Structural Dependent Eu3+ Luminescence, Photoelectric and Hysteresis Eﬀects in Porous Strontium Titanate

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Article Structural Dependent Eu3+ Luminescence, Photoelectric and Hysteresis Eﬀects in Porous Strontium Titanate

Maryia Rudenko 1,*, Nikolai Gaponenko 1,2, Vladimir Litvinov 3, Alexander Ermachikhin 3 , Eugene Chubenko 1 , Victor Borisenko 1,2, Nikolay Mukhin 4,5 , Yuriy Radyush 6, Andrey Tumarkin 4 and Alexander Gagarin 4 1 Department of Micro- and Nanoelectronics, Belarusian State University of Informatics and Radioelectronics, P. Browka Str. 6, 220013 Minsk, Belarus; [email protected] (N.G.); [email protected] (E.C.); [email protected] (V.B.) 2 Department of Condensed Matter Physics, National Research Nuclear University MEPhI, Kashirskoe Shosse 31, 115409 Moscow, Russia 3 Department of Micro and Nano Electronics, Ryazan State Radio Engineering University, Gagarin Str. 59, 390005 Ryazan, Russia; [email protected] (V.L.); [email protected] (A.E.) 4 Faculty of Electronics, Saint Petersburg Electrotechnical University “LETI”, Professor Popov Str. 5, 197376 Saint Petersburg, Russia; [email protected] (N.M.); [email protected] (A.T.); [email protected] (A.G.) 5 Department of Engineering, University of Applied Sciences Brandenburg, Magdeburger Str. 50, 14770 Brandenburg an der Havel, Germany 6 Scientiﬁc-Practical Materials Research Centre of NASB, P. Browka Str. 19, 220072 Minsk, Belarus; [email protected] * Correspondence: [email protected]

Received: 18 November 2020; Accepted: 14 December 2020; Published: 17 December 2020

3+ Abstract: Eu doped porous nanostructured SrTiO3 films and powder fabricated by sol-gel route without using any precursor template are characterized by different morphology and phase composition. The films and the power show red and yellow luminescence with the most intensive photoluminescence (PL) bands at 612 nm and 588 nm, respectively. Raman, secondary ion mass spectrometry (SIMS), and X-ray diffraction (XRD) analysis of undoped nanostructured porous SrTiO3 films showed the presence of TiO2, SrO, and SrTiO3 phases and their components. The undoped porous SrTiO3 films are photosensitive and demonstrate resistive switching. The capacitance-voltage hysteresis loops with the width of about 6 V in the frequency range of 2 kHz—2 MHz were observed.

Keywords: europium luminescence; porous ﬁlm; strontium titanate; resistive switching hysteresis; photocurrent

1. Introduction Materials containing Eu3+ ions are promising as red phosphors in optoelectronic devices, e.g., LEDs and lasers, due to their excellent photoluminescence properties [1–3]. Optical radiation in the europium emission band at 612 nm is perceived by the human eye as red light, which makes these materials particularly important. The luminescence spectrum can differ in different crystal matrices. Material composition, crystal structure, and presence of defects have a significant influence. Oxide compounds with a perovskite structure, such as strontium titanate (SrTiO3), are widely used in nonlinear optics [4], electro-optical modulators [5], photocatalysis [6,7], thin-film capacitors [8,9], and memory devices [10,11]. It is a good host matrix for Eu3+ due to its radiation resistance and thermal stability.

Materials 2020, 13, 5767; doi:10.3390/ma13245767 www.mdpi.com/journal/materials Materials 2020, 13, 5767 2 of 11

Properties of SrTiO3 depend not only on the chemical composition, but also on the structure and morphology [12,13]. As a result of the modification of titanium and layered metal oxide systems, fabricated oxide coatings have different compositions, surface morphology, and properties that are determined by the structural parameters, in particular, nanosized grains [14,15], which is also right for more complex oxide systems. A decrease in the grain size of SrTiO3 at the nanoscale leads to distinctive properties as compared to the bulk material [16,17]. In recent years, several attempts were made to synthesize porous perovskites SrTiO3 and BaTiO3 [18,19]. Nanostructured perovskites have great potential for applications. Nanostructured SrTiO3 is a promising material for water purification from heavy metals [19] and photocatalysis [20]. Grain size and porosity affect the electrophysical parameters of perovskites. For example, porous barium titanate has a lower dielectric constant [21], and the Curie temperature increased up to 350 ◦C as compared to the bulk material [18]. Hysteresis of capacitance-voltage (C-V) characteristics of bulk monocrystralline SrTiO3 with Ni and Au electrodes was investigated before and after irradiation in [22,23], where C-V hysteresis was considered from the point of the drift of nonequilibrium charge carriers in the crystal. The hysteresis effects were observed not only in dielectric properties but also in magnetization loops [24,25]. Thus, the study of nanostructured porous perovskites is of particular interest for obtaining new properties of materials and creating novel structures based on them. The synthesis of such nanostructured composites is possible by chemical methods, which include the sol-gel technology. This technology has a low cost, allows variation of the grain size, phase composition, concentration of dopants. The use of titanium alkoxides is the most promising for the synthesis of nanostructured SrTiO3, since they easily hydrolyze, forming hydrated oxides. By varying the temperature of dehydration, one can control the dispersion and morphology, the phase composition and the physical properties of the resulting xerogels. The use of metal alkoxides in the synthesis of multicomponent oxides ensures a high chemical homogeneity of the product, significantly reducing the temperature of oxide phases formation [26,27]. One of the synthesis techniques includes using templates, which control formation periodical porous SrTiO3 structure and repeatable combs form [28]. Another approach is based on using alcoxides and influence of water on the morphology of xerogels [18,29–31]. This approach allows for the fabrication of porous perovskites with grains larger than 100 nm. Nevertheless, the formation of porous perovskites with much smaller grains is a problem to be solved. In this paper we present structural, luminescent and dielectric properties of porous undoped and Eu-doped SrTiO3 films with grains as small as 30 nm fabricated on monocrystalline silicon substrates by the sol-gel technique without any templates and compare them with those of identically synthesized bulk materials.

2. Materials and Methods

Porous SrTiO3 films doped with Eu, undoped ones and SrTiO3:Eu powder were synthesized using water containing sols. The sols were synthesized in the following way. First titanium isopropoxide was dissolved in a mixture of ethylene glycol monomethyl ether and nitric acid to prevent gelation initiated by titanium isopropoxide. Then, strontium nitrate was dissolved in the distilled water, followed by the addition of ethylene glycol monomethyl ether. Finally, both solutions were mixed to obtain the sols. To fabricate europium doped sols europium nitrate was added. The sols were deposited by spinning at the rate of 2700 rpm for 30 s on monocrystalline (111) silicon wafers. The samples were then dried at 200 ◦C for 10 min and next film was identically deposited and dried. The deposition was repeated 5–10 times in order to fabricate the film of an appropriate thickness. Final calcination was performed at 800 ◦C for 40 min in air. Porous SrTiO3:Eux (x = 0.053) films containing 5 layers and undoped SrTiO3 films containing 10 layers were fabricated on silicon substrates. Powder materials were synthesized by the same heat processing of the precursors in a ceramic crucible. Materials 2020, 13, 5767 3 of 11 Materials 2020, 13, x FOR PEER REVIEW 3 of 11

Silicon substrate covered by TiOx/Pt layers with Pt playing a role of an isolated from the substrate Silicon substrate covered by TiOx/Pt layers with Pt playing a role of an isolated from the substrate electric contact was used for fabrication of the heterostructure Si/TiOx/Pt/SrTiO3/Ni for electrical electric contact was used for fabrication of the heterostructure Si/TiOx/Pt/SrTiO3/Ni for electrical measurements.measurements. The The top top round round nickel nickel electrode electrode of of ab aboutout 330 330 µmµm in in diameter diameter was was formed formed by by ion-beam ion-beam evaporationevaporation technique technique [9]. [9]. TheThe morphology morphology of of the the experimental experimental samples samples was was examined examined with with scanning scanning electron electron microscope microscope S-4800S-4800 (Hitachi, (Hitachi, Tokyo, Tokyo, Japan). The The photoluminescenc photoluminescencee spectra spectra were were recorded recorded at at room temperature usingusing a a laser spectroscopic complex based based on a Solar TII MS 7504i spectrograph-monochromatorspectrograph-monochromator (Solar(Solar TII,TII, Minsk, Minsk, Belarus) Belarus) in which in which a 1 kW xenona 1 kW lamp xenon was used lamp for excitation.was used The for photoluminescence excitation. The photoluminescencelight was detected bylight a digital was detected camera by (Proscan, a digital Minsk, camera Belarus) (Proscan, with Minsk, a cooled Belarus) silicon with CCD a matrix.cooled siliconTo isolate CCD monochromatic matrix. To isolate lines frommonochromatic a wide spectrum lines from of the a lamp, wide wespectrum used a Solarof the TII lamp, DM 160we used double a Solarmonochromator TII DM 160 (Solar double TII, monochromator Minsk, Belarus). The(Solar photoluminescence TII, Minsk, Belarus). was excitedThe photoluminescence by the monochromatic was excitedlight with by the wavelengthmonochromatic of 345 light nm. with the wavelength of 345 nm. XRDXRD (BOUREVESTNIK JSC, SaintSaint Petersburg,Petersburg, Russia)Russia) studiesstudies of of the the ﬁlms films were were carried carried out out with with a aDRON-3 DRON-3 di diffractometerﬀractometer using using monochromatized monochromatized CuK CuKα αradiation. radiation. RamanRaman spectra spectra were were measured with 3D Scanning Laser Confocal Raman Microscope Confotec NR500NR500 (SOL(SOL Instruments, Instruments, Minsk, Minsk, Belarus) Belarus) using 473using nm 473 laser nm radiation laser forradiation excitation. for The excitation. measurements The measurementswere performed were at room performed temperature. at room temperature. TheThe depth depth distributions distributions of of the the elements elements were were in investigatedvestigated by by secondary-ion secondary-ion mass mass spectrometry spectrometry (SIMS)(SIMS) (TOF.SIMS (TOF.SIMS 5, IONTOF, Munster,Munster, Germany).Germany). Secondary Secondary positive positive ions ions were were produced produced by by the the + bombardmentbombardment ofof thethe samples samples with with 30 keV30 keV Bi+ ionsBi (1ions pA). (1 InpA). situ In etching situ ofetching the samples of the was samples performed was + performedby sputtering by sputtering with a focused with 2 a keV focused Cs+ ion2 keV beam Cs (100ion beam nA) raster (100 nA) scanned raster across scanned a sample across surfacea sample of 2 surface150 µm 2of. 150 µm . Capacitance-voltageCapacitance-voltage (C-V) (C-V) and and cu current-voltagerrent-voltage (I-V) (I-V) characteristics characteristics were were recorded recorded at at room temperaturetemperature using using an an Agilent Agilent E4980A E4980A Precision Precision LCR LCR meter meter (Agilent (Agilent Technologies, Technologies, Bayan BayanLepas, Lepas,Pulau Pinang,Pulau Pinang, Malaysia) Malaysia) integrated integrated with Keithley with Keithley 6517B (K 6517Beithley (Keithley Instruments, Instruments, Solon, Ohio Solon, USA) Ohio voltage USA) sourcevoltage and source digital and V7-23 digital voltmeter, V7-23 voltmeter,V7-27A ammete V7-27Ar and ammeter stabilized and TEC-23 stabilized power TEC-23 supply, power respectively. supply, Therespectively. C-V measurements The C-V were measurements performed werein the performed frequency range in the of frequency 2 kHz–2 MHz. range All of 2measurements kHz–2 MHz. wereAll measurements done in automatic were mode, done implemented in automatic usin mode,g the implemented LabVIEW engineering using the LabVIEWgraphic programming engineering environmentgraphic programming [32]. environment [32].

3.3. Results Results and and Discussion Discussion

3.1.3.1. Porous Porous Films Films and and Powder Powder Containing Containing Strontium Strontium Titanate and Europium Ions 3+ TheThe SEM images of the five-layerfive-layer SrTiO3:Eu:Eu3+ filmfilm with with the the thickness thickness of of about about 500 500 nm nm and identicallyidentically synthesizedsynthesized powderpowder are are given given in in Figure Figure1. The 1. The films films clearly clearly reveal reveal high porosityhigh porosity and highly and highlydeveloped developed surface. surface. Grains Grains in the powderin the powder are larger are thanlarger in than the film.in the film.

(a) (b) (c)

Figure 1. SEM images of the five-layer SrTiO3:Eu33++ film (a,b) and SrTiO3:Eu3+3+ powder (c). Figure 1. SEM images of the ﬁve-layer SrTiO3:Eu ﬁlm (a,b) and SrTiO3:Eu powder (c).

PLPL spectra spectra of of the the films ﬁlms presented in in Figure 22aa demonstratedemonstrate thethe bandsbands atat 607,607, 612612 andand 619619 nmnm 5 7 7 correspondingcorresponding to electrical dipole transition DD0 → FF2. . 0 → 2 Due to the high solubility of europium ions and the ability to deploy in defective areas of the crystal, the incorporation of these ions can contribute to the phase formation in the material [33,34].

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The intense PL band corresponding to the 5D0 → 7F2 electrical dipole transitions of Eu3+ indicates that europium occupies the position in the SrTiO3 crystallites that does not coincide with the symmetry center, most likely in defect regions [35]. PL spectra of the synthesized powder contain the bands at 588, 599, 607, 611, 626, 695, 700, 720 and 726 nm corresponding to the 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F4 transitions (Figure 2a). In this case the most intensive PL band is observed for magnetic dipole transition 5D0 → 7F1 at 588 nm. It indicates that Eu3+ ions occupy the positions with the symmetry center [35]. The XRD analysis of the powder presented in Figure 2b shows the diffraction peaks typical for SrTiO3 (PDF 01-089-4934) and Sr2TiO4 (PDF 00-039-1471). There are no signals belonging to the europium oxide. In addition to the identified peaks, there are ones that can be attributed to unreacted initial oxides and related solid solutions. The observed formation of Sr2TiO4 phase can be a result of Eu substitution by Sr leading to the enrichment of the powder with Sr. XRD pattern of the five-layer SrTiO3:Eu3+ film contains the intense peak associated with the Materials 2020silicon, 13, 5767 substrate (Figure 2b). Apparently, the SrTiO3:Eu3+ film has a diffuse line in the region of 32.2 4 of 11 deg (influx to the left of the first sharp peak).

(a) (b)

Figure 2. PL spectra of the SrTiO3:Eu+ 3+ powder and five-layer SrTiO3:Eu3+ film3 +(a) and diffraction Figure 2. PL spectra of the SrTiO3:Eu powder and five-layer SrTiO3:Eu film (a) and diffraction 3+ patterns of the SrTiO3+ 3:Eu powder and the SrTiO3 film (b). patterns of the SrTiO3:Eu powder and the SrTiO3 film (b). The powder synthesized from the same sol as the porous film has much larger grains as Due tocompared the high with solubility the porous offilm europium and is characterized ions and by the obvious ability phase to deploy formation. in defectiveAs a result, the areas of the crystal, thePL incorporation spectra of the powder of these and ionsthe film can differ. contribute The presence to the of defects phase and formation disordering in of the the material crystal [33,34]. structure certainly affects the luminescence5 spectr7 um of europium. The most intensive peak3 +at 588 The intense PL band corresponding to the D0 F2 electrical dipole transitions of Eu indicates that nm corresponding to the magnetic dipole transition→ 5D0 → 7F1 confirms that Eu3+ occupies the position europiumwith occupies the symmetry the position center to in be the consistent SrTiO with3 crystallites the XRD data. that The does typical not doping coincide concentration with the for symmetry center, mostlanthanides likely in in defect solids is regions about 1 [atomic35]. %. In this case, it is not mandatory that trivalent lanthanide PL spectraion substitute of the only synthesized trivalent ion powder in host matrix, contain and the there bands are no at other 588, local 599, states 607, for 611, lanthanides. 626, 695, 700, 720 Luminescence of trivalent lanthanide5 7 ions 5with typical7 transitions5 7 between the corresponding terms and 726 nm corresponding to the D0 F1, D0 F2, D0 F4 transitions (Figure2a). In this case was observed in inorganic perovskites:→ Tb3+ in YAlO→ 3 [36], Er3+→ in LiNbO3 corresponds to a [37], Eu3+ the most intensive PL band is observed for magnetic dipole transition 5D 7F at 588 nm. It indicates in BaTiO3 [38], as well as Eu3+ in sol-gel derived amorphous yttrium alumina0 1composites [39]. 3+ → that Eu ionsEuropium occupy ion theis the positions dopant withthat can the exhibit symmetry divalent center or trivalent [35]. valence state [40,41]. The The XRDluminescence analysis spectra of the correspond powder to presentedPL bands of Eu in3+ Figure, as the PL2b spectra shows of theEu2+ diliesff inraction the region peaks of typical about 380–500 nm [40]. In the case when Eu3+ occupies the position of strontium while remaining for SrTiO3 (PDF 01-089-4934) and Sr2TiO4 (PDF 00-039-1471). There are no signals belonging to the trivalent, the charge is compensated by the field of local environment, for example, titanium ions [42]. europium oxide.Replacement In addition of divalent to ions the with identified trivalent peaks, leads to there distortions are ones of the that crystal can lattice be attributed of the structure to unreacted initial oxides[43,44]. and Since related the degree solid of solutions. doping of the The image observed with europium formation is low; ofdoping Sr2TiO of the4 phase sample candoes benot a result of Eu substitutionintroduce by significant Sr leading distortions to the enrichment of the crystallin ofe theenvironment powder of with europium Sr. ion, which corresponds to the presence of a narrow intense magnetic3+ transition 5D0 → 7F1 (588 nm) in the spectrum. The electric XRD pattern of the five-layer SrTiO3:Eu film contains the intense peak associated with the silicon 5 → 7 3+ dipole transition D0 F2, observed at 599–6263+ nm, is allowed only when Eu is located at a substrate (Figurenoncentrosymmetric2b). Apparently, crystallographic the SrTiO site. Otherwise3:Eu film considering has a low diff solid-solubilityuse line in the limit region of Eu3+ in of 32.2 deg (influx to the left of the first sharp peak). The powder synthesized from the same sol as the porous film has much larger grains as compared with the porous film and is characterized by the obvious phase formation. As a result, the PL spectra of the powder and the film differ. The presence of defects and disordering of the crystal structure certainly affects the luminescence spectrum of europium. The most intensive peak at 588 nm corresponding to the magnetic dipole transition 5D 7F confirms that Eu3+ occupies the position with the symmetry 0 → 1 center to be consistent with the XRD data. The typical doping concentration for lanthanides in solids is about 1 atomic %. In this case, it is not mandatory that trivalent lanthanide ion substitute only trivalent ion in host matrix, and there are no other local states for lanthanides. Luminescence of trivalent lanthanide ions with typical transitions between the corresponding terms was observed in inorganic 3+ 3+ 3+ perovskites: Tb in YAlO3 [36], Er in LiNbO3 corresponds to a [37], Eu in BaTiO3 [38], as well as Eu3+ in sol-gel derived amorphous yttrium alumina composites [39]. Europium ion is the dopant that can exhibit divalent or trivalent valence state [40,41]. The luminescence spectra correspond to PL bands of Eu3+, as the PL spectra of Eu2+ lies in the region of about 380–500 nm [40]. In the case when Eu3+ occupies the position of strontium while remaining trivalent, the charge is compensated by the field of local environment, for example, titanium ions [42]. Replacement of divalent ions with trivalent leads to distortions of the crystal lattice of the structure [43,44]. Since the degree of doping of the image with europium is low; doping of the sample does not introduce significant distortions of the crystalline environment of europium ion, which corresponds to the presence of a narrow intense magnetic transition 5D 7F (588 nm) in the spectrum. The electric dipole transition 5D 7F , 0 → 1 0 → 2 observed at 599–626 nm, is allowed only when Eu3+ is located at a noncentrosymmetric crystallographic Materials 2020, 13, 5767 5 of 11 site. Otherwise considering low solid-solubility limit of Eu3+ in the perovskite matrix, such PL spectra 2+ 4+ 3+ is probablyMaterials the 2020 result, 13, x of FOR the PEER simultaneous REVIEW exchange substitution of Sr and Ti positions5 of 11 by Eu ions [42]. the perovskite matrix, such PL spectra is probably the result of the simultaneous exchange substitution of Sr2+ and Ti4+ positions by Eu3+ ions [42]. 3.2. Heterostructures Si/SrTiO3/Ni and Si/TiOx/Pt/SrTiO3/Ni

3.2. Heterostructures Si/SrTiO3/Ni and Si/TiOx/Pt/SrTiO3/Ni Figure3 shows cross-sections and plane views of Si /TiOx/Pt/SrTiO3/Ni and Si/SrTiO3/Ni Figure 3 shows cross-sections and plane views of Si/TiOx/Pt/SrTiO3/Ni and Si/SrTiO3/Ni heterostructures. The structures contain ten-layer porous SrTiO3 ﬁlms with the total thickness heterostructures. The structures contain ten-layer porous SrTiO3 films with the total thickness of of about 630about nm 630 and nm theand the average average grain grain size of of about about 30 nm. 30 nm.

(a) (b)

Figure 3. SEM images of Si/TiOx/Pt/SrTiO3/Ni (a), Si/SrTiO3/Ni (b) heterostructures, the surfaces of Figure 3. SEM images of Si/TiOx/Pt/SrTiO3/Ni (a), Si/SrTiO3/Ni (b) heterostructures, the surfaces of SrTiO3 (c) and Ni electrode (d). SrTiO3 (c) and Ni electrode (d). A Raman spectrum of the Si/TiOx/Pt/SrTiO3 heterostructure is presented in Figure 4. Several A Ramanbandsspectrum corresponding of the to TiO Si/2TiO andx SrTiO/Pt/SrTiO3 phases3 heterostructure are observed. is presented in Figure4. Several bands Three bands at 395, 515 and 635 cm−1 correspond to TiO2 anatase crystalline phase. The band at corresponding to TiO2 and SrTiO3 phases are observed. 395Materials cm− 12020 corresponds, 13, x FOR PEER to B1g REVIEW vibration mode, the band at 635 cm−1 corresponds to Eg and the band6 of at11 515 cm−1 is a doublet of A1g and B1g modes [45]. The bands at 261 and 797 cm−1 are an indication of cubic SrTiO3 phase in the film. The band at 261 cm−1 is associated with first-order transversal TO3 phonon mode and the band at 797 cm−1 is a longitudinal LO4 phonon mode [46–48]. The activation of TO3 and LO4 bands indicates that structural distortions extended over a distance of the order of the phonon wavelength in the synthesized material [46]. Other SrTiO3 bands may be veiled by strong bands of TiO2. The single band at 865 cm−1 can be associated with SrO 2LO mode [49]. Two lines located at 1443 and 1625 cm−1 are related to carbon presenting in the structure [50]. A summary of the band identification is given in Table 1.

Figure 4. Raman shift spectra of Si/TiOx/Pt/SrTiO3/Ni heterostructure. Figure 4. Raman shift spectra of Si/TiOx/Pt/SrTiO3/Ni heterostructure. Table 1. Raman spectra bands.

Peak, cm−1 Material Mode Reference 261 SrTiO3 (cubic) TO3 (O–Sr–O bending) [46–48] 395 TiO2 (Anatase) B1g [45] 515 TiO2 (Anatase) A1g + B1g [45] 635 TiO2 (Anatase) Eg [45] 797 SrTiO3 (cubic) LO4 (Ti–O stretching) [46–48] 865 SrO 2LO [49] 1443 Carbon Disorder [50] 1625 Carbon E2g [50]

Porous films with small grains go through difficulties with the formation of the SrTiO3 phase in conditions of limiting grain growth because it is accompanied by the development of defects. They make TiO2 to dominate over SrTiO3. The XRD pattern of Si/TiOx/Pt/SrTiO3 heterostructure contains intense peaks associated with silicon substrate and platinum layer (Figure 5). The five-layer SrTiO3:Eu3+ film XRD pattern of Si/TiOx/Pt/SrTiO3 heterostructure contains a diffuse line in the region of 32.2 deg associated with SrTiO3. They also contain unidentified lines that can be attributed to oxides and oxide composites, including components of the precursors.

Figure 5. XRD spectra of Si/TiOx/Pt/SrTiO3/Ni heterostructure.

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1 Three bands at 395, 515 and 635 cm− correspond to TiO2 anatase crystalline phase. The band at 1 1 395 cm− corresponds to B1g vibration mode, the band at 635 cm− corresponds to Eg and the band at 1 1 515 cm− is a doublet of A1g and B1g modes [45]. The bands at 261 and 797 cm− are an indication of 1 cubic SrTiO3 phase in the film. The band at 261 cm− is associated with first-order transversal TO3 1 phonon mode and the band at 797 cm− is a longitudinal LO4 phonon mode [46–48]. The activation of TO3 and LO4 bands indicates that structural distortions extended over a distance of the order of the phonon wavelength in the synthesized material [46]. Other SrTiO3 bands may be veiled by strong 1 bands of TiO2. The single band at 865 cm− can be associated with SrO 2LO mode [49]. Two lines 1 located at 1443 andFigure 1625 cm4. Raman− are relatedshift spectra to carbon of Si/TiO presentingx/Pt/SrTiO in3/Ni the heterostructure. structure [50]. A summary of the band identification is given in Table1. Table 1. Raman spectra bands. Table 1. Peak, cm−1 Material Raman spectra Mode bands. Reference 261 1 SrTiO3 (cubic) TO3 (O–Sr–O bending) [46–48] Peak, cm− Material Mode Reference 395 TiO2 (Anatase) B1g [45] 261 SrTiO3 (cubic) TO3 (O–Sr–O bending) [46–48] 2 515395 TiO TiO (Anatase)2 (Anatase) A1g B1g + B1g [ [45]45] 635515 TiO TiO2 (Anatase)2 (Anatase) A1g + EgB1g [ [45]45] 797635 SrTiO TiO23(Anatase) (cubic) LO4 (Ti–O Eg stretching) [46–48] [45] 797 SrTiO (cubic) LO4 (Ti–O stretching) [46–48] 865 SrO3 2LO [49] 865 SrO 2LO [49] 14431443 Carbon Carbon Disorder Disorder [ [50]50] 16251625 Carbon Carbon E2g E2g [ [50]50]

Porous films with small grains go through difficulties with the formation of the SrTiO3 phase in Porous films with small grains go through difficulties with the formation of the SrTiO3 phase conditions of limiting grain growth because it is accompanied by the development of defects. They in conditions of limiting grain growth because it is accompanied by the development of defects. make TiO2 to dominate over SrTiO3. They make TiO2 to dominate over SrTiO3. The XRD pattern of Si/TiOx/Pt/SrTiO3 heterostructure contains intense peaks associated with The XRD pattern of Si/TiOx/Pt/SrTiO3 heterostructure contains intense peaks associated with silicon substrate and platinum layer (Figure 5). The five-layer SrTiO3:Eu3+3+ film XRD pattern of silicon substrate and platinum layer (Figure5). The five-layer SrTiO 3:Eu film XRD pattern of Si/TiOx/Pt/SrTiO3 heterostructure contains a diffuse line in the region of 32.2 deg associated with Si/TiOx/Pt/SrTiO3 heterostructure contains a diffuse line in the region of 32.2 deg associated with SrTiO3. They also contain unidentified lines that can be attributed to oxides and oxide composites, SrTiO3. They also contain unidentified lines that can be attributed to oxides and oxide composites, including components of the precursors. including components of the precursors.

Figure 5. XRD spectra of Si/TiOx/Pt/SrTiO3/Ni heterostructure. Figure 5. XRD spectra of Si/TiOx/Pt/SrTiO3/Ni heterostructure. Figure6 shows SIMS depth distribution proﬁles of the components in the porous nanostructured

SrTiO3 ﬁlm formed on Pt/TiOx/Si substrate. Materials 2020, 13, x FOR PEER REVIEW 7 of 11

Materials 2020, 13,Figure 5767 6 shows SIMS depth distribution profiles of the components in the porous nanostructured 7 of 11 SrTiO3 film formed on Pt/TiOx/Si substrate.

Figure 6. SIMS depth distribution of the precursor components in the Si/TiOx/Pt/SrTiO3 Figure 6. SIMS depth distribution of the precursor components in the Si/TiOx/Pt/SrTiO3 heterostructure. heterostructure.

The thickness of the SrTiO3 film estimated from the SIMS data is about 450 nm. It is smaller than The thickness of the SrTiO3 film estimated from the SIMS data is about 450 nm. It is smaller than the one determinedthe one determined by SEM by which SEM which is explainable is explainable noting noting that the the sputtering sputtering rate rateof porous of porous structures structures is larger thanis oflarger dense than filmsof dense [51 films]. Qualitatively, [51]. Qualitatively, the th SIMSe SIMS data are are in a in good a good agreement agreement with the withresults the results of XRD and Raman spectroscopy analysis. TiO2, SrO, and SrTiO3 are formed homogeneously in the of XRD and Raman spectroscopy analysis. TiO2, SrO, and SrTiO3 are formed homogeneously in the nanostructured porous films. nanostructured porous films. The SIMS depth profiles of the components in the Si/TiOx/Pt/SrTiO3 heterostructure illustrate the The SIMSpresence depth of several profiles layers. of The the layer components corresponding in the to the Si/ TiOsputteringx/Pt/SrTiO time of3 1800heterostructure s is enriched with illustrate the presence ofSrO several we associate layers. with The the porous layer correspondingSrTiO3. It contains tosome the carbon sputtering that is a timeresidue of of 1800 the incomplete s is enriched with elimination of the carbon containing isopropoxy groups and organic groups of the solvent [51,52]. SrO we associate with the porous SrTiO3. It contains some carbon that is a residue of the incomplete The next layers corresponding to the sputtering time in the ranges of 1800–2500 and 2500–3000 s are elimination of the carbon containing isopropoxy groups and organic groups of the solvent [51,52]. associated with the Pt and TiOx, respectively. Silicon ions of the substrate were not detected under The next layersthese experimental corresponding conditions. to the sputtering time in the ranges of 1800–2500 and 2500–3000 s are associated withFigure the 7 Pt shows and C-V TiO characteristicsx, respectively. of the Si/SrTiO Silicon3/Ni ions heterostructure of the substrate after several were measurement not detected under these experimentalcycles at different conditions. frequencies. Initially, the structure was in the state with the maximum capacity followed by the high frequency bypass performed from 0 V counterclockwise. The voltage polarity Figure7 shows C-V characteristics of the Si /SrTiO /Ni heterostructure after several measurement corresponds to the sign of the electric potential on one top3 electrode relative to the other top electrode. cycles at diThefferent charge frequencies. of the capacitor Initially,basically occurs the structureonly by applying was ina positive the state voltag withe. The the capacity maximum of this capacity followed bystructure the high with frequencytwo upper electrodes bypass varies performed from 6 pF from to 22 0 pF. V The counterclockwise. width of the hysteresis The is voltage~6 V in polarity correspondsthe tofrequency the sign range of theof the electric test measurement potential signal on one from top 2 kHz electrode to 2 MHz. relative The width to of the the other hysteresis top electrode. loop is about 6 V. The charge of the capacitor basically occurs only by applying a positive voltage. The capacity of this structure with two upper electrodes varies from 6 pF to 22 pF. The width of the hysteresis is ~6 V in the frequency range of the test measurement signal from 2 kHz to 2 MHz. The width of the hysteresis loop is aboutMaterials 6 V. 2020, 13, x FOR PEER REVIEW 8 of 11

(a) (b)

Figure 7. Capacitance-voltage characteristics of the Si/SrTiO3/Ni heterostructure recorded in the Figure 7. Capacitance-voltage characteristics of the Si/SrTiO3/Ni heterostructure recorded in the frequency ranges of 100 kHz-2 MHz (a) and 2 kHz-50 kHz (b). frequency ranges of 100 kHz-2 MHz (a) and 2 kHz-50 kHz (b). The nonlinearity of the C–V characteristics is caused by the presence of electrically active deep energy levels in the band gap, created by various defects. As the capacity growth is observed only with one positive polarity, electrons are trapped by the defect states [22]. The nonlinear dependence of the capacitance on voltage applied can be associated with the formation of potential wells in the surface layer of nanostructured grains due to the porosity. The frequency dependence of the hysteresis loop area indicates the presence of fast and slow energy states involved in the recharging of electrical capacitance. A sharp increase of the capacitance at a positive bias voltage with a decrease in the frequency of the test signal to 2 kHz indicates the electrical charge accumulation on slow energy states. A shift of the C-V characteristics towards negative bias voltages by approximately the same value indicates the presence of a fixed positive charge in the SrTiO3 film or near the interfaces. The nature of the traps and energy states is still not clear and may be the goal of a separate study by electric thermally stimulated methods. In-plane I-V measurements between two top Ni electrodes of the structure Si/SrTiO3/Ni show that illumination of the samples with the light of a halogen lamp at the intensity of 39 mW/cm2 leads to significant changes in the forward and reverse branches of the I-V curves. This is illustrated in Figure 8a. Under illumination, the curves are characterized by hysteresis for both forward and reverse biasing, which was also early observed in [53]. The transversal electrical measurements on the Si/TiOx/Pt/SrTiO3/Ni structure shows resistance switching even without illumination (Figure 8b) when the bias was applied between top Ni and bottom Pt electrodes.

(a) (b)

Figure 8. In-plane (a) and transversal (b) I-V characteristics of Si/SrTiO3/Ni (a) and

Si/TiOx/Pt/SrTiO3/Ni (b) containing ten layers of SrTiO3 under and without illumination.

Materials 2020, 13, x FOR PEER REVIEW 8 of 11

(a) (b)

Figure 7. Capacitance-voltage characteristics of the Si/SrTiO3/Ni heterostructure recorded in the Materials 2020, 13, 5767 8 of 11 frequency ranges of 100 kHz-2 MHz (a) and 2 kHz-50 kHz (b).

The nonlinearity of the C–V characteristics is caus causeded by the presence of electrically active deep energy levels in the band gap, created by variousvarious defects.defects. As the capacity growth is observed only with one positive polarity, electronselectrons areare trappedtrapped byby thethe defectdefect statesstates [[22].22]. The nonlinear dependence of the capacitance on voltage applied can be associated with the formation of potential wells in the surface layer of nanostructured grains due to the porosity.porosity. The frequency dependence of the hysteresis loop area indicates the presence of fast and slow energy statesstates involvedinvolved inin thethe recharging recharging of of electrical electrical capacitance. capacitance. A A sharp sharp increase increase of of the the capacitance capacitance at ata positive a positive bias bias voltage voltage with with a decrease a decrease in the in frequency the frequency of the testof the signal test to signal 2 kHz to indicates 2 kHz indicates the electrical the electricalcharge accumulation charge accumulation on slow energy on slow states. energy A shift states. of the A C-Vshift characteristics of the C-V characteristics towards negative towards bias voltagesnegative bybias approximately voltages by approximately the same value the indicates same value the presenceindicates ofthe a presence fixed positive of a fixed charge positive in the charge in the SrTiO3 film or near the interfaces. The nature of the traps and energy states is still not SrTiO3 film or near the interfaces. The nature of the traps and energy states is still not clear and may be clearthe goal and of may a separate be the goal study of bya separate electric thermallystudy by electric stimulated thermally methods. stimulated methods. In-plane I-V measurements between two top Ni electrodes of the structure Si/SrTiO3/Ni show In-plane I-V measurements between two top Ni electrodes of the structure Si/SrTiO3/Ni show 2 that illumination illumination of of the the samples samples with with the the light light of ofa halogen a halogen lamp lamp at the at intensity the intensity of 39 ofmW/cm 39 mW leads/cm2 toleads significant to significant changes changes in the in forward the forward and andreverse reverse branches branches of the of theI-V I-Vcurves. curves. This This is illustrated is illustrated in Figurein Figure 8a.8 a.Under Under illumination, illumination, the the curves curves are are ch characterizedaracterized by by hysteresis hysteresis for both forwardforward andand reverse biasing,biasing, whichwhich was was also also early early observed observed in in [53 [53].]. The The transversal transversal electrical electrical measurements measurements on theon the Si/TiOx/Pt/SrTiO3/Ni structure shows resistance switching even without illumination (Figure 8b) Si/TiOx/Pt/SrTiO3/Ni structure shows resistance switching even without illumination (Figure8b) when whenthe bias the was bias applied was applied between between top Ni top and Ni bottom and bottom Pt electrodes. Pt electrodes.

(a) (b)

Figure 8. In-plane (a) and transversal (b) I-V characteristics of Si/SrTiO3/Ni (a) and Figure 8. In-plane (a) and transversal (b) I-V characteristics of Si/SrTiO3/Ni (a) and Si/TiOx/Pt/SrTiO3/Ni Si/TiOx/Pt/SrTiO3/Ni (b) containing ten layers of SrTiO3 under and without illumination. (b) containing ten layers of SrTiO3 under and without illumination.

4. Conclusions

We described the sol-gel approach to the fabrication of porous Eu doped SrTiO3 multilayer film structures. The technique does not require any organic template precursors in the sol for making the material porous. Both porous nanostructured films and bulk powders demonstrate the most intensive PL bands at 612 nm and 588 nm, respectively. Thus, this material can be used as correspondingly yellow and red phosphors in optoelectronic devices depending on the structural state of Eu3+ ions. XRD, Raman spectroscopy, and SIMS analysis are in a good agreement with each other and confirm the formation of TiO2, SrO, and SrTiO3 phases and their components in the porous SrTiO3 material. The thin film structures, including porous SrTiO3, exhibit hysteresis of C-V and I-V characteristics, which is of interest for nonvolatile memory elements.

Author Contributions: Data curation, E.C. and Y.R.; formal analysis, V.L., A.E., E.C., V.B., N.M. and Y.R.; investigation, M.R. and N.G.; resources, M.R., A.T. and A.G.; supervision, N.G.; writing—original draft, M.R.; Materials 2020, 13, 5767 9 of 11 writing—review & editing, V.B. and N.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the grant T19-MLDG of State Committee on Science and Technology of the Republic of Belarus. Nikolay Gaponenko and Victor Borisenko acknowledge the partial financial support of the “Improving of the Competitiveness” Program of the National Research Nuclear University MEPhI—Moscow Engineering Physics Institute. The part of work was supported by the Russian Foundation for Basic Research under the project № 19-07-00600 A. Conflicts of Interest: The authors declare no conflict of interest.

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