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Influence of reactive oxygen ambience on the structural, morphological and optical properties of pulsed laser ablated potassium lithium niobate thin films
Thin Solid Films 517 (2008) 603–608
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Influence of reactive oxygen ambience on the structural, morphological and optical properties of pulsed laser ablated potassium lithium niobate thin films V. Jayasree a, R. Ratheesh b, V. Ganesan c, V.R. Reddy c, C. Sudarsanakumar d, V.P. Mahadevan Pillai a,⁎, V.U. Nayar a a
Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram 695 581, Kerala, India Centre for Materials for Electronics Technology, Athani P. O, Thrissur, Kerala, India UGC-DAE Consortium for Scientific Research, Indore Centre, Madhya Pradesh, India d School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India b c
a r t i c l e
i n f o
Article history: Received 10 October 2007 Received in revised form 7 June 2008 Accepted 6 July 2008 Available online 17 July 2008 Keywords: Potassium lithium niobate Nonlinear optic materials Ferroelectric Tungsten bronze materials Electro-optic materials Pulsed laser deposition Transmittance spectra Optical band gap
a b s t r a c t The effect of oxygen ambience on the structure and properties of potassium lithium niobate (K3Li2Nb5O15: KLN) films prepared on glass substrates by pulsed laser ablation technique (PLD) are studied. The influence of annealing on the properties of vacuum deposited films is also investigated. The Gracing Incidence X-ray Diffraction (GIXRD) data suggests the tetragonal structure for the KLN film whose grain sizes increase on thermal annealing. The Atomic Force Microscopic (AFM) analysis reveals the four-fold symmetric nature of the grains in the films. Self assembly of grains in the form of rings and rods are observed in AFM images of the films deposited in an oxygen ambience of 2 Pa. The films deposited at higher oxygen ambience show a blue shift in optical band gap. The direct current (DC) resistance measurement on the films deposited at non-reactive ambience reveals resistivity in the range of kΩ m. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As a class of compounds, ferroelectric oxides have very attractive and potential properties such as wide band gap (N3 eV), large electrooptic and nonlinear optic coefficients and have the possibility of sustaining the spontaneous polarizations [1]. Ferroelectric tungsten– bronze crystals with general formula (A1)2(A2)4C4(B1)2(B2)8O30 have generated much interest, especially for nonlinear optical applications and for their large optical damage threshold originating from their crystal structure [2]. Potassium lithium niobate (K3Li2Nb5O15: KLN) with a completely filled tungsten–bronze type structure is a very promising material for various optical applications owing to its large electro-optic, nonlinear optic, piezoelectric and pyroelectric properties [3] However the growth of large single crystals of KLN is still a challenging problem as they crack easily while cooling down through paraelectric to ferroelectric phase. Moreover the crystallographic and dielectric properties are severely affected by the Nb2O5 content in the melt. Hence, the growth of epitaxial thin films of KLN is of particular interest, as high quality thin films of KLN are suitable for many of its applications. Different thin film depositions technique such as epitaxial growth by melting [4], RF magnetron sputtering [5,6] on ⁎ Corresponding author. E-mail address: [email protected] (V.P.M. Pillai). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.07.004
substrates like potassium bismuth niobate [5] and sapphire have been used for the epitaxial or polycrystalline growth of KLN thin films. However, preparation of KLN thin film using pulsed laser deposition technique has not yet been reported. Pulsed laser deposition (PLD) is an effective tool for the growth of quantum structures with high chemical purity and controlled stoichiometry. In PLD, one can control size distribution in nanocrystals by varying the parameters like target to substrate distance, laser fluence, background gas pressure etc. This paper presents a report of preparation of KLN thin film using pulsed laser deposition technique in non-reactive and reactive atmosphere. The as-deposited films prepared under vacuum condition, the films annealed in air atmosphere and the as-deposited films prepared under different reactive oxygen pressures, are characterised using Gracing Incidence X-Ray Diffraction (GIXRD), Atomic Force Microscopy (AFM), UV–Vis spectroscopy and direct current (DC) resistance measurements to study the effect of annealing and that of background oxygen pressure on the properties of the films. 2. Experiment 2.1. Preparation of stoichiometric target for PLD operation Stoichiometric composition of K2CO3, Li2CO3 and Nb2O5 (Aldrich, purity 99.99%) is mixed well in an agate mortar for an hour using
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distilled water as solvent. This mixture is then dried by heating in an oven at 100 °C and is transferred to an alumina crucible and calcined for 2 h at 1000 °C. The analysis of powder X-ray diffraction (XRD) patterns of this compound show a tetragonal tungsten–bronze structure with space group P4bm, Z = 2 and cell parameters a =b = 12.562 Å and c = 4.019 Å which are in good agreement with the reported data [7]. The compound thus prepared is powdered well using an agate motor and is pressed into pellet of size 7 mm thickness and 10 mm diameter using a hydraulic press. The pellet used as target for ablation, is prepared by sintering it at 1050 °C for 4 h. 2.2. KLN thin film preparation using PLD The deposition of the films is carried out inside a multiport stainless steel vacuum chamber equipped with a gas inlet, a rotating multitarget and a temperature controlled substrate holder. The irradiations are performed using a Q-switched Nd: YAG laser with frequency doubled 532 nm radiation of energy 70 mJ, pulse width 7 ns and repetition frequency of 10 Hz (Quanta – Ray INDI – series, Spectra Physics). The laser impinged on the target at 45° with respect to the normal in a dynamic flow of oxygen. Before irradiations, the deposition chamber is evacuated down to a base pressure of 4 × 10− 4 Pa using a diffusion pump and two rotary pumps. The depositions of the films are done on glass substrates kept at an on-axis distance of 70 mm from the target for a deposition time of 20 min. Glass is chosen as substrate to avoid the influence of single crystal substrate on the orientation of thin films. The films are deposited in non-reactive and reactive oxygen atmospheres with oxygen pressures of 0.2, 2 and 20 Pa. During ablation the target is rotated at a constant speed to avoid pitting of target at any given spot and to obtain uniform thin films. The films deposited in the non-reactive atmosphere are annealed at 473 K and 673 K. The asdeposited and annealed films are milky white in appearance. The crystalline nature and orientations of the deposited films are investigated by GIXRD (Siemens D5000 Diffractometer) measurements employing Cu Kα radiation with wavelength of 0.15406 nm. The surface morphology of the deposited films has been investigated using the AFM images recorded by a Digital Instrument Nanoscope E atomic force microscope. AFM tip of Si3N4 having a force constant of 0.58 N/m in contact mode operation has been employed for the measurements. The data are measured in 256 × 256 pixel format and the scan rate is 5.086 Hz. The rms roughness and the grain size are measured using the software associated with the instrument. For the grain size calculation, 4–5 frames are selected and 20 measurements are taken in each frame and the grain size is obtained with an accuracy of ± 4 nm. The optical transmission spectra of the films are recorded using a JASCO V 550 UV– VIS double beam spectrophotometer in the wavelength range of 190– 900 nm. The DC resistivity of the deposited films is determined using two probe method. Thickness of the as-deposited films are measured using a profilometer (XP stylus profile, Ambios Technology, USA). The as-deposited film prepared under non-reactive atmosphere yields a thickness value of 520 nm, whereas the thickness of the films deposited under the reactive oxygen pressures of 0.2, 2 and 20 Pa are found to be 500, 200 and 90 nm respectively. Quantitative energy dispersive spectra (EDS) of the films are recorded (standard less analysis with ZAF correction, acceleration voltage = 15 keV, live time for data acquisition = 62 s) using scanning electron microscope (SEM) JEOL JSM 5600LV. The EDS spectra show the peaks corresponding to potassium (13.5 at.%), niobium (39.2 at.%) and oxygen (47.1 at.%). 3. Results and discussions 3.1. Effect of thermal annealing 3.1.1. GIXRD and AFM results Fig. 1 shows the GIXRD patterns of the films prepared by PLD in the non-reactive atmosphere. The GIXRD pattern of the as-deposited film
Fig. 1. GIXRD patterns of laser ablated KLN thin films deposited on glass substrate in non-reactive ambience as a function of the annealing temperature.
shows three well defined peaks. The broad peak located at 22.25° has a d spacing corresponding to the c-axis lattice constant in the tetragonal tungsten–bronze structure range. Therefore this peak can be identified as the (001) lattice plane reflection of tetragonal tungsten–bronze crystalline phase of KLN. The broadness of this peak may be due to the closeness in d spacing of (001) and (310) reflection planes [8]. It may be noted that poor crystalline nature of the films can also cause broadness [9]. The other peaks at 29.3° and 31.75° correspond to (410) and (420) lattice reflection planes. The peak corresponding to the plane (001) appears with maximum intensity and this suggests that the preferred orientation of growth in KLN film is along (001). In the reported XRD data of KLN powder [7], the intensities of the peaks corresponding to the planes (001), (410) and (420) bear a ratio 6:75:2. However in the present case these peaks show an intensity ratio of 310:259:247. The growth mechanism of KLN thin film can be explained on the basis of cleavages observed in the KLN crystals during growth. In KLN crystal, cleavages are formed along the (001) plane during its growth [10,11]. Cleavages are understood as the planes where the bonding energy is weaker than the energy between the other planes [12,13]. The vacant octahedral sites present in the cleavage planes reduce the interfacial energy between the deposited film and the substrate. This plane of minimum surface energy promotes the nucleation and thereby preferential directional growth during crystallization [14]. Thus, without having any contribution from the substrate, KLN thin film grows preferentially along the (001) plane leading to higher intensity for this peak. In the film annealed at 473 K, the peaks corresponding to lattice reflection planes (001), (410) and (420) show a considerable reduction in intensity. A new KLN peak corresponding to (320) is also observed. The positions of all other peaks, except the peak corresponding to (410) lattice plane are shifted to higher 2θ values on annealing to 473 K. The development of new (320) KLN peak can be due to either the re-crystallization process or the stress relaxation process during annealing. For films annealed at 673 K, the peaks corresponding to (001), (410) and (420) planes appear with a still lesser intensity. At this temperature, the positions of the peaks corresponding to (410) and (320) planes shifted slightly to lower 2θ value. During annealing, the stress relaxation process takes place, which results in the shift in the position of peaks and the decrease in FWHM [14]. The average size of the crystalline grains in the films are determined by the following Scherer equation [9] Dhkl ¼
0:9λ β hkl cosðθhkl Þ
ð1Þ
where λ is the X-ray wavelength, θhkl is the Bragg diffraction angle and βhkl is the full width at half-maximum (FWHM) of the main peak
V. Jayasree et al. / Thin Solid Films 517 (2008) 603–608
Fig. 2. Lattice constants of the c-axis and FWHM values of 520-nm thick laser ablated KLN films deposited on glass substrate in non-reactive ambience as a function of the annealing temperature.
in the X-ray diffraction pattern. The average size of the crystalline grains of 238, 179 and 363 nm is obtained for the as-deposited and films annealed at 473 and 673 K respectively. Similar variations in
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grain size with annealing temperature have been reported [15,16] for films deposited on glass substrates. The lattice constant of the c-axis versus annealing temperature with the FWHM values are plotted in Fig. 2. When annealed at a temperature of 473 K, the lattice constant of c-axis is found to be within 3.98 Å–4.04 Å which is corresponding to the tetragonal structure range. On further annealing to a temperature of 673 K, the lattice constant changes to 3.975 Å. The AFM images of the as-deposited film and the films annealed at 473 and 673 K are shown in Fig. 3. A close analysis of the AFM images of the deposited films reveal that the grains are four-fold symmetric in nature. The average size of the grains is found to be 455, 447 and 529 nm respectively for the as-deposited and the films annealed at 473 and 673 K. The rms surface roughness of the films deposited at room temperature is 38.6 nm, whereas it is found to be 35.4 nm and 38.1 nm respectively for the films annealed at 473 K and 673 K. The large surface roughness of the as-deposited films may be due to the stress in the films induced by the growth of films on amorphous substrate at low temperature (room temperature) [17]. 3.1.2. Optical and electrical properties The optical transmittance spectra of the KLN film deposited on glass substrate, as grown and after annealing at 473 K and 673 K in air, are given in Fig. 4. The transmittance of the film annealed at 473 K is less compared to the as-deposited and the film annealed at 673 K. Due to the low rms surface roughness exhibited by the film annealed at 473 K, one can expect a higher value of transmittance compared to other films. We are not able to give an exact explanation to the mechanism by which this film exhibits lower value of transmittance compared to others. This can be due to some micro voids or imerfections developed in the film due to the effect of glass substrate at this particular temperature. The optical band gap energy is determined from the transmission spectra using the relation [18], αhm ¼ B hm−Eg
n
ð2Þ
where B is a constant and Eg is the optical band gap energy. The transmittance spectra do not show a sharp absorption edge, probably due to the scattering from the different lattice planes [19]. This is in agreement with the GIXRD result which suggests the existence of well defined peaks corresponding to lattice planes (001), (410) and (420). In the case of film annealed at 673 K the absorption edge is approximately 388 nm which is in agreement with the bulk KLN crystal (354–396 nm) [20,21].
Fig. 3. AFM images of as-deposited and annealed 520 nm thick laser ablated KLN films deposited on glass substrate in non-reactive ambience.
Fig. 4. Transmittance spectra of laser ablated KLN films on glass substrate in nonreactive ambience.
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Fig. 5. (αhv)2 versus hν plot of as-deposited and annealed 520 nm thick laser ablated KLN films on glass substrate in non-reactive ambience (a) as-deposited (b) annealed at 473 K (c) annealed at 673 K. Fig. 7. GIXRD patterns of as-deposited laser ablated KLN films on glass substrates at different oxygen pressures.
Our attempt to measure the DC resistivity of the films using four probe technique was not successful as the resistance of the films are much above the maximum measurable range (1 MΩ) of the instrument (Keithly (Model 6430) source-meter and a nano voltmeter (Model 2182A)). Hence the DC resistivity of the films is measured by two probe method, using Keithly 6517A Electrometer in which resistance up to 10 PΩ can be measured. The DC resistivity of the asdeposited films prepared under non-reactive atmosphere is found to be 1196 Ω m. The accurate measurement of the resistivity of the film annealed at 473 K cannot be obtained due to high fluctuations in the meter reading. However it is seemed to be in the range of 5408– 5616 kΩ m. However the film annealed at 673 K shows a resistivity comparable to that of the as-deposited film. The higher value of DC
Fig. 6. Extinction coefficient (k) of as-deposited and annealed 520 nm thick laser ablated KLN films on glass substrate in non-reactive ambience as a function of wavelength.
To determine the value of band gap (Eg) a graph of (αhν)1/n versus hν is plotted in accordance with Eq. (2). The best linear plot that covers the widest range of data is obtained for the n = 1/2 and the kind of optical transition in these samples are identified as direct allowed inter band transition [22]. The optical band gap energy (Eg) of these films is evaluated from the plot of (αhν)2 versus hν and is depicted in Fig. 5. The extinction coefficient k is also evaluated for the as-deposited and the annealed films using the relation k¼
αλ 4π
ð3Þ
where the symbols have their usual meaning. The variation of k with wavelength is presented in Fig. 6. The variations in optical band gap, optical density and extinction coefficient with annealing is given in Table 1.
Table 1 Optical parameters of laser ablated KLN thin films Annealing temperature (K)
Band gap (eV)
Absorption edge (nm)
Extinction coefficient
Optical density
As-deposited 473 673
3.64 3.7 3.2
341 336 388
0.1466 0.1425 0.1852
2.8166 2.4895 3.6123
Fig. 8. AFM images of as-deposited laser ablated KLN film exhibiting rings and rods.
V. Jayasree et al. / Thin Solid Films 517 (2008) 603–608
Fig. 9. Optical transmittance spectra of as-deposited laser ablated KLN thin films on glass substrate at different background oxygen pressures.
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As a result, there is no efficient material transformation on the films deposited at high oxygen pressures which can be the cause of the poor crystalline quality of these films. The investigations on the AFM images of the films deposited at various oxygen pressures show a decrease in rms surface roughness and grain size with increase in oxygen pressures. The poor crystallinity of the films prepared at oxygen ambience of 2 and 20 Pa as observed from XRD data, and smaller grain size obtained from AFM images, are associated with the kinetics of atomic arrangements during deposition. It is also noted that AFM images (Fig. 8) of the films deposited at oxygen pressure of 2 Pa shows the self assembly of grains in the form of rings and rods. The diameters of the rings vary from 890 nm to 1453 nm. Long rod like structures are also observed which vary in length from 1330 to 1549 nm and breadth from 563 nm to 633 nm. However, AFM images of the films deposited at oxygen pressures 0.2 Pa and 20 Pa do not exhibit any ring or rod like structures. The self
resistivity observed for the films annealed at 473 K, compared to other films, can be due to the presence of voids or imperfections in the film and is supporting the earlier prediction drawn from the optical transmittance spectra. 3.2. Effect of reactive oxygen pressure 3.2.1. GIXRD and AFM results Fig. 7 shows the GIXRD patterns of the films deposited at oxygen pressures 0.2, 2, 20 Pa. The film deposited at an oxygen pressure of 0.2 Pa is polycrystalline in nature as indicated by the presence of well defined XRD peaks. The diffraction peaks observed at (001), (320), (211), (410), (330), and (420) are in good agreement with the diffraction pattern observed in the powder diffraction file [7]. The peak corresponding to (410) lattice reflection plane shows the maximum intensity. The films deposited at oxygen pressures 2 Pa and 20 Pa show no prominent XRD peaks suggesting that the crystalline properties of these films are not good. Structural and optical properties of KLN strongly depend on Li content in the material. At a given temperature, the incorporation of volatile atoms such as Li and K into the film greatly depends on the reactive atmosphere [23]. From the XRD analysis, it can be inferred that a slight background oxygen pressure (0.2 Pa) improves the crystalline property of the films considerably, as it can suppress re-evaporation of volatile elements such as K and Li in the film [14]. But a considerable increase in the background oxygen pressure makes the mean free path shorter. This result in enhanced scattering of the ejected particles in the laser produced plasma by the oxygen gas molecules and prevents them from reaching the substrate.
Fig. 10. Extinction coefficient (k) of as-deposited laser ablated KLN thin films on glass substrate at different background oxygen pressures as a function of wavelength.
Fig. 11. Optical band gap (Eg) of as-deposited laser ablated KLN thin films on glass substrate at different background oxygen pressures as a function of photon energy.
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Table 2 Variation of the thickness and optical band gap of the as-deposited laser ablated KLN films as a function of oxygen pressure Oxygen pressure (Pa)
Band gap (eV)
Thickness of the film (nm)
0.2 2 20
3.1 3.8 4.2
500 200 90
assembly of grains leading to the formation of rings is a complex mechanism. When the oxygen concentration is less, the scattering of species ejected from laser produced plasma among the gas molecules is much smaller and this may cause the effective material transformation to the substrate. As the oxygen pressure increases, the scattering also increases. The lighter species which are at the edges of the plume will be scattered more and suffer larger loss in kinetic energy due to collisions. The heavier ones which are confined to the axis of the plume suffer lesser amount of scattering. They strike the substrate with greater kinetic energy and produce re-sputtering from the film. The particles at the edges of the plume may not have sufficient energy to re-sputter matter from the film. This phenomenon results in the self assembly of particles in the form of rings. When the oxygen pressure increases to a higher value (20 Pa), collision of particles in the plume with background oxygen molecules become more frequent and kinetic energies of the particles become too low to make re-sputtering effect [24]. Also, at higher oxygen pressures there is a possibility of redeposition of resputtered material back on the substrate due to the reflection/scattering by oxygen molecules. Therefore the ring formation is not observed at higher oxygen pressure. No evidence of grain growth in the form of thin sticks as reported by Park et al. [14] has been observed at any stages of growth of thin films in the conditions adopted in this experiment. 3.2.2. Optical properties Fig. 9 shows the optical transmittance spectra of the film deposited at different oxygen pressures. Films deposited at 0.2 Pa oxygen pressure shows a transmittance of more than 50% in the wavelength range 536–900 nm. The optical transmittance of the films is found to increase with increase in oxygen pressure, owing to the decrease in thickness of the films with increase in oxygen pressure. The higher transparency of the films at higher oxygen pressures agrees with smoother surfaces observed in AFM images. The extinction coefficient k (Fig. 10) and the optical band gap energies (Fig. 11) of the films deposited at three oxygen pressures are also evaluated. The variation of the thickness and band gap of the films as a function of oxygen pressures is given in Table 2. It can be seen that the optical energy band gap increases as the oxygen pressure increases. The increase in band gap energy with oxygen pressure can be attributed to the decrease in average grain size as observed in AFM measurements [25]. If the films have less thickness and smaller grain size, the widening of respective conduction and valence bands will be less and this results in wider energy gap [26,27]. 4. Conclusions Potassium lithium niobate films are prepared by pulsed laser deposition technique. GIXRD data suggests the tetragonal structure of
KLN film. The AFM analysis of the films reveals that the grains in the films are four fold symmetric in nature whose size increases on thermal annealing. Also it can be concluded that a slight oxygen background pressure improves the crystallinity of the films. The rms surface roughness and the grain size of the films decrease as the oxygen pressure increases. AFM images of the films deposited at oxygen pressure of 2 Pa shows the self assembly of particles in the form of rings and rods of sub-micron sizes. The optical transmittance of the films is found to increase with increase in oxygen pressure, owing to the decrease in thickness of the films with increase in oxygen pressure. The DC resistance measurement of the films reveals that the films have resistivity in the range of kΩ m. Acknowledgements The authors at University of Kerala would like to thank The Director, UGC-DAE CSR, Indore for providing various facilities and Ms. Deepti Jain and Mr. Mohan Gangrade for their assistance in AFM measurements. V Jayasree is grateful to University Grants Commission, India, for providing financial assistance under faculty improvement program. References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
M. Di Domenico Jr., H. Wemple, J. Appl. Phys. 40 (1969) 720. K. Chikuma, A. Onoe, A. Yoshida, Jpn. J. Appl. Phys. 37 (1998) 5582. S.K. Park, Thin Solid Films 457 (2004) 397. M. Adachi, T. Shiosaki, A. Kawabata, Jpn. J. Appl. Phys. 18 (1979) 193. M. Adachi, T. Shiosaki, A. Kawabata, Ferroelectrics 27 (1980) 89. S.K. Park, M.S. Baek, S.C. Bae, S.Y. Kwon, J.H. Kim, K.W. Kim, Jpn. J. Appl. Phys. 38 (1999) 6483. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, North Dakoda State University, USA, 1982 Card 34-0122. A. Donnadieu, in: C. M Lampert, C. G Grangvist (Eds.), Large-area Chromogenics: Materials and Devices for Transmittance Control, IS4 SPIE Opt. Eng. Press, Bellingham, 1990, p. 191. D.B. Cullity, Elements of X-ray Diffraction, Addison-Wesley Inc., Massachusetts, 1956. Q. Jiang, T.P.J. Han, H.G. Gallaghei, J. Mater. Sci. Mater. Electron. 9 (1998) 193. T. Karaki, K. Miyashita, M. Nakatsuji, M. Adachi, Jpn. J. Appl. Phys. 37 (1998) 5277. X.L. Guo, Z.G. Liu, J.M. Liu, S. Nzhu, Y.Y. Zhu, Phys. Status Solidi A 154 (1996) 615. K. Chikuma, A. Onoe, A. Yoshida, Jpn. J. Appl. Phys. 37 (1998) 5582. S.K. Park, S.C. Bae, B.J. Choi, G.H. Nam, Y.J. Kim, K.W. Kim, Jpn. J. Appl. Phys. 39 (2000) 1303. W. Tang, D.C. Cameron, Thin Solid Films 238 (1994) 95. E. Senadim, H. Kavak, R. Esen, J. Phys. Condens. Matter 18 (2006) 6391. C. Julien, A. Khelfia, O.M. Hussain, J. Cryst. Growth 156 (1995) 235. A.Z. Moshfegh, R. Azimirad, O. Akhavan, Thin Solid Films 484 (2005) 124. C.V. Ramana, R.J. Smith, O.M. Hussain, Phys. Status Solidi, A 199 (2003) R4. H.R. Xia, L.J. Hu, J.Q. Wei, J.Y. Wang, Y.G. Liu, Cryst. Res. Technol. 32 (1997) 311. K. Imai, M. Imaeda, S. Uda, T. Taniuchi, T. Fukuda, J. Cryst. Growth 177 (1997) 79. J.C. Tauc, Optical Properties of Solids, North- Holland, Amsterdam, The Netherlands, 1972, p. 372. D.B. Chrisey, G.K. Hubbler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, NY, 1994, p. 478. M. Migliuolo, A. K Stamper, D.W. Greve, T.E. Schelsinger, Appl. Phys. Lett. 54 (1989) 859. J.S. Zhu, X.M. Lu, W. Jiang, W. Tian, M. Zhu, M.S. Zhang, X.B. Chen, X. Liu, Y.N. Wang, J. Appl. Phys. 81 (1997) 1392. I.P. Batra, P. Wurfel, B.D. Silverman, Phys. Rev. Lett. 30 (1973) 384. I.P. Batra, P. Wurfel, B.D. Silverman, Phys. Rev. B 8 (1973) 3257.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Influence of reactive oxygen ambience on the structural, morphological and optical properties of pulsed laser ablated potassium lithium niobate thin films V. Jayasree a, R. Ratheesh b, V. Ganesan c, V.R. Reddy c, C. Sudarsanakumar d, V.P. Mahadevan Pillai a,⁎, V.U. Nayar a a
Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram 695 581, Kerala, India Centre for Materials for Electronics Technology, Athani P. O, Thrissur, Kerala, India UGC-DAE Consortium for Scientific Research, Indore Centre, Madhya Pradesh, India d School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India b c
a r t i c l e
i n f o
Article history: Received 10 October 2007 Received in revised form 7 June 2008 Accepted 6 July 2008 Available online 17 July 2008 Keywords: Potassium lithium niobate Nonlinear optic materials Ferroelectric Tungsten bronze materials Electro-optic materials Pulsed laser deposition Transmittance spectra Optical band gap
a b s t r a c t The effect of oxygen ambience on the structure and properties of potassium lithium niobate (K3Li2Nb5O15: KLN) films prepared on glass substrates by pulsed laser ablation technique (PLD) are studied. The influence of annealing on the properties of vacuum deposited films is also investigated. The Gracing Incidence X-ray Diffraction (GIXRD) data suggests the tetragonal structure for the KLN film whose grain sizes increase on thermal annealing. The Atomic Force Microscopic (AFM) analysis reveals the four-fold symmetric nature of the grains in the films. Self assembly of grains in the form of rings and rods are observed in AFM images of the films deposited in an oxygen ambience of 2 Pa. The films deposited at higher oxygen ambience show a blue shift in optical band gap. The direct current (DC) resistance measurement on the films deposited at non-reactive ambience reveals resistivity in the range of kΩ m. © 2008 Elsevier B.V. All rights reserved.
1. Introduction As a class of compounds, ferroelectric oxides have very attractive and potential properties such as wide band gap (N3 eV), large electrooptic and nonlinear optic coefficients and have the possibility of sustaining the spontaneous polarizations [1]. Ferroelectric tungsten– bronze crystals with general formula (A1)2(A2)4C4(B1)2(B2)8O30 have generated much interest, especially for nonlinear optical applications and for their large optical damage threshold originating from their crystal structure [2]. Potassium lithium niobate (K3Li2Nb5O15: KLN) with a completely filled tungsten–bronze type structure is a very promising material for various optical applications owing to its large electro-optic, nonlinear optic, piezoelectric and pyroelectric properties [3] However the growth of large single crystals of KLN is still a challenging problem as they crack easily while cooling down through paraelectric to ferroelectric phase. Moreover the crystallographic and dielectric properties are severely affected by the Nb2O5 content in the melt. Hence, the growth of epitaxial thin films of KLN is of particular interest, as high quality thin films of KLN are suitable for many of its applications. Different thin film depositions technique such as epitaxial growth by melting [4], RF magnetron sputtering [5,6] on ⁎ Corresponding author. E-mail address: [email protected] (V.P.M. Pillai). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.07.004
substrates like potassium bismuth niobate [5] and sapphire have been used for the epitaxial or polycrystalline growth of KLN thin films. However, preparation of KLN thin film using pulsed laser deposition technique has not yet been reported. Pulsed laser deposition (PLD) is an effective tool for the growth of quantum structures with high chemical purity and controlled stoichiometry. In PLD, one can control size distribution in nanocrystals by varying the parameters like target to substrate distance, laser fluence, background gas pressure etc. This paper presents a report of preparation of KLN thin film using pulsed laser deposition technique in non-reactive and reactive atmosphere. The as-deposited films prepared under vacuum condition, the films annealed in air atmosphere and the as-deposited films prepared under different reactive oxygen pressures, are characterised using Gracing Incidence X-Ray Diffraction (GIXRD), Atomic Force Microscopy (AFM), UV–Vis spectroscopy and direct current (DC) resistance measurements to study the effect of annealing and that of background oxygen pressure on the properties of the films. 2. Experiment 2.1. Preparation of stoichiometric target for PLD operation Stoichiometric composition of K2CO3, Li2CO3 and Nb2O5 (Aldrich, purity 99.99%) is mixed well in an agate mortar for an hour using
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distilled water as solvent. This mixture is then dried by heating in an oven at 100 °C and is transferred to an alumina crucible and calcined for 2 h at 1000 °C. The analysis of powder X-ray diffraction (XRD) patterns of this compound show a tetragonal tungsten–bronze structure with space group P4bm, Z = 2 and cell parameters a =b = 12.562 Å and c = 4.019 Å which are in good agreement with the reported data [7]. The compound thus prepared is powdered well using an agate motor and is pressed into pellet of size 7 mm thickness and 10 mm diameter using a hydraulic press. The pellet used as target for ablation, is prepared by sintering it at 1050 °C for 4 h. 2.2. KLN thin film preparation using PLD The deposition of the films is carried out inside a multiport stainless steel vacuum chamber equipped with a gas inlet, a rotating multitarget and a temperature controlled substrate holder. The irradiations are performed using a Q-switched Nd: YAG laser with frequency doubled 532 nm radiation of energy 70 mJ, pulse width 7 ns and repetition frequency of 10 Hz (Quanta – Ray INDI – series, Spectra Physics). The laser impinged on the target at 45° with respect to the normal in a dynamic flow of oxygen. Before irradiations, the deposition chamber is evacuated down to a base pressure of 4 × 10− 4 Pa using a diffusion pump and two rotary pumps. The depositions of the films are done on glass substrates kept at an on-axis distance of 70 mm from the target for a deposition time of 20 min. Glass is chosen as substrate to avoid the influence of single crystal substrate on the orientation of thin films. The films are deposited in non-reactive and reactive oxygen atmospheres with oxygen pressures of 0.2, 2 and 20 Pa. During ablation the target is rotated at a constant speed to avoid pitting of target at any given spot and to obtain uniform thin films. The films deposited in the non-reactive atmosphere are annealed at 473 K and 673 K. The asdeposited and annealed films are milky white in appearance. The crystalline nature and orientations of the deposited films are investigated by GIXRD (Siemens D5000 Diffractometer) measurements employing Cu Kα radiation with wavelength of 0.15406 nm. The surface morphology of the deposited films has been investigated using the AFM images recorded by a Digital Instrument Nanoscope E atomic force microscope. AFM tip of Si3N4 having a force constant of 0.58 N/m in contact mode operation has been employed for the measurements. The data are measured in 256 × 256 pixel format and the scan rate is 5.086 Hz. The rms roughness and the grain size are measured using the software associated with the instrument. For the grain size calculation, 4–5 frames are selected and 20 measurements are taken in each frame and the grain size is obtained with an accuracy of ± 4 nm. The optical transmission spectra of the films are recorded using a JASCO V 550 UV– VIS double beam spectrophotometer in the wavelength range of 190– 900 nm. The DC resistivity of the deposited films is determined using two probe method. Thickness of the as-deposited films are measured using a profilometer (XP stylus profile, Ambios Technology, USA). The as-deposited film prepared under non-reactive atmosphere yields a thickness value of 520 nm, whereas the thickness of the films deposited under the reactive oxygen pressures of 0.2, 2 and 20 Pa are found to be 500, 200 and 90 nm respectively. Quantitative energy dispersive spectra (EDS) of the films are recorded (standard less analysis with ZAF correction, acceleration voltage = 15 keV, live time for data acquisition = 62 s) using scanning electron microscope (SEM) JEOL JSM 5600LV. The EDS spectra show the peaks corresponding to potassium (13.5 at.%), niobium (39.2 at.%) and oxygen (47.1 at.%). 3. Results and discussions 3.1. Effect of thermal annealing 3.1.1. GIXRD and AFM results Fig. 1 shows the GIXRD patterns of the films prepared by PLD in the non-reactive atmosphere. The GIXRD pattern of the as-deposited film
Fig. 1. GIXRD patterns of laser ablated KLN thin films deposited on glass substrate in non-reactive ambience as a function of the annealing temperature.
shows three well defined peaks. The broad peak located at 22.25° has a d spacing corresponding to the c-axis lattice constant in the tetragonal tungsten–bronze structure range. Therefore this peak can be identified as the (001) lattice plane reflection of tetragonal tungsten–bronze crystalline phase of KLN. The broadness of this peak may be due to the closeness in d spacing of (001) and (310) reflection planes [8]. It may be noted that poor crystalline nature of the films can also cause broadness [9]. The other peaks at 29.3° and 31.75° correspond to (410) and (420) lattice reflection planes. The peak corresponding to the plane (001) appears with maximum intensity and this suggests that the preferred orientation of growth in KLN film is along (001). In the reported XRD data of KLN powder [7], the intensities of the peaks corresponding to the planes (001), (410) and (420) bear a ratio 6:75:2. However in the present case these peaks show an intensity ratio of 310:259:247. The growth mechanism of KLN thin film can be explained on the basis of cleavages observed in the KLN crystals during growth. In KLN crystal, cleavages are formed along the (001) plane during its growth [10,11]. Cleavages are understood as the planes where the bonding energy is weaker than the energy between the other planes [12,13]. The vacant octahedral sites present in the cleavage planes reduce the interfacial energy between the deposited film and the substrate. This plane of minimum surface energy promotes the nucleation and thereby preferential directional growth during crystallization [14]. Thus, without having any contribution from the substrate, KLN thin film grows preferentially along the (001) plane leading to higher intensity for this peak. In the film annealed at 473 K, the peaks corresponding to lattice reflection planes (001), (410) and (420) show a considerable reduction in intensity. A new KLN peak corresponding to (320) is also observed. The positions of all other peaks, except the peak corresponding to (410) lattice plane are shifted to higher 2θ values on annealing to 473 K. The development of new (320) KLN peak can be due to either the re-crystallization process or the stress relaxation process during annealing. For films annealed at 673 K, the peaks corresponding to (001), (410) and (420) planes appear with a still lesser intensity. At this temperature, the positions of the peaks corresponding to (410) and (320) planes shifted slightly to lower 2θ value. During annealing, the stress relaxation process takes place, which results in the shift in the position of peaks and the decrease in FWHM [14]. The average size of the crystalline grains in the films are determined by the following Scherer equation [9] Dhkl ¼
0:9λ β hkl cosðθhkl Þ
ð1Þ
where λ is the X-ray wavelength, θhkl is the Bragg diffraction angle and βhkl is the full width at half-maximum (FWHM) of the main peak
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Fig. 2. Lattice constants of the c-axis and FWHM values of 520-nm thick laser ablated KLN films deposited on glass substrate in non-reactive ambience as a function of the annealing temperature.
in the X-ray diffraction pattern. The average size of the crystalline grains of 238, 179 and 363 nm is obtained for the as-deposited and films annealed at 473 and 673 K respectively. Similar variations in
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grain size with annealing temperature have been reported [15,16] for films deposited on glass substrates. The lattice constant of the c-axis versus annealing temperature with the FWHM values are plotted in Fig. 2. When annealed at a temperature of 473 K, the lattice constant of c-axis is found to be within 3.98 Å–4.04 Å which is corresponding to the tetragonal structure range. On further annealing to a temperature of 673 K, the lattice constant changes to 3.975 Å. The AFM images of the as-deposited film and the films annealed at 473 and 673 K are shown in Fig. 3. A close analysis of the AFM images of the deposited films reveal that the grains are four-fold symmetric in nature. The average size of the grains is found to be 455, 447 and 529 nm respectively for the as-deposited and the films annealed at 473 and 673 K. The rms surface roughness of the films deposited at room temperature is 38.6 nm, whereas it is found to be 35.4 nm and 38.1 nm respectively for the films annealed at 473 K and 673 K. The large surface roughness of the as-deposited films may be due to the stress in the films induced by the growth of films on amorphous substrate at low temperature (room temperature) [17]. 3.1.2. Optical and electrical properties The optical transmittance spectra of the KLN film deposited on glass substrate, as grown and after annealing at 473 K and 673 K in air, are given in Fig. 4. The transmittance of the film annealed at 473 K is less compared to the as-deposited and the film annealed at 673 K. Due to the low rms surface roughness exhibited by the film annealed at 473 K, one can expect a higher value of transmittance compared to other films. We are not able to give an exact explanation to the mechanism by which this film exhibits lower value of transmittance compared to others. This can be due to some micro voids or imerfections developed in the film due to the effect of glass substrate at this particular temperature. The optical band gap energy is determined from the transmission spectra using the relation [18], αhm ¼ B hm−Eg
n
ð2Þ
where B is a constant and Eg is the optical band gap energy. The transmittance spectra do not show a sharp absorption edge, probably due to the scattering from the different lattice planes [19]. This is in agreement with the GIXRD result which suggests the existence of well defined peaks corresponding to lattice planes (001), (410) and (420). In the case of film annealed at 673 K the absorption edge is approximately 388 nm which is in agreement with the bulk KLN crystal (354–396 nm) [20,21].
Fig. 3. AFM images of as-deposited and annealed 520 nm thick laser ablated KLN films deposited on glass substrate in non-reactive ambience.
Fig. 4. Transmittance spectra of laser ablated KLN films on glass substrate in nonreactive ambience.
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Fig. 5. (αhv)2 versus hν plot of as-deposited and annealed 520 nm thick laser ablated KLN films on glass substrate in non-reactive ambience (a) as-deposited (b) annealed at 473 K (c) annealed at 673 K. Fig. 7. GIXRD patterns of as-deposited laser ablated KLN films on glass substrates at different oxygen pressures.
Our attempt to measure the DC resistivity of the films using four probe technique was not successful as the resistance of the films are much above the maximum measurable range (1 MΩ) of the instrument (Keithly (Model 6430) source-meter and a nano voltmeter (Model 2182A)). Hence the DC resistivity of the films is measured by two probe method, using Keithly 6517A Electrometer in which resistance up to 10 PΩ can be measured. The DC resistivity of the asdeposited films prepared under non-reactive atmosphere is found to be 1196 Ω m. The accurate measurement of the resistivity of the film annealed at 473 K cannot be obtained due to high fluctuations in the meter reading. However it is seemed to be in the range of 5408– 5616 kΩ m. However the film annealed at 673 K shows a resistivity comparable to that of the as-deposited film. The higher value of DC
Fig. 6. Extinction coefficient (k) of as-deposited and annealed 520 nm thick laser ablated KLN films on glass substrate in non-reactive ambience as a function of wavelength.
To determine the value of band gap (Eg) a graph of (αhν)1/n versus hν is plotted in accordance with Eq. (2). The best linear plot that covers the widest range of data is obtained for the n = 1/2 and the kind of optical transition in these samples are identified as direct allowed inter band transition [22]. The optical band gap energy (Eg) of these films is evaluated from the plot of (αhν)2 versus hν and is depicted in Fig. 5. The extinction coefficient k is also evaluated for the as-deposited and the annealed films using the relation k¼
αλ 4π
ð3Þ
where the symbols have their usual meaning. The variation of k with wavelength is presented in Fig. 6. The variations in optical band gap, optical density and extinction coefficient with annealing is given in Table 1.
Table 1 Optical parameters of laser ablated KLN thin films Annealing temperature (K)
Band gap (eV)
Absorption edge (nm)
Extinction coefficient
Optical density
As-deposited 473 673
3.64 3.7 3.2
341 336 388
0.1466 0.1425 0.1852
2.8166 2.4895 3.6123
Fig. 8. AFM images of as-deposited laser ablated KLN film exhibiting rings and rods.
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Fig. 9. Optical transmittance spectra of as-deposited laser ablated KLN thin films on glass substrate at different background oxygen pressures.
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As a result, there is no efficient material transformation on the films deposited at high oxygen pressures which can be the cause of the poor crystalline quality of these films. The investigations on the AFM images of the films deposited at various oxygen pressures show a decrease in rms surface roughness and grain size with increase in oxygen pressures. The poor crystallinity of the films prepared at oxygen ambience of 2 and 20 Pa as observed from XRD data, and smaller grain size obtained from AFM images, are associated with the kinetics of atomic arrangements during deposition. It is also noted that AFM images (Fig. 8) of the films deposited at oxygen pressure of 2 Pa shows the self assembly of grains in the form of rings and rods. The diameters of the rings vary from 890 nm to 1453 nm. Long rod like structures are also observed which vary in length from 1330 to 1549 nm and breadth from 563 nm to 633 nm. However, AFM images of the films deposited at oxygen pressures 0.2 Pa and 20 Pa do not exhibit any ring or rod like structures. The self
resistivity observed for the films annealed at 473 K, compared to other films, can be due to the presence of voids or imperfections in the film and is supporting the earlier prediction drawn from the optical transmittance spectra. 3.2. Effect of reactive oxygen pressure 3.2.1. GIXRD and AFM results Fig. 7 shows the GIXRD patterns of the films deposited at oxygen pressures 0.2, 2, 20 Pa. The film deposited at an oxygen pressure of 0.2 Pa is polycrystalline in nature as indicated by the presence of well defined XRD peaks. The diffraction peaks observed at (001), (320), (211), (410), (330), and (420) are in good agreement with the diffraction pattern observed in the powder diffraction file [7]. The peak corresponding to (410) lattice reflection plane shows the maximum intensity. The films deposited at oxygen pressures 2 Pa and 20 Pa show no prominent XRD peaks suggesting that the crystalline properties of these films are not good. Structural and optical properties of KLN strongly depend on Li content in the material. At a given temperature, the incorporation of volatile atoms such as Li and K into the film greatly depends on the reactive atmosphere [23]. From the XRD analysis, it can be inferred that a slight background oxygen pressure (0.2 Pa) improves the crystalline property of the films considerably, as it can suppress re-evaporation of volatile elements such as K and Li in the film [14]. But a considerable increase in the background oxygen pressure makes the mean free path shorter. This result in enhanced scattering of the ejected particles in the laser produced plasma by the oxygen gas molecules and prevents them from reaching the substrate.
Fig. 10. Extinction coefficient (k) of as-deposited laser ablated KLN thin films on glass substrate at different background oxygen pressures as a function of wavelength.
Fig. 11. Optical band gap (Eg) of as-deposited laser ablated KLN thin films on glass substrate at different background oxygen pressures as a function of photon energy.
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Table 2 Variation of the thickness and optical band gap of the as-deposited laser ablated KLN films as a function of oxygen pressure Oxygen pressure (Pa)
Band gap (eV)
Thickness of the film (nm)
0.2 2 20
3.1 3.8 4.2
500 200 90
assembly of grains leading to the formation of rings is a complex mechanism. When the oxygen concentration is less, the scattering of species ejected from laser produced plasma among the gas molecules is much smaller and this may cause the effective material transformation to the substrate. As the oxygen pressure increases, the scattering also increases. The lighter species which are at the edges of the plume will be scattered more and suffer larger loss in kinetic energy due to collisions. The heavier ones which are confined to the axis of the plume suffer lesser amount of scattering. They strike the substrate with greater kinetic energy and produce re-sputtering from the film. The particles at the edges of the plume may not have sufficient energy to re-sputter matter from the film. This phenomenon results in the self assembly of particles in the form of rings. When the oxygen pressure increases to a higher value (20 Pa), collision of particles in the plume with background oxygen molecules become more frequent and kinetic energies of the particles become too low to make re-sputtering effect [24]. Also, at higher oxygen pressures there is a possibility of redeposition of resputtered material back on the substrate due to the reflection/scattering by oxygen molecules. Therefore the ring formation is not observed at higher oxygen pressure. No evidence of grain growth in the form of thin sticks as reported by Park et al. [14] has been observed at any stages of growth of thin films in the conditions adopted in this experiment. 3.2.2. Optical properties Fig. 9 shows the optical transmittance spectra of the film deposited at different oxygen pressures. Films deposited at 0.2 Pa oxygen pressure shows a transmittance of more than 50% in the wavelength range 536–900 nm. The optical transmittance of the films is found to increase with increase in oxygen pressure, owing to the decrease in thickness of the films with increase in oxygen pressure. The higher transparency of the films at higher oxygen pressures agrees with smoother surfaces observed in AFM images. The extinction coefficient k (Fig. 10) and the optical band gap energies (Fig. 11) of the films deposited at three oxygen pressures are also evaluated. The variation of the thickness and band gap of the films as a function of oxygen pressures is given in Table 2. It can be seen that the optical energy band gap increases as the oxygen pressure increases. The increase in band gap energy with oxygen pressure can be attributed to the decrease in average grain size as observed in AFM measurements [25]. If the films have less thickness and smaller grain size, the widening of respective conduction and valence bands will be less and this results in wider energy gap [26,27]. 4. Conclusions Potassium lithium niobate films are prepared by pulsed laser deposition technique. GIXRD data suggests the tetragonal structure of
KLN film. The AFM analysis of the films reveals that the grains in the films are four fold symmetric in nature whose size increases on thermal annealing. Also it can be concluded that a slight oxygen background pressure improves the crystallinity of the films. The rms surface roughness and the grain size of the films decrease as the oxygen pressure increases. AFM images of the films deposited at oxygen pressure of 2 Pa shows the self assembly of particles in the form of rings and rods of sub-micron sizes. The optical transmittance of the films is found to increase with increase in oxygen pressure, owing to the decrease in thickness of the films with increase in oxygen pressure. The DC resistance measurement of the films reveals that the films have resistivity in the range of kΩ m. Acknowledgements The authors at University of Kerala would like to thank The Director, UGC-DAE CSR, Indore for providing various facilities and Ms. Deepti Jain and Mr. Mohan Gangrade for their assistance in AFM measurements. V Jayasree is grateful to University Grants Commission, India, for providing financial assistance under faculty improvement program. References [1] [2] [3] [4] [5] [6] [7] [8]
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