Miriyala, Kumaraswamy and Ramadurai, Ranjit
(2019)
Texture and Microstructural Influence on Piezoelectric Properties of Na0.5Bi0.5TiO3 Thin Films: A Lead Free Piezoelectric Material.
PhD thesis, Indian institute of technology Hyderabad.
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Abstract
Among the lead free piezoelectric compounds Sodium Bismuth Titanate
(Na0.5Bi0.5TiO3: NBT) show promising features that could replace the existing lead based
piezoelectric materials in various applications. In this work, we report the effect of substrate
temperature (400-650 °C), oxygen partial pressures (50-200mTorr) on the crystallographic
orientations and microstructural evolution of NBT thin films grown on (111) Pt/TiO2/SiO2/Si
(100) substrate by pulsed laser deposition (PLD) technique. The films grown at low partial
pressures exhibits a preferred orientation along the <220> direction and at high pressures they
exhibit polycrystalline nature. The variation of the growth temperature and oxygen partial
pressures are allowed the microstructural features tuning from coarse faceted grains to fine
spherical grains. The ferroelectric domain studies reveal that in case of fine spherical grain
structure, microstructural features dominate the domain distribution and in case of coarse
faceted grain structure, domain features are independent of its morphology. A control over
the crystallographic orientations facilitate the tunability over the polarization vector
components, which allows us to attain the dominating planar piezoresponse in case of highly
oriented (<220>) samples. Further, in the case of polycrystalline films there are certain
regions at which the ferroelectric domains are extending beyond the grain boundaries. Such
an extension could plausibly arise due to the close crystallographic relation between the
adjacent grains. The estimated grain boundary angles between the planes (-211), (-111) and
(-311), (-111) are showing that there exist some of the planes that forms a low angle grain
boundary (≤15°) with polarization components which are favoring the domain walls to cross
over the adjacent grain boundaries in NBT polycrystalline films. Moreover from the empirical
observation of surface roughness, when the surface roughness is larger than the average
roughness (~ 2nm), the grain boundary acts as a physical boundary for the domain. Thus, the
surface roughness also play a crucial role on defining the domain pattern by introducing a
physical boundary for the coherent interaction of the polarization across grains. Fast Fourier
Transform (FFT) spectrum analysis of the domain patterns confirmed that only highly
oriented films possess periodic domain distribution. Moreover, the nano scale
piezocoefficient values (d33) are increased from 16±0.4 to 30±0.46 pm/V with increasing the
oxygen partial pressures and growth temperatures.
Further the temperature dependent leakage current studies are performed for NBT
thin films with different microstructures. For faceted grain structure, Schottky emission is
dominated in the temperature range from 30°C to 120°C. Further in the temperature range
130-200 °C Ohmic conduction is dominated. In case of spherical grain structure 30-120 °C
Poole-Frenkel emission and from 130-200 °C, Ohmic conductions are dominated. More over
the highly oriented film with needle shaped grain structure exhibits Schottky emission in the
temperature of 30-140 °C and from 150-200 °C, ohmic conductions was dominated. Further,
we also performed the variation in the leakage currents by changing the oxygen partial
pressures for a given temperature. The leakage current density under an applied electric field
for the films grown at 650 ºC with various oxygen partial pressures. At a given field as the
oxygen partial pressure increases, the leakage current density decreases. This is an indicative
of the role of oxygen vacancies on leakage currents in NBT thin films. Hence, the optimized
oxygen partial pressures could result in improved electrical properties in NBT thin films.
Further, the microstructure influence on leakage currents studies reveal that coarse faceted
columnar structure exhibit less leakage currents than the spherical grain structure. In addition,
the phase transition studies indicates the plausible antiferroelectric transition at 120 °C and
paraelectric transition in the range of 350-370 °C.
In addition, the epitaxial NBT thin films are fabricated on LNO coated STO single
crystal substrates with (100) (110) and (111) crystallographic orientations by pulsed laser
deposition technique. The in- plane and out-of-lattice parameters and epitaxial strains are
calculated from the reciprocal space map studies. Further, the FE-SEM studies shows that the
surface energies of the substrate orientations play a crucial role in deciding the morphology
of the respective film. We have observed a smooth 2D growth morphology in case of (100)
orientation whereas (110) and (111) acquired 3D growth morphology. The ellipsometry
studies reveal that the onset of the absorption is highly depends on the planar densities of the
respective orientations. For all the three orientations, the calculated absorption coefficient
values are in the range of 3.1-3.5 eV. The PFM studies reveals that the OP phase contrast is
weak in case of (100) orientation and it is strong in case of (111) orientation. Further the IPPhase
is strong in case of (100) than the (111) orientation. This is explained based on the
polarization components that are participating in deriving the different types of domains in all
three orientations. The single point piezoresponse measurements confirms that (111) oriented
film is pocessing a large piezoelectric coefficient (d33) value of 63 pm/V compared with the
other two orientations. The obtained d33 values for (100) and (110) are 42 pm/V and 51 pm/V
respectively. This suggest that NBT (111) oriented film can be a potential candidate for
replacing the lead based compounds for device applications.
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