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Applied Surface Science
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n the formation of an interface amorphous layer in nanostructured ferroelectrica0.8Sr0.2TiO3 thin films integrated on Pt–Si and its effect on the electricalroperties
.P.B. Silvaa,∗, K.C. Sekhara, S.A.S. Rodriguesa, M. Pereiraa, A. Parisinib, E. Alvesc,.P. Barradasc, M.J.M. Gomesa
Centre of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, PortugalCNR-IMM Sezione di Bologna, via P. Gobetti 101, 40129 Bologna, ItalyIST/ITN, Instituto Superior Técnico, EN10, 2686-953 Sacavém, Portugal
r t i c l e i n f o
rticle history:eceived 28 June 2012eceived in revised form 16 October 2012ccepted 28 November 2012vailable online 7 December 2012
eywords:
a b s t r a c t
The thin films of Ba0.8Sr0.2TiO3 (BST) investigated in this work were produced by pulsed laser deposi-tion at different pulse-repetition frequencies (PRFs). First measurements by X-ray diffraction suggested acrystalline nature of the deposited films. However, scanning transmission electron microscopy and trans-mission electron microscopy images have revealed that a BST amorphous layer of considerable thicknessis formed at the interface between the film and the Pt layer in the films deposited at 10 Hz. Moreover,energy-dispersive X-ray spectroscopy shows that the composition of the BST layer is the same in both the
ulsed laser depositionaTiO3 and titanatesielectric properties
nterfacial amorphous layer
amorphous and the crystalline phases whereas Rutherford backscattering spectrometry measurementshave revealed a stoichiometry of the films identical to that of the target. A new interpretation is proposedto explain the formation of this amorphous layer, based on the PRF used during the deposition. Finally,measurements of dielectric and electric properties were performed on as-grown and annealed samples.The results of these measurements are explained by a model, where a low-permittivity amorphous layeris connected in series with the crystalline BST layer.
. Introduction
Barium strontium titanate (Ba1−xSrxTiO3) materials are promis-ng candidates for a wide range of applications such as highielectric constant capacitors, non-volatile memories, infrared sen-ors and tunable microwave applications due to their excellenterroelectric, piezoelectric and pyroelectric properties in additiono their eco-friendly nature [1–3]. The important limitation ineduction of size and operating power of these devices is the thick-ess of the films since their functional properties degrades as thehickness decreases. This could be due to the presence of certainow-dielectric interfacial layers at the metal–film interface. Thisnterfacial layer acts as a parasitic capacitor connected in series
ith the bulk of the film, lowering their functional properties. Itas been reported that growth conditions have also shown strong
nfluence on the formation of interface. For example, the thickness
f amorphous layer formed at the interface of bottom platinumlectrode and Ba0.6Sr0.4TiO3 crystalline thin films produced by RFputtering was decreased with decreasing the RF power [4].
Despite experimental proof of the existence of the interfacelayer, its physical origin and the issues incurring from inter-face layer are still unclear. The proposed possible explanationsinclude the existence of low dielectric constant space-charge lay-ers caused by strain due to different thermal expansion coefficientsof substrate, electrode, and BST thin film; oxygen-depletion zonesadjacent to metals with a high oxygen affinity; formation of surfacestates; local diffusion of electrode material into the ferroelectricthin films; lattice mismatch induced ion vacancy formation; chem-ically distinct surface phase; intrinsic surface polarization effects[5–7]. Unfortunately, none of the above is well accepted by thescientific community and is free to be contradicted.
In this work, we have deposited Ba0.8Sr0.2TiO3 thin films on pla-tinized substrate at different pulse-repetition frequencies (PRFs) of1 and 10 Hz and the influence of PRF on the growth and propertiesof films have been investigated. The composition and crystallinityof different regions of the films have been investigated with high-resolution transmission electron microscopy. Interestingly, a 50 nmthick amorphous layer was observed at the interface in the films
deposited at 10 Hz. Moreover, the amorphous layer disappearedwhen the films are deposited at 1 Hz. A crystal growth mechanismis proposed in order to explore the formation of amorphous layer.The effect of annealing on thickness of interfacial layer was also
gen defects in pervoskite films like SrTiO3, will lead to incrementin the lattice parameters [11,12]. Fig. 3 shows the RBS spectra ofBST thin films grown at 1 and 10 Hz. The relative amounts of thebarium (Ba) and strontium (Sr) in the deposited BST thin films were
Table 1Lattice parameters of the BST films and target.
Sample a (A) c (A)
ig. 1. Optical transmittance spectra of BST films deposited at different frequenciesn glass substrates. The inset shows the plot of the refractive index versus frequencyf deposition.
nvestigated. The effective dielectric constant of bulk, amorphousayers has been extracted by applying the model, where a low-ermittivity interface layer is connected in series with the bulkegion of the film.
. Experimental techniques
A Ba0.8Sr0.2TiO3 (BST) target was synthesized by a conventionalolid state reaction using a stoichiometric mixture of BaCO3, SrCO3nd TiO2, and sintered at 1200 ◦C. This target was used for the depo-ition of thin films. The BST thin films were grown on Pt/Ti/SiO2/Si attemperature of 700 ◦C using a KrF excimer laser of 248 nm wave-
ength at the pulse-repetition frequencies (PRFs) of 1 and 10 Hz. Theaser fluence was kept at 4 J/cm2. During the deposition, an oxygenas pressure of 2.5 × 10−2 mbar was kept constant. After the depo-ition, the film was cooled down to the room temperature under theame pressure. The obtained thickness was found to be ≈380 nm.he BST thin films were structurally characterized by X-ray diffrac-ion with a Philips X-ray diffractometer (model PW1710) usinguK� radiation (� = 1.54056 A). Rutherford back scattering (RBS)pectroscopy was performed to estimate the exact composition ofhin films. The measurements were done at 2 MeV with He4+. Theransmission electron microscopy (TEM), scanning transmissionlectron microscopy (STEM), energy-dispersive X-ray spectroscopyEDX) and electron micro-diffractions were performed with FEIecnai F20 ST system.
Gold circular electrodes having a diameter of 1 mm wereeposited by thermal evaporation to perform the electrical andielectric characterization. The electrical properties were mea-ured using a Keithley 617 programmable electrometer. Dielectriceasurements were done with a QuadTech 7600b Precision LCReter with high accuracy (±0.05%), and an applied voltage 30 mV,
t room temperature.
. Results and discussion
Before depositing the films on Pt/Ti/SiO2/Si substrate, the filmsere first deposited on the glass with different laser PRFs of 1, 2, 5
nd 10 Hz. Their transmittance spectra were recorded in the wave-ength range of 300–1000 nm. All the films are transparent in theisible region. The plots of transmittance as a function of wave-
ength are shown in Fig. 1. The refractive indices of the thin filmsre calculated from the transmittance spectrum using the envelopeethod [8]. The variation of refractive index with laser PRF used for
he deposition is shown in inset of Fig. 1. It suggests that there is
Fig. 2. XRD patterns of as-grown (deposited at 1 and 10 Hz) and annealed BST(deposited at 10 Hz) thin films and the BST target.
significant variation in the refractive index of the films grown at1 and 10 Hz. With decreasing the laser PRF, the refractive indexshifted toward the bulk value [3]. In order to explore the effect oflaser PRF on growth and properties of the films, we have grown thefilms on Pt/Ti/SiO2/Si with laser PRF of 1 and 10 Hz and have beenused for the further characterization.
The crystallinity and stoichiometry of BST thin films deposited atPRFs of 1 and 10 Hz were studied with XRD and RBS. Fig. 2 displaystypical XRD patterns of the BST films deposited on the platinizedsubstrate and the target. The presence of all the peaks correspond-ing to bulk BST reveals that the films have polycrystalline perovskitephase without any other impurity phase [9]. However, the peaksposition in films slightly shifts toward the lower side as comparedto the target and causes the change in lattice parameters. The cal-culated values of lattice parameters (a and c) for the BST films andtarget were shown in Table 1.The increased value of the latticeparameter in the films compared to the bulk could be attributedto the tensile stress, that might be due to difference in the ther-mal expansion coefficient of BST (˛BST = 10.5 × 10−6 ◦C−1) and thesubstrate (˛Pt = 9 × 10−6 ◦C−1) [10] and/or the presence of oxygendefects in the films [11,12]. It is shown that the presence of oxy-
BST film grown at 1 Hz 4.000 4.006BST film grown at 10 Hz 4.002 4.006Annealed film (grown at 10 Hz) 3.995 3.996Target 3.976 3.990
ig. 3. Rutherford back-scattering spectra of the BST films deposited at 1 and 10 Hz.
educed from the data using the standard data analysis code NDF13], including all relevants effects such as double scattering [14]nd pile-up [15] in order to calculate the backgrounds accurately,hat affect in particular the signal in the low energy range, requiredo determine the oxygen content. The stoichiometry of both the filmas found to be Ba0.82Sr0.18TiO3, which is very close to the desireda0.8Sr0.2TiO3 film stoichiometry. The simulated signal from eachlement is marked in Fig. 3. The inset in Fig. 3 shows an expansionf the low energy signals, where it is clear that the O signal is super-mposed to the Si substrate signal. However, both the front and theack edges of the O are clearly visible, and, moreover, an increasedmount of oxygen leads to corresponding decrease in the amount
f metal, which would be visible in the high energy part of the spec-rum. A Bayesian inference uncertainty analysis [16] indicates thathe error in the O content is around 3 at.%, or 3 ± 0.09. The error inhe Ba and Sr is around 1 at.%, and it is around 2 at.% in the Ti.
ig. 5. (a) Cross-sectional Z-contrast STEM image of the BST film grown at PRF of 10 Hz. Ceen obtained. (b) EDX spectra of the BST and Pt layers.
Fig. 4. Plot of dielectric permittivity and dielectric loss versus frequency for differentBST films.
The dielectric properties of the films deposited at PRFs of 1 and10 Hz were shown in Fig. 4. The dielectric properties were measuredin the frequency range of 100 Hz to 1 MHz, at room temperature.Both dielectric permittivity (εr) and losses (tan ı) have shown asmall dispersion with frequency. The value of εr was found to be≈281 in the films deposited at PRF of 1 Hz and is in good agree-ment with reported values for BST films [17,18]. However, thefilm deposited at 10 Hz has shown only half value (ε = 132) andhigher dielectric losses as that of the films deposited at 1 Hz. Thissuggests that PRF has shown a strong influence on the growthof films and thus consequently on their dielectric and electricalproperties.
To understand the effect of PRF on the structure and morphologyof the film, STEM was performed on the cross-section of the BST thinfilm deposited at PRF of 10 Hz and is shown in Fig. 5(a). It is clear thatthe most of film has a dense columnar structure, but this columnarstructure has not initiated from the surface of the substrate (bot-tom Pt electrode). The EDX was performed on the different parts offilms (marked as 1, 2 and 3 in Fig. 5(a)) to explore the possibilityof diffusion and/or the composition variation with the depth of thefilm and are shown in Fig. 5(b). The relative intensity ratio is found
to be the same in all regions of films. It reveals that there is no sig-nificant change in the Ba/Sr ratio between the top of the film andnear the bottom platinum electrode and also, confirms that thereis no diffusion between BST film and the platinized substrate. As
ircles indicate the probe positions where the spectra on the right of the figure have
Fig. 6. (a) Bright field TEM image of BST films. The number indicates the probe positions where the electron micro-diffractions were obtained. (b) Electron micro-diffractionso ce; (3
tfeedtarfiostIea
btained on the different layers: (1) BST top layer; (2) BST layer near Pt/film interfa
he electrode/film interface is one of the most important causesor the failure of the devices, it is worthy to understand the exist-nce of two different regions in the film. The TEM was employed toxplore crystalline nature of these two different regions. Fig. 6(a)epicts the bright field TEM image of the BST film. It is noticed thathe lower part of the film has a homogeneous layer of thicknessround 50 nm (marked as 2), which has a contrast that differs fromest of the film (marked as 1 in Fig. 6(a)). This upper part of thelm has columnar structure. Electron micro-diffractions obtainedn the different layers marked in the bright-field TEM image werehown in Fig. 6(b). This result indicates that the regions 1 and 2 ofhe films have an amorphous and crystalline nature respectively.
n addition, the electron micro-diffractions also confirm the pres-nce of the Pt layer (marked as 3), amorphous SiO2 (marked as 4)nd Si substrate (marked as 5). However, the films deposited with
low PRF did not exhibit any amorphous region near the surface ofsubstrate. Fig. 7 shows the cross-section SEM image of a BST thinfilm deposited at PRF of 1 Hz. It clearly shows that the columnarstructure has initiated from the bottom platinum electrode. Theexistence of amorphous layer in the films deposited at 10 Hz willbe discussed further. As both the films have the same thicknessand all other conditions are same except the laser PRF used forthe deposition, the main processing difference between them isthe only the time needed for deposition. Hence, the film grown at1 Hz may have ample time to crystallize fully due to longer timescale of deposition as that of 10 Hz. Whereas, in the films grownat 10 Hz, the first ablated specimens may not have enough time to
reorganize and to form the BST crystalline phase before the newspecimens arrived during the deposition. However, in the coolingprocess, the crystallization process may partially occur. From the
140 J.P.B. Silva et al. / Applied Surface S
Fig. 7. SEM image of a BST thin film grown at PRF of 1 Hz.
Ftagc[1tatdtpTiPtatbtmp
the values of εi and ε, the thickness the amorphous layer estimated
ig. 6, it seems that the crystallization process may be initiated fromhe top of the film and proceed further toward the substrate [19,20],s a result an amorphous layer is observed. This interpretation is inood agreement with the results obtained in yttria stabilized zir-onia (YSZ) films produced by pulsed laser deposition technique21]. It is observed that the YSZ films of same thickness grown atand 20 Hz were shown different textures and crystallinities. Yet
he film grown at 20 Hz has shown similar texture and crystallinitys that of the film grown at 1 Hz, by keeping it at the depositionemperature for an excess time (equivalent to the deposition timeifference) before cooling. Therefore, it may be possible to reducehe thickness of the amorphous interfacial layer, and even com-letely eliminated by adjusting the growth process of BST films.he film deposited with PRF of 10 Hz was post-annealed at 200 ◦Cn the presence of air during 30 min to improve the interface oft-BST film [22]. In order to see the annealing effect, the XRD pat-ern of annealed film was also studied and shown in Fig. 1. It haslso shown the perovskite phase, as expected. However, the crys-allite size, calculated by the Scherrer formula [23], was found toe 24 nm and 20 nm in as-grown and annealed films. This suggests
he improvement at the interface, i.e. some part of amorphous layer
ay be crystalline after annealing. The calculated values of latticearameters for the annealed film were also shown in Table 1 and
Fig. 8. (a) Leakage current versus applied voltage of different BST thin films; (b) S
cience 278 (2013) 136–141
were close to the bulk values as compared to the grown films. Thissuggests that there might be incorporation of oxygen into films andthus oxygen defects decreases.
The current–voltage (I–V) characteristics of the as grown andannealed films were shown in Fig. 8(a). The leakage currents werefound to be lowered in the annealed film as compared to thegrown films and could be attributed decrease of oxygen vacanciesas suggested by XRD analysis. At low fields (0–1 V), the conductionmechanism follows the Ohmic nature. But, the conduction mech-anism at higher fields follows the Schottky mechanism in both asgrown and as annealed films. The equation for the Schottky mech-anism is as follows:
J = A ∗ ∗T2 exp
[−q(ϕB −
√qV/4�ε0εiti)
kT
](1)
where J is the leakage current density, A** the effective Richardsonconstant, T the temperature, q the electronic charge, ϕB the Schottkybarrier height, V the applied voltage, ε0 the free space dielectricconstant, ε the optical dielectric constant, w the depletion layerwidth and k the Boltzmann constant [24–26]. Fig. 8(b) depicts theSchottky plots of the bias voltage dependence of the leakage currentdensity (ln J versus V1/2) for the as-grown and annealed BST films.According to Eq. (1), the plot of ln J versus V1/2 should be linearand its slope gives the value of q/kT (q/4�ε0εiti)1/2. The estimatedvalue of slop for the film grown at 10 Hz is about 1.35 and from this,the value of ti × εi is obtained as 1.2 × 10−6. By assuming that theamorphous layer (50 nm as seen in TEM) is the main source of theleakage current [27], the estimated value of dielectric constant ofthe amorphous layer (εi) is found to be ≈25.0.
Since the film contains both the amorphous and the crystallinelayers, which are connected in series, the permittivity of wholestructure can be written as:t
ε= ti
εi+ t − ti
εb(2)
where t and ti are the thickness of the entire film and the amor-phous layer, and ε, εi and εb are the dielectric permittivity of theentire film, amorphous layer and crystalline regions, respectively.Since we know all the terms, the value of εb is found to be 316,which is in good agreement with the literature value [28] and alsocomparable with the values obtained in the films deposited at a PRFof 1 Hz, where there is no amorphous layer. In the similar way, withconsidering the measured dielectric constant (ε = 159) (Fig. 4) and
chottky plots of the bias voltage dependence of the leakage current density.
from Eq. (2) in the annealed film is found to be 34 nm. Hence it con-cludes that the annealing process effectively reduces the thicknessof the amorphous layer by 32%.
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. Conclusions
The BST thin films were grown on the platinized substrate atRF of 1 and 10 Hz and their structural, morphological and electri-al properties were studied. RBS analysis suggests that the targetsed for deposition and film have the same composition, whileRTEM analysis reveals a 50 nm thin amorphous layer was formedt the interface in the films deposited at high PRF. Moreover, themorphous layer disappears when the films were deposited at lowRF and is discussed in terms of growth mechanism. The dielectricnd leakage current properties were improved after annealing andttributed to the decrease of the amorphous layer thickness. Theielectric behavior of these films was explained by a model, wherehe low-permittivity interface layer is connected in series with theulk region of the film.
cknowledgments
J.P.B.S. thanks FCT for the financial support (grantFRH/BD/44861/2008). S.A.S.R. thanks FCT for the financialupport (grant SFRH/BD/30531/2006). K.C.S thanks to FCT forost-doc grant (SFRH/BPD/68489/2010). This work was par-ially supported by the Project ANNA – European Commission
6th Framework Programme (ref. TA Research Project RP013(2009–2010)) and by Portuguese funds through Portuguese
oundation for Science and Technology (FCT) in the frame of theroject PEst-C-FIS/UI607/2011–2012.
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