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Optical properties of (Mn, Co) co-doped ZnO films prepared by dual-radio frequency magnetron sputtering
Thin Solid Films 515 (2006) 2361 – 2365 www.elsevier.com/locate/tsf
Optical properties of (Mn, Co) co-doped ZnO films prepared by dual-radio frequency magnetron sputtering Zheng-bin Gu, Ming-hui Lu, Jing Wang, Chao-ling Du, Chang-sheng Yuan, Di Wu, Shan-tao Zhang, Yong-yuan Zhu, Shi-ning Zhu, Yan-feng Chen ⁎ National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China Received 18 July 2005; received in revised form 18 January 2006; accepted 12 April 2006 Available online 22 June 2006
Abstract Dual-radio frequency magnetron sputtering was employed to prepare (Mn, Co) co-doped ZnO films on sapphire (0001). X-ray diffraction and X-ray photoelectron spectroscopy results indicate that the films are highly c-axis oriented and the doped Mn and Co ions in films are both in the divalent states. A dominant photoluminescence peak was observed at the wavelength of 405 nm. Ultraviolet–Visible absorption spectra show band gaps of 2.80, 2.90, 3.27, and 3.22 eV for (Mn0.02, Co0.04) Zn0.94O, Zn0.90Co0.10O, Zn0.97Mn0.03O and ZnO films, respectively. A red-shift more than 500 meV was observed in a (Mn, Co) co-doped ZnO film. The Raman spectra measurement of (Mn, Co)ZnO films revealed additional vibrational modes at 276.6, 525.6, 634.6, and 643.9 cm− 1 compared to the host phonons of ZnO. © 2006 Elsevier B.V. All rights reserved. PACS: 61.72.-y; 78.20.-e; 78.30.-j Keywords: Sputtering; Zinc oxide; Doped II–VI semiconductor; Optical properties
1. Introduction ZnO is a well-known optoelectronic material, which belongs to the wide-band-gap semiconductor family, with a relatively large exciton binding energy (∼60 meV). It has been extensively investigated for various applications such as varistors [1], transparent transistors [2,3], sensors [4], and UV light emitting devices [5]. The prediction [6] that p-type ZnO and GaN may exhibit ferromagnetism above room temperature on doping with Mn has initiated intensive experimental work on a variety of doped dilute magnetic semiconductors (DMS) [7–13]. Recently, room temperature ferromagnetism has been reported in transition metal-doped ZnO-based films using a variety of deposition techniques [11–13], although the origin of ferromagnetism in DMS has not been clearly elucidated. So far, much of work has concentrated on the growth of high quality Co- or Mn-doped ZnO films and characterization of ⁎ Corresponding author. Tel.: +86 25 83593355; fax: +86 25 83595535. E-mail address: [email protected] (Y. Chen). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.04.030
their magnetic and optical properties. However, little attention was paid to (Mn, Co) co-doped ZnO film system, and the luminescence and Raman scattering properties of (Mnx, Coy) Zn1 − x − yO films have rarely been reported. We have prepared (Mnx, Coy)Zn1 − x − yO films on c-sapphire substrates using a dual-radio frequency (RF) magnetron sputtering system, which show room temperature ferromagnetism [14]. In this system, two targets could be sputtered simultaneously and the sputtering powers could be regulated separately during the deposition. The crystal structure, photoluminescence, absorption, and Raman scattering characteristics of (Mnx, Coy)Zn1 − x − yO films were investigated in this work. A remarkable characteristic of this sputtering system is that the doping concentrations (x and y) could be adjusted by tuning the two targets sputtering power ratio. Some interesting properties were observed in these codoped films. For instance, a relatively small band gap which is usually less than 3.0 eV was found for (Mn, Co) co-doped ZnO films. Comparing with the band gap of ZnO films (3.22 eV in our work), the largest red-shift of (Mn, Co)ZnO films is over 500 meV in our experiments.
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2. Experiment Fig. 1 shows a schematic diagram for the experimental set-up of the dual-RF magnetron sputtering system in our research. There are two sputtering targets (2 in. in diameter) in the system and their RF powers can be adjusted independently. Then the dopant concentrations in the films could be adjusted with different RF power ratios. The sputtering chamber is evacuated by a turbomolecular pump to a base pressure below 4.5 × 10− 4 Pa. The (Mnx, Coy)Zn1 − x − yO films were prepared on sapphire (α-Al2O3) (0001) substrate using two targets: Zn0.95Mn0.05O and Zn0.80Co0.20O. The targets were prepared using the standard solid-state reaction process. Fine powders of high purity (N 99.9%) Co2O3, MnO2 and ZnO with different molar ratios were mixed thoroughly and sintered at 1000 °C for 5 h in air. Argon was used as working gas and the total gas flow rate was 25 sccm. During deposition, the substrate temperature and the chamber pressure were fixed at 550 °C and 1.6 Pa, respectively. The film thickness measured using a profilometer was about 300 nm. The crystal structure of the films was characterized by Xray diffraction (XRD) using Cu Kα radiation. (Mnx, Coy)Zn1 − x − yO films were also analyzed by X-ray photoelectron spectroscopy (XPS) using a ESCALAB MK-II instrument with Mg Kα X-ray source (hν = 1253.6 eV). The concentrations of Zn, Mn, and Co ions are determined using the inductively coupled plasma–mass (ICP) spectrometer (J-A1100). To investigate the optical properties, photoluminescence (PL) spectrum in the ultraviolet–visible range was carried out using a He–Cd laser (λ = 325 nm) at room temperature. The absorption spectra of the films were measured at room temperature on an ultraviolet–visible (UV–VIS) spectrometer. Raman spectra were obtained at room temperature under a quasi-backscattering geometry using a T64000 triple Raman system of Jobin-Yvon. 3. Structures of the films Fig. 2 shows the concentrations of Mn and Co ions in films, determined by ICP measurements, as a function of RF power
Fig. 2. Variation of the Mn, Co ions concentration as a function of various sputtering powers.
ratios (Zn0.95Mn0.05O/Zn0.80Co0.20O). The figure illustrates that the concentrations vary approximately linearly as the changing of two RF power ratios. Fig. 3 shows a typical XRD pattern of (Mnx, Coy)Zn1 − x − yO films for x = 0.02 and y = 0.04. Only ZnO wurtzite phase with (0002) preferential orientation is observed. This indicates that the (Mnx, Coy)Zn1 − x − yO films are predominately caxis-oriented texture. No manganese or cobalt oxide phases were detected. A shift of (0002) peak position related to lattice spacing changes was observed [14], suggesting that the Mn and Co atoms might uniformly substitute the Zn sites in the lattice. The in-plane alignment of the films was examined using the Φ scan technique, and the result is shown in the inset of Fig. 3. Only six peaks with 60° separation are clearly observed, indicating that the wurtzite (102) plane of the films exhibits sixfold symmetry for the sapphire (0001) substrates. The incorporation of Mn, Co ions into the ZnO lattice and their valence states was also characterized by XPS measurements. The binding energies of the Zn 2p3/2, Mn 2p3/2 and Co 2p3/2 peak are located at 1020.8, 640.2, and 779.0 eV, respectively, which shows that the valence state of Mn and Co ions in (Mn, Co) co-doped ZnO films are mainly in divalent states [15–17]. Above results demonstrate an excellent growth of Mn and Co co-doped ZnO films by dual-RF magnetron sputtering technique. 4. Optical properties of the films The ZnO thin films are transparent and colorless, whereas the Zn1 − xMnxO and Zn1 − xCoxO films are transparent with a straw
Fig. 1. Schematic diagram of the dual-RF magnetron sputtering system.
Fig. 3. XRD pattern of (Mn0.02, Co0.04)Zn0.94O thin films; Inset shows the Φ scan of the wurtzite (102) plane of the (Mn0.02, Co0.04)Zn0.94O film.
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yellow and a dark green color, respectively. The color of (Mnx, Coy)Zn1 − x − yO films are close to that of Zn1 − xCoxO films due to the relatively high concentration of cobalt. The room temperature PL spectrum of the (Mn0.02, Co0.04)Zn0.94O film is shown in Fig. 4. The film reveals a broad band PL emission ranging from 360 to 600 nm, centered at about 405 nm (3.06 eV). When the spectrum was extracted by Gauss fitting, three peaks at the wavelength of 397, 431, and 502 nm were observed, respectively. The peak at the wavelength of 397 nm (3.12 eV) might be assigned to the near-band edge transition of free excitons. The deep level emissions around 431 nm (2.88 eV) and 502 nm (2.47 eV) might be ascribed to the intrinsic defects in ZnO material, for instance, oxygen vacancy (VO), interstitial zinc (Zni), and antisite oxygen (OZn), etc. Fan et al. [18] had calculated the energy levels of the intrinsic defects in ZnO by applying the fullpotential linear muffin-tin orbital method. In their results the energy interval from the top of the valence band to the Zni level is 2.9 eV, which is coincidence with the energy of the deep level emission at 431 nm in our experiments. The energy of the deep level emission at 502 nm is in coincidence with the energy interval from the bottom of the conduction band to the OZn (2.38 eV) according to the calculated results and other reports [19]. We believe that the deep level emission peaks at 431 nm and 502 nm originate mainly from Zni and OZn defects, respectively. The UV–VIS absorption spectrum of the (Mn0.02, Co0.04) Zn0.94O film is shown in Fig. 5(a). The spectra of a ZnO film and a Zn0.90Co0.10O film are also shown as reference. The ZnO film shows a sharp absorption edge at about 385 nm (3.22 eV). In contrast, the absorption edge of the (Mn0.02, Co0.04)Zn0.94O thin film is red-shifted to 440 nm (2.80 eV) and the absorption edge is less sharp due to the Mn and Co states extending into the band gap. The absorption spectra of (Mn0.02, Co0.04)Zn0.94O and Zn0.90Co0.10O films showed the significant band tailing in the near-band-edge region. This band tailing is attributable to point defects in the films. The absorption bands at around 566, 611, and 660 nm, as shown in Fig. 5(a), are attributed to the d–d transitions of tetrahedrally coordinated Co2+ [20,21]. They are assigned as 4A2(F)→2E(G) (1.88 eV), 4A2(F)→4T1(P) (2.02 eV), and 4A2
Fig. 5. (a) Absorption spectrum of (Mn0.02, Co0.04)Zn0.94O film compared with that of ZnO and Zn0.90Co0.10O films. (b) (αhí)2 vs. hí plot of (Mn0.02, Co0.04) Zn0.94O film compared with that of ZnO, Zn0.90Co0.10O, and Zn0.97Mn0.03O films. (c) (αhí)2 vs. hí plots of (Mnx, Coy)Zn1 − x − yO films with different dopant content x and y.
Fig. 4. Room-temperature photoluminescence spectrum of (Mn0.02, Co0.04) Zn0.94O.
(F)→2A1(G) (2.18 eV) transitions in high spin state Co2+ (d7), respectively [20]. These d–d transitions indicate that Co ions are in the divalent state at the Zn site, which is consistent with XPS result. The d–d transitions of tetrahedral Mn2+ between the crystal-field-split 3d5 multiplet levels: 6A1(S)→4E(G) (2.97 eV) and/
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Fig. 6. Raman spectra of ZnO, Zn0.90Co0.10O, Zn0.97Mn0.03O, and (Mn0.02, Co0.04)Zn0.94O films. Dashed and dotted circles indicated the wave numbers of ZnO and sapphire substrate modes, respectively. Solid circles denote additional modes.
or 6A1(S)→4A1(G) (2.99 eV), are not observed in (Mn, Co)ZnO films, which probably have overlapped with the direct absorption edge. The absorption of ZnO film is a direct transition and it has an absorption coefficient (α) obeying the following relation when exciting photon energies (hν) is relatively high [22]: (hνα)2 = A (hν − Eg), where Eg is the band gap, and A is a constant. The curves of (hνα)2 vs. hν of (Mn0.02, Co0.04)Zn0.94O, Zn0.90Co0.10O, Zn0.97Mn0.03O and ZnO films are shown in Fig. 5(b). The values of Eg are evaluated by extrapolation of the linear part to be about 2.80, 2.90, 3.27 and 3.22 eV for (Mn0.02, Co0.04)Zn0.94O, Zn0.90Co0.10O, Zn0.98Mn0.03O and ZnO films, respectively. These results illustrate that the band gap will increase (blue-shift) if Mn2+ ions are doped into the ZnO film, while the doping of Co2+ ions into the ZnO film will result in a decrease of the band gap (red-shift). Contrary to the predictions, the band gap of (Mn0.02, Co0.04) Zn0.94O film is not higher than that of Zn0.90Co0.10O film, though a relatively higher content of Mn2+ ions in the film. The red-shift of Eg edge is interpreted as being due to the spin-exchange interaction between the sp band and localized spins of the transition-metal ions [23]. The exchange interaction between the transition-metal ions and the band electrons (the so-called s–d and p–d interactions) gives rise to a negative and a positive correction to the energy of the conduction and valence bands, respectively, and lead to decreasing of the band gap. The larger red-shift in (Mnx, Coy)Zn1 − x − yO compared to that in Zn1 − xCoxO indicates a higher spin-exchange interaction in (Mnx, Coy)Zn1 − x − yO than that in Zn1 − xCoxO. Furthermore, the increase of the lattice constant due to the large Mn2+ ions into the lattice might also contribute to the red-shift of the band gap of (Mnx, Coy)Zn1 − x − yO films. When altering the sputtering power ratios, the values of the band gap of (Mnx, Coy)Zn1 − x − yO films were shifting in the range of 2.56 to 3.00 eV due to the different dopant contents (x and y), as shown in Fig. 5(c). ZnO has a hexagonal wurtzite structure and belongs to the C6ν symmetry group with two formula units in the primitive cell. According to the selection rule, only E2 and A1(LO) modes can be observed in the unpolarized Raman spectra of bulk ZnO under backscattering configuration. However, this is not always the case. When the dimension is reduced to nanoscale, not only
the first-order vibration modes will appear with shifting and broadening but also some vibration modes will appear which is forbidden in symmetry geometries. Fig. 6 shows the unpolarized Raman scattering spectra performed at room temperature. For ZnO films, the Raman band at 437.1 cm− 1 is attributed to the high frequency E2 mode [24–26], however, the low frequency E2 mode is out of the detecting range in this work. The Raman bands at 381.1 and 577.5 cm− 1 might be ascribed to the transverse and longitudinal optical phonon modes with A1 symmetry, respectively. However, these bands might be partially overlapped with sapphire modes (Eg: 379 and 576 cm− 1) [25]. The bands at 418.8, 449.8 and 750.8 cm− 1 are ascribed to sapphire substrate. The Raman bands from sapphire have a low intensity in doped ZnO film. This is due to the higher absorption of dopant in films. In the Raman spectra of Mn-doped and (Mn, Co) co-doped ZnO films, two additional modes at 276.6 cm− 1 and 525.6 cm− 1 could be observed. Furthermore, in the (Mn, Co) co-doped ZnO films, other two modes at 634.6 and 643.9 cm− 1 could be found. The additional mode at 276.6 cm− 1 is often observed in doped ZnO films [24–26]. Thus we think it is related to the intrinsic host defects, for instance, oxygen vacancies. A higher intensity of the vibration mode at 276.6 cm− 1 was observed for (Mn, Co) co-doped ZnO films, indicating a higher concentration of oxygen vacancies might be in (Mn, Co)ZnO films than that in ZnMnO and ZnCoO films. As a donor, more oxygen vacancies lead to higher conductive band electron density, which will enhance the sp–d interactions and lead to a red-shift of the band gap. The vibration mode at 525.6 cm− 1 was also observed at Mndoped nanostructures at 522 cm− 1 [24], and it was considered to be associated with Mn impurities. Because the Mn2+ ionic radius (80 pm) is larger than that of Zn2+ (74 pm), some new lattice defects are introduced or intrinsic host-lattice defects are activated when Mn2+ ions occupy the Zn sites. In the contrary, no additional vibration modes could be found in the Raman spectra of Zn0.90Co0.10O films compared with the vibration modes of ZnO films. This result indicated that Co-doped ZnO films keep better crystallinity than that of the Mn-doped ZnO films because the Co2+ ionic radius (72 pm) is close to that of Zn2+. However, for Zn0.90Co0.10O film, the full width at half maximum (FWHM) of the vibration mode at 577.5 cm− 1 is larger than that of ZnO film. The disorder caused by the doping of Co ions into the ZnO lattice result in the broadening of this band. Furthermore, the lighter ionic weight would result in the lattice relaxation and the shift of Raman band, which is also a reason for this broadening. For the vibration mode at 643.9 cm− 1, it was also observed in Fe-, Sb-, Al-, Ga-, and N-doped ZnO films by Bundesmann et al. [25], and they proposed that this mode is related to intrinsic host-lattice defects. They also observed some additional vibration mode at 531, 631 and 720 cm− 1 in Sb-, Ga- and Fedoped ZnO films and proposed that these modes are related to a single dopant and may be used as indicators for identifying their incorporation. However, the vibration mode of 634.6 cm− 1 in (Mn, Co) co-doped ZnO films is much closer to the reported mode of 631 cm− 1 in the Ga-doped ZnO films. More work is still needed to exactly identify its origin.
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5. Conclusions High-quality (Mn, Co) co-doped ZnO films were fabricated on c-sapphire by magnetron co-sputtering technique. The (Mn, Co)ZnO films have a dominant photoluminescence peak at the wavelength of 405 nm. The band gap of (Mn, Co)ZnO films is usually less than 3.0 eV. The red-shift of band gap indicates a higher spin-exchange interaction in (Mn, Co)ZnO films. In addition to the host phonons of ZnO, there are four additional vibrational modes at 276.6, 525.6, 634.6, and 643.9 cm− 1 in the Raman spectra of (Mn, Co)ZnO films. The origin of the vibration mode of 634.6 cm− 1 is not clearly identified. Acknowledgment This work was jointly supported by the program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), the State Key Program for Basic Research of China and the National Nature Science Foundation of China. References [1] Y. Sato, M. Yodogawa, T. Yamamoto, N. Shibata, Y. Ikuhara, Appl. Phys. Lett. 86 (2005) 152112. [2] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonçalves, A.J.S. Marques, R.F.P. Martins, L.M.N. Pereira, Appl. Phys. Lett. 85 (2004) 2541. [3] Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 87 (2005) 153504. [4] P.D. Batista, M. Mulato, Appl. Phys. Lett. 87 (2005) 143508. [5] H. Ohta, M. Hirano, K. Nakahara, H. Maruta, T. Tanabe, M. Kamiya, T. Kamiya, H. Hosono, Appl. Phys. Lett. 83 (2003) 1029.
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[6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [7] J. Honga, R.Q. Wu, J. Appl. Phys. 97 (2005) 63911. [8] M.H. Kane, K. Shalini, C.J. Summers, R. Varatharajan, J. Nause, C.R. Vestal, Z.J. Zhang, I.T. Fergusona, J. Appl. Phys. 97 (2005) 23906. [9] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M.O. Guillen, B. Johansson, G.A. Gehring, Nat. Mater. 2 (2003) 673. [10] N. Hasuike, H. Fukumura, H. Harima, K. Kisoda, M. Hashimoto, Y.K. Zhou, H. Asahi, J. Phys., Condens. Matter 16 (2004) S5811. [11] A.B. Pakhomov, B.K. Roberts, A. Tuan, V. Shutthanandan, D. McCready, S. Thevuthasan, S.A. Chambers, K.M. Krishnana, J. Appl. Phys. 95 (2004) 7393. [12] Y.W. Heo, M.P. Ivill, K. Ip, D.P. Norton, S.J. Pearton, J.G. Kelly, R. Rairigh, A.F. Hebard, T. Steiner, Appl. Phys. Lett. 84 (2004) 2292. [13] S.W. Jung, S.-J. An, G.-C. Yi, C.U. Jung, S.-I. Lee, S. Cho, Appl. Phys. Lett. 80 (2002) 4561. [14] Z.-B. Gu, C.-S. Yuan, M.-H. Lu, J. Wang, D. Wu, S.-T. Zhang, S.-N. Zhu, Y.-Y. Zhu, Y.-F. Chen, J. Appl. Phys. 98 (2005) 53908. [15] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura, M. Kawasaki, Phys. Rev., B 65 (2002) 85209. [16] G. Lee, S.-J. Oh, Phys. Rev., B 43 (1991) 14674. [17] J. Okabayashi, K. Ono, M. Mizuguchi, M. Oshima, S.S. Guptab, D.D. Sarma, T. Mizokawa, A. Fujimori, M. Yuri, C.T. Chen, T. Fukumura, M. Kawasaki, H. Koinuma, J. Appl. Phys. 95 (2004) 3573. [18] X.M. Fan, J.S. Lian, Z.X. Guo, H.J. Lu, Appl. Surf. Sci. 239 (2005) 176. [19] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [20] P. Koidl, Phys. Rev., B 15 (1977) 2493. [21] Y.-Z. Yoo, T. Fukumura, Z. Jin, K. Hasegawa, M. Kawasaki, P. Ahmet, T. Chikyow, H. Koinuma, J. Appl. Phys. 90 (2001) 4246. [22] M.N. Kamalasanan, N.O. Kumar, S. Chandra, J. Appl. Phys. 74 (1993) 679. [23] Y.R. Lee, A.K. Ramdas, R.L. Aggarwal, Phys. Rev., B 38 (1988) 10600. [24] L.W. Yang, X.L. Wu, G.S. Huang, T. Qiu, Y.M. Yang, J. Appl. Phys. 97 (2003) 14308. [25] C. Bundesmann, N. Ashkenov, M. Schubert, D. Spemann, T. Butz, E.M. Kaidashev, M. Lorenz, M. Grundmann, Appl. Phys. Lett. 83 (2003) 1974. [26] N. Hasuike, H. Fukumura, H. Harima, K. Kisoda, H. Matsui, H. Saeki, H. Tabata, J. Phys., Condens. Matter 16 (2004) S5807.
Optical properties of (Mn, Co) co-doped ZnO films prepared by dual-radio frequency magnetron sputtering Zheng-bin Gu, Ming-hui Lu, Jing Wang, Chao-ling Du, Chang-sheng Yuan, Di Wu, Shan-tao Zhang, Yong-yuan Zhu, Shi-ning Zhu, Yan-feng Chen ⁎ National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China Received 18 July 2005; received in revised form 18 January 2006; accepted 12 April 2006 Available online 22 June 2006
Abstract Dual-radio frequency magnetron sputtering was employed to prepare (Mn, Co) co-doped ZnO films on sapphire (0001). X-ray diffraction and X-ray photoelectron spectroscopy results indicate that the films are highly c-axis oriented and the doped Mn and Co ions in films are both in the divalent states. A dominant photoluminescence peak was observed at the wavelength of 405 nm. Ultraviolet–Visible absorption spectra show band gaps of 2.80, 2.90, 3.27, and 3.22 eV for (Mn0.02, Co0.04) Zn0.94O, Zn0.90Co0.10O, Zn0.97Mn0.03O and ZnO films, respectively. A red-shift more than 500 meV was observed in a (Mn, Co) co-doped ZnO film. The Raman spectra measurement of (Mn, Co)ZnO films revealed additional vibrational modes at 276.6, 525.6, 634.6, and 643.9 cm− 1 compared to the host phonons of ZnO. © 2006 Elsevier B.V. All rights reserved. PACS: 61.72.-y; 78.20.-e; 78.30.-j Keywords: Sputtering; Zinc oxide; Doped II–VI semiconductor; Optical properties
1. Introduction ZnO is a well-known optoelectronic material, which belongs to the wide-band-gap semiconductor family, with a relatively large exciton binding energy (∼60 meV). It has been extensively investigated for various applications such as varistors [1], transparent transistors [2,3], sensors [4], and UV light emitting devices [5]. The prediction [6] that p-type ZnO and GaN may exhibit ferromagnetism above room temperature on doping with Mn has initiated intensive experimental work on a variety of doped dilute magnetic semiconductors (DMS) [7–13]. Recently, room temperature ferromagnetism has been reported in transition metal-doped ZnO-based films using a variety of deposition techniques [11–13], although the origin of ferromagnetism in DMS has not been clearly elucidated. So far, much of work has concentrated on the growth of high quality Co- or Mn-doped ZnO films and characterization of ⁎ Corresponding author. Tel.: +86 25 83593355; fax: +86 25 83595535. E-mail address: [email protected] (Y. Chen). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.04.030
their magnetic and optical properties. However, little attention was paid to (Mn, Co) co-doped ZnO film system, and the luminescence and Raman scattering properties of (Mnx, Coy) Zn1 − x − yO films have rarely been reported. We have prepared (Mnx, Coy)Zn1 − x − yO films on c-sapphire substrates using a dual-radio frequency (RF) magnetron sputtering system, which show room temperature ferromagnetism [14]. In this system, two targets could be sputtered simultaneously and the sputtering powers could be regulated separately during the deposition. The crystal structure, photoluminescence, absorption, and Raman scattering characteristics of (Mnx, Coy)Zn1 − x − yO films were investigated in this work. A remarkable characteristic of this sputtering system is that the doping concentrations (x and y) could be adjusted by tuning the two targets sputtering power ratio. Some interesting properties were observed in these codoped films. For instance, a relatively small band gap which is usually less than 3.0 eV was found for (Mn, Co) co-doped ZnO films. Comparing with the band gap of ZnO films (3.22 eV in our work), the largest red-shift of (Mn, Co)ZnO films is over 500 meV in our experiments.
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2. Experiment Fig. 1 shows a schematic diagram for the experimental set-up of the dual-RF magnetron sputtering system in our research. There are two sputtering targets (2 in. in diameter) in the system and their RF powers can be adjusted independently. Then the dopant concentrations in the films could be adjusted with different RF power ratios. The sputtering chamber is evacuated by a turbomolecular pump to a base pressure below 4.5 × 10− 4 Pa. The (Mnx, Coy)Zn1 − x − yO films were prepared on sapphire (α-Al2O3) (0001) substrate using two targets: Zn0.95Mn0.05O and Zn0.80Co0.20O. The targets were prepared using the standard solid-state reaction process. Fine powders of high purity (N 99.9%) Co2O3, MnO2 and ZnO with different molar ratios were mixed thoroughly and sintered at 1000 °C for 5 h in air. Argon was used as working gas and the total gas flow rate was 25 sccm. During deposition, the substrate temperature and the chamber pressure were fixed at 550 °C and 1.6 Pa, respectively. The film thickness measured using a profilometer was about 300 nm. The crystal structure of the films was characterized by Xray diffraction (XRD) using Cu Kα radiation. (Mnx, Coy)Zn1 − x − yO films were also analyzed by X-ray photoelectron spectroscopy (XPS) using a ESCALAB MK-II instrument with Mg Kα X-ray source (hν = 1253.6 eV). The concentrations of Zn, Mn, and Co ions are determined using the inductively coupled plasma–mass (ICP) spectrometer (J-A1100). To investigate the optical properties, photoluminescence (PL) spectrum in the ultraviolet–visible range was carried out using a He–Cd laser (λ = 325 nm) at room temperature. The absorption spectra of the films were measured at room temperature on an ultraviolet–visible (UV–VIS) spectrometer. Raman spectra were obtained at room temperature under a quasi-backscattering geometry using a T64000 triple Raman system of Jobin-Yvon. 3. Structures of the films Fig. 2 shows the concentrations of Mn and Co ions in films, determined by ICP measurements, as a function of RF power
Fig. 2. Variation of the Mn, Co ions concentration as a function of various sputtering powers.
ratios (Zn0.95Mn0.05O/Zn0.80Co0.20O). The figure illustrates that the concentrations vary approximately linearly as the changing of two RF power ratios. Fig. 3 shows a typical XRD pattern of (Mnx, Coy)Zn1 − x − yO films for x = 0.02 and y = 0.04. Only ZnO wurtzite phase with (0002) preferential orientation is observed. This indicates that the (Mnx, Coy)Zn1 − x − yO films are predominately caxis-oriented texture. No manganese or cobalt oxide phases were detected. A shift of (0002) peak position related to lattice spacing changes was observed [14], suggesting that the Mn and Co atoms might uniformly substitute the Zn sites in the lattice. The in-plane alignment of the films was examined using the Φ scan technique, and the result is shown in the inset of Fig. 3. Only six peaks with 60° separation are clearly observed, indicating that the wurtzite (102) plane of the films exhibits sixfold symmetry for the sapphire (0001) substrates. The incorporation of Mn, Co ions into the ZnO lattice and their valence states was also characterized by XPS measurements. The binding energies of the Zn 2p3/2, Mn 2p3/2 and Co 2p3/2 peak are located at 1020.8, 640.2, and 779.0 eV, respectively, which shows that the valence state of Mn and Co ions in (Mn, Co) co-doped ZnO films are mainly in divalent states [15–17]. Above results demonstrate an excellent growth of Mn and Co co-doped ZnO films by dual-RF magnetron sputtering technique. 4. Optical properties of the films The ZnO thin films are transparent and colorless, whereas the Zn1 − xMnxO and Zn1 − xCoxO films are transparent with a straw
Fig. 1. Schematic diagram of the dual-RF magnetron sputtering system.
Fig. 3. XRD pattern of (Mn0.02, Co0.04)Zn0.94O thin films; Inset shows the Φ scan of the wurtzite (102) plane of the (Mn0.02, Co0.04)Zn0.94O film.
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yellow and a dark green color, respectively. The color of (Mnx, Coy)Zn1 − x − yO films are close to that of Zn1 − xCoxO films due to the relatively high concentration of cobalt. The room temperature PL spectrum of the (Mn0.02, Co0.04)Zn0.94O film is shown in Fig. 4. The film reveals a broad band PL emission ranging from 360 to 600 nm, centered at about 405 nm (3.06 eV). When the spectrum was extracted by Gauss fitting, three peaks at the wavelength of 397, 431, and 502 nm were observed, respectively. The peak at the wavelength of 397 nm (3.12 eV) might be assigned to the near-band edge transition of free excitons. The deep level emissions around 431 nm (2.88 eV) and 502 nm (2.47 eV) might be ascribed to the intrinsic defects in ZnO material, for instance, oxygen vacancy (VO), interstitial zinc (Zni), and antisite oxygen (OZn), etc. Fan et al. [18] had calculated the energy levels of the intrinsic defects in ZnO by applying the fullpotential linear muffin-tin orbital method. In their results the energy interval from the top of the valence band to the Zni level is 2.9 eV, which is coincidence with the energy of the deep level emission at 431 nm in our experiments. The energy of the deep level emission at 502 nm is in coincidence with the energy interval from the bottom of the conduction band to the OZn (2.38 eV) according to the calculated results and other reports [19]. We believe that the deep level emission peaks at 431 nm and 502 nm originate mainly from Zni and OZn defects, respectively. The UV–VIS absorption spectrum of the (Mn0.02, Co0.04) Zn0.94O film is shown in Fig. 5(a). The spectra of a ZnO film and a Zn0.90Co0.10O film are also shown as reference. The ZnO film shows a sharp absorption edge at about 385 nm (3.22 eV). In contrast, the absorption edge of the (Mn0.02, Co0.04)Zn0.94O thin film is red-shifted to 440 nm (2.80 eV) and the absorption edge is less sharp due to the Mn and Co states extending into the band gap. The absorption spectra of (Mn0.02, Co0.04)Zn0.94O and Zn0.90Co0.10O films showed the significant band tailing in the near-band-edge region. This band tailing is attributable to point defects in the films. The absorption bands at around 566, 611, and 660 nm, as shown in Fig. 5(a), are attributed to the d–d transitions of tetrahedrally coordinated Co2+ [20,21]. They are assigned as 4A2(F)→2E(G) (1.88 eV), 4A2(F)→4T1(P) (2.02 eV), and 4A2
Fig. 5. (a) Absorption spectrum of (Mn0.02, Co0.04)Zn0.94O film compared with that of ZnO and Zn0.90Co0.10O films. (b) (αhí)2 vs. hí plot of (Mn0.02, Co0.04) Zn0.94O film compared with that of ZnO, Zn0.90Co0.10O, and Zn0.97Mn0.03O films. (c) (αhí)2 vs. hí plots of (Mnx, Coy)Zn1 − x − yO films with different dopant content x and y.
Fig. 4. Room-temperature photoluminescence spectrum of (Mn0.02, Co0.04) Zn0.94O.
(F)→2A1(G) (2.18 eV) transitions in high spin state Co2+ (d7), respectively [20]. These d–d transitions indicate that Co ions are in the divalent state at the Zn site, which is consistent with XPS result. The d–d transitions of tetrahedral Mn2+ between the crystal-field-split 3d5 multiplet levels: 6A1(S)→4E(G) (2.97 eV) and/
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Fig. 6. Raman spectra of ZnO, Zn0.90Co0.10O, Zn0.97Mn0.03O, and (Mn0.02, Co0.04)Zn0.94O films. Dashed and dotted circles indicated the wave numbers of ZnO and sapphire substrate modes, respectively. Solid circles denote additional modes.
or 6A1(S)→4A1(G) (2.99 eV), are not observed in (Mn, Co)ZnO films, which probably have overlapped with the direct absorption edge. The absorption of ZnO film is a direct transition and it has an absorption coefficient (α) obeying the following relation when exciting photon energies (hν) is relatively high [22]: (hνα)2 = A (hν − Eg), where Eg is the band gap, and A is a constant. The curves of (hνα)2 vs. hν of (Mn0.02, Co0.04)Zn0.94O, Zn0.90Co0.10O, Zn0.97Mn0.03O and ZnO films are shown in Fig. 5(b). The values of Eg are evaluated by extrapolation of the linear part to be about 2.80, 2.90, 3.27 and 3.22 eV for (Mn0.02, Co0.04)Zn0.94O, Zn0.90Co0.10O, Zn0.98Mn0.03O and ZnO films, respectively. These results illustrate that the band gap will increase (blue-shift) if Mn2+ ions are doped into the ZnO film, while the doping of Co2+ ions into the ZnO film will result in a decrease of the band gap (red-shift). Contrary to the predictions, the band gap of (Mn0.02, Co0.04) Zn0.94O film is not higher than that of Zn0.90Co0.10O film, though a relatively higher content of Mn2+ ions in the film. The red-shift of Eg edge is interpreted as being due to the spin-exchange interaction between the sp band and localized spins of the transition-metal ions [23]. The exchange interaction between the transition-metal ions and the band electrons (the so-called s–d and p–d interactions) gives rise to a negative and a positive correction to the energy of the conduction and valence bands, respectively, and lead to decreasing of the band gap. The larger red-shift in (Mnx, Coy)Zn1 − x − yO compared to that in Zn1 − xCoxO indicates a higher spin-exchange interaction in (Mnx, Coy)Zn1 − x − yO than that in Zn1 − xCoxO. Furthermore, the increase of the lattice constant due to the large Mn2+ ions into the lattice might also contribute to the red-shift of the band gap of (Mnx, Coy)Zn1 − x − yO films. When altering the sputtering power ratios, the values of the band gap of (Mnx, Coy)Zn1 − x − yO films were shifting in the range of 2.56 to 3.00 eV due to the different dopant contents (x and y), as shown in Fig. 5(c). ZnO has a hexagonal wurtzite structure and belongs to the C6ν symmetry group with two formula units in the primitive cell. According to the selection rule, only E2 and A1(LO) modes can be observed in the unpolarized Raman spectra of bulk ZnO under backscattering configuration. However, this is not always the case. When the dimension is reduced to nanoscale, not only
the first-order vibration modes will appear with shifting and broadening but also some vibration modes will appear which is forbidden in symmetry geometries. Fig. 6 shows the unpolarized Raman scattering spectra performed at room temperature. For ZnO films, the Raman band at 437.1 cm− 1 is attributed to the high frequency E2 mode [24–26], however, the low frequency E2 mode is out of the detecting range in this work. The Raman bands at 381.1 and 577.5 cm− 1 might be ascribed to the transverse and longitudinal optical phonon modes with A1 symmetry, respectively. However, these bands might be partially overlapped with sapphire modes (Eg: 379 and 576 cm− 1) [25]. The bands at 418.8, 449.8 and 750.8 cm− 1 are ascribed to sapphire substrate. The Raman bands from sapphire have a low intensity in doped ZnO film. This is due to the higher absorption of dopant in films. In the Raman spectra of Mn-doped and (Mn, Co) co-doped ZnO films, two additional modes at 276.6 cm− 1 and 525.6 cm− 1 could be observed. Furthermore, in the (Mn, Co) co-doped ZnO films, other two modes at 634.6 and 643.9 cm− 1 could be found. The additional mode at 276.6 cm− 1 is often observed in doped ZnO films [24–26]. Thus we think it is related to the intrinsic host defects, for instance, oxygen vacancies. A higher intensity of the vibration mode at 276.6 cm− 1 was observed for (Mn, Co) co-doped ZnO films, indicating a higher concentration of oxygen vacancies might be in (Mn, Co)ZnO films than that in ZnMnO and ZnCoO films. As a donor, more oxygen vacancies lead to higher conductive band electron density, which will enhance the sp–d interactions and lead to a red-shift of the band gap. The vibration mode at 525.6 cm− 1 was also observed at Mndoped nanostructures at 522 cm− 1 [24], and it was considered to be associated with Mn impurities. Because the Mn2+ ionic radius (80 pm) is larger than that of Zn2+ (74 pm), some new lattice defects are introduced or intrinsic host-lattice defects are activated when Mn2+ ions occupy the Zn sites. In the contrary, no additional vibration modes could be found in the Raman spectra of Zn0.90Co0.10O films compared with the vibration modes of ZnO films. This result indicated that Co-doped ZnO films keep better crystallinity than that of the Mn-doped ZnO films because the Co2+ ionic radius (72 pm) is close to that of Zn2+. However, for Zn0.90Co0.10O film, the full width at half maximum (FWHM) of the vibration mode at 577.5 cm− 1 is larger than that of ZnO film. The disorder caused by the doping of Co ions into the ZnO lattice result in the broadening of this band. Furthermore, the lighter ionic weight would result in the lattice relaxation and the shift of Raman band, which is also a reason for this broadening. For the vibration mode at 643.9 cm− 1, it was also observed in Fe-, Sb-, Al-, Ga-, and N-doped ZnO films by Bundesmann et al. [25], and they proposed that this mode is related to intrinsic host-lattice defects. They also observed some additional vibration mode at 531, 631 and 720 cm− 1 in Sb-, Ga- and Fedoped ZnO films and proposed that these modes are related to a single dopant and may be used as indicators for identifying their incorporation. However, the vibration mode of 634.6 cm− 1 in (Mn, Co) co-doped ZnO films is much closer to the reported mode of 631 cm− 1 in the Ga-doped ZnO films. More work is still needed to exactly identify its origin.
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5. Conclusions High-quality (Mn, Co) co-doped ZnO films were fabricated on c-sapphire by magnetron co-sputtering technique. The (Mn, Co)ZnO films have a dominant photoluminescence peak at the wavelength of 405 nm. The band gap of (Mn, Co)ZnO films is usually less than 3.0 eV. The red-shift of band gap indicates a higher spin-exchange interaction in (Mn, Co)ZnO films. In addition to the host phonons of ZnO, there are four additional vibrational modes at 276.6, 525.6, 634.6, and 643.9 cm− 1 in the Raman spectra of (Mn, Co)ZnO films. The origin of the vibration mode of 634.6 cm− 1 is not clearly identified. Acknowledgment This work was jointly supported by the program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), the State Key Program for Basic Research of China and the National Nature Science Foundation of China. References [1] Y. Sato, M. Yodogawa, T. Yamamoto, N. Shibata, Y. Ikuhara, Appl. Phys. Lett. 86 (2005) 152112. [2] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonçalves, A.J.S. Marques, R.F.P. Martins, L.M.N. Pereira, Appl. Phys. Lett. 85 (2004) 2541. [3] Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 87 (2005) 153504. [4] P.D. Batista, M. Mulato, Appl. Phys. Lett. 87 (2005) 143508. [5] H. Ohta, M. Hirano, K. Nakahara, H. Maruta, T. Tanabe, M. Kamiya, T. Kamiya, H. Hosono, Appl. Phys. Lett. 83 (2003) 1029.
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