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High optical transmittance and anomalous electronic transport in flexible transparent conducting oxides Ba0.96La0.04SnO3 thin films
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High optical transmittance and anomalous electronic transport in flexible transparent conducting oxides Ba 0.96La 0.04SnO3 thin films ⁎
Weifeng Suna, Jiyu Fana, , Ruixing Xua, Xiyuan Zhanga, Caixia Kana, Wei Liub, Lei Zhangb, Chunlan Mac, Dazhi Hua, Yanda Jia, Yan Zhua, Hao Yanga a
Department of Applied Physics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China c School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical property Transport property Heteroepitaxy Flexible substrate
Wide bandgap semiconducting perovskite La:BaSnO3 is a promising candidate for next-generation transparent conducting oxides due to its high electron mobility and excellent oxygen stability. In this work, in order to realize flexible optoelectronics, an epitaxial growth of Ba 0.96La 0.04 SnO3 (BLSO) film on a flexible mica via van der Waals epitaxy is established. The high quality heteroepitaxy and crystallinity of BLSO films are confirmed by a combination of X-ray diffraction and atomic force microscopy. Results show that the flexible BLSO films not only retain a high transmittance of more than 85% in the visible region under unbending conditions, but also exhibit a remarkable transmittance of 90% under bending conditions. Due to the fixed lattice mismatch and misfit strain, an anomalous electronic transport behavior, showing as a continuous enhancement of resistivity dependence on the decreasing temperatures from high to low, was observed for all BLSO films. Although the resistivity of flexible BLSO films is a slight larger than that of growth on rigid counterparts, the resistivity of 7–10 mΩ cm is also satisfied with actual application for optoelectronic devices at room-temperature. Our study marks that the technological advancements toward realizing flexible optoelectronics are promising by utilizing perovskite oxides La-doped BaSnO3 and mica substrate.
1. Introduction Over the past few years, transparent conducting oxides (TCO) have attracted a great deal of interests owing to increasing demands for the widespread application of current optoelectronic devices, e.g. solar cells, flat panel displays, light emitting diodes, and transparent logic devices, and so on. [1–5] Many binary oxide materials including the pure and impurity doped ZnO, In2 O3, as well as SnO2 were studied for these purposes, successfully demonstrating passive transparent conductive windows to active semiconducting devices, such as pn junctions, field effect transistors, and UV lasers [5–8]. However, with the ongoing development of optoelectronic technology, exploration of new alternative transparent materials but with more outstanding performance or special properties are becoming more and more important. In view of actual applications, most of TCO films are generally required to have a wide band gap of more than 3.0 eV, necessary for high transmittance in the visible region [6]. Meanwhile, wide band gap semiconductors with the perovskite structure are becoming the favorable TCO due to their rich physical properties and compatibility for
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multilayer structures together with the potential applications in hightemperature and high-power electronics. For example, La(In,Sb)-doped SrTiO3 and Nb-doped CaTiO3 have been reported in the context of new TCO materials [9–12]. In this work, we focus on another wide band gap semiconductor, perovskite barium stannate oxide BaSnO3 (BSO). BSO is known to form an ideal cubic perovskite structure in which the Sn-O-Sn bonding angle is close to 180 ° . The pristine BSO can be regarded as an insulating material since for its negligible conductivity. Its conduction band is mainly Sn 5s orbitals, and the valence band is mainly oxygen 2p orbitals with a large bandgap of 3.4 eV [13]. For the enhancement of conductivity, some different doped elements have been substituted, such as La and Gd doping on Ba site, Sb doping on Sn site [14–17]. In particular, La-doped BSO (BLSO) has been recently gained numerous attentions due to its excellent electron mobility and high chemical stability. Kim et al. reported that BLSO film and single crystal have high carrier mobility of 70 and 320 cm2 /V s at room temperature, respectively [15]. Recent investigations have proved that such a high mobility is due to its smaller carrier effective mass (m* = 0.40 m e ) and longer
Corresponding author. E-mail addresses: [email protected] (J. Fan), [email protected] (H. Yang).
https://doi.org/10.1016/j.ceramint.2018.07.001 Received 17 June 2018; Received in revised form 30 June 2018; Accepted 1 July 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sun, W., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.07.001
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more than 700 °C. The good chemical stability of mica under high temperature was very important for BLSO growth. However, mica wafers are non-perovskite substrate. The perovskite substrates are good in promoting the growth of perovskite type film. On the contrary, nonperovskite substrates difficultly achieve an ideal epitaxial growth of it. Therefore, BLSO can't be directly deposited on mica substrate (In fact, we have tried doing it but only polycrystalline film is obtained.). Thus, we need to first deposit one kind of perovskite materials as buffer layer and then deposit BLSO. In this paper, perovskite oxides BaTiO3(BTO) was chosen to deposit on mica substrate as buffer layer through comparing different materials. We found BTO was easy to realize epitaxial growth on mica substrate along the direction of (111). In the following work for all BLSO films, before depositing BLSO, BTO buffer layer with the thickness near 30 nm was first deposited on mica substrate. In order to guarantee the epitaxial growth of BLSO films, some basic film properties of BTO buffer layer, such as its appropriate orientation, crystallinity, and smooth surface have been also checked beforehand. The super-smooth surface and high crystallinity of BTO buffer layer can be prepared by using the optimal growth condition. As we know, many deposition conditions determine the growth quality of epitaxial films, including laser energy density, repetition rate (number of laser pulses per second), ambient gas and its pressure, target-substrate distance, substrate temperature, and annealing time. In this work, we found that the last two factors were more prominently related with the growth quality than others. Fig. 1(a) shows the XRD patterns of BLSO films deposited on mica (00l) substrate with the inserted buffer layer BTO as the substrate temperature is set in the range of 720–780 °C. We find that the BTO film exactly grows along the direction (111) on mica substrate and BLSO film epitaxially grows on top of BTO film. Both (111) and (222) peaks of BLSO film were clearly shown in Fig. 1(a). Two obvious characteristics can be also observed with the increase of substrate temperatures. (i) At T = 720 °C, except for the normal diffraction peaks of mica, BTO, and BLSO, its XRD pattern also exhibits many impurity peaks. As the substrate temperatures was increased to 780 °C, the impurity peaks significantly decrease and almost disappear. (ii) With the enhancement of substrate temperatures, the peak intensity of BLSO film (222) becomes stronger, indicating a better crystallization of BLSO film at 780 °C than 720 °C. The rise of substrate temperatures improves the mobility and diffusion of the adatoms, which is propitious for BLSO atoms to arrange themselves in highly ordered manner. Fig. 1(b) shows the magnified Bragg peaks of BLSO film (222) around 2 θ∼ 81°. Besides the rise of peak intensity, the full width at half-maximum of BLSO peak (222) also becomes smaller
carrier relaxation time [18,19]. The high mobility and a wide band gap indicate that BLSO is a promising candidate for transparent conductor applications. Furthermore, Wadekar et al. found that the room temperature resistivity could be effectively decreased from 7.8 to 4.4 mΩ cm by choosing SmScO3 substrate comparing with SrTiO3 substrate [20]. The main reason has been attributed to reduction in dislocation density due to the lower lattice mismatch. In addition, epitaxial growth BLSO film on the other variety of substrates, including PrScO3 [21], LaAlO3 [22], BaSnO3 [23], MgO [24], have been also reported. We noticed that all reported substrates were rigid wafers instead of flexible substrates. In present, flexible devices, i.e., devices fabricated on flexible substrates, are very attractive in application due to their stretchable, biocompatible, light-weight, and portable [25–29]. Therefore, in order to further expand the application scope and give rise to new functionalities of BLSO film on optoelectronic field, it is necessary to study the BLSO film growth on the flexible substrates. Here, we reported our recent results about the epitaxial growth BLSO film on flexible mica substrates, including detailed fabrication conditions, optical transmittance spectra of unbending and bending conditions, as well as electronic transport properties. The obtained results indicate that BLSO films have excellent optical transmittance as high as more than 85% in the visible region regardless of under bending or unbending state. Different from the conductivity properties of BLSO films grown on SrTiO3 substrate, an anomalous electronic transport showing as the enhancement of resistivity with the decrease of temperature occurred in all of present films. The experiments confirmed that this behavior was unrelated to the inserted buffer layer but depended on misfit strain due to lattice mismatch. Although BLSO films grown on the flexible mica substrate show a larger resistivity than that grown on rigid SrTiO3(001) substrate, the value of 7–10 mΩ cm is also satisfied with actual application in optoelectronic devices at room temperature. Our work will pave a way for the future development of flexible TCO film. 2. Experiment A series of BLSO films with various thicknesses were fabricated on mica substrates using pulsed laser deposition (PLD) technique. Dense ceramics targets of Ba 0.96La 0.04 SnO3 were prepared by standard solid state reactions. A 248 nm KrF excimer laser with the repetition rate of 4 Hz and laser energy density irradiated on the rotating targets of 1.6 J/cm2 were used for the film fabrication. During deposition, the temperature of substrate was set in the range of 720–780 °C to change the quality of epitaxial films. Then, the films were in situ annealed before being cooled down in the same oxygen ambient. The crystalline structure and the epitaxial characteristics of the BLSO films were examined by X-ray diffraction (XRD, PANalytical) using Cu K α radiation at room temperature. The surface morphology of the films was studied by atomic force microscopy (AFM) in the tapping mode. The optical transmittance were measured on a spectrophotometer UV3600 at the room temperature. Temperature-dependent resistivity was investigated by the standard four-terminal method. 3. Results and discussion Up to present, BLSO films were mostly deposited on the rigid wafers SrTiO3 (001) which is one of the most widely substrate for depositing perovskite oxides films [14,15]. Some reported results have confirmed that it is easy to realize epitaxial growth of BLSO films on SrTiO3 (001) substrate with a superior crystalline quality due to both of all perovskite structures and an ignorable lattice mismatch. Here, we choose mica as the substrate mainly based on three reasons [30,31]: (i)Mica is a flexible material and has been extensively utilized to fabricate flexible devices; (ii) Mica is a transparent material which is beneficial for high light transmission; (iii) Mica can bear high temperature. During the deposition of BLSO films, the substrate was heated to high temperature
Fig. 1. (a) The X-ray θ -2θ profile of the grown BLSO/BTO/mica films at different substrate temperatures and the bottom XRD pattern is for empty mica (The symbol * indicates the diffraction peaks of impurity phase.). (b) The varied FWHM of (222) Bragg peaks at three various substrate temperatures. 2
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heterostructure films. The average transmittances of all films including a clean mica exhibited a high transparency of more than 85% in the visible region(380–780 nm). The detailed comparison of them can be seen in Table 1. Except for a linear behavior observed for pure mica, the others transmission spectra of heterostructure films all show a wave line. Here, BLSO/BTO/mica films can be considered as a multilayer film. Therefore, the transmissivity inevitably exhibits some maximum and the minimum values due to Fabry-Pérot interference effect. Similar to other TCO films, we can find a subtle decrease of transmittance with increasing film thickness in the current samples. This is a kind of thickness effect, in which the thicker films absorb more light and have lower transmittance. However, in the the short-wavelength near-infrared range (780–1100 nm), the average transmittance of all the films do not reveal an obvious discrepancy. The main reason was attributed to another transmission mechanism. In this region, the plasma resonance effect plays a main role for optical transmission. The plasma frequency dependence on the square root of carrier concentration. As the film thickness exceeds to a certain level, such as 120 nm in this case, the carrier concentration is basically independent on the variation of film thickness. Due to the nearly same carrier concentration in three films, there are no obvious change in their average transmittances. Relationship between the optical absorption coefficient (α ) and the photo energy (h ν ) is commonly described by the Taucs relation [34], (hνα )m = A(hν − Eg ) , where h, ν , A, and Eg are the Plank constant, photo frequency, ordinary constant, and optical band gap width of the material, respectively, and m is the exponent which is determined by the type of electronic transition causing the optical absorption and equals 1/2 and 2 for the indirect and direct optical band transition, respectively. BSO belongs to direct optical band transition. the value of m was chosen to be 2. The inset of Fig. 4 shows the plot of (hνα )2 against h ν of BLSO/BTO/mica film with different thicknesses. By fitting linear portion of (hνα )2 to energy axis h ν , the optical band gap Eg can be deduced. For clarity, we only listed the fitting results of 120 and 200 nm due to too small change of h ν among these films. Here, three optical band gaps, Eg = 3.26, 3.22, 3.19 eV , are obtained for the films thickness of 120, 160, and 200 nm, respectively. These values are basically consistent with Eg = 3.1 eV reported for the BLSO film deposited on SrTiO3 substrate, indicating that the induced strain from lattice mismatch does not influent seriously BLSO film due to the non-perovskite mica substrate. Moreover, high optical transmission observed in current films also means that mica substrate possibly possesses more potential applications for the developments of optoelectronic devices. Besides transparency, flexibility is another superiority for mica when its thickness is reduced to few layers by mechanical cleavage. In order to clarify the possible variation of optical transmission when BLSO film is bent due to the external environment, we also studied the transmission spectra of BLSO/BTO/mica film under bending conditions. There are two bending patterns. One is inward bending and the other is outward bending. The former produces an in-plane compressive strain but the latter leads to an in-plane tensile strain. In this study, we only choose the outward bending pattern to measure the optical transmission because this bending mode is easier to be performed for our measurement on spectrophotometer. Fig. 5 shows the measurement results of transmission spectra for the film under bended state and unbended state. For clarity, we only plotted one optical transmission for bended film because the transmission basically had no any obvious change under different bending states. The insets show the actual shapes and transparency of BLSO/BTO/mica heterostructure with bended and undended conditions. From two photographs, one can find that BLSO/BTO/mica flexible film shows a high optical transmittance regardless of flat or warped state. This result indicates that the bending effects do not significantly change the film transparency and all of them retain near 90% transparency in the visible region. Here, in comparison with the transparency of Fig. 4, we can note that there is a slight increase of it (about 3–5% ) for the BLSO film in Fig. 5. The reason is due to the reduction of mica thickness by mechanical cleavage. Only when
Fig. 2. The X-ray θ –2θ profile of the grown BLSO/BTO/mica films with the various annealing times (The symbol * indicates the diffraction peaks of impurity phase.).
and smaller. Consistent with the properties on SrTiO3 substrate, high substrate temperatures are also beneficial for BLSO film growth on mica substrate. Generally, the annealing process also plays a key role for the film growth. Here, an in situ annealing method with varied annealing times was applied in the process of film fabrication. Fig. 2 shows the XRD pattern of BLSO/BTO/mica film as the annealing time is 5, 10, 15 min, respectively. With the prolongation of annealing time, the extra peaks of impurity phase become gradually weakening and completely disappear as the annealing time is prolonged to 15 min. Similar to the increase of substrate temperatures, the longer annealing time also favored the enhancement of crystallinity. At present, although the great potential of BLSO has been reported to have extensive applications in TCO, a relatively smaller carrier mobility ( μe < 40 cm2V−1s−1) of BLSO films is incomparable with the value of BLSO single crystals ( μe < 320 cm2V−1s−1) [14,32]. The main reason is due to the high density of extended defects in films, e.g., threading dislocations, which are referred to be scattering centers for carriers [15]. Therefore, some reported results have confirmed that an appropriate annealing method can effectively decrease the threading dislocations and is indispensable to improve the carrier mobility of epitaxial BLSO films [33]. Thus, the crystallinity quality of as-grown films gain an obvious improvement by utilized annealing technology. This result can be also reflected from the upper XRD pattern of Fig. 2, where except for the normal diffraction peaks of mica substrate, two sets of separate diffraction peaks of epitaxial films, BLSO (111)/(222) and BTO (111)/(222) are clearly observed, indicating an ideal BLSO epitaxial film growth on mica substrate. Normally, the properties of highly oriented films approximate the properties of single crystals. The single crystal epitaxial films have a large homogeneity with well-defined material properties and correct oxygen stoichiometry. Based on the optimization of deposition parameters, a high quality BLSO/BTO/mica epitaxial film has been fabricated and shown in Fig. 3(a). Using the same parameters, we fabricated a series of BLSO/ BTO/mica film with different thicknesses and all of them were confirmed to be single crystal films. From the inset of Fig. 3(a), the full width at half-maximum of BLSO peak (222) is only 0.268°. Although the value is uncomparable with (∼ 0.022°) of BSO single crystal, it is still smaller than (∼ 0.57°) reported in BLSO film deposited on SrTiO3 (001) substrates [17]. Fig. 3(b) shows the surface morphology of the film with the thickness 120 nm by an atomic force microscopy over a measurement area of 10 × 10 μm2 . It shows densely packed surface structures and homogeneous grains. As shown in Fig. 3(c), a maximum height difference and a root mean square are no more than 2 nm and 1.3 nm, respectively, indicating a fairly smooth surface of BLSO/BTO/ mica film. Fig. 4 shows the transmission spectra in the wavelength range of 200–1200 nm for pure mica substrates and BLSO/BTO/mica
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Fig. 3. (a) The θ –2θ scan results of the grown BLSO/BTO/mica film. The inset shows the magnified Bragg peaks near (222). (b) the surface morphology of film obtained by AFM. (c) the height profile along the red solid line.
the thickness of mica substrate is decreased to few layers, the bending/ flexible state is easy to be realized. The pristine BSO is an insulting material and has a very large resistance. The substitutions of La3 + for Ba2 + ions generate free carriers and transfer it into n-type semiconductors. The increasing carrier density will reduce the barrier width and enhance the carrier mobility. However, more La-doping in BSO can cause an increase in the activation energy of the donor due to increased disorder, which bring a decrease in carrier density. Up to present, La-doping concentration of x = 0.04 is generally referred to be the optimal one which exhibits a very high mobility of 320 cm2 /V.s (carrier concentration: 8 × 1019 cm−3 for single crystals) at room temperature [14,15]. Therefore, in this
Table 1 Comparison of average optical transmittance(OT), optical band gap(Eg ), and resistivity at room temperature(∼ 300 K) for BLSO/BTO/mica film with different thickness (T = thickness; OTmax = maximal optical transmittance; OTmin = minimal optical transmittance). BLSO film
OTmax (% )
OTmin (% )
Eg (eV)
resistivity(mΩ cm)
T(120 nm) T(160 nm) T(200 nm)
91.37 90.26 89.12
86.15 88.61 82.39
3.26 3.22 3.19
10.29 8.41 7.32
Fig. 4. Optical transmission of pure mica substrate and the BLSO/BTO/mica film. Inset shows the plot of (hνα )2 vs. h ν . 4
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increasing the pressure of oxygen. However, their resistances also show an increase with the decrease of temperature, completely similar to the variation as in Fig. 6(a). Because most of BLSO films are deposited directly on SrTiO3(001) substrate without any buffer layer, we doubt whether the inserted buffer layer of BaTiO3 and growth orientation(111) are main reasons for the abnormal variation observed in the present heterostructure films. For clarifying this problem, two different heteroepitaxy films of BLSO/BTO/STO(001) and BLSO/BTO/STO(111) have been fabricated and their resistivity curves are shown in Fig. 6(b). Obviously, both values shows a steady decline with the decrease of temperature, consistent with the electronic transport behavior reported on BLSO/ SrTiO3(001) [14]. This result indicates that the BaTiO3 buffer layer plays no role on the abnormal variation of resistance in BLSO/BTO/ mica films. The only difference is that the resistivity of substrate orientation(111) is larger than that of (001)orientation. This point is consistent with the large resistivity observed in BLSO/BTO/mica films of Fig. 6(a). From the view point of crystal structure, BaSnO3 has an almost ideal cubic perovskite with its lattice constant of 4.116 Å . We can consider that the cubic structure of BLSO has not been changed by a slight substitutions of 4% La3 + ions for Ba3 +. Thus, compared with (001) plane, the distance between Sn-Sn is more longer for the (111) crystal surface. For such a doping semiconductor material, because the carrier transport behaviors are more favorable for band-gap model, we suggest that the carrier transport in (111) plane need more larger thermal activation energy against that in (001) plane, showing a larger resistivity observed in the current BLSO/BTO/mica films. Therefore, the BaTiO3 buffer layer and epitaxial growth along orientation(111) are not reasons for the anomalous electronic transport in Fig. 6(b). Although the insertion of BaTiO3 buffer layer can realize a BLSO epitaxial growth on mica substrate, the fixed lattice mismatch among them is unavoidable. Such a mismatch necessarily affects the actual crystal structure of epitaxial film more or less. We propose the fundamental reasons for the anomalous resistivity originates from misfit strain due to the substrate clamping and hampering effect which parasitize in BLSO epitaxial growth. Therefore, how to effectively eliminate misfit strain in BLSO/BTO/mica heterostructure films is the key to solve the problem. Some recent reports have confirmed that the carrier transports of BLSO film can be elevated obviously by the
Fig. 5. Optical transmission of bending and unbending BLSO/BTO/mica film. Inset shows the transparency photographs under unbending (left) and bending (right) conditions.
work, all of BLSO flexible films were deposited with the target Ba 0.96La 0.04 SnO3 compound. Fig. 6(a) shows the temperature dependence of resistivity for BLSO/BTO/mica films with different film thickness. Three samples basically exhibit a similar variation with the decrease of temperature. At the same temperature, the resistivity of thicker film is smaller but the thinner film is larger. For example (as showing a dot line in Fig. 6(a)), at T = 300 K, the resistivity of thicknesses 200 and 120 nm are 7.32 and 10.29 mΩ cm, respectively. This is a normal variation and most of films also obey this regular pattern. With the increase of film thickness, the physical properties are more and more close to the nature of bulk material. Here, an anomalous variation is worthy of our attention. In the previous reports, the resistivity of BLSO films always decrease as the temperature decrease from room temperature to low temperature. In Fig. 6(a), however, all curves increase with the decrease of temperatures, exhibiting an insulting behavior. Although the observed value of resistance is comparable with the reported data in the same temperature range, their variation tendencies are different. In order to further verify this variation, we also measured other BLSO/BTO/mica heterostructure films which were prepared with the changed fabrication condition(no shown here), such as prolonging the annealing time or
Fig. 6. Temperature-dependent resistivity ρ : (a) BLSO/BTO/mica film and dot line is visual guides for resistivity variation at T = 300 K for different thickness, (b) BLSO/BTO/STO(001) and (111) films. 5
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insertion of BaSnO3 buffer layer or annealing treatment under N2 rather than O2 environment due to an annihilation of extended defects [23,33]. More recently, from comparison BLSO epitaxial film grown on SrTiO3(001) and MgO(001) substrate, Sanchela et al. found that the carrier mobility strongly depends on the film thickness instead of the film/substrate lattice mismatch [35]. This result implies that the increase of BLSO film thickness may be a convenient and valid method to enhance the carrier transport properties in the BLSO epitaxial film. However, the enhancement of film thickness and adding another buffer layer of BaSnO3 are bound to decrease transparency. Therefore, combined with the consideration of optical transmittance and conductivity, exploiting new methods to improve the carrier transport properties is an open issue for the future flexible BLSO heteroepitaxy films.
[10] H.H. Wang, D.F. Cui, S.Y. Dai, H.B. Lu, Y.L. Zhou, Z.H. Chen, G.Z. Yang, Optical and transport properties of Sb-doped SrTiO3 thin films, J. Appl. Phys. 90 (2001) 4664. [11] H.Z. Guo, L.F. Liu, Y.Y. Fei, W.F. Xiang, H.B. Lu, S.Y. Dai, Y.L. Zhou, Z.H. Chen, Optical properties of p-type In-doped SrTiO3 thin films, J. Appl. Phys. 94 (2003) 4558. [12] R.P. Wang, C.J. Tao, Nb-doped CaTiO3 transparent semiconductor thin films, J. Cryst. Growth 245 (2002) 63. [13] H.R. Liu, J.H. Yang, H.J. Xiang, X.G. Gong, S.H. Wei, Origin of the superior conductivity of perovskite Ba(Sr)SnO3, Appl. Phys. Lett. 102 (2013) 112109. [14] H.J. Kim, U. Kim, H.M. Kim, T.H. Kim, H.S. Mun, B.-G. Jeon, K.T. Hong, W.-J. Lee, C. Ju, K.H. Kim, K. Char, High mobility in a stable transparent perovskite oxide, Appl. Phys. Express 5 (2012) 061102. [15] H.J. Kim, U. Kim, T.H. Kim, J. Kim, H.M. Kim, B.G. Jeon, W.J. Lee, H.S. Mun, K.T. Hong, J. Yu, K. Char, K.H. Kim, Physical properties of transparent perovskite oxides (Ba,La)SnO3 with high electrical mobility at room temperature, Phys. Rev. B 86 (2012) 165205. [16] Q.Z. Liu, J.M. Dai, H. Li, B. Li, Y.X. Zhang, K. Dai, S. Chen, Optical and transport properties of Gd doped BaSnO3 epitaxial films, J. Alloy. Compd. 647 (2015) 959. [17] Q.Z. Liu, J.M. Dai, Z.L. Liu, X.B. Zhang, G.P. Zhu, G.H. Ding, J. Phys. D: Appl. Phys. 43 (2010) 455401. [18] A.V. Sanchela, T. Onozato, B. Feng, Y. Ikuhara, H. Ohta, Thermopower modulation clarification of the intrinsic effective mass in transparent oxide semiconductor BaSnO3, Phys. Rev. Mater. 1 (2017) 034603. [19] K. Krishnaswamy, B. Himmetoglu, Y. Kang, A. Janotti, C.G. Van de Walle, Firstprinciples analysis of electron transport in BaSnO3, Phys. Rev. B 95 (2017) 205202. [20] P.V. Wadekar, J. Alaria, M. OSullivan, N.L.O. Flack, T.D. Manning, L.J. Phillips, K. Durose, O. Lozano, S. Lucas, J.B. Claridge, M.J. Rosseinsky, Improved electrical mobility in highly epitaxial La:BaSnO3 films on SmScO3(110) substrates, Appl. Phys. Lett. 105 (2014) 052104. [21] S. Raghavan, T. Schumann, H. Kim, J.Y. Zhang, T.A. Cain, S. Stemmer, High-mobility BaSnO3 grown by oxide molecular beam epitaxy, APL Mater. 4 (2016) 016106. [22] F.A. Chambers, T.C. Kaspar, A. Prakash, G. Haugstad, B. Jalan, Band alignment at epitaxial BaSnO3/SrTiO3(001) and BaSnO3/LaAlO3(001) heterojunctions, Appl. Phys. Lett. 108 (2016) 152104. [23] W.J. Lee, H.J. Kim, E. Sohn, T.H. Kim, J.Y. Park, W. Park, H. Jeong, T. Lee, J.H. Kim, K.Y. Choi, K.H. Kim, Enhanced electron mobility in epitaxial (Ba,La)SnO3 films on BaSnO3(001) substrates, Appl. Phys. Lett. 108 (2016) 082105. [24] J. Shin, Y.M. Kim, Y. Kim, C. Park, K. Char, High mobility BaSnO3 films and field effect transistors on non-perovskite MgO substrate, Appl. Phys. Lett. 109 (2016) 262102. [25] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, T. Sakurai, A large-area, flexible pressure sensor matrix with organic field-effect transistors for artificial skin applications, Proc. Natl. Acad. Sci. USA 101 (2004) 9966. [26] D.Y. Khang, H. Jiang, Y. Huang, J. Rogers, A stretchable form of single-crystal silicon for high-performance electronics on rubber substrates, Science 311 (2006) 208. [27] D. Grimm, C.C.B. Bufon, C. Deneke, P. Atkinson, D.J. Thurmer, F. Schäffel, S. Gorantla, A. Bachmatiuk, O.G. Schmidt, Rolled-up nanomembranes as compact 3D architectures for field effect transistors and fluidic sensing applications, Nano Lett. 13 (2013) 213. [28] L.Y. Yuan, J.J. Dai, X.H. Fan, T. Song, Y.T. Tao, K. Wang, Z. Xu, J. Zhang, X.D. Bai, P.X. Lu, J. Chen, J. Zhou, Z.L. Wang, Flexible solid-state supercapacitors based on carbon nanoparticles/MnO2 nanorods hybrid structure, ACS Nano 5 (2011) 4007. [29] S.R. Bakaul, C.R. Serrao, O. Lee, Z.Y. Lu, A. Yadav, C. Carraro, R. Maboudian, R. Ramesh, S. Salahuddin, High speed epitaxial perovskite memory on flexible substrates, Adv. Mater. 29 (2017) 1605699. [30] Yuxi Yang, Guoliang Yuan, Zhibo Yan, Yaojin Wang, Xubing Lu, Junming Liu, Flexible, semitransparent, and inorganic resistive memory based on BaTi0.95Co0.05 O3 film, Adv. Mater. 29 (2017) 1700425. [31] Ying-Hao Chu, Van der Waals oxide heteroepitaxy, NPJ Quantum Materials 2 (2017) 67. [32] H. Mun, U. Kim, H. Min Kim, C. Park, T. Hoon Kim, H. Joon Kim, K. Hoon Kim, K. Char, Large effects of dislocations on high mobility of epitaxial perovskite Ba 0.96La 0.04 SnO3 films, Appl. Phys. Lett. 102 (2013) 252105. [33] S. Yu, D. Yoon, J. Son, Enhancing electron mobility in La-doped BaSnO3 thin films by thermal strain to annihilate extended defects, Appl. Phys. Lett. 108 (2016) 262101. [34] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solidi b 15 (1966) 627. [35] A.V. Sanchela, M. Wei, H. Zensyo, B. Feng, J.Y. Lee, G. Kim, H.J. Jeen, Y. Ikuhara, H. Ohta, Large thickness dependence of the carrier mobility in a transparent oxide semiconductor, La-doped BaSnO3, Appl. Phys. Lett. 112 (2018) 232102.
4. Conclusion In summary, a series of high quality flexible BLSO heteroepitaxy films with different thickness have been fabricated on mica substrate by PLD technique. The measurement of transmission spectra indicates that all of these flexible films exhibit an excellent transmittance with more than 85 % in the visible region regardless of bending or unbending states. Compared with the electronic transport behavior of films deposited on rigid wafer STO, we find the anomalous increase of resistivity dependence on temperature for flexible BLSO film is due to the fixed misfit strain rather than the insertion of BaTiO3 buffer layer or the film growth along orientation(111). We suggest that the further optimization of film fabrication on mica substrate will play a crucial role in improving the electronic transport for heteroepitaxy BLSO flexible films. Acknowledgment One of the authors Jiyu Fan would like to thank Dr. Qinzhuang Liu for the fruitful discussions and enlightening suggestions about BLSO film growth. This work was supported by the Fundamental Research Funds for the Central Universities (Grant nos. NE2016102 and NP2017103), and the National Natural Science Foundation of China (Grant nos. 11574322, 11774171, 11774172 and U1632122). References [1] K. Hayashi, S. Matsuishi, T. Kamiya, M. Hirano, H. Hosono, Light-induced conversion of an insulating refractory oxide into a persistent electronic conductor, Nature 419 (2002) 462. [2] J.F. Wager, Transparent electronics, Science 300 (2003) 1245. [3] G. Thomas, Materials science: invisible circuits, Nature 389 (1997) 907. [4] O.N. Mryasov, A.J. Freeman, Electronic band structure of indium tin oxide and criteria for transparent conducting behavior, Phys. Rev. B 64 (2001) 233111. [5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor, Science 300 (2003) 1269. [6] H. Hosono, Recent progress in transparent oxide semiconductors: materials and device application, Thin Solid Films 515 (2007) 6000. [7] H. Ohta, M. Orita, M. Hirano, H. Tanji, H. Kawazoe, H. Hosono, Highly electrically conductive indium-tin-oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by pulsed-laser deposition, Appl. Phys. Lett. 76 (2000) 2740. [8] T. Makino, Y. Segawa, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Electron transport in ZnO thin films, Appl. Phys. Lett. 87 (2005) 022101. [9] M.D. McDaniel, A. Posadas, T.Q. Ngo, C.M. Karako, J. Bruley, M.M. Frank, V. Narayanan, A.A. Demkov, J.G. Ekerdt, Incorporation of La in epitaxial SrTiO3 thin films grown by atomic layer deposition on SrTiO3-buffered Si (001) substrates, J. Appl. Phys. 115 (2014) 224108.
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High optical transmittance and anomalous electronic transport in flexible transparent conducting oxides Ba 0.96La 0.04SnO3 thin films ⁎
Weifeng Suna, Jiyu Fana, , Ruixing Xua, Xiyuan Zhanga, Caixia Kana, Wei Liub, Lei Zhangb, Chunlan Mac, Dazhi Hua, Yanda Jia, Yan Zhua, Hao Yanga a
Department of Applied Physics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China c School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical property Transport property Heteroepitaxy Flexible substrate
Wide bandgap semiconducting perovskite La:BaSnO3 is a promising candidate for next-generation transparent conducting oxides due to its high electron mobility and excellent oxygen stability. In this work, in order to realize flexible optoelectronics, an epitaxial growth of Ba 0.96La 0.04 SnO3 (BLSO) film on a flexible mica via van der Waals epitaxy is established. The high quality heteroepitaxy and crystallinity of BLSO films are confirmed by a combination of X-ray diffraction and atomic force microscopy. Results show that the flexible BLSO films not only retain a high transmittance of more than 85% in the visible region under unbending conditions, but also exhibit a remarkable transmittance of 90% under bending conditions. Due to the fixed lattice mismatch and misfit strain, an anomalous electronic transport behavior, showing as a continuous enhancement of resistivity dependence on the decreasing temperatures from high to low, was observed for all BLSO films. Although the resistivity of flexible BLSO films is a slight larger than that of growth on rigid counterparts, the resistivity of 7–10 mΩ cm is also satisfied with actual application for optoelectronic devices at room-temperature. Our study marks that the technological advancements toward realizing flexible optoelectronics are promising by utilizing perovskite oxides La-doped BaSnO3 and mica substrate.
1. Introduction Over the past few years, transparent conducting oxides (TCO) have attracted a great deal of interests owing to increasing demands for the widespread application of current optoelectronic devices, e.g. solar cells, flat panel displays, light emitting diodes, and transparent logic devices, and so on. [1–5] Many binary oxide materials including the pure and impurity doped ZnO, In2 O3, as well as SnO2 were studied for these purposes, successfully demonstrating passive transparent conductive windows to active semiconducting devices, such as pn junctions, field effect transistors, and UV lasers [5–8]. However, with the ongoing development of optoelectronic technology, exploration of new alternative transparent materials but with more outstanding performance or special properties are becoming more and more important. In view of actual applications, most of TCO films are generally required to have a wide band gap of more than 3.0 eV, necessary for high transmittance in the visible region [6]. Meanwhile, wide band gap semiconductors with the perovskite structure are becoming the favorable TCO due to their rich physical properties and compatibility for
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multilayer structures together with the potential applications in hightemperature and high-power electronics. For example, La(In,Sb)-doped SrTiO3 and Nb-doped CaTiO3 have been reported in the context of new TCO materials [9–12]. In this work, we focus on another wide band gap semiconductor, perovskite barium stannate oxide BaSnO3 (BSO). BSO is known to form an ideal cubic perovskite structure in which the Sn-O-Sn bonding angle is close to 180 ° . The pristine BSO can be regarded as an insulating material since for its negligible conductivity. Its conduction band is mainly Sn 5s orbitals, and the valence band is mainly oxygen 2p orbitals with a large bandgap of 3.4 eV [13]. For the enhancement of conductivity, some different doped elements have been substituted, such as La and Gd doping on Ba site, Sb doping on Sn site [14–17]. In particular, La-doped BSO (BLSO) has been recently gained numerous attentions due to its excellent electron mobility and high chemical stability. Kim et al. reported that BLSO film and single crystal have high carrier mobility of 70 and 320 cm2 /V s at room temperature, respectively [15]. Recent investigations have proved that such a high mobility is due to its smaller carrier effective mass (m* = 0.40 m e ) and longer
Corresponding author. E-mail addresses: [email protected] (J. Fan), [email protected] (H. Yang).
https://doi.org/10.1016/j.ceramint.2018.07.001 Received 17 June 2018; Received in revised form 30 June 2018; Accepted 1 July 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Sun, W., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.07.001
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more than 700 °C. The good chemical stability of mica under high temperature was very important for BLSO growth. However, mica wafers are non-perovskite substrate. The perovskite substrates are good in promoting the growth of perovskite type film. On the contrary, nonperovskite substrates difficultly achieve an ideal epitaxial growth of it. Therefore, BLSO can't be directly deposited on mica substrate (In fact, we have tried doing it but only polycrystalline film is obtained.). Thus, we need to first deposit one kind of perovskite materials as buffer layer and then deposit BLSO. In this paper, perovskite oxides BaTiO3(BTO) was chosen to deposit on mica substrate as buffer layer through comparing different materials. We found BTO was easy to realize epitaxial growth on mica substrate along the direction of (111). In the following work for all BLSO films, before depositing BLSO, BTO buffer layer with the thickness near 30 nm was first deposited on mica substrate. In order to guarantee the epitaxial growth of BLSO films, some basic film properties of BTO buffer layer, such as its appropriate orientation, crystallinity, and smooth surface have been also checked beforehand. The super-smooth surface and high crystallinity of BTO buffer layer can be prepared by using the optimal growth condition. As we know, many deposition conditions determine the growth quality of epitaxial films, including laser energy density, repetition rate (number of laser pulses per second), ambient gas and its pressure, target-substrate distance, substrate temperature, and annealing time. In this work, we found that the last two factors were more prominently related with the growth quality than others. Fig. 1(a) shows the XRD patterns of BLSO films deposited on mica (00l) substrate with the inserted buffer layer BTO as the substrate temperature is set in the range of 720–780 °C. We find that the BTO film exactly grows along the direction (111) on mica substrate and BLSO film epitaxially grows on top of BTO film. Both (111) and (222) peaks of BLSO film were clearly shown in Fig. 1(a). Two obvious characteristics can be also observed with the increase of substrate temperatures. (i) At T = 720 °C, except for the normal diffraction peaks of mica, BTO, and BLSO, its XRD pattern also exhibits many impurity peaks. As the substrate temperatures was increased to 780 °C, the impurity peaks significantly decrease and almost disappear. (ii) With the enhancement of substrate temperatures, the peak intensity of BLSO film (222) becomes stronger, indicating a better crystallization of BLSO film at 780 °C than 720 °C. The rise of substrate temperatures improves the mobility and diffusion of the adatoms, which is propitious for BLSO atoms to arrange themselves in highly ordered manner. Fig. 1(b) shows the magnified Bragg peaks of BLSO film (222) around 2 θ∼ 81°. Besides the rise of peak intensity, the full width at half-maximum of BLSO peak (222) also becomes smaller
carrier relaxation time [18,19]. The high mobility and a wide band gap indicate that BLSO is a promising candidate for transparent conductor applications. Furthermore, Wadekar et al. found that the room temperature resistivity could be effectively decreased from 7.8 to 4.4 mΩ cm by choosing SmScO3 substrate comparing with SrTiO3 substrate [20]. The main reason has been attributed to reduction in dislocation density due to the lower lattice mismatch. In addition, epitaxial growth BLSO film on the other variety of substrates, including PrScO3 [21], LaAlO3 [22], BaSnO3 [23], MgO [24], have been also reported. We noticed that all reported substrates were rigid wafers instead of flexible substrates. In present, flexible devices, i.e., devices fabricated on flexible substrates, are very attractive in application due to their stretchable, biocompatible, light-weight, and portable [25–29]. Therefore, in order to further expand the application scope and give rise to new functionalities of BLSO film on optoelectronic field, it is necessary to study the BLSO film growth on the flexible substrates. Here, we reported our recent results about the epitaxial growth BLSO film on flexible mica substrates, including detailed fabrication conditions, optical transmittance spectra of unbending and bending conditions, as well as electronic transport properties. The obtained results indicate that BLSO films have excellent optical transmittance as high as more than 85% in the visible region regardless of under bending or unbending state. Different from the conductivity properties of BLSO films grown on SrTiO3 substrate, an anomalous electronic transport showing as the enhancement of resistivity with the decrease of temperature occurred in all of present films. The experiments confirmed that this behavior was unrelated to the inserted buffer layer but depended on misfit strain due to lattice mismatch. Although BLSO films grown on the flexible mica substrate show a larger resistivity than that grown on rigid SrTiO3(001) substrate, the value of 7–10 mΩ cm is also satisfied with actual application in optoelectronic devices at room temperature. Our work will pave a way for the future development of flexible TCO film. 2. Experiment A series of BLSO films with various thicknesses were fabricated on mica substrates using pulsed laser deposition (PLD) technique. Dense ceramics targets of Ba 0.96La 0.04 SnO3 were prepared by standard solid state reactions. A 248 nm KrF excimer laser with the repetition rate of 4 Hz and laser energy density irradiated on the rotating targets of 1.6 J/cm2 were used for the film fabrication. During deposition, the temperature of substrate was set in the range of 720–780 °C to change the quality of epitaxial films. Then, the films were in situ annealed before being cooled down in the same oxygen ambient. The crystalline structure and the epitaxial characteristics of the BLSO films were examined by X-ray diffraction (XRD, PANalytical) using Cu K α radiation at room temperature. The surface morphology of the films was studied by atomic force microscopy (AFM) in the tapping mode. The optical transmittance were measured on a spectrophotometer UV3600 at the room temperature. Temperature-dependent resistivity was investigated by the standard four-terminal method. 3. Results and discussion Up to present, BLSO films were mostly deposited on the rigid wafers SrTiO3 (001) which is one of the most widely substrate for depositing perovskite oxides films [14,15]. Some reported results have confirmed that it is easy to realize epitaxial growth of BLSO films on SrTiO3 (001) substrate with a superior crystalline quality due to both of all perovskite structures and an ignorable lattice mismatch. Here, we choose mica as the substrate mainly based on three reasons [30,31]: (i)Mica is a flexible material and has been extensively utilized to fabricate flexible devices; (ii) Mica is a transparent material which is beneficial for high light transmission; (iii) Mica can bear high temperature. During the deposition of BLSO films, the substrate was heated to high temperature
Fig. 1. (a) The X-ray θ -2θ profile of the grown BLSO/BTO/mica films at different substrate temperatures and the bottom XRD pattern is for empty mica (The symbol * indicates the diffraction peaks of impurity phase.). (b) The varied FWHM of (222) Bragg peaks at three various substrate temperatures. 2
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heterostructure films. The average transmittances of all films including a clean mica exhibited a high transparency of more than 85% in the visible region(380–780 nm). The detailed comparison of them can be seen in Table 1. Except for a linear behavior observed for pure mica, the others transmission spectra of heterostructure films all show a wave line. Here, BLSO/BTO/mica films can be considered as a multilayer film. Therefore, the transmissivity inevitably exhibits some maximum and the minimum values due to Fabry-Pérot interference effect. Similar to other TCO films, we can find a subtle decrease of transmittance with increasing film thickness in the current samples. This is a kind of thickness effect, in which the thicker films absorb more light and have lower transmittance. However, in the the short-wavelength near-infrared range (780–1100 nm), the average transmittance of all the films do not reveal an obvious discrepancy. The main reason was attributed to another transmission mechanism. In this region, the plasma resonance effect plays a main role for optical transmission. The plasma frequency dependence on the square root of carrier concentration. As the film thickness exceeds to a certain level, such as 120 nm in this case, the carrier concentration is basically independent on the variation of film thickness. Due to the nearly same carrier concentration in three films, there are no obvious change in their average transmittances. Relationship between the optical absorption coefficient (α ) and the photo energy (h ν ) is commonly described by the Taucs relation [34], (hνα )m = A(hν − Eg ) , where h, ν , A, and Eg are the Plank constant, photo frequency, ordinary constant, and optical band gap width of the material, respectively, and m is the exponent which is determined by the type of electronic transition causing the optical absorption and equals 1/2 and 2 for the indirect and direct optical band transition, respectively. BSO belongs to direct optical band transition. the value of m was chosen to be 2. The inset of Fig. 4 shows the plot of (hνα )2 against h ν of BLSO/BTO/mica film with different thicknesses. By fitting linear portion of (hνα )2 to energy axis h ν , the optical band gap Eg can be deduced. For clarity, we only listed the fitting results of 120 and 200 nm due to too small change of h ν among these films. Here, three optical band gaps, Eg = 3.26, 3.22, 3.19 eV , are obtained for the films thickness of 120, 160, and 200 nm, respectively. These values are basically consistent with Eg = 3.1 eV reported for the BLSO film deposited on SrTiO3 substrate, indicating that the induced strain from lattice mismatch does not influent seriously BLSO film due to the non-perovskite mica substrate. Moreover, high optical transmission observed in current films also means that mica substrate possibly possesses more potential applications for the developments of optoelectronic devices. Besides transparency, flexibility is another superiority for mica when its thickness is reduced to few layers by mechanical cleavage. In order to clarify the possible variation of optical transmission when BLSO film is bent due to the external environment, we also studied the transmission spectra of BLSO/BTO/mica film under bending conditions. There are two bending patterns. One is inward bending and the other is outward bending. The former produces an in-plane compressive strain but the latter leads to an in-plane tensile strain. In this study, we only choose the outward bending pattern to measure the optical transmission because this bending mode is easier to be performed for our measurement on spectrophotometer. Fig. 5 shows the measurement results of transmission spectra for the film under bended state and unbended state. For clarity, we only plotted one optical transmission for bended film because the transmission basically had no any obvious change under different bending states. The insets show the actual shapes and transparency of BLSO/BTO/mica heterostructure with bended and undended conditions. From two photographs, one can find that BLSO/BTO/mica flexible film shows a high optical transmittance regardless of flat or warped state. This result indicates that the bending effects do not significantly change the film transparency and all of them retain near 90% transparency in the visible region. Here, in comparison with the transparency of Fig. 4, we can note that there is a slight increase of it (about 3–5% ) for the BLSO film in Fig. 5. The reason is due to the reduction of mica thickness by mechanical cleavage. Only when
Fig. 2. The X-ray θ –2θ profile of the grown BLSO/BTO/mica films with the various annealing times (The symbol * indicates the diffraction peaks of impurity phase.).
and smaller. Consistent with the properties on SrTiO3 substrate, high substrate temperatures are also beneficial for BLSO film growth on mica substrate. Generally, the annealing process also plays a key role for the film growth. Here, an in situ annealing method with varied annealing times was applied in the process of film fabrication. Fig. 2 shows the XRD pattern of BLSO/BTO/mica film as the annealing time is 5, 10, 15 min, respectively. With the prolongation of annealing time, the extra peaks of impurity phase become gradually weakening and completely disappear as the annealing time is prolonged to 15 min. Similar to the increase of substrate temperatures, the longer annealing time also favored the enhancement of crystallinity. At present, although the great potential of BLSO has been reported to have extensive applications in TCO, a relatively smaller carrier mobility ( μe < 40 cm2V−1s−1) of BLSO films is incomparable with the value of BLSO single crystals ( μe < 320 cm2V−1s−1) [14,32]. The main reason is due to the high density of extended defects in films, e.g., threading dislocations, which are referred to be scattering centers for carriers [15]. Therefore, some reported results have confirmed that an appropriate annealing method can effectively decrease the threading dislocations and is indispensable to improve the carrier mobility of epitaxial BLSO films [33]. Thus, the crystallinity quality of as-grown films gain an obvious improvement by utilized annealing technology. This result can be also reflected from the upper XRD pattern of Fig. 2, where except for the normal diffraction peaks of mica substrate, two sets of separate diffraction peaks of epitaxial films, BLSO (111)/(222) and BTO (111)/(222) are clearly observed, indicating an ideal BLSO epitaxial film growth on mica substrate. Normally, the properties of highly oriented films approximate the properties of single crystals. The single crystal epitaxial films have a large homogeneity with well-defined material properties and correct oxygen stoichiometry. Based on the optimization of deposition parameters, a high quality BLSO/BTO/mica epitaxial film has been fabricated and shown in Fig. 3(a). Using the same parameters, we fabricated a series of BLSO/ BTO/mica film with different thicknesses and all of them were confirmed to be single crystal films. From the inset of Fig. 3(a), the full width at half-maximum of BLSO peak (222) is only 0.268°. Although the value is uncomparable with (∼ 0.022°) of BSO single crystal, it is still smaller than (∼ 0.57°) reported in BLSO film deposited on SrTiO3 (001) substrates [17]. Fig. 3(b) shows the surface morphology of the film with the thickness 120 nm by an atomic force microscopy over a measurement area of 10 × 10 μm2 . It shows densely packed surface structures and homogeneous grains. As shown in Fig. 3(c), a maximum height difference and a root mean square are no more than 2 nm and 1.3 nm, respectively, indicating a fairly smooth surface of BLSO/BTO/ mica film. Fig. 4 shows the transmission spectra in the wavelength range of 200–1200 nm for pure mica substrates and BLSO/BTO/mica
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Fig. 3. (a) The θ –2θ scan results of the grown BLSO/BTO/mica film. The inset shows the magnified Bragg peaks near (222). (b) the surface morphology of film obtained by AFM. (c) the height profile along the red solid line.
the thickness of mica substrate is decreased to few layers, the bending/ flexible state is easy to be realized. The pristine BSO is an insulting material and has a very large resistance. The substitutions of La3 + for Ba2 + ions generate free carriers and transfer it into n-type semiconductors. The increasing carrier density will reduce the barrier width and enhance the carrier mobility. However, more La-doping in BSO can cause an increase in the activation energy of the donor due to increased disorder, which bring a decrease in carrier density. Up to present, La-doping concentration of x = 0.04 is generally referred to be the optimal one which exhibits a very high mobility of 320 cm2 /V.s (carrier concentration: 8 × 1019 cm−3 for single crystals) at room temperature [14,15]. Therefore, in this
Table 1 Comparison of average optical transmittance(OT), optical band gap(Eg ), and resistivity at room temperature(∼ 300 K) for BLSO/BTO/mica film with different thickness (T = thickness; OTmax = maximal optical transmittance; OTmin = minimal optical transmittance). BLSO film
OTmax (% )
OTmin (% )
Eg (eV)
resistivity(mΩ cm)
T(120 nm) T(160 nm) T(200 nm)
91.37 90.26 89.12
86.15 88.61 82.39
3.26 3.22 3.19
10.29 8.41 7.32
Fig. 4. Optical transmission of pure mica substrate and the BLSO/BTO/mica film. Inset shows the plot of (hνα )2 vs. h ν . 4
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increasing the pressure of oxygen. However, their resistances also show an increase with the decrease of temperature, completely similar to the variation as in Fig. 6(a). Because most of BLSO films are deposited directly on SrTiO3(001) substrate without any buffer layer, we doubt whether the inserted buffer layer of BaTiO3 and growth orientation(111) are main reasons for the abnormal variation observed in the present heterostructure films. For clarifying this problem, two different heteroepitaxy films of BLSO/BTO/STO(001) and BLSO/BTO/STO(111) have been fabricated and their resistivity curves are shown in Fig. 6(b). Obviously, both values shows a steady decline with the decrease of temperature, consistent with the electronic transport behavior reported on BLSO/ SrTiO3(001) [14]. This result indicates that the BaTiO3 buffer layer plays no role on the abnormal variation of resistance in BLSO/BTO/ mica films. The only difference is that the resistivity of substrate orientation(111) is larger than that of (001)orientation. This point is consistent with the large resistivity observed in BLSO/BTO/mica films of Fig. 6(a). From the view point of crystal structure, BaSnO3 has an almost ideal cubic perovskite with its lattice constant of 4.116 Å . We can consider that the cubic structure of BLSO has not been changed by a slight substitutions of 4% La3 + ions for Ba3 +. Thus, compared with (001) plane, the distance between Sn-Sn is more longer for the (111) crystal surface. For such a doping semiconductor material, because the carrier transport behaviors are more favorable for band-gap model, we suggest that the carrier transport in (111) plane need more larger thermal activation energy against that in (001) plane, showing a larger resistivity observed in the current BLSO/BTO/mica films. Therefore, the BaTiO3 buffer layer and epitaxial growth along orientation(111) are not reasons for the anomalous electronic transport in Fig. 6(b). Although the insertion of BaTiO3 buffer layer can realize a BLSO epitaxial growth on mica substrate, the fixed lattice mismatch among them is unavoidable. Such a mismatch necessarily affects the actual crystal structure of epitaxial film more or less. We propose the fundamental reasons for the anomalous resistivity originates from misfit strain due to the substrate clamping and hampering effect which parasitize in BLSO epitaxial growth. Therefore, how to effectively eliminate misfit strain in BLSO/BTO/mica heterostructure films is the key to solve the problem. Some recent reports have confirmed that the carrier transports of BLSO film can be elevated obviously by the
Fig. 5. Optical transmission of bending and unbending BLSO/BTO/mica film. Inset shows the transparency photographs under unbending (left) and bending (right) conditions.
work, all of BLSO flexible films were deposited with the target Ba 0.96La 0.04 SnO3 compound. Fig. 6(a) shows the temperature dependence of resistivity for BLSO/BTO/mica films with different film thickness. Three samples basically exhibit a similar variation with the decrease of temperature. At the same temperature, the resistivity of thicker film is smaller but the thinner film is larger. For example (as showing a dot line in Fig. 6(a)), at T = 300 K, the resistivity of thicknesses 200 and 120 nm are 7.32 and 10.29 mΩ cm, respectively. This is a normal variation and most of films also obey this regular pattern. With the increase of film thickness, the physical properties are more and more close to the nature of bulk material. Here, an anomalous variation is worthy of our attention. In the previous reports, the resistivity of BLSO films always decrease as the temperature decrease from room temperature to low temperature. In Fig. 6(a), however, all curves increase with the decrease of temperatures, exhibiting an insulting behavior. Although the observed value of resistance is comparable with the reported data in the same temperature range, their variation tendencies are different. In order to further verify this variation, we also measured other BLSO/BTO/mica heterostructure films which were prepared with the changed fabrication condition(no shown here), such as prolonging the annealing time or
Fig. 6. Temperature-dependent resistivity ρ : (a) BLSO/BTO/mica film and dot line is visual guides for resistivity variation at T = 300 K for different thickness, (b) BLSO/BTO/STO(001) and (111) films. 5
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insertion of BaSnO3 buffer layer or annealing treatment under N2 rather than O2 environment due to an annihilation of extended defects [23,33]. More recently, from comparison BLSO epitaxial film grown on SrTiO3(001) and MgO(001) substrate, Sanchela et al. found that the carrier mobility strongly depends on the film thickness instead of the film/substrate lattice mismatch [35]. This result implies that the increase of BLSO film thickness may be a convenient and valid method to enhance the carrier transport properties in the BLSO epitaxial film. However, the enhancement of film thickness and adding another buffer layer of BaSnO3 are bound to decrease transparency. Therefore, combined with the consideration of optical transmittance and conductivity, exploiting new methods to improve the carrier transport properties is an open issue for the future flexible BLSO heteroepitaxy films.
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4. Conclusion In summary, a series of high quality flexible BLSO heteroepitaxy films with different thickness have been fabricated on mica substrate by PLD technique. The measurement of transmission spectra indicates that all of these flexible films exhibit an excellent transmittance with more than 85 % in the visible region regardless of bending or unbending states. Compared with the electronic transport behavior of films deposited on rigid wafer STO, we find the anomalous increase of resistivity dependence on temperature for flexible BLSO film is due to the fixed misfit strain rather than the insertion of BaTiO3 buffer layer or the film growth along orientation(111). We suggest that the further optimization of film fabrication on mica substrate will play a crucial role in improving the electronic transport for heteroepitaxy BLSO flexible films. Acknowledgment One of the authors Jiyu Fan would like to thank Dr. Qinzhuang Liu for the fruitful discussions and enlightening suggestions about BLSO film growth. This work was supported by the Fundamental Research Funds for the Central Universities (Grant nos. NE2016102 and NP2017103), and the National Natural Science Foundation of China (Grant nos. 11574322, 11774171, 11774172 and U1632122). References [1] K. Hayashi, S. Matsuishi, T. Kamiya, M. Hirano, H. Hosono, Light-induced conversion of an insulating refractory oxide into a persistent electronic conductor, Nature 419 (2002) 462. [2] J.F. Wager, Transparent electronics, Science 300 (2003) 1245. [3] G. Thomas, Materials science: invisible circuits, Nature 389 (1997) 907. [4] O.N. Mryasov, A.J. Freeman, Electronic band structure of indium tin oxide and criteria for transparent conducting behavior, Phys. Rev. B 64 (2001) 233111. [5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor, Science 300 (2003) 1269. [6] H. Hosono, Recent progress in transparent oxide semiconductors: materials and device application, Thin Solid Films 515 (2007) 6000. [7] H. Ohta, M. Orita, M. Hirano, H. Tanji, H. Kawazoe, H. Hosono, Highly electrically conductive indium-tin-oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by pulsed-laser deposition, Appl. Phys. Lett. 76 (2000) 2740. [8] T. Makino, Y. Segawa, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Electron transport in ZnO thin films, Appl. Phys. Lett. 87 (2005) 022101. [9] M.D. McDaniel, A. Posadas, T.Q. Ngo, C.M. Karako, J. Bruley, M.M. Frank, V. Narayanan, A.A. Demkov, J.G. Ekerdt, Incorporation of La in epitaxial SrTiO3 thin films grown by atomic layer deposition on SrTiO3-buffered Si (001) substrates, J. Appl. Phys. 115 (2014) 224108.
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