- Email: [email protected]
Multiferroic Bi0.7Dy0.3FeO3 thin films directly integrated on Si for integrated circuit compatible devices
Thin Solid Films 518 (2010) 5866–5870
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Multiferroic Bi0.7Dy0.3FeO3 thin films directly integrated on Si for integrated circuit compatible devices K. Prashanthi a,⁎, B.A. Chalke b, R.D. Bapat b, S.C. Purandare b, V.R. Palkar a a b
Centre for Nanoelectronics, Dept. of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India Dept. of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research, Mumbai 400005, India
a r t i c l e
i n f o
Available online 31 May 2010 Keywords: Thin films Multiferroics Pulse laser deposition Magnetic properties Ferroelectric properties
a b s t r a c t Magnetoelectric multiferroic Bi0.7Dy0.3FeO3 (BDFO) thin films deposited on p-type Si (100) substrate using pulsed laser deposition technique demonstrated a saturated ferroelectric and ferromagnetic hysteresis loop at room temperature. More interestingly, the observed change in electric polarization with applied magnetic field in these films indicated the presence of room temperature magnetoelectric coupling behavior. Using high-frequency capacitance–voltage measurements, the fixed oxide charge density, interface trap density and dielectric constant were estimated on Au/BDFO/Si capacitors. These results suggest the integrated circuit compatible application potential of BDFO films in the field of micro-electro-mechanical systems and nonvolatile memories. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnetoelectric multiferroics are the materials which simultaneously exhibit magnetic and ferroelectric ordering in the same phase at certain temperature ranges. This phenomenon derives from the fact that electrons have spin as well as charge, giving an extra level of complexity to the physics of these materials, making it a very fascinating subject. Moreover, in this class of materials, the presence of magnetoelectric (M–E) coupling between electric and magnetic order parameters has been theoretically predicted. Hence, there is intense interest in its implementation in device architectures by taking advantage of the said properties [1–4]. The perovskite BiFeO3 (BFO) has recently emerged as a promising room temperature magnetoelectric material. It simultaneously exhibits antiferromagnetic (TN ∼ 380 °C) and ferroelectric (TC ∼ 810 °C) ordering, which enables BFO to be the most promising candidate for high temperature device applications. The structure and properties of pure and modified BFO have been extensively studied [5–8]. Though BFO has an antiferromagnetic spin ordering, it displays a weak magnetic moment arising from a canted spin structure [9,10]. However, from the application point of view, it is not sufficient. Hence, it is required to enhance the magnetic properties of BFO without disturbing the ferroelectric properties by substitutions at cation sites. It has been demonstrated that ferromagnetism could be
⁎ Corresponding author. Tel.: + 91 22 25764482; fax: + 91 22 25723707. E-mail address: [email protected] (K. Prashanthi). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.060
induced in BFO without disturbing its ferroelectric behavior by partially substituting Bi with Dy [11–15]. Moreover, the presence of M–E coupling at the macroscopic as well as microscopic level has also been verified [13]. However, for the simplification of device fabrication and to bring compatibility with the semiconductor industry, it is essential to integrate room temperature multiferroic thin films directly on silicon substrate. In this paper, we report the room temperature magnetoelectric multiferroic properties with M–E coupling in BDFO thin films directly deposited on p-type conducting Si (100) substrates using pulsed laser deposition (PLD) technique. We have further shown the memory properties of Au/BDFO/Si capacitors by means of high-frequency (1 MHz) capacitance–voltage (C–V) measurement.
2. Experimental details BDFO films of 200 nm thickness were deposited on p-type conducting (0.01–0.02 Ω cm) Si (100) substrates using PLD. The standard RCA (Radio Corporation of America) cleaning was performed on silicon wafers before loading them in PLD process chamber. Compex Pro 201, 248 KrF excimer laser from Coherent, was used for ablation process. The optimized ablation conditions used for deposition of BDFO films on Si are: laser density — 2.7 J/cm2, target to substrate distance — 4.5 cm, substrate temperature — 690 °C and oxygen pressure — 53 Pa. The deposition was performed for 30 min to obtain approximately a 200 nm thick film. X-ray diffraction (XRD) from an X'pert PRO (Philips Inc.), (Cu-Kα radiation, λ = 1.5405 Å) was used for structural phase identification. Scanning electron microscopy
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
5867
(SEM) was done using Raith150TWO tool to determine the grain morphology of BDFO thin films. High-resolution transmission electron microscopy (HRTEM) studies using Tecnai 200 kV TEM from FEI were performed to understand more about the BDFO/Si interface. The sample preparation for TEM imaging was done as follows: samples were cut in 2.3 × 5 mm sizes. Film surfaces of two samples were stuck using GATAN glue. This assembly was pressed using GATAN Kit and heated for 30 min at 130 °C. This sample pack was then inserted in 3 mm stainless steel rod with a grove of 2.5 × 0.7 × 5 mm in the center. Sides were filled with GATAN glue, which then cured for 30 min at 130 °C. It was then sliced using diamond saw to get thin slices in the range of 250 to 300 μm. This slice was thinned down to 90 µm using a disk grinder. In the center, it was dimpled up to depth of 20 μm using a dimple grinder. It was then ion polished using GATAN PIPS (precession Ion Polishing System) for 2 h. It was further ion polished at low energies and low angles. The sample was then imaged in TEM. The ferroelectric hysteresis and magnetic hysteresis measurements were carried out using a commercially available RT Precision LC system from Radiant Technology, USA and vibrating sample magnetometer (VSM) 7400 series from Lakeshore, USA, respectively. The highly conducting Si served as the bottom electrode for ferroelectric measurements of BDFO film. The M–E effect was determined by measuring the change in electric polarization developed across the sample in the presence of dc magnetic field. For electrical measurements, gold dots of 100 μm diameter were patterned on the top surface of the film by using lithography. Aluminum was evaporated on the backside for substrate contact. The high-frequency C–V characteristics of Au/BDFO/Si capacitor were measured using Agilent 4284A impedance analyzer. The results were compared to those of BDFO films deposited under identical conditions on Pt/TiO2/SiO2/Si and also other non-silicon substrates. 3. Results and discussion Fig. 1 shows the XRD pattern obtained for BDFO thin film deposited on p-type Si (100) substrate. The peak positions of BDFO thin films match well with the rhombohedrally distorted (R3c space group) BFO structure. It helps to demonstrate that the BDFO thin films can be directly grown well on Si substrates without the formation of any secondary phases. SEM image of the film shown in Fig. 2(a) indicates uniform grain morphology with the grain size of the order of 50–70 nm. The surface root-mean-square (rms) roughness evaluated by AFM was about 4 nm. The grain size and rms roughness of BDFO film on Si are comparable with those reported by Yao Wang and Ce-Wen Nan [16]. Yao Wang and Ce-Wen Nan have reported the growth of (110)textured BFO thin films with high quality on Si (100) substrate by a
Fig. 1. XRD pattern obtained for BDFO thin film grown on Si.
Fig. 2. (a) SEM image and (b) HRTEM image of BDFO thin film grown on Si.
seeding technique via a simple sol–gel method. However, in the present case, it has been observed that BDFO films on Si do not show column grain and preferential growth. A high resolution TEM image shown in Fig. 2(b) illustrates the growth of the BDFO films on Si, with an amorphous interface of the order of 3 nm. The composition of this interface layer is unknown at present. The electrical consequence of this interface layer is the modification of the dielectric constant of the BDFO film, which alters the flat-band voltage of the associated capacitor and consequently the threshold voltage of the device [17]. Therefore, the dielectric constant of BDFO on Si was calculated and compared with dielectric constant values obtained for BDFO on non-Si substrates for verification. The close match of effective dielectric constant value with films grown on Pt/TiO2/SiO2/Si and non-Si based substrates like MgO and LaAlO3 [14] suggested the absence of a well-defined interfacial SiO2 layer. However, a detailed study by us to determine the layer-by-layer interface composition is presently in progress. Ferroelectric hysteresis loop (P–E) obtained for BDFO thin film grown on silicon is shown in Fig. 3(a). Well-saturated ferroelectric hysteresis loop having polarization (Ps) of ∼ 9.5 μC/cm2, remanent polarization (Pr) of ∼ 3.8 μC/cm2 and coercive field (Ec) of 10 kV/cm for maximum applied electric field of 25 kV/cm was observed. In BFO related systems, it is generally accepted that Ec is about 100 kV/cm. However, ferroelectric measurements in this study suggested a much smaller Ec and smaller Pr. This could be due to the fact that the coercive field, the spontaneous, remanent polarization, and the shape of the loop are affected by various factors including the presence of charged defects, mechanical stresses, preparation conditions and
5868
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
less, it was reported in the case of sintered polycrystalline samples that, dielectric anomaly at the magnetic transition temperature may not necessarily result from multiferroicity of the bulk sample only; as it may have significant contributions from space charge effects at the interfacial layers i.e. grain boundaries, grain–electrode interfaces of different resistivities [27]. In this study, we have tried to prove the M– E coupling behavior of BDFO films by examining the effect of magnetic field on ferroelectric hysteresis loop. Fig. 4(a) indicates changes in the ferroelectric hysteresis loops with the applied dc magnetic field in BDFO thin films. Initially, a low electric field (∼10 kV/cm) was applied to the sample to start the experiment. After that, electric field was kept constant and the magnetic field was varied. As the magnetic field increased, there was an improvement in ferroelectric hysteresis loops. Saturation polarization value Ps gradually increased, and at a magnetic field of 0.02 T, a well-saturated ferroelectric hysteresis loop was obtained. This trend is identical to what is normally obtained with the applied electric field (as it can be seen from Fig. 3(a)). Fig. 4(b) shows the change in the Pr/ Ps ratio with the applied magnetic field. Pr/Ps ratio gradually increases with the applied magnetic field up to 0.012 T and saturates after that. This is an unambiguous confirmation of the presence of M–E coupling in this system. Nevertheless, the results positively hint at the orientation of ferroelectric domains by means of magnetic field in BDFO thin films and hence the presence of coupling.
Fig. 3. (a) Ferroelectric hysteresis loop and (b) magnetic hysteresis curve (M–H) obtained for BDFO thin film grown on Si.
thermal treatment [18]. Magnetic hysteresis loop (M–H) observed for BDFO film on silicon is shown in Fig. 3(b). The film exhibits a saturation magnetization (Ms) value of ∼80 × 103 A/m, remanent magnetization (Mr) of 30 × 103 A/m and coercive field (Mc) of 0.45 T for maximum applied magnetic field of 1 T. The high values of Ms and Mr obtained in this study could be attributed to stress developed during the growth of the film as observed for BFO films by Wang et al. [19]. The observed saturated magnetic hysteresis loop demonstrates the presence of ferromagnetism in the films. Overall, the results confirm the co-existence of ferromagnetic and ferroelectric properties at room temperature in BDFO films grown on silicon. However, the presence of M–E coupling in the sample is important to achieve flexibility in device design. The co-existence of magnetic and electric dipoles is the primary requirement to observe the M–E effect in magnetoelectric materials. Therefore, the M–E effect could be realized in a composite consisting of both ferroelectric and ferromagnetic phases by using the product property. Several composite materials and bilayers/multilayered structures, consisting of separate piezoelectric and magnetic phases, have been reported to show M–E coupling at room temperature [20,21]. However, most of the known single phase bulk magnetoelectric materials rarely exhibit strong room temperature M–E coupling. In general, magnetoelectric materials show linear and/or higher-order effects as reported by various authors [22,23]. In some papers, the observed anomaly in the dielectric constant at the magnetic transition temperature is considered as an evidence of multiferroic M–E coupling [24–26]. Neverthe-
Fig. 4. (a) Change in ferroelectric polarization induced by applied magnetic field. (b) Change in Pr/Ps ratio with applied magnetic field obtained for BDFO films deposited on Si.
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
The memory characteristics of BDFO thin films were investigated on Au/BDFO/Si capacitors. Fig. 5(a) shows high-frequency (1 MHz) C–V characteristics of Au/BDFO/Si capacitor. A small AC signal (Amplitude of 25 mV) was applied to measure the C–V characteristics, with dc sweep voltage from ± 3 V to ± 6 V. The hysteresis in the C–V characteristics is attributed to the ferroelectric behavior of BDFO film required for a memory device. The ferroelectric polarization delays the change in capacitance from accumulation to depletion. Therefore, both the absolute values of the flat-band voltage in forward bias, VFBF (shift to right) and the flat-band voltage in reverse bias VFBR (shift to left) increase with the sweeping voltage. The memory window increases from 0.1 to 0.8 V when the sweeping voltage is increased from ± 3 to ± 6 V. This increase in memory window with sweep voltage is due to the ferroelectric polarization without charge injection [28]. In case of charge injection, it neutralizes the ferroelectric effect, resulting in decrease in the VFBF (shift to left) and increase in the VFBR (shift to right) so that the memory window decreases [29]. The effect of frequency (varied from 1 kHz to 1 MHz) on the C–V characteristics of Au/BDFO/Si capacitor, is shown in Fig. 5(b). When voltage was applied, a slight shift in the C–V curve towards the left was observed with increase in frequency. This stretch-out in capacitance could be attributed to interface-trapped charges because the interface trap occupancy varies with the applied sweep voltage.
5869
Since the effective oxide charge density NOX is related to the oxide/ semiconductor interface defects, it can be calculated by the Terman's method [30]: NOX =
COX ðΔVMG −Φms Þ qA
ð1Þ
where COX is the capacitance of BDFO thin film and ΔVMG is the difference of mid-gap voltage between the measured and the theoretical (ideal) VMG values. Φms is the work function difference between metal electrode and semiconductor substrate, q is the electronic charge, A is the area of electrode and ϕB is the bulk potential of Si. Some studies [31] have suggested that it is preferable to estimate NOX from ΔVMG (as opposed to shift in flat-band voltage, ΔVFB) as ΔVMG is mostly unaffected by the presence of interface states. The estimated NOX value was 3.7 × 1011 cm−2, which is indicative of a good electronic interface. Interface-state density (Nit) is an important parameter which is used to monitor how good the semiconductor–insulator interface is. In a good metal oxide semiconductor device, Nit should be of the order of 1 × 1010 cm−2. Devices which have undergone some type of stress, such as high-field or radiation stress may have interface-state densities higher than 1 × 1012 cm−2. Interfaces other than Si/SiO2 generally have densities greater than 1 × 1011 cm−2. It is possible to get an estimate of the average interface-state density, D it̅ (cm−2 eV−1) by: P
Dit = −
COX ðΔVFB Þ qAðϕB Þ
ð2Þ
where ΔVFB is the difference of flat-band voltage between the measured and the theoretical (ideal) VFB values. The estimated minimum interface-state density around the mid-gap was 4.6 × 1011 cm−2 eV−1. The measured NOX, D ̅it and dielectric constant of Au/BDFO/Si capacitor remained stable over an extended period of testing (N1000 h). 4. Conclusions Magnetoelectric multiferroic BDFO thin films directly deposited on silicon using PLD demonstrated a saturated ferroelectric and ferromagnetic hysteresis loops. Moreover, a change in electric polarization with application of magnetic field has been observed in these films. The memory characteristics of Au/BDFO/Si capacitors were studied. The electrical parameters like effective oxide charge density, interface density and dielectric constant have been determined. Overall, the results suggest the integrated circuit (IC) compatible application potential of multiferroic BDFO films in the field of micro-electromechanical systems (MEMS) as well as memory applications. Acknowledgment The authors wish to acknowledge partial funding received from the Department of Information Technology (Grant No. 05IT006), Government of India, through the Centre of Excellence in Nanoelectronics. References [1] [2] [3] [4]
Fig. 5. (a) C–V characteristics for Au/BDFO/Si capacitor at different gate voltages. (b) C–V characteristics for Au/BDFO/Si capacitor at different frequencies.
R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21. W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. M. Fiebig, Th. Lottermoser, D. Frohlich, A.V. Goltsev, R.V. Pisarev, Nature 419 (2002) 818. [5] Yu.E. Roginskaya, Yu.Ya. Tomashpol'skii, Yu.N. Venevtsev, V.M. Petrov, G.S. Zhdanov, Sov. Phys. JETP 23 (1966) 47. [6] S.V. Kiselev, R.P. Ozerov, G.S. Zhdanov, Sov. Phys. Dokl. 7 (1963) 742. [7] P. Kharel, S. Talebi, B. Ramachandran, A. Dixit, V.M. Naik, M.B. Sahana, C. Sudakar, R. Naik, M.S.R. Rao, G. Lawes, J. Phys.: Condens. Matter. 21 (2009) 036001.
5870
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
[8] C.J. Cheng, D. Kan, S.H. Lim, W.R. McKenzie, P.R. Munroe, L.G. Salamanca-Riba, R.L. Withers, I. Takeuchi, Phys. Rev. B 80 (2009) 014109. [9] I. Sosnovska, T. Peterlin-Neumaier, E. Steichele, J. Phys. C: Solid State Phys. 15 (1982) 4835. [10] C. Blaauw, F. Van der Woude, J. Phys. C: Solid State Phys. 6 (1973) 1422. [11] V.R. Palkar, K. Prashanthi, S.P. Duttagupta, J. Phys. D Appl. Phys. 41 (2008) 045003. [12] K. Prashanthi, B.A. Chalke, K.C. Barick, A. Das, I. Dhiman, V.R. Palkar, Solid State Commun. 149 (2009) 188. [13] V.R. Palkar, K. Prashanthi, Appl. Phys. Lett. 93 (2008) 132906. [14] P.K. Petrov, V.R. Palkar, A.K. Tagantsev, H.I. Chien, K. Prashanthi, A.K. Axelsson, S. Bhattacharya, N.M. Alford, J. Mater. Res. 22 (2007) 2179. [15] F.Z. Qian, J.S. Jiang, S.Z. Guo, D.M. Jiang, W.G. Zhang, J. Appl. Phys. 106 (2009) 084312. [16] Yao Wang, Ce-Wen Nan, Thin Solid Films 517 (2009) 4484. [17] Michel Houssa, High-k Gate Dielectrics, Series in Materials Science and Engineering, IOP Publishing, U.K., 2004. [18] I. Mayergoyz, G. Bertotti, The Science of Hysteresis, Acadamic Press is an imprint of Elsevier, Oxford, U.K., 2006. [19] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaidyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Robe, M. Wutting, R. Ramesh, Science 299 (2003) 1719.
[20] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L.M. Ardabili, T. Zhao, L.S. Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 661. [21] K. Ban, M. Gomi, T. Shundo, N. Nishimura, IEEE Trans. Magn. 41 (2005) 2793. [22] A.K. Zvezdin, A.M. Kadomtseva, S.S. Krotov, J. Magn. Magn. Mater. 300 (2006) 224. [23] Y.F. Popov, A.M. Kadomtseva, S.S. Krotov, Low Temp. Phys. 27 (2001) 478. [24] M. Kumar, K.L. Yadav, J. Appl. Phys. 100 (2006) 074111. [25] K. Taniguchi, N. Abe, T. Takenobu, Y. Iwasa, T. Arima, Phys. Rev. Lett. 97 (2006) 097203. [26] V.R. Palkar, J. John, R. Pinto, Appl. Phys. Lett. 80 (2002) 1628. [27] G. Catalan, Appl. Phys. Lett. 88 (2006) 102902. [28] K. Prashanthi, S.P. Duttagupta, R. Pinto, V.R. Palkar, Electron. Lett. 45 (2009) 821. [29] K. Neelam, P. Jayanta, K.B.R. Varma, S.B. Krupanidhi, Solid State Commun. 137 (2006) 566. [30] L.M. Terman, Solid State Electon. 5 (1962) 285. [31] E.H. Nicollian, J.R. Brews, MOS (Metal Oxide Semiconductor) Physics and Technology, John Wiley and Sons, New York, NY, 1982.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Multiferroic Bi0.7Dy0.3FeO3 thin films directly integrated on Si for integrated circuit compatible devices K. Prashanthi a,⁎, B.A. Chalke b, R.D. Bapat b, S.C. Purandare b, V.R. Palkar a a b
Centre for Nanoelectronics, Dept. of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India Dept. of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research, Mumbai 400005, India
a r t i c l e
i n f o
Available online 31 May 2010 Keywords: Thin films Multiferroics Pulse laser deposition Magnetic properties Ferroelectric properties
a b s t r a c t Magnetoelectric multiferroic Bi0.7Dy0.3FeO3 (BDFO) thin films deposited on p-type Si (100) substrate using pulsed laser deposition technique demonstrated a saturated ferroelectric and ferromagnetic hysteresis loop at room temperature. More interestingly, the observed change in electric polarization with applied magnetic field in these films indicated the presence of room temperature magnetoelectric coupling behavior. Using high-frequency capacitance–voltage measurements, the fixed oxide charge density, interface trap density and dielectric constant were estimated on Au/BDFO/Si capacitors. These results suggest the integrated circuit compatible application potential of BDFO films in the field of micro-electro-mechanical systems and nonvolatile memories. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnetoelectric multiferroics are the materials which simultaneously exhibit magnetic and ferroelectric ordering in the same phase at certain temperature ranges. This phenomenon derives from the fact that electrons have spin as well as charge, giving an extra level of complexity to the physics of these materials, making it a very fascinating subject. Moreover, in this class of materials, the presence of magnetoelectric (M–E) coupling between electric and magnetic order parameters has been theoretically predicted. Hence, there is intense interest in its implementation in device architectures by taking advantage of the said properties [1–4]. The perovskite BiFeO3 (BFO) has recently emerged as a promising room temperature magnetoelectric material. It simultaneously exhibits antiferromagnetic (TN ∼ 380 °C) and ferroelectric (TC ∼ 810 °C) ordering, which enables BFO to be the most promising candidate for high temperature device applications. The structure and properties of pure and modified BFO have been extensively studied [5–8]. Though BFO has an antiferromagnetic spin ordering, it displays a weak magnetic moment arising from a canted spin structure [9,10]. However, from the application point of view, it is not sufficient. Hence, it is required to enhance the magnetic properties of BFO without disturbing the ferroelectric properties by substitutions at cation sites. It has been demonstrated that ferromagnetism could be
⁎ Corresponding author. Tel.: + 91 22 25764482; fax: + 91 22 25723707. E-mail address: [email protected] (K. Prashanthi). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.060
induced in BFO without disturbing its ferroelectric behavior by partially substituting Bi with Dy [11–15]. Moreover, the presence of M–E coupling at the macroscopic as well as microscopic level has also been verified [13]. However, for the simplification of device fabrication and to bring compatibility with the semiconductor industry, it is essential to integrate room temperature multiferroic thin films directly on silicon substrate. In this paper, we report the room temperature magnetoelectric multiferroic properties with M–E coupling in BDFO thin films directly deposited on p-type conducting Si (100) substrates using pulsed laser deposition (PLD) technique. We have further shown the memory properties of Au/BDFO/Si capacitors by means of high-frequency (1 MHz) capacitance–voltage (C–V) measurement.
2. Experimental details BDFO films of 200 nm thickness were deposited on p-type conducting (0.01–0.02 Ω cm) Si (100) substrates using PLD. The standard RCA (Radio Corporation of America) cleaning was performed on silicon wafers before loading them in PLD process chamber. Compex Pro 201, 248 KrF excimer laser from Coherent, was used for ablation process. The optimized ablation conditions used for deposition of BDFO films on Si are: laser density — 2.7 J/cm2, target to substrate distance — 4.5 cm, substrate temperature — 690 °C and oxygen pressure — 53 Pa. The deposition was performed for 30 min to obtain approximately a 200 nm thick film. X-ray diffraction (XRD) from an X'pert PRO (Philips Inc.), (Cu-Kα radiation, λ = 1.5405 Å) was used for structural phase identification. Scanning electron microscopy
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
5867
(SEM) was done using Raith150TWO tool to determine the grain morphology of BDFO thin films. High-resolution transmission electron microscopy (HRTEM) studies using Tecnai 200 kV TEM from FEI were performed to understand more about the BDFO/Si interface. The sample preparation for TEM imaging was done as follows: samples were cut in 2.3 × 5 mm sizes. Film surfaces of two samples were stuck using GATAN glue. This assembly was pressed using GATAN Kit and heated for 30 min at 130 °C. This sample pack was then inserted in 3 mm stainless steel rod with a grove of 2.5 × 0.7 × 5 mm in the center. Sides were filled with GATAN glue, which then cured for 30 min at 130 °C. It was then sliced using diamond saw to get thin slices in the range of 250 to 300 μm. This slice was thinned down to 90 µm using a disk grinder. In the center, it was dimpled up to depth of 20 μm using a dimple grinder. It was then ion polished using GATAN PIPS (precession Ion Polishing System) for 2 h. It was further ion polished at low energies and low angles. The sample was then imaged in TEM. The ferroelectric hysteresis and magnetic hysteresis measurements were carried out using a commercially available RT Precision LC system from Radiant Technology, USA and vibrating sample magnetometer (VSM) 7400 series from Lakeshore, USA, respectively. The highly conducting Si served as the bottom electrode for ferroelectric measurements of BDFO film. The M–E effect was determined by measuring the change in electric polarization developed across the sample in the presence of dc magnetic field. For electrical measurements, gold dots of 100 μm diameter were patterned on the top surface of the film by using lithography. Aluminum was evaporated on the backside for substrate contact. The high-frequency C–V characteristics of Au/BDFO/Si capacitor were measured using Agilent 4284A impedance analyzer. The results were compared to those of BDFO films deposited under identical conditions on Pt/TiO2/SiO2/Si and also other non-silicon substrates. 3. Results and discussion Fig. 1 shows the XRD pattern obtained for BDFO thin film deposited on p-type Si (100) substrate. The peak positions of BDFO thin films match well with the rhombohedrally distorted (R3c space group) BFO structure. It helps to demonstrate that the BDFO thin films can be directly grown well on Si substrates without the formation of any secondary phases. SEM image of the film shown in Fig. 2(a) indicates uniform grain morphology with the grain size of the order of 50–70 nm. The surface root-mean-square (rms) roughness evaluated by AFM was about 4 nm. The grain size and rms roughness of BDFO film on Si are comparable with those reported by Yao Wang and Ce-Wen Nan [16]. Yao Wang and Ce-Wen Nan have reported the growth of (110)textured BFO thin films with high quality on Si (100) substrate by a
Fig. 1. XRD pattern obtained for BDFO thin film grown on Si.
Fig. 2. (a) SEM image and (b) HRTEM image of BDFO thin film grown on Si.
seeding technique via a simple sol–gel method. However, in the present case, it has been observed that BDFO films on Si do not show column grain and preferential growth. A high resolution TEM image shown in Fig. 2(b) illustrates the growth of the BDFO films on Si, with an amorphous interface of the order of 3 nm. The composition of this interface layer is unknown at present. The electrical consequence of this interface layer is the modification of the dielectric constant of the BDFO film, which alters the flat-band voltage of the associated capacitor and consequently the threshold voltage of the device [17]. Therefore, the dielectric constant of BDFO on Si was calculated and compared with dielectric constant values obtained for BDFO on non-Si substrates for verification. The close match of effective dielectric constant value with films grown on Pt/TiO2/SiO2/Si and non-Si based substrates like MgO and LaAlO3 [14] suggested the absence of a well-defined interfacial SiO2 layer. However, a detailed study by us to determine the layer-by-layer interface composition is presently in progress. Ferroelectric hysteresis loop (P–E) obtained for BDFO thin film grown on silicon is shown in Fig. 3(a). Well-saturated ferroelectric hysteresis loop having polarization (Ps) of ∼ 9.5 μC/cm2, remanent polarization (Pr) of ∼ 3.8 μC/cm2 and coercive field (Ec) of 10 kV/cm for maximum applied electric field of 25 kV/cm was observed. In BFO related systems, it is generally accepted that Ec is about 100 kV/cm. However, ferroelectric measurements in this study suggested a much smaller Ec and smaller Pr. This could be due to the fact that the coercive field, the spontaneous, remanent polarization, and the shape of the loop are affected by various factors including the presence of charged defects, mechanical stresses, preparation conditions and
5868
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
less, it was reported in the case of sintered polycrystalline samples that, dielectric anomaly at the magnetic transition temperature may not necessarily result from multiferroicity of the bulk sample only; as it may have significant contributions from space charge effects at the interfacial layers i.e. grain boundaries, grain–electrode interfaces of different resistivities [27]. In this study, we have tried to prove the M– E coupling behavior of BDFO films by examining the effect of magnetic field on ferroelectric hysteresis loop. Fig. 4(a) indicates changes in the ferroelectric hysteresis loops with the applied dc magnetic field in BDFO thin films. Initially, a low electric field (∼10 kV/cm) was applied to the sample to start the experiment. After that, electric field was kept constant and the magnetic field was varied. As the magnetic field increased, there was an improvement in ferroelectric hysteresis loops. Saturation polarization value Ps gradually increased, and at a magnetic field of 0.02 T, a well-saturated ferroelectric hysteresis loop was obtained. This trend is identical to what is normally obtained with the applied electric field (as it can be seen from Fig. 3(a)). Fig. 4(b) shows the change in the Pr/ Ps ratio with the applied magnetic field. Pr/Ps ratio gradually increases with the applied magnetic field up to 0.012 T and saturates after that. This is an unambiguous confirmation of the presence of M–E coupling in this system. Nevertheless, the results positively hint at the orientation of ferroelectric domains by means of magnetic field in BDFO thin films and hence the presence of coupling.
Fig. 3. (a) Ferroelectric hysteresis loop and (b) magnetic hysteresis curve (M–H) obtained for BDFO thin film grown on Si.
thermal treatment [18]. Magnetic hysteresis loop (M–H) observed for BDFO film on silicon is shown in Fig. 3(b). The film exhibits a saturation magnetization (Ms) value of ∼80 × 103 A/m, remanent magnetization (Mr) of 30 × 103 A/m and coercive field (Mc) of 0.45 T for maximum applied magnetic field of 1 T. The high values of Ms and Mr obtained in this study could be attributed to stress developed during the growth of the film as observed for BFO films by Wang et al. [19]. The observed saturated magnetic hysteresis loop demonstrates the presence of ferromagnetism in the films. Overall, the results confirm the co-existence of ferromagnetic and ferroelectric properties at room temperature in BDFO films grown on silicon. However, the presence of M–E coupling in the sample is important to achieve flexibility in device design. The co-existence of magnetic and electric dipoles is the primary requirement to observe the M–E effect in magnetoelectric materials. Therefore, the M–E effect could be realized in a composite consisting of both ferroelectric and ferromagnetic phases by using the product property. Several composite materials and bilayers/multilayered structures, consisting of separate piezoelectric and magnetic phases, have been reported to show M–E coupling at room temperature [20,21]. However, most of the known single phase bulk magnetoelectric materials rarely exhibit strong room temperature M–E coupling. In general, magnetoelectric materials show linear and/or higher-order effects as reported by various authors [22,23]. In some papers, the observed anomaly in the dielectric constant at the magnetic transition temperature is considered as an evidence of multiferroic M–E coupling [24–26]. Neverthe-
Fig. 4. (a) Change in ferroelectric polarization induced by applied magnetic field. (b) Change in Pr/Ps ratio with applied magnetic field obtained for BDFO films deposited on Si.
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
The memory characteristics of BDFO thin films were investigated on Au/BDFO/Si capacitors. Fig. 5(a) shows high-frequency (1 MHz) C–V characteristics of Au/BDFO/Si capacitor. A small AC signal (Amplitude of 25 mV) was applied to measure the C–V characteristics, with dc sweep voltage from ± 3 V to ± 6 V. The hysteresis in the C–V characteristics is attributed to the ferroelectric behavior of BDFO film required for a memory device. The ferroelectric polarization delays the change in capacitance from accumulation to depletion. Therefore, both the absolute values of the flat-band voltage in forward bias, VFBF (shift to right) and the flat-band voltage in reverse bias VFBR (shift to left) increase with the sweeping voltage. The memory window increases from 0.1 to 0.8 V when the sweeping voltage is increased from ± 3 to ± 6 V. This increase in memory window with sweep voltage is due to the ferroelectric polarization without charge injection [28]. In case of charge injection, it neutralizes the ferroelectric effect, resulting in decrease in the VFBF (shift to left) and increase in the VFBR (shift to right) so that the memory window decreases [29]. The effect of frequency (varied from 1 kHz to 1 MHz) on the C–V characteristics of Au/BDFO/Si capacitor, is shown in Fig. 5(b). When voltage was applied, a slight shift in the C–V curve towards the left was observed with increase in frequency. This stretch-out in capacitance could be attributed to interface-trapped charges because the interface trap occupancy varies with the applied sweep voltage.
5869
Since the effective oxide charge density NOX is related to the oxide/ semiconductor interface defects, it can be calculated by the Terman's method [30]: NOX =
COX ðΔVMG −Φms Þ qA
ð1Þ
where COX is the capacitance of BDFO thin film and ΔVMG is the difference of mid-gap voltage between the measured and the theoretical (ideal) VMG values. Φms is the work function difference between metal electrode and semiconductor substrate, q is the electronic charge, A is the area of electrode and ϕB is the bulk potential of Si. Some studies [31] have suggested that it is preferable to estimate NOX from ΔVMG (as opposed to shift in flat-band voltage, ΔVFB) as ΔVMG is mostly unaffected by the presence of interface states. The estimated NOX value was 3.7 × 1011 cm−2, which is indicative of a good electronic interface. Interface-state density (Nit) is an important parameter which is used to monitor how good the semiconductor–insulator interface is. In a good metal oxide semiconductor device, Nit should be of the order of 1 × 1010 cm−2. Devices which have undergone some type of stress, such as high-field or radiation stress may have interface-state densities higher than 1 × 1012 cm−2. Interfaces other than Si/SiO2 generally have densities greater than 1 × 1011 cm−2. It is possible to get an estimate of the average interface-state density, D it̅ (cm−2 eV−1) by: P
Dit = −
COX ðΔVFB Þ qAðϕB Þ
ð2Þ
where ΔVFB is the difference of flat-band voltage between the measured and the theoretical (ideal) VFB values. The estimated minimum interface-state density around the mid-gap was 4.6 × 1011 cm−2 eV−1. The measured NOX, D ̅it and dielectric constant of Au/BDFO/Si capacitor remained stable over an extended period of testing (N1000 h). 4. Conclusions Magnetoelectric multiferroic BDFO thin films directly deposited on silicon using PLD demonstrated a saturated ferroelectric and ferromagnetic hysteresis loops. Moreover, a change in electric polarization with application of magnetic field has been observed in these films. The memory characteristics of Au/BDFO/Si capacitors were studied. The electrical parameters like effective oxide charge density, interface density and dielectric constant have been determined. Overall, the results suggest the integrated circuit (IC) compatible application potential of multiferroic BDFO films in the field of micro-electromechanical systems (MEMS) as well as memory applications. Acknowledgment The authors wish to acknowledge partial funding received from the Department of Information Technology (Grant No. 05IT006), Government of India, through the Centre of Excellence in Nanoelectronics. References [1] [2] [3] [4]
Fig. 5. (a) C–V characteristics for Au/BDFO/Si capacitor at different gate voltages. (b) C–V characteristics for Au/BDFO/Si capacitor at different frequencies.
R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21. W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. M. Fiebig, Th. Lottermoser, D. Frohlich, A.V. Goltsev, R.V. Pisarev, Nature 419 (2002) 818. [5] Yu.E. Roginskaya, Yu.Ya. Tomashpol'skii, Yu.N. Venevtsev, V.M. Petrov, G.S. Zhdanov, Sov. Phys. JETP 23 (1966) 47. [6] S.V. Kiselev, R.P. Ozerov, G.S. Zhdanov, Sov. Phys. Dokl. 7 (1963) 742. [7] P. Kharel, S. Talebi, B. Ramachandran, A. Dixit, V.M. Naik, M.B. Sahana, C. Sudakar, R. Naik, M.S.R. Rao, G. Lawes, J. Phys.: Condens. Matter. 21 (2009) 036001.
5870
K. Prashanthi et al. / Thin Solid Films 518 (2010) 5866–5870
[8] C.J. Cheng, D. Kan, S.H. Lim, W.R. McKenzie, P.R. Munroe, L.G. Salamanca-Riba, R.L. Withers, I. Takeuchi, Phys. Rev. B 80 (2009) 014109. [9] I. Sosnovska, T. Peterlin-Neumaier, E. Steichele, J. Phys. C: Solid State Phys. 15 (1982) 4835. [10] C. Blaauw, F. Van der Woude, J. Phys. C: Solid State Phys. 6 (1973) 1422. [11] V.R. Palkar, K. Prashanthi, S.P. Duttagupta, J. Phys. D Appl. Phys. 41 (2008) 045003. [12] K. Prashanthi, B.A. Chalke, K.C. Barick, A. Das, I. Dhiman, V.R. Palkar, Solid State Commun. 149 (2009) 188. [13] V.R. Palkar, K. Prashanthi, Appl. Phys. Lett. 93 (2008) 132906. [14] P.K. Petrov, V.R. Palkar, A.K. Tagantsev, H.I. Chien, K. Prashanthi, A.K. Axelsson, S. Bhattacharya, N.M. Alford, J. Mater. Res. 22 (2007) 2179. [15] F.Z. Qian, J.S. Jiang, S.Z. Guo, D.M. Jiang, W.G. Zhang, J. Appl. Phys. 106 (2009) 084312. [16] Yao Wang, Ce-Wen Nan, Thin Solid Films 517 (2009) 4484. [17] Michel Houssa, High-k Gate Dielectrics, Series in Materials Science and Engineering, IOP Publishing, U.K., 2004. [18] I. Mayergoyz, G. Bertotti, The Science of Hysteresis, Acadamic Press is an imprint of Elsevier, Oxford, U.K., 2006. [19] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaidyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Robe, M. Wutting, R. Ramesh, Science 299 (2003) 1719.
[20] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L.M. Ardabili, T. Zhao, L.S. Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 661. [21] K. Ban, M. Gomi, T. Shundo, N. Nishimura, IEEE Trans. Magn. 41 (2005) 2793. [22] A.K. Zvezdin, A.M. Kadomtseva, S.S. Krotov, J. Magn. Magn. Mater. 300 (2006) 224. [23] Y.F. Popov, A.M. Kadomtseva, S.S. Krotov, Low Temp. Phys. 27 (2001) 478. [24] M. Kumar, K.L. Yadav, J. Appl. Phys. 100 (2006) 074111. [25] K. Taniguchi, N. Abe, T. Takenobu, Y. Iwasa, T. Arima, Phys. Rev. Lett. 97 (2006) 097203. [26] V.R. Palkar, J. John, R. Pinto, Appl. Phys. Lett. 80 (2002) 1628. [27] G. Catalan, Appl. Phys. Lett. 88 (2006) 102902. [28] K. Prashanthi, S.P. Duttagupta, R. Pinto, V.R. Palkar, Electron. Lett. 45 (2009) 821. [29] K. Neelam, P. Jayanta, K.B.R. Varma, S.B. Krupanidhi, Solid State Commun. 137 (2006) 566. [30] L.M. Terman, Solid State Electon. 5 (1962) 285. [31] E.H. Nicollian, J.R. Brews, MOS (Metal Oxide Semiconductor) Physics and Technology, John Wiley and Sons, New York, NY, 1982.