Tutorial 8

1. Tutorial 8 Derek Wright Wednesday, March 9th, 2005

2. Logic Devices • Ferroelectric FETs • Resonant Tunneling Quantum Devices • Single-Electron Devices • Carbon Nanotubes

3. FeFETs • Structure similar to a MOSFET • Substrate with source/drain, dielectric, gate • Dielectric has magnetic dipoles • VGS can “flip” the dipole moment • The dipole is either pointing towards or away from the substrate • One direction creates a channel of minority carriers (inversion  ON) • One direction pulls majority carriers towards the gate (accumulation  OFF)

4. FeFET Operation • Structure shows hysteresis • State stores as which side of hystersis curve FET is on • Must be programmed on/off

5. FeFET Structures

6. a) b) c) FeRAMs • Nonvolatile RAMs can be made that use FeFETs and Fe capacitors • a) DRAM • b) FeRAM using Fe capacitor • c) FeRAM using FeFET

7. FeRAMs • For smaller cell, instead of 1T1C, fold ferroelectric capacitor into gate dielectric • Challenge is dielectric to silicon interface • Buffer layer required  series capacitance

8. FeRAMs • By using High-k dielectric (LaAlO3), series capacitance issue is reduced • New stack shows good memory window

9. FeRAMs • With the improved stack, good storage characteristics are observed

10. Resonant Tunneling Quantum Devices • When structures are on the order of the wavelength of an electron, quantum effects become important • Tunneling is one effect that is useful • Since electrons are waves, they can have resonance properties, too • We can use resonance and tunneling together to make devices with interesting transfer characteristics

11. Resonant Tunneling • Thin barriers allow tunneling • However, the distance between two barriers limits the electron’s energy to discrete values • This results in discrete electron energies (lower than the barrier) being allowed to pass • It also distorts the transmission of energies higher than the barrier due to interference effects

12. Resonant Tunneling

13. Single Electron Devices • Single electron devices: • Benefit from scaling • Dramatically reduce power • Simple device has: • a quantum dot • a capacitively coupled gate • a tunnel barrier • Gate draws in or pushes out an e- through the tunnel barrier on the other side

14. Single Electron Devices • More than one electron can enter the box under discrete gate bias • Can accurately control the number of electrons in the dot

15. Single Electron Transistor

16. Single Electron Transistor

17. Single Electron Transistor • Compared to MOSFETs, SETs: • Consume less power • Are more easily scalable • Are easier to operate at low temperatures • Must have a smaller source-drain voltage

18. What is A Carbon Nanotube? • A cylinder of graphite (carbon) • Capped by hemispherical ends • Composed of pentagons and hexagons • Diameter from 0.5 – 2.0 nm • Discovered by Sumio Ijyma

19. Single- and Multi-Wall Nanotubes • MWNT is made from layers of SWNTs • MWNTs can have a diameter of tens of nm • Length can be micrometers SWNT MWNT

20. Mechanical Properties of CNTs • 100x stronger than steel but 6x lighter • Highly flexible, unlike carbon fibers • Expansion when in E-field • High thermal resistance

21. Physical Properties of CNTs • High surface area: 100s of m2/g • Hollow CNTs enable molecule storage inside • Chemical treatment of CNTs allows other molecules to be fixed to the surface

22. Electrical Properties of CNTs • Metallic or semiconductor behavior based on chirality • Can be more conductive than copper • Mobility  = 100,000 cm2/Vs • Standard n-FET  = 1,500 cm2/Vs • Carrier density (conductance) can by electrostatically tuned • Tunable field emission

23. CNT Chirality • Graphite sheets have 2D E-k diagrams • Semiconducting along some vectors and conducting along others • CNT rolled from graphite forces 1D E-k behaviour • Forces either semiconducting or conducting behaviour a) Graphite Sheet b) CNT made from graphite roll

24. Carbon sublimates onto catalyst Catalyst Bulk Synthesis of CNTs • CNTs are grown by bulk synthesis then deposited on a substrate by spinning or drying (liquid epitaxy) • Arc Synthesis • Laser-assisted Growth • Tubes are bundled together in “ropes” and are highly tangled • Must be cut apart before deposition (ultrasonication) • Creates tubes of varying lengths and many defects

25. Growth of CNTs • Nanotubes can be grown directly on the substrate using CVD • PECVD • Thermal CVD • Alcohol Catalytic CVD • Vapour Phase Growth (no substrate) • Aero gel-supported CVD • Laser-assisted thermal CVD • SWNT diameter controllable • Simple process on existing equipment

26. CNT Gas Sensors • Carbon nanotubes can have extremely high E-fields near the tip • Great field emission • Can be used to measure the discharge currents of different gasses Anode Insulator CNTs (Cathode) Substrate

27. CNT Field-Emission Displays • CNTs can shoot electrons at a phosphorous screen Phosphorous CNTs Insulator Substrate

28. CNT Field-Effect Transistors • CNT is used as the channel between source and drain • Works as a FET • Very small feature size ideal for advanced digital circuits

29. CNT Force Measurement • Use a CNT as a cantilever on an atomic force microscope (AFM) to improve resolution AFM Cantilever CNT

30. Transparent Electrode CNTs Substrate CNT Zoom Lenses • CNT Index of refraction can be adjusted with the application of an E-field (n ~0.9) Convex Zoom Lens Concave Zoom Lens Variable Phase Shifter

31. CNT Radiative Recombination

32. Thank You! • This presentation will be available on the web.