Multi-layer film structures BaTiO3/SiO2 with a thickness of ~1 μm containing up to 14 pairs of layers were synthesized by the sol–gel method with sequential heat treatment. It is shown that the synthesized structures are X-ray amorphous. The formation of bands in the transmission and reflection spectra caused by interference effects is demonstrated. A more regular structure exhibits a photon band gap (opacity band) in the visible range with main minimum at 636 nm and corresponding maximum in the reflection spectra. Dispersion characteristics of barium titanate films with different concentrations of initial sols were studied and an increase in the refractive index with an increase in the concentration of sol was demonstrated. For a sol with a concentration of 60 mg/ml, the refractive index in the spectral range of 390–1600 nm is 1.88–1.81. The prospects of sol–gel technology for the formation of BaTiO3/SiO2 structures for nanophotonics and solar radiation converters are discussed

Gaponenko, N. V.

Kholov, P. A.

Mudryi, A. V.

Raichenok, T. F.

Soma Raju, K. R. C.

Subasri, R.

Sukalin, K. S.

Tikhomirov, S. A.

Belarusian State University of Informatics and Radioelectronics Repository

ISSN 1063-7834,  Physics of the Solid State, 2019, Vol. 61, No. 3, pp. 398–402. © Pleiades Publishing, Ltd., 2019. Original Russian Text © N.V. Gaponenko, P.A. Khokhlov, K.S. Sykalin, T.F. Raichenok, S.A. Tikhomirov, R. Subasri, K.R.C. Soma Raju, A.V. Mydrii, 2019, published in Fizika Tverdogo Tela, 2019, Vol. 61, No. 3, pp. 528–532.     OPTICAL    PROPERTIES   Optical Properties of Multilayer BaTiO3/SiO2 Film Structures Formed by the Sol–Gel Method  N. V. Gaponenkoa, b, *, P. A. Khokhlova, K. S. Sykalina, T. F. Raichenokc, S. A. Tikhomirovc, R. Subasrid, K. R. C. Soma Rajud, and A. V. Mydriie aBelarusian State University of Informatics and Radioelectronics, Minsk, Belarus bNational Research Nuclear University MEPhI, Moscow, Russia cStepanov Institute of Physics, Minsk, Belarus dCentre for Sol-Gel Coatings, International  Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad, Telangana State, India eScientific-Practical Materials Research Centre of the National Academy of Sciences of Belarus,  Minsk, Belarus *e-mail: nik@nano.bsuir.edu.by Received July 4, 2018; revised August 17, 2018  Abstract—Multi-layer film structures BaTiO3/SiO2 with a thickness of ~1 μm containing up to 14 pairs of lay- ers were synthesized by the sol–gel method with sequential heat treatment. It is shown that the synthesized structures are X-ray amorphous. The formation of bands in the transmission and ref lection spectra caused by interference effects is demonstrated. A more regular structure exhibits a photon band gap (opacity band) in the visible range with main minimum at 636 nm and corresponding maximum in the ref lection spectra. Dis- persion characteristics of barium titanate films with different concentrations of initial sols were studied and an increase in the refractive index with an increase in the concentration of sol was demonstrated. For a sol with a concentration of 60 mg/ml, the refractive index in the spectral range of 390–1600 nm is 1.88–1.81. The prospects of sol–gel technology for the formation of BaTiO3/SiO2 structures for nanophotonics and solar radiation converters are discussed.   DOI: 10.1134/S1063783419030120   INTRODUCTION  Interest in development and improvement of the methods for the forming multilayer film structures with periodically changing refractive indices of layers is primarily due to a wide range of applied problems which can be solved using such structures. Multilayer interference coatings, also known as Bragg mirrors or one-dimensional photonic crystals, with a ref lection band localized in different areas of the optical range, are widely used in laser technology  and optical instru- mentation   as laser mirrors, antiref lection coatings, interference  filters,  etc.  Multilayer  coatings,  which ref lect IR and transmit visible radiation, are of consid- erable interest for manufacturing windows for build- ings and vehicles  especially  in countries with high annual average temperature.  Coating components (layers) may contain luminescent lanthanide ions radiating in different spectral regions. The reduction of the ref lection coefficient in a wide spectral ref lec- tion region in combination with intense luminescence of doped ions in the sensitivity range of solar cells is used to increase the efficiency of photovoltaic cells with such coatings [1]. On the other hand, structures such as Bragg mir- ror/microresonator with excited ions/Bragg mirror are of great interest for studying fundamental problems of spontaneous emission of excited light centers in the conditions when the density of photonic states is mod- ified [2]. Technologies of formation of multilayer structures with periodically changing refractive index using vari- ous  dielectric and semiconductor  materials  are the subject of permanent development. There are a num- ber  of methods  including magnetron and electron beam evaporation [3, 4], plasma chemical gas-phase deposition [5], electrochemical formation of porous silicon [6–8] and the sol–gel method [9–12]. Among the various methods of forming both undoped and lanthanide-doped multilayer coatings, the sol–gel method is of particular interest. The method does not require energy-demanding vacuum equipment and, accordingly,  can be applied on large areas at a relatively low cost. The refractive index and the film thickness can be varied by choice of precur- sor, the viscosity and concentration of the sol, deposi- tion rate, regimes of heat treatment, and other factors. An essential condition for forming a multilayer  peri-  398 OPTICAL PROPERTIES OF MULTILAYER BaTiO3/SiO2 399 PHYSICS OF THE SOLID STATE Vol. 61 No. 3 2019    odic structure is that the sol should preserve its prop- erties for a sufficiently  long time, since any changes in the manufacturing process of the structure result in uncontrolled changes  in the film  thickness,  which ultimately leads to a deterioration in the quality of the multilayer structure. The development of sol–gel technologies for the formation of high-quality multi- layer structures is of interest. The components of pho- tonic crystals should differ in refractive indices. Per- ovskites-aluminum-titanium composites, strontium titanate, barium titanate, etc., have relatively high val- ues of refractive indices (about two); however, in con- trast to titanium dioxide [9–11], their application for the synthesis of photonic crystals has not been studied in detail. The values of the refractive indices of thin films depend on the methods of their production, heat treatment modes, and other factors. In this paper we analyze optical transmission and ref lection spectra of multilayer film structures of bar- ium titanate/silicon oxide (BaTiO3/SiO2) formed by the  sol–gel  method  using  centrifugation  and heat treatment.   EXPERIMENTAL  The sol for forming films of kerogel SiO2 included eth- anol (C2H5OH), tetraethylorthosilicate (Si(C2H5O)4), distilled water (H2O), and hydrochloric acid (HCl). To prepare the sol, ethanol and distilled water were mixed,  then  the resulting solution  was  brought to pH =  1 by adding concentrated  hydrochloric acid. After that, tetraethylortosilicate was added to the solu- tion. The  sols prepared  for forming films of kerogel BaTiO3 had a concentration  of 45, 50, and 60 mg/ml. First, a solution containing titanium isopropoxide and acetic acid was prepared,  which  was thoroughly mixed until all components were dissolved. Subsequent addi- tion of barium acetate to the solution resulted in a sta- ble film-forming sol. Barium titanate and silicon oxide films were centri- fuged at 2700 rpm for 30 s. Each layer was dried at a temperature of 200°C for 10 min, then heat treated at a temperature of 450°C for 30 min. A multilayer structure  containing 14 alternating layers of BaTiO3 and SiO2  was formed on a quartz glass substrate using a barium titanate sol with a concentra- tion of 60 mg/ml. Transmission spectra were recorded in the process of sample preparation after the addition of each pair  of BaTiO3/SiO2 layers  and subsequent heat treatment. After  applying 14 layers,  additional heat treatment was carried  out at 600°C with a dura- tion of 30 min; then the morphology of the sample was analyzed by scanning electron microscopy (SEM). Sample No. 2 was prepared using a modified tech- nology of applying the layers. First, one layer of BaTiO3 was applied.  The second BaTiO3 layer was applied  after  drying and heat treatment of the first layer at a tem- perature of 450°C for 30 min. Next, the second BaTiO3 layer was subjected to drying and heat treatment  at a temperature of 450°C for 30 min. After the formation of barium titanate film by this method,  a silicon oxide film was formed in a similar mode. As a result, a five- layer  BaTiO3/SiO2/BaTiO3/SiO2/BaTiO3  structure was formed,  in which each layer of the material con- sisted of two successive layers. For ellipsometric measurements, single-layer bar- ium titanate films were formed on polished monocrystalline silicon substrates with a size of approximately 15 × 15 mm, using sols with a concen- tration of 45, 50, and 60 mg/ml. Thickness of films and refractive index were mea- sured using a M-2000V ellipsometer (J. A. Woollam, Inc., United States) in the range of 350–1700 nm. The data were processed using the Cauchy model. Trans- mission spectra were measured on a CARY-500  Scan UV-VIS-NIR  spectrophotometer (Varian, United States & Australia). X-ray diffraction studies were car- ried out on a DRON-3 automated X-ray diffractome- ter using monochromatic CuKα radiation. Morphol- ogy of the films was analyzed using a Hitachi S-4800 scanning electron microscope. Optical  ref lection spectra  were  recorded on  a MDR-23Y    monochromator   equipped    with    a 1200 lines/mm diffraction grating. The spectral reso- lution was 0.5 Å. The light source was a tungsten lamp with a ribbon filament with a power  up to 170W. A R9110 photoelectron multiplier (Hamamatsu, Japan) was used as a detector of optical signals. The spectra were corrected taking into account the spectral depen- dence of the radiation detector sensitivity, the spectral characteristics of the light source and the monochro- mator.   RESULTS AND DISCUSSION  Figure 1 shows dispersion graphs obtained for films formed of three barium titanate sols on monocrystal- line silicon. According to ellipsometry data, the thick- ness of barium titanate films ranges from 48 to 63 nm and increases monotonically with increasing sol con- centration (Table 1). The refractive index also depends on the concentration of sol and for the concentration of 60 mg/ml varies between 1.8–1.9.   Table 1. Thickness values of single-layer BaTiO3 films obtained from sols of different  concentrations  measured at three points of each sample Sol concentration, mg/ml Layer thickness, nm  45 48; 48; 49 50 56; 57; 58 60 61; 61; 63 400 GAPONENKO et al.  n n n    2.0 (a)   1.8   1.6   1.4    500 1000    1500 2000  1  m     2.0 Wavelength, nm  (b)  Fig. 2. SEM image of a structure containing 14 layers of BaTiO3/SiO2 after  heat treatment  at a temperature  of 600°C.      1.8     1.6          500 1000          1500 2000 An increase in the number of layers leads to forma- tion of an opacity band, the intensity  of which increases with the number of layers. For a final sample with 14 pairs of layers, the minimum of the band is at 693 nm (Fig. 3a) and there is a broad band in the ref lection spectrum which corresponds to the opacity band (Fig. 3b). In our opinion, these bands are due to the interference of radiation on a multilayer  structure. The mismatch between the transmittance minimum  1.90   1.88   1.86   1.84   1.82 Wavelength, nm   (c) and ref lectance  maximum in the  spectra (Fig. 3c) points to the insufficient quality of the structure, as evidenced by the SEM image (see Fig. 2). An additional increase in the heat treatment tem- perature from 450°C to 600°C leads to a significant shift of the minimum from 693 to 652 nm and a reduc- tion in transmittance in a broad range. As it turned out, the increase in the heat treatment temperature results in cracking of the upper layer, which reduces the transmission due to diffuse scattering. According to the modeling  carried out by the matrix element method, the observed shift in the spectrum can be related to the change in the optical thickness of the films as a result  of the increase in the heat treatment 500 1000 1500 2000 temperature. Wavelength, nm   Fig. 1. Wavelength dependences of the refractive index of BaTiO3 films synthesized from sols with a concentration of 45–60 mg/ml:  (a) 45 mg/ml,  (b) 50 mg/ml,  and (c) 60 mg/ml.   Figure 2 shows a SEM image of the structure con- taining 14 layers. The irregularity of the structure can be caused by changes in the viscosity of the sols during the  process of preparing the structure; this question requires additional research. The structure  is  X-ray amorphous both after heat treatment at 450°C and at 600°C.  Figure 4 shows  a REM image of sample No. 2. After  heat treatment  at 450°C, this sample  is also X-ray amorphous  and, in comparison  with sample No. 1 containing 14 layers, exhibits a more clear opac- ity band with a minimum at 636 nm and a correspond- ing band in the ref lection spectrum (Fig. 5). It should be noted that the positions of extrema in the ref lec- tance  and transmittance   spectra of sample  No.  2 match. The observed monotonic increase in the thick- ness of the layers of both materials from the interface with the substrate to the sample surface (Fig. 4a) can be caused both by an increase in the viscosity of the sols and a smaller number of heat treatment operations of the upper layers compared to the lower ones. Con- sideration of these factors can lead to higher reproduc-  PHYSICS OF THE SOLID STATE Vol. 61 No. 3 2019 OPTICAL  PROPERTIES OF MULTILAYER  BaTiO3/SiO2 401 PHYSICS OF THE SOLID STATE Vol. 61 No. 3 2019   Transmission, % Transmission, % Reflection, %  100 (a)   80   60 Quartz 3 layers 40 5 layers 7 layers 9 layers 20 13 layers 14 layers  0 500 1000 1500 2000 2500 3000 Wavelength, nm        (a) 1   m 100 (b) 100  80 80   60 60   40 40   20 20   0 0   (b) 500 nm   Fig. 4. SEM images of a five-layer BaTiO3/SiO2  structure (sample No. 2): (a) heat treatment of each layer at a tem- 300 400 500 600 700 800 900 Wavelength, nm perature  of 450°C, (b) additional  heat treatment  of the sample at a temperature of 800°C for 30min.    Fig. 3. Optical spectra of structures containing 3 to 14 lay- ers of BaTiO3/SiO2  on a quartz substrate, after heat treat- ment  of each  layer at 450°C: (a) transmittance  at 3 to 14 layers, (b) transmittance and ref lection at 14 layers.   ibility of film thickness. Additional heat treatment  of the formed structure at 800°C reduces the deviation in the thickness of BaTiO3/SiO2 layers (Fig. 4b).  The results obtained show that after further improvements, the sol–gel technology of forming the multilayer BaTiO3/SiO2 structures can be used to obtain single dimensional crystals and microresoan- tors. According to the results of the simulation carried out by the matrix element method, optimization of the synthesis of multilayer BaTiO3/SiO2  structure for achieving the thickness of BaTiO3  and SiO2  layers of about 84 and 110 nm, respectively (Fig. 6a) can be rec- ommended for forming a photon band gap with an extremum at 636 nm, similar to the structure shown in Fig. 4, sample No. 2. The simulation was performed for films with refractive indices 1.9 (as in amorphous barium titanate film) and 1.45 for silicon oxide. According to the simulation, if the film thicknesses are maintained as constant after reaching values of about 142 nm for BaTiO3  and 283 nm for SiO2 (the average values for the layers of the structure shown in Fig. 4a, sample No. 2), the photon band gap will have an extre- mum at 1320 nm and 1332 nm,  respectively for the structures of 5 and 14 alternating layers of BaTiO3 and SiO2.  Thus, for the multilayer structure formed by the sol–gel method consisting of 14 BaTiO3/SiO2 layers, the formation of bands in the transmission and ref lec- tion spectra due to interference  effects was demon- strated.  A  five-layer  BaTiO3/SiO2   structure  on  a quartz glass substrate was formed by an improved sol– gel method. The structure is characterized spectrally coinciding extrema in the transmission and ref lection spectra in the optical range. Additional heating of the BaTiO3/SiO2 structure at a temperature of 800°C reduces the deviation in the thickness of both BaTiO3 and SiO2 layers. In the future, it will be of interest to study the values of refractive indices of barium titanate films formed at high processing temperatures. Taking into account the intensive luminescence of lantha- nides in silicon oxide [13] and barium titanate [14] films formed by the sol–gel method and the low cost 402 GAPONENKO et al.  Transmission, % Transmission, % Transmission, % Reflection, % Reflection, % Reflection, %  80                                                                             80   60                                                                             60   40                                                                             40   20                                                                             20   0 0 300 400 500 600 700 800 900 Wavelength, nm   Fig. 5. Optical transmission and ref lection spectra of sam- ple No.  2 (BaTiO3/SiO2/BaTiO3/SiO2/BaTiO3  films on quartz).    (a) of this method, the aim of further development of this technology can be the creation of lanthanide-doped luminescent BaTiO3/SiO2 structures and microreso- nators, as well as the development of large-area radia- tion converters luminesing in the visible and IR ranges for solar cells.   ACKNOWLEDGMENTS  The work was supported by a Belarussian–Indian grant from the State Committee on Science and Tech- nology of the Republic of Belarus, No.  F17INDG- 001.   REFERENCES  1.  A. Shalav,  B.  S.  Richards,  and  M.  A. Green,  Sol. Energy Mater. Sol. Cells 1, 829 (2007).  2.  S. V. Gaponenko, Introduction to Nanophotonics (Cam- bridge Univ. Press, Cambridge, UK, 2010). 100  80  60  40  20  0     100  80  60  40  20  0     T - 5 layers R - 5 layers T - 14 layers R - 14 layers      300 400 500 600 700 800 900 Wavelength, nm (b)     T - 5 layers R - 5 layers T - 14 layers R - 14 layers 100  80  60  40  20  0     100  80  60  40  20  0 3.  E.  F.  Schubert,  A. M.  Vredenberg, N.  E.  J.  Hunt, Y. H. Wong, P. C. Becker, J. M. Poate, D. C. Jacobson, L. C. Feldman, and G. J. Zydzik, Appl. Phys. Lett. 61, 1381 (1992).  4.  A. M.  Vredenberg, N.  E.  J.  Hunt,  E.  F.  Schubert, D. C. Jacobson, J. M. Poate, and G. J. Zydzik, Phys. Rev. Lett. 71, 517 (1993).  5.  V.  G.   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(solid line) Five-layer BaTiO3/SiO2/BaTiO3  structure,  (dotted  line) 14-layer BaTiO3/SiO2  structure (7 pairs of layers): (a) BaTiO3 with a thickness of 83.8 nm and SiO2 with a thick- ness of 109.8 nm, (b) BaTiO3 with a thickness of 142nm and SiO2 with a thickness of 283 nm.  12.  G. J. Hu, J. Chen, D. L. An, J. H. Chu, and N. Dai, Appl. Phys. Lett. 86, 162905 (2005).  13.  N.  V. Gaponenko, V. M.  Parkun,  O. S. Katernoga, V. E. Borisenko,   A.  V.  Mudryi,   E.  A.  Stepanova, A. I. Rat’ko, M. Cavanagh, B. O’Kelly, and J. F. McGilp, Thin Solid Films 297, 202 (1997).  14. J. Li and M. Kuwabara, Sci. Technol. Adv. Mater. 4, 143 (2003).  Translated by V. Alekseev  PHYSICS  OF THE SOLID  STATE Vol. 61 No. 3 2019    SPELL: OK 

Optical properties of multilayer BaTiO3/SiO2 film structures formed by the sol–gel method

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Optical properties of multilayer BaTiO3/SiO2 film structures formed by the sol–gel method

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