Dielectric, optical and electric studies on nanocrystalline Ba5Nb4O15 thin films deposited by RF magnetron sputtering

Applied Surface Science 340 (2015) 56–63

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Dielectric, optical and electric studies on nanocrystalline Ba5 Nb4 O15 thin films deposited by RF magnetron sputtering C. Anil Kumar, D. Pamu ∗ Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India

a r t i c l e

i n f o

Article history: Received 13 September 2014 Received in revised form 13 January 2015 Accepted 25 February 2015 Available online 6 March 2015 Keywords: BNO thin films RF magnetron sputtering Microstructure Electrical and dielectric properties

a b s t r a c t We report the fabrication of nanocrystalline Ag/BNO/Pt/Ti/SiO2 /Si thin film capacitors by RF magnetron sputtering with different film thicknesses. The effect of Ba5 Nb4 O15 (BNO) thickness on structural, microstructural, electrical, optical and dielectric properties is investigated for the first time. BNO sputtering target prepared is by mechanochemical synthesis method to eliminate the subordinate phases. As deposited thin films were X-ray amorphous and crystallinity is induced after annealing at 700 ◦ C. Upon annealing, refractive indices of the films enhanced whereas the bandgap is decreased and are in the range of 1.89–2.16 and 4.07–4.24, respectively. With an increase in thickness, the dielectric properties improved substantially, which is described by the representation of a dead layer connected in series with a bulk region of the BNO film. The extracted values of thickness and dielectric constant for the dead layer found to be 15.21 nm and 37.03, correspondingly. The activation energy of the mobile charge carriers obtained using the Arrhenius relation are found to be 0.254, 0.036 and 0.027 eV, for the films with 150, 250 and 450 nm, respectively. The leakage current density found to decrease with thickness and found to be 2.5 × 10−6 A/cm2 at applied voltage of 50 kV/cm. The J–E characteristics of the BNO films show a combined response of grain, grain boundaries and film–electrode interfaces. It is interesting to note that in the negative electric field region, conduction is ohmic in nature whereas in the positive field region BNO films exhibit both ohmic and the space charge-limited current mechanisms. The achieved dielectric, electrical and optical properties make these films suitable for MIC, CMOS and optoelectronic applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The microwave materials have been widely used in a variety of applications ranging from communication devices to satellite services. The rapid development of communication technology has increased the demand for dielectric materials for microwave resonators and capacitors, because these materials facilitate miniaturization of microwave devices. The microwave dielectric properties for such materials are a high dielectric constant (εr ), an extremely low dielectric loss (high Q value) and a small temperature coefficient of resonant frequency [1,2]. With the tremendous increase in the development of microwave-based communication technologies, the production of dielectric resonators has emerged as one of the most rapid growth areas in electronic ceramic manufacturing [3,4]. Further, the use of metal oxide surface coatings is performing a wide range of applications in the antireflection

∗ Corresponding author. Tel.: +91 361 2582721; fax: +91 361 2582749. E-mail address: [email protected] (D. Pamu). http://dx.doi.org/10.1016/j.apsusc.2015.02.172 0169-4332/© 2015 Elsevier B.V. All rights reserved.

coatings; microwave integrated circuits (MICs), complementary metal oxide semiconductor (CMOS) and optoelectronic devices [5,6]. Due to the comparatively eminent inertness of ceramic materials will make this type of thin films coatings used for wear and corrosion-resistant coatings [7]. Besides the thin films having been lowering crystallization temperature as compared to bulk samples and the manufacturing of high-energy storage thin film capacitors with high dielectric constant will make applications in the miniaturization of microwave constituents. Further, it is also known that the physical and chemical properties of the films drastically deviate from the bulk properties due to the confined geometry and the properties of thin films significantly changes with the processing parameters such as RF power, deposition time, substrate to target distance, O2 /(O2 + Ar) %, and working pressure. Hence consequently, it is important to study and understand the behavior of the films. Hence, it is important to understand the thickness dependence of these properties. Ba5 Nb4 O15 (BNO) ceramics are promising for the above applications due to its excellent dielectric properties in bulk form: high dielectric constant (εr ) ∼ 39, quality factor (Q × f) ∼ 23,700 GHz and

C. Anil Kumar, D. Pamu / Applied Surface Science 340 (2015) 56–63

temperature coefficient of resonance frequency ( f ∼ +78 ppm/◦ C). ¯ BNO ceramics reveals hexagonal perovskites structure with P3ml space group [8]. Nevertheless, the disadvantage with this material is the formation of subordinate phases when the samples were prepared using solid-state reaction method and higher sintering temperature, which limits the applications of this material in various applications [8,9]. To address the above problems, we have chosen to prepare the BNO ceramics using the mechanochemical synthesis method and small initial particle sizes. However, dielectric studies on bulk form of these BNO ceramics were reported by few researchers [10–12]. In addition, it is well known that optical, electrical and dielectric properties are strongly dependent on thickness and the processing technique, and it is important to understand how the response of a material varies with thickness. In contrast, to the best of the authors’ knowledge, there is no report available on the BNO thin films; this motivated us to pursue this study. Therefore, this work focuses on deposition and characterization of nanocrystalline BNO thin films by RF reactive magnetron sputtering. Study the effect of thickness on the structural, electrical and dielectric properties of BNO films. 2. Experimental details The BNO polycrystalline sputtering target was prepared using the mechanical alloying method to avoid the subordinate phases. The high-purity powders of BaCO3 (99.99%), Nb2 O5 (99.95%) were used as starting materials and were weighed in accordance with their stoichiometry. The weighed powders were ball milled for 50 h using tungsten vial and balls in a planetary ball mill (Pulrverisette, Fritsch GmbH, Germany). It was found that the pure BNO phase is obtained for the samples milled for 50 h. Afterwards BNO powders were pressed into a cylindrical target of 62 mm in diameter with 4 mm in thickness. The BNO target was sintered at 1250 ◦ C for 5 h. The BNO thin films were deposited by RF magnetron sputtering on 1 0 0 oriented Pt/Ti/SiO2 /Si and amorphous SiO2 substrates at ambient temperatures. The deposition chamber was pumped down to a base pressure of 5.0 × 10−6 Torr. The substrate to target distance was maintained at 6 cm. A mixture of high pure argon (99.999%) and oxygen (99.999%) were introduced into deposition chamber using mass-flow controllers. The sputtering pressure of 5 × 10−3 mTorr is maintained constantly throughout the deposition process by varying the argon/oxygen ratio in a standard cubic centimeter per minute (SCCM) in the sputtering gas, at a fixed RF power of 40 W for a constant duration of 3 h. The BNO target was

57

pre-sputtered in argon ambience for 10 min to remove the impurities on the target surface. The rate of deposition under different (O2 /(O2 + Ar)) % has been optimized to achieve the constant thickness of the film. The thickness of the films deposited on amorphous SiO2 substrates and Pt coated film was estimated using the surface profilometer (Vecco Dektak 150). The as deposited BNO films were X-ray amorphous and to obtain the crystallinity; BNO films were annealed at 700 ◦ C for 3 h in air. Phase purity of the BNO sputtering target and the crystallinity of the films was studied using the XRD patterns and the patterns were obtained using a Rigaku high power XRD machine with CuK␣ radia˚ The X-ray diffraction pattern of sputtering target tion ( = 1.5406 A). prepared by mechanical alloying was refined by using the Rietvield refinement technique and fullprof program. Surface morphology of the thin films was examined by field emission scanning electron microscope (FESEM, Zeiss Sigma). The energy dispersive spectra (EDS) of sputtering target and BNO films were obtained by using scanning electron microscope (SEM, Leo 1430vp) equipped with EDS. To study the electrical and dielectric properties Ag–BNO–Pt (MIM) capacitor structures were fabricated under different film thicknesses. Silver top electrodes of diameter 0.4 mm and 150 nm thick were deposited by dc sputtering using a shadow mask. The dielectric constants and dielectric loss of the films were calculated from the capacitance–frequency response as a function of temperature in frequency range of 20 Hz to 1 MHz using the LCR meter (Wayne Kerr Electronics Pvt. Ltd., Model 1J43100). The leakage current mechanism of BNO film capacitors at room temperature was measured using a parameter analyzer (Keithely Instruments INC., 4200-SCS). Spectral transmission characteristics in the wavelength range 200–800 nm were measured using a UV-VIS-NIR spectrophotometer (UV 3101PC, SHIMDZU). 3. Results and discussion 3.1. Structural and morphological studies To prevent the formation of secondary phases and to reduce the sintering temperature, BNO ceramics were prepared using mechanochemical synthesis method. The phase formation of the BNO ceramics was occurred after milling for 50 h. The X-ray diffraction pattern of BNO ceramics along with the Rietveld refinement data of the sputtering target, sintered at 1250 ◦ C for 5 h is shown in ¯ Fig. 1a. The refinement was carried out by considering P3ml space group. The lattice parameters, atomic positions of the Ba, Nb, and O

Fig. 1. (a) XRD pattern along with Rietveld refinement of Ba5 Nb4 O15 ceramic target material. The circles and solid line represent the experimental and the Rietveld refined data, respectively. The dotted line at the bottom shows the difference between the experimental and the refined data. The vertical bars correspond to the allowed Bragg’s peaks. (b) XRD patterns of BNO sputtering target and thin films deposited on Pt–Si substrates at different (O2 /(O2 + Ar)) % and annealed at 700 ◦ C for 3 h (() ␣-Ba4 Nb2 O9 , (*) BaNb2 O6 ,).

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Fig. 2. FESEM micrographs of the BNO films deposited on Pt–Si substrates at 50 (O2 /(O2 + Ar)) %, and annealed at 700 ◦ C for a constant duration of 3 h with different thicknesses (a) 150 nm, (b) 250 nm, and (c) 450 nm.

atoms, and occupancy are refined. The corresponding lattice con˚ c = 11.77 ± 0.002 A˚ which are stant values are a = b = 5.79 ± 0.002 A, in agreement with earlier reports [JCPDF file # 17-0794]. The fitting parameters (i) 2 ≈ 8.583, (ii) RBrag factor ≈ 7.718 and (iii) Rf factor ≈ 9.125 for BNO target system. BNO thin films deposited on platinized silicon substrates at ambient temperatures under various (O2 /(O2 + Ar)) % (0, 25, 50, 75 and 100%) depicts amorphous nature. It is known that oxide films naturally grow in an amorphous state unless the energy is supplied in the form of ion bombardment or in the form of temperature during the deposition and also due to the lattice mismatch between the substrate and the film [13]. However, postannealing at 700 ◦ C for 3 h in air, all the thin films are partially crystalline in the hexagonal perovskites crystal structure and the XRD patterns of the BNO sputtering target and films deposited on platinized silicon substrates are shown in Fig. 1b. It is interesting to note that the films deposited in pure argon exhibit ␣-Ba4 Nb2 O9 [JCPDF file # 35-1154] and BaNb2 O6 [JCPDF file # 13-0451] as subordinate phases along with the BNO phase. With an increase in (O2 /(O2 + Ar)) % from 25 to 100% in the deposition chamber, the secondary phases disappear. Further the crystallinity of the films increases with an increase in (O2 /(O2 + Ar)) % up to 50% above that the crystallinity of the films diminishes. Films deposited at 50 (O2 /(O2 + Ar)) % exhibit better crystallinity as compared to the films deposited at 25 (O2 /(O2 + Ar)) %. It is interesting to note that BNO films were obtained even in the pure oxygen atmosphere which shows that the target poisoning is partial. However, even after annealing at 700 ◦ C, the films were partially crystalline with strong glassy background. The obtained better crystallinity under pure argon ambience may be due to the large momentum transfer of ions (heavier argon atoms). With an increase in (O2 /(O2 + Ar)) %, the rate of deposition decreases due to the oxidation of the target and also due to lighter oxygen ions, which result poor crystallinity. Further, it is also observed that with an increment in (O2 /(O2 + Ar)) %, the relative intensities of the emission lines changes and may be due to the variations in the energies of ions in the plasma. These variations may cause slight changes during the growth and orientations of the films in a particular plane. In addition, it is as well observed that with an increment in (O2 /(O2 + Ar)) %, the full width at half maximum declines, which shows the decrease

Table 1 The deposition rates of as-deposited BNO films on platinized silicon substrates as a function of different (O2 /(O2 + Ar)) %, for a constant duration of 3 h. BNO films as-deposited at different (O2 /(O2 + Ar)) %

RF power (W)

Thickness (nm)

0 25 50 75 100

40 40 40 40 40

568 534 497 461 429

± ± ± ± ±

5 5 5 5 5

Deposition rate (nm/min) 3.1 2.9 2.7 2.5 2.4

in crystallite size. The average crystallite sizes obtained using the Scherer’s formula and are in the range of 21–43 nm. Furthermore, films deposited in pure argon atmosphere exhibit secondary phases whereas the films deposited beyond 50 (O2 /(O2 + Ar)) % exhibit poor crystallinity. Consequently, it is decided to study the electric and dielectric properties of the BNO films as a function of thickness deposited at 50 (O2 /(O2 + Ar)) %. For the films deposited at 50 (O2 /(O2 + Ar)) %, as the thickness increases, the degree of crystallinity of the films enhanced (not shown). This may be due to formation of nucleation centers on the substrates assembles tiny crystals also due to the shorter deposition times. At smaller thickness, these small crystals cannot form the neck with neighboring crystals. Hence, a film with lower thickness exhibits tiny crystals. Further, as the deposition time increases, thickness of the films also increases, which facilitate the nucleation of microscopic crystals into large grains. The thickness and deposition rates of as-deposited BNO films on platinized silicon substrate at different (O2 /(O2 + Ar)) % shown in Table 1. FESEM images of the BNO films deposited on platinized silicon substrates and annealed at 700 ◦ C for a constant time duration of 3 h under 50 (O2 /(O2 + Ar)) % with different thicknesses are shown in Fig. 2a–c, which corresponds to the films’ thickness of 150, 250 and 450 nm, respectively. The FESEM images of as-deposited films and the films deposited below 100 nm did not show any significant features in the surface morphology. As the thickness increases, the grain size improved gradually. Interestingly, all the annealed films reveal the nanogranulars and nanorods, which are distributed uniformly on the film surface. The average diameter of nanogranulars

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Table 2 Elemental analysis for the sputtering target and the films with different thicknesses. Bulk BNO target/BNO films thickness (nm) at 50 (O2 /(O2 + Ar)) %

Element

Weight

Atomic (%)

Required

Obtained

Bulk BNO target

O (K) Nb (L) Ba (L)

18.53 28.70 52.77

62.55 16.69 20.76

15 4 5

15.01 4.00 4.99

150 nm BNO film

O (K) Nb (L) Ba (L)

18.45 28.73 52.82

62.44 16.74 20.82

15 4 5

14.99 4.02 4.99

250 nm BNO film

O (K) Nb (L) Ba (L)

18.50 28.89 52.61

62.50 16.80 20.70

15 4 5

15.00 4.03 4.97

450 nm BNO film

O (K) Nb (L) Ba (L)

18.46 28.66 52.88

62.46 16.70 20.84

15 4 5

14.99 4.01 5.00

and nanorods are found to be in the range of 20–45 nm. Further, the grains of BNO films have well defined boundaries, denser in structure with an increase in the thickness of the film. Furthermore, as the thickness of the films enhances the nucleation of the small crystals takes place, this tends to increase the grain size. In addition, with an increase in thickness, the formation of nanogranulars and nanorods is prominent. The elemental analysis of the annealed BNO film deposited at 50 (O2 /(O2 + Ar)) % with different thicknesses of 150, 250 and 450 nm, respectively are summarized in Table 2. It is found that the composition of the target is nearly achieved in the BNO films. The present study confirms that the multicomponent material can be easily grown in stoichiometric state by using RF magnetron sputtering technique.

interference. The strong absorption was observed at wavelengths below 350 nm, 400 nm as deposited and annealed films, respectively and the annealed films exhibit the red shift. In addition, the annealed films exhibit lower transmittance as compared to the as deposited films and this may be attributed to the increase in average grain size and grain boundary, which scatters the light. The property of high transmittance in the visible region makes BNO films a good candidate for transparent window layer applications. The refractive index (n) of the BNO films can be derived by employing the envelop method [14] on the basis of the following expression

3.2. Optical properties

where

The optical behavior of the nanocrystalline BNO films were studied for the films deposited on amorphous SiO2 substrates. Further, the optical response of the films with respect to thickness is minimal and hence the optical constants of the BNO films are studied as a function of (O2 /(O2 + Ar)) %. The spectral transmittance of amorphous and nanocrystalline BNO films is depicted in Fig. 3a and b, respectively. The transmittance spectra of nanostructured BNO thin films show a transparency about 90% and 80% for as-deposited and annealed films in the visible region, correspondingly. Further, a transparent oscillating and a strong absorption were observed in the transmittance spectra of both the films. Both the films exhibit peaks and valleys that are associated with the reflections from the incident light from the air–film, and film–substrate due to

N = 2s

 = [N + (N 2 − 2s )

T

0.5 0.5

]

− Tmin Tmax Tmin

max



+

(1)

2s + 1 2

(2)

here Tmax and Tmin are the corresponding transmittance maxima and minima at certain wavelength and ns is the refractive index of the substrate used. The variation in a refractive index of both the as deposited and annealed films as a function of (O2 /(O2 + Ar)) % is shown in Fig. 4a. It is observed that the refractive index of the annealed films is higher compared with as deposited films. On annealing, the increase in the refractive index is attributed to the crystallinity of the films. This is obvious because the as deposited films were deposited at ambient temperature and at this temperature films will be highly disorder due to the low adatom mobility. This causes the poor nucleation

Fig. 3. Transmittance spectra of (a) as deposited and (b) annealed BNO films at 700 ◦ C for 3 h on amorphous SiO2 substrates as a function of (O2 /(O2 + Ar)) %.

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Fig. 4. Variation in (a) refractive index, (b) bandgap of BNO films as a function of (O2 /(O2 + Ar)) % and (c) absorption edges of as deposited and annealed BNO film deposited at 50 (O2 /(O2 + Ar)) %.

which results in lower refractive indices. Further, the refractive index of both the films increases linearly up to 50 (O2 /(O2 + Ar)) % above that it started decreasing, and this is attributed to the crystallinity of the films which is complemented by the obtained XRD patterns of these films. The obtained refractive index values are in the range of 1.82–2.05, and 1.89–2.16 at 550 nm for the as-deposited and annealed films, respectively. The variation in an optical band gap as a function of pressure ratio in (O2 /(O2 + Ar)) % for the as-deposited and annealed films was shown in Fig. 4b. The Tauc equation [15] was used to estimate the optical band gap of films, which was given by (˛h) = c(h − Eg )



(3)

where ˛ is the optical absorption coefficient, h is the incident photon energy, c is energy independent constant, Eg is the optical bandgap and  is a constant which characterizes the nature of band transition.  = 0.5, 1.5 corresponds to direct allowed and direct forbidden transition and  = 2, 3 corresponds to indirect allowed and indirect forbidden transitions respectively. The optical absorption coefficient (˛) was calculated from the transmittance (T) using the following equation ˛=

− ln(T ) d

was confirmed from optical transmittance spectra shown in Fig. 3. Another possibility is lowering of bandgap can be due to electro negativity of transition metal Nb5+ ions and increased width of a conduction band [16]. The estimated optical bandgap values for the as deposited and annealed films are in the range of 4.13–4.36 eV and 4.07–4.24 eV, respectively. The obtained bandgap values are higher when compared to the bulk bandgap of the BNO ceramics (Eg = 3.63 eV and 3.9 eV) [11,17]. In the present study, the achieved higher bandgap of the BNO films are attributed to the partial crystallinity of the films. It is known that when the crystalline phase of the film is embedded in the predominantly amorphous matrix, there are large deviations in the optical bandgap of the films from the bulk values [18]. The obtained optical bandgap values indicate that the deposited films were in the stoichiometric state because the optical bandgap is an indirect estimate of the stoichiometry of the films. The absorption edges of the as deposited and annealed film deposited at 50 (O2 /(O2 + Ar)) % is shown in Fig. 4c which displays the variations in the optical bandgap of the as deposited and annealed films and complements the variations in the absorption edges in the transmittance spectrums. 3.3. Dielectric properties

(4)

where d = BNO film thickness in nm and T is defined by T = IT /Io where IT and Io are the intensities of radiation coming out of the film and incident radiation, respectively. The values of optical bandgap Eg can be obtained from the extrapolation of the straightline portion of the (˛h)2 vs h plot to h = 0. It is observed that the optical bandgap of both as-deposited and annealed films increases with an increase in (O2 /(O2 + Ar)) % and as-deposited films showed higher values as compared to annealed films. The observed variation in bandgap may be attributed to the improvement in the crystallinity, increase in particle size of the films which is associated to decrease of energy levels within the optical bandgap and

To study the dielectric response of the BNO films, Ag/BNO/Pt/Ti/SiO2 /Si thin film capacitors were fabricated with three different (150, 250,450 nm) thicknesses and the films were deposited at 50 (O2 /(O2 + Ar)) % at ambient temperature for the time duration of 65, 105 and 180 min, respectively. The frequency dependence of dielectric constant (εr ) and loss tangents (tan ı) for the Ag/BNO/Pt/Ti/SiO2 /Si capacitors are shown in Fig. 5a and b, respectively. It is observed that at low frequencies, all the samples exhibit larger dielectric constants and higher tan ı and beyond 200 kHz, there is a little dispersion in εr . This can be attributed to the interfacial polarization in this frequency range. Beyond 200 kHz, the reduction in both the dielectric constant and higher

Fig. 5. Variation in (a) dielectric constant, (b) loss tangent of BNO thin films deposited at different thicknesses as a function of frequency and (c) plot of dielectric constant and loss tangent vs film thickness, measured at 1 MHz.

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tan ı could be due to the dipole polarization which active in this frequency range (200 kHz to 1 MHz), which is a typical behavior of the linear dielectrics and can be explained on the basis of Maxwell’s polarization theory and Koop’s two-layer model [19]. Further, with an increase in thickness, the dielectric constant of the films increases whereas the loss decreases. The variation in dielectric constant and tan ı measured at 1 MHz as a function of thickness is shown in Fig. 5c. It is evident that with an increase in thickness, the dielectric constant of the films increases exponentially whereas the dielectric loss is found to decrease linearly. The simultaneous increase in dielectric constant and decrease in tan ı with an increase in thickness can be attributed to the enhancement in degree of crystallinity of the films, improvement in the microstructure, and reduction in rms roughness. Further, Ag/BNO/Pt/Ti/SiO2 /Si structures can be considered as the two parallel plate capacitors coupled in series. This assembly can be interrelated to the electrode–BNO films interfacial layer (dead layer) and bulk. Hence, the total capacitance (c) of Ag/BNO/Pt/Ti/SiO2 /Si structure can be written as 1/c = (1/ci ) + (1/cb ) where ci and cb designate the capacitance of the interfacial layer and bulk sample, respectively. In addition, the dielectric constant of BNO thin film capacitor with separate thickness can be expressed as t t − ti t t t ≈ i + = i + ε εi εb εi εb

(5)

where t and ti are the total thickness of the film and the interfacial layer and ε, εi and εb are the dielectric constants of the film, interfacial layer and bulk BNO, respectively [20,21]. The approximation in Eq. (5) is reasonable because the total thickness of the films is much higher as compared to interfacial (dead) layer. Further, the contributions from the dead layer and bulk BNO to the dielectric constant can be estimated by plotting t/ε vs t, gives a linear curve which is depicted in Fig. 6. The values of εb and ti /εi can be extracted from the reciprocal of the slope and y-intercept through the linear fitting. The obtained values for dead layer and bulk BNO for dielectric constant is found to be 15.21 nm and 37.03, respectively. The dead layer occurs between the BNO film and of the electrodes, which arise due to the oxygen diffusion and changes in the surface chemistry of electrode interfaces or may be due to the formation of reactive layer at the electrode interface. Further with an increase in thickness, the surface diffusion reduces at the film–electrode interface and the influence of dead layer diminish, which result in low losses. The temperature dependent dielectric constants and tan ␦ of BNO films fabricated with different thicknesses are shown in Fig. 7a and b, respectively. It is observed that as the temperature increases, both the dielectric constant and tan ı of BNO films increases progressively. Further, the loss tangent is higher for the films with

61

Fig. 6. Plot of t/ε vs thickness of film (discrete points) and solid line represents linear fit.

lower thickness. The increase in dielectric constant with temperature can be explained as follows: thermal energy facilitates the alignment of the dipoles in the field direction which enhances the dielectric constant of the films. On the other hand, with an increase in thermal energy, mobility, conductivity and the vibration of atoms result in enhanced dielectric loss. The temperature stability of the dielectric constant of the BNO films is evaluated by calculating the temperature coefficient of dielectric constant (TC␧). TC␧ is obtained by using the following expression TC␧ =

εr (ppm/◦ C) ε0 T

(6)

where εr is the change in εr with respect to the value ε0 (T = 300 K) and T is the change in temperature relative to 300 K. The variation in TC␧ as a function of thickness is shown in Fig. 7c. It observed that all BNO films revealed positive TC␧ values across. It is evident that as the film thickness increases, the TC␧ of BNO films decreases linearly and the film with 450 nm thickness exhibits a better stability (53 ppm/K). Fig. 8 shows the variation of AC conductivity ( ac ) of BNO films with different thicknesses vs 1000/T, measured at 1 MHz. The slope of each curve gives the activation energy of the mobile charge carriers and calculated using the Arrhenius relation which is given below

ac = 0 exp(−Ea /kB T )

(7)

Fig. 7. Variation in (a) dielectric constant, (b) loss tangent of BNO thin films deposited at different film thicknesses as a function of temperature and (c) plot of TC␧ vs film thicknesses.

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Fig. 8. Variation of (ac) vs 1000/T.

where o is pre-exponential factor, Ea is the activation energy, kB is the Boltzmann constant and T is the absolute temperature. The extracted energies are found to be Ea = 0.254, 0.036 and 0.027 eV for the films with 150, 250 and 450 nm, respectively. The Ea depends on the average film thickness. The activation energy for film of 150 nm is one order more than 250, 450 nm films. So these reflections may be due to size effects that are appeared because of quantum confinement of charge carriers within the particles. 3.4. Electrical properties of BNO film capacitors Fig. 9 shows the room-temperature leakage current density mechanism for the Ag/BNO/Pt/Ti/SiO2 /Si (MIM) capacitors as a function of thickness. The leakage currents were measured with a voltage step of 1 V and step duration of 10 s. The average leakage current density is about less than 10−6 A/cm2 at an applied electric field of 50 kV/cm. As the thickness of the film increases, the leakage current density of the BNO films decreases. The leakage current density increases with applying electric field, which may be due to the structured grain boundaries and crystal defects. The J–E characteristics of the BNO films showed a combined response of grain, grain boundaries and film–electrode interfaces. The asymmetric J–E curves may be associated with the different interfacial layer indentations between the bottom electrodes–film and film–top electrode interfaces. Fig. 10 shows the logarithmic dependence of current density as a function of logarithmic electric field for BNO thin films. Here

Fig. 9. Leakage current densities of BNO films measured at different applied electric field ranges with different film thicknesses.

the conduction behavior of BNO films with different thicknesses is described on the basis of ohmic and space charge-limited current (SCLC) model [22,23]. In the negative electric field region, the BNO films follow an Ohmic conduction (J ∼ E˛ : ˛ ∼ 1) behavior. The obtained ˛ values are found to be 0.84, 0.88 and 0.96 for film thickness of 150, 250 and 450 nm, respectively. On the other hand, in the positive electric field region, BNO films exhibit both the Ohmic and SCLC (J ∼ E˛ : ˛ 1) nature. The extracted ˛ values are 1.19, 1.39 and 1.14 for film thickness of 150, 250 and 450 nm, respectively in the positive electric field region and ˛ = 3.45 observed for film thickness of 450 nm which shows the SCLC conduction. The equivalent oxide thickness (EOT) of the Ba5 Nb4 O15 thin films and the required thickness of the high – εr thin film when the EOT is taken as 1 nm can be calculated [24] using the formula teq =

thigh−εr × 3.9 εhigh−εr

(8)

where thigh-εr is the thickness, εhigh-εr is the dielectric constant of the high-εr material and teq is the equivalent oxide thickness. The obtained value is 10.5 nm for Ba5 Nb4 O15 films with thickness 450 nm. The obtained excellent electric and dielectric properties of BNO films are promising for MIC and CMOS applications.

Fig. 10. Log J as a function of log E under the (a) negative electric field and (b) positive electric field.

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4. Conclusions BNO thin films were prepared by RF magnetron sputtering at different oxygen pressure ratios in (O2 /(O2 + Ar)) % on to Pt–Si and amorphous SiO2 substrates. The effects of annealing and thickness on the structural, dielectric and electrical properties were discussed. As deposited thin films were amorphous in nature and after annealing they were crystallined. The optical constants of the films studies as function of (O2 /(O2 + Ar)) ratio. With increasing thickness of the film, dielectric constant advances whereas the loss tangent decreases. The best dielectric properties were obtained for film with 450 nm are εr = 41 and tan ı = 0.0005 at 1 MHz. The dielectric response of BNO capacitors has been explained by considering a dead layer connected in series with dielectric constant of the films. The extracted values of dielectric constant and thickness for the dead layer are found to be 37.03 and 15.21 nm, respectively. The leakage current characteristics found to decrease with thickness and the leakage current density is about less than 10−6 A/cm2 at an applied electric field of 50 kV/cm. The leakage current studies revealed ohmic behavior during the negative cycle whereas space charge limited current and ohmic response during a positive cycle. Acknowledgments The authors acknowledge the financial support from DST [SR/FTP/PS-109/2009] India. Facilities provided by DAE BRNS [2010/20/37P/14BRNS], DRDO [ERIP/ER/0900371/M/01/1264], BRFST [NFP-RF-A12-01] and XRD under FIST program (Ref. No: SR/FST/PSII-020/2009) is acknowledged. References [1] K. Sudheendran, M.K. Singh, K.C.J. Raju, R.S. Katiyar, The effect of an oxygen atmosphere on the microwave dielectric properties of Bi2 Zn2/3 Nb4/3 O7 thin films, Solid State Commun. 150 (2010) 1928–1931. [2] D. Pamu, P.D. Raju, A.K. Bhatnagar, Structural and optical properties of Ba(Zn1/3 Ta2/3 )O3 thin films deposited by pulsed laser deposition, Solid State Commun. 149 (2009) 1932–1935. [3] M.V. Jacob, D. Pamu, K.C.J. Raju, Cryogenic microwave dielectric properties of sintered (Zr0.8 Sn0.2 )TiO4 doped with CuO and ZnO, J. Am. Ceram. Soc. 90 (5) (2007) 1511–1514. [4] K.P. Surendran, M.T. Sebastian, P. Mohanan, M.V. Jacob, The effect of dopants on the microwave dielectric properties of Ba(Mg0.33 Ta0.67 )O3 ceramics, J. Appl. Phys. 98 (2005) 094114.

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