Appl. Phys. A DOI 10.1007/s00339-013-8085-5

A novel Sr3 Pb6 Ce2 Ti12 O36 ferroelectric thin film grown by pulsed laser ablation P. S. Krishnaprasad · P. Mohanan · G. Subodh · M. T. Sebastian · M. K. Jayaraj

Received: 22 July 2013 / Accepted: 14 October 2013 © Springer-Verlag Berlin Heidelberg 2013

Abstract Complex perovskite oxide ferroelectric thin films are of great technological interest because of their high dielectric constant and large tunability. In this paper, we report the structural and electrical properties of Sr3 Pb6 Ce2 Ti12 O36 (SPCTO) thin films grown by pulsed laser deposition. The role of oxygen pressure and substrate temperature on the microstructure, dielectric properties and leakage current mechanism of SPCTO thin films was investigated. Strong oxygen partial pressure dependence on the microcrystalline properties and leakage current conduction mechanism was observed. Both Raman spectra and C-V characteristics show a ferroelectric phase rather than paraelectric phase for the deposited thin films. Investigations on the leakage current showed that SPCTO thin films deposited at different oxygen pressure have different dominant conduction mechanism at various electric fields. The low field conduction mechanism is governed by Ohmic and space charge limited conduction mechanisms, whereas at high fields, the conduction process is dominated by Schottky emission mechanism. The dielectric constant as well as the tunability is found to increase with increase in the crystallite size.

P. S. Krishnaprasad (B) · M. K. Jayaraj Department of Physics, Cochin University of Science and Technology, Kochi 682022, India e-mail: [email protected] M. K. Jayaraj e-mail: [email protected]

1 Introduction Developments in the microwave communication led to much interest in the field of perovskite material engineering, especially in complex perovskites. Materials with high temperature stability, moderate dielectric constant, reasonable tunability and low dielectric loss have received attention of the industry for the generation of high density dynamic random access memories and tunable microwave devices. Materials with high dielectric constant, high thermal stability together with low loss are indeed a challenging problem in the field of microwave electronics [1,2]. Pure SrTiO 3 (STO) is an intrinsic quantum paraelectric, which exhibits high dielectric constant and low microwave dielectric dispersion at low temperatures [3]. Ferroelectricity can be induced in STO by mechanical stress [4] or by introducing substitutional defects into the lattice [5,6]. Even though STO is a highly stable material with low dielectric loss, the temperature coefficient of the resonant frequency (τ f ) is very high which hinders it from practical applications. Recently, Subodh et al. [7] reported a new ceramic system Sr 2+n Ce2 Ti5+n O15+3n (n = 0–7) (SCTO). This ceramic system shows εr ranging from 113 to 185 with relatively low dielectric loss. The temperature dependence of resonant frequency value is five times less than that observed in STO. The charge compensation in cerium-doped STO compositions occurs via A-site vacancy formation with Ce3+ dopant cations occupying Sr2+ sites and creating A-site vacancies in the ABO3 structure through the reaction

P. Mohanan Department of Electronics, Cochin University of Science and Technology, Kochi 682022, India

x Ce2 O3 −−−−→ 2Ce... Sr + VSr + 3O0 .

G. Subodh · M. T. Sebastian National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, India

It has also been experimentally verified that Ce doping decreases the leakage current in some tunable perovskite materials like Ba0.5 Sr0.5 TiO3 thin films. The reduction of

SrTiO3



(1)

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the leakage current is ascribed to the effect of acceptor Ce3+ doping [8]. As such SCTO is relatively inexpensive and thermally stable candidates in the telecommunication field. Similar ABO3 structures previously reported are NdAlO3 [9], LaNiO3 [10], LaAlO3 [11], LaCoO3 [12], and LaCuO3 [13,14]. More recently, Kamba et al. [15] investigated the crystal symmetry and dielectric properties of Pb substituted Sr9 Ce2 Ti12 O36 ceramics. The samples with low content of Pb exhibit paraelectric behavior, while higher content of Pb induces displacive ferroelectric phase transition and Curie–Weiss temperature TC increases linearly with Pb concentration. Among these materials, Sr3 Pb6 Ce2 Ti12 O36 ceramics possess highest dielectric constant and a reasonably low dielectric loss at microwave frequencies. Hence Sr3 Pb6 Ce2 Ti12 O36 ceramic system was chosen for investigating the tunable dielectric properties. In the present article, we report for the first time the structural and dielectric properties of SPCTO thin films and its dependence on deposition parameters. In the application point of view, the current flow through the film should be as low as possible, since the high leakage current means that the data stored in the DRAM capacitor cell are easy to lose and require frequent refreshing, resulting in increased power consumption. Also the miniaturization of electronic devices leads to higher electric field across the devices and hence understanding of leakage current mechanism through the device and its dependence on various deposition parameters is very important. The conduction in the SPCTO thin films shows that they can be effectively used in tunable microwave and memory devices.

pressure [16]. Thin films were deposited at different substrate temperatures varying from 400 to 700 ◦ C. The substrate temperature was optimized to have good crystallinity for the films. The films were deposited at that optimized substrate temperature at various oxygen partial pressures ranging from 0.05 to 0.5 mbar. After deposition, films were maintained at the same temperature in the chamber for 30 min, but the oxygen pressure was increased to 0.5 mbar, then the films were allowed to cool to 50 ◦ C at the rate of 10 ◦ C/min keeping the oxygen pressure at 0.5 mbar before they were further cooled down to room temperature. The crystallinity of thin films was determined by X-ray diffraction (XRD) with CuKα radiation (λ = 1.5418 Å) in θ – 2θ geometry. Thickness of the samples was measured using Dektak 6M stylus profiler. Room temperature Raman spectra were measured using Horiba Jobin Yvon LabRam HR with He-Ne laser (632.8 nm) as excitation source. Electrical characterization of SPCTO thin films was carried out with a metal-insulator-metal (MIM) structure of PtSi/SPCTO/Cr/Au. Au top electrodes of 2 mm in diameter were deposited through a shadow mask by RF sputtering and a very thin layer of chrome was deposited as a sticking layer before the deposition of gold electrode. Leakage current characterizations were carried out using Keithley 236 SMU unit. The dielectric constant of SPCTO thin films was calculated from the capacitance measured using 4192A HP impedance analyzer. Measurements of C-V curves are made by applying a DC field which changes as a step function and an AC voltage of relatively high frequency with a small amplitude of about 30 mV, simultaneously on the sample.

2 Experiment

3 Results and discussion

SPCTO ceramic target for pulsed laser deposition was prepared by solid-state reaction. Stoichiometric amount of highpurity CeO2 , PbO, SrCO3 , and TiO2 powders was ball-milled in distilled water medium. The slurry was dried and the ground material was then calcined at 1100◦ C. The calcined material was ground well and 4 wt% polyvinyl alcohol (PVA) was added to the powder, and again ground into fine powder. The PLD targets were prepared as pellets of 1cm diameter. The targets were sintered at temperature 1,350 ◦ C for 2 h. The pellets were muffled in the same powder to prevent escape of the volatile Pb. The SPCTO thin films were grown on Pt-Si (Pt/TiO2 / SiO2 /Si) substrate by pulsed laser deposition. The fourth harmonics of Q-switched Nd: YAG laser (266 nm) was used for ablation. The repetition frequency was 10 Hz with a pulse width of 7–8 ns. The ablation was carried out at an optimized laser fluence of 1.8 J/cm2 . The oxygen partial pressure in the chamber was kept at 0.1 mbar, since dielectric oxide materials generally have better growth at higher oxygen partial

The SPCTO thin films were grown on platinized silicon (Pt/TiO2 /SiO2 /Si) wafers by pulsed laser deposition. All the deposited films have good adhesion and has an average thickness of 300 nm. Figure 1 shows the XRD pattern of the SPCTO thin films deposited on Pt-Si substrate at various substrate temperatures. The XRD of the as-deposited films shows only a pronounced peak of tetragonal SPCTO, besides reflection from the Pt-Si substrate. The film deposited at 600 ◦ C has smaller full width at half maximum (FWHM) in the XRD pattern indicating better crystallinity. The occurrence of (100) peak in the XRD pattern confirms the formation of oxygenrich SPCTO film at this growth condition. The SPCTO thin films deposited at room temperature were amorphous. Crystalline films with (110) orientation were obtained only above 600 ◦ C substrate temperature. From the FWHM of XRD spectra, the optimum substrate temperature for the growth of crystalline SPCTO thin films was found to be 600 ◦ C. After optimizing the substrate temperature at 600 ◦ C, SPCTO films were deposited at various

123

A novel Sr3 Pb6 Ce2 Ti12 O36 ferroelectric thin film

Fig. 1 The XRD pattern of SPCTO thin films grown on Pt-Si substrates at an oxygen partial pressure of 0.1 mbar and at different substrate temperatures

substrate. The insufficient oxidation due to the low oxygen pressure in the deposition chamber causes oxygen vacancies and as a result the lattice is expanded by the elongation of Ti-O bond length [17]. The oxygen introduced in to the deposition chamber increases the collisions of the ablated species and thus reduces the energy of the ablated species and facilitates the oriented growth and thus more particles accumulate to form larger crystallites. There is a small shift in the (110) peak towards the lower angle with decrease in the oxygen partial pressure. This can be due to the larger lattice constant of the thin films occurred due to the oxygen vacancies induced during the film growth, as a result of reducing the Coulomb attractive force between cation and anion [18]. At higher oxygen pressures greater than 0.3 mbar, the ablated particles do not get enough energy due to the larger collision of these ablated particles with oxygen molecules and the adatoms may not have sufficient energy to crystallise. Therefore, the crystallinity decreases above 0.3mbar. The crystallite size of SPCTO thin films was determined by Scherrer’s formula [19]. d=

Fig. 2 The XRD pattern of the SPCTO thin films deposited on Pt-Si substrate at substrate temperature 600◦ C and at various oxygen partial pressures

oxygen partial pressures. Compared to the XRD spectrum from SPCTO target (Fig. 2), the SPCTO (110) and (100) diffraction peaks in the thin films, deposited at different oxygen partial pressures, have been shifted to the lower angle, due to the increased lead content in the thin films. Since ionic radius of Pb (1.49Å) is larger than Sr (1.44Å), the d-spacing and corresponding lattice parameter increase with increase in Pb concentration in the thin film [15]. Also the XRD spectrum of deposited thin films shows that FWHM for the (110) peak decreases with increase in oxygen partial pressure up to 0.3 mbar, thereafter it decreases. In vacuum most of the kinetic energy and internal energy of the ablated particles are converted to migration energy on the substrate due to relatively low rate of collision with oxygen atom. Thus, the inability to grow crystalline films at higher vacuum can be due to the fact that the high energy of the adatoms in the ablated plume is knocking out the deposited atoms from the

0.9λ , β cos θ

(2)

where d is the crystallite size, λ is the wavelength of X-rays used, β is the FWHM in radians of the peak at the diffraction angle θ . Table 1 shows the variation in crystallite size as a function of oxygen partial pressure within the chamber. The crystallinity as well as the crystallite size is maximum for the film deposited at 0.3 mbar. Reported phase diagram for Sr9−x Pbx Ce2 Ti12 O36 is shown in Fig. 3 [15]. The phase diagram shows that the Curie temperature increases linearly with Pb concentration. From the phase diagram, it is evident that Sr9−x Pbx Ce2 Ti12 O36 for x ≤ 6 exists in the paraelectric R3cphase at room temperature. As the Pb content increases beyond x = 6, Sr9−x Pbx Ce2 Ti12 O36 structure changes to monoclinic ferroelectric phase even at room temperature. EDX analysis was carried out for quantifying the Pb concentration in SPCTO thin films. Results (Table 1) shows that all the SPCTO thin films contain Pb in greater concentration than in the target (x = 6). Raman spectroscopy is an important tool for the structural study of ferroelectric materials, since the structural changes associated with ferroelectric phase transition will have significant effects in Raman spectrum. The room temperature Raman spectra from SCTO bulk, SPCTO bulk and SPCTO thin films deposited at various oxygen partial pressures are shown in the Fig. 4. Only few low intense Raman bands are observed in the SCTO bulk sample. Compared to Raman spectra from SCTO bulk sample, many new modes have been activated in the SPCTO thin films. These new modes correspond to ferroelectric phase [15]. Even though the SPCTO bulk with x = 6 corresponds to the paraelectric R3c group

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P. S. Krishnaprasad et al. Table 1 Summary of structural and dielectric studies on the SPCTO thin films on Pt-Si Thin film material

O2 Pressure (mbar)

FWHM-2 Theta (◦ )

Crystallite size (nm)

Pb content % = Pb Ce+Pb+Sr

(At.%)

Corresponding concentration of x in Sr9−x Pbx Ce2 Ti12 O36

Dielectric constant at 1 V

Tunability %

SPCTO

0.05

0.42

19.31

69

7.60

64

2



0.1

0.32

25.90

75

8.25

201

12



0.3

0.27

30.56

79

8.70

614

40

72

7.92

179

11

78

70



0.5

0.34

23.60

SCTO

0.3

0.40

20

0

Fig. 3 The phase diagram for Sr9−x Pbx Ce2 Ti12 O36 with x = 0–9 [15]

0

surements shown the Table 1. This is in well agreement with the previously reported results on SPCTO bulk material [15]. The intensity of Raman peak due to the ferroelectric phases increased with increase in Pb content. Thus, the Raman spectra suggest different crystal structures in paraelectric and ferroelectric phases of SPCTO. Leakage current directly limits the charge retention of ferroelectric capacitor which influences the ferroelectric hysteresis loop as well as the C-V characteristics. The leakage current is also a sensitive electrical probe for characterizing the material quality of the heterostructure as it is strongly dependent on material aspects of the ferroelectric film and of electrode-ferroelectric interfaces [20]. The current transport in ferroelectric films is generally governed by Ohmic conduction at low applied electric fields, besides that several non-linear conduction mechanisms such as Schottky emission [21], Poole Frenkel emission [22], space charge limited conduction [23] and grain boundary limited conduction [24] have been proposed as responsible for leakage current in the dielectric thin films at relatively high electric field. The leakage current due to Schottky emission for ferroelectric oxide thin films is given by Sudhama et al. [21] and Simmons [25].  J = AT 2 exp

Fig. 4 Room temperature Raman spectra from a Pt-Si substrate, b Sr9 Ce2 Ti12 O36 bulk, c Sr3 Pb6 Ce2 Ti12 O36 bulk and d Sr3 Pb6 Ce2 Ti12 O36 thin films deposited at various oxygen partial pressures

123

√  q V /4π εε0 w) , kT

(3)

where J is the leakage current density, A the effective Richardson constant, T the temperature, q the electronic charge, ϕ B the Schottky barrier height, V the applied voltage, ε0 the free space dielectric constant, ε the optical dielectric constant of SPCTO, w the depletion layer width and k is the Boltzmann constant. If the conduction is governed by carriers that are thermally emitted from trapped centers under a strong electric field, the Schottky emission model can be modified to Pool Frenkel model where the current is given by Simmons [25].  J = AV exp

at room temperature, the thin films deposited with this target show ferroelectric phase. This is due to the increased lead content in the films, which is evident from the EDX mea-

−q(ϕ B −

−q(ϕt −

√  q V /π εε0 w) , kT

(4)

where ϕt is the barrier height of trapped level. If the current transport at high voltages is governed by either one of these

A novel Sr3 Pb6 Ce2 Ti12 O36 ferroelectric thin film

Fig. 5 log J vs. E 1/2 plot of SPCTO thin films deposited at different oxygen partial pressure

Fig. 6 The log J vs. log E plot for the films deposited at different oxygen partial pressures

two mechanisms, the natural logarithm of current should linearly depend on the square root of applied voltage. The Schottky plot (log J vs. E 1/2 : E is the mean applied field) (Fig. 5) shows that the leakage current increases with increase of field. If the conduction current at high electric field (for which the internal space charge field is of less important) is governed by Schottky emission, the log J vs. E 1/2 plot should be a straight line. For the thin films deposited above 0.1 mbar, the log J is increasing linearly with E 1/2 above 300 (V/cm)1/2 , which indicates that above this field, conduction is governed by Schottky emission. For the film deposited at 0.05 mbar, the Schottky emission starts at 200 (V/cm)1/2 , which may be due to the higher carrier concentration in the film, originating from higher oxygen deficiency. To confirm the results, the same data are replotted as log J vs. log E (Fig. 6). For the films grown at 0.5 and 0.3 mbar oxygen partial pressures, the current increases linearly with external electric field (Fig. 6) at low electric field (slope m = 1 and 1.13, respectively), which suggests ohmic conduction through the film [17,26]. At progressively higher electrical field above 100 kV/cm, a transition region characterized by a larger slope is observed. Thus here, the current increases nonlinearly which implies that conduction is governed by Schottky emission. But for the film deposited at 0.1 and 0.05 mbar, even at the low field the slope is about 2, which shows SCL conduction. Here, the density of free electrons due to carrier injection becomes greater than the density of thermally activated free electrons [16,17,27]. The leakage current increases rapidly above 100 kV/cm for the films grown at 0.1 mbar and it is about 50 kV/cm for the film deposited at 0.05 mbar. Different slopes under positive bias demonstrate the difference in band bending conditions of the top and bottom electrode-film interface. The leakage current of SPCTO thin films grown at

0.3 and 0.5 mbar varies slowly at low electric field region in comparison with the films grown at 0.1 and 0.05 mbar. This may be due to the large grain boundaries with high resistivity impeding the conduction through the film [21]. Thus, it is evident from the above discussions, different conduction mechanisms dominate at different electric field regions. At low electric field, the conduction mechanism is governed by Ohmic or SCLC. At high fields, the conduction process is dominated by Schottky emission. The different conduction processes may be due to different crystallite size of SPCTO thin films and different defect levels in SPCTO thin films deposited at various conditions. Thus, the conduction mechanism in a thin film largely depends on the growth parameters like O2 pressure and substrate temperature. The time dependence of the leakage current of SPCTO thin films deposited at various oxygen partial pressures (Fig. 7) shows no resistance degradation in these thin films. Generally, the ferroelectric materials undergo a slow polarization when a voltage is applied to the capacitor [28]. The polarization results in a current that decreases with time. In highpermittivity materials, the flow of polarization charge caused by the application of a step voltage is large enough that it is easily measured as time-dependent current flow. The value of log J for an applied step voltage decreases linearly with log t over many decades in time, which can be fitted by Curie–Von Schweidler type current J = J0 t −n (0 < n < 1) [26,29,30], where n equal to the slope of the log–log plot and t is the time. The value of n is found to be 0.1 and 0.046 for the thin films deposited at 0.1 and 0.3 mbar, respectively. The current decreases more rapidly with time for the film deposited at lower oxygen partial pressure, which may be due to the higher oxygen vacancy concentration in the film. The C-V measurements were carried out on the PtSi/SPCTO/Cr/Au, M-I-M capacitor configuration. Pt-Si is

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Fig. 7 Time dependence of leakage current of SPCTO thin films at an applied voltage of 1 V

one of the most commonly used bottom electrodes for the integration of ferroelectric materials. The ferroelectric thin films deposited on Pt-Si substrate possess better charge storage density than that of films deposited on Si substrate. A very strong correlation between the growth process and dielectric properties of the SPCTO film is observed. The dielectric constant was calculated from the capacitance Vs applied voltage sweep across the capacitor at an applied frequency of 1 MHz. The applied bias voltage-dielectric constant plots of SCTO thin film deposited at 0.3 mbar (Fig. 8a) shows a paraelectric behavior, whereas SPCTO thin films (Fig. 8b–d) grown at different oxygen partial pressures confirm the ferroelectric behavior. The non-linearity of the capacitance or dielectric constant with electric field at the ferroelectric phase of the SPCTO film results from the anharmonic interaction of Ti4+ ion in the perovskite structure. The dielectric constant for the films deposited at different oxygen partial pressures is shown

Fig. 8 The bias voltage dependence of the dielectric constant of a SCTO film deposited at 0.3 mbar and SPCTO films deposited at b 0.3 mbar, c 0.1 mbar and d 0.05 mbar oxygen partial pressures

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A novel Sr3 Pb6 Ce2 Ti12 O36 ferroelectric thin film

in Table 1. The corresponding tunability for these devices was calculated using the formula Tunability =

Cmax − Cmin , Cmax

(5)

where Cmax is the maximum capacitance obtained and Cmin is the minimum capacitance over the investigated voltage range. The crystallinity as well as the crystallite size plays an important role in the dielectric properties as well as the tunability of the devices. The SPCTO film grown at 0.3 mbar has larger crystallite size leading to maximum dielectric constant compared with the films grown at other oxygen pressures. It is due to the fact that the larger grain size means domains with dipoles aligned in the direction of applied electric field is large causing greater dielectric constant [31–33]. All the SPCTO thin films have dielectric loss of the order of 10−1 . The bias voltage dependence of the dielectric loss of SPCTO films shows that the dielectric loss is larger for the film grown at 0.3 mbar, which may be due to the increased lead content in the film. The absence of Pb in SCTO thin film gives a lower dielectric loss of the order of 10−2 compared to SPCTO thin films. SCTO thin film grown at optimized temperature of 600 ◦ C and oxygen partial pressure of 0.05 mbar has a maximum dielectric constant of 78 and tunability 70%. Compared to dielectric constant for SCTO thin film at optimized conditions, SPCTO thin films have larger dielectric constant but lower tunability. The dielectric constant of the thin films was found to be low compared to that of bulk (≈ 3500 for bulk SPCTO and ≈185 for bulk SCTO at room temperature), which is a consequence of the small crystallites in the film. The presence of unavoidable dead layer of lower dielectric constant on the substrate and this layer in series with the crystalline SPCTO would result in a much lower capacitance value for the entire capacitor.

4 Conclusion Ferroelectric SPCTO thin films were deposited by pulsed laser ablation and the dependence of the structural and electrical properties of SPCTO thin films on the oxygen pressure and substrate temperature was investigated. The growth conditions were optimized. The optimum substrate temperature for the growth of SPCTO thin films was found to be 600 ◦ C. The crystallinity and crystallite size increases with increase in the oxygen partial pressure up to 0.3 mbar, thereafter it decreases. Investigations on the leakage current show that SPCTO thin films deposited at different oxygen pressure have different dominant conduction mechanism at various electric fields. At low electric field, the conduction mechanism is governed by Ohmic and SCLC. At high fields, the conduction process is dominated by Schottky emission mechanism.

The different conduction processes may be due to different crystallite size and different defect levels in SPCTO thin films deposited at various conditions. The time dependence of the leakage current of SPCTO thin films deposited at various oxygen partial pressures show no resistance degradation. The C-V characteristics revealed the ferroelectric nature of SPCTO thin films. The dielectric constant as well as the tunability is found to increase with increase in the crystallite size. Both Raman spectra and C-V characteristics shows a ferroelectric phase rather than paraelectric phase for the deposited thin films. The origin of ferroelectric phase is attributed to the increased Pb content in the film than in the target, which is in agreement with reported results on bulk SPCTO material. The conduction mechanism in the SPCTO thin films shows that they can be effectively used in tunable microwave and memory devices. Acknowledgments The authors wish to thank the Department of Science and Technology for financial assistance.

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