Sol-gel BST thin films for FeFET applications : fabrication and characterization
The effect of the chemical composition and film thickness of the ferroelectric barium strontium titanate (BST) at the memory window behavior of Al/BST/SiO2/Si-Gatefield effect transistor structure has been investigated. BaxSr1-xTiO3 thin films with different x values and film thickness have been f...
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| Format: | Thesis |
| Language: | English |
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| Online Access: | http://dspace.unimap.edu.my:80/xmlui/bitstream/123456789/31170/1/Page%201-24.pdf http://dspace.unimap.edu.my:80/xmlui/bitstream/123456789/31170/2/Full%20text.pdf |
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| Summary: | The effect of the chemical composition and film thickness of the ferroelectric
barium strontium titanate (BST) at the memory window behavior of Al/BST/SiO2/Si-Gatefield effect transistor structure has been investigated. BaxSr1-xTiO3 thin films with
different x values and film thickness have been fabricated as metal-ferroelectricinsulator- semiconductor (MFIS) and metal-ferroelectric-metal (MFM) configurations using a sol-gel technique. The surface morphology and grain size have been studied in detail using an atomic force microscope (AFM). The AFM micrographs show that BST films have good surface properties with increasing in the grain size and surface roughness as both Ba content and film thickness increase. The grain size of BST for different Ba:Sr ratio and film thickness varies between 48 ± 3.56 nm and 120 ± 3.56 nm.
In order to confirm the ferroelectric behavior of BST, the ferroelectric hysteresis
characteristics of the films have been studied with MFM structure using both Sawyer-
Tower circuit and capacitance-voltage (C-V) characteristic tests. The results show that
the ferroelectric hysteresis strengthens as both Ba content and film thickness increase.
The conduction mechanism and dielectric relaxation mechanism for BST film within
MFM structure have been investigated using an impedance spectroscopy in the frequency
range of 1 Hz to 1 MHz. The results show that the conductivity of the films decreases as
both the Ba content and film thickness increase which are attributed to the grain size
effect. The complex impedance (Z*) plane for all BST thin films possesses two regions,
which attributed to the film’s regions conduction mechanism i.e. grains, and grain
boundaries and electrodes. The variation of the dielectric constant (ε) and tan δ with
frequency have been studied to ensure the dielectric quality of the material. The results
show that the dielectric constant for BST films decreases as both of the Ba content and
film thickness increase, which also attributed to the grain size effect. The values of the
dielectric permittivity constant of the tested samples vary between 151 and 335 at 105 Hz
frequency for different Ba:Sr ratio and film thickness. The electric modulus results show
that the relaxation mechanism within BST films is non-Debye type. The frequency
dependent conductivity plots show three regions of conduction processes, i.e. lowfrequency
region due to DC conduction, mid-frequency region due to translational
hopping motion and high-frequency region due to localized hopping and/or
reorientational motion. In addition, depending on the complex impedance and complex
modulus plots, an equivalent circuit has been suggested for each film in the current study
to obtain further understanding for the conduction mechanism of BST film. The
ferroelectric memory behavior of the MFIS samples has been investigated using
capacitance-voltage (C-V) characteristics. The C-V characteristics show that the memory
window width increases with the increase both of Ba content and film thickness. This is
attributed to the grain size and dipole dynamics effect. The values of the memory window
for the studied films vary between 0.3 V and 3.3 V for different Ba:Sr ratios and film
thickness. Furthermore, the width of the memory window increases as the applied voltage
increases; however, it is not affected by the voltage sweeping rate. In addition, the
operating mode of the gate structure, i.e. accumulation, depletion and inversion
conditions, has been also discussed in detail. |
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