Investigation of structure, dielectrical, optical and electronic properties for Y0.225Sr0.775CoO3 thin films deposited on different single crystal substrates and prepared by Pulsed Laser Deposition (PLD) method.AbstractThe structure of Y0.225Sr0.775CoO3 (YSCO) thin films deposited on different single crystal substrayes such as SrTiO3 (STO)(100), SrTiO3 (STO)(111), MgO(111) and LaAlO3 (LAO)(100) using pulsed laser deposition (PLD) method, was carried out using X-Ray Diffraction (XRD) to determine The crystallite size (Cs), dislocation density (), The lattice strain (Ls) the number of crystallites per unit area (N).
The The surface topography using was carried out Atomic Force Electron Microscope (AFM), which shows that, all these films had a polycrystalline structure.The optical properties of these films were measured using UV/Vis spectrophotometer. The optical parameters; energy gap (Eg), refractive index (n), extinction coefficient (k), the optical energy gap affected strongly by changing the substrate type. Moreever both of oscillating energy (Eo), dispersion energy (Ed), The effective mass of these films with different substrate had been calculated optically, another important paremeters such as The dielectric constant (µ) and dielectric loss (µ\) and both of the real part (1) and imaginary part (2) of optical conductivity were determined.
The ratio of VEL to SEL as functions of photon energy for Y0.225Sr0.775CoO3 films were calculated optically. Finally the density of both valence band (NV) and conduction band (NC) for these films has been calculatedThe sielectric constants for these films were determinedKeywords: Y0.225Sr0.775CoO3, PLD, Different substrates, optical properties, effective mass, density of states, dispersion energy, oscillating energy, dielectric results. 1. Introduction The interesting physical properties of transition metal oxides, which consists of transition metal such as Co and Mn widely investigated due to exhibiting interesting behaviors, including; superconductivity, thermoelectricity, ferromagnetism, and giant magneto resistance due to electron-electron interaction [1-5]. Among them cobalt oxides with perovskite structure Ln1€’xMxCoO3 (Ln: lanthanides, M: alkaline-earth metals) have attracted much interest because they exhibit spin-state transitions and unusual magnetic properties [3, 5]. The doping level for A-site order perovskite play an important rule in changing the transport and magnetic properties of Sr1-xRxCoO3 materials, which give an expiation of transition metal oxide and their industrial application as a magnetic material [6-7]. Interest the YxSrx-1CoO3 composition has magnetic, electric, and spin state transitions with temperature and Sr doping [5- 6, 8]. The most popular composition in the Sr1-xRxCoO3 family is YxSrx-1CoO3, because it exhibits a ferromagnetic behavior at room temperature with a Curie temperature of 335 K with a composition ratio of 0.2 ‰¤ x ‰¤0.25 [8-9]. The metal oxides promising material as as result of their optoelectronic applications [10-11]. The physical properties of metal oxides and rare earth had been studied widely [12-18]. The optical properties of (YSCO) were not investigated in details yet which motivated us to study it on epitaxial thin films[19]. In this paper, we have investigated the optical properties of (YSCO) thin films that grown on different substrates by (PLD) method. Optical parameters such as; optical energy gap, refractive index, extinction coefficient, and complex dielectric constants for these films were estimated. In addition an oscillating energy, dispersion energy as well as effective mass and the density of (NV) and conduction band (NC) for these films have been calculated.3. Results and Discussion3.1. structure X-ray Diffraction (XRD) patterns of a YSCO thin film which are deposited on different substrates [STO (100), STO (111), MgO (111) and LAO (100)] are shown in Fig. 1( a, b, c and d). From this Fig. it was noticed that the kind of substrate effect on the crystallinity of the studied films, for Fig. 1a,b, the YSCO peak position has been changed as a result of changing the orientation of the substrate. Fig. 2 shows the AFM studies for these samples; from this figure it was shown that, the used substrates affected strongly on both of size and the orientation of these films, and all these films for different substrates had a polycrystalline structure nature [21-22] .The crystallite size (Cs) of these films has been calculated from the full width at halfmaximum (FWHM) of the most intense peak of these crystals using Sherrer’s formula [23] (1)where both ” is the wavelength of the used X-ray, is the Bragg’s angle and І (the FWHM of the peak). While the dislocation density (), which refers to the number of defects in the films, is as [24]: (2)The lattice strain (Ls) was determined using [25] (3)Another important factor for these studied samples was determined , the number of crystallites per unit area (N), which has been determined using the following equation [26] (4)where t is the crystal thickness. The calculate values for these studied crystals are shown in table 1The surface topography pictures for these films are shown in Figs. 2. From this Fig. it is clear that all these films had a polycrystalline structure, and the type of substrate plays an important role for increasing the grain size of the deposited material. 3.2. Optical, dielectrical and optical conductivity results Figs 3 (a, b) show the optical transmission (T) and reflection (R) for theses studied. As we noticed all films have a similar behavior of (R and T) spectra. In addition the other film deposited on the MgO(111) substrate has the highest (T) and lowest (R) compared to the films deposited on another substrates. The absorption coefficient (±) of these films were calculated from the following equation [27] (5)where d is the film thickness. As we can see that, the film with both SrTiO3(100) and MgO(111) substrates have the highest and lowest values of (±), respectively. The optical energy gap (Eg) was determined from the optical absorption curve using the imperial equation [28]: (6)Where A is a constant, Eg is the energy band gap, Ѕ is the frequency of the incident radiation and h is Planck’s constant. Fig. 3 (c) shows the relation between (±hЅ)2 and photon energy (hЅ) in order to determine the direct transition for these samples. It is found that for all films showed a direct energy gap. The values of Eg for these films is shown in table 2. The orientation and the nature of substrates have clear effects on the optical band gap energies. The value of activation energy I was calculated according to the Urbach equation [29] (7)The Ea values were determined from the slopes of the linear parts of the logarithmic dependence of (±) on (hЅ) that is drawn as representative for these studied samples as shown in Fig 3(d). The calculated values of the activation energy of these samples are shown in Table 2. The extinction coefficients (k) of these films was calculated optically using the relation: (8)Fig. 4.a shows the relation between k and (hЅ) for these films were As from the experimental data, the value of k is more than zero, which means the electromagnetic waves confirmed to be absorbed by these films. At lower energies E < 2.5eV the imaginary part of the refractive index is low, which means there is a small attenuation of the light that incident on the samples. While at higher energies the extinction coefficient values increased linearly. Here also the Y0.225Sr0.775CoO3 film on MgO(111) substrate has the most lowest extinction coefficient value. The refractive index (n) of these films with the different substrates was calculated using the following equation [30] (9) The refractive index (n) of films for all films on different substrates has been estimated. The relation between (n)2 and (“)2 for these amples is shown in Fig.4 (b). First of all, the films have the same behavior with small and nearly constant (n) at lower (“) and exhibited a big peak around “2/106 = 2.5 nm2 for film on MgO(111) substrate shifting to “2/106 = 5.5 nm2 indicating the relaxation of the (n) with different substrates. Moreover the film deposited on STO(100) substrate had the highest (n) peak at higher (“), while the other films on the other substrates had the same value of the (n) which is similar to the behavior of the (R)spectra. The dispersion of refractive index in Y0.225Sr0.775CoO3 thin film was analyzed using the concept of the single oscillator and can be expressed by the Wemple”DiDomenico relationship [31]: (10)Where E is the photon energy, Eo is the oscillator energy and Ed is the dispersion energy. The parameter E, which is a measure of the intensity of the inter-band optical transition, does not depend signicantly on the band gap. The values of Eo and Ed are obtained from the intercept and the slope resulting from the extrapolation of the line. The values of Eo and Ed for all samples are shown in Table 2. The effective mass of the investigated thin films of different substrates was determined optically from Fig. 2(b) using the following equation [32] (11)Where µL is the lattice dielectric constant and it has different values with different substrates, e is the charge of the electron, N is the free carrier concentration of theses investigated films which were determined using a Hall method [20], and c is the speed of light. From this data the slope equal to the value of (e.N/4C2µ0m*), since the effective mass of these films was calculated as shown in Table 2. It is clear that the type of substrates affects strongly on the calculated effective mass values for these films.The dielectric parameters for the Y0.225Sr0.775CoO3 thin films deposited on SrTiO3(100), SrTiO3(111), LaAlO3(100), and MgO (111) substrates were calculated optically, the lattice dielectric constant µL was determined by drawing the relation between (n2) and photon energy, the extend line will intercept with the y-axis at a value of (n2), this will be the µL of the films [28]. The µL values of these films are shown in Table 2.The dielectric constant (µ) and dielectric loss (µ\) for these films were calculated using the equations [33]: (14) (15) Figs 4 (c, d) shows both of (µ) and (µ\) dependence on photon energies for these films. The real part of the dielectric constant ( ) is high at lower energy (less than 2 eV), and there is a large peak at 1.75 eV for film on STO(100) shifted to higher energies with the other films, exhibiting relaxation. This peak is shifted to energy higher energy (2 eV) for the film deposited on MgO(111) substrate. While (µ\) is showing the same behavior at low energy for all films except the film deposited on MgO(111) substrate. At higher energy (E > 2 eV), there is a sudden increase in the dielectric loss for the film deposited on MgO(111) substrate. This indicated that the semiconductor behavior of the film due to the MgO substrate, which generated more carriers and increased the conductivity at higher energy. Figs 5(a, b) shows the real part (1) and imaginary part (2) of optical conductivity dependence on photon energy for studied thin films. The optical conductivity was calculated from the following equations [34]: (16) (17)It is clear that the conductivity has similar behavior of dielectric constant and loss, where all samples have similar values and peak position except the film grown on MgO substrate, and this due to the semiconductor-like behavior of the MgO substrate. The ratio of volume energy loss to surface energy loss (VEL/SEL) as functions of photon energy for these samples shown in Fig. 5(c). Both of (VEL, SEL) were calculated from the imperial equations [35-36]: (18) (19)Finally the values and behaviors of the optical parameters are depending strongly on the type of substrates. The effective mass ratio on rest mass (m*/me)of these films with different substrate had led to determine the density of both valence and conduction bands (Table 2). The experimental results confirmed that it is possible to control the structure [20], optical parameters of Y0.225Sr0.775CoO3 thin films by changing the used substrate, this leads to important industrial applications such as, electronic and optoelectronic devices. 3.3. electronic results The values of density for both valence and conduction states are in Table2, which were calculated from the equations [37] (12) (13)From this table it clear that the type of substrate affected strongly on the calculated electronic results, such as both of values of (Nv and Nc),(me/m*) the ratio between electron rest mass and the effective mass of the studied samples. 4. Conclusion Y0.225Sr0.775CoO3 thin films were simultaneously grown on single crystals STO(100), STO(111), MgO(111) and LAO(100) substrates using PLD method. The X-Ray diffraction showed that the kind of substrate affect on the film crystallinty and film orientation, while the SEM images showed that the type of substrate play important rule for the film crystal size. The optical properties were measured using UV/Vis spectrophotometer, the optical parameters values and behaviors depends strongly on the type of substrates. Eg affected strongly by changing the type of substrates, which were in values (2.32 to 2.50) which give a facility to control and change the optical energy gap only by change the type of substrate. m* of these films with different substrate had been calculated optically which was found that, these determined factor depends strongly on the type of substrate, another important optical parameters such as VEL/SEL as functions of photon energy for Y0.225Sr0.775CoO3 films were calculated optically. The experimental results revealed that it is possible to control the optical parameters of materials by changing the substrate, this leads to important industrial applications such as, electronic and optoelectronic devices with low cost.Finally the electronic results such as the Nc and Nvand the the ratio between electron rest mass and the effective mass of the studied samples were calculated. . AcknowledgmentsThis study was supported financially by the Basic Science Department, Faculty of Industrial Education, Helwan University, Cairo, Egypt. Brain Pool Program (152S-3-3-1372) through the Korean Federation of Science and Technology Societies (KOFST), through the National Research Foundation of Korea (NRF), funded by Korean government. LAO(100) MgO(111)STO(111) (STO)(100)Lattice strain Ls The number of crystallit-es per unit area (N) / cm2 Disloc-ation denidty () line/cm2 Grain size Cs (nm) І (FWHM) Lattice strain Ls The number of crystallit-es per unit area (N) / cm2 Disloc-ation denidty () line/cm2 Grain size Cs (nm) І (FWHM) Lattice strain Ls The number of crystallit-es per unit area (N) / cm2 Disloc-ation denidty () line/cm2 Grain size Cs (nm) І (FWHM) Lattice strain Ls The number of crystallit-es per unit area (N) / cm2 Disloc-ation denidty () line/cm2 Grain size Cs (nm) І (FWHM)20.0 2.1E+17 3.0E+13 17.0 0.20 12.0 4.0E+20 1.2E+15 4.20 0.60 9.0 3.3E+21 6.9E+14 3.70 0.4 4.0 2.0E+21 4.0E+14 7.2 0.2031.0 2.2E+18 1.8E+14 7.50 0.30 1.0 1.2E+22 8.4E+13 10.8 0.2 6.0 6.0E+21 3.2E+14 5.6 0.60 12.0 4.0E+22 1.2E+15 2.90 0.50Type of substrate Activation energy (Eact) eV desperation energy (Ed) (eV) Oscillating energy (E0) (eV) N/m* m*/me Density of conduction state Nc (cm-3) Density of valence state Nv (cm-3) lattice dielectric constant µLSTO(100) 0.20 07.60 04.60 4.20€™1048 0.086 1.3E+21 8.8E+20 16.0STO(111) 0.19 08.40 04.80 4.70€™1048 0.070 1.8E+21 1.23E+21 11.0MgO(111) 0.17 10.20 06.80 5.80€™1048 0.030 2.1E+21 1.45E+21 06.50LAO(100) 0.27 8.50 05.50 6.90€™1048 0.020 2.3E+21 1.6E+21 12.40Fig CapturesFig. 1. X-Ray Diffraction pattern for YSCO thin film with (a) STO (100), (b) STO (111), (c) MgO (111) and (d) LAO (100) substrates. Fig. 2. AFM images of YSCO thin films on (a) STO (100), (b) STO (111), (c) MgO(111) and (d) LAO (100) substrates .Fig.3. Optical transmittance (a)and reflectance (b) dependence on wave length , (±.hЅ)2(c) and ln(±) (d) dependence on photon energies for Y0.225Sr0.775CoO3 thin films grown on STO(100), STO(111), MgO(111) and LAO(100) substrates.Fig.4. (a) Relation between extinction coefficient and photon energy, (b) the relation between (n)2 and (wave length)2, and (c, d) the dependence of dielectric loss and dielectric tangent loss on photon energies for Y0.225Sr0.775CoO3 thin films grown on STO(100), STO(111), MgO(111) and LAO(100) substrates.Fig.5. (a, b) Real and imaginary part of optical conductivity and (c) the ratio of VEL/SEL dependence on photon energies for Y0.225Sr0.775CoO3 thin films grown on STO(100), STO(111), MgO(111) and LAO(100) substrates.