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Effect of Zn-doping on the structural and optical properties of BaTiO3 thin films grown by pulsed laser deposition
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Thin Solid Films 516 (2008) 6226 – 6232 www.elsevier.com/locate/tsf
Effect of Zn-doping on the structural and optical properties of BaTiO3 thin films grown by pulsed laser deposition A.Y. Fasasi a,b,⁎, M. Maaza b , E.G. Rohwer c , D. Knoessen d , Ch. Theron b , A. Leitch e , U. Buttner f b
a Centre for Energy Research & Development, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Nano-Sciences Laboratories, Materials Research Group, iThemba LABS, National Research Foundation, P. O. Box 722, Somerset West 7129, South Africa c Laser Research Institute, Department of Physics, University of Stellenbosch, Stellenbosch, Western Cape, South Africa d Department of Physics, University of Western Cape, Private Bag X1001, Belville, South Africa e Department of Physics, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa f Department of Electrical Engineering, University of Stellenbosch, Stellenbosch, Western Cape, South Africa
Received 8 August 2006; received in revised form 5 September 2007; accepted 28 November 2007 Available online 10 January 2008
Abstract Thin films of zinc oxide doped barium titanate (BaZnxTi1 − xO3) have been prepared by pulsed laser ablation using different targets having zinc composition varying between x = 1 to 5 wt.% at a step of 1 wt.% on corning glass microscope slide and silicon substrates. X-ray diffraction analyses showed films to be of tetragonal phase with an average grain size of 20 nm and c/a ratio of 1.08 indicating lattice expansion due to ZnO incorporation. Atomic force microscopy studies of the prepared thin films indicated smooth surfaces with average roughness of 1.84 and 4.6 nm for as-deposited and sintered specimens respectively. Scanning electron microscopy showed films to be smooth and uniform. UV–Visible as well as Fourier Transform Infrared transmission measurements showed a transmission of more than 80% in the visible and 5–20% in the near infrared. The transmittance is strongly affected by annealing. There is a dependence of band gap energy on film thickness as well as on the amount of ZnO added. High ZnO dopant level led to an increase in the band gap. © 2007 Elsevier B.V. All rights reserved. Keywords: Barium titanate; Zinc oxide; Optical properties; Band gap; Refractive index dispersion; Laser ablation
1. Introduction Barium titanate (BaTiO3) is one of the best-known perovskite ferroelectric compounds (formula A2+B4+O3) that have been extensively studied due to the simplicity of its crystal structure, which can accommodate different types of dopant. This has led to the possibility of tailoring the properties [1] of doped barium titanate for specific technological applications which include optical memory and optical processing devices which is based on a large photorefractive effect in doped materials [2], ferroelectric dynamic random access memory, thin film high dielectric capacitors, pyroelectric detectors, surface acoustic wave ⁎ Corresponding author. Centre for Energy Research & Development, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. E-mail address: [email protected] (A.Y. Fasasi). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.11.123
devices, thin film electroluminescent display and infrared imagers [3]. These improved properties, through thin film as well as bulk samples, have been achieved by employing different methods of preparation which include sputtering [4–6] (radio frequency (rf) plasma sputtering), laser ablation [7–9], sol–gel [1,10–12], screen printing [13] and spin coating [14]. In order to improve the functional qualities and extend the areas of application of barium titanate, different elements have been employed as dopants to occupy the A2+ or the B4+ site [15–18], though the improvement observed is strongly dependent on the processing route, the type of dopant and the occupied site in the BaTiO3 lattice. In spite of the fact that zinc oxide showed an excellent optoelectronic [19–21] as well as a good piezoelectric property, few reports are available on the use of zinc oxide as a dopant in barium titanate. Therefore, this report deals with the preparation
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and characterization of zinc oxide doped barium titanate thin films and its influence on the structural as well as the optical properties. 2. Experimental procedure Barium titanate powder (99.99% pure) and zinc oxide (99.99% pure) were purchased from Alfa Aesar and weighed in the right proportion to produce xZnO:(1 − x)BaTiO3 with “x” varied between 1 and 5 wt.%. Each composition was thoroughly ground in a mortar to produce a homogeneous distribution of zinc oxide. This was followed by producing a disk shape of 1 cm diameter with a pelletizer at a pressure of 2 × 109 Pa. Sintering of the pellets was effected at a constant heating and cooling rate of 5 °C per minute in oxygen ambient. The maximum sintering temperature was 1150 °C with a dwell time of 6 h. Laser ablation was carried out at the Laser Research Institute, Physics Department, University of Stellenbosch, with an excimer laser from Lambda Physik (Model EMG-203 MSC) having a wavelength of 308 nm. Preliminary studies carried out on BaTiO3–1 wt.% ZnO on the influence of temperature, oxygen pressure and laser power on the integrity of the films led to the adoption of the following optimisation parameters for film deposition; laser power (105 mJ per pulse), base pressure (5 × 10− 3 Pa), oxygen pressure (2 Pa), substrate temperature (400 °C), substrate–target-distance (40 mm), repetition rate (5 Hz) and wavelength (308 nm). Films of different thicknesses were deposited on Corning glass microscope slides and silicon substrates of 10 mm × 10 mm dimensions. From this point and for ease of reference, Corning glass microscope slide substrate will simply be referred to as glass substrate. In order to observe the influence of deposition time on film properties, some films were deposited on glass with deposition time varying between 20 and 180 min while other films were deposited for a constant time of 60 min to observe the influence of ZnO. Undoped BaTiO3 samples were also prepared on glass and gold/glass system for a constant deposition time of 60 min for the purpose of comparison. The gold layer which was 0.5 µm thick was deposited by electron beam evaporation. Samples prepared on glass substrates were annealed in air at a constant temperature of 550 °C for 12 h while samples deposited on silicon substrates were annealed at 800 °C for 6 h. From the interference fringes obtained through the transmission spectra, the optical parameters of the film, especially the thickness and the variation of refractive index “n” with wavelength “λ” can be determined using the envelope method developed by Swanepoel [22]. This method, carried out on the transmission spectra of BaTiO3–1 wt.% ZnO films deposited on glass substrates showed that film deposition times of 20, 40, 60, 80, 100 and 180 min correspond to film thickness of 100, 160, 255, 320, 369 and 677 nm respectively. The energy gap (Eg) was estimated by assuming a direct transition between the valence and conduction bands using the expression αhυ = A(hυ − Eg)1/2 where A is a constant. The absorption coefficient “α” was then determined using the relation a ¼ d1 ln T1 where “d” is the film thickness determined from the envelope method and T is the transmittance of the film.
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The optical band gap (Eg) can now be determined by extrapolations of the straight regions of the plot of (αhυ)2 vs hυ to intercept the energy axis. The structures of the films were determined through the use of X-ray diffraction (XRD) analyses carried out with X-ray Diffractometer model Bruker AXS D8 Advance and Philips PW 1729. The morphological aspects of the samples deposited on glass substrates were observed by employing a Leo-StereoScan 440 Scanning Electron Microscope (SEM) operated at an accelerating voltage of 20 kV and a current of 50 pA. The surface topography was observed with an atomic force microscopy (AFM) from Digital Instruments, VeecoMetrology Group with a silicon tip in a contact mode operation. Optical transmission measurements of the films on glass substrates in the wavelength range 200 to 3500 nm were performed with Varian Spectrometer (Models Cary 1E), Shimadzu UV-3100 and Perkin-Elmer Fourier Transform Infrared (FTIR) Spectrometer (Model Paragon 1000PC) with air taken as a reference medium. 3. Result and discussion 3.1. XRD XRD analyses showed that all samples deposited on glass and silicon substrates, irrespective of deposition temperature were amorphous without annealing. XRD analyses of some of the annealed films and pellets employed for ablation are shown in Fig. 1a–e. Fig. 1a and b represents the diffraction pattern of undoped BaTiO3 and BaTiO3–5 wt.% ZnO pellets employed for ablation. The [002] splitting is a strong indication that the tetragonal nature is maintained even after the incorporation of ZnO, though there is the presence of an unidentified peak at 2θ = 29° in the diffraction pattern of pure BaTiO3. The addition
Fig. 1. X-ray diffraction pattern obtained with 2θ ranging between 20 and 80° of (a) undoped and (b) 5 wt.%–zinc-doped BaTiO3 polycrystalline BaTiO3 pellet, 225-nm thick (c) 5 wt.% and (d) 1 wt.% ZnO doped BaTiO3 films deposited on glass and annealed in air at 550 °C for 12 h and (e) 255-nm thick 5 wt.% ZnO doped BaTiO3 film deposited on silicon and annealed in air at 800 °C for 6 h.
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of ZnO eliminates this peak but causes the appearance of a new unidentified peak at 2θ = 24° for all the doped pellet samples. The effect of annealing on the crystallinity of the film is shown through the typical XRD patterns in Fig. 1c–e for 255-nm film of BaTiO3–5 wt.% ZnO and 677-nm film of BaTiO3–1 wt.% ZnO deposited on glass respectively and 255-nm film of BaTiO3–5 wt.% ZnO deposited on silicon substrate. Comparison between Fig. 1c and d shows the film thickness effect and annealing in air at 550 °C for 12 h for film of BaTiO3–5 wt.% ZnO. These films showed a preferred (110) crystallographic orientation with the number of peaks increasing as the film thickness increases. The position of the diffraction peaks observed on 677-nm thick BaTiO3–1 wt.% ZnO film indicated a tetragonal phase with the absence of the unwanted peaks that were observed in the X-ray diffraction pattern of the pellets. No peaks belonging to zinc oxide were detected. V. Vaithianathan et al. [21] also observed a similar absence of arsenic (As) in XRD pattern of As-doped p-type ZnO grown on c-plane sapphire substrates at 600 °C by pulsed laser deposition (PLD), the presence of which was later confirmed by X-ray photoelectron spectroscopy. The nature of the peaks indicated that even after 12 h of annealing, the film did not totally crystallise and that some amorphous phase were still coexisting with the crystalline phase. This is understandable since recrystallisation is a function of temperature and time and it is not possible to raise the annealing temperature due to the melting point of the glass substrate, which is around 600 °C. On the other hand, comparison between Fig. 1c and e shows the influence of higher annealing temperature on the crystallinity of the film. The films deposited on silicon and annealed at 800 °C for 6 h showed a higher degree of crystallinity with diffraction peaks at 2θ = 33.01, 44.43 and 61.84° corresponding to (211), (002) and (103) crystallographic directions of silicon, barium titanate and zinc oxide respectively. On the influence of ZnO content on the crystallography of the film, the diffraction pattern of the 255-nm thick films deposited on glass substrates with varying ZnO content (not shown) indicates a very strong noise to signal ratio with only a peak corresponding to (110) crystallographic direction as the prominent peak in all the samples irrespective of the quantity of ZnO. The effect of annealing on the crystallite size of films deposited on glass substrates has been determined using Scherrer's equation (D = kA / βCosθ) where D is the grain size, λ is the wavelength (1.5418 Å for CuKα radiation), β is the full width at half maximum (expressed in radians) and k is the shape factor approximately equal to unity. The calculated average value of the annealed crystallite size using this equation for samples doped with 1 wt.% ZnO deposited at a constant substrate temperature of 400 °C with film thickness varying between 160 and 677 nm is approximately 20 nm. The film thickness apparently has no noticeable influence on the grain size. The lattice parameters “a” and “c” calculated from the (101) and (111) peaks of the 677-nm film of BaTiO3–1 wt.% ZnO are ´ respectively. These parameters give a “c/a” 3.869 and 4.214 Å ratio of 1.08. Comparison between the c/a ratio obtained in this study and the c/a ratio of 1.04 obtained from other source [23], shows that there is lattice expansion in the film due to the
incorporation of ZnO. This expansion can be explained if the difference in ionic size is taken into consideration. Zinc (with an ionic radius of 74 pm) in BaTiO3 can either substitute for Ba or Ti and given the ionic radius of Ba as 135 pm and that of Ti as 68 pm, then if Zn substitutes for Ba, the layer will contract while it will expand if substituted for Ti. Therefore, the magnitude of the c/a can be employed in determining the position that zinc occupies in the lattice of BaTiO3. Similar observation has been made with Al3+ employed as a substitutional point defect on the Zn lattice site in the Al:ZnO system [19]. 3.2. Surface analyses — AFM The surface roughness of the undoped 255-nm film of BaTiO3 samples has been studied with the aid of an AFM for
Fig. 2. AFM images of 255-nm thick BaTiO3 sample grown on (a) glass, annealed and taken with AFM z-range of 131 nm and (b) glass/gold, unannealed and taken with AFM z-range of 48 nm.
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Fig. 3. SEM micrograph of 320-nm thick film of BaTiO3–1 wt.% ZnO deposited on glass substrate.
two different surfaces and the result is shown in Fig. 2a and b. The measurement was performed on an area of the sample having 10 µm × 10 µm dimensions. Fig. 2a is depicting an undoped BaTiO3 deposited on glass substrate and annealed at 550 °C for 12 h in air while Fig. 2b is showing the sample deposited on glass/gold system without annealing. The influence of annealing can be seen on the large grain size of the annealed sample (manifestation of grain growth through recrystallisation) compared with the smaller grain size of the unannealed samples. Moreover, the surface roughness (Ra) has been determined to be 4.6 and 1.84 nm for the annealed and asdeposited films respectively with an uncertainty of ~ 5%. The surface roughness of the annealed sample may probably be an indication of atomic rearrangement and pore elimination leading to a more compact film.
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in the infrared is well above 80%. The effect of annealing on the transmittance in the UV–Visible region is negligible as shown in Fig. 4a. On the other hand, there is an increase of about 20% infrared (IR) transmittance of the same 100-nm film due to annealing as shown in Fig. 4b. In addition to this, Fig. 4b also shows the effect of substrate temperature varied between 200 and 500 °C. For as-deposited samples, the FTIR transmittance increases as the substrate temperature increases, while the reverse is the case after annealing. There is an overall increase in IR transmittance of the annealed samples compared with the asdeposited ones. Moreover, this high IR transmittance occurs between wavenumber 2000 and 3500 cm− 1 which is an indication of the insulating property of the film. The effect of increased film thickness and annealing can be observed in the FTIR transmission spectra of 369-nm thick film of BaTiO3–1 wt.% ZnO deposited on glass that is presented in Fig. 5. Comparing Fig. 5b with Fig. 4b, it can be seen that thicker films manifest a low IR transmittance. The reason for this low IR transmittance of thicker films compared with the high transmittance of thinner films after annealing is presently not understood. Furthermore, annealing in air ambient for 12 h at 550 °C reduced the transmission further; maximum transmittance of 60% for as-deposited compared with 30% for annealed samples. Moreover, the high transmittance region has
3.3. Morphology — SEM Typical morphology of the 320-nm thick BaTiO3–1 wt.% ZnO film deposited on glass substrates is shown in Fig. 3. The micrograph shows a uniform film with spherical-shaped droplets scattered over the surface. Analysis on the droplets carried out by (i) measuring the average diameter and hence the area of a droplet (ii) determining the number of droplets per unit surface area and (iii) calculating the total area covered by the droplets showed that the area covered by the droplets is 0.112%. Therefore we may deduce that the droplets have negligible influence on the properties of the films. Point compositional analysis carried out on the same film using energy dispersive X-ray analyses (not shown) indicated the presence of Si, Ti, Ba and oxygen with negligible amount of Zn. The presence of Si may be a contribution from the glass substrate. 3.4. Optical properties 3.4.1. Transmission studies Fig. 4 shows typical transmission spectra of a 100-nm thin BaTiO3–1 wt.% ZnO film taken with a UV–Visible spectrophotometer (Fig. 4a) as well as with FTIR spectroscopy (Fig. 4b). It can be observed that the average optical transmittance of the as-deposited films in the UV–Visible as well as
Fig. 4. Comparison of the transmission spectra of 100-nm thick as-deposited and annealed film samples of BaTiO3–1 wt.% ZnO on glass substrate (a) UV–Visible transmittance of samples deposited at a substrate temperature of 400 °C and (b) FTIR transmittance of the sample of the same composition deposited at different substrate temperatures as indicated in the legend.
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an increase of Eg with thickness. They concluded that the increase in film thickness results in a homogenous network with low density of defects and this is responsible for the observed increase in the band gap with thickness. Since the XRD pattern of the doped BaTiO3 in the present study indicated the presence of amorphous phase in the annealed film and since SrTiO3 and BaTiO3 have the same structure, it can be concluded that the presence of the amorphous phase must be responsible for the variation of band gap energies with thickness. The amorphous nature of the as-deposited doped films is manifested by an average band gap of 4.3 eV, a little lower than that of undoped films and this is in good agreement with the value reported by McClure et al. [27] for rf-sputtered films made from hot-pressed BaTiO3 targets and higher than 3.95 eV reported by Sreenivas et al. [28]. The influence of ZnO addition on the optical property of the film has been studied by varying the ZnO content from 1 to 5 wt.% with a constant film thickness of 255 nm. The result gives a constant band gap of 3.79 eV up to 3 wt.% ZnO and then the band gap increased to 3.91 and 3.93 eV for 4 and 5 wt.% ZnO additions respectively. Noting that the lattice constant
Fig. 5. Effect of increased thickness on the transmission spectra of the as-deposited and annealed samples of 369-nm thick BaTiO3–1 wt.% ZnO film on glass substrate (a) FTIR and (ii) UV–Visible transmission spectra of the same sample.
shifted to the 2750–3500 cm− 1 range, which may indicate an improvement in the crystallinity of the film due to annealing. The interference fringes present in the UV–Visible region (Fig. 5a) are an indication of a thick film with smooth surface. The optical band gap for deposited and annealed undoped BaTiO3 samples with a thickness of 255 nm is presented in Fig. 6a. It can be seen that the band gap decreased upon annealing from 4.52 eV to 3.97 eV. The result of the analyses on annealed 160-, 255-, 320-, 369- and 677-nm BaTiO3–1 wt.% ZnO films gave corresponding optical band gap of 3.9, 3.79, 3.78, 3.74 and 3.58 eV. Comparing these values with the reported band gap of single crystalline BaTiO3, which is 3.6 eV [24], we can then say that thicker films have band gap energies close to that of a single crystal. This type of band gap dependence on thickness has been observed by Bao et al. [25] who studied the band gap of sol–gel derived SrTiO3 in terms of annealing temperature and film thickness. They concluded that the band gap showed a strong dependence on film thickness and also that there is a critical thickness (~ 200 nm) above which the films had band gap energies close to those of crystals or bulk but below which the value shifted largely. This dependence was attributed to quantum size effect and the existence of amorphous phase in the films. Sati et al. [26] also studied the influence of thickness on optical properties of a:As2Se3 thin films but instead of observing a decrease in Eg with film thickness, they observed
Fig. 6. Influence of annealing on the optical band gap of 255-nm thick film deposited on glass substrate (a) annealed and as-deposited BaTiO3 (b) annealed zinc-doped BaTiO3.
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calculation indicated that Zn2+ substituted for Ti4+ in the BaTiO3 lattice and using Kroger–Vink notation, BaO þ ZnO→BaBa þ Zn UU Ti þ 2Oo þ Vo then the presence of Zn2+ on Ti4+ site will lead to the formation of BaTi1 − xZnxO3 − x and acceptor states that trap the unlocalised electrons thereby creating doubly charged acceptor centers ZnUU Ti that compensates for the oxygen vacancy formed. This substitution will provoke compositional and structural disorder because of (i) the difference in charge, (ii) the difference in ionic radii and (iii) modification of the bond length from Ti–O to Zn–O with a resultant change in bond energy. The increase in band gap from 4 wt.% ZnO occurred when the higher content of Zn has substituted for Ti causing higher degree of disorder. Similar observation, but on crossover from ferroelectric to diffuse phase transition, was made by Victor et al. [29] when a higher composition of Ca substituted for Ti in BaTiO3 lattice. Moreover, the result obtained in this study is further corroborated by Ishu et al. [30] who carried out a study on the influence of composition on the optical band gap of a:Ge20Se80 − xInx thin films. They concluded that the optical band gap increased in the first instance up to 10 at.% indium and thereafter it decreased with increase in indium content of the film. This trend was explained on the basis of changes in average bond energy of the system because the optical band gap is a bond sensitive property. 3.4.2. Refractive index dispersion The results obtained for the variation of refractive index with wavelength on samples with varying ZnO content and samples having varying film thickness are shown in Fig. 7a and b respectively. Fig. 7a indicates that the addition of ZnO changed the dispersion trend and that undoped BaTiO3 manifests lower refractive index of 2.33 at 600 nm than doped samples. This value of the refractive index for undoped BaTiO3 is lower than 2.4 obtained by Kaiser et al. [2] who employed MOCVD technique but agrees well with Aulika et al. [31] who obtained n = 2.31 at 633 nm on 320-nm thick BaTiO3 prepared by PLD. On the other hand, doped samples show increased refractive index as the concentration of dopant increased. From this result, it is probable that the mechanism responsible for the variation of band gap energy with ZnO content observed is also responsible for the variation of refractive index with ZnO content. The effect of film thickness on the refractive index dispersion, depicted in Fig. 7b indicated a slight change in the variation of refractive indices with wavelength up to 369-nm thick film. On the other hand, 677-nm film of BaTiO3–1 wt.% ZnO manifests a different dispersion trend compared with others; higher refractive index at low wavelength and lower refractive index at higher wavelength. The variation of the refractive index with thickness is not regular. Apparently, the observed variation of refractive index with thickness is not limited to BaTiO3 system. Girgis et al. [32] observed the same variation of refractive index with thickness on HgSe thin films grown by reactive solution. In the same vein, Senthilarasu et al. [33] observed the same dependence of refractive index on
Fig. 7. Influence of (a) ZnO content and (b) film thickness on the dispersion of refractive index with wavelength on samples deposited on glass substrates. (BT signifies BaTiO3 and BT1 signifies BaTiO3–1 wt.% ZnO).
thickness in zincphthalocyanine (ZnPc) thin films prepared by thermal evaporation technique. 4. Conclusion The study synthesized zinc oxide doped barium titanate for its optical properties. The results showed the film to be chemically stable and highly transparent with transmittance of over 80%. This high transmittance in the near infrared makes doped BaTiO3 suitable as an efficient window material. The addition of ZnO did not modify the room temperature structure of BaTiO3, which remains tetragonal, but led to lattice expansion with a “c/a” ratio of 1.08. There is a dependence of band gap energy on film thickness as well as on the amount of ZnO addition. The band gap decreases as the thickness increases while high dopant level increases the optical band gap. The refractive index obtained is greater than 2.3 for doped and undoped samples and coupling this with the fact that the refractive index of the glass substrate lies between 1.5 and 1.7, and then the presence of doped BaTiO3 on glass substrate will greatly enhance wave guiding. Moreover, since it retains its non-centro symmetric nature, it will equally be useful for second harmonic generation. This high refractive index makes doped BaTiO3 suitable for antireflection coating.
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Acknowledgements One of the authors, A. Y. Fasasi wishes to thank the African Laser Centre, National Research Foundation and iThemba LABS in South Africa for giving the opportunity and all the necessary support needed to carry out this study. Special appreciation goes to Prof. Botha and all the staff of the Physics Department, Nelson Mandela Metropolitan University in Port Elizabeth and the Laser Research Institute, University of Stellenbosch, Stellenbosch in South Africa. References [1] A. Jana, T.K. Kundu, S.K. Pradhan, D. Chakravorty, J. Appl. Phys. 97/4 (2005) 044311. [2] D.L. Kaiser, M.D. Vaudin, L.D. Rotter, Z.L. Wang, J.P. Cline, C.S. Hwang, R.B. Marinenko, J.G. Gillen, Appl. Phys. Lett. 66/21 (1995) 2801. [3] M.N. Kamalasanan, N.D. Kumar, S. Chandra, J. Appl. Phys. 74/9 (1993) 5679. [4] Y. Park, A. Grichener, J. Jensen, S.H. Choi, SPIE Photonics West 2005 Conference, Jan. 22–27, 2005, San Jose, CA, SPIE Proc., 5728, 2005, p. 67. [5] N.M. Abuhadba, C.R. Aita, J. Appl. Phys. 71/6 (1992) 3045. [6] Y.W. Cho, S.K. Choi, G.V. Rao, Appl. Phys. Lett. 86/20 (2005) 202905. [7] W.F. Zhang, Y.B. Huang, M.S. Zhang, Z.G. Liu, Appl. Phys. Lett. 76/8 (2000) 1003. [8] P. Victor, S.B. Krupanidhi, Semicond. Sci. Technol. 20/2 (2005) 250. [9] Y. Liu, Z. Chen, C. Li, D. Cui, Y. Zhou, G. Yang, Y. Zhu, J. Appl. Phys. 81/9 (1997) 6328. [10] D. Hreniak, E. Lukowiak, K. Maruszewski, R. Pazik, W. Strek, Mater. Sci. 20 (2002) 43. [11] R. Pazik, D. Hreniak, W. Strek, Mater. Sci.-Poland 22/3 (2004) 219. [12] T. Aitasalo, J. Holsa, H. Jungner, M. Lastuusari, J. Nittykoski, Mater. Sci. 20 (1) (2002) 15.
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Thin Solid Films 516 (2008) 6226 – 6232 www.elsevier.com/locate/tsf
Effect of Zn-doping on the structural and optical properties of BaTiO3 thin films grown by pulsed laser deposition A.Y. Fasasi a,b,⁎, M. Maaza b , E.G. Rohwer c , D. Knoessen d , Ch. Theron b , A. Leitch e , U. Buttner f b
a Centre for Energy Research & Development, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Nano-Sciences Laboratories, Materials Research Group, iThemba LABS, National Research Foundation, P. O. Box 722, Somerset West 7129, South Africa c Laser Research Institute, Department of Physics, University of Stellenbosch, Stellenbosch, Western Cape, South Africa d Department of Physics, University of Western Cape, Private Bag X1001, Belville, South Africa e Department of Physics, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa f Department of Electrical Engineering, University of Stellenbosch, Stellenbosch, Western Cape, South Africa
Received 8 August 2006; received in revised form 5 September 2007; accepted 28 November 2007 Available online 10 January 2008
Abstract Thin films of zinc oxide doped barium titanate (BaZnxTi1 − xO3) have been prepared by pulsed laser ablation using different targets having zinc composition varying between x = 1 to 5 wt.% at a step of 1 wt.% on corning glass microscope slide and silicon substrates. X-ray diffraction analyses showed films to be of tetragonal phase with an average grain size of 20 nm and c/a ratio of 1.08 indicating lattice expansion due to ZnO incorporation. Atomic force microscopy studies of the prepared thin films indicated smooth surfaces with average roughness of 1.84 and 4.6 nm for as-deposited and sintered specimens respectively. Scanning electron microscopy showed films to be smooth and uniform. UV–Visible as well as Fourier Transform Infrared transmission measurements showed a transmission of more than 80% in the visible and 5–20% in the near infrared. The transmittance is strongly affected by annealing. There is a dependence of band gap energy on film thickness as well as on the amount of ZnO added. High ZnO dopant level led to an increase in the band gap. © 2007 Elsevier B.V. All rights reserved. Keywords: Barium titanate; Zinc oxide; Optical properties; Band gap; Refractive index dispersion; Laser ablation
1. Introduction Barium titanate (BaTiO3) is one of the best-known perovskite ferroelectric compounds (formula A2+B4+O3) that have been extensively studied due to the simplicity of its crystal structure, which can accommodate different types of dopant. This has led to the possibility of tailoring the properties [1] of doped barium titanate for specific technological applications which include optical memory and optical processing devices which is based on a large photorefractive effect in doped materials [2], ferroelectric dynamic random access memory, thin film high dielectric capacitors, pyroelectric detectors, surface acoustic wave ⁎ Corresponding author. Centre for Energy Research & Development, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. E-mail address: [email protected] (A.Y. Fasasi). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.11.123
devices, thin film electroluminescent display and infrared imagers [3]. These improved properties, through thin film as well as bulk samples, have been achieved by employing different methods of preparation which include sputtering [4–6] (radio frequency (rf) plasma sputtering), laser ablation [7–9], sol–gel [1,10–12], screen printing [13] and spin coating [14]. In order to improve the functional qualities and extend the areas of application of barium titanate, different elements have been employed as dopants to occupy the A2+ or the B4+ site [15–18], though the improvement observed is strongly dependent on the processing route, the type of dopant and the occupied site in the BaTiO3 lattice. In spite of the fact that zinc oxide showed an excellent optoelectronic [19–21] as well as a good piezoelectric property, few reports are available on the use of zinc oxide as a dopant in barium titanate. Therefore, this report deals with the preparation
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and characterization of zinc oxide doped barium titanate thin films and its influence on the structural as well as the optical properties. 2. Experimental procedure Barium titanate powder (99.99% pure) and zinc oxide (99.99% pure) were purchased from Alfa Aesar and weighed in the right proportion to produce xZnO:(1 − x)BaTiO3 with “x” varied between 1 and 5 wt.%. Each composition was thoroughly ground in a mortar to produce a homogeneous distribution of zinc oxide. This was followed by producing a disk shape of 1 cm diameter with a pelletizer at a pressure of 2 × 109 Pa. Sintering of the pellets was effected at a constant heating and cooling rate of 5 °C per minute in oxygen ambient. The maximum sintering temperature was 1150 °C with a dwell time of 6 h. Laser ablation was carried out at the Laser Research Institute, Physics Department, University of Stellenbosch, with an excimer laser from Lambda Physik (Model EMG-203 MSC) having a wavelength of 308 nm. Preliminary studies carried out on BaTiO3–1 wt.% ZnO on the influence of temperature, oxygen pressure and laser power on the integrity of the films led to the adoption of the following optimisation parameters for film deposition; laser power (105 mJ per pulse), base pressure (5 × 10− 3 Pa), oxygen pressure (2 Pa), substrate temperature (400 °C), substrate–target-distance (40 mm), repetition rate (5 Hz) and wavelength (308 nm). Films of different thicknesses were deposited on Corning glass microscope slides and silicon substrates of 10 mm × 10 mm dimensions. From this point and for ease of reference, Corning glass microscope slide substrate will simply be referred to as glass substrate. In order to observe the influence of deposition time on film properties, some films were deposited on glass with deposition time varying between 20 and 180 min while other films were deposited for a constant time of 60 min to observe the influence of ZnO. Undoped BaTiO3 samples were also prepared on glass and gold/glass system for a constant deposition time of 60 min for the purpose of comparison. The gold layer which was 0.5 µm thick was deposited by electron beam evaporation. Samples prepared on glass substrates were annealed in air at a constant temperature of 550 °C for 12 h while samples deposited on silicon substrates were annealed at 800 °C for 6 h. From the interference fringes obtained through the transmission spectra, the optical parameters of the film, especially the thickness and the variation of refractive index “n” with wavelength “λ” can be determined using the envelope method developed by Swanepoel [22]. This method, carried out on the transmission spectra of BaTiO3–1 wt.% ZnO films deposited on glass substrates showed that film deposition times of 20, 40, 60, 80, 100 and 180 min correspond to film thickness of 100, 160, 255, 320, 369 and 677 nm respectively. The energy gap (Eg) was estimated by assuming a direct transition between the valence and conduction bands using the expression αhυ = A(hυ − Eg)1/2 where A is a constant. The absorption coefficient “α” was then determined using the relation a ¼ d1 ln T1 where “d” is the film thickness determined from the envelope method and T is the transmittance of the film.
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The optical band gap (Eg) can now be determined by extrapolations of the straight regions of the plot of (αhυ)2 vs hυ to intercept the energy axis. The structures of the films were determined through the use of X-ray diffraction (XRD) analyses carried out with X-ray Diffractometer model Bruker AXS D8 Advance and Philips PW 1729. The morphological aspects of the samples deposited on glass substrates were observed by employing a Leo-StereoScan 440 Scanning Electron Microscope (SEM) operated at an accelerating voltage of 20 kV and a current of 50 pA. The surface topography was observed with an atomic force microscopy (AFM) from Digital Instruments, VeecoMetrology Group with a silicon tip in a contact mode operation. Optical transmission measurements of the films on glass substrates in the wavelength range 200 to 3500 nm were performed with Varian Spectrometer (Models Cary 1E), Shimadzu UV-3100 and Perkin-Elmer Fourier Transform Infrared (FTIR) Spectrometer (Model Paragon 1000PC) with air taken as a reference medium. 3. Result and discussion 3.1. XRD XRD analyses showed that all samples deposited on glass and silicon substrates, irrespective of deposition temperature were amorphous without annealing. XRD analyses of some of the annealed films and pellets employed for ablation are shown in Fig. 1a–e. Fig. 1a and b represents the diffraction pattern of undoped BaTiO3 and BaTiO3–5 wt.% ZnO pellets employed for ablation. The [002] splitting is a strong indication that the tetragonal nature is maintained even after the incorporation of ZnO, though there is the presence of an unidentified peak at 2θ = 29° in the diffraction pattern of pure BaTiO3. The addition
Fig. 1. X-ray diffraction pattern obtained with 2θ ranging between 20 and 80° of (a) undoped and (b) 5 wt.%–zinc-doped BaTiO3 polycrystalline BaTiO3 pellet, 225-nm thick (c) 5 wt.% and (d) 1 wt.% ZnO doped BaTiO3 films deposited on glass and annealed in air at 550 °C for 12 h and (e) 255-nm thick 5 wt.% ZnO doped BaTiO3 film deposited on silicon and annealed in air at 800 °C for 6 h.
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of ZnO eliminates this peak but causes the appearance of a new unidentified peak at 2θ = 24° for all the doped pellet samples. The effect of annealing on the crystallinity of the film is shown through the typical XRD patterns in Fig. 1c–e for 255-nm film of BaTiO3–5 wt.% ZnO and 677-nm film of BaTiO3–1 wt.% ZnO deposited on glass respectively and 255-nm film of BaTiO3–5 wt.% ZnO deposited on silicon substrate. Comparison between Fig. 1c and d shows the film thickness effect and annealing in air at 550 °C for 12 h for film of BaTiO3–5 wt.% ZnO. These films showed a preferred (110) crystallographic orientation with the number of peaks increasing as the film thickness increases. The position of the diffraction peaks observed on 677-nm thick BaTiO3–1 wt.% ZnO film indicated a tetragonal phase with the absence of the unwanted peaks that were observed in the X-ray diffraction pattern of the pellets. No peaks belonging to zinc oxide were detected. V. Vaithianathan et al. [21] also observed a similar absence of arsenic (As) in XRD pattern of As-doped p-type ZnO grown on c-plane sapphire substrates at 600 °C by pulsed laser deposition (PLD), the presence of which was later confirmed by X-ray photoelectron spectroscopy. The nature of the peaks indicated that even after 12 h of annealing, the film did not totally crystallise and that some amorphous phase were still coexisting with the crystalline phase. This is understandable since recrystallisation is a function of temperature and time and it is not possible to raise the annealing temperature due to the melting point of the glass substrate, which is around 600 °C. On the other hand, comparison between Fig. 1c and e shows the influence of higher annealing temperature on the crystallinity of the film. The films deposited on silicon and annealed at 800 °C for 6 h showed a higher degree of crystallinity with diffraction peaks at 2θ = 33.01, 44.43 and 61.84° corresponding to (211), (002) and (103) crystallographic directions of silicon, barium titanate and zinc oxide respectively. On the influence of ZnO content on the crystallography of the film, the diffraction pattern of the 255-nm thick films deposited on glass substrates with varying ZnO content (not shown) indicates a very strong noise to signal ratio with only a peak corresponding to (110) crystallographic direction as the prominent peak in all the samples irrespective of the quantity of ZnO. The effect of annealing on the crystallite size of films deposited on glass substrates has been determined using Scherrer's equation (D = kA / βCosθ) where D is the grain size, λ is the wavelength (1.5418 Å for CuKα radiation), β is the full width at half maximum (expressed in radians) and k is the shape factor approximately equal to unity. The calculated average value of the annealed crystallite size using this equation for samples doped with 1 wt.% ZnO deposited at a constant substrate temperature of 400 °C with film thickness varying between 160 and 677 nm is approximately 20 nm. The film thickness apparently has no noticeable influence on the grain size. The lattice parameters “a” and “c” calculated from the (101) and (111) peaks of the 677-nm film of BaTiO3–1 wt.% ZnO are ´ respectively. These parameters give a “c/a” 3.869 and 4.214 Å ratio of 1.08. Comparison between the c/a ratio obtained in this study and the c/a ratio of 1.04 obtained from other source [23], shows that there is lattice expansion in the film due to the
incorporation of ZnO. This expansion can be explained if the difference in ionic size is taken into consideration. Zinc (with an ionic radius of 74 pm) in BaTiO3 can either substitute for Ba or Ti and given the ionic radius of Ba as 135 pm and that of Ti as 68 pm, then if Zn substitutes for Ba, the layer will contract while it will expand if substituted for Ti. Therefore, the magnitude of the c/a can be employed in determining the position that zinc occupies in the lattice of BaTiO3. Similar observation has been made with Al3+ employed as a substitutional point defect on the Zn lattice site in the Al:ZnO system [19]. 3.2. Surface analyses — AFM The surface roughness of the undoped 255-nm film of BaTiO3 samples has been studied with the aid of an AFM for
Fig. 2. AFM images of 255-nm thick BaTiO3 sample grown on (a) glass, annealed and taken with AFM z-range of 131 nm and (b) glass/gold, unannealed and taken with AFM z-range of 48 nm.
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Fig. 3. SEM micrograph of 320-nm thick film of BaTiO3–1 wt.% ZnO deposited on glass substrate.
two different surfaces and the result is shown in Fig. 2a and b. The measurement was performed on an area of the sample having 10 µm × 10 µm dimensions. Fig. 2a is depicting an undoped BaTiO3 deposited on glass substrate and annealed at 550 °C for 12 h in air while Fig. 2b is showing the sample deposited on glass/gold system without annealing. The influence of annealing can be seen on the large grain size of the annealed sample (manifestation of grain growth through recrystallisation) compared with the smaller grain size of the unannealed samples. Moreover, the surface roughness (Ra) has been determined to be 4.6 and 1.84 nm for the annealed and asdeposited films respectively with an uncertainty of ~ 5%. The surface roughness of the annealed sample may probably be an indication of atomic rearrangement and pore elimination leading to a more compact film.
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in the infrared is well above 80%. The effect of annealing on the transmittance in the UV–Visible region is negligible as shown in Fig. 4a. On the other hand, there is an increase of about 20% infrared (IR) transmittance of the same 100-nm film due to annealing as shown in Fig. 4b. In addition to this, Fig. 4b also shows the effect of substrate temperature varied between 200 and 500 °C. For as-deposited samples, the FTIR transmittance increases as the substrate temperature increases, while the reverse is the case after annealing. There is an overall increase in IR transmittance of the annealed samples compared with the asdeposited ones. Moreover, this high IR transmittance occurs between wavenumber 2000 and 3500 cm− 1 which is an indication of the insulating property of the film. The effect of increased film thickness and annealing can be observed in the FTIR transmission spectra of 369-nm thick film of BaTiO3–1 wt.% ZnO deposited on glass that is presented in Fig. 5. Comparing Fig. 5b with Fig. 4b, it can be seen that thicker films manifest a low IR transmittance. The reason for this low IR transmittance of thicker films compared with the high transmittance of thinner films after annealing is presently not understood. Furthermore, annealing in air ambient for 12 h at 550 °C reduced the transmission further; maximum transmittance of 60% for as-deposited compared with 30% for annealed samples. Moreover, the high transmittance region has
3.3. Morphology — SEM Typical morphology of the 320-nm thick BaTiO3–1 wt.% ZnO film deposited on glass substrates is shown in Fig. 3. The micrograph shows a uniform film with spherical-shaped droplets scattered over the surface. Analysis on the droplets carried out by (i) measuring the average diameter and hence the area of a droplet (ii) determining the number of droplets per unit surface area and (iii) calculating the total area covered by the droplets showed that the area covered by the droplets is 0.112%. Therefore we may deduce that the droplets have negligible influence on the properties of the films. Point compositional analysis carried out on the same film using energy dispersive X-ray analyses (not shown) indicated the presence of Si, Ti, Ba and oxygen with negligible amount of Zn. The presence of Si may be a contribution from the glass substrate. 3.4. Optical properties 3.4.1. Transmission studies Fig. 4 shows typical transmission spectra of a 100-nm thin BaTiO3–1 wt.% ZnO film taken with a UV–Visible spectrophotometer (Fig. 4a) as well as with FTIR spectroscopy (Fig. 4b). It can be observed that the average optical transmittance of the as-deposited films in the UV–Visible as well as
Fig. 4. Comparison of the transmission spectra of 100-nm thick as-deposited and annealed film samples of BaTiO3–1 wt.% ZnO on glass substrate (a) UV–Visible transmittance of samples deposited at a substrate temperature of 400 °C and (b) FTIR transmittance of the sample of the same composition deposited at different substrate temperatures as indicated in the legend.
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an increase of Eg with thickness. They concluded that the increase in film thickness results in a homogenous network with low density of defects and this is responsible for the observed increase in the band gap with thickness. Since the XRD pattern of the doped BaTiO3 in the present study indicated the presence of amorphous phase in the annealed film and since SrTiO3 and BaTiO3 have the same structure, it can be concluded that the presence of the amorphous phase must be responsible for the variation of band gap energies with thickness. The amorphous nature of the as-deposited doped films is manifested by an average band gap of 4.3 eV, a little lower than that of undoped films and this is in good agreement with the value reported by McClure et al. [27] for rf-sputtered films made from hot-pressed BaTiO3 targets and higher than 3.95 eV reported by Sreenivas et al. [28]. The influence of ZnO addition on the optical property of the film has been studied by varying the ZnO content from 1 to 5 wt.% with a constant film thickness of 255 nm. The result gives a constant band gap of 3.79 eV up to 3 wt.% ZnO and then the band gap increased to 3.91 and 3.93 eV for 4 and 5 wt.% ZnO additions respectively. Noting that the lattice constant
Fig. 5. Effect of increased thickness on the transmission spectra of the as-deposited and annealed samples of 369-nm thick BaTiO3–1 wt.% ZnO film on glass substrate (a) FTIR and (ii) UV–Visible transmission spectra of the same sample.
shifted to the 2750–3500 cm− 1 range, which may indicate an improvement in the crystallinity of the film due to annealing. The interference fringes present in the UV–Visible region (Fig. 5a) are an indication of a thick film with smooth surface. The optical band gap for deposited and annealed undoped BaTiO3 samples with a thickness of 255 nm is presented in Fig. 6a. It can be seen that the band gap decreased upon annealing from 4.52 eV to 3.97 eV. The result of the analyses on annealed 160-, 255-, 320-, 369- and 677-nm BaTiO3–1 wt.% ZnO films gave corresponding optical band gap of 3.9, 3.79, 3.78, 3.74 and 3.58 eV. Comparing these values with the reported band gap of single crystalline BaTiO3, which is 3.6 eV [24], we can then say that thicker films have band gap energies close to that of a single crystal. This type of band gap dependence on thickness has been observed by Bao et al. [25] who studied the band gap of sol–gel derived SrTiO3 in terms of annealing temperature and film thickness. They concluded that the band gap showed a strong dependence on film thickness and also that there is a critical thickness (~ 200 nm) above which the films had band gap energies close to those of crystals or bulk but below which the value shifted largely. This dependence was attributed to quantum size effect and the existence of amorphous phase in the films. Sati et al. [26] also studied the influence of thickness on optical properties of a:As2Se3 thin films but instead of observing a decrease in Eg with film thickness, they observed
Fig. 6. Influence of annealing on the optical band gap of 255-nm thick film deposited on glass substrate (a) annealed and as-deposited BaTiO3 (b) annealed zinc-doped BaTiO3.
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calculation indicated that Zn2+ substituted for Ti4+ in the BaTiO3 lattice and using Kroger–Vink notation, BaO þ ZnO→BaBa þ Zn UU Ti þ 2Oo þ Vo then the presence of Zn2+ on Ti4+ site will lead to the formation of BaTi1 − xZnxO3 − x and acceptor states that trap the unlocalised electrons thereby creating doubly charged acceptor centers ZnUU Ti that compensates for the oxygen vacancy formed. This substitution will provoke compositional and structural disorder because of (i) the difference in charge, (ii) the difference in ionic radii and (iii) modification of the bond length from Ti–O to Zn–O with a resultant change in bond energy. The increase in band gap from 4 wt.% ZnO occurred when the higher content of Zn has substituted for Ti causing higher degree of disorder. Similar observation, but on crossover from ferroelectric to diffuse phase transition, was made by Victor et al. [29] when a higher composition of Ca substituted for Ti in BaTiO3 lattice. Moreover, the result obtained in this study is further corroborated by Ishu et al. [30] who carried out a study on the influence of composition on the optical band gap of a:Ge20Se80 − xInx thin films. They concluded that the optical band gap increased in the first instance up to 10 at.% indium and thereafter it decreased with increase in indium content of the film. This trend was explained on the basis of changes in average bond energy of the system because the optical band gap is a bond sensitive property. 3.4.2. Refractive index dispersion The results obtained for the variation of refractive index with wavelength on samples with varying ZnO content and samples having varying film thickness are shown in Fig. 7a and b respectively. Fig. 7a indicates that the addition of ZnO changed the dispersion trend and that undoped BaTiO3 manifests lower refractive index of 2.33 at 600 nm than doped samples. This value of the refractive index for undoped BaTiO3 is lower than 2.4 obtained by Kaiser et al. [2] who employed MOCVD technique but agrees well with Aulika et al. [31] who obtained n = 2.31 at 633 nm on 320-nm thick BaTiO3 prepared by PLD. On the other hand, doped samples show increased refractive index as the concentration of dopant increased. From this result, it is probable that the mechanism responsible for the variation of band gap energy with ZnO content observed is also responsible for the variation of refractive index with ZnO content. The effect of film thickness on the refractive index dispersion, depicted in Fig. 7b indicated a slight change in the variation of refractive indices with wavelength up to 369-nm thick film. On the other hand, 677-nm film of BaTiO3–1 wt.% ZnO manifests a different dispersion trend compared with others; higher refractive index at low wavelength and lower refractive index at higher wavelength. The variation of the refractive index with thickness is not regular. Apparently, the observed variation of refractive index with thickness is not limited to BaTiO3 system. Girgis et al. [32] observed the same variation of refractive index with thickness on HgSe thin films grown by reactive solution. In the same vein, Senthilarasu et al. [33] observed the same dependence of refractive index on
Fig. 7. Influence of (a) ZnO content and (b) film thickness on the dispersion of refractive index with wavelength on samples deposited on glass substrates. (BT signifies BaTiO3 and BT1 signifies BaTiO3–1 wt.% ZnO).
thickness in zincphthalocyanine (ZnPc) thin films prepared by thermal evaporation technique. 4. Conclusion The study synthesized zinc oxide doped barium titanate for its optical properties. The results showed the film to be chemically stable and highly transparent with transmittance of over 80%. This high transmittance in the near infrared makes doped BaTiO3 suitable as an efficient window material. The addition of ZnO did not modify the room temperature structure of BaTiO3, which remains tetragonal, but led to lattice expansion with a “c/a” ratio of 1.08. There is a dependence of band gap energy on film thickness as well as on the amount of ZnO addition. The band gap decreases as the thickness increases while high dopant level increases the optical band gap. The refractive index obtained is greater than 2.3 for doped and undoped samples and coupling this with the fact that the refractive index of the glass substrate lies between 1.5 and 1.7, and then the presence of doped BaTiO3 on glass substrate will greatly enhance wave guiding. Moreover, since it retains its non-centro symmetric nature, it will equally be useful for second harmonic generation. This high refractive index makes doped BaTiO3 suitable for antireflection coating.
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