Tutorial 8 Derek Wright Wednesday, March 9th, 2005 Logic Devices • • • • Ferroelectric FETs Resonant Tunneling Quantum Devices Single-Electron Devices Carbon Nanotubes 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) FeFET Operation • Structure shows hysteresis • State stores as which side of hystersis curve FET is on • Must be programmed on/off FeFET Structures 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) FeRAMs • For smaller cell, instead of 1T1C, fold ferroelectric capacitor into gate dielectric • Challenge is dielectric to silicon interface – Buffer layer required series capacitance FeRAMs • By using High-k dielectric (LaAlO3), series capacitance issue is reduced • New stack shows good memory window FeRAMs • With the improved stack, good storage characteristics are observed 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 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 Resonant Tunneling 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 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 Single Electron Transistor Single Electron Transistor 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 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 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 Mechanical Properties of CNTs • • • • 100x stronger than steel but 6x lighter Highly flexible, unlike carbon fibers Expansion when in E-field High thermal resistance 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 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 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 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 Catalyst Carbon sublimates onto catalyst • 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 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 CNT Gas Sensors • Carbon nanotubes can have extremely high Efields near the tip • Great field emission • Can be used to measure the discharge currents of different gasses Anode Insulator CNTs (Cathode) Substrate CNT Field-Emission Displays • CNTs can shoot electrons at a phosphorous screen Phosphorous CNTs Insulator Substrate 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 CNT Force Measurement • Use a CNT as a cantilever on an atomic force microscope (AFM) to improve resolution AFM Cantilever CNT CNT Zoom Lenses • CNT Index of refraction can be adjusted with the application of an E-field (n ~0.9) Convex Zoom Lens Transparent Electrode CNTs Concave Zoom Lens Variable Phase Shifter Substrate CNT Radiative Recombination Thank You! • This presentation will be available on the web.