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Red luminescence in ZnO films prepared by a glycol-based Pechini method
Physics Letters A 372 (2008) 4104–4108 www.elsevier.com/locate/pla
Red luminescence in ZnO films prepared by a glycol-based Pechini method J.H. Cai a,∗ , G. Ni b , G. He a , Z.Y. Wu a a Department of Physics, Shanghai Jiaotong University, Shanghai 200240, China b Department of Optics Science and Engineering, Fudan University, Shanghai 200433, China
Received 19 January 2008; received in revised form 5 March 2008; accepted 7 March 2008 Available online 14 March 2008 Communicated by R. Wu
Abstract ZnO thin films on fused quartz substrates were prepared by a glycol-based Pechini method. The structural and optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance spectrum, and photoluminescence (PL) spectrum. A red emission around 700 nm was found in PL spectrum, and its peak intensity gained a strong enhancement (∼140%) while annealing temperature increased from 700 ◦ C to 800 ◦ C. The red emission was ascribed to the possible high defect density in boundary layers of nanocrystalline grains. © 2008 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.66.Hf; 71.55.Gs Keywords: Luminescence; Optical materials and properties; Thin films; Defects; Sol-gel preparation
1. Introduction ZnO is a II–VI group compound semiconductor, and is widely recognized as an ideal material for application in exciton-related optoelectronic devices because of its wide band gap of 3.37 eV at room temperature and large exciton binding energy of 60 meV [1]. The photoluminescent properties of ZnO films have been extensively investigated. The photoluminescence (PL) spectrum is normally composed of two parts: near-band-edge (NBE) ultraviolet (UV) emission and deeplevel (DL) yellow–green (YG) emission [2]. The NBE UV emission is usually ascribed to free excitons, bound excitons, and their phonon replicas [3–7]. The DL YG emission is usually ascribed to various defects, such as VO , VZn , Oi , Zni , OZn [8–13], etc. The characteristic of PL spectrum usually depends on the fabrication methods, growth conditions, and annealing treatments. In epitaxial ZnO films with a low density of DL defects, such as prepared by molecular beam epitaxy [4,7,12], chemical vapor deposition [3], pulsed laser deposition [14], * Corresponding author. Tel.: +86 21 54743165; fax: +86 21 54741040.
E-mail address: [email protected] (J.H. Cai). 0375-9601/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2008.03.011
etc., the NBE UV emission dominates the PL spectrum. In most polycrystalline thin films prepared by magnetron sputtering [11,15], spray pyrolysis [16,17], sol-gel method [18–21], significant DL emissions can be observed, and in some cases the DL emissions dominate the PL spectrum [17]. However, some exceptions should be mentioned here. In some polycrystalline thin films prepared by electrodeposition [22], sol-gel method [23], the NBE emissions dominate the PL spectrum. Even more, in polycrystalline thin films prepared by the oxidation of metallic Zn film [24–26] and compound ZnS film [27–29], a strong and narrow NBE emission accompanied by very weak DL emissions in the PL spectrum was observed. The high PL intensity ratio of the NBE emission to DL emission was regarded as an indication of the high quality of the nanocrystalline ZnO films prepared by the oxidation of the corresponding precursor films. In comparison with the works on the NBE UV emission and DL YG emission, the works on the red emission in ZnO film are very scarce [17,30–32]. In ZnO film prepared by electrodeposition [30], the PL spectrum at low temperature (25 K to 200 K) shows a broad emission centered at about 620 nm. In doped ZnO film prepared by a sol-gel method [31], the PL spectrum shows a broad emission centered at about 680 nm in the
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range of 400 to 700 nm, and the emission is attributed to host material ZnO. In ZnO film grown by spray pyrolysis [32], the PL spectrum also shows a broad emission centered at 700 nm and 800 nm in the range of 550 to 850 nm. In this Letter, the structural and optical properties of ZnO film prepared by a glycol-based Pechini method [33] are investigated, a narrow red emission centered at 700 nm is found, and possible origins are discussed. 2. Experimental Zinc nitrate, citric acid (CA), and ethylene glycol (EG) were employed as raw materials. CA was used as complexation agent, and EG was used either as polymerization agent or as solvent agent. The 0.02 mol zinc nitrate and 0.03 mol CA were dissolved in 50 ml EG at 50 ◦ C, then the solution was evaporated at 80 ◦ C until the final volume reached 20 ml. The precursor solution was spin-coated on fused quartz substrate in 3000 rpm for 30 s, then the precursor film was baked on a hot plate at 110 ◦ C for 10 min to remove the moisture, and fired in a furnace at 400 ◦ C for 30 min to remove organic components. The above procedure was repeated six times. Finally the films were annealed at 500, 600, 700, and 800 ◦ C for 2 h. The obtained film has a thickness 0.3 µm, which was estimated directly from the cross section photograph by scanning electron microscopy (SEM). The crystalline structures of ZnO films were studied by X-ray diffraction (XRD) (Bruker D8 Advance) with CuKα radiation (λ = 1.5406 Å). The surface morphology was observed by SEM (FEI SIRION 200). The optical transmittance spectra were recorded by a spectrometer (7-STAR 7ISW302) in the wavelength range of 200 to 850 nm. The PL measurements were performed on a Jobin Yvon LabRAM HR 800 microRaman system, which was equipped with an Andor DU420 classic charge-coupled device detector, in the wavelength range of 330 to 800 nm. For excitation, the 325 nm line of a 3 mW He–Cd laser was used. The excitation light was focused on the surface of sample, and a light spot with size ∼100 µm was yielded. 3. Results and discussion Fig. 1 shows the XRD patterns. A broad peak centered at about 22◦ (2θ value) should be assigned to amorphous silica substrate [34]. When annealing temperature Ta is over 600 ◦ C, the peaks corresponding to (100), (002), (101) planes of wurtzite ZnO phase emerge. The intensity of these peaks shows no significant enhancement with further increase of Ta . At a high temperature, ZnO is able to react with SiO2 and form Zn2 SiO4 or β-Zn2 SiO4 . In SiO2 -coated ZnO ultrafine particles or a ZnO/SiO2 nanocomposite film, the formation temperature of Zn2 SiO4 is about 700 ◦ C [35,36]. In a ZnO/SiO2 nanocomposite, the formation temperature of β-Zn2 SiO4 is about 800 ◦ C [34]. In a Tb-doped ZnO film on the amorphous SiO2 substrate, the formation temperature of Zn2 SiO4 is about 850 ◦ C [37]. In our samples only wurtzite ZnO phase is found, and no secondary phases, such as Zn2 SiO4 or β-Zn2 SiO4 , are found. However, this conclusion for secondary phase suffers from the
Fig. 1. XRD patterns of ZnO films annealed at different temperatures.
low ratio of signal to noise in XRD measurement. We will discuss this problem again in the analysis of transmittance spectra. Fig. 2 shows the SEM photographs. The photographs show the ZnO films consist of nanoscaled crystalline grains. Apparently the grain size increases significantly while Ta is changed from 600 to 700 ◦ C. In the film annealed at 800 ◦ C, some crystalline grains agglomerate, simultaneously, more unoccupied spaces around these grains are left. The morphology indicates that the ZnO film annealed at 800 ◦ C becomes porous, at least in the vicinity of surface. In addition to the agglomeration between crystalline grains, the surface of grain becomes slightly coarse at Ta = 800 ◦ C. Fig. 3 shows transmittance spectra. In the range of 200 to 375 nm, the transmittance slightly decreases while Ta is changed from 600 to 700 ◦ C. This should be attributed to the increase of crystallinity since the transition of valence band to conduction band should be responsible for the absorption in this range. The transmittance in the range from 220 to 375 nm shows no significant increase with Ta , and this indicates that no secondary phase, such as Zn2 SiO4 , forms while Ta is increased. This is because that the Eg of zinc silicate is about 5.7 eV [37] and the transmittance in this range should increases with the amount of zinc silicate transformed from zinc oxide and fused quartz [36,37]. In the range of 400 to 850 nm, the transmittance decreases while Ta increases from 500 to 700 ◦ C. However, the transmittance at 800 ◦ C slightly increases in the range of 420 to 850 nm, and decreases rapidly below 500 nm. The DL defects should be responsible for the absorption in this range. From transmittance spectrum, the absorption coefficient α can be obtained as α = − ln(T )/d, where d is the thickness of film and T is transmittance. For the direct transition in a semiconductor, α ∝ (hν − Eg )1/2 /(hν) for hν approaches Eg , where hν is photon energy and Eg is optical band gap energy. Hence the Eg satisfies equation (hν)2 ln2 (T ) = C(hν − Eg ), where C is a constant. Usually Eg is obtained by a linear fitting to (hν)2 ln2 (T ). The inset in Fig. 3 shows a plot of (hν)2 ln2 (T )
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Fig. 3. Transmittance spectra of ZnO films annealed at different temperatures. The inset shows a plot of (ln(T )hν)2 against hν for film annealed at 700 ◦ C. In the inset the circle (◦) denotes experimental data, and the straight line is a linear fitting to them.
Fig. 2. SEM images of ZnO films annealed at different temperatures (a) 500 ◦ C, (b) 600 ◦ C, (c) 700 ◦ C, and (d) 800 ◦ C.
against hν for film annealed at 700 ◦ C, and the obtained Eg is 3.27 eV. The dependence of Eg on annealing temperature is very weak, and the band gap energy Eg at Ta = 500, 600, 800 ◦ C is 3.29, 3.28, 3.25 eV, respectively. Here the obtained Eg is consistent with many reported values, such as in the films prepared by sol-gel method [19–21,38], thermal oxidation [39], pulsed laser deposition [14]. Fig. 4 shows PL spectra. The PL spectrum is composed of three emissions: an UV emission, a YG emission, and a red emission. The film annealed at 700 ◦ C exhibits a maximum peak value and a minimum full-width at half-maximum (FWHM) in UV emission range. This indicates that the best crystal quality is obtained at Ta = 700 ◦ C. A red emission around 700 nm is observed in ZnO films. The red emission becomes strong while annealing temperature increases from 700 ◦ C to 800 ◦ C, and the peak intensity gains an enhancement about 140%. Compared with the reported red luminescence in ZnO films [17,30–32], here the red emission shows a narrow range from 620 nm to 800 nm and it can be distinguished clearly from YG emission.
Fig. 4. PL spectra of ZnO films annealed at different temperatures.
The red emission should be mainly contributed by ZnO film, not by fused quartz substrate, because the luminescence in fused quartz substrate shows no significant changes with annealing temperature (not showed here). However, the transmittance of the ZnO films has a value about 45% at 325 nm (see Fig. 3), so the emission from the substrate may contribute to the collected luminescence. In order to clearly exclude the contribution of substrate, we attempt to deposit a layer of ZnO film with very low transmittance at 325 nm. A precursor solution with a Zn2+ concentration 1.5 M is prepared for this purpose, and all other parameters, such as the ratio of CA to Zn2+ , are kept unchanged as described in Section 2. Fig. 5(a) shows the XRD pattern of ZnO film yielded from this precursor solution. Compared with the XRD patterns in Fig. 1, the XRD pattern in Fig. 5(a) has a high ratio of signal to noise, and clearly shows that the film is with a wurtzite ZnO structure and without any secondary phases, such as Zn2 SiO4 and β-Zn2 SiO4 . The
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Ta = 800 ◦ C can be explained as that more defects are generated in the agglomerating procedure between crystalline grains. However, more experimental evidence is required to confirm such an interpretation. 4. Conclusion In a summary, ZnO films on fused quartz substrates are prepared by a glycol-based Pechini method, and relevant structural and optical properties are investigated. The ZnO films have wurtzite structure. The band gap energy Eg of these films is about 3.27 eV. A narrow red emission in the PL spectra of ZnO films is observed, and its peak intensity gains an enhancement (∼140%) while annealing temperature is changed from 700 to 800 ◦ C. The red emission is explained by assuming an − up-shifting of defect (V+ O , Oi ) levels in the boundary layer of crystalline grain, and the enhancement of red emission is explained by the increase of the amount of defects in the boundary layer. Acknowledgements
Fig. 5. XRD pattern, PL spectrum, and transmittance spectrum of ZnO film prepared from the precursor solution with a Zn2+ concentration 1.5 M and annealed at 700 ◦ C. (a) XRD pattern. (b) PL spectrum and transmittance spectrum.
film also shows a transmittance as low as about 0.01 at 325 nm (see Fig. 5(b)). The luminescence contributed by the substrate should be significantly suppressed. However, the red emission around 700 nm shows no significant decrease in Fig. 5(b). This indicates that the red emission around 700 nm should be ascribed to the ZnO film on fused quartz substrate. The red emission and YG emission are two distinguishable bands, and they should be contributed by two or more independent radiative transitions. In a crystalline grain, the atoms in the vicinity of surface or boundary form a boundary layer, in which the potential field has a significant difference with that in an ideal crystal. In a nanoscaled grain, the boundary layer cannot be neglected since the ratio of surface to volume increases with the decreased grain size. The boundary layer usually prohibits electrons from transferring over it, so it works approximately as a short-range repulsive potential acting on electrons. Because the DL defect state is usually a localized state, the defect level in boundary layer obtains an up-shifting relative to valence band. − V+ O (Oi ) is at 2.05 eV (2.07 eV) below conduction band in an ideal crystal [12,40]. If a 0.3 eV up-shifting of defect levels in boundary layer is assumed, the red emission can be well explained by transition of conduction band to these defect levels in boundary layer. The YG emission still can be explained by transition of conduction band to these defect levels [9,10,12] or surface states to valence state [41] in inner layer of crystalline grain. The enhancement of red emission and YG emission at
This work was supported by National Natural Science Foundation of China through Grant No. 10374066. The SEM observation was performed in Instrumental Analysis Center of Shanghai Jiaotong University. One of the authors (J.H. Cai) would also like to acknowledge Mr. A.B. Shi and Prof. W.Z. Shen for the measurement of PL spectra. References [1] C. Klingshirn, Phys. Status Solidi B 71 (1975) 547. [2] See, for example, the review by Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do˘gan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [3] S. Bethke, H. Pan, B.W. Wessels, Appl. Phys. Lett. 52 (1988) 138. [4] Y. Chen, D.M. Bagnall, Z. Zhu, T. Sekiuchi, K.-T. Park, K. Hiraga, T. Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [5] D.M. Bagnall, Y.F. Chen, M.Y. Shen, Z. Zhu, T. Goto, T. Yao, J. Cryst. Growth 184/185 (1998) 605. [6] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 456. [7] H.J. Ko, Y.F. Chen, Z. Zhu, T. Yao, I. Kobayashi, H. Uchiki, Appl. Phys. Lett. 76 (2000) 1905. [8] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35. [9] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [10] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285. [11] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. [12] Y.W. Heo, D.P. Norton, S.J. Pearton, J. Appl. Phys. 98 (2005) 073502. [13] F.K. Shan, G.X. Liu, W.J. Lee, B.C. Shin, J. Appl. Phys. 101 (2007) 053106. [14] F.K. Shan, B.C. Shin, S.W. Jang, Y.S. Yu, J. Eur. Ceram. Soc. 24 (2004) 1015. [15] P.-T. Hsieh, Y.-C. Chen, K.-S. Kao, C.-M. Wang, Physica B 403 (2008) 178. [16] A. Ortíz, C. Falcony, J.A. Hernández, M. Garcia, J.C. Alonso, Thin Solid Films 293 (1997) 103. [17] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [18] A. Chatterjee, C.H. Shen, A. Ganguly, L.C. Chen, C.W. Hsu, J.Y. Hwang, K.H. Chen, Chem. Phys. Lett. 391 (2004) 278. [19] Y.-S. Kim, W.-P. Tai, S.-J. Shu, Thin Solid Films 491 (2005) 153.
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Red luminescence in ZnO films prepared by a glycol-based Pechini method J.H. Cai a,∗ , G. Ni b , G. He a , Z.Y. Wu a a Department of Physics, Shanghai Jiaotong University, Shanghai 200240, China b Department of Optics Science and Engineering, Fudan University, Shanghai 200433, China
Received 19 January 2008; received in revised form 5 March 2008; accepted 7 March 2008 Available online 14 March 2008 Communicated by R. Wu
Abstract ZnO thin films on fused quartz substrates were prepared by a glycol-based Pechini method. The structural and optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance spectrum, and photoluminescence (PL) spectrum. A red emission around 700 nm was found in PL spectrum, and its peak intensity gained a strong enhancement (∼140%) while annealing temperature increased from 700 ◦ C to 800 ◦ C. The red emission was ascribed to the possible high defect density in boundary layers of nanocrystalline grains. © 2008 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.66.Hf; 71.55.Gs Keywords: Luminescence; Optical materials and properties; Thin films; Defects; Sol-gel preparation
1. Introduction ZnO is a II–VI group compound semiconductor, and is widely recognized as an ideal material for application in exciton-related optoelectronic devices because of its wide band gap of 3.37 eV at room temperature and large exciton binding energy of 60 meV [1]. The photoluminescent properties of ZnO films have been extensively investigated. The photoluminescence (PL) spectrum is normally composed of two parts: near-band-edge (NBE) ultraviolet (UV) emission and deeplevel (DL) yellow–green (YG) emission [2]. The NBE UV emission is usually ascribed to free excitons, bound excitons, and their phonon replicas [3–7]. The DL YG emission is usually ascribed to various defects, such as VO , VZn , Oi , Zni , OZn [8–13], etc. The characteristic of PL spectrum usually depends on the fabrication methods, growth conditions, and annealing treatments. In epitaxial ZnO films with a low density of DL defects, such as prepared by molecular beam epitaxy [4,7,12], chemical vapor deposition [3], pulsed laser deposition [14], * Corresponding author. Tel.: +86 21 54743165; fax: +86 21 54741040.
E-mail address: [email protected] (J.H. Cai). 0375-9601/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2008.03.011
etc., the NBE UV emission dominates the PL spectrum. In most polycrystalline thin films prepared by magnetron sputtering [11,15], spray pyrolysis [16,17], sol-gel method [18–21], significant DL emissions can be observed, and in some cases the DL emissions dominate the PL spectrum [17]. However, some exceptions should be mentioned here. In some polycrystalline thin films prepared by electrodeposition [22], sol-gel method [23], the NBE emissions dominate the PL spectrum. Even more, in polycrystalline thin films prepared by the oxidation of metallic Zn film [24–26] and compound ZnS film [27–29], a strong and narrow NBE emission accompanied by very weak DL emissions in the PL spectrum was observed. The high PL intensity ratio of the NBE emission to DL emission was regarded as an indication of the high quality of the nanocrystalline ZnO films prepared by the oxidation of the corresponding precursor films. In comparison with the works on the NBE UV emission and DL YG emission, the works on the red emission in ZnO film are very scarce [17,30–32]. In ZnO film prepared by electrodeposition [30], the PL spectrum at low temperature (25 K to 200 K) shows a broad emission centered at about 620 nm. In doped ZnO film prepared by a sol-gel method [31], the PL spectrum shows a broad emission centered at about 680 nm in the
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range of 400 to 700 nm, and the emission is attributed to host material ZnO. In ZnO film grown by spray pyrolysis [32], the PL spectrum also shows a broad emission centered at 700 nm and 800 nm in the range of 550 to 850 nm. In this Letter, the structural and optical properties of ZnO film prepared by a glycol-based Pechini method [33] are investigated, a narrow red emission centered at 700 nm is found, and possible origins are discussed. 2. Experimental Zinc nitrate, citric acid (CA), and ethylene glycol (EG) were employed as raw materials. CA was used as complexation agent, and EG was used either as polymerization agent or as solvent agent. The 0.02 mol zinc nitrate and 0.03 mol CA were dissolved in 50 ml EG at 50 ◦ C, then the solution was evaporated at 80 ◦ C until the final volume reached 20 ml. The precursor solution was spin-coated on fused quartz substrate in 3000 rpm for 30 s, then the precursor film was baked on a hot plate at 110 ◦ C for 10 min to remove the moisture, and fired in a furnace at 400 ◦ C for 30 min to remove organic components. The above procedure was repeated six times. Finally the films were annealed at 500, 600, 700, and 800 ◦ C for 2 h. The obtained film has a thickness 0.3 µm, which was estimated directly from the cross section photograph by scanning electron microscopy (SEM). The crystalline structures of ZnO films were studied by X-ray diffraction (XRD) (Bruker D8 Advance) with CuKα radiation (λ = 1.5406 Å). The surface morphology was observed by SEM (FEI SIRION 200). The optical transmittance spectra were recorded by a spectrometer (7-STAR 7ISW302) in the wavelength range of 200 to 850 nm. The PL measurements were performed on a Jobin Yvon LabRAM HR 800 microRaman system, which was equipped with an Andor DU420 classic charge-coupled device detector, in the wavelength range of 330 to 800 nm. For excitation, the 325 nm line of a 3 mW He–Cd laser was used. The excitation light was focused on the surface of sample, and a light spot with size ∼100 µm was yielded. 3. Results and discussion Fig. 1 shows the XRD patterns. A broad peak centered at about 22◦ (2θ value) should be assigned to amorphous silica substrate [34]. When annealing temperature Ta is over 600 ◦ C, the peaks corresponding to (100), (002), (101) planes of wurtzite ZnO phase emerge. The intensity of these peaks shows no significant enhancement with further increase of Ta . At a high temperature, ZnO is able to react with SiO2 and form Zn2 SiO4 or β-Zn2 SiO4 . In SiO2 -coated ZnO ultrafine particles or a ZnO/SiO2 nanocomposite film, the formation temperature of Zn2 SiO4 is about 700 ◦ C [35,36]. In a ZnO/SiO2 nanocomposite, the formation temperature of β-Zn2 SiO4 is about 800 ◦ C [34]. In a Tb-doped ZnO film on the amorphous SiO2 substrate, the formation temperature of Zn2 SiO4 is about 850 ◦ C [37]. In our samples only wurtzite ZnO phase is found, and no secondary phases, such as Zn2 SiO4 or β-Zn2 SiO4 , are found. However, this conclusion for secondary phase suffers from the
Fig. 1. XRD patterns of ZnO films annealed at different temperatures.
low ratio of signal to noise in XRD measurement. We will discuss this problem again in the analysis of transmittance spectra. Fig. 2 shows the SEM photographs. The photographs show the ZnO films consist of nanoscaled crystalline grains. Apparently the grain size increases significantly while Ta is changed from 600 to 700 ◦ C. In the film annealed at 800 ◦ C, some crystalline grains agglomerate, simultaneously, more unoccupied spaces around these grains are left. The morphology indicates that the ZnO film annealed at 800 ◦ C becomes porous, at least in the vicinity of surface. In addition to the agglomeration between crystalline grains, the surface of grain becomes slightly coarse at Ta = 800 ◦ C. Fig. 3 shows transmittance spectra. In the range of 200 to 375 nm, the transmittance slightly decreases while Ta is changed from 600 to 700 ◦ C. This should be attributed to the increase of crystallinity since the transition of valence band to conduction band should be responsible for the absorption in this range. The transmittance in the range from 220 to 375 nm shows no significant increase with Ta , and this indicates that no secondary phase, such as Zn2 SiO4 , forms while Ta is increased. This is because that the Eg of zinc silicate is about 5.7 eV [37] and the transmittance in this range should increases with the amount of zinc silicate transformed from zinc oxide and fused quartz [36,37]. In the range of 400 to 850 nm, the transmittance decreases while Ta increases from 500 to 700 ◦ C. However, the transmittance at 800 ◦ C slightly increases in the range of 420 to 850 nm, and decreases rapidly below 500 nm. The DL defects should be responsible for the absorption in this range. From transmittance spectrum, the absorption coefficient α can be obtained as α = − ln(T )/d, where d is the thickness of film and T is transmittance. For the direct transition in a semiconductor, α ∝ (hν − Eg )1/2 /(hν) for hν approaches Eg , where hν is photon energy and Eg is optical band gap energy. Hence the Eg satisfies equation (hν)2 ln2 (T ) = C(hν − Eg ), where C is a constant. Usually Eg is obtained by a linear fitting to (hν)2 ln2 (T ). The inset in Fig. 3 shows a plot of (hν)2 ln2 (T )
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Fig. 3. Transmittance spectra of ZnO films annealed at different temperatures. The inset shows a plot of (ln(T )hν)2 against hν for film annealed at 700 ◦ C. In the inset the circle (◦) denotes experimental data, and the straight line is a linear fitting to them.
Fig. 2. SEM images of ZnO films annealed at different temperatures (a) 500 ◦ C, (b) 600 ◦ C, (c) 700 ◦ C, and (d) 800 ◦ C.
against hν for film annealed at 700 ◦ C, and the obtained Eg is 3.27 eV. The dependence of Eg on annealing temperature is very weak, and the band gap energy Eg at Ta = 500, 600, 800 ◦ C is 3.29, 3.28, 3.25 eV, respectively. Here the obtained Eg is consistent with many reported values, such as in the films prepared by sol-gel method [19–21,38], thermal oxidation [39], pulsed laser deposition [14]. Fig. 4 shows PL spectra. The PL spectrum is composed of three emissions: an UV emission, a YG emission, and a red emission. The film annealed at 700 ◦ C exhibits a maximum peak value and a minimum full-width at half-maximum (FWHM) in UV emission range. This indicates that the best crystal quality is obtained at Ta = 700 ◦ C. A red emission around 700 nm is observed in ZnO films. The red emission becomes strong while annealing temperature increases from 700 ◦ C to 800 ◦ C, and the peak intensity gains an enhancement about 140%. Compared with the reported red luminescence in ZnO films [17,30–32], here the red emission shows a narrow range from 620 nm to 800 nm and it can be distinguished clearly from YG emission.
Fig. 4. PL spectra of ZnO films annealed at different temperatures.
The red emission should be mainly contributed by ZnO film, not by fused quartz substrate, because the luminescence in fused quartz substrate shows no significant changes with annealing temperature (not showed here). However, the transmittance of the ZnO films has a value about 45% at 325 nm (see Fig. 3), so the emission from the substrate may contribute to the collected luminescence. In order to clearly exclude the contribution of substrate, we attempt to deposit a layer of ZnO film with very low transmittance at 325 nm. A precursor solution with a Zn2+ concentration 1.5 M is prepared for this purpose, and all other parameters, such as the ratio of CA to Zn2+ , are kept unchanged as described in Section 2. Fig. 5(a) shows the XRD pattern of ZnO film yielded from this precursor solution. Compared with the XRD patterns in Fig. 1, the XRD pattern in Fig. 5(a) has a high ratio of signal to noise, and clearly shows that the film is with a wurtzite ZnO structure and without any secondary phases, such as Zn2 SiO4 and β-Zn2 SiO4 . The
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Ta = 800 ◦ C can be explained as that more defects are generated in the agglomerating procedure between crystalline grains. However, more experimental evidence is required to confirm such an interpretation. 4. Conclusion In a summary, ZnO films on fused quartz substrates are prepared by a glycol-based Pechini method, and relevant structural and optical properties are investigated. The ZnO films have wurtzite structure. The band gap energy Eg of these films is about 3.27 eV. A narrow red emission in the PL spectra of ZnO films is observed, and its peak intensity gains an enhancement (∼140%) while annealing temperature is changed from 700 to 800 ◦ C. The red emission is explained by assuming an − up-shifting of defect (V+ O , Oi ) levels in the boundary layer of crystalline grain, and the enhancement of red emission is explained by the increase of the amount of defects in the boundary layer. Acknowledgements
Fig. 5. XRD pattern, PL spectrum, and transmittance spectrum of ZnO film prepared from the precursor solution with a Zn2+ concentration 1.5 M and annealed at 700 ◦ C. (a) XRD pattern. (b) PL spectrum and transmittance spectrum.
film also shows a transmittance as low as about 0.01 at 325 nm (see Fig. 5(b)). The luminescence contributed by the substrate should be significantly suppressed. However, the red emission around 700 nm shows no significant decrease in Fig. 5(b). This indicates that the red emission around 700 nm should be ascribed to the ZnO film on fused quartz substrate. The red emission and YG emission are two distinguishable bands, and they should be contributed by two or more independent radiative transitions. In a crystalline grain, the atoms in the vicinity of surface or boundary form a boundary layer, in which the potential field has a significant difference with that in an ideal crystal. In a nanoscaled grain, the boundary layer cannot be neglected since the ratio of surface to volume increases with the decreased grain size. The boundary layer usually prohibits electrons from transferring over it, so it works approximately as a short-range repulsive potential acting on electrons. Because the DL defect state is usually a localized state, the defect level in boundary layer obtains an up-shifting relative to valence band. − V+ O (Oi ) is at 2.05 eV (2.07 eV) below conduction band in an ideal crystal [12,40]. If a 0.3 eV up-shifting of defect levels in boundary layer is assumed, the red emission can be well explained by transition of conduction band to these defect levels in boundary layer. The YG emission still can be explained by transition of conduction band to these defect levels [9,10,12] or surface states to valence state [41] in inner layer of crystalline grain. The enhancement of red emission and YG emission at
This work was supported by National Natural Science Foundation of China through Grant No. 10374066. The SEM observation was performed in Instrumental Analysis Center of Shanghai Jiaotong University. One of the authors (J.H. Cai) would also like to acknowledge Mr. A.B. Shi and Prof. W.Z. Shen for the measurement of PL spectra. References [1] C. Klingshirn, Phys. Status Solidi B 71 (1975) 547. [2] See, for example, the review by Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do˘gan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [3] S. Bethke, H. Pan, B.W. Wessels, Appl. Phys. Lett. 52 (1988) 138. [4] Y. Chen, D.M. Bagnall, Z. Zhu, T. Sekiuchi, K.-T. Park, K. Hiraga, T. Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [5] D.M. Bagnall, Y.F. Chen, M.Y. Shen, Z. Zhu, T. Goto, T. Yao, J. Cryst. Growth 184/185 (1998) 605. [6] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 456. [7] H.J. Ko, Y.F. Chen, Z. Zhu, T. Yao, I. Kobayashi, H. Uchiki, Appl. Phys. Lett. 76 (2000) 1905. [8] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35. [9] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [10] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285. [11] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. [12] Y.W. Heo, D.P. Norton, S.J. Pearton, J. Appl. Phys. 98 (2005) 073502. [13] F.K. Shan, G.X. Liu, W.J. Lee, B.C. Shin, J. Appl. Phys. 101 (2007) 053106. [14] F.K. Shan, B.C. Shin, S.W. Jang, Y.S. Yu, J. Eur. Ceram. Soc. 24 (2004) 1015. [15] P.-T. Hsieh, Y.-C. Chen, K.-S. Kao, C.-M. Wang, Physica B 403 (2008) 178. [16] A. Ortíz, C. Falcony, J.A. Hernández, M. Garcia, J.C. Alonso, Thin Solid Films 293 (1997) 103. [17] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [18] A. Chatterjee, C.H. Shen, A. Ganguly, L.C. Chen, C.W. Hsu, J.Y. Hwang, K.H. Chen, Chem. Phys. Lett. 391 (2004) 278. [19] Y.-S. Kim, W.-P. Tai, S.-J. Shu, Thin Solid Films 491 (2005) 153.
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