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VINIKA MANGLANI1*, ASHISH AGNIHOTRI2
1Research Scholar, Suresh Gyan Vihar University, Jaipur 2Department of Physics, Sri Balaji College of Engineering & Technology, Jaipur
Corresponding Author’s Email: vinikamanglani@gmail.comAbstract: Bi3.25La0.75Ti3O12 (BLT) thin film was prepared on Pt (111)/TiO2/SiO2/Si (100) substrate by the Sol gel spin coating technique at annealing temperature of 6000 C and annealing time of 2 hours. Various characteristics have been discussed in this paper which is suitable to make FRAM.
Keywords: Perovskite thin film, ferroelectric, Sol gel technique, FRAM, polarization, annealing time, FRAM
INTRODUCTION
Memories can be divided into two broad categories volatile and non-volatile. Volatile memories are the memories in which the data is lost when the power is removed from the electronic system. These memories are expensive and very fast in read and write access [1]. On the other hand, non-volatile memory retains the data even when the electric power is interrupted [2].Ferroelectric random access memory (FRAM) has features consistent with a RAM technology, but it is non-volatile like ROM technology. FRAM is a RAM-based device that uses the ferroelectric effect for the storage mechanism [3-5]. The writing speed of FRAMs is 1000 times faster and they consume lower power (1/1000th power) than EEPROMs. FRAM is a type of non volatile read/write random access semiconductor memory. FRAM has similar applications to EEPROM, but can be written much faster. The ferroelectric film of the memory cell capacitor is made of PZT, PLZT or SBT, and permitting high storage density [6].
At present, the ferroelectric materials suitable for these devices like PZT (Lead Zirconium Titanate) system, SBT(Strontium Bismuth Tentalate), BLT(Bismuth Lanthanum Titanate), BT(Barium Titanate), are studied with a great deal of interest [7]. Sol-gel technique, which is a versatile solution process for making thin films, is used to deposit thin films of BLT [8-9].
EXPERIMENTAL PROCEEDURE
A. Sol Preparation
25 ml of Bi3.25 La0.75 Ti3 O12 precursor solution of 0.1 M molarity was prepared. To prepare sol 4.7294gm of Bi (NO3)3.5H2O was dissolved in 25 ml of acetic acid at 1400C and the net volume was reduced to 12.5 ml. 0.8119gm of La (NO3)3.6H2O was dissolved in 25 ml of 2-Methoxy ethanol at 1600C .Volume was reduced to 12.5 ml. Both the solutions were mixed at room temperature and 2.2324ml of Ti – (isopropoxide) was added. Sol was stabilized with 2 ml of acetyl acetone. 10% wt of Bi (NO3) 3.5H2O and 10%wt of Ti- (isopropoxide) was added to compensate for possible Bi loss during high temperature process.B. Film Deposition
BLT films were prepared on cleaned Pt (111)/TiO2/SiO2/Si (100) substrate. The precursor solution was spin coated on substrate at rotational speed of 5000 rpm for 45 sec. The baked films were spin coated several times to yield desired thickness of 350 nm. For each coating films were dried at 1100C for 5 min and after each two coating films were fired at 4000C for 20 min. All coated BLT films were annealed at about 6000C in air 2hours.RESULTS AND DISCUSSIONS
A. Phase AnalysisBLT thin films were prepared on cleaned Pt (111)/TiO2/SiO2/Si (100) substrate. These films were annealed at about 6000C in air for 2 hours. Fig 1 shows XRD pattern. No secondary phase was observed which shows that the prepared sample is crystalline.
B. Field Induced Polarization
The polarization was found to increase with increasing applied field for prepared film. P-E hysteresis loop was measured against 200 kV/cm applied field at 100 Hz is shown in fig.2.
The Pr was found to increase more steeply with increasing the applied field. The Pr value was ~2.6 µC/cm2 for test sample. The Psat value was found to increase linearly with increasing field. Ec value was found to increase with the applied field. The polarization values in sample film are not saturated up to the applied field as high as ~200 kV/cm. It is clear that thin film can sustain much higher applied field which is because of the low probability of finding a defect under the electrode in thin films. Frequency dependent P-E hysteresis loop is shown in fig.3. A P-E loop was recorded at room temperature when the field 200 kV/cm was applied at different frequencies. The prepared film showed frequency dispersion which may be due to the presence of high space charge polarization in film.
Fig.4 shows Pr, Ec and Psat values against frequency for sample. It should be noticed that these values decrease exponentially with increasing frequencies.
C. Leakage Current
Time dependent leakage current density was measured at different field for prepared sample as shown in fig. 5. The soak time and measurement time was 1 ms. The leakage current density was found to decrease exponentially for film at low DCfield (up to ~ 100 kV/cm). An increase in time dependent current density at higher field is because of the removal of traps at the domain boundary and losses due to the movement of the domain boundaries under at very high DC-field.
D. I-V characteristics
I-V characteristic has been obtained for the prepared sample for a triangle field profile. The leakage current density has been found to increase exponentially with increasing field film as shown in fig. 6. A sudden increase in the current density can be seen for prepared thin film which is due to trap charges. This trap charges may be near to the domain boundary, which are pinning these boundaries. The traps are deep for this film, which is consistent with the field at which charges are escaping from these traps for film. When the DCfield was decreased from ~200 kV/cm to 0 kV/cm for film, it is observed that the boundary is pinned again at lower field. The enclosed loop confirms the ferroelectric behaviour of the film. A small enclosed area was observed for BLTF1 film, which is due to the presence of shallow traps.
E. Microstructure
Fig. 7 shows the microstructure of the top surface of the prepared thin film. The average grain size was in nm range.
CONCLUSION
This study clearly shows that the BLT film prepared at annealing temperature at at 600oC for 2 hours exhibits crystalline phase , good ferroelectric properties, low leakage current density , good IV characteristics. SEM result shows that thin film is order of 350 nm which is suitable to make FRAM for various applications.
ACKNOWLEDGEMENT
This paper is based upon work supported by the Raja Ramanna Centre for Advanced Technology, Indore, India. Authors acknowledge the support of Mrs. Rachna Selvamani, scientific officer, Ramanna Centre for Advanced Technology, Indore for his support at various stage of this work.
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