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Tunable Visible Photoluminescence from ZnO Thin Films through Mg-Doping and Annealing Shinobu Fujihara,* Yusuke Ogawa, and Asayo Kasai Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received March 11, 2004. Revised Manuscript Received May 13, 2004
Visible photoluminescence (PL) from ZnO has been found to be tunable in a wide range from blue to green and orange through chemical doping and annealing. Mg-doped, (Al, Li)doped, and undoped ZnO thin films were deposited on glass substrates by a metal-organic decomposition method at temperatures around 600 °C. The films were annealed under different atmospheres, including air, oxygen, nitrogen, and hydrogen/nitrogen. X-ray diffraction analysis and field-emission scanning electron microscope observations revealed that the films consisted of large ZnO grains 50-100 nm in size. When the Mg-doped ZnO films were annealed in nitrogen or hydrogen/nitrogen, unusual blue or bluish-white PL, respectively, was observed in response to an ultraviolet light excitation. We confirmed the band-gap broadening (approximately 0.25 eV) of the Mg-doped ZnO films as compared to that of the undoped films through observation of the absorption edge. The blue-related PL therefore appeared to be caused by energetic shifts of the valence band and/or the conduction band of ZnO. Films annealed in the oxidizing atmospheres, on the other hand, showed yellow/ orange PL. We ascribed this PL to electronic transitions between shallow and deep defect levels. Yellow PL was also observed in the (Al, Li)-doped ZnO films, suggesting that shallow donor/acceptor levels due to extrinsic defects also contributed to the yellow PL.
1. Introduction Zinc oxide (ZnO) is an n-type semiconductor with a direct band gap. It is well-known that chemical doping, as well as intrinsic lattice defects, greatly influences electronic and optical properties of ZnO. Control of defects is therefore key in achieving viable applications of ZnO. Doped ZnO thin films are of technological importance because of their great potential for applications to transparent conducting electrodes (doping group III B elements or fluorine)1,2 and insulating or ferroelectric layers (doping Li or Mg)3 in optoelectronic devices. Another important application is as luminescent layers because reduced forms of ZnO, often represented as ZnO:Zn, exhibit bright green luminescence.4,5 Generally, ZnO exhibits two kinds of emissions: one is an ultraviolet (UV) near-band-edge emission at approximately 380 nm and the other a visible deep-level emission with a peak anywhere in the range from 450 to 730 nm.6,7 The visible emissions are related to intrinsic defects or dopants in the ZnO crystal and * Corresponding author. Fax: +81-45-566-1551. E-mail: shinobu@ applc.keio.ac.jp. (1) Minami, T.; Nanto, H.; Takata, S. Jpn. J. Appl. Phys. 1984, 23, L280. (2) Hu, J.; Gordon, R. G. Sol. Cells 1991, 30, 437. (3) Joseph, M.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 1999, 74, 2534. (4) Jeon, B. S.; Yoo, J. S.; Lee, J. D. J. Electrochem. Soc. 1996, 143, 3923. (5) Nakanishi, Y.; Miyake, A.; Kominami, H.; Aoki, T.; Hatanaka, Y.; Shimaoka, G. Appl. Surf. Sci. 1999, 142, 233. (6) Vanheusden, K.; Seager, C. H.; Warren, W. L.; Tallant, D. R.; Voigt, J. A. Appl. Phys. Lett. 1996, 68, 403. (7) Nyffenegger, R. M.; Craft, B.; Shaaban, M.; Gorer, S.; Erley, G.; Penner, R. M. Chem. Mater. 1998, 10, 1120.
depend greatly on the preparation methods and conditions. For example, red luminescence can be observed in ZnO doped with Li, Na, N, P, Ne, or Bi.8-10 However, visible luminescence (except green) has relatively low intensity and its origin remains unclear. Recently, Mg-doped ZnO materials have attracted much attention because of their unique UV-luminescent properties based on radiative recombination of the electron-hole pairs.11 A peak wavelength of the UV emissions is slightly blue-shifted because of the bandgap broadening of ZnO:Mg.12-14 Recently, Zhang et al.15 have reported a new luminescent phenomenon of Zn1- xMgxO (x < 0.15) ceramic specimens sintered at temperatures of 1300-1600 °C. Bright orange photoluminescence induced by the UV excitation persists for approximately 10 min in the darkness, which can be attributed to a precipitate trapping effect of MgO on ZnO luminescence. Until now, however, visible PL has not been reported for the ZnO:Mg thin-film system. In this paper, we first report bluish and orangish-white PL from Mg-doped ZnO thin films prepared by metal(8) Schirmer, O. F.; Zwingel, D. Solid State Commun. 1970, 8, 1559. (9) Pierce, B. J.; Hengehold, R. L. J. Appl. Phys. 1976, 47, 644. (10) Garcia, J. A.; Remon, A.; Piqueras, J. J. Appl. Phys. 1987, 62, 3058. (11) Makino, T.; Tamura, K.; Chia, C. H.; Segawa, Y.; Kawasaki, M.; Ohtomo, A.; Koinuma, H. Appl. Phys. Lett. 2002, 81, 2355. (12) Kang, J. H.; Park, Y. R.; Kim, K. J. Solid State Commun. 2000, 115, 127. (13) Choopun, S.; Vispute, R. D.; Yang, W.; Sharma, R. P.; Venkatesan, T.; Shen, H. Appl. Phys. Lett. 2002, 80, 1529. (14) Heo, Y. W.; Kaufman, M.; Pruessner, K.; Norton, D. P.; Ren, F.; Chisholm, M. F.; Fleming, P. H. Solid-State Electron. 2003, 47, 2269. (15) Zhang, J.; Zhang, Z.; Wang, T. Chem. Mater. 2004, 16, 768.
10.1021/cm049599i CCC: $27.50 © 2004 American Chemical Society Published on Web 07/02/2004
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Figure 1. XRD patterns for ZnO:Mg thin films deposited on glass substrates and annealed at 600 °C in air, O2, N2, and H2/N2 atmospheres.
organic decomposition (MOD). It was found that PL colors could be tuned by annealing the films under different atmospheres such as air, O2, N2, or H2(4%)/ N2(96%). To confirm that these unique PL properties are due to the Mg-doping, undoped or (Al, Li)-doped ZnO films were also prepared for comparison. The results demonstrated that the bluish PL is caused by the bandgap broadening (approximately 0.25 eV) of the ZnO:Mg films. 2. Experimental Methods Zinc acetylacetonate monohydrate, Zn(CH3COCHCOCH3)2‚ H2O, was added to methanol and stirred for 2 h. This precursor solution was used to prepare undoped ZnO films. Magnesium acetate tetrahydrate, Mg(CH3COO)2‚4H2O, was added to a mixture of 2-propanol and water (12.5:1 in volume) and stirred for 2 h. An MOD precursor was then prepared by mixing the Zn and Mg precursors and stirring further for 24 h. A dopant concentration of Mg in ZnO was fixed at 10 at. %. The MOD precursors were deposited on quartz glass substrates by spincoating at 2000 rpm. The coated substrates were then immediately placed in a furnace preheated to 600 or 700 °C and annealed there for 10 min in air, O2, N2, or H2/N2 atmospheres, followed by quenching. This coating/heating procedure was repeated two additional times to increase the film thickness to ca. 100 nm. In preparing (Al, Li)-doped ZnO films, AlCl3‚H2O and LiCl were simultaneously added to the zinc acetylacetonate precursor. The coated substrates were dried at 250 °C before annealing to suppress lithium volatization. Phase identification of the films was performed with an X-ray diffractometer equipped with a thin-film attachment using Cu KR radiation (Rigaku). The film morphology was observed with a field-emission scanning electron microscope (FESEM) (Hitachi, type S-4700). Optical transmission spectra were recorded with a UV-visible spectrophotometer (Hitachi, type U-3300). PL spectra were measured at room temperature with a spectrofluorophotometer (Shimadzu, type RF-5300PC) using a Xe lamp (150 W) as a light source. A filter was used to remove a second-order peak of the excitation light in the PL measurement.
Figure 2. FESEM images for ZnO:Mg thin films annealed at 600 °C in (a) O2 and (b) H2/N2.
of the acetylacetonate precursor.16,17 The well-known Scherrer crystallite size was calculated to be 17.4, 14.1, 13.6, and 16.8 nm for the films annealed in air, O2, N2, and H2/N2, respectively. Undoped and (Al, Li)-doped ZnO thin films also showed similar XRD patterns indicative of the wurtzite structure. Typical FESEM images indicating the surface morphology are shown in Figure 2 for the ZnO:Mg thin films annealed at 600 °C in O2 and H2/N2. A dense microstructure can be observed for both the films with uniformly grown grains 50-100 nm in size. These structural analyses demonstrate that the ZnO:Mg thin films are composed of crystalline grains large enough to be out of the “quantum size”. That is, quantum size effects such as the blue shift of the band gap are observed in ZnO having particle sizes less than 7 nm.18 These experimental findings are of great significance, in that at least the bluish-white luminescence from the present ZnO:Mg thin film, as described later, may not be due to the quantum size effects. The optical absorption coefficient R of the direct bandgap semiconductor ZnO can be derived using the following equation,
R ) A(hν - Eopt)1/2
(1)
where A is the proportional constant, hν is the photon energy, and Eopt is the optical band gap.19 Near the absorption edge, the transmittance T is expressed by
T = exp(-Rd)
(2)
3. Results and Discussion
where d is the film thickness. Because (ln T)2 is proportional to R2,20
3.1. Film Structure and Optical Properties. Figure 1 shows the X-ray diffraction (XRD) patterns of ZnO:Mg thin films annealed at 600 °C in air, O2, N2, and H2/N2. All the patterns indicate the formation of the wurtzite-type ZnO phase. No preferential orientation was observed in these films, which was attributed to the low decomposition temperature (typically 250 °C)
(16) Fujihara, S.; Sasaki, C.; Kimura, T. Appl. Surf. Sci. 2001, 180, 341. (17) Fujihara, S.; Suzuki, A.; Kimura, T. J. Appl. Phys. 2003, 94, 2411. (18) Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507. (19) Sarkar, A.; Ghosh, S.; Chaudhuri, S.; Pal, A. K. Thin Solid Films 1991, 204, 255. (20) Hu, J.; Gordon, R. G. J. Electrochem. Soc. 1992, 139, 2014.
Tunable Photoluminescence from ZnO Thin Films
(ln T)2 ∝ R2d2 ) A2d2(hν - Eopt)
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(3)
one can determine the optical band gap by plotting (ln T)2 as a function of the photon energy and drawing a tangential line near the absorption edge. Figure 3 shows the (ln T)2 vs hν plots for the ZnO:Mg and the undoped ZnO films annealed at 600 °C in air, O2, N2, and H2/N2. While the undoped ZnO films exhibit Eopt values of 3.20-3.23 eV (Figure 3b) that agree with the bulk band gap, the ZnO:Mg films have larger values of 3.45-3.51 eV (Figure 3a). This blue shift (approximately 0.25 eV) of the band-gap energy by the 10 at. %-doping of Mg coincides with that reported for the Mg0.1Zn0.9O thin film prepared by pulsed laser deposition.21 Therefore, the present results clearly indicate that Mg can be doped in ZnO. The presence of Mg in the films has also been confirmed by X-ray photoelectron spectroscopy. 3.2. Photoluminescent Properties. Figure 4a shows the PL spectra of the ZnO:Mg films annealed at 600 °C in air, O2, N2, or H2/N2. The excitation wavelength used was 290 nm. Relatively weak UV emissions at wavelengths of 360-370 nm can be observed for all of the films. This quenching of the UV emissions indicates that crystalline ZnO:Mg prepared by the MOD method contains a large number of defects that can trap photogenerated free electrons and/or holes. Strong visible emissions can also be seen in Figure 4a, with the peak wavelengths of these emissions depending greatly on the heating atmosphere. The films annealed in air or O2 have emissions centered at 520 or 526 nm, respectively, exhibiting yellow emission colors to the naked eye. When annealed at 700 °C in air, the films exhibit an enhanced orangish-white emission with a peak wavelength of 550 nm. In contrast to the oxidizing atmospheres, annealing in the N2 and H2/N2 atmosphere produced remarkable blue-shift effects on visible emissions. As seen in Figure 4a, the film annealed in N2 has an emission peak at 468 nm, producing a blue emission color. Furthermore, a broader emission centered at 477 nm and extending up to 650 nm can be attained by annealing the film in H2/N2. This emission was found to be twice as strong as that of the N2-annealed film and provided a bluish-white color. Blue-related emissions have not been observed from the undoped ZnO films annealed under the same conditions. As shown in Figure 4b, the ZnO film annealed at 600 °C in H2/N2 usually provides a green emission with a peak wavelength of 498 nm. The film can therefore be considered to be the same as that in the ZnO:Zn form. The film annealed in N2 also exhibited a 498-nm green emission, although its intensity was much weaker. Under oxidizing (air and O2) conditions, the films exhibited weak yellow emissions with a peak at 517 or 522 nm. Thus, the visible luminescence of ZnO was found to be tunable with blue, bluish-white, green, yellow, and orangishwhite by doping Mg and changing the annealing atmosphere. Oxygen vacancies have been believed to be the main defects causing the green PL in ZnO.6,22 Among the (21) Schmidt, R.; Rheinlander, B.; Schubert, M.; Spemann, D.; Butz, T.; Lenzner, J.; Kaidashev, E. M.; Lorenz, M.; Rahm, A.; Semmelhack, H. C.; Grundmann, M. Appl. Phys. Lett. 2003, 82, 2260. (22) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983.
Figure 3. Square of ln T (T: transmittance) vs the photon energy for (a) ZnO:Mg and (b) ZnO thin films annealed in air, O2, N2, and H2/N2 atmospheres.
Figure 4. PL for (a) ZnO:Mg and (b) ZnO thin films annealed in air, O2, N2, and H2/N2 atmospheres. The excitation wavelength was 290 nm.
three different charge states, namely VO, V•O, and V••O, the singly ionized oxygen vacancy (V•O) is thought to be responsible for the green PL. Although Vanheusden et al.6,22 have claimed that the visible emissions arise from recombination of electrons in the V•O center with photoexcited holes (h+) in the valence band, Wu et al.23 have proposed a mechanism of recombination of photoexcited electrons (e-) in the conduction band with deeply trapped holes in the V•O. With temporary adoption of the former mechanism, an energy difference between the V•O level and the valence band can be estimated to be 2.49 eV (corresponding to 498 nm of the green emission peak). If we assume that the blue PL in the reduced ZnO:Mg films is also caused by the V•O-h+ recombination, the gap between the V•O level and the valence band can be derived experimentally from the present results as 2.65 eV (468 nm) or 2.60 eV (477 nm). It is interesting to note that adding half of the band(23) Wu, X. L.; Siu, G. G.; Fu, C. L.; Ong, H. C. Appl. Phys. Lett. 2001, 78, 2285.
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gap broadening (0.25/2 ) 0.125 eV), which results from the Mg-doping, to 2.49 eV (the green emission) gives 2.615 eV (the blue emission). This result may imply that the Mg-doping shifts the conduction-band and valenceband levels to the same degree in relation to the V•O level. It is obvious from the present work that the reduced and the oxidized ZnO:Mg films have similar band-gap energies. However, the oxidized films have yellow emissions (approximately 2.36 eV). It has been suggested that the yellow emission from ZnO is related to the single negatively charged interstitial oxygen ion (Oi′).23 This assignment was made because the enhancement of the yellow emission was accompanied by a reduction in the green emissions in the oxidized ZnO. This tendency has also been observed in our Mg-doped, as well as undoped, ZnO films, as indicated in Figure 4a,b. The emission peaks shift to a slightly lower energy if the heating atmosphere is changed from air to O2, indicating that the yellow emissions are possibly related to the Oi′ center. An important result regarding the yellow emissions is that the band-gap broadening due to the Mg doping has a reverse effect on the peak shift; the peaks show a red shift in response to the Mg doping. Furthermore, the shift is as small as 0.018 eV, which is approximately one-tenth the blue shift of the green emission (0.125 eV). This result encouraged us to consider that the yellow emissions could be caused by recombination of deeply trapped electrons (or holes) in the Oi′ center with the shallowly trapped holes (or electrons) at the acceptor (or donor) level. In the undoped ZnO, shallow acceptor and donor levels can be generated by intrinsic V′′Zn and Zni•• defects, respectively. It is well-known that extrinsic defects due to Li and Al doping in ZnO also produce acceptor and donor levels, respectively. We therefore examined PL properties of the (Al, Li)-doped ZnO thin films in which the Al and Li concentrations were varied independently between 0 and 3.0 at. % with an interval of 1.0 at. %, providing 16 samples in total. Figure 5 shows the typical PL spectra of the films with the Al concentration fixed to 1 at. %, with the Li concentration varied between 0, 1.0, 2.0, and 3.0 at. %. These films exhibited a yellow emission centered at approximately 525 nm (2.36 eV), although its intensity decreased with increases in the Li concentrations. All other films also showed a similar yellow emission. Basically, the emission intensities of the (Al, Li)-doped ZnO films, except the highly doped film (Al 3.0 at. % and Li 3.0 at. %), were higher than that of the undoped ZnO film. It can therefore be said that the shallow levels can indeed produce yellow emissions. We have also examined the PL properties of the (Al, Li)-doped ZnO films annealed under air, O2, N2, and H2/N2 atmospheres, respectively. However, none of
Fujihara et al.
Figure 5. PL for ZnO:(Al, Li) thin films with the Al concentration fixed to 1 at. % and the Li concentration varied between 0, 1.0, 2.0, and 3.0 at. %.
the films exhibited bluish PL. Even the films annealed in the reducing atmosphere exhibited a yellow emission centered at 525 nm. It should be noted that undoped ZnO annealed under a reducing atmosphere can emit intense green PL centered at 498 nm, as indicated in Figure 4b. Doping of the aliovalent cations in ZnO therefore quenches the green emissions. This effect is in great contrast to that of the Mg doping, which leads to blue or bluish-white emissions. 4. Conclusions Mg-doped, (Al, Li)-doped, and undoped ZnO thin films were prepared by the metal-organic decomposition method. The films were annealed under different atmospheres, air, O2, N2, or H2/N2. Doping the films with Mg resulted in band-gap broadening of approximately 0.125 eV, as compared to the undoped ZnO films. When the Mg-doped films were annealed in the reducing atmospheres, they exhibited blue or bluish-white PL in response to the UV excitation. The films annealed in the oxidizing atmospheres, on the other hand, exhibited yellow or orangish-white PL. Because the undoped ZnO film annealed in H2/N2 produced green emissions, the visible PL of ZnO could be tuned over a wide color range through the Mg doping and annealing. We discussed herein the mechanisms of the visible PL based on the energy levels of the valence and conduction bands, the shallow and deep intrinsic defects, and the extrinsic acceptors and donors. Our results suggest that the green PL from the undoped ZnO and the blue PL from the Mg-doped ZnO are related to the photoexcited electrons in the conduction band or to holes in the valence band. The yellow and orange PL are due to the shallow defect levels and the deep defect centers. Acknowledgment. This work was supported by the Mizuho Foundation for the Promotion of Sciences, Japan. CM049599I
