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Regulating effect of SiO2 interlayer on optical properties of ZnO thin films
Journal of Luminescence 136 (2013) 307–312
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Regulating effect of SiO2 interlayer on optical properties of ZnO thin films Linhua Xu a,b,n, Gaige Zheng a,b, Juhong Miao a, Jing Su a,b, Chengyi Zhang a,b, Hua Shen c, Lilong Zhao a,b a b c
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China Optics and Photonic Technology Laboratory, Nanjing University of Information Science and Technology, Nanjing 210044, China Institute of Electronic Engineering and Photo-electric Technology, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e i n f o
abstract
Article history: Received 25 June 2012 Received in revised form 30 September 2012 Accepted 4 December 2012 Available online 13 December 2012
ZnO/SiO2 nanocomposite films with periodic structure were prepared by electron beam evaporation technique. Regulating effect of SiO2 interlayer with various thicknesses on the optical properties of ZnO/SiO2 thin films was investigated deeply. The analyses of X-ray diffraction show that the ZnO layers in ZnO/SiO2 nanocomposite films have a wurtzite structure and are preferentially oriented along the c-axis while the SiO2 layers are amorphous. The scanning electron microscope images display that the ZnO layers are composed of columnar grains and the thicknesses of ZnO and SiO2 layers are all very uniform. The SiO2 interlayer presents a significant modulation effect on the optical properties of ZnO thin films, which is reflected in the following two aspects: (1) the transmittance of ZnO/SiO2 nanocomposite films is increased; (2) the photoluminescence (PL) of ZnO/SiO2 nanocomposite films is largely enhanced compared with that of pure ZnO thin films. The ZnO/SiO2 nanocomposite films have potential applications in light-emitting devices and flat panel displays. & 2012 Elsevier B.V. All rights reserved.
Keywords: ZnO/SiO2 nanocomposite films Electron beam evaporation Periodic structure Optical transmittance Photoluminescence
1. Introduction ZnO is an important direct band gap semiconductor, which has a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV. In recent years, the ZnO thin film has attracted lots of attention due to its excellent optical and electrical properties. It can be used in many optoelectronic devices such as lightemitting diodes [1], ultraviolet lasers [2], ultraviolet photoconductive detectors [3], optical waveguides [4], optical storages [5], and so on. In practical applications, people always want to control or adjust the optical properties of ZnO thin films according to their practical needs, such as changing the optical band gap, increasing the transmittance in the visible region, enhancing the excitonic emission, adjusting the wavelength of the visible emissions, etc. Many methods can be utilized to adjust the optical properties of ZnO thin films, such as doping other atoms in ZnO thin films [6], adjusting the deposition parameters [7], performing post-annealing treatments [8], adopting surface modification [9], and forming composite films with other metal oxide [10,11]. The former three methods have been extensively used and deeply studied, while the latter two methods are widely adopted only lately. For instance, many researchers adopted metal nanolayers or nanoparticles to cap the ZnO thin films in order to enhance n Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Ningliu Road 219#, Nanjing 210044, China. Tel./fax: þ86 25 58731174. E-mail addresses: [email protected], [email protected] (L. Xu).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.009
ultraviolet emission through surface plasmon resonance. Also, some researchers prepared ZnO-based composite films to adjust the optical properties of ZnO thin films. For example, Wang et al. [10] prepared ZnO/MgO multilayer films by pulsed laser deposition and studied the optical properties of the samples. Pankratov et al. [12] fabricated ZnO/SiO2 multilayer films by RF-magnetron sputtering and found that ZnO nanocrystal size had an important influence on the emission wavelength of the composite films. Among the materials which are often used to form composite films with ZnO, the SiO2 is one of the most used. A lot of researchers prepared ZnO/SiO2 nanocomposite materials with ZnO nanocrystals embedded in the SiO2 matrix and adjusted the optical properties by controlling the growth conditions of ZnO nanocrystals. For example, Fu et al. [13] prepared ZnO–SiO2 nanocomposites by sol–gel method and found that the ultraviolet emission was greatly enhanced compared with that of pure ZnO crystals. Chakrabarti et al. [14] deposited ZnO–SiO2 nanocomposite films on quartz glass by sol–gel method. They investigated the mechanisms of visible emissions and found that the optical band gap decreased with the increase of annealing temperature. Sharma et al. [15] fabricated ZnO quantum dots embedded in SiO2 matrix by a wet chemical method and found that surface effects significantly influenced the luminescence of this material. Hong et al. [16] prepared amorphous ZnO/silica composites by sol–gel method and studied the influence of calcination temperature on the photoluminescence. They found that the PL emission spectra consisted of near-UV (365 nm), violet (404 nm), blue (443 nm) and green (517 nm) bands, and the sample calcined at 700 1C
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showed the strongest PL emission. Recently, Panigrahi et al. [17] obtained ordered dispersion of ZnO quantum dots in SiO2 matrix by Stober method and found that these materials had excellent luminescent properties which could be used to fabricate flat panel displays. From the above-mentioned results, it can be noticed that many ZnO/SiO2 nanocomposite materials have been prepared by wet chemical methods while they are rarely prepared by physical vapor methods. Especially for the ZnO/SiO2 multilayer films with periodic structure deposited by physical method, they are seldom reported to our knowledge. Considering the ZnO/SiO2 nanocomposite films have important potential applications in flat panel displays [12,18] and biosensors [19], it is very important to deeply investigate the preparation and the optical properties of ZnO/SiO2 nanocomposite films. In this work, ZnO/SiO2 multilayer films with periodic structure were prepared by electron beam evaporation technique and the regulating effect of the SiO2 interlayer thickness on the optical properties was studied intensively. The electron beam evaporation is a widely used film deposition method, which is especially suitable for preparation of multicomponent metal oxide thin films. The films prepared by this technique are very uniform and high-purity. Compared with magnetron sputtering and pulsed laser deposition, it does not need to compress the metal oxide powders into a sheet-like target beforehand; the metal oxide powders can be directly used as the evaporation source materials.
2. Experiments ZnO/SiO2 multilayer films were prepared by electron beam evaporation equipments (PMC90S, Protech Korea Ltd.) on Si and glass substrates. One ZnO layer and one SiO2 layer is a unit; each composite film consists of four units. The raw materials are highpurity ZnO and SiO2 particles. Before deposition, the substrates have been rinsed by acetone, ethanol and deionized water. Evaporation source is about 1.5 m from the sample holder. The rinsed and dried substrates are placed on the holder. When the deposition is in progress, two evaporation sources are used alternately. The substrates are rotated at 40 rpm in order to obtain uniform films. The substrate temperature is 250 1C and the working pressure is 3.0 10 4 Pa. The thickness of each layer
is controlled by a quartz crystal thickness controller (IC5). First a SiO2 layer is deposited on substrates, and then a ZnO layer is deposited on the SiO2 layer. The deposition process is repeated four times. In order to study the influence of the SiO2 interlayer thickness on the optical properties of the composite film, three samples with the SiO2 interlayer thickness of 4, 8 and 16 nm, were prepared. The thickness of each ZnO layer is 50 nm. A pure ZnO thin film with 200 nm thickness is also deposited under the same deposition conditions for comparison with the ZnO/SiO2 multilayer film. All the samples were not annealed. The cross-section morphology of the samples was analyzed by a field emission scanning electron microscope (S4800). The crystal structures of the samples were analyzed by an X-ray diffractometer (XRD) (D/max-2500/PC). The transmittance was recorded by a spectrophotometer (UV-3600). The luminescence behavior was investigated by a PL spectrometer (LS50B) with an excitation wavelength of 325 nm.
3. Results and discussion Fig. 1 shows the cross-sectional images of the samples. Fig. 1(a) is the cross-sectional image of the pure ZnO thin film, from which it can be seen that the ZnO thin film is composed of columnar grains and its structure is very dense. Fig. 1(b)–(d) displays the crosssectional images of ZnO/SiO2 multilayer films. From these images, we can clearly see that the ZnO/SiO2 multilayer films consist of four units. Each unit includes one ZnO layer and one SiO2 layer. The thickness of the SiO2 layer in the three samples is 4, 8 and 16 nm, respectively. The thicknesses of ZnO layers and SiO2 layers are very uniform in every sample. The ZnO layers are also composed of columnar grains. Fig. 2 presents the XRD patterns of the pure ZnO thin film and ZnO/SiO2 multilayer films. All the samples show a strong (002) peak of ZnO, indicating that the ZnO layers have a hexagonal wurtzite structure and are preferentially oriented along the c-axis direction while the SiO2 layers are amorphous. The crystallite size of ZnO is calculated from XRD patterns using Scherrer’s equation: d¼
0:9 l bcosy
ð1Þ
Fig. 1. Cross-sectional morphology images of the pure ZnO thin film (a) and ZnO/SiO2 composite films with the SiO2 layer thicknesses of 4 nm (b), 8 nm (c) and 16 nm (d).
L. Xu et al. / Journal of Luminescence 136 (2013) 307–312
309
Fig. 4. Transmittance spectra of the samples.
Fig. 2. XRD patterns of the samples.
Fig. 5. Surface roughness and average transmittance of the samples.
Fig. 3. Crystallite size and stress of the samples.
where d is the crystallite size, l is the wavelength of X-ray, b is the full width at half maximum (FWHM) of the (002) peak, and y is the Bragg angle. The crystallite size of the samples is shown in Fig. 3. Compared with the pure ZnO thin film, the ZnO crystallite sizes in ZnO/SiO2 multilayer films obviously decreased. This is probably connected with the stress in the films. The stress is estimated according to the following formula [20]:
s ¼ 233
cc0 GPa c0
ð2Þ
where c is the lattice parameter of the ZnO deposited in this study, c0 is the lattice parameter of single crystal ZnO without stress. The stress values are shown in Fig. 3. It can be seen that when the thickness of the SiO2 layer is thinner, the stress in the ZnO layers is larger. The stress value of ZnO/SiO2 multilayer films is negative, meaning that the lattice parameter c increased. The origin of the stress is mainly ascribed to the imperfection of ZnO crystal structure. The imperfection of ZnO crystal structure should mostly result from the native point defects. Considering that the deposition environment is oxygen-insufficient, the main point
defects ought to be oxygen vacancies and Zn interstitials. On the other hand, the existence of interstitial defects can increase the lattice parameter c. Therefore, we think that the main point defects in our samples should be Zn interstitials. According to the variation of the stress in the films, we speculate that the Zn interstitials are much more when the SiO2 interlayer is thinner. Fig. 4 displays the transmittance spectra of the samples. All the samples exhibit high transmittance in the visible region and a sharp absorption edge at 365 nm or so. It is interesting that in spite of the thickness of the SiO2 layer (within the range from 4 to 16 nm), the transmittance of the ZnO/SiO2 multilayer films increases compared with that of pure ZnO thin film. The average transmittance in the visible and near-infrared region (390 800 nm) is shown in Fig. 5. Generally speaking, if the film is thicker, the more light will be absorbed; if the interlayer is more in the composite film, the scattering of the light will be more serious due to more interfaces. The above-mentioned factors will both lead to the decline of the transmittance of the films. However, the situation here is not the case. The higher transmittance is obtained in the ZnO/SiO2 multilayer films rather than in the pure ZnO thin film. We think that because the thicknesses of the ZnO/SiO2 multilayer films and the pure ZnO thin film are relatively thin and the ZnO grains are preferentially oriented along the c-axis perpendicular to the substrate surface, the interface scattering and grain boundary scattering have little effect on the transmittance as the light propagates in the films. However, the surface scattering probably has a significant
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impact on the visible transmittance. In order to further verify this speculation, the average surface roughness of the samples is measured by an atomic force microscope in contact mode as shown in Fig. 5. For the pure ZnO thin film, its grains are comparatively big, leading to the comparatively large surface roughness. As for the ZnO/SiO2 multilayer films, the ZnO grains are comparatively small, accordingly leading to comparatively small surface roughness. When the thickness of the SiO2 interlayer increases from 4 to 16 nm, the average roughness of ZnO/SiO2 multilayer films is increased slightly due to the increase of the grain size. Therefore, the improvement of the transmittance of ZnO/SiO2 multilayer films should be attributed to the decrease of the surface roughness. Furthermore, the lower refractive index of SiO2 than that of ZnO is probably another reason for the enhanced transmittance of ZnO/ SiO2 multilayer films [21]. If the thickness of SiO2 interlayer is further increased or the thickness of ZnO layer is changed, whether the high transmittance of the composite films in the visible region is still obtained is unknown. It needs further study. These results are helpful for improving the transmittance of window materials in solar cells. From Fig. 4, it also can be seen that the oscillation of the transmittance in the visible region become more obvious with the increase of the thickness of the SiO2 interlayer, which is caused by optical interference at the interface between ZnO and SiO2 layers. What is more, the absorption edge of ZnO/SiO2 multilayer films has a slight red-shift compared with that of pure ZnO thin film. As for the red-shift of the absorption edge, it has been observed in some ZnO-based multilayer films [11]. It is deemed that the shift of the absorption edge is associated with the ratio of crystallized/disordered ZnO [6]. Owing to the introduction of SiO2 interlayers, the disorder of ZnO in ZnO/SiO2 multilayer films is increased compared with that of single ZnO thin film. It is known that disorder induces band tails in absorption spectrum of disordered film [6]. Therefore, the red-shift of the absorption edge of ZnO/SiO2 composite films is attributed to the increase of ZnO disorder. The red-shift of absorption edge means that the optical band gap has a change. Fig. 6 shows the plot of (ahv)2 hn. Extrapolation of linear portion of the plot to the horizontal axis at (ahv)2 ¼ 0 gives the band gap energy value. By this method, we obtained the optical band gaps of 3.28 and 3.26 eV for pure ZnO thin film and ZnO/SiO2 multilayer films, respectively. An interesting phenomenon is found that the optical band gap of ZnO/SiO2 multilayer films is basically stable when the SiO2 interlayer thickness is within the range of 416 nm. Fig. 7 shows the room-temperature photoluminescence spectra of pure ZnO thin film and ZnO/SiO2 multilayer films. It can be seen that the photoluminescence intensity of ZnO/SiO2 multilayer
Fig. 6. The plot of (ahv)2 hv for obtaining optical band gaps of the samples.
Fig. 7. (a and b) Photoluminescence spectra of the samples.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
films is markedly enhanced compared with that of pure ZnO thin film. Although the emission intensity is much changed, the shape of the spectrum is similar. In order to better analyze the luminescence behavior of the samples, we performed a Gaussian curve fitting for the photoluminescence spectrum of the pure ZnO thin film. It can be seen that the fitting curve (red line) is well in agreement with the original curve (black line). The result of fitting curve indicates that the five emission bands with peaks at 393 nm, 420 nm, 431 nm, 487 nm and 522 nm are superposed to form the broad emission band. The ultraviolet emission centered at 393 nm is attributed to the recombination of free excitons. For the ZnO/SiO2 multilayer films, the excitonic emission is obviously enhanced despite the thickness of SiO2 interlayer. Previously, Hong et al. [21] found that when a MgF2 layer was sandwiched in ZnO thin films, the ultraviolet emission was also enhanced. Hong et al. deemed that the enhanced ultraviolet emission resulted from the improvement of the crystalline quality of ZnO thin film due to the smaller lattice mismatch between MgF2 and ZnO than that between fused silica and ZnO. In the samples prepared by Hong et al., the thickness of ZnO layers and the single ZnO thin film is identical. However, for our samples, the thickness of ZnO layers in composite films and the single ZnO thin
L. Xu et al. / Journal of Luminescence 136 (2013) 307–312
film is different. Therefore, it is considered that the ultraviolet emission enhancement mechanism for our samples is different from that proposed by Hong et al. It can be seen from Fig. 4 that the ultraviolet absorption of ZnO/SiO2 multilayer films is obviously enhanced in comparison with that of the pure ZnO thin film. This is because the light wavelength is relatively short in the UV region, it is more easily scattered at the interface between ZnO and SiO2, leading to UV light to be more absorbed by ZnO layers. Since ZnO layers absorbed more UV photons which have a energy higher than the optical band gap, the excitonic emission of ZnO is enhanced accordingly. This explains why the excitonic emission of ZnO/SiO2 multilayer films is stronger than that of pure ZnO thin film. Furthermore, as a insulation layer, the SiO2 interlayers limit the motion range of electrons [18], which makes electron and hole recombine in the shorter distance hence improving the probability of radiative recombination. This can also improve the excitonic emission. Furthermore, we can see that the sample with the SiO2 interlayer of 4 nm thickness has the strongest photoluminescence while the photoluminescence intensity decreases with the increase of SiO2 interlayer thickness. May be it is connected with the size effect resulting from the variation of ZnO crystallite size [22]. The emissions centered at 420 and 431 nm are violet emissions which have been observed in some PL or CL spectra of ZnO/ SiO2 composites. For instance, Hong et al. [16] found that a violet emission centered at 404 nm occurred in their prepared amorphous ZnO/SiO2 composites. Peng et al. [23] found that ZnO quantum dots–SiO2 nanocomposite films deposited by them also showed a strong violet emission. In fact, the violet emission has been observed not only in ZnO/SiO2 composites but also in pure or doped ZnO thin films. For example, Rao and Kumar [24] prepared Ga-doped ZnO thin films by spray pyrolysis. They found that the samples had a very strong violet emission peak located at 419 nm and they deemed that the violet emission was probably due to the radiative defects related to the interface traps existing at the grain boundaries. Ahn et al. [25] prepared ZnO thin films by various growth methods and studied the deep level emissions of the samples. They found that the ZnO thin film prepared by magnetron sputtering showed a strong violet emission. Although the violet emission has been observed in some ZnO materials and widely studied, the emission mechanism is still controversial. Some researchers deem that the violet emission is probably due to radiative defects related to the interface traps existing at the grain boundaries [24,26], others ascribe the violet emission to Zn interstitial defects [25,27], and still others believe that the violet emission is mainly connected with Zn vacancy defects [28]. As for our samples, they are deposited in an oxygen-insufficient environment. Therefore, the point defects in these films should be oxygen vacancy and zinc interstitial. A lot of studies show that the green emission of ZnO is mainly connected with the oxygen vacancy defects [17,25]. However, the green emission in our samples is weak. Therefore, the zinc interstitial could be the main point defects. What is more, considering that the pure ZnO thin film shows the violet emission as strong as the ultraviolet emission and the position of this violet emission peak is the same as that of ZnO/SiO2 multilayer films, the violet emission could not mainly result from interface defects due to the introduction of SiO2 layers. However, the introduction of SiO2 layers may lead to more zinc interstitial defects which cause the violet emission. The former analyses of XRD also support that the Zn interstitials should be the main point defects. Furthermore, the Zn interstitial is a shallow donor [29]. Based on the above analyses, the authors attribute the violet emission to electron–hole recombination between the Zn interstitial level and valence band [30]. When the thickness of SiO2 interlayer is thinner, more Zn interstitials exist in ZnO layers, accordingly leading to stronger violet emission as shown in Fig. 7(a). The blue–green emission centered at 487 nm probably
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results from the electron transition from Zn interstitial level to Zn vacancy level [25]. The green emission centered at 522 nm is generally attributed to electron–hole radiative recombination at the oxygen vacancy (VO) center [25,29]. The difference on the violet and blue emissions of ZnO/SiO2 multilayer films mainly results from different defect density of Zn interstitials.
4. Conclusion In this work, we prepared pure ZnO thin film and ZnO/SiO2 composite films. Compared with the pure ZnO thin film, ZnO/SiO2 composite films present higher transmittance in the visible region and stronger absorption in the ultraviolet region. Photoluminescence spectra show that due to the SiO2 interlayers, excitonic emission efficiency of ZnO/SiO2 multilayer films is largely improved in comparison with that of pure ZnO thin film. We think that the multilayer structure can make excitation light (325 nm) be more absorbed owing to multiple scattering at the interface of ZnO and SiO2, hence improving the excitonic emission efficiency of ZnO. Furthermore, the sandwich of SiO2 layer leads to more Zn interstitial defects. Accordingly, the violet and blue emissions related to the Zn interstitials are also enhanced. The violet emission is attributed to the electron transition from Zn interstitial defect level to valence band and the blue emission probably results from the electron transition from Zn interstitial level to Zn vacancy level. The photoluminescence results indicate that the SiO2 interlayer can improve luminescence efficiency of ZnO thin films. The ZnO/SiO2 multilayer films have potential applications in future flat panel displays and light-emitting devices.
Acknowledgments This work is supported by the Scientific Research Foundation of Nanjing University of Information Science and Technology (Grant nos. S8110141001 (20100400) and S8111118001 (20110384)) and the National Natural Science Foundation of China (Grant no. 51002079). References [1] Sheng Chu, Jianze Zhao, Zheng Zuo, Jieying Kong, Lin Li, Jianlin Liu, J. Appl. Phys. 109 (2011) 123110. [2] Yong-ning He, Chang-chun Zhu, Jing-wen Zhang, Microelectron. J. 35 (2004) 389. [3] Q.A. Xu, J.W. Zhang, K.R. Ju, X.D. Yang, X. Hou, J. Cryst. Growth 289 (2006) 44. [4] Navina Mehan, Monika Tomar, Vinay Gupta, Abhai Mansingh, Opt. Mater. 27 (2004) 241. [5] K.C. Hui, C.W. Lai, H.C. Ong, Thin Solid Films 483 (2005) 222. ˇ [6] F. Kadi Allah, S. Yapi Abe´, C.M. Nu´nez, A. Khelil, L. Cattin, M. Morsli, J.C. Berne de, A. Bougrine, M.A. del Valle, F.R. Dı´az, Appl. Surf. Sci. 253 (2007) 9241. [7] Y. Hu, Y.Q. Chen, Y.C. Wu, M.J. Wang, G.J. Fang, C.Q. He, S.J. Wang, Appl. Surf. Sci. 255 (2009) 9279. [8] M.P. Bole, D.S. Patil, J. Phys. Chem. Solids 70 (2009) 466. [9] A.P. Abiyasa, S.F. Yu, S.P. Lau, Eunice S.P. Leong, H.Y. Yang, Appl. Phys. Lett. 90 (2007) 231106. [10] Zhao-yang Wang, Li-zhong Hu, Jie Zhao, He-qiu Zhang, Zhi-jun Wang, Vacuum 80 (2006) 977. [11] Daeil Kim, Opt. Commun. 285 (2012) 1212. [12] V. Pankratov, V. Osinniy, A. Nylandsted Larsen, B. Bech Nielsen, Physica B 404 (2009) 4827. [13] Zhengping Fu, Beifang Yang, Lin Li, Weiwei Dong, Chong Jia, Wan Wu, J. Phys: Condens. Matter 15 (2003) 2867. [14] S. Chakrabarti, D. Ganguli, S. Chaudhuri, Phys. Stat. Sol. (a) 201 (2004) 2134. [15] Prashant K. Sharma, Ranu K. Dutta, Manvendra Kumar, Prashant K. Singh, Avinash C. Pandey, J. Lumin. 129 (2009) 605. [16] Jian-He Hong, Yan-Fen Wang, Gang He, Jun-Xia Wang, J. Non-Cryst. Solids 356 (2010) 2778. [17] Shrabani Panigrahi, Ashok Bera, Durga Basak, J. Colloid Interface Sci. 353 (2011) 30.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Regulating effect of SiO2 interlayer on optical properties of ZnO thin films Linhua Xu a,b,n, Gaige Zheng a,b, Juhong Miao a, Jing Su a,b, Chengyi Zhang a,b, Hua Shen c, Lilong Zhao a,b a b c
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China Optics and Photonic Technology Laboratory, Nanjing University of Information Science and Technology, Nanjing 210044, China Institute of Electronic Engineering and Photo-electric Technology, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e i n f o
abstract
Article history: Received 25 June 2012 Received in revised form 30 September 2012 Accepted 4 December 2012 Available online 13 December 2012
ZnO/SiO2 nanocomposite films with periodic structure were prepared by electron beam evaporation technique. Regulating effect of SiO2 interlayer with various thicknesses on the optical properties of ZnO/SiO2 thin films was investigated deeply. The analyses of X-ray diffraction show that the ZnO layers in ZnO/SiO2 nanocomposite films have a wurtzite structure and are preferentially oriented along the c-axis while the SiO2 layers are amorphous. The scanning electron microscope images display that the ZnO layers are composed of columnar grains and the thicknesses of ZnO and SiO2 layers are all very uniform. The SiO2 interlayer presents a significant modulation effect on the optical properties of ZnO thin films, which is reflected in the following two aspects: (1) the transmittance of ZnO/SiO2 nanocomposite films is increased; (2) the photoluminescence (PL) of ZnO/SiO2 nanocomposite films is largely enhanced compared with that of pure ZnO thin films. The ZnO/SiO2 nanocomposite films have potential applications in light-emitting devices and flat panel displays. & 2012 Elsevier B.V. All rights reserved.
Keywords: ZnO/SiO2 nanocomposite films Electron beam evaporation Periodic structure Optical transmittance Photoluminescence
1. Introduction ZnO is an important direct band gap semiconductor, which has a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV. In recent years, the ZnO thin film has attracted lots of attention due to its excellent optical and electrical properties. It can be used in many optoelectronic devices such as lightemitting diodes [1], ultraviolet lasers [2], ultraviolet photoconductive detectors [3], optical waveguides [4], optical storages [5], and so on. In practical applications, people always want to control or adjust the optical properties of ZnO thin films according to their practical needs, such as changing the optical band gap, increasing the transmittance in the visible region, enhancing the excitonic emission, adjusting the wavelength of the visible emissions, etc. Many methods can be utilized to adjust the optical properties of ZnO thin films, such as doping other atoms in ZnO thin films [6], adjusting the deposition parameters [7], performing post-annealing treatments [8], adopting surface modification [9], and forming composite films with other metal oxide [10,11]. The former three methods have been extensively used and deeply studied, while the latter two methods are widely adopted only lately. For instance, many researchers adopted metal nanolayers or nanoparticles to cap the ZnO thin films in order to enhance n Corresponding author at: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science and Technology, Ningliu Road 219#, Nanjing 210044, China. Tel./fax: þ86 25 58731174. E-mail addresses: [email protected], [email protected] (L. Xu).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.009
ultraviolet emission through surface plasmon resonance. Also, some researchers prepared ZnO-based composite films to adjust the optical properties of ZnO thin films. For example, Wang et al. [10] prepared ZnO/MgO multilayer films by pulsed laser deposition and studied the optical properties of the samples. Pankratov et al. [12] fabricated ZnO/SiO2 multilayer films by RF-magnetron sputtering and found that ZnO nanocrystal size had an important influence on the emission wavelength of the composite films. Among the materials which are often used to form composite films with ZnO, the SiO2 is one of the most used. A lot of researchers prepared ZnO/SiO2 nanocomposite materials with ZnO nanocrystals embedded in the SiO2 matrix and adjusted the optical properties by controlling the growth conditions of ZnO nanocrystals. For example, Fu et al. [13] prepared ZnO–SiO2 nanocomposites by sol–gel method and found that the ultraviolet emission was greatly enhanced compared with that of pure ZnO crystals. Chakrabarti et al. [14] deposited ZnO–SiO2 nanocomposite films on quartz glass by sol–gel method. They investigated the mechanisms of visible emissions and found that the optical band gap decreased with the increase of annealing temperature. Sharma et al. [15] fabricated ZnO quantum dots embedded in SiO2 matrix by a wet chemical method and found that surface effects significantly influenced the luminescence of this material. Hong et al. [16] prepared amorphous ZnO/silica composites by sol–gel method and studied the influence of calcination temperature on the photoluminescence. They found that the PL emission spectra consisted of near-UV (365 nm), violet (404 nm), blue (443 nm) and green (517 nm) bands, and the sample calcined at 700 1C
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showed the strongest PL emission. Recently, Panigrahi et al. [17] obtained ordered dispersion of ZnO quantum dots in SiO2 matrix by Stober method and found that these materials had excellent luminescent properties which could be used to fabricate flat panel displays. From the above-mentioned results, it can be noticed that many ZnO/SiO2 nanocomposite materials have been prepared by wet chemical methods while they are rarely prepared by physical vapor methods. Especially for the ZnO/SiO2 multilayer films with periodic structure deposited by physical method, they are seldom reported to our knowledge. Considering the ZnO/SiO2 nanocomposite films have important potential applications in flat panel displays [12,18] and biosensors [19], it is very important to deeply investigate the preparation and the optical properties of ZnO/SiO2 nanocomposite films. In this work, ZnO/SiO2 multilayer films with periodic structure were prepared by electron beam evaporation technique and the regulating effect of the SiO2 interlayer thickness on the optical properties was studied intensively. The electron beam evaporation is a widely used film deposition method, which is especially suitable for preparation of multicomponent metal oxide thin films. The films prepared by this technique are very uniform and high-purity. Compared with magnetron sputtering and pulsed laser deposition, it does not need to compress the metal oxide powders into a sheet-like target beforehand; the metal oxide powders can be directly used as the evaporation source materials.
2. Experiments ZnO/SiO2 multilayer films were prepared by electron beam evaporation equipments (PMC90S, Protech Korea Ltd.) on Si and glass substrates. One ZnO layer and one SiO2 layer is a unit; each composite film consists of four units. The raw materials are highpurity ZnO and SiO2 particles. Before deposition, the substrates have been rinsed by acetone, ethanol and deionized water. Evaporation source is about 1.5 m from the sample holder. The rinsed and dried substrates are placed on the holder. When the deposition is in progress, two evaporation sources are used alternately. The substrates are rotated at 40 rpm in order to obtain uniform films. The substrate temperature is 250 1C and the working pressure is 3.0 10 4 Pa. The thickness of each layer
is controlled by a quartz crystal thickness controller (IC5). First a SiO2 layer is deposited on substrates, and then a ZnO layer is deposited on the SiO2 layer. The deposition process is repeated four times. In order to study the influence of the SiO2 interlayer thickness on the optical properties of the composite film, three samples with the SiO2 interlayer thickness of 4, 8 and 16 nm, were prepared. The thickness of each ZnO layer is 50 nm. A pure ZnO thin film with 200 nm thickness is also deposited under the same deposition conditions for comparison with the ZnO/SiO2 multilayer film. All the samples were not annealed. The cross-section morphology of the samples was analyzed by a field emission scanning electron microscope (S4800). The crystal structures of the samples were analyzed by an X-ray diffractometer (XRD) (D/max-2500/PC). The transmittance was recorded by a spectrophotometer (UV-3600). The luminescence behavior was investigated by a PL spectrometer (LS50B) with an excitation wavelength of 325 nm.
3. Results and discussion Fig. 1 shows the cross-sectional images of the samples. Fig. 1(a) is the cross-sectional image of the pure ZnO thin film, from which it can be seen that the ZnO thin film is composed of columnar grains and its structure is very dense. Fig. 1(b)–(d) displays the crosssectional images of ZnO/SiO2 multilayer films. From these images, we can clearly see that the ZnO/SiO2 multilayer films consist of four units. Each unit includes one ZnO layer and one SiO2 layer. The thickness of the SiO2 layer in the three samples is 4, 8 and 16 nm, respectively. The thicknesses of ZnO layers and SiO2 layers are very uniform in every sample. The ZnO layers are also composed of columnar grains. Fig. 2 presents the XRD patterns of the pure ZnO thin film and ZnO/SiO2 multilayer films. All the samples show a strong (002) peak of ZnO, indicating that the ZnO layers have a hexagonal wurtzite structure and are preferentially oriented along the c-axis direction while the SiO2 layers are amorphous. The crystallite size of ZnO is calculated from XRD patterns using Scherrer’s equation: d¼
0:9 l bcosy
ð1Þ
Fig. 1. Cross-sectional morphology images of the pure ZnO thin film (a) and ZnO/SiO2 composite films with the SiO2 layer thicknesses of 4 nm (b), 8 nm (c) and 16 nm (d).
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Fig. 4. Transmittance spectra of the samples.
Fig. 2. XRD patterns of the samples.
Fig. 5. Surface roughness and average transmittance of the samples.
Fig. 3. Crystallite size and stress of the samples.
where d is the crystallite size, l is the wavelength of X-ray, b is the full width at half maximum (FWHM) of the (002) peak, and y is the Bragg angle. The crystallite size of the samples is shown in Fig. 3. Compared with the pure ZnO thin film, the ZnO crystallite sizes in ZnO/SiO2 multilayer films obviously decreased. This is probably connected with the stress in the films. The stress is estimated according to the following formula [20]:
s ¼ 233
cc0 GPa c0
ð2Þ
where c is the lattice parameter of the ZnO deposited in this study, c0 is the lattice parameter of single crystal ZnO without stress. The stress values are shown in Fig. 3. It can be seen that when the thickness of the SiO2 layer is thinner, the stress in the ZnO layers is larger. The stress value of ZnO/SiO2 multilayer films is negative, meaning that the lattice parameter c increased. The origin of the stress is mainly ascribed to the imperfection of ZnO crystal structure. The imperfection of ZnO crystal structure should mostly result from the native point defects. Considering that the deposition environment is oxygen-insufficient, the main point
defects ought to be oxygen vacancies and Zn interstitials. On the other hand, the existence of interstitial defects can increase the lattice parameter c. Therefore, we think that the main point defects in our samples should be Zn interstitials. According to the variation of the stress in the films, we speculate that the Zn interstitials are much more when the SiO2 interlayer is thinner. Fig. 4 displays the transmittance spectra of the samples. All the samples exhibit high transmittance in the visible region and a sharp absorption edge at 365 nm or so. It is interesting that in spite of the thickness of the SiO2 layer (within the range from 4 to 16 nm), the transmittance of the ZnO/SiO2 multilayer films increases compared with that of pure ZnO thin film. The average transmittance in the visible and near-infrared region (390 800 nm) is shown in Fig. 5. Generally speaking, if the film is thicker, the more light will be absorbed; if the interlayer is more in the composite film, the scattering of the light will be more serious due to more interfaces. The above-mentioned factors will both lead to the decline of the transmittance of the films. However, the situation here is not the case. The higher transmittance is obtained in the ZnO/SiO2 multilayer films rather than in the pure ZnO thin film. We think that because the thicknesses of the ZnO/SiO2 multilayer films and the pure ZnO thin film are relatively thin and the ZnO grains are preferentially oriented along the c-axis perpendicular to the substrate surface, the interface scattering and grain boundary scattering have little effect on the transmittance as the light propagates in the films. However, the surface scattering probably has a significant
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impact on the visible transmittance. In order to further verify this speculation, the average surface roughness of the samples is measured by an atomic force microscope in contact mode as shown in Fig. 5. For the pure ZnO thin film, its grains are comparatively big, leading to the comparatively large surface roughness. As for the ZnO/SiO2 multilayer films, the ZnO grains are comparatively small, accordingly leading to comparatively small surface roughness. When the thickness of the SiO2 interlayer increases from 4 to 16 nm, the average roughness of ZnO/SiO2 multilayer films is increased slightly due to the increase of the grain size. Therefore, the improvement of the transmittance of ZnO/SiO2 multilayer films should be attributed to the decrease of the surface roughness. Furthermore, the lower refractive index of SiO2 than that of ZnO is probably another reason for the enhanced transmittance of ZnO/ SiO2 multilayer films [21]. If the thickness of SiO2 interlayer is further increased or the thickness of ZnO layer is changed, whether the high transmittance of the composite films in the visible region is still obtained is unknown. It needs further study. These results are helpful for improving the transmittance of window materials in solar cells. From Fig. 4, it also can be seen that the oscillation of the transmittance in the visible region become more obvious with the increase of the thickness of the SiO2 interlayer, which is caused by optical interference at the interface between ZnO and SiO2 layers. What is more, the absorption edge of ZnO/SiO2 multilayer films has a slight red-shift compared with that of pure ZnO thin film. As for the red-shift of the absorption edge, it has been observed in some ZnO-based multilayer films [11]. It is deemed that the shift of the absorption edge is associated with the ratio of crystallized/disordered ZnO [6]. Owing to the introduction of SiO2 interlayers, the disorder of ZnO in ZnO/SiO2 multilayer films is increased compared with that of single ZnO thin film. It is known that disorder induces band tails in absorption spectrum of disordered film [6]. Therefore, the red-shift of the absorption edge of ZnO/SiO2 composite films is attributed to the increase of ZnO disorder. The red-shift of absorption edge means that the optical band gap has a change. Fig. 6 shows the plot of (ahv)2 hn. Extrapolation of linear portion of the plot to the horizontal axis at (ahv)2 ¼ 0 gives the band gap energy value. By this method, we obtained the optical band gaps of 3.28 and 3.26 eV for pure ZnO thin film and ZnO/SiO2 multilayer films, respectively. An interesting phenomenon is found that the optical band gap of ZnO/SiO2 multilayer films is basically stable when the SiO2 interlayer thickness is within the range of 416 nm. Fig. 7 shows the room-temperature photoluminescence spectra of pure ZnO thin film and ZnO/SiO2 multilayer films. It can be seen that the photoluminescence intensity of ZnO/SiO2 multilayer
Fig. 6. The plot of (ahv)2 hv for obtaining optical band gaps of the samples.
Fig. 7. (a and b) Photoluminescence spectra of the samples.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
films is markedly enhanced compared with that of pure ZnO thin film. Although the emission intensity is much changed, the shape of the spectrum is similar. In order to better analyze the luminescence behavior of the samples, we performed a Gaussian curve fitting for the photoluminescence spectrum of the pure ZnO thin film. It can be seen that the fitting curve (red line) is well in agreement with the original curve (black line). The result of fitting curve indicates that the five emission bands with peaks at 393 nm, 420 nm, 431 nm, 487 nm and 522 nm are superposed to form the broad emission band. The ultraviolet emission centered at 393 nm is attributed to the recombination of free excitons. For the ZnO/SiO2 multilayer films, the excitonic emission is obviously enhanced despite the thickness of SiO2 interlayer. Previously, Hong et al. [21] found that when a MgF2 layer was sandwiched in ZnO thin films, the ultraviolet emission was also enhanced. Hong et al. deemed that the enhanced ultraviolet emission resulted from the improvement of the crystalline quality of ZnO thin film due to the smaller lattice mismatch between MgF2 and ZnO than that between fused silica and ZnO. In the samples prepared by Hong et al., the thickness of ZnO layers and the single ZnO thin film is identical. However, for our samples, the thickness of ZnO layers in composite films and the single ZnO thin
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film is different. Therefore, it is considered that the ultraviolet emission enhancement mechanism for our samples is different from that proposed by Hong et al. It can be seen from Fig. 4 that the ultraviolet absorption of ZnO/SiO2 multilayer films is obviously enhanced in comparison with that of the pure ZnO thin film. This is because the light wavelength is relatively short in the UV region, it is more easily scattered at the interface between ZnO and SiO2, leading to UV light to be more absorbed by ZnO layers. Since ZnO layers absorbed more UV photons which have a energy higher than the optical band gap, the excitonic emission of ZnO is enhanced accordingly. This explains why the excitonic emission of ZnO/SiO2 multilayer films is stronger than that of pure ZnO thin film. Furthermore, as a insulation layer, the SiO2 interlayers limit the motion range of electrons [18], which makes electron and hole recombine in the shorter distance hence improving the probability of radiative recombination. This can also improve the excitonic emission. Furthermore, we can see that the sample with the SiO2 interlayer of 4 nm thickness has the strongest photoluminescence while the photoluminescence intensity decreases with the increase of SiO2 interlayer thickness. May be it is connected with the size effect resulting from the variation of ZnO crystallite size [22]. The emissions centered at 420 and 431 nm are violet emissions which have been observed in some PL or CL spectra of ZnO/ SiO2 composites. For instance, Hong et al. [16] found that a violet emission centered at 404 nm occurred in their prepared amorphous ZnO/SiO2 composites. Peng et al. [23] found that ZnO quantum dots–SiO2 nanocomposite films deposited by them also showed a strong violet emission. In fact, the violet emission has been observed not only in ZnO/SiO2 composites but also in pure or doped ZnO thin films. For example, Rao and Kumar [24] prepared Ga-doped ZnO thin films by spray pyrolysis. They found that the samples had a very strong violet emission peak located at 419 nm and they deemed that the violet emission was probably due to the radiative defects related to the interface traps existing at the grain boundaries. Ahn et al. [25] prepared ZnO thin films by various growth methods and studied the deep level emissions of the samples. They found that the ZnO thin film prepared by magnetron sputtering showed a strong violet emission. Although the violet emission has been observed in some ZnO materials and widely studied, the emission mechanism is still controversial. Some researchers deem that the violet emission is probably due to radiative defects related to the interface traps existing at the grain boundaries [24,26], others ascribe the violet emission to Zn interstitial defects [25,27], and still others believe that the violet emission is mainly connected with Zn vacancy defects [28]. As for our samples, they are deposited in an oxygen-insufficient environment. Therefore, the point defects in these films should be oxygen vacancy and zinc interstitial. A lot of studies show that the green emission of ZnO is mainly connected with the oxygen vacancy defects [17,25]. However, the green emission in our samples is weak. Therefore, the zinc interstitial could be the main point defects. What is more, considering that the pure ZnO thin film shows the violet emission as strong as the ultraviolet emission and the position of this violet emission peak is the same as that of ZnO/SiO2 multilayer films, the violet emission could not mainly result from interface defects due to the introduction of SiO2 layers. However, the introduction of SiO2 layers may lead to more zinc interstitial defects which cause the violet emission. The former analyses of XRD also support that the Zn interstitials should be the main point defects. Furthermore, the Zn interstitial is a shallow donor [29]. Based on the above analyses, the authors attribute the violet emission to electron–hole recombination between the Zn interstitial level and valence band [30]. When the thickness of SiO2 interlayer is thinner, more Zn interstitials exist in ZnO layers, accordingly leading to stronger violet emission as shown in Fig. 7(a). The blue–green emission centered at 487 nm probably
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results from the electron transition from Zn interstitial level to Zn vacancy level [25]. The green emission centered at 522 nm is generally attributed to electron–hole radiative recombination at the oxygen vacancy (VO) center [25,29]. The difference on the violet and blue emissions of ZnO/SiO2 multilayer films mainly results from different defect density of Zn interstitials.
4. Conclusion In this work, we prepared pure ZnO thin film and ZnO/SiO2 composite films. Compared with the pure ZnO thin film, ZnO/SiO2 composite films present higher transmittance in the visible region and stronger absorption in the ultraviolet region. Photoluminescence spectra show that due to the SiO2 interlayers, excitonic emission efficiency of ZnO/SiO2 multilayer films is largely improved in comparison with that of pure ZnO thin film. We think that the multilayer structure can make excitation light (325 nm) be more absorbed owing to multiple scattering at the interface of ZnO and SiO2, hence improving the excitonic emission efficiency of ZnO. Furthermore, the sandwich of SiO2 layer leads to more Zn interstitial defects. Accordingly, the violet and blue emissions related to the Zn interstitials are also enhanced. The violet emission is attributed to the electron transition from Zn interstitial defect level to valence band and the blue emission probably results from the electron transition from Zn interstitial level to Zn vacancy level. The photoluminescence results indicate that the SiO2 interlayer can improve luminescence efficiency of ZnO thin films. The ZnO/SiO2 multilayer films have potential applications in future flat panel displays and light-emitting devices.
Acknowledgments This work is supported by the Scientific Research Foundation of Nanjing University of Information Science and Technology (Grant nos. S8110141001 (20100400) and S8111118001 (20110384)) and the National Natural Science Foundation of China (Grant no. 51002079). References [1] Sheng Chu, Jianze Zhao, Zheng Zuo, Jieying Kong, Lin Li, Jianlin Liu, J. Appl. Phys. 109 (2011) 123110. [2] Yong-ning He, Chang-chun Zhu, Jing-wen Zhang, Microelectron. J. 35 (2004) 389. [3] Q.A. Xu, J.W. Zhang, K.R. Ju, X.D. Yang, X. Hou, J. Cryst. Growth 289 (2006) 44. [4] Navina Mehan, Monika Tomar, Vinay Gupta, Abhai Mansingh, Opt. Mater. 27 (2004) 241. [5] K.C. Hui, C.W. Lai, H.C. Ong, Thin Solid Films 483 (2005) 222. ˇ [6] F. Kadi Allah, S. Yapi Abe´, C.M. Nu´nez, A. Khelil, L. Cattin, M. Morsli, J.C. Berne de, A. Bougrine, M.A. del Valle, F.R. Dı´az, Appl. Surf. Sci. 253 (2007) 9241. [7] Y. Hu, Y.Q. Chen, Y.C. Wu, M.J. Wang, G.J. Fang, C.Q. He, S.J. Wang, Appl. Surf. Sci. 255 (2009) 9279. [8] M.P. Bole, D.S. Patil, J. Phys. Chem. Solids 70 (2009) 466. [9] A.P. Abiyasa, S.F. Yu, S.P. Lau, Eunice S.P. Leong, H.Y. Yang, Appl. Phys. Lett. 90 (2007) 231106. [10] Zhao-yang Wang, Li-zhong Hu, Jie Zhao, He-qiu Zhang, Zhi-jun Wang, Vacuum 80 (2006) 977. [11] Daeil Kim, Opt. Commun. 285 (2012) 1212. [12] V. Pankratov, V. Osinniy, A. Nylandsted Larsen, B. Bech Nielsen, Physica B 404 (2009) 4827. [13] Zhengping Fu, Beifang Yang, Lin Li, Weiwei Dong, Chong Jia, Wan Wu, J. Phys: Condens. Matter 15 (2003) 2867. [14] S. Chakrabarti, D. Ganguli, S. Chaudhuri, Phys. Stat. Sol. (a) 201 (2004) 2134. [15] Prashant K. Sharma, Ranu K. Dutta, Manvendra Kumar, Prashant K. Singh, Avinash C. Pandey, J. Lumin. 129 (2009) 605. [16] Jian-He Hong, Yan-Fen Wang, Gang He, Jun-Xia Wang, J. Non-Cryst. Solids 356 (2010) 2778. [17] Shrabani Panigrahi, Ashok Bera, Durga Basak, J. Colloid Interface Sci. 353 (2011) 30.
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