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

SWNT MWNT

• MWNT is made from layers of SWNTs

• MWNTs can have a diameter of tens of nm

• Length can be micrometers

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

• 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

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 E-fields 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

InsulatorSubstrate

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)

Transparent Electrode

CNTs

Substrate

Convex Zoom Lens

Concave Zoom Lens

Variable Phase Shifter

CNT Radiative Recombination

Thank You!

• This presentation will be available on the web.