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200 MeV Ag15+ ion induced surface modification and transport behaviour in manganite based thin film devices
Applied Surface Science 258 (2012) 4203–4206
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
200 MeV Ag15+ ion induced surface modification and transport behaviour in manganite based thin film devices Ashish Ravalia a , Megha Vagadia a , P.S. Vachhani a,1 , R.J. Choudhary b , D.M. Phase b , K. Asokan c , D.G. Kuberkar a,∗ a
Department of Physics, Saurashtra University, Rajkot 360 005, India UGC-DAE CSR, Indore 452 017, India c Inter University, Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India b
a r t i c l e
i n f o
Article history: Available online 23 June 2011 Keywords: SHI irradiation Surface morphology Manganite Devices
a b s t r a c t We report the effect of 200 MeV Ag15+ ion irradiation with fluence of ∼5 × 1011 ions/cm2 on the structure and surface morphology of thin film devices of La0.6 Pr0.2 Sr0.2 MnO3 (named as LPSMO) (p)/SrNb0.2 Ti0.8 O3 (named as SNTO) (n) manganite grown by Pulsed Laser Deposition (PLD) technique. The n-type manganite layers studied were of thickness 50, 100 and 200 nm. After ion irradiation, the structural studies revealed improved crystallinity in the LPSMO/SNTO devices with the thickness of 50 and 200 nm. But the 100 nm films showed the formation of regular tracks like feature in their surface morphology after irradiation. All the irradiated films show suppression of resistivity at all the temperatures but it was different rate in 100 nm LPSMO films which may be attributed to the formation of linear track like defects. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Divalent cation (hole)-doped manganites with the general stoichiometry (La1−x Mx )MnO3 (M = Ca, Sr, Ba) have attracted the attention of many researchers due to their property of colossal magnetoresistance (CMR) phenomenon. These materials exhibit rich phase diagram involving metal–insulator (M–I) transition and ferromagnetic–paramagnetic (FM–PM) transitions [1]. Mixed valent manganites fascinates due to the basic phenomena underlying the CMR behaviour and their potential applications. Varieties of devices based on thin films of these materials on different substrates have been studied [2,3]. Swift heavy ion (SHI) is becoming an important tool to create defects in controlled manner and localize strain in various materials. Irradiation results in the structural and morphological modifications which in turn are reflected in the surface and physical properties [4–8]. It has been reported that the electronic transport and magnetic properties of the La0.5 Pr0.2 Sr0.3 MnO3 thin films are extremely sensitive to strain at the interface of the film and substrate [9]. Our previous studies have shown that irradiation results in the suppression in resistivity of La0.5 Pr0.2 Sr0.3 MnO3 /SrTiO3 thin film [10]. Present study investigates the effect of 200 MeV Ag15+ in irradiation on the structural, microstructural and transport behaviour of LPSMO films grown
∗ Corresponding author. Tel.: +91 281 2588428; fax: +91 281 2576347. E-mail address: [email protected] (D.G. Kuberkar). 1 School of Physics, University of Hyderabad, Hyderabad 500 046, India. 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.079
on n-type semiconducting Nb-SrTiO3 substrate for device applications. We have shown the thickness dependent irradiation effects on La0.6 Pr0.2 Sr0.2 MnO3 /SrNb0.2 Ti0.8 O3 thin film devices with special reference to their structure, surface morphology and resistivity under applied field. 2. Experimental Polycrystalline bulk target of La0.6 Pr0.2 Sr0.2 MnO3 (LPSMO) target was synthesized using conventional solid-state reaction method reported earlier [11]. Thin films of p-type LPSMO with various thicknesses were deposited on n-type SrNb0.2 Ti0.8 O3 (SNTO) substrate using pulsed laser deposition technique (PLD). 248 nm KrF laser having energy density of 2 J/cm2 at 10 Hz repetition rate was used for the ablation. The ejected plum was deposited on to the polished SNTO (1 0 0) single crystal substrates by keeping a distance 5.5 cm from the target. The deposition was carried out at the substrate temperature of ∼700 ◦ C in oxygen ambient at partial pressure ∼400 mTorr. The growth rate of p-type layer on SNTO substrate was ∼0.28 nm/s. By keeping the deposition time of 3 min, 6 min and 12 min, LPSMO films with 50,100 and 200 nm thicknesses were deposited. The 15 UD Tandem Accelerator, at Inter University Accelerator Centre (IUAC), New Delhi was used for irradiating the LPSMO/SNTO devices at 200 MeV Ag 15+ ions with fluence of 5 × 1011 ions/cm2 . Irradiation was performed at a low angle with respect to the ion beam to avoid the channeling effect. The ion beam was focused on to a spot of ∼1 mm diameter on the sample and
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Fig. 1. XRD patterns of pristine and irradiated LPSMO/SNTO thin films.
was scanned over a 10 mm × 10 mm area using a magnetic scanner in order to ensure that irradiation on the bi-layered device is uniform. The devices under study were characterized for structural phase and purity by using X-ray diffraction (XRD) and the surface morphology by Atomic field microscopy (AFM). –T measurements were carried out using standard dc four-probe current in plane (CIP) geometry with and without applied magnetic field.
3. Results and discussion The XRD pattern of pristine and irradiated LPSMO/SNTO devices were indexed using orthorhombic cell parameters of bulk compound. Fig. 1 shows that, the growth of LPSMO/SNTO is oriented in (h 0 0) direction. Figure shows, (expanded scale) the indexed (2 0 0) peak of 50, 100 and 200 nm pristine and irradiated LPSMO/SNTO
Fig. 2. 2D and 3D (inset) images of 50 nm (a) pristine (b) irradiated,100 nm (c) pristine (d)irradiated and 200 nm (e) pristine (f) irradiated LPSMO/SNTO thin films. Enlarged view [(g), (h), (i) and (j)] shows modified portion of irradiated film.
A. Ravalia et al. / Applied Surface Science 258 (2012) 4203–4206
4205
Fig. 3. Resistivity versus temperature plots of the LPSMO/SNTO pristine and irradiated thin films.
devices. As evident from the figure, after irradiation, the peak intensity increased almost double in 50 nm film and six times in 200 nm, while in 100 nm film, it reduced ∼37%. All the films possess single crystalline nature with lattice mismatch between film and substrate leading to structural strain ı which can be quantified using the relation, ı(%) = [(dsubstrate − dthin film )/dsubstrate ] × 100. Positive values of ı correspond to tensile strain while negative values correspond to compressive strain. In the case of pristine 50, 100 and 200 nm films, strain values were respectively ∼0.243%, ∼0.241% and ∼0.23%. This suggests the reduction in strain with film thickness [9]. After irradiation, strain becomes ∼0.241%, 0.23% and 0.243% for 50, 100 and 200 nm films respectively. This implies that after irradiation, strain is reduced in 50 and 100 nm films but increased in 200 nm films. This is confirmed by the sharpness of XRD peak and reduction in FWHM 200 nm films irradiated with 5 × 1011 ions/cm2 ions. Previous studies have shown the formation of track like defect in manganites [9]. Based on this, 200 MeV Ag15+ ions were used for the modifications of various properties of LPSMO/SNTO. Using SRIM2008 calculation [12], the calculated value of electronic energy loss (Se ) and nuclear energy loss (Sn ) were Se ∼ 13.61 KeV/nm; Sn ∼ 37.78 eV/nm in LPSMO/SNTO and the projected range of Ag15+ ions was ∼23.07 m which is very high as compared to maximum film thickness ∼200 nm, ensuring that the bombarded ions pass through the film completely and finally get implanted in the substrate
Fig. 2 shows the 2D AFM micrographs (3D inset) of 50, 100 and 200 nm pristine and irradiated devices. As seen in the images, all pristine films exhibit island like grain growth. After irradiation, track like defects are formed on surface of 50 and 200 nm films while 100 nm film shows regular linear tracks of defects. The surface modifications can be clearly seen in Fig. 2, due to the Ag15+ ion irradiation. Inset in Fig. 2 shows the enlarged view of the defects having average diameter ∼150 nm. The size of the defects created is much larger than the reported columnar track like defects. In 100 nm film, small spike like outgrowth can be seen near the defects, possibly arising due to energetic ions (∼200 MeV) striking the surface producing spike like structures. RMS surface roughness of 50 and 200 nm films increases on irradiation while decreases in 100 nm film, which may be attributed to the regular track like defect formation. The temperature dependence of resistivity (), of pristine and irradiated LPSMO/SNTO (50, 100 and 200 nm) films under 0, 1, 5 and 8 T are shown in Fig. 3. In 50 nm pristine LPSMO/SNTO device, the resistivity increases with applied field at all temperatures while in 100 nm, it decreases below ∼218 K after which it increases under the applied field. In 200 nm pristine film, the decrease in resistivity with field is observed below ∼106 K, above which, it increases upto RT. This behaviour of –T under field, is understood in the light of thickness dependent modifications in the resistivity measured in current in plane (CIP) mode. In 50 nm LPSMO/SNTO, the contribution of n-type SNTO substrate to the
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resistivity is more prominent as compared to the 100 and 200 nm films wherein, the nature of LPSMO manganite has major contribution to resistivity. Earlier studies reported the observation of some interesting effects of 200 MeV Ag-ion irradiation on the modification in the structural and magnetotransport in La0.7 Ba0.3 MnO3 (LBMO) and La0.5 Pr0.2 Sr0.3 MnO3 (LPSMO) manganite film, with the ion fluence of 5 × 1011 ions/cm2 [7,10]. In the present work, all the LPSMO/SNTO devices irradiated with 5 × 1011 ions/cm2 ion fluence of 200 MeV Ag15+ ion also exhibit suppression in resistivity at all the temperatures under increasing field which is similar to earlier studies [10]. The rate of suppression in resistivity in 100 nm LPSMO/SNTO device is comparatively less when compared to 50 and 200 nm films due to the formation of regular track like defect structure after irradiation of fluence of 5 × 1011 ions/cm2 . It is proposed that, these regular spikuler structure introduce scattering centres which suppresses the spin polarized tunneling (SPT) of electron at low temperature, thereby improving, slightly the conductivity in 100 nm film. In addition, it is observed that, the insulator–metal transition temperature (TIM ) decreases in 100 nm LPSMO/SNTO device irradiated with 5 × 1011 ions/cm2 200 MeV Ag-ions (Fig. 3) which can be ascribed to the increase in defect concentration in the irradiated film [10]. 4. Conclusion In summary, we observed the thickness dependent effects in LPSMO/SNTO devices as a result of 200 MeV Ag15+ ion irradiation with fluence of 5 × 1011 ions/cm2 on their structural, surface morphological and resistivity behaviours. The difference in the
transport behaviour of 50 and 200 nm films, and 100 nm LPSMO films after irradiation may be attributed to the formation of regular track like defects in 100 nm films. Acknowledgements Authors are thankful to Inter University Accelerator Centre, New Delhi for research project (UFR-44316) and SHI irradiation experiment. The experimental help by Dr. R. Rawat, Dr. V.R. Reddy from UGC DAE-CSR, Indore and Mrs. Indra, Mr. Jayprakash from IUAC, New Delhi are gratefully acknowledged. References [1] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [2] C. Mitra, P. Raychaudari, G. Kobernik, K. Dorr, K.H. Muller, L. Schultz, R. Pinto, Appl. Phys. Lett. 79 (2001) 2408. [3] K.-J. Jin, H.-B. Lu, Q.-L. Zhao, B.-L. Cheng, Z.-H. Chen, Y.-L. Zhou, G.-Z. Yang, Phys. Rev. B 71 (2005) 184428. [4] S.B. Ogale, Y.F H. Li, M. Rajeswari, L. Salamanca Riba, R. Ramesh, T. Venkatesan, A.J. Mills, R. Kumar, G.K. Mehta, R. Bathe, S.I. Patil, J. Appl. Phys. 87 (2000) 4210. [5] R.J. Choudary, R. Kumar, S.I. Patil, S. Husain, J.P. Srivastava, S.K. Malik, Appl. Phys. Lett. 86 (2005) 3846. [6] R.N. Parmar, J.H. Markna, R. Kumar, D.S. Rana, V.C. Bagve, S.K. Malik, D.G. Kuberkar, Appl. Phys. Lett. 8 (2008) 4146. [7] J.H. Markna, R.N. Parmar, D.G. Kuberkar, R. Kumar, D.S. Rana, S.K. Malik, Appl. Phys. Lett. 88 (2006) 152503. [8] A.V. Krasheninnikov, K. Nardlund, J. Appl. Phys. Rev. 107 (2010) 071301. [9] J.Z. Sun, D.W. Abraham, R.A. Rao, C.B. Eom, Appl. Phys. Lett. 74 (1999) 2231. [10] J.H. Markna, R.N. Parmar, D.S. Rana, R. Kumar, P. Misra, L.M. Kukareja, D.G. Kuberkar, S.K. Malik, NIM B 256 (2005) 693. [11] D.S. Rana, C.M. Thaker, K.R. Mavani, D.G. Kuberkar, S.K. Malik, Solid State Commun. 133 (2005) 505. [12] J.F. Ziegler, J.P. Biersack, M.D. Ziegler, Program SRIM, http://www.srim.org.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
200 MeV Ag15+ ion induced surface modification and transport behaviour in manganite based thin film devices Ashish Ravalia a , Megha Vagadia a , P.S. Vachhani a,1 , R.J. Choudhary b , D.M. Phase b , K. Asokan c , D.G. Kuberkar a,∗ a
Department of Physics, Saurashtra University, Rajkot 360 005, India UGC-DAE CSR, Indore 452 017, India c Inter University, Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India b
a r t i c l e
i n f o
Article history: Available online 23 June 2011 Keywords: SHI irradiation Surface morphology Manganite Devices
a b s t r a c t We report the effect of 200 MeV Ag15+ ion irradiation with fluence of ∼5 × 1011 ions/cm2 on the structure and surface morphology of thin film devices of La0.6 Pr0.2 Sr0.2 MnO3 (named as LPSMO) (p)/SrNb0.2 Ti0.8 O3 (named as SNTO) (n) manganite grown by Pulsed Laser Deposition (PLD) technique. The n-type manganite layers studied were of thickness 50, 100 and 200 nm. After ion irradiation, the structural studies revealed improved crystallinity in the LPSMO/SNTO devices with the thickness of 50 and 200 nm. But the 100 nm films showed the formation of regular tracks like feature in their surface morphology after irradiation. All the irradiated films show suppression of resistivity at all the temperatures but it was different rate in 100 nm LPSMO films which may be attributed to the formation of linear track like defects. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Divalent cation (hole)-doped manganites with the general stoichiometry (La1−x Mx )MnO3 (M = Ca, Sr, Ba) have attracted the attention of many researchers due to their property of colossal magnetoresistance (CMR) phenomenon. These materials exhibit rich phase diagram involving metal–insulator (M–I) transition and ferromagnetic–paramagnetic (FM–PM) transitions [1]. Mixed valent manganites fascinates due to the basic phenomena underlying the CMR behaviour and their potential applications. Varieties of devices based on thin films of these materials on different substrates have been studied [2,3]. Swift heavy ion (SHI) is becoming an important tool to create defects in controlled manner and localize strain in various materials. Irradiation results in the structural and morphological modifications which in turn are reflected in the surface and physical properties [4–8]. It has been reported that the electronic transport and magnetic properties of the La0.5 Pr0.2 Sr0.3 MnO3 thin films are extremely sensitive to strain at the interface of the film and substrate [9]. Our previous studies have shown that irradiation results in the suppression in resistivity of La0.5 Pr0.2 Sr0.3 MnO3 /SrTiO3 thin film [10]. Present study investigates the effect of 200 MeV Ag15+ in irradiation on the structural, microstructural and transport behaviour of LPSMO films grown
∗ Corresponding author. Tel.: +91 281 2588428; fax: +91 281 2576347. E-mail address: [email protected] (D.G. Kuberkar). 1 School of Physics, University of Hyderabad, Hyderabad 500 046, India. 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.079
on n-type semiconducting Nb-SrTiO3 substrate for device applications. We have shown the thickness dependent irradiation effects on La0.6 Pr0.2 Sr0.2 MnO3 /SrNb0.2 Ti0.8 O3 thin film devices with special reference to their structure, surface morphology and resistivity under applied field. 2. Experimental Polycrystalline bulk target of La0.6 Pr0.2 Sr0.2 MnO3 (LPSMO) target was synthesized using conventional solid-state reaction method reported earlier [11]. Thin films of p-type LPSMO with various thicknesses were deposited on n-type SrNb0.2 Ti0.8 O3 (SNTO) substrate using pulsed laser deposition technique (PLD). 248 nm KrF laser having energy density of 2 J/cm2 at 10 Hz repetition rate was used for the ablation. The ejected plum was deposited on to the polished SNTO (1 0 0) single crystal substrates by keeping a distance 5.5 cm from the target. The deposition was carried out at the substrate temperature of ∼700 ◦ C in oxygen ambient at partial pressure ∼400 mTorr. The growth rate of p-type layer on SNTO substrate was ∼0.28 nm/s. By keeping the deposition time of 3 min, 6 min and 12 min, LPSMO films with 50,100 and 200 nm thicknesses were deposited. The 15 UD Tandem Accelerator, at Inter University Accelerator Centre (IUAC), New Delhi was used for irradiating the LPSMO/SNTO devices at 200 MeV Ag 15+ ions with fluence of 5 × 1011 ions/cm2 . Irradiation was performed at a low angle with respect to the ion beam to avoid the channeling effect. The ion beam was focused on to a spot of ∼1 mm diameter on the sample and
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Fig. 1. XRD patterns of pristine and irradiated LPSMO/SNTO thin films.
was scanned over a 10 mm × 10 mm area using a magnetic scanner in order to ensure that irradiation on the bi-layered device is uniform. The devices under study were characterized for structural phase and purity by using X-ray diffraction (XRD) and the surface morphology by Atomic field microscopy (AFM). –T measurements were carried out using standard dc four-probe current in plane (CIP) geometry with and without applied magnetic field.
3. Results and discussion The XRD pattern of pristine and irradiated LPSMO/SNTO devices were indexed using orthorhombic cell parameters of bulk compound. Fig. 1 shows that, the growth of LPSMO/SNTO is oriented in (h 0 0) direction. Figure shows, (expanded scale) the indexed (2 0 0) peak of 50, 100 and 200 nm pristine and irradiated LPSMO/SNTO
Fig. 2. 2D and 3D (inset) images of 50 nm (a) pristine (b) irradiated,100 nm (c) pristine (d)irradiated and 200 nm (e) pristine (f) irradiated LPSMO/SNTO thin films. Enlarged view [(g), (h), (i) and (j)] shows modified portion of irradiated film.
A. Ravalia et al. / Applied Surface Science 258 (2012) 4203–4206
4205
Fig. 3. Resistivity versus temperature plots of the LPSMO/SNTO pristine and irradiated thin films.
devices. As evident from the figure, after irradiation, the peak intensity increased almost double in 50 nm film and six times in 200 nm, while in 100 nm film, it reduced ∼37%. All the films possess single crystalline nature with lattice mismatch between film and substrate leading to structural strain ı which can be quantified using the relation, ı(%) = [(dsubstrate − dthin film )/dsubstrate ] × 100. Positive values of ı correspond to tensile strain while negative values correspond to compressive strain. In the case of pristine 50, 100 and 200 nm films, strain values were respectively ∼0.243%, ∼0.241% and ∼0.23%. This suggests the reduction in strain with film thickness [9]. After irradiation, strain becomes ∼0.241%, 0.23% and 0.243% for 50, 100 and 200 nm films respectively. This implies that after irradiation, strain is reduced in 50 and 100 nm films but increased in 200 nm films. This is confirmed by the sharpness of XRD peak and reduction in FWHM 200 nm films irradiated with 5 × 1011 ions/cm2 ions. Previous studies have shown the formation of track like defect in manganites [9]. Based on this, 200 MeV Ag15+ ions were used for the modifications of various properties of LPSMO/SNTO. Using SRIM2008 calculation [12], the calculated value of electronic energy loss (Se ) and nuclear energy loss (Sn ) were Se ∼ 13.61 KeV/nm; Sn ∼ 37.78 eV/nm in LPSMO/SNTO and the projected range of Ag15+ ions was ∼23.07 m which is very high as compared to maximum film thickness ∼200 nm, ensuring that the bombarded ions pass through the film completely and finally get implanted in the substrate
Fig. 2 shows the 2D AFM micrographs (3D inset) of 50, 100 and 200 nm pristine and irradiated devices. As seen in the images, all pristine films exhibit island like grain growth. After irradiation, track like defects are formed on surface of 50 and 200 nm films while 100 nm film shows regular linear tracks of defects. The surface modifications can be clearly seen in Fig. 2, due to the Ag15+ ion irradiation. Inset in Fig. 2 shows the enlarged view of the defects having average diameter ∼150 nm. The size of the defects created is much larger than the reported columnar track like defects. In 100 nm film, small spike like outgrowth can be seen near the defects, possibly arising due to energetic ions (∼200 MeV) striking the surface producing spike like structures. RMS surface roughness of 50 and 200 nm films increases on irradiation while decreases in 100 nm film, which may be attributed to the regular track like defect formation. The temperature dependence of resistivity (), of pristine and irradiated LPSMO/SNTO (50, 100 and 200 nm) films under 0, 1, 5 and 8 T are shown in Fig. 3. In 50 nm pristine LPSMO/SNTO device, the resistivity increases with applied field at all temperatures while in 100 nm, it decreases below ∼218 K after which it increases under the applied field. In 200 nm pristine film, the decrease in resistivity with field is observed below ∼106 K, above which, it increases upto RT. This behaviour of –T under field, is understood in the light of thickness dependent modifications in the resistivity measured in current in plane (CIP) mode. In 50 nm LPSMO/SNTO, the contribution of n-type SNTO substrate to the
4206
A. Ravalia et al. / Applied Surface Science 258 (2012) 4203–4206
resistivity is more prominent as compared to the 100 and 200 nm films wherein, the nature of LPSMO manganite has major contribution to resistivity. Earlier studies reported the observation of some interesting effects of 200 MeV Ag-ion irradiation on the modification in the structural and magnetotransport in La0.7 Ba0.3 MnO3 (LBMO) and La0.5 Pr0.2 Sr0.3 MnO3 (LPSMO) manganite film, with the ion fluence of 5 × 1011 ions/cm2 [7,10]. In the present work, all the LPSMO/SNTO devices irradiated with 5 × 1011 ions/cm2 ion fluence of 200 MeV Ag15+ ion also exhibit suppression in resistivity at all the temperatures under increasing field which is similar to earlier studies [10]. The rate of suppression in resistivity in 100 nm LPSMO/SNTO device is comparatively less when compared to 50 and 200 nm films due to the formation of regular track like defect structure after irradiation of fluence of 5 × 1011 ions/cm2 . It is proposed that, these regular spikuler structure introduce scattering centres which suppresses the spin polarized tunneling (SPT) of electron at low temperature, thereby improving, slightly the conductivity in 100 nm film. In addition, it is observed that, the insulator–metal transition temperature (TIM ) decreases in 100 nm LPSMO/SNTO device irradiated with 5 × 1011 ions/cm2 200 MeV Ag-ions (Fig. 3) which can be ascribed to the increase in defect concentration in the irradiated film [10]. 4. Conclusion In summary, we observed the thickness dependent effects in LPSMO/SNTO devices as a result of 200 MeV Ag15+ ion irradiation with fluence of 5 × 1011 ions/cm2 on their structural, surface morphological and resistivity behaviours. The difference in the
transport behaviour of 50 and 200 nm films, and 100 nm LPSMO films after irradiation may be attributed to the formation of regular track like defects in 100 nm films. Acknowledgements Authors are thankful to Inter University Accelerator Centre, New Delhi for research project (UFR-44316) and SHI irradiation experiment. The experimental help by Dr. R. Rawat, Dr. V.R. Reddy from UGC DAE-CSR, Indore and Mrs. Indra, Mr. Jayprakash from IUAC, New Delhi are gratefully acknowledged. References [1] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [2] C. Mitra, P. Raychaudari, G. Kobernik, K. Dorr, K.H. Muller, L. Schultz, R. Pinto, Appl. Phys. Lett. 79 (2001) 2408. [3] K.-J. Jin, H.-B. Lu, Q.-L. Zhao, B.-L. Cheng, Z.-H. Chen, Y.-L. Zhou, G.-Z. Yang, Phys. Rev. B 71 (2005) 184428. [4] S.B. Ogale, Y.F H. Li, M. Rajeswari, L. Salamanca Riba, R. Ramesh, T. Venkatesan, A.J. Mills, R. Kumar, G.K. Mehta, R. Bathe, S.I. Patil, J. Appl. Phys. 87 (2000) 4210. [5] R.J. Choudary, R. Kumar, S.I. Patil, S. Husain, J.P. Srivastava, S.K. Malik, Appl. Phys. Lett. 86 (2005) 3846. [6] R.N. Parmar, J.H. Markna, R. Kumar, D.S. Rana, V.C. Bagve, S.K. Malik, D.G. Kuberkar, Appl. Phys. Lett. 8 (2008) 4146. [7] J.H. Markna, R.N. Parmar, D.G. Kuberkar, R. Kumar, D.S. Rana, S.K. Malik, Appl. Phys. Lett. 88 (2006) 152503. [8] A.V. Krasheninnikov, K. Nardlund, J. Appl. Phys. Rev. 107 (2010) 071301. [9] J.Z. Sun, D.W. Abraham, R.A. Rao, C.B. Eom, Appl. Phys. Lett. 74 (1999) 2231. [10] J.H. Markna, R.N. Parmar, D.S. Rana, R. Kumar, P. Misra, L.M. Kukareja, D.G. Kuberkar, S.K. Malik, NIM B 256 (2005) 693. [11] D.S. Rana, C.M. Thaker, K.R. Mavani, D.G. Kuberkar, S.K. Malik, Solid State Commun. 133 (2005) 505. [12] J.F. Ziegler, J.P. Biersack, M.D. Ziegler, Program SRIM, http://www.srim.org.