Ferroelectric Field Effect Transistor - The Memory Technology of Tomorrow

1. Ferroelectric Field Effect Transistor-The Memory Technology of Tomorrow? Antoine Brugere Arndt von Bieren Florent Boyer Chammard Michael Kallenberg

2. 2 Introduction on FeFET Combination of (Si MOSFET) transistor technology and ferroelectric materials Like a conventional transistor, but it can �remember� its state Provides wide spectrum of possible applications, e.g.

3. 3 Contents Ferroelectrics Principles of FeFET Problems and Improvements Conclusion

4. 4 Contents Ferroelectrics Basic Properties Ferroelectric Domains & Hysteresis Important Ferroelectric Materials Principles of FeFET Problems and Improvements Conclusion

5. 5 1. Ferroelectrics Basic Properties Ferroelectrics: dielectric, ionic crystals, which exhibit spontaneous polarization defined states depending on structure, switchable by external electric fields occurs only below material-specific �Curie�-temperature Example (figure): PZT Lead zirconate titanate (Pb[ZrxTi1-x]O3 0<x<1, also Lead zirconium titanate) is a ceramic perovskite material Where: Perovskite (calcium titanium oxide, CaTiO3) Polarization: positive and negative charges are displaced, orientated dipole moments are created. Vanishes in normal dielectric crystals immediately after switching off the external field. Ability to exhibit spont. Polarization is related to its symmetry. For example: spont. Pol. Is not possible in crystals with an inversion center. Unique polar axis exists -> spont. Pol. Parallel to this axis possible (BaTiO3) More crystals than only the ferroelectrics exhibit piezo and pyroelectric effects Short summary of applications� Example (figure): PZT Lead zirconate titanate (Pb[ZrxTi1-x]O3 0<x<1, also Lead zirconium titanate) is a ceramic perovskite material Where: Perovskite (calcium titanium oxide, CaTiO3) Polarization: positive and negative charges are displaced, orientated dipole moments are created. Vanishes in normal dielectric crystals immediately after switching off the external field. Ability to exhibit spont. Polarization is related to its symmetry. For example: spont. Pol. Is not possible in crystals with an inversion center. Unique polar axis exists -> spont. Pol. Parallel to this axis possible (BaTiO3) More crystals than only the ferroelectrics exhibit piezo and pyroelectric effects Short summary of applications�

6. 6 1. FerroelectricsOrigin of Spontaneous Polarization splitting of optical vibration modes in ionic crystals softening of TO mode due to partial force compensation (elastic & electrostatic) Lecture: Explanation of ferroelectrics with mean field concept Here: Sketch of the soft phonon-mode approach! LO: shifted to higher frequencies, enhancing of mechanical restoring forces TO: partial compensation of short-range lattice forces and long-range electric fields softening very material-specific, depends on the crystal structure homogeneous polarization only in planes whose thickness is small compared to lambda/2. homogeneous region becomes infinite for q = 0 (c) ! no vibration for frozen out TO mode -> permanent spont. pol. -> ferroelectric displacive phase transition takes place! Asoft phonon is a vibrational mode of a crystalline material whose frequency decreases as T falls, eventually reaching zero. At this point the crystal is unstable in relation to the corresponding atomic displacements and undergoes a transition to a lower symmetry phase. Typical examples are the high-low transition in quartz, the ferroelectric transition in BaTiO3, and the ferroelastic transition in SrTiO3 [1]. =================================== NATURE article (ferroelectrics.pdf) Phonons are important in the phase transitions in the ferroelectric perovskite titanates SrTiO3 (STO) and (Ba,Sr)TiO3 (BST)4,5. As temperature decreases, the eigenfrequency of the lowest optical mode (the soft mode) falls and approaches zero at a critical temperature Tc where a lattice instability leads to a ferroelectric phase transition6. The Lyddane�Sachs�Teller (LST) relation for a crystal with N infrared-active optical modes (N = 3 for STO) is: This relates the static dielectric constant, e(0), and the highfrequency dielectric constant, e(`), to the eigenfrequencies, qLOj and qTOj, of the longitudinal (LO) and transverse (TO) opticalphonon modes, respectively. It is generally found that the eigenfrequencies of the higher optical modes exhibit no sizeable variation with temperature. In bulk crystals the LST relation has been proven experimentally, and the dramatic increase of e(0) is directly related to the soft-mode behaviour. The decrease of the soft-mode eigenfrequency with temperature in bulk STO suggests a Tc of 32 K and causes e(0) to increase to values above 20,000 (ref. 5), although zeropoint quantum �uctuations of Ti ions prevent a ferroelectric phase transition from occurring7. For BST, the most commonly used ferroelectric material for DRAM applications, Tc can be adjusted by the Ba/Sr ratio to up to approximately 130 8C and e(0) can be as high as 15,000 (ref. 8). This high dielectric constant should allow the production of very compact capacitor structures. In reality, however, much lower values have been reported in thin �lms. For example, a dielectric constant of 150 was observed for a 24-nm-thick polycrystalline BST �lm in a DRAM device structure3, while a value of e(0) of ,250 was found in an epitaxial 25-nm-thick STO �lm9. Lecture: Explanation of ferroelectrics with mean field concept Here: Sketch of the soft phonon-mode approach! LO: shifted to higher frequencies, enhancing of mechanical restoring forces TO: partial compensation of short-range lattice forces and long-range electric fields softening very material-specific, depends on the crystal structure homogeneous polarization only in planes whose thickness is small compared to lambda/2. homogeneous region becomes infinite for q = 0 (c) ! no vibration for frozen out TO mode -> permanent spont. pol. -> ferroelectric displacive phase transition takes place! Asoft phonon is a vibrational mode of a crystalline material whose frequency decreases as T falls, eventually reaching zero. At this point the crystal is unstable in relation to the corresponding atomic displacements and undergoes a transition to a lower symmetry phase. Typical examples are the high-low transition in quartz, the ferroelectric transition in BaTiO3, and the ferroelastic transition in SrTiO3 [1]. =================================== NATURE article (ferroelectrics.pdf) Phonons are important in the phase transitions in the ferroelectric perovskite titanates SrTiO3 (STO) and (Ba,Sr)TiO3 (BST)4,5. As temperature decreases, the eigenfrequency of the lowest optical mode (the soft mode) falls and approaches zero at a critical temperature Tc where a lattice instability leads to a ferroelectric phase transition6. The Lyddane�Sachs�Teller (LST) relation for a crystal with N infrared-active optical modes (N = 3 for STO) is: This relates the static dielectric constant, e(0), and the highfrequency dielectric constant, e(`), to the eigenfrequencies, qLOj and qTOj, of the longitudinal (LO) and transverse (TO) opticalphonon modes, respectively. It is generally found that the eigenfrequencies of the higher optical modes exhibit no sizeable variation with temperature. In bulk crystals the LST relation has been proven experimentally, and the dramatic increase of e(0) is directly related to the soft-mode behaviour. The decrease of the soft-mode eigenfrequency with temperature in bulk STO suggests a Tc of 32 K and causes e(0) to increase to values above 20,000 (ref. 5), although zeropoint quantum �uctuations of Ti ions prevent a ferroelectric phase transition from occurring7. For BST, the most commonly used ferroelectric material for DRAM applications, Tc can be adjusted by the Ba/Sr ratio to up to approximately 130 8C and e(0) can be as high as 15,000 (ref. 8). This high dielectric constant should allow the production of very compact capacitor structures. In reality, however, much lower values have been reported in thin �lms. For example, a dielectric constant of 150 was observed for a 24-nm-thick polycrystalline BST �lm in a DRAM device structure3, while a value of e(0) of ,250 was found in an epitaxial 25-nm-thick STO �lm9.

7. 7 1. Ferroelectrics Ferroelectric Domains & Hysteresis domains are regions with uniform direction of spontaneous polarization separated by domain walls (1-10 a thick) which appear along specific crystal planes Domain = region with uniform direction of spontaneous polarization depends on the crystallography of the system PZT & Barium-Titanate: 6 possible direction for the Ti atom to move -> six possible domain states domain walls: 1-10 lattice parameters thick, polarization rotates in domain wall -> no opposite polarizations next to each other -> minimization of energy why domains? compensated ferroelectric single crystal (i.e. one which is electroded�) -> only one domain leads to lowest energy i.e.: domain formation driven by electrical boundary formations: field energy is reduced by creation of domain walls and formation of different ferroelectric domains putting a domain wall in a system costs energy -> subdividing into smaller domains stops when balance reachedDomain = region with uniform direction of spontaneous polarization depends on the crystallography of the system PZT & Barium-Titanate: 6 possible direction for the Ti atom to move -> six possible domain states domain walls: 1-10 lattice parameters thick, polarization rotates in domain wall -> no opposite polarizations next to each other -> minimization of energy why domains? compensated ferroelectric single crystal (i.e. one which is electroded�) -> only one domain leads to lowest energy i.e.: domain formation driven by electrical boundary formations: field energy is reduced by creation of domain walls and formation of different ferroelectric domains putting a domain wall in a system costs energy -> subdividing into smaller domains stops when balance reached

8. 8 1. Ferroelectrics Ferroelectric Domains & Hysteresis Hysteresis is caused by irreversible polarization processes �pinning� of domain walls at lattice defects newly created domains do not disappear after removal of field small displacements in weak fields are reversible wall movement can be described by a potential major processes: PINNING: lattice defects interact with domain walls and prevent them from returning into their initial position after removing the electric field that initiated the domain wall motion SECOND: nucleation and growth of new domains which do not disappear after the field is removed again motion of walls under an external field takes place in a statistical potential generated by their interaction with the lattice, point defects, dislocations, and neighboring walls major processes: PINNING: lattice defects interact with domain walls and prevent them from returning into their initial position after removing the electric field that initiated the domain wall motion SECOND: nucleation and growth of new domains which do not disappear after the field is removed again motion of walls under an external field takes place in a statistical potential generated by their interaction with the lattice, point defects, dislocations, and neighboring walls

9. 9 1. Ferroelectrics Important Ferroelectric Materials PZT - Pb(ZrxTi1-x)O3 large available spontaneous polarization, high piezoelectric coefficients high transition temperature (~370�C) SBT � SrBi2Ta2O9 only few allowed directions of spontaneous polarization low remanent polarization very high transition temperature (~570�C) to come slowly to fefets: two important ferroelectric materials used in fefets PZT: Lead Zirconate Titanate a particular phase transition appears at a certain mixture ratio � the so-called morphotropic phase boundary marks this transition. leads to 14 different polarization directions -> highly polarizable state, which is often exploited for certain applications SBT: Strontium Bismuth Tantalate to come slowly to fefets: two important ferroelectric materials used in fefets PZT: Lead Zirconate Titanate a particular phase transition appears at a certain mixture ratio � the so-called morphotropic phase boundary marks this transition. leads to 14 different polarization directions -> highly polarizable state, which is often exploited for certain applications SBT: Strontium Bismuth Tantalate

10. 10 Contents Ferroelectrics Principles of FeFET Problems and Improvements Conclusion

11. 11 Contents Ferroelectrics Principles of FeFET Function and Properties Non-Volatile Writing Process Non-Destructive Reading Process Requirements Problems and Improvements Conclusion

12. 12 2. Principles of FeFET Function and Properties

13. 13 2. Principles of FeFET Non-Volatile Writing Process Data stored in the orientation of the polarization P By applying an electric field (gate voltage) higher than the coercitive field Ec (|V| > |Vc|) .

14. 14 2. Principles of FeFET Non-Volatile Writing Process Data stored in the orientation of the polarization P By applying an electric field (gate voltage) higher than the coercitive field Ec (|V| > |Vc|) . After turning off the power, P becomes equals to Pr

15. 15 2. Principles of FeFET Non-Destructive Reading Process Due to the polarization, charges appear at the interface F/Si

16. 16 2. Principles of FeFET Non-Destructive Reading Process Due to the polarization, charges appear at the interface F/Si Those charges influence the resistivity of the FET channel.

17. 17 2. Principles of FeFET Non-Destructive Reading Process Due to the polarization, charges appear at the interface F/Si Those charges influence the resistivity of the FET channel. The reading is processed by measuring this resistivity

18. 18 2. Principles of FeFET Requirements Compatibility with CMOS technology -> Integration of the material without change of ferroelectric properties No retention loss -> conservation of the polarization Pr (more than 10 years) Easily switchable -> switch must be fast and not need much power. High cycle endurance -> more than 1015 writing processes

19. 19 Contents Ferroelectrics Principles of FeFET Problems and Improvements Conclusion

20. 20 Contents Ferroelectrics Principles of FeFET Problems and Improvements Interface Issues Threshold Voltage Retention Time Fatigue Effect Conclusion

21. 21 3. Problems and Improvements Interface Issues Problems Interdiffusion between the ferroelectric layer and Si during the deposition process. Charge injection from Si to the ferroelectric during the switching of P.

22. 22 3. Problems and Improvements Threshold Voltage Problems For MFIS and MFMIS (only), the system is equivalent to two serial capacitors (voltage divider).

23. 23 3. Problems and Improvements Retention Time Problems For MFIS and MFMIS (only), apparition of an electric field opposed to the polarization Ferroelectrics materials with a low Vc shows a unstable polarization

24. 24 3. Problems and Improvements Fatigue Effect Problems Pr decreases with increasing number of cycles (reduction of 50% after 1012 cycles) no distinction between on and off state

25. 25 Contents Ferroelectrics Principles of FeFET Problems and Improvements Conclusion

26. 26 4. Conclusion  Big potential for memory application : non-volatile data storage, non-destructive readout... Solutions proposed to solve the different problems Here we study high-performance solution-processed polymer FeFETs consisting of a poly(vinylidene fl uoride/trifl uoroethylene) (P(VDF/TrFE)) ferroelectric copolymer as gate insulator and poly[2-methoxy, 5-(2?-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) as a semiconductor. P(VDF/TrFE) is a wide-bandgap insulator and a ferroelectric material17. A heteropolymer, also called a copolymer, is a polymer formed when two (or more) different types of monomer are linked in the same polymer chain, as opposed to a homopolymer where only one monomer is used. If exactly three monomers are used, it is called a terpolymer. RETENTION: The on/off ratio after a week is 10^4 and, apart from the initial off-state increase, is completely stable. Figure 5a shows that the on-state is stable for a week whereas the off-current has an initial increase that stops after 1 day. This increase is largely due to an increased gate current and not due to the channel conductance, as shown by the similar increase of gate and drain current. Here we study high-performance solution-processed polymer FeFETs consisting of a poly(vinylidene fl uoride/trifl uoroethylene) (P(VDF/TrFE)) ferroelectric copolymer as gate insulator and poly[2-methoxy, 5-(2?-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) as a semiconductor. P(VDF/TrFE) is a wide-bandgap insulator and a ferroelectric material17. A heteropolymer, also called a copolymer, is a polymer formed when two (or more) different types of monomer are linked in the same polymer chain, as opposed to a homopolymer where only one monomer is used. If exactly three monomers are used, it is called a terpolymer. RETENTION: The on/off ratio after a week is 10^4 and, apart from the initial off-state increase, is completely stable. Figure 5a shows that the on-state is stable for a week whereas the off-current has an initial increase that stops after 1 day. This increase is largely due to an increased gate current and not due to the channel conductance, as shown by the similar increase of gate and drain current.

27. 27 4. Conclusion  Novel approach : FeFET based on organic materials

28. 28 The End Thank you for your attention References can be found in our report