pure ZnO film. The mechanisms for blue–green emission by ZnO:Cu films were analyzed and the intensity of the emission in relation to the chemical state of the Cu ion was studied in detail. The results revealed that Cu+ and Cu2+ substitutions and interstitial Cu2+ in the ZnO:Cu films affect the concentration of intrinsic defects, leading to blue–green emission. It is reasonable to conclude that Cu ions are incorporated in the ZnO lattice and are responsible for the blue–green emission of ZnO:Cu films. The higher photocurrent observed for ZnO:Cu films under UV illumination reflects significant UV photosensitivity. High sensitivity and fast response and recovery times are important sensor parameters that are usually determined by the microstructure of the nanomaterial. The high UV sensitivity and fast response and recovery times for our ZnO:Cu films can be attributed to intrinsic donor defects and adsorption of oxygen species (O−, O2−). Our ZnO:Cu films are promising candidates for UV sensors.
Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 10874140), the College Basic Scientific Research Operation Cost of Gansu Province (manufacture and characteristic research of optical gassensing films and Y series superconducting materials), and the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry. References
Fig. 8. Photocurrent transients for undoped ZnO and ZnO:Cu films prepared at various O2/Ar ratios. Excitation was at 380 nm and the light was turned on and off as indicated. The applied potential was 0 V.
Fig. 9. Photocurrent behavior for ZnO:Cu film prepared at an O2/Ar ratio of 16:10 sccm under periodic UV illumination at 380 nm.
[1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J. Vac. Sci. Technol. B 22 (2004) 932. [2] D.M. Bagall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [3] J.H. He, Y.H. Lin, M.E. McConney, W. Tsukrek, Z.L. Wang, J. Appl. Phys. 102 (2007) 084303. [4] M.C. Jeong, B.Y. Oh, W. Lee, J.M. Myoung, Appl. Phys. Lett. 86 (2005) 103105. [5] Z.Y. Fan, P.C.M. Chang, J.G. Lu, Appl. Phys. Lett. 85 (2004) 6128. [6] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158. [7] H.K. Yadav, K. Sreenivas, V. Gupta, J. Appl. Phys. 107 (2010) 044507. [8] C.W. Lai, J. An, H.C. Ong, Appl. Phys. Lett. 86 (2005) 51105. [9] J. Song, X.Y. An, J.Y. Zhou, et al., Appl. Phys. Lett. 97 (2010) 122103. [10] K.W. Liu, Y.D. Tang, C.X. Cong, et al., Appl. Phys. Lett. 94 (2009) 151102. [11] D.M. Schaadt, B. Feng, E.T. Yu, Appl. Phys. Lett. 86 (2005) 063106. [12] M. El Jouad, M. Alaoui Lamrani, Z. Sofiani, et al., Opt. Mater. 31 (2009) 1357. [13] S. Tokito, T. Lijinma, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 2459. [14] L.G. Ma, S.Y. Ma, H.X. Chen, X.Q. Ai, X.L. Huang, Appl. Surf. Sci. 257 (2011) 10036. [15] T. Yamada, A. Miyake, S. Kishimoto, et al., Surf. Coat. Technol 202 (2007) 973. [16] R. Cebulla, R. Wendt, K. Ellmer, J. Appl. Phys. 83 (1998) 1087. [17] J.L. Sun, J. Xu, J.L. Zhu, Appl. Phys. Lett. 90 (2007) 201119. [18] M.N. Islam, T.B. Ghosh, K.L. Chopra, H.N. Acharya, Thin Solid Films 280 (1996) 20. [19] P.S. Xu, Y.M. Sun, C.S. Shi, F.Q. Xu, H.B. Pan, N.u.c.l. Instrum., Methods Phys. Res. B 199 (2003) 286.
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[20] K. Vanheusden, C.H. Seager, W.L. Warren, et al., Appl. Phys. Lett. 68 (1996) 403. [21] S. Dhara, P.K. Giri, Chem. Phys. Lett. 541 (2012) 39. [22] Q.H. Li, T. Gao, Y.G. Wang, T.H. Wang, Appl. Phys. Lett. 86 (2005) 123117.
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[23] X.G. Zheng, Q.S. Li, W. Hu, D. Chen, N. Zhang, J.J. Wang, L.C. Zhang, J, Luminescence 123 (2007) 198. [24] S. Dhara, P. Giri, Nanoscale Res. Lett. 6 (2011) 504. [25] J.V. Bellini, M.R. Morelli, R.H.G.A. Kiminami, J. Mater. Sci. 13 (2001) 285.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Investigation of the blue–green emission and UV photosensitivity of Cu-doped ZnO films F.M. Li, C.T. Zhu, S.Y. Ma n, A.M. Sun, H.S. Song, X.B. Li, X. Wang College of Physics and Electronic Engineering, Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, Northwest Normal University, Lanzhou, Gansu 730070, China
a r t i c l e i n f o
abstract
Available online 3 May 2013
Cu-doped zinc oxide (ZnO:Cu) films were deposited on p-Si (100) substrates using radiofrequency reactive magnetron sputtering. The structure and optical properties of the films were characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and fluorescence spectroscopy. XRD and SEM results revealed that ZnO:Cu film had a better preferential orientation along the c-axis compared with pure ZnO film. The chemical state of copper and oxygen in ZnO:Cu films was investigated by XPS. The results suggest that the Cu ion has a mixed univalent and bivalent state. The integrated Cu2+/Cu+ intensity ratio increased with the O2 partial pressure. Photoluminescence measurements at room temperature revealed a double peak in the blue regions and a green emission peak. The close relationship between the valence state of Cu ions and the blue–green emission is discussed in detail. A higher photocurrent was observed for ZnO:Cu films under UV illumination. UV photodetectors based on ZnO: Cu films have high sensitivity and fast response and recovery times. Under periodic UV illumination at 380 nm the ZnO:Cu films showed stable photocurrent growth and decay, so the films are potential candidate materials for UV photodetectors. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: ZnO:Cu film Blue–green emission I–V characteristics UV photosensitivity
1. Introduction Owing to its wide direct bandgap (3.37 eV) and high exciton binding energy (60 meV), ZnO has attracted considerable attention for the fabrication of short-wavelength optoelectronic devices, such as light-emitting and laser diodes [1], blue and UV light emitters [2], and UV photodetectors [3–7]. Doping and surface modification are regarded as effective approaches for optimizing the optical and electrical properties of ZnO films. Surface modifications include capping with various metals or oxides (e.g., Ti, TiO2) [8,9] and embedding of metal nanoparticles (e.g., Ag, Au, Pt) [10,11] to enhance the ZnO UV emission via surface passivation and surface plasmon coupling, respectively. ZnO films have been doped with various ions to obtain different luminescent materials.
n
Corresponding author. Tel.: +86 15293153606; fax: +86 9317971503. E-mail address: [email protected] (S.Y. Ma).
El Jouad et al. observed emission (650–750 nm) for ZnO:Ce films [12]. In general, white light is obtained by combining red, green, and blue light emitted by different molecules [13]. Therefore, if emission in the red region from ZnO:Ce can be combined with blue and green emission from ZnO:Cu films, it would be possible to obtain white light using a bi-layer emitting structure, which is significant for display applications. To obtain good white light emission, we first evaluated the effect of Cu ions on blue and green emission by ZnO:Cu films. UV sensors are used in many industries, and high photosensitivity and low response and recovery times are important parameters for these sensors. ZnO:Cu films with a wide bandgap could be used as UV sensors. ZnO:Cu films were deposited on p-Si (100) substrates under various oxygen partial pressures. The mechanism of the blue and green emission by ZnO:Cu films was analyzed. In particular, the intensity of the blue–green emission in relation to the chemical state of the Cu ion was studied in detail. The ZnO:Cu films showed UV sensitivity
1369-8001/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.03.012
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F.M. Li et al. / Materials Science in Semiconductor Processing 16 (2013) 1079–1085
similar to that of undoped ZnO films. The UV sensitivity of ZnO:Cu films is discussed. 2. Experimental 2.1. Preparation of ZnO:Cu films A high-purity Zn target ( 99.9%, 60 mm in diameter) and p-Si (100) substrates were used to prepare the films. Cu foils were pasted on the Zn target to yield a Cu coverage area of 2% of the effective sputtering area of the Zn target. The films were deposited at 200 1C under RF power of 100 W during sputtering for 1 h. The atmosphere for film growth was changed by varying the O2/Ar ratio (8:10, 12:10, 16:10, and 20:10 sccm) at a constant working pressure of 1.0 Pa. 2.2. Instrumentation The Cu doping concentration in ZnO:Cu films was 1.80 70.2 at%, as determined on an energy-dispersive X-ray spectrometer (S-4800). The crystal structures were studied by X-ray diffraction (XRD, D/Max-2400) using Cu Kα radiation (λ ¼0.15406 nm). The surface morphology was characterized by scanning electronic microscopy (SEM; S-4800). The chemical state of Cu in the ZnO:Cu films was investigated by X-ray photoelectron spectroscopy (XPS; ESCA LAB 220-XL). Photoluminescence (PL) measurements were carried out using a xenon lamp (RF-5301, wavelength 250 nm) for excitation. Photocurrent was measured on a CHI 600D electrochemical workstation using a 150-W xenon lamp as the illumination source and a grating monochromator (1200 grooves/mm) to provide monochromatic light at wavelengths of 300–800 nm. Current–voltage measurements were carried out using a dual-channel digital power source (Keithley 2612). Two circular gold contacts (diameter 2 mm) were sputtered on samples.
It is evident that the grain size on the surface of pure ZnO film was not uniform. However, the ZnO:Cu film exhibited a uniform grain size and a smooth surface, indicating good crystal quality. Fig. 3a shows the (002) diffraction peak for ZnO:Cu films fabricated at different O2 partial pressures. As the O2/Ar ratio increased, the intensity of the (002) diffraction peak first increased and then decreased, reaching a maximum at 16:10 sccm. This indicates that the (002) orientation of ZnO:Cu films was promoted by an appropriate O2 partial pressure. Moreover, the (002) peak shifted to lower diffraction angles with increasing O2/Ar ratio and reached a minimum of 33.981 at 12:10 sccm. According to the Bragg formula λ ¼2d sin θ, the decrease in diffraction angle corresponds to an increase in the interplanar spacing (d002). In our experiment, the valence of Cu could be assumed to be +1 or/and +2 in the ZnO:Cu films. The atomic radius of Cu+, Cu2+, and Zn2+ is 0.096, 0.072, and 0.074 nm, respectively. Thus, Cu+ substitution for Zn2+ ions increased as the O2/Ar ratio increased from 8:10 to 12:10 sccm. However, a further increase in the O2/Ar ratio shifted the (002) peak to higher diffraction angles, reaching 34.441 at 20:10 sccm. This represents a slight shift to higher angle compared with the standard XRD spectrum for ZnO powder. It is possible that Cu+ ions decrease and Cu2+ ions increase when the O2/Ar ratio exceeds 12:10 sccm, and Cu2+ substitution for Zn2+ is predominant in the ZnO:Cu film prepared at 20:10 sccm. Therefore, we can hypothesize that more Cu+ (Cu2+) substitution could occur in ZnO:Cu films grown in an oxygen-poor (-enriched) environment. Fig. 3b shows the full width at half-maximum (FWHM), strain, and grain size from XRD patterns of the films as a function of the O2 partial pressures. FWHM first decreased, reaching a minimum of 0.351 for 16:10 sccm, and then increased (Fig. 3b). This indicates that the ZnO:Cu film fabricated at 16:10 sccm had the best crystal quality among all the ZnO:Cu films. The crystallite size of the samples was 16–24 nm, as calculated using the Scherrer equation [15]
3. Results and discussion D¼ 3.1. Structure and optical properties Fig. 1 shows XRD patterns for pure ZnO and ZnO:Cu films prepared at an O2/Ar ratio of 8:10. Both samples exhibit (100), (002), and (101) diffraction peaks, indicating that the films were polycrystalline with a hexagonal wurtzite ZnO structure. The intensity of the (002) diffraction peak decreased and the other diffraction peaks almost disappeared after Cu doping, indicating that the ZnO:Cu film had a better preferential orientation along the c-axis compared with pure ZnO film. It is clear from the inset in Fig. 1 that the (002) diffraction peak of ZnO:Cu shifted to a lower angle compared with pure ZnO, which implies that the lattice constant changed after Cu doping. Cu+ substitution for Zn2+ ions in the lattice results in an increase in the lattice constant and the (002) crystalline plane distance, which would lead to a decrease in diffraction angle compared with undoped ZnO [14]. The surface texture of pure ZnO and ZnO:Cu films prepared at 8:10 sccm were observed by SEM (Fig. 2).
0:9λ ; β cosθ
where λ is the X-ray wavelength, θ is the diffraction angle and β is the FWHM for the (002) peak. The plane stress of ZnO:Cu films was calculated according to [16] s ¼ −233
c−c0 ; c0
where c0 is the lattice constant of ZnO film without defects (c0 ¼0.5205 nm) and c is the lattice constant for our ZnO: Cu films. Samples grown in an oxygen-enriched environment (e.g., O2/Ar¼20:10) had the lowest strain (Fig. 3b). Narrow-scan XPS spectra and simulated lines for Cu 2p3/2 and O 1s for ZnO:Cu films prepared at 8:10 and 20:10 sccm are shown in Fig. 4. It is evident that the Cu 2p3/2 spectrum can be Gaussian fitted to Cu+ (932.7 eV) and Cu2+ (933.8 eV) components (Fig. 4a,b) [17]. Thus, the results reveal that Cu ions exist predominantly in a univalent and bivalent state in films prepared at 8:10 and 20:10 sccm, respectively. This conclusion is consistent with the XRD results. The XPS spectrum for O 1s can be
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Fig. 1. XRD patterns for pure ZnO and ZnO:Cu films prepared at an O2/Ar ratio of 8:10 sccm. The inset shows an enlargement of the (002) diffraction peak.
Fig. 2. SEM images of (a) pure ZnO and (b) ZnO:Cu films prepared at an O2/Ar ratio of 8:10 sccm.
Fig. 3. (a) Enlargement of the (002) diffraction peak for ZnO:Cu films prepared at various O2/Ar ratios. (b) Plot of FWHM, strain and grain size from XRD patterns for the films as a function of the O2/Ar ratio.
divided into three peaks centered at 529.8, 531.6, and 532.4 eV (Fig. 4c,d). According to Islam et al., the peaks at low and intermediate binging energy can be attributed to O2 − ions at intrinsic sites (Oi) and O2− ions in oxygen-deficient (Vo) regions, respectively[18]. The peak at 532.4 eV is usually attributed to loosely bound oxygen (So) on the film surface.
Therefore, major oxygen vacancies and interstitial deficiencies may be present in the ZnO:Cu films prepared at 8:10 and 20:10 sccm, respectively. The PL spectrum of pure ZnO film in Fig. 5a shows a strong emission peak at 380 nm due to exciton transition and a weak broad band in the range 410–480 nm. Fig. 5b
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Fig. 4. High-resolution XPS spectra (open squares) and simulated lines for Cu 2p3/2 and O 1s regions on the surface of ZnO:Cu films prepared at O2/Ar ratios of 8:10 and 20:10 sccm.
Fig. 5. PL spectra for (a) pure ZnO film prepared at an O2/Ar ratio of 8:10 sccm and (b) ZnO:Cu films prepared at various O2/Ar ratios.
shows PL spectra for ZnO:Cu films prepared at various O2/Ar ratios. A double peak at approximately 440 nm (2.82 eV) and 480 nm (2.57 eV) is evident in the blue region. However, when the O2/Ar ratio exceeds 16:10 sccm, the peak at 440 nm shifts to longer wavelength and that at 480 nm disappeared. A peak located at approximately 530 nm in the green region is also evident in Fig. 5b. According to the XRD and XPS results, Cu+ and Cu2+ substitutions and interstitial Cu2+ ions may be the main
impurities in the ZnO:Cu films. Cu+ ions increased monotonically as the O2/Ar ratio increased from 8:10 to 12:10 sccm. On the one hand, interstitial Zn might increase with Cu+ substitution. On the other hand, Zn vacancies might increase with interstitial Cu2+ because ZnO is a self-assembling oxide. According to results calculated by Xu et al., the double peak at 440 nm (2.82 eV) and 480 nm (2.57 eV) can be attributed to transition of Zn vacancies to the bottom of the conduction band and to interstitial
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Zn and Zn vacancies, respectively [19]. Therefore, this double-peak emission became strong. It is evident that the ZnO:Cu film prepared at 16:10 sccm had the best crystal quality, which resulted in weakened blue–green emission due to a decrease in point defects. As the O2/Ar ratio increased, Cu2+ ions became predominant in the ZnO: Cu films, especially at 20:10 sccm. It is possible that greater Cu2+ substitution in the ZnO:Cu films might increase interstitial Zn defects and decrease Zn vacancies for films grown in an oxygen-enriched environment. The shift of the blue emission peak to 460 nm (Fig. 5b) can be attributed to electron transition from interstitial Zn levels to the top of the valence band. In addition, the green emission by ZnO might due to electron transition from deep oxygen vacancy levels to the top of the valance band [20]. In our films, Cu2+ ions exist as interstitials that share oxygen with Zn atoms; this increases O vacancies and interstitial defects, which is the main reason for the green emission. 3.2. UV photosensitivity of ZnO:Cu films Fig. 6a shows I–V curves for the ZnO:Cu film prepared at 16:10 sccm in the dark and on illumination with light of various wavelengths. The inset shows the set-up for I–V measurements. As soon as the film was illuminated, the magnitude of the current increased compared to the film in dark in a wavelength-dependent manner. As the illuminating wavelength was blue-shifted, the current increased for the same voltage. The higher current observed for the films under UV illumination reflects significant UV photosensitivity. As shown in Fig. 6a, the absolute value of the current increased slowly at first and then rapidly increased when the voltage exceeded 71 V. The main reason may be that a potential barrier of 1 eV exists between the Fermi level of the Au electrode and the conduction band of the ZnO film, as shown in Fig. 7. The work function (W) of Au is 5.1 eV and the electron affinity energy (X) of ZnO film is approximately 4.1 eV. Therefore, the current is lower for absolute voltage magnitudes o1 V. Fig. 6b shows log I–V curves for
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the ZnO:Cu films. The behavior of the current is almost symmetric for positive and negative voltages and the higher dark current is primarily due to the increase in free carrier concentration caused by Cu doping in the ZnO lattice [21]. ZnO is a wide-bandgap semiconductor and can be excited by UV irradiation for photocurrent generation. Changes in photocurrent for ZnO films are primarily caused by adsorption and desorption of oxygen molecules on the film surface [6,22,23]. In air, oxygen molecules adsorb on the surface of ZnO films. Adsorbed oxygen molecules capture electrons from the conduction band and change to O2− or O− (O2− are believed to predominate). Under UV illumination, electron–hole pairs generated by light absorption take part in reducing O2− or O− and release O2 gas and free electrons [24], resulting in an increase in film photocurrent. Photocurrent results under illumination at 380 nm for undoped ZnO and ZnO:Cu films prepared at various O2/Ar ratios are shown in Fig. 8. The photocurrent rapidly increased at first and then slowly increased asymptotically over time. When the UV light was turned off, the photocurrent rapidly decreased, followed by a slower decrease. The film prepared at 16:10 sccm had the strongest photocurrent response (Fig. 8). According the O 1s XPS analysis, the film prepared at 16:10 sccm had more O2− ions on the surface. The XRD analysis suggests that the ZnO:Cu film prepared at 16:10 sccm had the best crystal quality.
Fig. 7. Schematic of the band alignment for ZnO and the Au electrode.
Fig. 6. (a) Linear and (b) log I–V curves for ZnO:Cu films prepared at an O2/Ar ratio of 16:10 sccm in the dark and illuminated by light of various wavelengths. Inset: set-up for I–V measurements.
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Therefore, electrons from the valence band transfer to the conduction band and then the holes participate in reducing O2− or O−, leading to the release of more free electrons, under illumination at 380 nm. The lower photocurrent for films prepared at 8:10, 12:10 and 20:10 sccm can be explained as follows. O2− vacancies and interstitial deficiencies recombine with holes in the valence band under UV illumination, whereas O2− ions absorbed on the film surface are unavailable for recombination with holes and release of free electron. Moreover, empty lowerenergy Cu 3d states and [CuZn+Zni]x complex defects could capture free electrons [25], resulting in a decrease in photocurrent. For the ZnO film, the electron concentration would increase after Cu doping, so the photocurrent is higher for ZnO:Cu film prepared at 16:10 than for pure ZnO film. Fig. 9 shows the photocurrent for ZnO:Cu film (16:10 sccm) under periodic UV illumination at 380 nm. The photocurrent shows stable increases and decreases. Therefore, our ZnO:Cu films are promising candidates for UV sensors.
4. Conclusion Pure ZnO and ZnO:Cu films were deposited on p-Si (100) substrates under various oxygen partial pressures. XRD and SEM results revealed that ZnO:Cu films had a better preferential orientation along the c-axis compared with pure ZnO film. The mechanisms for blue–green emission by ZnO:Cu films were analyzed and the intensity of the emission in relation to the chemical state of the Cu ion was studied in detail. The results revealed that Cu+ and Cu2+ substitutions and interstitial Cu2+ in the ZnO:Cu films affect the concentration of intrinsic defects, leading to blue–green emission. It is reasonable to conclude that Cu ions are incorporated in the ZnO lattice and are responsible for the blue–green emission of ZnO:Cu films. The higher photocurrent observed for ZnO:Cu films under UV illumination reflects significant UV photosensitivity. High sensitivity and fast response and recovery times are important sensor parameters that are usually determined by the microstructure of the nanomaterial. The high UV sensitivity and fast response and recovery times for our ZnO:Cu films can be attributed to intrinsic donor defects and adsorption of oxygen species (O−, O2−). Our ZnO:Cu films are promising candidates for UV sensors.
Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 10874140), the College Basic Scientific Research Operation Cost of Gansu Province (manufacture and characteristic research of optical gassensing films and Y series superconducting materials), and the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry. References
Fig. 8. Photocurrent transients for undoped ZnO and ZnO:Cu films prepared at various O2/Ar ratios. Excitation was at 380 nm and the light was turned on and off as indicated. The applied potential was 0 V.
Fig. 9. Photocurrent behavior for ZnO:Cu film prepared at an O2/Ar ratio of 16:10 sccm under periodic UV illumination at 380 nm.
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