Investigation of optical properties of bismuth titanate
(Bi4Ti3O12) thin films, prepared by spin coating
method
Ebrahim Attaran kakhki *,1, Malihe Rostamyari**,2
1
Physics Department, Faculty of Sciences, Ferdowsi University of Mashhad, Iran.
2 Physics Department, Faculty of Sciences, Azad University of Mashhad, Iran.
Abstract
Ferroelectric Bismuth Titanate Bi4Ti3O12 (BTO) thin film was prepared on glass
substrate using spin coating method. The structure and the transmitted spectrum of as
prepared films were investigated by X-ray diffraction and UV-vis-photometer. The
optical properties of films such as refractive index n, absorption coefficient α, and
extinction coefficient k were obtained by applying the data from the transmitted
spectrum.
Keywords: optical properties, optical memories, ferroelectric, non-volatile memory.
1 Introduction Many of optical and opto-electronic devices are made in the form of
thin films. A group of materials which in recent years are under highly developed and
investigation are ferroelectric media. Because these materials have proper dielectric,
pizo-electric and electronic properties. They have many applications in manufacturing
devices such as multilayer capacitor; pyro-electric detectors, non-volatile memory,
and optical memories devices.
Ferroelectric thin film have attracted much attention for the application in
nonvolatile random memory (NvFRAM), microelectronic mechanical systems
(MEMS), non-linear optical devices, surface acoustic wave devices, sensing
application and pyroelectric detectors [1]. Among various ferroelectric, Bi4Ti3O12
(BTO) is currently regarded as one of the most promising candidate materials for
NvFRAM.
Ferroelectric bismuth layer compounds were first studied by Aurivillius and
Fang [2, 3]. These structure consist of alternative bismuth oxide and
perovskite type layers represented by the general formula (Bi2o2)2+(Am21BmO3m+1) , where A can be occupied by mono -,di-,or trivalent cations like
Na+, Ba2+, Ca2+, Sr2+,Bi3+ and rare-earth cations. B is usually occupied by
smaller cations such as Ti4+, Ta5+, Nb5+ and W6+ [4]. As a member of
Aurivillius family compounds, which are well known ferroelectric materials,
bismuth titanate Bi4Ti3O12 is of particular interest due to its unique properties
such as high dielectric constant, Curie temperature (676ºC), breakdown
strength and having good electro-optic switching behaviour [5].
Crystal structure, space group and lattice parameter of Bi4Ti3O12 are given in table 1.
* Corresponding author: e-mail: attarankak@yahoo.com, Phone: +98511 8796983, Fax: +98511 8796416
Table 1 crystal structure, space group and
Lattice parameter of Bismuth titanate.
Crystal
structure
Space
group
Lattice parameters (Aº)
a
b
c
Tc<T
monoclinic
Blal
5.45
5.4056
32.832
Tc<T
orthorhombic
fmmm,B2
cb
5.432
5.394
32.94
Tc>T
tetragonal
I4/mmm
3.86
3.86
32.8
3. preparation procedure Bismuth titanate thin films were prepared by the spincoating method. For preparation of the solution we used the materials which listed in
the Table 2.
Table 2 starting material used for preparation of Bi4Ti3O12 thin film.
Nitrate bismuth –pentahydrate(Bi(NO3)3.5H2O , 99%) , Fluka
Titanium butoxide (Ti(OC4H9)4,98%) , Merck
2-methoxy ethanol (C3H8O2,99%) , Scharlau
An important point in preparation of precursor solution is that in bismuth layered
ferroelectrics, volatilization of bismuth occurs during annealing. To avoid this, excess
bismuth content is usually taken into the precursor material [6].
First, bismuth nitrate penta-hydrate [Bi (NO3)3.5H2O] (15%, excess) was dissolved in
2-methoxyethanol. The reason for using C3H8O2 as a solution is that the Nitrate
bismuth–pentahydrate is dissolved in it completely without any require of heatting.
Then titanium butoxide [Ti (OC4H9)4] was added into the solution to form BTO
precursor solution under stirring. Then the solution was directly deposited onto a glass
substrate by spin coating at 3000 r/min for 30 s, then dried at 100ºC for 1 h to form gel
film by hydrolysis and polymerization. Spin coating process was repeated for several
times to obtained films with desired thickness, followed by pyrolysis of each layer at
580ºC for 10 minutes.
In this work we repeated the coating process in 1, 2, 3 and 4 stages until the desired
thickness was obtained. All the preparation procedure was done at room temperature.
4 Results and discussion
4.1 X-ray pattern the crystal structure of the films were examined by X-ray
diffractometer with CuKα radiation (λ = 1.5405 Aº). Also, the optical transmittance of
the BTO thin film on glass substrates was recorded on a UV-Vis-NIR
spectrophotometer. All measurement was carried out at room temperature.
After examine different annealing temperatures we chose tow proper temperature
which show the crystal growth.
Fig. 1 shows the XRD patterns of BTO thin films at 520 ºC and 580 ºC on glass
substrate. The annealed time at each temperature was 20 minutes. As the Fig 1 (b)
show growth of Bismuth titanate Bi4Ti3O12 is completed at 580 °C.
4.2 Optical properties Fig.2 shows the experimental results of transmitted
spectrum measured by spectro-photometer. The transmission T is given by following
Relations [7]:
T
A
B  C cos  D
(1)
2
Where
A  16 n 2 nS
(2a)
(2b)
B  (n  1)3 (n  nS2 )
C  2(n 2  1)(n 2  nS2 )
(2c)
(2d)
D  (n  1)3 (n  nS2 )
  4nd 
(2e)
  exp( d )
(2f)
Where n and d denote the refractive index and thickness of film, respectively. ns and α
denote the refractive index of the transparent substrate and absorption coefficient of
film, respectively.
We consider the extremom values of the transmitted spectrum. If at a defined
wavelength, the maximum and minimum transmittance are TM and Tm then these
two parameters are obtained from the following equations.
Figure 1
X-ray pattern of Bi4Ti3O12 thin films on a glass substrate
after anealed at (a) 520 ºC (b) 580ºC for 20 min.
TM 
z
A
(3)
B  C  D 2
A
B  C  D
(4)
2
Substituting A and C from equations (2a) and (2c) in the above relations then the
refractive index is determine as
(5) n  [ N  ( N 2  nS2 )
1
2
1
]
2
Where
(6)
N  2nS
TM  Tm nS2  1

TM Tm
2
The film thickness d can be expressed by
(7)
d
 1 2
2[  2 n(  1 )   1n(  2 )]
Where n (1) and n (2) are the refractive indices at two consecutive maxima (or
minima) at 1 and 2.
The absorption coefficient α of the thin film is calculated by the formula
(8)
 
1
n ( c )
d
Where
(9)
c
(n  1)( n S  n)[1  (TM Tm )
1
(n  1)( n S  n)[1  (TM Tm )
1
2]
2]
The extinction coefficient  of a thin film is given by
K


4
(10)
With calculation of n and K, it is possible to determine the real and imaginary part of
dielectric constant and so the dielectric destruction of the layer. Therefore
(11)
r  n2  K 2
(12)
 ''  2nK
(13)
tan  
 ''
r
100
A
B
C
D
Transmittance(%)
90
80
70
60
50
40
30
20
10
400
500
600
700
800
900
1000
1100
1200
Wavelength(nm)
Figure 2 optical
transmittance spectra of Bi4Ti3O12 thin films
on glass substrates for the samples coated for (A): one, (B):
two, (C): three and (D): four, stages of coating process.
The obtained optical constants of bismuth titanate thin film are shown in Table 3. For
calculation of these parameters.
We used the sample (B) which is prepared in two coating stages. According to
equation (7), the thickness of the thin film is about 300 nm.
Table 3
The optical constants of Bi4Ti3O12 thin films
prepared on glass substrate.
(nm)
n
α (nm)-1×102
K×102
r
"
tanδ
384
2.5566
0.2759
0.0843
6.529
0.4314
0.066
439
2.5447
0.066
0.0230
6.4749
0.1174
0.0181
512
2.5302
0.2017
0.0822
6.3951
0.4160
0.0650
639
2.4108
0.0059
0.003
5.8121
0.0145
0.0025
The optical energy band-gap of BTO films can be obtained from the transmittance
spectra. For a crystalline material, it is shown that the optical absorption near the band
edge follows the following equation [8]
(14) ( h ) 2 n  C( h  E g )
Where , Eg, and C are light frequency, band-gap, and a constant, respectively. The
transition of light through a martial can be determined by refractive index n. films
( h ) 2 it is equal one The Eg value can be estimated from the diagram of
versus hv by extrapolating the linear
portion of the graph at higher energies to
2
As shown in Fig. 3 [9]. (h )  0
15900
13900
11900
9900
7900
5900
3900
hv, for Bi4Ti3O12
substrate.
1900
3.19
3.24
3.29
3.34
3.39
3.44
Photon energy (eV)
3.49
3.54
3.59
Plot of (αh)2 vs.
thin films on glass
Figure 3.
As shown in Fig. 3 the optical band gap energy is about 3.44 eV.
5 conclusions In summary, bismuth titanate thin films were prepared on glass
substrate by spin coating process. The optical constant and optical energy band-gap
were determined from the transmittance spectra using the envelope method. The
optical energy band gap was found to be 3.44 ev. During calculation only data from
the transmitted spectrum were used and procedure was simple, fast and very accurate.
In conclusion, this is a useful method to obtain the thin film’s optical constants.
References
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