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Preparation and optical properties of ZnGa2O4:Cr3+ thin films derived by sol–gel process
Applied Surface Science 256 (2010) 4702–4707
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and optical properties of ZnGa2 O4 :Cr3+ thin films derived by sol–gel process Weiwei Zhang, Junying Zhang ∗ , Yuan Li, Ziyu Chen, Tianmin Wang School of Physics and Nuclear Energy Engineering, Beihang University, No. 37 XueYuan Road, HaiDian District, Beijing 100191, PR China
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Article history: Received 22 December 2009 Received in revised form 23 February 2010 Accepted 23 February 2010 Available online 3 March 2010 Keywords: ZnGa2 O4 :Cr3+ Photoluminescence Thin film Sol–gel process
a b s t r a c t ZnGa2 O4 :Cr3+ thin films with bright red emission were synthesized using a sol–gel process, characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Auger electron spectroscopy (AES) and UV–vis and fluorescence spectrophotometry measurements. Effects of calcining temperature, film thickness, calcining duration and substrates on the crystal structure and photoluminescent property have been investigated. It is found that the crystallinity, Ga/Zn ratio and band gap energy (Eg ) are significant factors influencing optical characteristics, while the nature of substrates affect the surface morphologies of ZnGa2 O4 :Cr3+ thin films. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, oxide phosphors have been extensively investigated for their use in flat panel displays (FPDs). ZnGa2 O4 based phosphors have received considerable attention due to the good luminescent characteristics, chemical and thermal stabilities in high vacuum and absence of corrosive gas emission under electron bombardment in comparison with currently used sulfide based phosphors [1]. ZnGa2 O4 has a spinel structure (AB2 O4 ), in which Zn2+ ions occupy the tetrahedrally coordinated A-sites (Td ), while Ga3+ in the octahedrally coordinated B-sites (Oh ) [2]. Under excitation by both ultraviolet light and low voltage electron, ZnGa2 O4 exhibits a strong blue emission due to transition via a selfactivation (SA) center [3,4]. It also shows various emissions from green to red when doped with Mn2+ or Cr3+ , respectively [5–7]. Recently ZnGa2 O4 -based white light emitting phosphors has been developed [8]. Due to the equal valence and similar ionic radii of Ga3+ (0.62 Å) and Cr3+ (0.64 Å), Cr3+ can replace the Ga3+ sites in the host lattice and has a high doping concentration, promoting a high luminescent intensity. The use of thin films of ZnGa2 O4 :Cr3+ phosphor has some advantage in the FPDs application as compared to the powder form. The lower outgassing, higher image resolution and higher contrast ration can be realized from the thin films [9]. Another promising application of thin film phosphors is in alternating-current thin
∗ Corresponding author. Tel.: +86 10 82315351; fax: +86 10 82315351. E-mail address: [email protected] (J. Zhang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.077
film electroluminescent (ACTFEL) displays. Several achievements have been gained by Minami et al. [10,11] and Kitai et al. [12–14] at high luminant ACTFEL devices. A variety of techniques have been developed to prepare ZnGa2 O4 -based thin film phosphors, mainly including radio-frequency sputtering deposition (RFSD) [15–18] and pulsed laser deposition (PLD) [19,20]. The preparation and characteristics of red-emitting ZnGa2 O4 :Cr3+ thin film phosphors using a sol–gel process, to our best knowledge, are rarely reported. Compared to other techniques, the sol–gel process is considered as a convenient routine to prepare thin films for the relatively inexpensive raw materials, excellent material stoichiometry and omitting of expensive vacuum equipment. In this paper, we have investigated the properties of ZnGa2 O4 :Cr3+ film phosphors deposited on amorphous quartz glass and Si (1 0 0) substrates by a sol–gel process. The effects of calcining temperature, film thickness and the kind of substrates on microstructures, composition ratios Zn/Ga and optical properties of the polycrystalline ZnGa2 O4 :Cr3+ films are examined.
2. Experimental details ZnGa2 O4 :Cr3+ thin film phosphors were synthesized via a sol–gel process. The starting materials, gallium nitrate hydrate (Ga(NO3 )3 ·xH2 O, 0.02 mol), zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O, 0.011 mol) and (Cr(NO3 )3 ·9H2 O, 0.0001 mol), were dissolved in ethanol (40 ml). Then acetyl acetone (0.03 mol) and monoethanolamine (MEA, 0.01 mol) were added to the solution. The mixed solution was stirred at 50 ◦ C for 12 h until a transparent sol was formed. The amorphous quartz glass and
W. Zhang et al. / Applied Surface Science 256 (2010) 4702–4707
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R[Ga3+ ] = 0.062 nm, R[Cr3+ ] = 0.064 nm) [22] and did not introduce any detectable impurity phase seen from Fig. 1(a). However, the -Zn2 SiO4 (JCPDS, 14-0653) phase emerged with increasing calcining temperature to 1200 ◦ C. It has been reported that there existed thermal diffusion of Zn and Si elements at the boundary of ZnO film deposited on quartz substrates under high temperature thermal treatment [23]. Thus, high temperature in our experiments (1200 ◦ C) can cause Zn and Si elements diffusion across interface forming Zn2 SiO4 which can be understood by Fick’s thermal diffusion law given below [24]: L = (4D0 e−(Ea /kT ) t)1/2 where L is diffusion length, D0 the maximum diffusion coefficient at infinite temperature, Ea the activation energy, T the calcining temperature and t is the calcining duration. As the calcining temperature increased, the inter-diffusion length of Zn–Si increased exponentially and more zinc silica was formed. Therefore the Zn2 SiO4 was detected at the 1200 ◦ C-calcined sample. Besides, the (1 1 1) plane preferentially grew at 1200 ◦ C in the ZnGa2 O4 :Cr3+ thin films. This orientation transition may take place by two mechanisms. According to Lee et al. [1] and Bondar [25], ZnGa2 O4 exhibited different preferred orientations on different substrates, such as (4 0 0) orientation on (1 0 0) MgO single crystal substrates and (1 1 1) orientation on ZnO-interlayer substrates, respectively. The thermal treatment provided a Zn2 SiO4 interlayer between the ZnGa2 O4 :Cr3+ film and quartz substrate. The crystal structure of Zn2 SiO4 interlayer may cause these changes in crystallinity. Anoop et al. [15] suggested that the ZnGa2 O4 thin film exhibits (1 1 1) orientation deposited using magnetron sputtering with shorter
Fig. 1. (a) The XRD patterns of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C in air ( for -Zn2 SiO4 peaks). (b) The positions of (3 1 1) peak and lattice constant.
Si (1 0 0) substrates were dipped into the sol and withdrawn at the rate of 0.8 mm/s. Then the films were dried at 80 ◦ C to gelation and the gel films were subsequently heated at 500 ◦ C for 1 h in an electric furnace to eliminate the organic materials. The coating and heating processes were repeated several times until a required thickness was obtained. Eventually, the films were calcined at temperatures ranging from 800 to 1200 ◦ C in air for crystallization. The crystal structures of the films were characterized by an X-ray diffractometer (Cu K␣ D/max-2200, RIGAKU, Japan). The morphologies were obtained by a scanning electron microscope (S4200, Hitachi, Japan). The film thickness was determined using SEM and a profilometer (Dektak 6M, Veeco, USA). The elemental composition of the films was obtained using Auger electron spectroscopy (PHI-700 AES system, ULVAC-PHI, Japan) and energy dispersive spectrometer (EDS) attached with SEM. The transmittance spectra of the films were determined by a UV–vis spectrophotometer (U-3010, Hitachi, Japan). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded by a fluorescence spectrophotometer (F-4500, Hitachi, Japan). 3. Results and discussion Fig. 1(a) shows the XRD patterns of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C. All the films exhibited single spinel phase when calcining temperature increased up to 1100 ◦ C with the enhanced (3 1 1) peak which would contribute largely to photoluminescence behavior [21]. The dopant of Cr3+ was expected to substitute for octahedral sites of Ga3+ to minimize charge and size difference (R[Zn2+ ] = 0.074 nm,
Fig. 2. (a) PL and PLE spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C. (b) Ga/Zn ratio and excitation peaks as the functions of calcining temperature.
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substrate–target (S–T) distance, which implies that particles with higher energy produce films with (1 1 1) orientation. Since (1 1 1) is the plane with the lowest surface energy in the spinels [26], the high calcining temperature of 1200 ◦ C also supplied sufficient thermal nucleation dynamic. Therefore, the films grew with the densest direction and the random-type orientation of (3 1 1) transferred to (1 1 1). Fig. 1(b) shows the peak positions and lattice constants of ZnGa2 O4 :Cr3+ thin films with different calcining temperatures. When calcining temperature increased from 800 to 1000 ◦ C, the (3 1 1) peak position shifted from 35.708◦ to 35.761◦ , indicating that the lattice constant decreased from 8.335 to 8.318 Å. This is attributed to the vaporization of Zn ion with low vapor pressure [27], leaving Zn vacancies in the lattice. At temperature higher than 1000 ◦ C, Ga3+ will fill the Zn vacancy site, increasing the lattice constant to 8.333 Å at 1200 ◦ C. Since the Ga–O bond length is shorter than that of Zn–O [28], the lattice constant of ZnGa2 O4 :Cr3+ at 1200 ◦ C still showed slight shrinkage in comparison with that at 800 ◦ C. The Auger electron spectrum (AES, not shown herein) of ZnGa2 O4 :Cr3+ films reveals that there is no impurity other than C element. The photoluminescent (PL) and photoluminescent excitation (PLE) spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C are shown in Fig. 2(a). The broad band at about 250 nm is a charge-transfer band (CTB), originating from charge-transfer excitation from oxygen 2p orbitals to empty 4s4p orbitals of gallium [29]. The films exhibited strong emission band in the spectral range of 650–725 nm, with red emission maximum at 694 nm, assigned to 2 E → 4 A2 transitions of 3d electrons of Cr3+ which occupied the octahedrally coordinated Ga3+ sites in the host lattice [30]. In ZnGa2 O4 host Ga3+ always acts as a sensitizer [31]. Owing to a significant spectral overlap between sensitizer emission and activator absorption [2],
a non-radiative resonant energy transfer with electric multipole interaction took places between sensitizer (Ga3+ ) and activator (Cr3+ ). Consequently, the host lattice played an important role in the PL and PLE spectra of ZnGa2 O4 :Cr3+ thin films. Fig. 2(b) shows Ga/Zn ratio of the obtained film phosphors determined using EDS. Ga/Zn ratio is found to increase monotonously from 1.9 to 2.7 with increasing calcining temperature attributed to a relatively low vapor pressure of Zn compared with Ga. The excessive Ga3+ is speculated to fill the Zn vacancy sites surrounded by four oxygens. Either located on a tetrahedral Zn2+ site (Td ) or in a distorted octahedral configuration, Ga3+ exhibited a low symmetric crystal field, weakening the interaction between Ga3+ and its surrounding O2− . This weak interaction caused localization of electron clouds of O2− , indicating increasing ionicity [32]. Consequently, photon with higher energy is required to transfer electrons from O to Ga, resulting in blue shift in excitation spectra from 248.4 to 244.4 nm for ZnGa2 O4 :Cr3+ thin films, as shown in Fig. 2(b). However, the energy level of 2 E is insensitive to the crystal field strength, even parallel to the ground state 4 A2 [33]. Thus the Cr3+ emission spectra did not experience an obvious shift compared with large shifts in excitation spectra. Besides, it is well known that the luminescent intensity highly depends on the crystallinity and the chemical composition [18]. The crystallinity of ZnGa2 O4 :Cr3+ thin films experienced an obvious improvement with the calcining temperature increasing from 800 to 1000 ◦ C. At the calcining temperature above 1000 ◦ C, the thermal treatment cause vaporizing of Zn ions significantly. The non-stoichiometry and lattice expansion generated more defects sites, acting as luminescent quenching sites, which weakened the luminescent intensity for high calcining temperatures. The average film thicknesses are obtained by cross-sectional SEM images. Fig. 3(a) shows the relationship of film thickness and deposition layer number, which exhibited good linearity. The slope
Fig. 3. (a) The thickness vs. the deposition cycle and the linear fitness. (b) XRD patterns of ZnGa2 O4 :Cr3+ films with different thicknesses calcined at 1000 ◦ C. The impurity phase labeled with () indicates -Zn2 SiO4 . (c) The peak position and crystallite size of ZnGa2 O4 :Cr3+ films with different thicknesses. (d) The plots of (˛h)2 vs. the energy of light (h). The inset of (d) shows the optical transmittance spectra of ZnGa2 O4 :Cr3+ thin films with various thicknesses.
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films were estimated by extrapolating the straight line portion of the (˛h)2 vs. h plot according to the equations given below [37], since ZnGa2 O4 belongs to direct-gap material [38]. ˛=−
ln T d
2
(˛h) = A(h − Eg )
Fig. 4. PL and PLE spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at 1000 ◦ C with different thicknesses.
of the linear fitted function was 39.3, indicating the thickness of each layer. Fig. 3(b) shows the XRD patterns of ZnGa2 O4 :Cr3+ films with different thicknesses calcined at 1000 ◦ C. With increasing film thickness, the peak intensities of all the spinel structure increased, indicating improved crystallinity. The impurity phase of -Zn2 SiO4 emerged when the film thickness is 196 nm, whereas could not be detected for the films with larger thickness, implying that the diffusion length was less than 323 nm. The (3 1 1) peak shifted from 35.825◦ to 35.710◦ with the increase of film thickness as shown in Fig. 3(c), indicating that the lattice constant of ZnGa2 O4 films increased from 8.309 to 8.335 Å. The crystallite size also increased from 64 to 74 nm. The extension of lattice constant and grain size with increasing film thickness were also observed in other studies [34–36]. The optical transmittance spectra of ZnGa2 O4 :Cr3+ film phosphors were presented in Fig. 3(d), from which the band gap of the
where T is the transmittance of the films, d the film thickness, ˛ the absorption coefficient, h is the energy of incident photon. The band gap energy decreased from 4.95 to 4.88 eV with increasing the film thickness. The decrease of Eg with the increase of thickness is likely to be attributed to the increase of particle size or the lattice constant [39–42]. Meanwhile, the improved crystallinity with increasing thickness of films, as discussed above, promoted less defect and distortion in the crystallite structure, providing the high symmetry surrounding Ga3+ , which would decrease ionicity between Ga and O. Therefore, the optical absorption exhibited a significant red shift. The PL and PLE spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at 1000 ◦ C with different thicknesses are shown in Fig. 4. It is evident that the PL intensity increased with increasing the thickness of the thin films. The luminescent behavior indicates that the luminescence from excited ZnGa2 O4 :Cr3+ is not primarily a surface phenomenon but also arises from the interior of the film. Additionally, the penetration depth of UV light from Xe lamp in fluorescence spectrophotometer is 800 nm at the least. The thicker films provided more emission sources, enhancing the emission intensity. Moreover, the films had larger grain size, decreasing the grain boundary which usually acted as the non-radiative recombination centers, enhancing the PL intensity. It is observed that the excitation band exhibited a remarkable red shift from 237 to 247 nm, which is mainly attributed to the decreasing Eg and weaker ionicity between Ga3+ and O2− as discussed above.
Fig. 5. The SEM images of ZnGa2 O4 :Cr3+ thin film deposited on amorphous quartz (a) and Si (1 0 0) calcined at 1000 ◦ C for 1 h (b) and 5 h (c). (d) XRD patterns of ZnGa2 O4 :Cr3+ thin film deposited on Si (1 0 0) with different calcining durations.
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Fig. 6. (a) PL and PLE spectra of ZnGa2 O4 :Cr3+ films grown on Si (1 0 0) substrates calcined at 1000 ◦ C in vacuum and in air with duration time of 1, 5 and 10 h. (b) The compared luminescent intensity of ZnGa2 O4 :Cr3+ deposited on quartz and Si (1 0 0) substrates. (c) Decay curve of 694 nm emission of ZnGa2 O4 :Cr3+ and its biexponential fitting.
The SEM images of ZnGa2 O4 :Cr3+ thin film deposited on amorphous quartz and Si (1 0 0) calcined at 1000 ◦ C with various calcining duration were shown in Fig. 5(a–c). The variation in surface morphology was evident for ZnGa2 O4 :Cr3+ deposited on different substrates. The shape of grains on amorphous quartz and Si (1 0 0) were circular type and rod, respectively. The average grain size of ZnGa2 O4 :Cr3+ on quartz substrate was about 60 nm, much smaller than that of about 140 nm on Si (1 0 0). Although the lattice mismatch between ZnGa2 O4 and Si (1 0 0) was 12% [19], the SEM images confirmed that ZnGa2 O4 :Cr3+ films exhibited preferred orientation. As the calcining duration extended, the grain size grew larger and crystallinity improved as shown in Fig. 5(d). All peaks were consistent with the standard powder diffraction pattern, because the ZnGa2 O4 :Cr3+ rod grew randomly on the surface of Si (1 0 0) substrates as in our previous work [43]. Fig. 6(a) shows the PL and PLE spectra of ZnGa2 O4 :Cr3+ films grown on Si (1 0 0) substrates calcined in vacuum and in air with duration time of 1, 5 and 10 h, respectively. The brightness of ZnGa2 O4 :Cr3+ films increases monotonously with the calcining duration until 5 h, whereas no luminescent signal could be detected for the one calcined in vacuum. This is attributed to the fact that the sol–gel process formed through hydrolysis usually entrapped some water, alcohol, and other organic groups [44]. The residual organic can efficiently quench the radiative transition of luminescent active ions through multi-phonon relaxation. The prolonged calcining duration in air could provide more oxygen and thermal process to eliminate the organic group which enhanced luminescent intensity. However, the longer calcining duration yielded more loss of Zn ions and crack or pore formed in the internal films, weakening the brightness. The luminescent intensity of the film on the Si substrate enhanced slightly compared to that on quartz as shown in Fig. 6(b). This is the evidence that the rod-like morphology favors the emission intensity, probably due to efficient utility of incident UV photon and reducing of defects or boundary sites in regular grain size on the Si (1 0 0) substrate. The room-temperature decay time of ZnGa2 O4 :Cr3+ film at 694 nm is depicted in Fig. 6(c). The decay curve is well fitted with a biexponential decay function [45], viz. I(t) ∝ ˛e−t/1 + ˇe−t/2 , implying two luminescent centers. The longer decay time ( 1 = 11.9 ms), identical to the previous study [46], is considered as transition of Cr3+ on octahedrally coordinated (Oh ) sites, while the shorter one ( 2 = 2.6 ms) on distorted sites with lower symmetry. 4. Conclusions In conclusion, ZnGa2 O4 :Cr3+ thin film phosphors have been prepared by a sol–gel process. The crystallinity and luminescent properties strongly depend on calcining temperatures, film thickness, calcining duration and kinds of substrates. When calcined at
1000 ◦ C for 5 h, the thin film phosphors exhibit high photoluminescent intensity at 694 nm due to the 2 E → 4 A2 transitions. The remarkable blue shifts in excitation band are observed, either with increasing calcining temperature or decreasing the film thickness, attributed to the change in Ga/Zn ratio and Eg . The morphologies of rod and circular shape have been obtained with amorphous quartz and Si (100) substrates. The two decay times of ZnGa2 O4 :Cr3+ are assigned to octahedrally coordinated (Oh ) sites and distorted sites with lower symmetry, respectively. Acknowledgements This work is supported by the Program for New Century Excellent Talents in the University of China under grant No. NCET06-0179 and the Fok Ying Tong Education Foundation under grant No. 111050. References [1] Y.E. Lee, D.P. Norton, J.D. Budai, Appl. Phys. Lett. 74 (1999) 3155. [2] J.S. Kim, J.S. Kim, T.W. Kim, H.L. Park, Y.G. Kim, S.K. Chang, S.D. Han, Solid State Commun. 131 (2004) 493. [3] I.K. Jeong, H.L. Park, S.I. Mho, Solid State Commun. 105 (1998) 179. [4] L. Zou, X. Xiang, M. Wei, F. Li, David G. Evans, Inorg. Chem. 47 (2008) 1361. [5] J.S. Kim, H.L. Park, G.C. Kim, T.W. Kim, Y.H. Hwang, H.K. Kim, S.I. Mho, S.D. Han, Solid State Commun. 126 (2001) 55. [6] D.R. Philip, J.P. Jeffrey, D.P. Michael, P.J. Wounjhang, Mater. Res. 16 (2001) 836. [7] P. Dhak, U.K. Gayen, S. Mishra, P. Pramanik, A. Roy, J. Appl. Phys. 106 (2009) 063721. [8] D.P. Dutta, R. Ghildiyal, A.K. Tyagi, J. Phys. Chem. C 113 (2009) 16954. [9] J.H. Kim, P.H. Holloway, J. Vac. Sci. Technol. A 24 (2006) 2164. [10] T. Miyata, T. Nakatani, T. Minami, Thin Solid Films 373 (2000) 145. [11] T. Minami, Y. Kobayashi, T. Shirai, T. Miyata, S. Suzuki, Jpn. J. Appl. Phys. 41 (2002) L478. [12] X. Ouyang, A.H. Kitai, T. Xiao, J. Appl. Phys. 79 (1996) 3229. [13] T. Xiao, A.H. Kitai, G. Liu, A. Nakua, J. Barbier, Appl. Phys. Lett. 72 (1998) 3356. [14] G. Stuyven, P.D. Visschere, K. Neyts, A. Kitai, J. Appl. Phys. 93 (2003) 4622. [15] G. Anoop, K.M. Krishna, M.K. Jayaraj, Appl. Phys. A 90 (2008) 711. [16] S.H. Yang, T.J. Hsueh, S.J. Chang, J. Cryst. Growth 287 (2006) 194–198. [17] H.W. Choi, B.J. Hong, S.K. Lee, K.H. Kim, Y.S. Park, J. Lumin. 126 (2007) 359. [18] J.H. Park, S.H. Lee, J.S. Kim, H.W. Park, J.C. Choi, H.L. Park, G.C. Kim, J.H. Yoo, J. Cryst. Growth 299 (2007) 369. [19] J.S. Bae, B.K. Moon, B.C. Choi, J.H. Jeong, S.S. Yi, I.W. Kim, J.S. Lee, Thin Solid Films 424 (2003) 291. [20] R. Reshmi, K.M. Krishna, R. Manoj, M.K. Jayaraj, Surf. Coating Technol. 198 (2005) 345. [21] K.W. Park, Y.H. Yun, S.C. Choi, Solid State Ionics 177 (2006) 1878. [22] K.E. Sickafus, J.M. Wills, N.W. Grimes, J. Am. Ceram. Soc. 82 (1999) 3279. [23] K.I. Seo, J.H. Park, J.S. Kima, G.C. Kim, J.H. Yoo, J. Lumin. 129 (2009) 715. [24] R.C. Ropp, Lumin. Solid State, Elsevier, NY, 1991. [25] V. Bondar, Mater. Sci. Eng. B 69–70 (2000) 505. [26] D.M. Saylor, B.E. Dasher, Y. Pang, H.M. Miller, P. Wynblatt, A.D. Rollett, G.S. Rohrer, J. Am. Ceram. Soc. 87 (2004) 724. [27] Y.J. Kim, Y.H. Jeong, K.D. Kim, J. Vac. Sci. Technol. B 16 (1998) 1239. [28] J.S. Kim, H.L. Park, C.M. Chon, H.S. Moon, T.W. Kim, Solid State Commun. 129 (2004) 163. [29] K. Ikarashi, J. Sato, H. Kobayashi, N. Saito, H. Nishiyama, Y. Inoue, J. Phys. Chem. B 106 (2002) 9048. [30] J.S. Kim, J.S. Kim, H.L. Park, Solid State Commun. 131 (2004) 735.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and optical properties of ZnGa2 O4 :Cr3+ thin films derived by sol–gel process Weiwei Zhang, Junying Zhang ∗ , Yuan Li, Ziyu Chen, Tianmin Wang School of Physics and Nuclear Energy Engineering, Beihang University, No. 37 XueYuan Road, HaiDian District, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 22 December 2009 Received in revised form 23 February 2010 Accepted 23 February 2010 Available online 3 March 2010 Keywords: ZnGa2 O4 :Cr3+ Photoluminescence Thin film Sol–gel process
a b s t r a c t ZnGa2 O4 :Cr3+ thin films with bright red emission were synthesized using a sol–gel process, characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Auger electron spectroscopy (AES) and UV–vis and fluorescence spectrophotometry measurements. Effects of calcining temperature, film thickness, calcining duration and substrates on the crystal structure and photoluminescent property have been investigated. It is found that the crystallinity, Ga/Zn ratio and band gap energy (Eg ) are significant factors influencing optical characteristics, while the nature of substrates affect the surface morphologies of ZnGa2 O4 :Cr3+ thin films. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, oxide phosphors have been extensively investigated for their use in flat panel displays (FPDs). ZnGa2 O4 based phosphors have received considerable attention due to the good luminescent characteristics, chemical and thermal stabilities in high vacuum and absence of corrosive gas emission under electron bombardment in comparison with currently used sulfide based phosphors [1]. ZnGa2 O4 has a spinel structure (AB2 O4 ), in which Zn2+ ions occupy the tetrahedrally coordinated A-sites (Td ), while Ga3+ in the octahedrally coordinated B-sites (Oh ) [2]. Under excitation by both ultraviolet light and low voltage electron, ZnGa2 O4 exhibits a strong blue emission due to transition via a selfactivation (SA) center [3,4]. It also shows various emissions from green to red when doped with Mn2+ or Cr3+ , respectively [5–7]. Recently ZnGa2 O4 -based white light emitting phosphors has been developed [8]. Due to the equal valence and similar ionic radii of Ga3+ (0.62 Å) and Cr3+ (0.64 Å), Cr3+ can replace the Ga3+ sites in the host lattice and has a high doping concentration, promoting a high luminescent intensity. The use of thin films of ZnGa2 O4 :Cr3+ phosphor has some advantage in the FPDs application as compared to the powder form. The lower outgassing, higher image resolution and higher contrast ration can be realized from the thin films [9]. Another promising application of thin film phosphors is in alternating-current thin
∗ Corresponding author. Tel.: +86 10 82315351; fax: +86 10 82315351. E-mail address: [email protected] (J. Zhang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.077
film electroluminescent (ACTFEL) displays. Several achievements have been gained by Minami et al. [10,11] and Kitai et al. [12–14] at high luminant ACTFEL devices. A variety of techniques have been developed to prepare ZnGa2 O4 -based thin film phosphors, mainly including radio-frequency sputtering deposition (RFSD) [15–18] and pulsed laser deposition (PLD) [19,20]. The preparation and characteristics of red-emitting ZnGa2 O4 :Cr3+ thin film phosphors using a sol–gel process, to our best knowledge, are rarely reported. Compared to other techniques, the sol–gel process is considered as a convenient routine to prepare thin films for the relatively inexpensive raw materials, excellent material stoichiometry and omitting of expensive vacuum equipment. In this paper, we have investigated the properties of ZnGa2 O4 :Cr3+ film phosphors deposited on amorphous quartz glass and Si (1 0 0) substrates by a sol–gel process. The effects of calcining temperature, film thickness and the kind of substrates on microstructures, composition ratios Zn/Ga and optical properties of the polycrystalline ZnGa2 O4 :Cr3+ films are examined.
2. Experimental details ZnGa2 O4 :Cr3+ thin film phosphors were synthesized via a sol–gel process. The starting materials, gallium nitrate hydrate (Ga(NO3 )3 ·xH2 O, 0.02 mol), zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O, 0.011 mol) and (Cr(NO3 )3 ·9H2 O, 0.0001 mol), were dissolved in ethanol (40 ml). Then acetyl acetone (0.03 mol) and monoethanolamine (MEA, 0.01 mol) were added to the solution. The mixed solution was stirred at 50 ◦ C for 12 h until a transparent sol was formed. The amorphous quartz glass and
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R[Ga3+ ] = 0.062 nm, R[Cr3+ ] = 0.064 nm) [22] and did not introduce any detectable impurity phase seen from Fig. 1(a). However, the -Zn2 SiO4 (JCPDS, 14-0653) phase emerged with increasing calcining temperature to 1200 ◦ C. It has been reported that there existed thermal diffusion of Zn and Si elements at the boundary of ZnO film deposited on quartz substrates under high temperature thermal treatment [23]. Thus, high temperature in our experiments (1200 ◦ C) can cause Zn and Si elements diffusion across interface forming Zn2 SiO4 which can be understood by Fick’s thermal diffusion law given below [24]: L = (4D0 e−(Ea /kT ) t)1/2 where L is diffusion length, D0 the maximum diffusion coefficient at infinite temperature, Ea the activation energy, T the calcining temperature and t is the calcining duration. As the calcining temperature increased, the inter-diffusion length of Zn–Si increased exponentially and more zinc silica was formed. Therefore the Zn2 SiO4 was detected at the 1200 ◦ C-calcined sample. Besides, the (1 1 1) plane preferentially grew at 1200 ◦ C in the ZnGa2 O4 :Cr3+ thin films. This orientation transition may take place by two mechanisms. According to Lee et al. [1] and Bondar [25], ZnGa2 O4 exhibited different preferred orientations on different substrates, such as (4 0 0) orientation on (1 0 0) MgO single crystal substrates and (1 1 1) orientation on ZnO-interlayer substrates, respectively. The thermal treatment provided a Zn2 SiO4 interlayer between the ZnGa2 O4 :Cr3+ film and quartz substrate. The crystal structure of Zn2 SiO4 interlayer may cause these changes in crystallinity. Anoop et al. [15] suggested that the ZnGa2 O4 thin film exhibits (1 1 1) orientation deposited using magnetron sputtering with shorter
Fig. 1. (a) The XRD patterns of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C in air ( for -Zn2 SiO4 peaks). (b) The positions of (3 1 1) peak and lattice constant.
Si (1 0 0) substrates were dipped into the sol and withdrawn at the rate of 0.8 mm/s. Then the films were dried at 80 ◦ C to gelation and the gel films were subsequently heated at 500 ◦ C for 1 h in an electric furnace to eliminate the organic materials. The coating and heating processes were repeated several times until a required thickness was obtained. Eventually, the films were calcined at temperatures ranging from 800 to 1200 ◦ C in air for crystallization. The crystal structures of the films were characterized by an X-ray diffractometer (Cu K␣ D/max-2200, RIGAKU, Japan). The morphologies were obtained by a scanning electron microscope (S4200, Hitachi, Japan). The film thickness was determined using SEM and a profilometer (Dektak 6M, Veeco, USA). The elemental composition of the films was obtained using Auger electron spectroscopy (PHI-700 AES system, ULVAC-PHI, Japan) and energy dispersive spectrometer (EDS) attached with SEM. The transmittance spectra of the films were determined by a UV–vis spectrophotometer (U-3010, Hitachi, Japan). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded by a fluorescence spectrophotometer (F-4500, Hitachi, Japan). 3. Results and discussion Fig. 1(a) shows the XRD patterns of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C. All the films exhibited single spinel phase when calcining temperature increased up to 1100 ◦ C with the enhanced (3 1 1) peak which would contribute largely to photoluminescence behavior [21]. The dopant of Cr3+ was expected to substitute for octahedral sites of Ga3+ to minimize charge and size difference (R[Zn2+ ] = 0.074 nm,
Fig. 2. (a) PL and PLE spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C. (b) Ga/Zn ratio and excitation peaks as the functions of calcining temperature.
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substrate–target (S–T) distance, which implies that particles with higher energy produce films with (1 1 1) orientation. Since (1 1 1) is the plane with the lowest surface energy in the spinels [26], the high calcining temperature of 1200 ◦ C also supplied sufficient thermal nucleation dynamic. Therefore, the films grew with the densest direction and the random-type orientation of (3 1 1) transferred to (1 1 1). Fig. 1(b) shows the peak positions and lattice constants of ZnGa2 O4 :Cr3+ thin films with different calcining temperatures. When calcining temperature increased from 800 to 1000 ◦ C, the (3 1 1) peak position shifted from 35.708◦ to 35.761◦ , indicating that the lattice constant decreased from 8.335 to 8.318 Å. This is attributed to the vaporization of Zn ion with low vapor pressure [27], leaving Zn vacancies in the lattice. At temperature higher than 1000 ◦ C, Ga3+ will fill the Zn vacancy site, increasing the lattice constant to 8.333 Å at 1200 ◦ C. Since the Ga–O bond length is shorter than that of Zn–O [28], the lattice constant of ZnGa2 O4 :Cr3+ at 1200 ◦ C still showed slight shrinkage in comparison with that at 800 ◦ C. The Auger electron spectrum (AES, not shown herein) of ZnGa2 O4 :Cr3+ films reveals that there is no impurity other than C element. The photoluminescent (PL) and photoluminescent excitation (PLE) spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at temperatures ranging from 800 to 1200 ◦ C are shown in Fig. 2(a). The broad band at about 250 nm is a charge-transfer band (CTB), originating from charge-transfer excitation from oxygen 2p orbitals to empty 4s4p orbitals of gallium [29]. The films exhibited strong emission band in the spectral range of 650–725 nm, with red emission maximum at 694 nm, assigned to 2 E → 4 A2 transitions of 3d electrons of Cr3+ which occupied the octahedrally coordinated Ga3+ sites in the host lattice [30]. In ZnGa2 O4 host Ga3+ always acts as a sensitizer [31]. Owing to a significant spectral overlap between sensitizer emission and activator absorption [2],
a non-radiative resonant energy transfer with electric multipole interaction took places between sensitizer (Ga3+ ) and activator (Cr3+ ). Consequently, the host lattice played an important role in the PL and PLE spectra of ZnGa2 O4 :Cr3+ thin films. Fig. 2(b) shows Ga/Zn ratio of the obtained film phosphors determined using EDS. Ga/Zn ratio is found to increase monotonously from 1.9 to 2.7 with increasing calcining temperature attributed to a relatively low vapor pressure of Zn compared with Ga. The excessive Ga3+ is speculated to fill the Zn vacancy sites surrounded by four oxygens. Either located on a tetrahedral Zn2+ site (Td ) or in a distorted octahedral configuration, Ga3+ exhibited a low symmetric crystal field, weakening the interaction between Ga3+ and its surrounding O2− . This weak interaction caused localization of electron clouds of O2− , indicating increasing ionicity [32]. Consequently, photon with higher energy is required to transfer electrons from O to Ga, resulting in blue shift in excitation spectra from 248.4 to 244.4 nm for ZnGa2 O4 :Cr3+ thin films, as shown in Fig. 2(b). However, the energy level of 2 E is insensitive to the crystal field strength, even parallel to the ground state 4 A2 [33]. Thus the Cr3+ emission spectra did not experience an obvious shift compared with large shifts in excitation spectra. Besides, it is well known that the luminescent intensity highly depends on the crystallinity and the chemical composition [18]. The crystallinity of ZnGa2 O4 :Cr3+ thin films experienced an obvious improvement with the calcining temperature increasing from 800 to 1000 ◦ C. At the calcining temperature above 1000 ◦ C, the thermal treatment cause vaporizing of Zn ions significantly. The non-stoichiometry and lattice expansion generated more defects sites, acting as luminescent quenching sites, which weakened the luminescent intensity for high calcining temperatures. The average film thicknesses are obtained by cross-sectional SEM images. Fig. 3(a) shows the relationship of film thickness and deposition layer number, which exhibited good linearity. The slope
Fig. 3. (a) The thickness vs. the deposition cycle and the linear fitness. (b) XRD patterns of ZnGa2 O4 :Cr3+ films with different thicknesses calcined at 1000 ◦ C. The impurity phase labeled with () indicates -Zn2 SiO4 . (c) The peak position and crystallite size of ZnGa2 O4 :Cr3+ films with different thicknesses. (d) The plots of (˛h)2 vs. the energy of light (h). The inset of (d) shows the optical transmittance spectra of ZnGa2 O4 :Cr3+ thin films with various thicknesses.
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films were estimated by extrapolating the straight line portion of the (˛h)2 vs. h plot according to the equations given below [37], since ZnGa2 O4 belongs to direct-gap material [38]. ˛=−
ln T d
2
(˛h) = A(h − Eg )
Fig. 4. PL and PLE spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at 1000 ◦ C with different thicknesses.
of the linear fitted function was 39.3, indicating the thickness of each layer. Fig. 3(b) shows the XRD patterns of ZnGa2 O4 :Cr3+ films with different thicknesses calcined at 1000 ◦ C. With increasing film thickness, the peak intensities of all the spinel structure increased, indicating improved crystallinity. The impurity phase of -Zn2 SiO4 emerged when the film thickness is 196 nm, whereas could not be detected for the films with larger thickness, implying that the diffusion length was less than 323 nm. The (3 1 1) peak shifted from 35.825◦ to 35.710◦ with the increase of film thickness as shown in Fig. 3(c), indicating that the lattice constant of ZnGa2 O4 films increased from 8.309 to 8.335 Å. The crystallite size also increased from 64 to 74 nm. The extension of lattice constant and grain size with increasing film thickness were also observed in other studies [34–36]. The optical transmittance spectra of ZnGa2 O4 :Cr3+ film phosphors were presented in Fig. 3(d), from which the band gap of the
where T is the transmittance of the films, d the film thickness, ˛ the absorption coefficient, h is the energy of incident photon. The band gap energy decreased from 4.95 to 4.88 eV with increasing the film thickness. The decrease of Eg with the increase of thickness is likely to be attributed to the increase of particle size or the lattice constant [39–42]. Meanwhile, the improved crystallinity with increasing thickness of films, as discussed above, promoted less defect and distortion in the crystallite structure, providing the high symmetry surrounding Ga3+ , which would decrease ionicity between Ga and O. Therefore, the optical absorption exhibited a significant red shift. The PL and PLE spectra of ZnGa2 O4 :Cr3+ thin film phosphors calcined at 1000 ◦ C with different thicknesses are shown in Fig. 4. It is evident that the PL intensity increased with increasing the thickness of the thin films. The luminescent behavior indicates that the luminescence from excited ZnGa2 O4 :Cr3+ is not primarily a surface phenomenon but also arises from the interior of the film. Additionally, the penetration depth of UV light from Xe lamp in fluorescence spectrophotometer is 800 nm at the least. The thicker films provided more emission sources, enhancing the emission intensity. Moreover, the films had larger grain size, decreasing the grain boundary which usually acted as the non-radiative recombination centers, enhancing the PL intensity. It is observed that the excitation band exhibited a remarkable red shift from 237 to 247 nm, which is mainly attributed to the decreasing Eg and weaker ionicity between Ga3+ and O2− as discussed above.
Fig. 5. The SEM images of ZnGa2 O4 :Cr3+ thin film deposited on amorphous quartz (a) and Si (1 0 0) calcined at 1000 ◦ C for 1 h (b) and 5 h (c). (d) XRD patterns of ZnGa2 O4 :Cr3+ thin film deposited on Si (1 0 0) with different calcining durations.
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Fig. 6. (a) PL and PLE spectra of ZnGa2 O4 :Cr3+ films grown on Si (1 0 0) substrates calcined at 1000 ◦ C in vacuum and in air with duration time of 1, 5 and 10 h. (b) The compared luminescent intensity of ZnGa2 O4 :Cr3+ deposited on quartz and Si (1 0 0) substrates. (c) Decay curve of 694 nm emission of ZnGa2 O4 :Cr3+ and its biexponential fitting.
The SEM images of ZnGa2 O4 :Cr3+ thin film deposited on amorphous quartz and Si (1 0 0) calcined at 1000 ◦ C with various calcining duration were shown in Fig. 5(a–c). The variation in surface morphology was evident for ZnGa2 O4 :Cr3+ deposited on different substrates. The shape of grains on amorphous quartz and Si (1 0 0) were circular type and rod, respectively. The average grain size of ZnGa2 O4 :Cr3+ on quartz substrate was about 60 nm, much smaller than that of about 140 nm on Si (1 0 0). Although the lattice mismatch between ZnGa2 O4 and Si (1 0 0) was 12% [19], the SEM images confirmed that ZnGa2 O4 :Cr3+ films exhibited preferred orientation. As the calcining duration extended, the grain size grew larger and crystallinity improved as shown in Fig. 5(d). All peaks were consistent with the standard powder diffraction pattern, because the ZnGa2 O4 :Cr3+ rod grew randomly on the surface of Si (1 0 0) substrates as in our previous work [43]. Fig. 6(a) shows the PL and PLE spectra of ZnGa2 O4 :Cr3+ films grown on Si (1 0 0) substrates calcined in vacuum and in air with duration time of 1, 5 and 10 h, respectively. The brightness of ZnGa2 O4 :Cr3+ films increases monotonously with the calcining duration until 5 h, whereas no luminescent signal could be detected for the one calcined in vacuum. This is attributed to the fact that the sol–gel process formed through hydrolysis usually entrapped some water, alcohol, and other organic groups [44]. The residual organic can efficiently quench the radiative transition of luminescent active ions through multi-phonon relaxation. The prolonged calcining duration in air could provide more oxygen and thermal process to eliminate the organic group which enhanced luminescent intensity. However, the longer calcining duration yielded more loss of Zn ions and crack or pore formed in the internal films, weakening the brightness. The luminescent intensity of the film on the Si substrate enhanced slightly compared to that on quartz as shown in Fig. 6(b). This is the evidence that the rod-like morphology favors the emission intensity, probably due to efficient utility of incident UV photon and reducing of defects or boundary sites in regular grain size on the Si (1 0 0) substrate. The room-temperature decay time of ZnGa2 O4 :Cr3+ film at 694 nm is depicted in Fig. 6(c). The decay curve is well fitted with a biexponential decay function [45], viz. I(t) ∝ ˛e−t/1 + ˇe−t/2 , implying two luminescent centers. The longer decay time ( 1 = 11.9 ms), identical to the previous study [46], is considered as transition of Cr3+ on octahedrally coordinated (Oh ) sites, while the shorter one ( 2 = 2.6 ms) on distorted sites with lower symmetry. 4. Conclusions In conclusion, ZnGa2 O4 :Cr3+ thin film phosphors have been prepared by a sol–gel process. The crystallinity and luminescent properties strongly depend on calcining temperatures, film thickness, calcining duration and kinds of substrates. When calcined at
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