High energy photon induced Fermi-level shift of Ba0.5Sr0.5TiO3 thin films

Accepted Manuscript High energy photon induced Fermi-level shift of Ba0.5Sr0.5TiO3 thin films

Surendra Singh Barala, Vijendra Singh Bhati, Mahesh Kumar PII: DOI: Reference:

S0040-6090(17)30640-5 doi: 10.1016/j.tsf.2017.08.041 TSF 36183

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

19 June 2017 23 August 2017 23 August 2017

Please cite this article as: Surendra Singh Barala, Vijendra Singh Bhati, Mahesh Kumar , High energy photon induced Fermi-level shift of Ba0.5Sr0.5TiO3 thin films, Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.08.041

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT High energy photon induced Fermi-level shift of Ba0.5Sr0.5TiO3 thin films

Surendra Singh Barala

a, *

Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur-342011, India Defence Laboratory Jodhpur-342011, India

SC

RI

b

a

, Vijendra Singh Bhati and Mahesh Kumar

PT

a

a, b

*Corresponding Author

AC

CE

PT E

D

MA

NU

E-mail address: [email protected])

1

ACCEPTED MANUSCRIPT Abstract We have studied surface chemical states of Ba0.5Sr0.5TiO3 (BST) thin films as a function of high energy photon doses. BST thin films were deposited on Si substrates by Sputtering technique and post irradiations were carried out with high energy

60

Co gamma radiation.

Core-level and Fermi-level spectra were measured by X-ray photoelectron spectroscopy. The

PT

gamma-ray irradiated BST films showed a higher binding energy shift of Ba, Sr, and Ti core

RI

level with increasing gamma doses due to shift in Fermi level. The Fermi level is shifted

SC

towards conduction band by ~ 1.1 eV for 200 kGy gamma irradiated BST with respect to pristine. An increase in full width at half maximum of X-ray diffraction peak and surface

NU

roughness were also observed with increasing gamma doses. Higher leakage current and decrease in capacitance with gamma doses are further evidence of higher carrier

D

MA

concentration which is consistent with the shift in Fermi level.

PT E

Keywords: Barium strontium titanate; Fermi level; X-ray photoelectron spectroscopy;

AC

CE

Gamma irradiation; Leakage current

2

ACCEPTED MANUSCRIPT 1. Introduction Perovskite oxides such as BST have been demonstrated one of voltage tunable material for microwave-tunable applications [1]. Variable capacitors are widely used for tunable filters, phase shifter, and voltage controlled oscillators in microwave frequency applications. In addition, BST thin film based reconfigurable microwave components have

PT

been established with the capability to lower the weight, size, and power requirements of a

RI

next generation microwave communications and radar systems. High tunability of a BST

SC

varactor was observed in the frequency range from 1 MHz to 45 GHz which is comparable to the semiconductor analogy [2]. BST interdigitated capacitor with a tunability of 21% at 30V

NU

maximum bias voltage was reported by Yoon et al [3]. BST films usually have lower losses than PbZrTiO3 films and therefore favoured for higher-frequency applications. These BST

MA

based capacitor also reveal room temperature tuning ability and very small dispersion was observed in dielectric constant as a function of frequency [4]. The performance of tunable

PT E

and surface states.

D

devices governed not only by the film composition but also defects, design, strain, interface

The perovskite ferroelectric thin films are being used in electronic communication

CE

devices and have potential application in space and nuclear reactors. The operational strength of devices functioning in these harsh environments is main issue for circuit design. Therefore,

AC

these circuits could become principally important in space for radiation hardened device application. An enormous effort was involved to develop conventional silicon based radiation hard devices and hence similar effort may be required to accept the new devices for such applications [5]. Irradiation effects attracted in the past, when thin-film were in demand and the influence of irradiation on dielectric properties of Pb(Zr,Ti)O3 (PZT) films was investigated. A significant change in the polarization retention was observed which have been attributed to a modification of the space charge near the film surfaces by the irradiation3

ACCEPTED MANUSCRIPT induced charged defects [6, 7]. Some of these applications need the BST based devices to work under radiation environment and the devices will be exposed to a high flux of energetic charged and uncharged particles. The radiation environment mostly includes electrons, protons, neutrons and gamma (γ-) rays and hence radiation hardness is the important aspect to be studied in devices and materials science. The Si/SiO2 has been studied widely with good

PT

understanding of radiation induced defects and charge trapping mechanisms where gamma

RI

irradiation is acknowledged as a major concern to device failure [8, 9].

SC

The radiation effects involve the variety of changes in macroscopic and microscopic material property upon exposure to ionizing radiation. These effects are categorised as

NU

ionization effects and displacement effects [10]. The ionization effects are pertaining to the re-distribution of electrons within the solid while displacement effects related to the

MA

arrangement of atoms within a structure. High density of electron– hole pairs are generated by gamma radiation within the oxide and some of these electrons or holes can be trapped by

D

various defects in the bulk or at the interface. Ionising radiation induced changes were

PT E

observed in atomic configuration, and consequent change resulted in chemical and physical properties such as structure, electrical and optical properties of the material. A combination

CE

of electron–hole pair generation, interface state and charge trapping can describe the changes in the material due to irradiation [11]. The devices prepared of metal oxides and mixtures of

AC

two or more metal oxides have been found to be highly susceptible to gamma irradiation [12]. Dielectric properties of thin tantalum penta-oxide layers were degraded in terms of dielectric constant and leakage current after gamma irradiation with higher doses [13]. Among the various energetic particles, γ-rays are being considered a largely suitable source for defect investigation [14]. The results from gamma irradiation effects on metal oxides showed that the ionizing radiation causes structural defects which are mainly the oxygen vacancies in oxides [15]. However, there is presence of oxygen vacancies in every oxide in 4

ACCEPTED MANUSCRIPT form of Frenkel or Schottky defects and their concentration can be increased by gamma irradiation. Miao et al studied gamma irradiation changes of the oxide film which produces oxygen vacancies and oxygen ions, in turn affect I-V characteristics with consideration of the impact by the generated vacancies [16]. The dielectric properties of BST films were investigated under the low-energy electron beam irradiation and found to decrease after

PT

electron beam irradiation [17]. Glinsek et al also showed degraded dielectric permittivity of

RI

solution derived BST thin film after gamma and neutron irradiation [18]. Radiation induced

SC

accumulated damage, oxygen vacancy formation at the interface has been considered one of possible cause for high leakage current and degradation of BST capacitors. The energy level

NU

alignment at interfaces can be affected by the surface chemistry which controls charge carrier injection process.

MA

The chemical shifts are measured by x-ray Photoelectron Spectroscopy (XPS) and the core level themselves are significantly shifted by changes in the distribution of valence

D

electrons. Surface treatment for BaTiO3 and SrTiO3 thin films showed a change in position of

PT E

the oxide core levels and valence-band maximum by the treatment [19]. Surface modifications were also observed by XPS after reactive ion etching using SF6/Ar and

CE

CHF3/Ar plasma on BST thin films [20, 21]. An enhancement in binding energy of oxygen core level spectra was observed of Ag-doped BST which is consistent with high binding

AC

energy shift of Ag (Ag3d) core level. The high binding energy shift may be because of its possible surface relaxation effects, chemical shift, and Fermi-level shifts in the Ag- doped BST films, in turn gives higher leakage current due to increased electronic conduction [22]. Whereas, Wang et al presented XPS study in Ce-doped BST and demonstrated lower binding energy shift in core-level of BST, which shows acceptor nature of Ce doped BST film [23]. Lower leakage current in Ce-doped BST, exhibited an increased barrier to thermionic emission of electrons. Saravanan et al investigated the properties of (Ba0.5,Sr0.5)TiO3 thin 5

ACCEPTED MANUSCRIPT films using XPS and stated strong correlation between the process parameter, surface chemical states, microstructure, tunable dielectric properties of BST films [24]. The study revealed presence of three kinds of oxygen species i.e. dissociated oxygen ion O2− , lattice oxide ion O2− and adsorbed oxide ion O− for different oxygen mixing percentages on the BST film surface.

PT

Considerable progress has been carried out to understand the electronic structure at

RI

the surface of BST however effect of gamma irradiation is not explored much for such

SC

complex oxides. High energy radiation induced structural changes in device material are found to be associated with the change in the atomic concentration due to bond breaking and

NU

its possible reorganization after irradiation [25]. Therefore, it is expected that gamma treatment may affect the electronic structures of core-level and valence band maxima of BST

MA

films. In addition, electrical and optical properties are also correlated to the surface properties.

D

In the present study, we have investigated the electronic structure at the surface of γ-

PT E

ray irradiated BST thin films by XPS. Electrical and optical properties were investigated to evidence the correlation between microscopic changes to macroscopic properties due to

AC

2. Experimental

CE

gamma irradiation.

Ba0.5Sr0.5TiO3 thin films of 250 nm thickness were deposited on Si substrate at 6000C using RF magnetron sputtering technique. RF power, target to substrate distance, pressure and Ar flow were kept 70 W, 4.5 cm, 15 mTorr and 35 SCCM, respectively. BST films were treated under gamma radiation with

60

Co source at dose rate of 3 kGy/hr and exposed at

different gamma doses from 0 kGy to 200 kGy. XPS spectra were acquired on 0 kGy (pristine), 25 kGy and 200 kGy samples using Al Kα x-ray source (hυ = 1486.6 eV) with a 6

ACCEPTED MANUSCRIPT base pressure below 10-9 mbar. Shirley background was subtracted and a Voigt line-shape was used for the peaks to analyse the core-level spectra. Structural and surface morphology characterisations were carried out by X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively. Leakage current and capacitance voltage characteristics were measured on BST capacitor. Optical measurements were carried out by Perkin- Elmer Lambda

RI

PT

spectrophotometer by employing a dual-beam measurement method.

SC

3.0 Results and Discussion

Perovskite phase of BST thin films is confirmed by XRD and shown in Fig.1. All the

NU

peaks pertain to the BST phase, and appearance of a second phase was not observed in the gamma irradiated BST. A small change in FWHM of (110) peak was observed as 0.42°

MA

(Pristine), 0.47° (25 kGy) and 0.50° (200 kGy) with gamma irradiation. The increase in FWHM reflects decreasing crystallinity of BST thin films. The surface roughness of the films

D

is a key factor which can influence the electrical properties of devices. Fig. 2 shows AFM

PT E

surface morphology images of BST thin films before and after gamma irradiation with 3 µm × 3 µm scanning area. The surface roughness was found to increases with increasing gamma

CE

dose from 2.4 nm to 4.4 nm due the transfer of energy from gamma radiation to the film. A line profile is shown in Fig. 2 to emphasize the feature variation as a function of gamma dose.

AC

XPS analysis was carried out to identify the chemical bonding energies of constituting elements of the BST thin films. A typical XPS surface survey scan is shown in Fig. 3 of pristine BST thin film. Ba, Sr, Ti, O and elemental C were present in the surface of sputtered pristine thin film. C1s peak from adventitious carbon to 284.6 eV was considered as reference for binding energy scale to compensate of sample charging. Binding energies (BE) of Ba, Sr, Ti core levels of BST thin films were determined from the peak maxima of the spectra recorded by XPS. XPS narrow scan spectra were measured to determine the chemical states 7

ACCEPTED MANUSCRIPT of BST films after γ-irradiation at doses of 0 kGy, 25 kGy and 200 kGy and the photoemission core level spectra of Sr3d, Ti2p and Ba3d are shown in Fig. 4. The Ba3d line shape at all γ-ray doses can be described by two overlapping spin-orbit-split (SOS) pairs separated by 13.6 eV, similar to earlier reported data [26, 27]. The binding energy peaks at 778.1 eV (Ba 3d5/2) and 793.7 eV (Ba 3d3/2) of are assigned to Ba in perovskite phase as

PT

demonstrated in Fig.4 (c). The Sr3d spectra were fitted, which adequately represented by two

RI

overlapping Sr3d doublets (two Sr 3d SOS pairs), while a doublet splitting was observed 1.72

SC

eV as similar to average of doublet splitting from the NIST XPS database. The binding energy peaks 132.69 eV (Sr 3d5/2) and 134.41 eV (Sr 3d3/2) as shown in Fig.4(a) are

NU

represented as perovskite BST phase from the pristine sample. Ti2p(Ti4+) peaks are observed at 457.54 eV and 463.20 eV corresponding to 2p3/2 and 2p1/2 for pristine BST sample, shown

MA

in Fig.4(b). No shoulder was observed at lower binding energy side of Ti2p spectra, attributes Ti+3 species i.e. Ti3+ 2p3/2 (455.0 eV) and Ti3+ 2p1/2 (461.7 eV) , neither before nor after

D

gamma exposure.

PT E

Further, gamma-ray treated BST shows asymmetry towards higher energy as shown in Fig. 4 which indicates radiation induced changes [25], in near surface region of BST thin

CE

films, similar to HfO2, reported by Cheng et al [28]. Higher binding energy shifts of about 1.1 eV was observed for the all Sr, Ti and Ba atoms with increasing the dose of γ-ray irradiation.

AC

The higher binding energy shift in core level of Ba, Sr and Ti elements in gamma irradiated BST can be attributed to the variation in chemical environment of the elements or relaxation effect [26, 29]. There could be an involvement of the Fermi level (EF) shift apart from chemical and relaxation, since the original Fermi level of BST could be shifted because of radiation induced changes in the film after gamma irradiation. The shift in the Fermi level of gamma exposed BST film would contribute to the shift in BE, while undergoing an equilibration process with BST through space charge effect [30]. Therefore, it is important to 8

ACCEPTED MANUSCRIPT investigate the effect of gamma irradiation on the Fermi level EF, in addition to the analysis of the core level spectra. This has been studied through valence band edge spectra of the pristine and gamma treated BST as shown in Fig. 5. The energy of the valence band maxima is determined from the intercept of a linear fit. The top of the valence band is positioned at about 2.0 eV lower than the Fermi level for pristine BST thin film as shown in Fig. 5.

PT

The gamma ray treatment on BST film raised the EF level toward the conduction band

RI

by 0.4 eV and 1.1 eV for 25 kGy and 200 kGy doses respectively as compared to pristine

SC

BST films. Gautam et al also reported that ion irradiation study in niobium doped titanium dioxide which showed defect induced increase in conductivity [31]. These defects disrupt the

NU

orbital character by creating the density of such donor defects and causes EF level shift towards conduction band. Similarly the core levels of CH3NH3PbI3 perovskite thin film were

MA

observed to shift toward a higher binding energy after irradiation which reflects more n-type nature of film surface [32]. The shift in core level BE position is of Fermi level shift due to

D

irradiation rather than the usual chemical shift occur to any change in the chemical

PT E

environment. An increment in EF leads to a noticeable increase in binding energy for all core level peaks because EF is considered a reference for the scale of binding energy in XPS.

CE

The band gap of both as-deposited and irradiated films were estimated using the standard expression for direct transition (αhυ)2 = A (hυ−Eg). Fig. 6 shows an optical

AC

transmittance plot between hυ vs (αhυ)2 and band gap (Eg) was determined from an intercept on the energy axis hυ. The band-gap of the BST films was observed approximately 3.87 eV, which is similar to reported for the band-gap of Ba0.5Sr0.5TiO3 films [33]. A slight change of ~0.07 eV in bandgap was observed after irradiation of 200 kGy. This confirms increasing carrier concentration and hence repulsion between same types of charge carriers, which in turn reduces the band gap.

9

ACCEPTED MANUSCRIPT Kan et al showed that the ion beam irradiated STO crystal became metallic which indicates the radiation induced charge carriers in the conduction band [34]. Many theoretical and experimental efforts have been carried out to understand the role of oxygen vacancies in transport phenomena of perovskite oxide crystal and thin films, however research area is still active and lacks complete understanding [35]. Ohtomo et al observed vacancy induced

PT

shallow donor level leading to metallic behaviour of the epitaxial SrTiO3 films [36]. Various

RI

other studies suggested different pictures, including a shift of the Fermi level by creating

SC

oxygen vacancies which in turn give rise to additional donors in the conduction band [37]. While others suggested formation of impurity levels near the conduction band edge [38]. Ti

NU

3d empty states have also been observed in the formation of the oxygen vacancy [39, 40], to some degree or total localization of the electrons in cavity [41]. There is transfer of two

MA

electrons to the empty 3d states from the valence of the central O atom of the nearby neighbours Ti. Possibly, wave functions get overlap between these two electrons to a certain

D

extent. Various types of bonds be present in (Ba,Sr)TiO3 as for binary alkaline-earth oxides

PT E

and Ti-O bonding has a mixed ionic-covalent nature while (Ba,Sr)-O interactions are ionic. This gives a competition between the electrons localization on the 3d levels of the transition

CE

metal ion or electron trapping from the missing oxygen in the vacancy. In the present study, it is envisaged that gamma irradiation induced oxygen are responsible for the high-leakage

AC

current vacancies in the BST films. It is usually thought that oxygen vacancies are created by the following process [42]: O2- (oxygen ion at its normal site) =V0++ (oxygen vacancy) +2e-+1/2 O2 Mitra et. al. carried out computational analysis of oxygen vacancy in perovskite oxides and found that doubly charged vacancy (V++) state is the most stable defect for STO [43].

10

ACCEPTED MANUSCRIPT Conductivity variation due to gamma irradiation is also important and leakage current characteristics of Ni/BST/Ni (MIM) capacitor were measured at room temperature as shown in Fig. 7. It can be seen that leakage current increases with gamma-ray doses [16], which also favour gamma-ray induced changes in the surface core level state of Ti, Ba and Sr. Dih et. al. have reported the role of oxygen in increasing the conductivity because of the associated

PT

electrons with the oxygen vacancies [44]. With experimental analysis of the present study, we

RI

propose that the oxygen vacancies formed during the gamma irradiation accumulated in the

SC

BST films are responsible for the high-leakage current. The oxygen vacancies are on the surface and finely separated from each other. There was no localized in-gap states are formed

NU

autonomously whereas two electrons contribute to the conduction band [45]. An electron cloud can form among the vacancy and its nearest neighbouring cation to maintain the charge

MA

neutrality [45, 46]. Therefore, under the gamma irradiation, it is much more likely that defect formation by oxygen deficiency will dominate, resulting n-type conductivity in the irradiated

D

films. An additional confirmation for the radiation induced effects BST toward the surface

PT E

could be attributed from measured capacitance–voltage (C–V) measurements of gamma irradiated BST. Fig. 8 show a decreasing capacitance in the C–V curves with increasing

CE

gamma dose because of the surface modification of the gamma treated BST films. The capacitance of the film is directly correlated to dielectric behaviour of the capacitor. There is

AC

a significant reduction in dielectric capacitance of gamma treated BST at all gamma doses. The observed reduction in capacitance has been ascribed by radiation induce polarization pinning. Upon increasing the gamma dose, the leakage current increases rapidly due to an increase in the electron concentration which consequently decreases the capacitance of the BST film.

11

ACCEPTED MANUSCRIPT 4.0 Conclusion The effects of gamma irradiation on the structural, optical and electrical properties of BST thin film have been addressed. A small change in the FWHM of the XRD peak was observed with gamma irradiation. The surface roughness was found to increase with increasing gamma dose. The band-gap of the BST films was determined around 3.87 eV and

PT

a small change in band gap was observed after gamma irradiation which is related with

RI

increasing carrier concentration. Gamma ray exposed surface layer of BST films exhibit an

SC

increase in the binding energy of core-level spectra of Ba, Sr and Ti atoms which are consistent to the Fermi level shift. These radiation induced variations are responsible for the

NU

increase of film conductivity in the gamma irradiated BST film. The donor-like defects are predominantly generated during irradiation, shifting EF upward and gamma irradiated BST

MA

will be more n-type in character.

D

Acknowledgements

PT E

The author acknowledges to Dr. S. R. Vadera, Director and Dr. D. Gopalani of Defence Laboratory Jodhpur for their support during the work. Authors are grateful to

CE

Material Research Centre, MNIT, Jaipur for providing XPS characterisation. The authors are thankful to Dr V. S. Choudhary of Defence Laboratory Jodhpur for optical characterisation.

AC

The authors also acknowledge the financial support from Indian National Science Academy (INSA) Project No. SP/YSP/114/2015/1093.

12

ACCEPTED MANUSCRIPT References 1. G. Subramanyam, M. W. Cole, N. X. Sun, T. S. Kalkur, N. M. Sbrockey, G. S. Tompa, X. Guo, C. Chen, S. P. Alpay, G. A. J. Rossetti, K. Dayal, L. Q. Chen, D. I. G. Schlom, Challenges and opportunities for multi-functional oxide thin films for voltage tunable radio frequency/microwave components, J. Appl. Phys. 114 (2013)

PT

191301-35.

RI

2. A. Vorobiev, P. Rundquist, K. Khamchane, S. Gevorgian,Silicon substrate integrated

SC

high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave applications, Appl. Phy. Lett.. 83(2003) 3144-3146.

NU

3. Y. K. Yoon, D. Kim, M. G. Allen, J. S. Kenney, A reduced intermodulation distortion tunable ferroelectric capacitor: architecture and demonstration IEEE Transactions on

MA

Microw. Theor. Techno.. 51(2003) 2568 - 2576.

4. L. S. Demenicis, L. F. M. Conrado, W. Margulis, M. C. R. Carvalho, Transmission-

D

line transformers in multilayered high-dielectric-constant thin-film structures,

PT E

Microw. Optic. Techno. Lett. 47(2005) 290-293. 5. P.S. Winokur, H.E. Boesch, J.M. McGarrity, F.B. McLean, Field and time-dependent

CE

radiation effects at the SiO2/Si interface of hardened MOS capacitors, IEEE Trans. Nucl. Sci. 24 (1977) 2113-2118.

AC

6. J. M. Benedetto, R. A. Moore, F. B. Mclean, P. S. Brody, S. K. Dey, The effect of ionizing-radiation on sol-gel ferroelectric PZT capacitors, IEEE Trans. Nucl. Sci. 37(1990) 1713–1717. 7. S. C. Lee, G. Teowee, R. D. Schrimpf, D. P. Birnie, D. R. Uhlmann, K. F. Galloway, Total-dose radiation effects on solgel derived PZT thin-films, IEEE Trans. Nucl. Sci. 39(1992) 2036–2043.

13

ACCEPTED MANUSCRIPT 8. M. C. Busch, A. Slaoui, P. Siffert, E. Dooryhee, M. Toulemonde, Structural and electrical damage induced by high‐energy heavy ions in SiO2/Si structures, J. Appl. Phys. 71(1995) 2596-2601. 9. E. F. da Silva, Y. Nishioka, T. P. Ma, Two distinct interface trap peaks in radiation‐damaged metal/SiO2/Si structures, Appl. Phys. Lett. 51(1987) 270-272.

PT

10. V. A. Balakin, A. I. Dedik, S. F. Karmanenko, The influence if electron beams on

RI

dielectric properties of ferroelectric BSTO films, Pisma JTF 29(2003) 77-83.

SC

11. A. Indluru , K.E. Holbert, T.L. Alford, Gamma radiation effects on indium-zinc oxide thin-film transistors, Thin Sol. Film. 539(2013) 342-344.

NU

12. M. Mohil, G. A. Kumar, Gamma Radiation Induced Effects in TeO2 Thin Films, J. Nano Electron. Phys. 5(2013) 02018, 1-3.

MA

13. E. Atanassova, A. Paskaleva, R. Konakova, D. Spassov, V. F. Mitin, Influence of gamma-radiation on thin Ta2O5-Si structures, J. Microelec. 32(2001) 553–562.

D

14. S. Karatas, A. Turut, S. Altındal, Effects of

60

Co γ-ray irradiation on the electrical

PT E

characteristics of Au/n-GaAs (MS) structures, Nucl. Instrum. Meth. A 555(2005) 260265.

CE

15. N. Tugluoglu, Co-60 gamma-ray irradiation effects on the interface traps density of tin oxide films of different thicknesses on n-type Si (111) Substrates, Nucl. Instrum.

AC

Meth. B 254(2007) 118-124. 16. B. Miao, R. Mahapatra, R. Jenkins, J. Silvie, N. G. Wright, A. B. Horsfall, Radiation induced change in defect density in HfO2-Based MIM capacitors, IEEE Trans. Nucl. Sci. 56(2009) 2916–2924. 17. V. V. Buniatyan, V. M. Tsakanov, N. W. Martirosyan, G. S. Melikyan, H. R. Dashtoyan, Dielectric Characteristics of Pt/BaxSr1-xTiO3/Pt Thin Film Structure under the Electron Beam Irradiation, Armeni. J. Phys. 9(2016) 138-146. 14

ACCEPTED MANUSCRIPT 18. S. Glinšek, T. Peˇcnik, V. Cindro, B. Kmet, B. Rožiˇc, B. Maliˇc, Role of the microstructure in the neutron and gamma-ray irradiation stability of solution-derived Ba0.5Sr0.5TiO3 thin films, Acta. Mater. 88(2015) 34–40. 19. F. Amy, A. Wan, A. Kahn, F. J. Walker, R. A. McKee, Surface and interface chemical composition of thin epitaxial SrTiO3 and BaTiO3 films: Photoemission

PT

investigation,J. Appl. Phys. 96(2004) 1601-1606.

RI

20. L. P. Dai et al., XPS Study on Barium Strontium Titanate (BST) Thin Films Etching

SC

in SF6/Ar Plasma, Adv. Mater. Resea. 415(2012) 1964-1968.

21. D. LiPing, W. ShuYa, S. Ping, Z. ZhiQin, W. Gang, Z. GuoJun, Etching mechanism

NU

of barium strontium titanate (BST) thin films in CHF3/Ar plasma, Chinese Sci. Bull. 56(2011) 2267-2271.

MA

22. K. P. Jayadevan, Chi-Yi Liu, T. Y. Tsengw, Surface Chemical and Leakage Current Density Characteristics of Nanocrystalline Ag–Ba0.5Sr0.5TiO3 Thin Films, J. Am.

D

Ceram. Soc. 88(2005) 2456–2460.

PT E

23. S. Y. Wang, B. L. Cheng, C. Wang, S. A. T. Redfern, S. Y. Dai, K. J. Jin, H. B. Lu, Y. L. Zhou, Z. H. Chen G. Z. Yang, Influence of Ce doping on leakage current in

CE

Ba0.5Sr0.5TiO3 films, J. Phys. D: Appl. Phys. 38(2005) 2253–2257. 24. K. V. Saravanan, K. C. James Raju, Understanding the influence of surface chemical

AC

states on the dielectric tunability of sputtered Ba0.5Sr0.5TiO3 thin films, Mater. Res. Express 1(2014) 015706. 25. V. N. Rai, B. N. Raj Shekhar, S. Kher, S. K. Deb, Effect of gamma ray irradiation on optical properties of Nd doped phosphate glass, J. Luminesc. 130(2010) 582-586. 26. V. Craciun, R. K. Singh, Characteristics of the surface layer of barium strontium titanate thin films deposited by laser ablation, Appl. Phys. Lett. 76(2000) 1932-1934.

15

ACCEPTED MANUSCRIPT 27. J. D. Baniecki, M. Ishii, T. Shioga, K. Kurihara, S. Miyahara, Surface core-level shifts of strontium observed in photoemission of barium strontium titanate thin films, Appl. Phys. Lett. 89(2006) 162908-3. 28. Y. Cheng, M. Ding, X. Wu, X. Liu, K. Wu, Irradiation effect of HfO2 MOS structure under gamma-ray, IEEE International Conference on Solid Dielectrics, Bologna,

PT

Italy, 2013, 764-767.

RI

29. X. L. Li, B. Chen, H. Y. Jing, H. B. Lu, B. R. Zhao, Z. H. Mai, Experimental

SC

evidence of the “dead layer” at Pt∕BaTiO3Pt∕BaTiO3 interface, Appl. Phys. Lett. 87(2005) 222905-3.

NU

30. N. Halder, A. D. Sharma, S. K. Khan, A. Sen, H. S. Maiti, Effect of silver addition on the dielectric properties of barium titanate based low temperature processed

MA

capacitors, Mater. Resea. Bul. 34(1999) 545-550.

31. S. K. Gautam, A. Das, S. Ojha, D. K. Shukla, D. M. Phase, F. Singh, Electronic

D

structure modification and Fermi level shifting in Niobium doped anatase Titanium

PT E

dioxide thin films: A comparative study of NEXAFS, work function and stiffening of phonons, Phys. Chem. Chem. Phys. 18(2016) 3618-3627. Xu, C. Wang, B. Ecker, J. Yang, J. Huang, Y. Gao,

Light-Induced

CE

32. Y. Li , X.

Degradation of CH3NH3PbI3 Hybrid Perovskite Thin Film, J. Phys. Chem.

AC

C 121(2017) 3904–3910. 33. Z. Xu, M. Suzuki, S. Yokoyama, Structure and optical band-gap energies of Ba0:5Sr0:5TiO3 thin films fabricated by RF magnetron plasma sputtering, Japan. J. Appl. Phys. 44(2005) 8507–8511. 34. D. Kan, T. Terashima, R. Kanda, A. Masuno, K. Tanaka, S. Chu, H. Kan, A. Ishizumi, Y. Kanemitsu, Y. Shimakawa M. Takano, Blue-light emission at room temperature from Ar+-irradiated SrTiO3, Nature mater. 4(2005) 816-819. 16

ACCEPTED MANUSCRIPT 35. A. Kalabukhov, R. Gunnarsson, .J. Börjesson, E. Olsson, T. Claeson, D. Winkler, Effect of oxygen vacancies in the SrTiO3 substrate on the electrical properties of the LaAlO3∕SrTiO3 interface, Phy. Rev. B 75(2007) 121404(R). 36. A. Ohtomo, H. Y. Hwang, Growth mode control of the free carrier density in SrTiO3-δ films, J. Appl. Phys. 102(2007) 083704-6.

PT

37. S. Kimura, J. Yamauchi, M. Tsukada, S. Watanabe, First-principles study on

RI

electronic structure of the (001) surface of SrTiO3, Phys. Rev. B 51(2005) 11049.

SC

38. M. O. Selme, P. Pecheur, A tight-binding model of the oxygen-vacancy in SrTiO3, J. Phys. C: : Solid State Phys. 16(1983) 2559-2568.

NU

39. H. Park, D. J. Chadi, Microscopic study of oxygen-vacancy defects in ferroelectric perovskites, Phys. Rev. B 57(1998) R13961.

MA

40. H. Donnerberg, A. Birkholz, Ab initio study of oxygen vacancies in BaTiO3, J. Phys.: Condens. Matter 12(2000) 8239-8247.

D

41. A. Stashans, F. Vargas, Periodic LUC study of F centers in cubic and tetragonal

PT E

SrTiO3, Mater. Lett. 50(2001) 145-148. 42. M. Shen, Z. Dong, Z. Gan, S. Ge, Oxygen-related dielectric relaxation and leakage

CE

characteristics of Pt/(Ba,Sr)TiO3/PtPt/(Ba,Sr)TiO3/Pt thin-film capacitors, Appl. Phys. Lett. 80(2002) 2538-2540.

AC

43. C. Mitra, C. Lin, J. Robertson, A. A. Demkov,

Electronic structure of oxygen

vacancies in SrTiO3 and LaAlO3,. Phy. Rev. B 86(2012) 155105. 44. J. J. Dih, R. M. Fulrath, Electrical Conductivity in Lead Zirconate-Titanate Ceramics, J. Am. Ceram. Soc. 61(1978) 448-451. 45. H. O. Jeschke, J. Shen, R. Valentí, Localized versus itinerant states created by multiple oxygen vacancies in SrTiO3,New J. Phys. 17(2015) 023034.

17

ACCEPTED MANUSCRIPT 46. C. Lin, A. A. Demkov, Electron Correlation in Oxygen Vacancy in SrTiO3, Phys.

AC

CE

PT E

D

MA

NU

SC

RI

PT

Rev. Lett. 111(2013) 217601.

18

ACCEPTED MANUSCRIPT Figure Caption

Fig. 1 XRD 2θ-ω scan of gamma irradiated BST thin films Fig. 2 AFM images and line profile of BST thin film (a) pristine, (b) 25 kGy and (c) 200 kGy.

PT

Fig. 3 XPS wide scan spectra of pristine BST thin film. O1s and C1s spectra presented with

RI

Ba3d, Ti2p and Sr2p. XPS narrow scan of C1s is shown in inset.

SC

Fig. 4 XPS core level spectra of gamma irradiated BST thin film from 0 kGy(Pristine) to 200

NU

kGy (a) Sr3d (3d5/2 and 3d3/2), (b) Ti 2p1/2 and 2p3/2 (c) Ba3d (3d5/2 and 3d3/2).

MA

Fig. 5 Valence Band Spectra of BST thin film from 0 kGy(Pristine) to 200 kGy and

D

schematic representation of Fermi level shift

PT E

Fig.6 Optical bang gap of BST deposited on quartz substrate at different gamma doses.

CE

Fig. 7 Leakage current of Ni/BST/Ni devices at 0kGy, 25kGy and 200 kGy.

AC

Fig. 8 Capacitance –Voltage measurement on Ni/BST/Ni devices at 0, 25 and 200 kGy.

19

Fig. 1

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

20

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

Fig. 2

21

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

Fig. 3

22

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

Fig. 4

23

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 5

24

AC

Fig. 6

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

25

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

Fig. 7

26

AC

Fig. 8

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

27

ACCEPTED MANUSCRIPT Highlights  Ba0.5Sr0.5TiO3 (BST) thin films were deposited on Si substrates by sputtering technique.

PT

 BST samples were irradiated with high energy 60Co gamma radiation.  Higher binding energy shift of core level observed in gamma irradiated

RI

BST.

SC

 1.1 eV shift of the Fermi level towards the conduction band for 200 kGy

NU

irradiation.

AC

CE

PT E

D

MA

 Increase in leakage current was observed with higher gamma doses.

28