1. Tutorial 8 Derek Wright Wednesday, March 9 th, 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 V GS 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. FeRAMs Nonvolatile RAMs can be made that use FeFETs and Fe capacitors a) DRAM b) FeRAM using Fe capacitor c) FeRAM using FeFET a) b) c)

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 (LaAlO 3 ), 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

17. 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 SWNTMWNT MWNT is made from layers of SWNTs MWNTs can have a diameter of tens of nm Length can be micrometers

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 m 2 /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 cm 2 /Vs –Standard n-FET  = 1,500 cm 2 /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. 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 Catalyst Carbon sublimates onto catalyst

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

31. CNT Radiative Recombination

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