Implementation of magnesium doping in SrTiO3 for correlating electronic, structural and optical properties: A DFT study

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Implementation of Magnesium Doping in SrTiO3 for Correlating Electronic, Structural and Optical Properties: A DFT Study Muhammad Rizwan , Maida Anwar , Zahid Usman , Muhammad Shakil , S.S.A. Gillani , H.B. Jin , C.B. Cao , Uzma Mushtaq PII: DOI: Reference:

S0577-9073(19)30954-2 https://doi.org/10.1016/j.cjph.2019.09.036 CJPH 977

To appear in:

Chinese Journal of Physics

Received date: Revised date: Accepted date:

16 July 2019 4 September 2019 12 September 2019

Please cite this article as: Muhammad Rizwan , Maida Anwar , Zahid Usman , Muhammad Shakil , S.S.A. Gillani , H.B. Jin , C.B. Cao , Uzma Mushtaq , Implementation of Magnesium Doping in SrTiO3 for Correlating Electronic, Structural and Optical Properties: A DFT Study, Chinese Journal of Physics (2019), doi: https://doi.org/10.1016/j.cjph.2019.09.036

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Revised Highlights 

The inclusion of Mg at the Sr site in SrTiO3 alters band gap from 1.788 eV to 1.866 eV.



The absorption edge shifted towards lower energy value.



The refractive index increase by doping as 2.49 to 2.52.



Red shift occurs after doping in SrTiO3.

Implementation of Magnesium Doping in SrTiO3 for Correlating Electronic, Structural and Optical Properties: A DFT Study Muhammad Rizwana,d*;Maida Anwara; Zahid Usmanb,d; Muhammad Shakila; S. S. A. Gillanic; H. B. Jind, C. B. Caod, Uzma Mushtaqe

a

Department of Physics, University of Gujrat, Gujrat, 50700, Pakistan

b

Department of Physics, University of Education, Dera Ghazi Khan, 32200, Pakistan.

c

Department of Physics, Government College University Lahore, Lahore 54000, Pakistan

d

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China

e

Department of Technology (Electrical), The University of Lahore, Lahore, Pakistan

Corresponding author: Dr. Muhammad Rizwan (email: [email protected])

Abstract In present work the structural, electronic and optical properties of Pure and Mg-doped SrTiO3 perovskites are calculated via implementing density functional theory calculation. To explore these properties, ultra-soft pseudo-potential (USP) and generalized gradient approximation (GGA) is used. The inclusion of Mg at the Sr site in SrTiO3 not only affects the electronic band structure through generating new gamma points but also band gap increases from 1.788eV to 1.866eV. The introduction of Mg is well explained by the partial and total density of states which is affected by incorporating dopant in pure SrTiO 3. Optical properties also affected by doping. The absorption edge shifted towards lower value from 0.37eV to 0.06eV as Mgdoped in the pure SrTiO3 that represented a red shift. The refractive index increases by doping as of 2.49 to 2.52.The doping of Mg in SrTiO3 affects positively in electronic and optical properties and makes this material a very interesting candidate for optical devices. Keywords:Doping, Density Functional Theory, Band Gap, Density of states, Optical Properties, Refractive Index

1. Introduction The thirst of human being to discover new materials for the betterment of humanity is as needful as oxygen for human beings. Mineralogists, chemist and engineers work together to develop and discover new materials and try to explain the hidden facts which reveals the properties of matter which provides base to new era technology. Among these materials, variety of semiconductor materials such as TiO2, Fe3O4, SnS, metal organic frameworks, CaTiO3 and many more, are used in numerous applications including photocatlaysis, Nano/microelectronics, thin film based optoelectronic devices [1-5]. Their applications are strongly related to their diversity of optical and electronic properties, and their sensitivity to stoichiometry, which are helpful in tuning their properties. In order to enhance the performance of above mentioned devices, it is important to understand the relationship between the required property and oxide stiochiometry. In addition, the inclusion of defects plays an important role to tailor the system’s functionality and hence it becomes inevitable to understand the type of doping, density and distribution of defects, and the electronic structure of these oxide materials theoretically and experimentally. The optical properties, characterizing the response of semiconductor materials to incident electromagnetic radiation, furnish abundance of information about these aspects in general [6]. Comprehensive understanding of optical properties (absorption, emission, transmission, and reflection) is vital in various scientific and industrial applications such as laser technology, (mirrors, lenses and optical windows), contactless temperature measurement, optics, modeling, heat transfer, energy, photovoltaic and aerospace industry etc. [6]. Here we focus mainly on least studied CaTiO3 perovskite mineral, explored first time by geologists Gustave Rose in 1839, from Ural Mountains of Russia. The generic formula of perovskites is ABX3 where A and B represents the cation and have different sizes that surrounded by X anion [7-11]. The earth crust restrains numerous kinds of perovskites mainly abundant are MgSiO3 as well as FeSiO3. The Perovskite family has many oxides such as the transition metals oxides and that shows unusual behaviors such as BaTiO 3, LiNbO3 and NaTaO3 shows the insulating behavior, SrNbO3 and LaTiO3 shows metallic behavior and PbCrO3,

LaCrO3 and PbCrO3 shows the magnetic behavior[5]. The ABO3 types of Perovskite have a great importance in technological point of view due to their broad range of properties such as in laser frequency doubling, in electro-optics, image storage, photo chromic, water producing hydrogen and in high capacity computer memory cell [10,12-13]. The SrTiO3 is one of the oxide-Perovskite has an active material in theoretical and experimental research owing to unusual dielectric properties and has wide applications such as colossal magneto resistive manganite’s, oxygen gas sensor, optical witches, high-temperaturesuperconducting cuprates and grain boundary barrier layer capacitor[14-17]. The SrTiO3 shows insulating behavior at room temperature with band gap 3.22eV[18]. Cationic doping and its concentration in perovskite materials are beneficial to modify the structural, electronic and optical properties of STO. For example, doping concentration has crucial effect on the optical properties, as it enhances the conductivity of a material significantly. At temperature=0, the Fermi level lies exactly at the center of energy gap of a semiconductor. In an intrinsic semiconductor (with no doping at all), the Fermi level lies exactly at the middle of the energy band gap at T=0 Kelvin. As the doping concentration is increased, the Fermi level moves away from central position in order to conserve charge neutrality and mass neutrality. In case of n-type doping, it shifts towards conduction band showing greater electron density than holes. But it moves towards valence band in case of p-type doping with higher hole density than the electrons. For highest doping concentration, the semiconductor behaves like a conductor. Doping concentration affects the optical properties by introducing impurity bands within the band gap depending upon the nature, and position of impurity and its local host environment material, thus shifting the absorption edge towards lower/higher energies. When doping concentration is lower, the effect of impurity bands on the electronic band structure is of additive type by increasing the optical absorption and the host material might turn into a metallic material with highest doping levels after appearing of impurity band merging with the host material’s parent bands [19]. The conductivity and photo catalytic properties might be enhanced by reducing oxygen atom or by doping impurities such as Mg, Cr, Nb or Cu at Sr or Ti sites and such dopants alter the electronic and optical properties of SrTiO3 that are primary condition in device

applications[20]. The STO make n-type semiconductor by applying high electric field the oxygen vacancies are form positive space charge via detrapping electrons [21]. The electrical conductivity increases as La doping is increase in SrTiO3[22]. The photocatalytical activities of STO increased and novel absorption edge are created under the visible light by co-doping N/Laand the creation of oxygen vacancies also are reduced which could act like electron-hole pair off recombination center. The charge is balanced by the N/La codoping [23]. Recent study shows that Cr doping cation enhances the ability of visible light absorption with main involvement of absorption is on 450 nm with slight involvement of absorption is 800nm [24-29]. Experimental research shows Ag doped SrTiO3 exists the inaccessible impurity states within the band gap. In Pb doped SrTiO3 the valence band shifts toward higher energy states to decreases the band gap. So the absorption band showed in visible region [30]. By heavy doping of Nb, the SrTiO3 insulating behavior changes into n-type semiconductor [31]. The current study is based on density functional theory. The reason of this study is to examine the structural, electronic and optical properties of pure and Magnesium doped SrTiO 3. This method is useful to obtain better understanding of pure and Mg-doped STO on atomic scale. This study reveals that the doped system got wider band gap compared with its pure counterpart but the nature of the band gap remains unchanged which is indirect. This transformation is owing to introducing the new gamma points. The inclusion of Mg at Sr site affects the electronic property of the system which is responsible for the red shift observed in optical properties. This also effect the other optical properties as all the optical properties are linked with each other thus this material is a suitable candidate for optical devices. 2. Computational detail The SrTiO3 is ideal cubic structure at a room temperature with Pm-3m space group, where the Oxygen octahedron with Ti is located at center. In STO the Sr is situated at origin (0.0, 0.0, 0.0), Titanium atom is present at body centered (0.5, 0.5, 0.5) and oxygen atoms on face centers (0.5, 0.5, 0), (0.5, 0, 0.5) and (0, 0.5, 0.5). For the understanding of Mg doping, it is necessary to consider the supercell to neglect the boundary effect. In our computation, Mg atom is doped at Sr site. A supercell of 2×2×1 was considered for proceeding of calculation.

For dealing with electron ion interaction the ultasoftpseudopotential (USP) is utilized in CASTEP code [32,33]. The generalized gradient approximation is used to study electron exchange interaction. The Perdew-Burke_Ernzerhof (GGA_PBE) firstly proposed the generalized gradient approximation[34,35]. The plan wave functions apply as a set of bases. Electronic wave functions extended as a distinct plane-wave by means of cut off energy 340eV. It illustrate so as to results are converges at that cut off [36]. The Monkhorst-pack grid of 2×2×1 k point is designed meant for integration over symmetrized brillouin zone. Before calculating the single point energy the geometry optimization was done. The accuracy of self-consistent energy convergence is 5×

. The different properties such as optical along with electronic

properties of pure along with Mg-doped SrTiO3 were calculated after geometry optimization.

Figure-01: Super cell of Mg-doped SrTiO3.

3. Results and Discussion 3.1 Geometry Optimization By applying the Birch-Murnaghan equation of state [37, 38],the optimized lattice parameter of SrTiO3 is achieved that is a=b=c=3.906Å. The above equation of state is applying after geometry optimization. The calculated value of lattice parameter is approximately same through the experimentally attained value of lattice parameter a=b=c=3.908Å [39]. The only minute difference is 0.002(less than 0.1%) occurring. This shows the validity and high precision

of this study. The previously theoretically reported lattice parameter waas a=b=c=3.938Å [40, 41] that is also good in agreement with obtained value, the only difference is 0.032. The Mg doped SrTiO3 structure shows lattice parameter values (a=b=c=3.878Å) that are little reduce compare with pure STO lattice parameter. Table-01 shows the lattice parameters and volume of early reported experimental and theoretical values and current study of Pure and Doped system. The volume of unit cell for pure system is 59.59Å and for Mg-doped STO system is 58.32Å. This is due to the lower ionic radius of Mg=1.45Å as compared to the ionic radius of Sr (2.45Å). Table-01: Geometry optimization parameters and unit cell volume of SrTiO 3 Lattice Parameters (Å)

Earlier Reported

Volume (A3)

a

b

c

3.938

3.938

3.938

61.069

3.908

3.908

3.908

59.684

3.906

3.906

3.906

59.59

3.878

3.878

3.878

58.32

(Pure) Ref [40,41] Experimental Value (Pure) Ref[39] Current Study (Pure) Current Study (Doped) 3.2 Electronic properties The electronic band gap, for insulators as well as semiconductors measures the distinction among the maxima of valence band along with minima of conduction band. Electronic band structure calculation notify about the possible transition as of valence band to conduction band [42]. If the minima of conduction band as well as maxima of valence band are positioned on a same point,it is called direct band gap nature in which transition of electrons are easily happen without wasting incident energy. If the maxima and minima of valence and conduction band respectively,are not at same point,it indicatesindirect band gape nature. In

indirect band gap electron transition occur by wasting considerable amount of incident energy [43].

Figure-02: (a) Band Structure of pure SrTiO3 and(b) Mg-doped SrTiO3

Figure-02(a) indicates the band structure of pure SrTiO3, where the maximum of valence band is occurring at R point, along with a minimum of conduction band is at G point. This shows the indirect nature of the band gap and the disparity among maxima of valence band along with minima of conduction band magnitude is 1.788eV. The previously reported value is 1.812eV [43]. In figure-02(b), band structure of Mg-doped SrTiO3 is shown. The maxima of valence band are shifted from R point to Z point due to doping and minima of conduction band is occurring at T point. The disparity between both bands is 1.866eV. The band gap also increases as compared to pure SrTiO3. After doping the semiconductor band gap nature is observed. To identify the cause of alter of electronic band structure after doping, the partial and total density of state is calculated and discussed below.

Figure-03: Density of states before and after doping(a) PDOS for SrTiO3(b) PDOS of Sr(c) PDOS of Ti (d) PDOS of O (e) PDOS of Mg and (f)TDOS.

Figure-03(a) illustrates the partial density of states for pure and Mg dopped-SrTiO3. In pure strontium Titanate, the valence band is formed due to the 5s state and this is modified by Mg doping. Subsequent to doping, the top of valence band have main contribution of p shell. The O-p states contribute more after doing as compared to pure system which is indication that Mg really affects the electronic distribution in the system. In conduction band, the sharp peak occur due to d shell because of the Mg-s state has influence on Sr-d shell. The elemental partial density of states for pure and doped systemsis plotted in figure03(b-e). Where see the dissimilarity in valence band as well as conduction band state before and after doping. The Mg-s state has more influence on Oxygen and Strontium rather than the Titanium clearly observed from graph. In figure-03(f) full density of state has shown, where in conduction and valence band have significant difference due to emerging Mg-s state. The valence band is shifted from R point to Z point due to s-Mg. Mg s-state is mainly responsible for incrementof band gap in doped system. The top of the valence band as well as bottom of the conduction band is not lie on the same point before and after doping, consequently the band gap has indirect nature in both systems. So the Mg s-state affected the Pure SrTiO3 by increasing the

band gape. The SrTiO3 shows p type behavior before and after doping because Fermi level is remains towards the valence band. 3.3 Optical properties The SrTiO3 is vigorous element of optoelectronic devices, so it’s important to study optical properties of pure and Mg-doped SrTiO3Perovskite. Optical properties depend upon the dielectric constant. The STiO3 have extraordinary dielectric property to other Perovskite oxide. The optical properties illustrate the behavior of crystal during interaction with electromagnetic field. The various optical properties such as refractive index, extinction coefficient, reflectivity, energy loss function, absorption and dielectric functions (real and imaginary part) are discussed here.

Figure-04: A comparative analysis of the optical properties of pure (in black) and Mg doped (in red) SrTiO 3. (a&b) Real and Imaginary part of DF (c) Absorption spectrum (d) Energy Loss function (e) Reflectivity and (f) Refractive index with an inset of extinction coefficient.

These properties are interrelated to each other and explained by following equations, √

√

(1)

⁄

(2)

(3) ( ) √

(4)

√

(5) √

(6) (7) (8)

The above expression coefficient, index,

expresses the absorption coefficient,

complex refractive index, reflectivity coefficient,

extinction

shows energy loss function,

are dielectric function with

refractive real and

imaginary part [44,45]. The optical spectra are computed from the interband transition. The complex dielectric functions have two parts first is real part and the imaginary part

that defines the polarization shown in figure-04(a)

shows the absorption as shown in figure-04(b). In pure SrTiO3 the

imaginary part of the dielectric function is zero at 0eV that shows there is no absorption (dissipation) of energy but for the Mg-doped system

gives positive value at 0eV that

shows the dissipation occur for doped system. When the energy is increases for

there are

three prominent peaks. These peaks represent the symmetric points where interband transition occur that shown in figure-04(b). For the doped system peaks become sharper and shifted toward high energy value with respect to pure system but last peak is sharper for pure system rather than doped system. The energy values for different peaksare 4.1eV, 7.25eV and 23.1eV for pure system. These peaks also obtained at same energy level for absorption

. The peaks energy

values for doped system are 4.3eV, 8.2eV and 23eV and same peaks obtained for absorption in doped system. The absorption edge is shifted towards low energy from 0.37eV to 0.06eV after doping. The shifting of absorption edge towards low value shows the red shift. The plasma resonance condition is accomplish when the energy of imaginary part of complex dielectric function is crosses zero and Loss function

peak is arises. Upon light

contact the electrons typically not confined their lattice site and go to plasma oscillation [46-49]. In figure-04(d), the energy loss function sharpest peak is obtained at energy 26.8eV for the doped system, so plasma undergo oscillation at minimum value in contrast with pure system. It is also clear from equations (3& 4) that loss function

obtain maximum when imaginary part

of CDF is attaining a minimum value that is the major characteristics of the semiconductors. Same for reflectivity shown in figure-04(c,e), absorption and reflectance have converse relation. Wherever reflectance give the maximum value at the same point absorption and imaginary part of CDF give minimum value. For Mg-doped system the reflectance give sharp peak for low energy value in contrast to pure system but high energy value pure system have sharp peak with respect to doped system. The complex refractive index coefficient

has two parts, the refractive index

shown in figure-04(f).The refractive index

and extinction

for pure system is 2.49 and

doped system is 2.52. So refractive index increases after Mg-doping. The increasing of the refractive index after doping is the confirmation of the increases of band gap. By taking square root of CDF the refractive index is obtained. The extinction coefficient shows the absorption of light by the system. The graph of extinction coefficient shows that doped system has sharp peaks at low energy reveal that absorption is high for low energy for the doped system. Extinction coefficient has direct relation by the absorption. If

zero at given wavelengths then particle

doesnot absorb radiation at this wavelength. The above discussion shows that the optical properties are modified after Mg-doping in SrTiO3. 4. Conclusion The structural, electronic and optical properties of Pure and Mg-doped SrTiO3 were illustrated in detail by using DFT calculations. The structural properties are argued in context of Mg doping that shows the fine overall conformity with experimental and theoretical reported outcomes. The

electronic band structure of both pure and doped system was also explored. The band structures were explained by means of the concept of partial and full density of states. The band gap is increased and Fermi level remains toward valence band. The DOS shifts towards lesser energy and so the relations among Mg with its neighbor atoms are turned stronger. Generally, the partial density of state of Sr-4d state on the bottom of conduction band have main contribution to the Mg-doping as well as the top of valence band O-2p state have main contribution after Mg doping. The DOS is changed for doped structure shows that increased of band gap. The new gamma points appear in band structure after doping. For the optical properties, various properties i.e. complex dielectric function (polarization and absorption), energy loss function, reflectivity, complex refractive index (refractive index plus extinction coefficient) on behalf of both pure as well as doped systems are conversed in detail. Imaginary part of dielectric function shows positive value at 0ev after Mg doping. Red shift occurs into absorption edge after doping in SrTiO3 and refractive index is increases.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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