CHAPTER-1 INTRODUCTION

Transcription

1 CHAPTER-1 INTRODUCTION 1.1 OVERVIEW In today s microelectronics computer industry, various types of memories are used for the data storage. Generally, memories are categorized into volatile and non-volatile. Volatile memories are those in which the data is lost when the power is removed from the electronic system. Non-volatile memories are those that retain the data even when the electric power is interrupted. There is one more type of memory called FRAM (ferroelectric random access memory) which is non-volatile like ROM technology but has features similar like RAM technology. FRAM works on the principle of ferroelectric effect to store the data. Polarization of FRAM is done by applying external electric field to it. If this external field is removed from FRAM, it will remain polarized. This characteristic of FRAM is called remanent polarization. Hence, FRAM does not lose any data even if the power is disconnected from it. This property is known as non-volatility. FRAM is a superior memory that has fast speed of reading and writing the data. The writing speed of FRAM is fast and hence consumes less power. FRAM is developed by depositing a film of ferroelectric material in crystalline form between two electrode plates to form a capacitor. Two families of materials, perovskite and layered perovskites have been widely studied for memory applications. In the perovskites family, Lead Zirconium Titanate (PZT) has been used for memory application. The lead based ferroelectric memories research efforts are hampered due to fatigue environmental safety and health concerns. In comparison with non-layered perovskite ferroelectrics, layered- Perovskites offer several advantages such as fatigue free, lead free, low operating 1

2 voltages and better ferroelectric properties. Therefore, Bismuth Titanate (BIT) that is a Bi- based layered perovskite ferroelectrics has been selected in the research work to make thin films. However, BIT has high leakage current and high fatigue problem so BIT is being doped with suitable percentage of La to prepare BLT (Bismuth Lanthanum Titanate) thin films. Lanthanum modified Bismuth Titanate (BLT) has several advantages such as extremely low coercive field, low processing temperature, high remanent polarization, better mechanical strength and minimum field induced polarization switching fatigue. Since the percentage doping of La is very important therefore before making thin films, bulk ceramic is to be prepared to study the various characteristics of La doped Bismuth Titanate. Single crystal or bulk ceramic ferroelectric materials cannot be used for FRAM because of 5-volt (standard for silicone based logic circuits) constraints to switch the polarization. The current operating voltage will soon drop to 3.3, 2.5 and 1.1 V and that restrict the thickness of ferroelectric film to 400 nm or lesser in order to sustain the coercive fields of about 40 kv/cm. But this bulk ceramic helps to find the optimum value of La doing in BIT. 1.2 FERROELECTRICITY Ferroelectricity is a phenomenon in which electric polarization is shown even in the absence of an external applied electric field, together with the property that the direction of the polarization may be reversed by an electric field. Ferroelectricity is also named as Seignette electricity because it was the first material to show ferroelectric properties. It is also known as Rochelle Salt which have chemical formula of NaKC 4 H 4 O 6.4H 2 O. Just as ferromagnetic materials may exhibit a spontaneous magnetic moment at zero magnetic fields, ferroelectric materials exhibit a polarization at zero electric field [1]. This spontaneous polarization is actually the electric dipole 2

3 moment that is measured as per unit volume. A Ferroelectric is spontaneously polarized i.e., it is polarized in the absence of an external field, and the direction of the spontaneous polarization may be altered under the influence of an applied electric field. Reversal of the state of the polarization is known as switching. More than 250 materials exhibit ferroelectric properties and some of the common materials are Lead titanate(pbtio 3 ), Lead zirconate titanate (PZT), Lead lanthanum zirconate titanate (PLZT) etc. 1.3 CRYSTAL SYMMETRY Any crystal structure can be obtained when a suitable basis is attached to each lattice point of one of the fourteen Bravais lattices. When, three different symmetry operations i.e. rotation (proper and improper), reflection and centre of symmetry are performed on Bravais lattices, 32 crystal classes are possible (if translation operation is also performed 230 space groups are possible) [2]. These 32 crystal classes are called point groups and shown in fig

4 Figure 1.1: Crystal Symmetry Some materials have the ability to produce electricity when subjected to mechanical stress. This is called the piezoelectric effect. This stress can be caused by hitting or twisting the material just enough to deform its crystal lattice without fracturing it. The piezoelectricity is the asymmetry of the crystal structures. There are total 32 crystal classes in which 11 crystals are centrosymmetric. Centrosymmetric are those crystals in which small movement of charge is symmetrically distributed about the center of symmetry in a manner that brings about a full compensation of relative displacements when a uniform stress is applied to such a material [3]. But if the crystal is not centrosymmetric, then the application of stress may create a displacement of positive and negative charge, hence a local dipole is created which lead to the possibility of long-range polarization. The remaining 21 points group doesn t have a centre of symmetry (non-centrosymmetric). Out of 21 crystal classes, 20 of them show electric polarity when a stress is applied and are therefore categorized as piezoelectric. 4

5 Out of 20 piezoelectric crystal classes, 10 of them have a unique polar axis which leads to spontaneous polarization. Such polarization creates surface charge, which is very sensitive towards the change of temperature. This effect is termed pyroelectricity. 1.4 PYROELECTRICITY Pyroelectricity means electricity resulting from variation of heat. Whenever Pyroelectric materials are heated or cooled, they generate electric field [4]. As discussed above, the 10 unique polar axis crystals show spontaneous polarization P s and hence are called polar crystals. P s is the value of the dipole moment i.e per unit volume or the value of the charge per unit area on the surface perpendicular to the axis of spontaneous polarization [5]. The value of the spontaneous polarization depends on the temperature. This is called the pyroelectric effect or pyroelectric coefficient [6-9]. The point groups which show the pyroelectric effect and whose spontaneous polarization P s can be reversed by an external electric field are called ferroelectrics. Hence, ferroelectric materials are both piezoelectric and pyroelectric, while the converse is not always true [10]. Ferroelectric materials are also known as polar materials that possess at least two equilibrium orientations of the spontaneous polarization vector in the absence of an external electric field and in which the spontaneous polarization vector may be switched between those orientations by an electric field [11]. In ferroelectric crystals, the centres of positive and negative charges do not coincide with each other even in the absence of electric field, thus producing a nonzero value of the dipole moment. There are certain crystals showing ferroelectric 5

6 properties these crystals are called ferroelectric crystals. Most of the materials are polarized linearly with external electric field. This is called dielectric polarization. Some materials known as paraelectric materials show more pronounced nonlinear polarization. The electric permittivity, corresponding to the slope of the polarization curve, is thereby a function of the external electric field. In addition to being nonlinear, ferroelectric materials show a spontaneous (zero field) polarization. Such materials are generally called pyroelectrics. The distinguishing feature of ferroelectrics is that the direction of the spontaneous polarization can be reversed by an applied electric field and results in a hysteresis loop. The characteristics of dielectric, pyroelectric and ferroelectric polarization is shown in figures 1.2, 1.3 and 1.4 respectively. Figure 1.2 : Dielectric Polarization 6

7 Figure 1.3 : Paraelectric Polarization Figure 1.4 : Ferroelectric Polarization 7

8 1.5 FERROELECTRIC DOMAINS AND HYSTERESIS LOOP In ferroelectric crystals, there are some uniform polarization regions which are known as ferroelectric domains [12]. In this domain, the electric dipoles are aligned in the same direction. Figures 1.5 & 1.6 show the orientation of domains in the absence and presence of electric field respectively. A grown ferroelectric crystal has multiple ferroelectric domains that are separated by interfaces known as domain walls. A single domain can be obtained by applying appropriate electric field. In domain the polarization may get reversed on applying high value of electric field and this is called as domain switching [13-15]. The polarization reversal effect can be observed in hysteresis curve which has been obtained between polarization and electric field as shown in figure 1.7. On increasing electric field strength, the domains start to align in the positive direction results in rapid increase in the polarization (OB). At very high field levels, the polarization reaches a saturation value (P sat ). The polarization does not fall to zero when the external field is removed. At zero external field, some of the domains remain aligned in the positive direction and hence crystal shows remanent polarization P r. The crystal can be depolarized if a negative electric field of magnitude OF is applied. The external field needed to reduce the polarization to zero is called the coercive field strength Ec. If the field is further increased to a more negative value, the direction of polarization flips and hence a hysteresis loop is obtained. The value of the spontaneous polarization Ps (OE) can be obtained by extrapolating the curve onto the polarization axes (CE). 8

9 Figure 1.5: Orientation of Ferroelectric Domains in the Absence of Electric Field Figure 1.6: Orientation of Ferroelectric Domains in the Presence of Electric Field 9

10 Figure 1.7: P-E Hysteresis Loop 10

11 1.6 CURIE POINT AND PHASE TRANSITIONS The phase transitions of the ferroelectric materials depend on the temperature. If the ferroelectric material changes to paraelectric at a particular phase transition, then the corresponding temperature is called the Curie temperature (T c ) [16]. At a temperature T greater than T c, the crystal does not exhibit ferroelectricity. While for temperature T less than T c, the crystal is ferroelectric. Hence, on decreasing the temperature through the Curie point, a crystal undergoes a phase transition from a non-ferroelectric phase to a ferroelectric phase [17]. If there are more than one ferroelectric phases, the temperature at which the crystal transforms from one ferroelectric phase to another is called the transition temperature [18-23]. 1.7 STRUCTURAL CLASSIFICATION OF FERROELECTRIC MATERIAL Ferroelectric materials are characterized by properties such as a high dielectrical constant, high piezoelectric constants, relatively low dielectric loss and high electrical resistivity. Ferroelectric is classified into two groups. The first type of ferroelectric is displacive ferroelectric in which the polarization occurs along several axes and second type is order-disorder ferroelectric in which the polarization occurs along only one axis i.e either up or down [24-25]. A. Displacive Ferroelectric In this category, the polarization in the material is developed due to the displacement of ions of few atoms in the crystal lattice. The displacive ferroelectric materials have octahedral oxygen and hence it is also known as oxygen octahedral ferroelectrics. The commonly displacive ferroelectric materials are perovskite type, for example BaTiO 3, KNbO 3, PbTiO 3, KTaO 3, NaNbO 3, NaTaO 3, PbZrO 3, LiNbO 3,LiTiO 3 [26-28]. 11

12 The perovskite structure has a general formula ABO 3 as shown in figure 1.8, where A is a monovalent or bivalent metal (A + or A 2+ ), B is a tetra- or pentavalent one (B 4+ or B 5+ ) and O the oxygen atom. Figure 1.8: Cubic Structure of ABO 3 B. Order- Disorder Ferroelectric In order-disorder ferroelectric, the spontaneous polarization occurs in the material due to linear ordering of the proton ions in the structure [29]. There are two groups of order disorder ferroelectrics which are as follows: (i) The first group includes phosphates, sulphates, periodates, fluoroberyllates, cyanides and glycine compounds, where the spontaneous polarizations appears as a result of the ordering of protons in the hydrogen bonds. They are also known as hydrogen bonded ferroelectrics. (ii) The second group consists of tartrates, potassium nitrate, dicalcium strontium propionate, sodium nitrate and tetramethylammomium chloro- and bromomercurates. In this group, spontaneous polarization is caused by the arbitrary ordering of radicals which takes place from hindered rotation. 12

13 The typical examples of order-disorder ferroelectrics are sodium nitrite NaNO 2, potassium dihydrogen phosphate (KDP) KH 2 PO 4 and triglycine sulphate (TGS) (CH 2 NH 2 COOH) 3 H 2 SO MEMORIES There are several kinds of memories present in today s microelectronics computer industry. Generally, memories can be divided into two broad categories, volatile and non-volatile. Volatile memories in which the data is lost when the power is removed from the electronic system include dynamic RAM (DRAM) and static random access memory (SRAM). DRAM is an inexpensive memory and can easily retain the data by regularly refreshing its contents with stored data [30]. Hence, it is suitable for large storage capacity systems. The speed of SRAM is faster than DRAM and does not required any refreshing. However, the size of SRAM is more than DRAM. Hence, it is used in small and medium storage capacity applications. On the other hand, non-volatile memory retains the data even when the electric power is interrupted. Non volatile memory is divided into read-only memory (ROM) which can only be read the data and random access memory (RAM) which has the ability to read or write the data. These memories derive from read only memory (ROM) technology, which includes electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) and Flash EPROM. These memories are slow to write, use large amounts of power to write and are unusable after writing a small number of times. BBSRAM (Battery Backed-up SRAM) is also a non-volatile memory that provides a battery backup in case of power failure [31]. FRAM is a non-volatile random access memory as ROM but has it has the features like RAM. FRAM is a RAM-based device that uses the ferroelectric effect for the storage mechanism. FRAM is polarized by applying external electric field and it 13

14 remains polarized even if the external electric field is removed from it. This type of polarization is called remanent polarization. Due to this polarization characteristic, FRAM does not lose any data if the power is removed from it. This property is called non-volatility. On reversing the direction of the applied electric field, the direction of polarization of the ferroelectric material gets changed and hence, it can update the new data. Polarization is a phenomenon due to the ionic displacement of atoms of ferroelectric crystal structure [32]. Therefore, FRAM is a superior memory that has fast speed of reading and writing the data. The writing speed of FRAM is 1000 times faster and they consume lower power (1/1000 th power) than EEPROMs. FRAM is a ferroelectric memory and is not affected by magnetic field as there is no ferrous material [33]. FRAM is developed by depositing a film of ferroelectric material in crystalline form between two electrode plates to form a capacitor. This construction is very similar to that of the DRAM capacitor [34]. Rather than storing data as charge on a capacitor like a DRAM, a ferroelectric memory stores data within a crystalline structure [35]. For practical FRAM applications, it is necessary to obtain FRAM in form of films with high remanent polarization, low leakage current and are required to be thin [36-37]. On considering the higher integration and lower voltage operation of NV-FeRAM, the area of a memory cell and the thickness of a ferroelectric capacitor has to be reduced as required in ultrahigh integrated dynamic random access memories (DRAM) [38-41]. It is important that the films have a low coercive field, high remanent polarization, low leakage current and low polarization fatigue [42]. A low coercive field translates to a lower operating voltage, faster switching speed and better fatigue resistance. 1.9 CELL STRUCTURE OF FRAM FRAM as non volatile memory cells can be divided into two types [43]: (i) 1T/1C (2T/2C) Type [1 Transistor / 1 Capacitor (2 Transistor/2 Capacitor)] : This memory has two parts, one is storage capacitance to retain and another is 14

15 transistor to access like a DRAM cell. FRAM is available with two types of memory cell transistor-either SRAM-type 2T/2C or DRAM TYPE 1T/1C. All modern FRAM use typically a denser 2T-DRAM layout. Cell information is detected by reading the change in current that results from the change in polarization charges when a voltage is applied to a cell. Hence, cell information is lost in each reading cycle during information is read. Due to the destructive reading, cell information must be rewritten during the same cycle. (ii) MFSFET [Metal Ferroelectric Semiconductor FET]: This configuration can be achieved by using a ferroelectric film instead of silicon oxidized film for the gate oxidized film. On applying the voltage between gate electrode and substrate, the information is written on the cell which polarizes the ferroelectric film. This cause the change of the threshold value of the transistor according to the direction of polarity. Now, on applying a fixed gate voltage, the cell information will be retrieved corresponds to the transistor threshold caused by the direction of polarity. Hence, the design technologies for this memory have an increased compatibility with those of EEPROM and Flash Memory F-RAM OPERATION The basic storage element in FRAM is a ferroelectric capacitor. This capacitor can be polarized in either direction by applying an external field as shown in figure 1.9. Figure 1.9: Ferroelectric Capacitor Polarization 15

16 The ferroelectric capacitor is a variable and non linear capacitor. When an electric field is applied there will be no change in polarization due to which ferroelectric capacitor will not be switched off. Therefore, it behaves like a linear capacitor. If it is switched, the additional charge will be induced and hence the capacitance must be increased. The ferroelectric capacitor is combined with an access transistor, a bit line, and a plate line to form a memory cell [44] as shown in figure Figure 1.10: F-RAM Memory Cell CELL OPERATING PRINCIPLES FRAM is a memory that uses ferroelectric materials to for the polarization phenomenon. Principle of FRAM cell operation is based on the characteristics of ferroelectrics and hysteresis. These characteristics exhibit voltage dependency of a polarized electric charge Q and hence correspond to polarization conditions of a ferroelectric capacitor. If the applied voltage to the ferroelectric capacitor is V f and the voltage level of the bottom electrode is positive with respect to the top electrode, relationship between the hysteresis loop and the charge that accumulates in the ferroelectric capacitor can easily be obtained in the form of hysteresis loop. In figure 1.11, various points are indicated on the hysteresis loop which shows the condition of a ferroelectric capacitor. 16

17 At V f = 0 V, points A and D describe different polarization conditions with a remnant polarization of +/ P r. At V f = +/ V c, points B and E indicate a polarization of 0 volts where as C and F describe the conditions of V f = +/ V cc. On applying different value of voltages from 0 V to +V cc and from 0 V to -V cc, the polarization state changes progressively around the loop from point A to B to C, and point D to E to F respectively. When applying voltage varies from +V cc to 0 V and from -V cc to 0 V, the polarization state changes and moves from point C to D and from point F to A. In this condition, the polarization decrease slightly without reversing its direction. The amounts of polarization at points C and F are designated as Q s and -Q s, respectively, which show the amount of saturation polarization. To implement these ferroelectric characteristics to a memory, two polarization conditions "0" and "1" can be assumed with "0" as upward polarization and "1" as downward polarization. Now, if the voltage applied to ferroelectric capacitor is adjusted to 0V i.e power is cut-off, the polarization state becomes D and A point in the hysteresis loop as shown in the below figure. That is, the remanent polarization charge either +P r or P r allows the data to be stored. Figure 1.11 shows the change in ferroelectric polarization if voltage varies according to 0 to + V c to + V cc to 0 to V c to V cc to 0 17

18 Figure 1.11: Hysteresis Loop and Ferroelectric Capacitor Polarization Conditions WRITING AND READING CELL DATA (A) Writing: Writing "1" or "0" data to a cell requires the application of the voltage +Vcc or - Vcc to both electrodes of the ferroelectric capacitor. In figure 1.12 shows the principle of Writing to the 1T/1C Cell. For writing, the word line (WL) is selected (i.e the transistor is on) and a voltage (Vcc) is applied between the bit line (BL) and the plate line (PL). Applying this voltage to the ferroelectric capacitor causes data to be written. Writing "0" data is done by making BL = 0 V and PL = V cc, whereas "1" data is written by making BL = V cc and PL = 0 V. After writing, data is retained even if the selected word line becomes unselected (i.e the transistor is off). Hence, the data is nonvolatile 18

19 i.e the polarity remains as remanent polarization (+P r, -P r ) even if the applied voltage is removed. (B) Reading: Before selecting WL, BL must be precharged to 0V to retain the highimpedance condition if "1" or "0" data from a cell has to be read. Now, WL is selected and V cc is applied to PL. By applying a voltage to the ferroelectric capacitor, the data can be read out. In figure 1.13, if the cell holds "0" data, the polarization is not reversed but the relatively slight movement of the electric charge causes BL to charge up by ΔVL. If another cell holds "1" data, polarization is reversed, causing a major movement of the electric charge. This causes BL to charge up by ΔVH. The sense amplifier holds a reference voltage (V ref ) which is applied to BL. In this way, ΔVL which has a lower voltage than Vr ef will again reduce to 0V, and ΔVH which has a higher voltage level than Vr ef may again increase to V cc. After the amplification, the states of the ferroelectric capacitor will be: when reading "0", V f = +V cc with BL = 0V and PL = V cc. When reading "1", V f = 0 V, meaning that the cell has a zero) bias, with BL = V cc and PL = V cc. (C) Rewriting After Reading On reading "1" data, the reversed polarity destroy the data and hence "0" data state is being created. Therefore, the "1" data needs to be written again to restore the data to its correct value prior to reading. After reading "1" data, the BL voltage level becomes V cc. At this time, PL voltage level becomes 0 V, causing "1" data to be rewritten. When WL is turned off, the bias of the ferroelectric, capacitor becomes "0", and the "1" data is stored. Hence, the stored data returns to the original "1" data. On reading "0" data, there is no change in the polarity and hence the data is not destroyed and a value of "0" is retained. 19

20 Figure 1.12: Writing Operation 20

21 Figure 1.13: Reading Operation 21

22 Figure 1.14: Hysteresis Loop Figure 1.15: Rewriting After Reading Operation 1.11 RELIABILITY OF FERROELECTRICS MATERIALS The characteristic of ferroelectric materials used for FRAM cells that affect the reliability of data retention are: 1) Data retention Characteristics 22

23 Figure 1.16 shows data retention characteristics. As time (t) elapses, the polarization charge (Q) decreases (deterioration). This characteristic depends upon the material used. From a design point of view, this characteristic can be improved by optimizing the write voltage to the ferroelectric capacitor. Figure 1.16: Data Retention Characteristics 2) Fatigue Characteristics A fatigue characteristic refers to the tendency for the amount of polarization (Q) to decrease because of repeated polarization reversal. The horizontal axis of the graph shows the number of times polarization has reversed. The fatigue characteristic depends on the operating voltage; therefore the degradation is slower at low operating voltages. The fatigue characteristic can be improved by reducing the voltage ratings of FRAM devices. 23

24 Figure 1.17: Fatigue Characteristics 1.12 MATERIALS FOR FRAM For FRAM device applications, an ideal material should have the properties as follows [45-46] : 1. Low electrical conductivity 2. Good leakage and breakdown characteristics 3. Large switching polarization 4. A low switching time (5~200 ns) for faster devices 5. High Curie temperature 6. Good aging and retention (decrease in Pr with time) characteristics 7. Fatigue resistance up to a minimum of switching cycles 8. Low power consumption i.e. low switching voltage (1~5 V). 9. Low imprint (imprint is caused by development of an internal field in the ferroelectric capacitor which leads to a progressive shift of the hysteresis loop along the field axis). 10. Low coercive field 11. Low leakage current 12. Non-toxic 24

25 Two families of ferroelectric materials Perovskites and Layered- Perovskites have been widely investigated for memory applications. In the perovskites family, Lead Zirconium Titanate (PZT) has been used for memory application. The lead based ferroelectric memories research efforts are hampered due to fatigue [47] environmental safety and health concerns. Also, the PZT thin films on platinum electrode show serious problems of degradation due to oxygen vacancies created at the interface [48]. In comparison with non-layered perovskite ferroelectrics, Layered- Perovskites offer several advantages such as fatigue free, lead free, low operating voltages and better ferroelectric properties. Therefore, Bismuth Titanate (BIT) which is a Bi- based layered perovskite ferroelectrics has been widely used in memory applications [49]. Bismuth-layered perovskite materials are also called Aurivillius phases.the layer perovskite structured Bismuth compound ferroelectrics has the general formula: A n-1 Bi 2 B n O 3n+3, where A is a divalent ion, such as Bi and B is Ti 4+ or Ta 5+. If consider A=Bi, B=Ti, and n=3, it makes the composition Bi 4 Ti 3 O 12. The bismuth layer structure was originally described by the formula (Bi 2 O 2 ) 2+ (A m-1 B m O 3m+1 ) 2- [50]. The layered structure consists of alternative stacking of a triple layer of TiO 6 octahedral and a monolayer of (Bi 2 O 2 ) 2+. Single crystal BIT has a low dielectric permittivity and a very high Curie temperature (T c = C) [51]. In figure 1.18, two Bi 3+ ions are located in A sites of perovskite block and other two Bi atom are present in (Bi 2 O 2 ) 2+ layers. In bismuth oxide layer, Bi-O bonding is covalent [52]. The oxygen coordination of Bi in bismuth oxide layer is less than that of Bi in A site of perovskite block. The central octahedral is completely corner shared to 25

26 [TiO 6/2 ] 2 whereas end octahedral possess one corner in a direction perpendicular to the layers unshared oxygen. In (Bi 2 O 2 ) 2+ layers, [BiO 2/2 ] + are interconnected and Bi atoms are severely under coordinated to oxygen. Thus, in BIT there are at least two types of Ti atoms and five type of oxygen atoms and two type of Bi atoms which are structurally distinguishable. They are also chemically distinguishable because there are differences in the nature of bonding at these sites [53]. 26

27 Figure 1.18: Layered Perovskite Structure of BIT However, BIT compound has high leakage current and domain pinning due to defects such as Bi vacancies are accompanied by oxygen vacancies. In order to minimize these defects, a suitable doping on A site is required. This doping displaced 27

28 the volatile Bi with suitable doped element to suppress the A site vacancies that acts as space charge [54]. The suitable doped element as lanthanum La has been proposed in [55] which led to chemically stability of perovskite layers compared to oxygen vacancies after substitution of Bi by La atom. Now, the formula Bi 4 Ti 3 O 12 has been converted into Bi 4- xla x Ti 3 O 12 when La 3+ substitutes Bi 3+ atoms partially. Because of the fatigue-free behavior of lanthanum doped bismuth titanate [Bi x La 4-x Ti 3 O 12 ] (BLT)], Bi-layered perovskite oxide films with a platinum electrode are widely used as nonvolatile memory [56-57]. Lanthanum doped bismuth titanate (BLT) has advantages including low leakage current, low coercive field, low processing temperature, high remnant polarization, good electrical and optical properties [58-62]. 28

29 Figure 1.19: Half Section of Pseudo Tetragonal Unit Cell of Bi 4 Ti 3 O 12 29

30 1.13 ADVANTAGES OF FERROELECTRIC MEMORIES The various advantages of FRAM over other non-volatile memories are: (i) FRAM write and erase access times are very fast of the order of nanoseconds (ii) FRAM has long write/erase lifetime (106 times higher than flash memories and EEPROM) (iii) (iv) Operating voltages of FRMA is very low say around 5 V FRAM possess wide range of operating temperature (-180 to 350 degree centigrade) (v) FRAM has high radiation hardness which are required for military and space applications [63]. Because of the above advantages, FRAM can replace SRAM in cache memory, DRAM in main computer memory and EEPROM in the lookup tables and hard disk as mass storage device. Hence, high density RAM can be considered as future memories PROBLEMS ASSOCIATED WITH FERROELECTRIC MEMORIES Fatigue Fatigue leads to decrease in switchable polarization with repeated cycles and hence decreases of switching and non-switching. Figure 1.20 shows the change in hysteresis loop with number of switching. Thus, it becomes difficult to distinguish between 0 & 1. Fatigue limits the life of ferroelectric non-volatile memory, where write and read operation both relay on ferroelectric switching. Fatigue of Ferroelectric capacitor can be measured by applying a train of bipolar pulses followed by a negative pulse and two positive pulses and then two negative pulses. Decrease in switching polarization comes due to pinning of domain walls, which inhabits switching of the domains. A variety of mechanisms for domain pinning have been proposed, including pinning due to electronic charge trapping pining by oxygen vacancies [64-68]. 30

31 Figure 1.20: Comparison of P-E hysteresis Loop After 1 Cycle and Cycles Retention In contrast to fatigue, retention is simply matter of shelf life. Retention failure means that stored charge has decreased to a level where + or - state of polarization cannot be reliably sensed as shown in figure Figure 1.21: Reduction in 1 and 0 Value Due to Retention 31

32 In general charge does not leak of the ferroelectric capacitor. Internal decay of stored charge is very small, typically it would take 30,000 years for stored charge to flow ohmically through the film to point where 50% was lost. This is due to space charge effects. Retention in ferroelectric materials can be measured by poling the ferroelectric capacitor in negative polarization state (-P r ) by negative voltage pulse; this correspond to storing a binary digit 1. And then after predetermined time interval, 1 is read by measuring the charge switched by + ve pulse. Thus P s corresponding 1 is measured. In similar way 0 is written by poling the capacitor by +ve voltage pulse and reading it with ve voltage pulse after predetermined time. In this way Pn s corresponding to 0 is measured. Due to retention, ability of dipoles to get depoled when switching pulse is applied reduces. So no difference between P s & Pn s and hence 1 & 0 respectively can be observed [69] Aging Shift of hysteresis loop towards negative voltage with time is called aging of ferroelectric material and is shown in figure Aging occurs only in ferroelectric state. Increasing temperature, as long as temperature is below Curie temperature, accelerates it. The development of voltage shift in thin film left in given polarization state for an extended period of time is the basis of the phenomenon of imprint [70] which was originally used to describe the failure of capacitor in non volatile memories. A voltage shift can lead to failure of memory element if shift is large enough that difference in value of polarization between two remanent state drops below a threshold value. 32

33 Figure 1.22: Shift in Hysteresis Curve Due to Aging 1.15 APPLICATIONS OF FRAM (i) Because of low power consumption, FRAM can be used in making personal digital diary, power meters and security systems. (ii) FRAM is faster than flash memory. A secure system can be made by integrating FRAM with CPU in a chip which allows the data encryption for e-commerce transactions. (iii) The incorporation of large density FRAM makes it possible to carry out multiple applications and store large amounts of data and makes it ideal for use in multifunctional IC cards. (iv) Ferroelectric memories have wide applications in semiconductor memory and defense industries because of the compatibility with silicon process, high speed of operation and less power consumption [71-72] THESIS OBJECTIVES The specific objectives of this thesis are as follows: 1) Preparation of bulk ceramic samples by using suitable raw materials and techniques for different % doping of La in BIT. 33

34 2) To analysis various characteristics to find the optimal percentage of La doping in BIT. 3) To prepare thin films of different thickness using sol- gel method with the optimal percentage of La doping in BIT. 4) To find the optimum thickness of the film. 5) To compare various characteristics to obtain best annealing time of thin films ORGANIZATION OF THESIS Chapter 2 describes the various methods of preparing of bulk ceramic and thin films. In this chapter, various thin film deposition techniques have been discussed. In this chapter, coating, spinning and spraying methods have also be explained. Chapter 3 focuses on experimental setup of making various samples of ceramic samples and thin films samples. In this chapter, various equipments that are used in the research work has also been discussed. Chapter 4 describes the results obtained for bulk ceramic and thin films samples. In this chapter various characteristics like XRD, hysteresis loops, leakage current curves, SEM pictures etc. for bulk ceramic and thin films are analyzed to find the optimal thin film. Chapter 5 summarizes the conclusion of the work done including optimum doping of La, thickness of films, various characteristics and annealing time of thin film samples. In this chapter, applications & future scope have also been discussed for further analysis. 34