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SOL-GEL SYNTHESIS AND ROPERTIES OF HETEROSTRUCTURES CONTAINING NANOSTRUCTURED POROUS LAYER

M. V. Rudenko1, N. V. Gaponenko1, E. B. Chubenko1, V. E. Borisenko1, V. G. Litvinov2,

A. V. Ermachikhin2, N. V. Mukhin3, 4, E. V. Monaico5 __________________________________________________________________________________________

1) Belarusian State University of Informatics and Radioelectronics, P. Browka 6, 220013 Minsk, Belarus 2) Ryazan State Radio Engineering University, Gagarin Str. 59, 390005 Ryazan, Russia

3) Saint Petersburg Electrotechnical University "LETI", Professora Popova str. 5, 197376 Saint Petersburg, Russia,

4) Otto-von-Guericke University, Universitaetsplatz 2, 39106 Magdeburg, Germany 5) National Center for Materials Study and Testing, Technical University of Moldova,

Bv. Stefan cel Mare 168, Chisinau MD-2004, Republic of Moldova Corresponding author: M.V. Rudenko (rudmash@gmail.com)

Porous nanostructured films with total thickness of about 630 nm and grain size up to

30 nm were fabricated by sol-gel route on silicon substrate and silicon substrate with TiOx/Pt layers. Raman spectroscopy analysis of the porous film confirms the presence of TiO2, SrO and SrTiO3 phases. Sol-gel synthesized heterostructures containing porous nanostructured strontium titanate (ST) films formed on dense ST sublayers are prospective for components of photonic crystals with a reduced refractive index and absence of chemi-cal interaction between layers.

Key words: porous films; strontium titanate; nanostructured materials; sol-gel method. INTRODUCTION

Nanostructured materials are of great interest. Particular attention is paid to nanostruc-tured perovskites strontium titanate and barium titanate. The study of the luminescent, photocatalytic and dielectric properties of these materials [1], nanoscale effects and the interaction of nanoperovskites with light represent a wide field for research. Strontium ti-tanate (ST), which has a wide-bandgap perovskite structure, is a promising phosphor and matrix for introducing rare earth elements due to its radiation resistance. The porous ST:Eu xerogel films are prospective for optoelectronic devices through multiple scattering in a porous structure, which contributes to obtaining the optimum luminescent properties [2].

Nanostructured ST is a promising material for water purification from heavy metals and photocatalysis [1]. Grain size and porosity affect the electrophysical parameters of perovskites, for example, porous barium titanate has a lower dielectric constant, and the Curie temperature increases up to 350 °С compared to bulk material [3]. Hysteresis of ca-pacitance–voltage (C–V) characteristic of bulk monocrystralline ST with Ni and Au elec-trodes was investigated before and after irradiation in [4]. Thus, the study of nanostructured porous perovskites is of particular interest for obtaining new properties of materials.

In this work, we investigated structural properties of porous nanostructured films formed on monocrystalline silicon and Pt/TiOx/Si structure and heterostructures containing porous and dense ST layers formed on monocrystalline by the sol-gel route. We describe our synthesis of porous ST using sol-gel method without any templates.

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MATERIALS AND METHODS Two types of sols were synthesized accordingly. The first type of sol was used to fabri-

cate porous strontium titanate films. Two solutions were prepared for the synthesis. For the preparation of the first solution, titanium isopropoxide was dissolved in a mixture of ethyl-ene glycol monomethyl ether and a stabilizer to prevent gelation initiated by titanium iso-propoxide. As the stabilizer, acetylacetone or nitric acid was used. To prepare the second solution, strontium nitrate was dissolved in distilled water, followed by the addition of eth-ylene glycol monomethyl ether. Finally, both solutions were mixed to obtain the first type of sol.

The second type of sol was used to fabricate the dense strontium titanate layers. The sol was prepared from titanium isopropoxide Ti[(CH3)2CHO]4 and strontium acetate (Sr(CH3COO)2 ·H2O). Acetic acid was used as the solvents. First, appropriate amount of titanium isopropoxide was dissolved in acetic acid. Then strontium acetate was added to the solution and stirred at room temperature. In order to stabilize the solution to prevent fast gelation, acetyl acetone was added as a stabilizer.

The sols were deposited by spinning at a rate of 2700 rpm for 30 s on silicon substrate and silicon substrate with TiOx/Pt layers. The samples were then heat treated at the tem-perature of 200 °C for 10 min after each deposition, followed by a final heat treatment at 800 °C for 40 min.

HITACHI S4800 scanning electron microscope was used to obtain SEM images of the heterostructures.

Raman spectra were measured on 3D Scanning Laser Confocal Raman Microscope Confotec NR500 using 473 nm laser for excitation. Measurements were performed at room temperature.

RESULTS AND DISCUSSION Figure 1 shows Si/TiOx/Pt/SrTiO3/Ni and Si/SrTiO3/Ni capacitor structures with nickel

electrodes. The structures contain ten-layer porous films with total thickness about 630 nm. The films clearly reveal high porosity and highly developed surface (Fig. 1) with an aver-age grain size of about 30 nm.

Figure 1. – SEM images of Si/TiOx/Pt/SrTiO3/Ni (a) and Si/SrTiO3/Ni (b) heterostructures

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The heterostructures (Fig. 1) exhibited hysteresis on the forward and reverse curves of the current-voltage and capacitance-voltage characteristics in the frequency range from 2 kHz to 2 MHz, which is typical for materials having a memory effect.

Raman spectra of the heterostructure Si/TiOx/Pt/SrTiO3 containing porous film show sev-eral bands corresponding to TiO2 and SrTiO3 phases (see Table). Three bands at 395, 515 and 635 cm−1 correspond to TiO2 anatase crystalline phase. Band at 395 cm−1 corresponds to B1g vibration mode, band at 635 cm−1 corresponds to Eg and band at 515 cm−1 is a doublet of A1g and B1g modes [5]. Bands at 261 and 797 cm−1 are due to the presence of cubic SrTiO3 phase in the film. Band at 261 cm−1 is associated with first-order transversal TO3 phonon mode and band at 797 cm−1 is a longitudinal LO4 phonon mode [5]. The activation of TO3 and LO4 bands indicates that structural distortions extended over a distance of the order of the phonon wavelength of the obtained material [5]. Other SrTiO3 bands may be veiled by strong bands of TiO2. Single band at 865 cm−1 can be associated with SrO 2LO mode [6]. Two lines lo-cated at 1443 and 1625 cm−1 are related to carbon presented in the structure [7].

Table

Raman spectra bands

Peak, cm−1 Material Mode Reference

261 SrTiO3 (cubic) TO3 (O–Sr–O bending) [5] 395 TiO2 (Anatase) B1g [5] 515 TiO2 (Anatase) A1g + B1g [5] 635 TiO2 (Anatase) Eg [5] 797 SrTiO3 (cubic) LO4 (Ti–O stretching) [5] 865 SrO 2LO [6] 1443 Carbon Disorder [7] 1625 Carbon E2g [7]

Figure 2 shows SEM images of heterostructures containing porous nanostructured ST films formed on dense ST sublayers. The thicknesses of ST sublayers formed on monocrys-talline silicon after one and two spinning of the sol are of about 100 and 200 nm, respec-tively. The thickness of the porous nanostructured ST layers are of about 350 nm.

Porous nanostructured films are expected to have reduced refractive index due to high porosity. When the porosity of ST increases, that leads to the band gap and refractive index change from 3.96 to 4.2 eV and from 2.33 to 1.87 eV, respectively [8]. Heterostructures based on dense and porous ST with high and low refractive index are free from chemical interaction between layers. Three-layer heterostructure containing in the middle porous layer with low refractive index can be used as a stack with low near infrared transmittance, as was shown recently for BaTiO3/MgF2/BaTiO3 xerogel structures [9].

CONCLUSIONS We described the sol-gel method for fabrication of containing porous ST layer het-

erostructures. The method does not require any organic templates in the sol for making the material porous. Raman spectroscopy analyses confirm the formation of TiO2, SrO and SrTiO3 phases in porous nanostructured film. The synthesized porous films are prospective

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for sorption-type structures (sensors, catalysts) and components of photonic crystals with a reduced refractive index [9].

a

b

c

d

Figure 2. – SEM images of heterostructures containing porous ST layers formed on dense ST sublayers formed on monocrystalline silicon after one (a, b) and two (c, d) spinning on

ACKNOWLEDGMENT The authors acknowledge financial support from the State Committee on Science and

Technology of the Republic of Belarus and Ministry of Education, Culture and Research of Moldova under the Grants № T19MLDG-005 and № 19.80013.50.07.02A/BL.

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TECHNOLOGIES OF ELECTROCHEMICAL DEPOSITION OF METALS ON A SEMICONDUCTOR WAFER OF VARIOUS MEMS MICROFORMS

Y. V. Timoshkov1, A. V. Khanko2, V. I. Kurmashev3 ____________________________________________________________________________________________

1) Belarusian State University of Informatics and Radioelectronics P. Brovki St. 6, 220013 Minsk, Belarus, e-mail: timoshkov@bsuir.by

2) Belarusian State Academy of Communications, P. Brovki St., 14, 220013 Minsk, Belarus, e-mail: andreaaaASD94@gmail.com

3) Belarusian State Academy of Communications, P. Brovki St., 14, 220013 Minsk, Belarus, e-mail: kurmashev.vi@mail.ru

The technology of formation of nanostructured materials with high physical and me-

chanical properties is considered. It allows significantly improve the quality and reliability of the moving elements of systems. The results of the choice of the optimal bath for elec-trochemical deposition of copper and cobalt-phosphorus alloy to obtain metal structures on a substrate of high quality and strength are presented. For the formation of components of complex microsystems, filling by the method of galvanic codeposition is proposed. The technology of deposition of coatings from metals, alloys, as well as composite coatings on a surface with a complex nanometer configuration has been developed. Samples were ob-tained and showed absence of typical defects such as voids, seams, as well as increased coarse grained structure.

Key words: microelectromechanical system; electrochemical deposition; LIGA-like technology; carbon nanotube; galvanic deposition.

INTRODUCTION At a high rate of development in the creation of modern microelectronics and optoelec-

tronics, elements and systems are in demand, which are increasingly small and complex in configuration. These include: micro-optical components (microlens array), microelectro-mechanical systems (microcompressors, accelerometers, microactivation devices), micro-channels, microsystem connecting elements and others, for which a structure is obtained in geometry that changes in three directions. Their formation is a combination of various technical approaches based on the design and creation of polymerization microforms, the minimum dimensions of the elements used, on the basis of the creation of polymer micro-forms and their subsequent filling with functional materials.