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P. E. Russell: russllp@appstate.edu


Nanoscience and Nanotechnology


Lecture 19

PHY5540 Spring 2006

Only 8 lectures remaining!

(including this one)

Nanoelectronics

Course notes are at:

http://www.phys.appstate.edu/nanotech/NanoCourseLecturePPTs2008/


P. E. Russell: russllp@appstate.edu

Back to Nanoelectronics

We briefly started this earlier

P. E. Russell: russllp@appstate.edu

IBM: Atomic
-
sized graphene has use in nanoelectronics

Electronic News, 3/6/2008
IBM

researchers at the
company's Yorktown Heights, NY
-
based lab today detailed
a discovery that could allow graphite to be used as a
material for building nanoelectonic circuits smaller than
those in today's silicon
-
based computer chips

Single layer:

Noisy, as
predicted by

Double
layer: Noise
reduced to
acceptable
level

P. E. Russell: russllp@appstate.edu

IBM: Atomic
-
sized graphene has use in nanoelectronics

IBM'S ATOMIC 'CHICKENWIRE' FOR NANOELECTRONICS: The image on the
left shows a single layer, or sheet of carbon molecules known as Graphene.
The noise that occurs from electrical signals bouncing around in the material
as a current is passed through it is greater as the device is made smaller and
smaller, impeding the performance for nanoscale electronics. In the image on
the right, the IBM scientists demonstrated for the first time that adding a
second sheet of Graphene reduces the noise significantly, giving promise to
this material for potential use in future nanoelectronics.

P. E. Russell: russllp@appstate.edu

Speed Bumps Less Important Than Potholes For Graphene

National Institute of Standards and Technology. "Speed Bumps
Less Important Than Potholes For Graphene."
ScienceDaily

16 July 2007. 17 March 2008 <
http://www.sciencedaily.com
/
releases/2007/07/070713131450.htm
>.

For electrical charges racing through an atom
-
thick sheet of
graphene, occasional hills and valleys are no big deal, but
the potholes
--
single
-
atom defects in the crystal
--
they're
killers. That's one of the conclusions reached by
researchers from the National Institute of Standards and
Technology (NIST) and the Georgia Institute of Technology
who created detailed maps of electron interference patterns
in graphene to understand how defects in the two
-
dimensional carbon crystal affect charge flow through the
material.

P. E. Russell: russllp@appstate.edu

The Nanotube transistor

Carbon
-
atom model of singlewall carbon nanotubes.
Singlewall carbon nanotubes exist in a variety of
structures corresponding to the many ways a sheet of
graphite can be wrapped into a seamless tube. Each
structure has a specific diameter and chiral (or
wrapping) angle.

The armchair structures (


= 30
o
?
) have metallic
character.

The zigzag tubes (


= 0
o
) can be either semimetallic or
semiconducting.

Nanotubes with chiral angles of 0
?
to 30
o
?
] include both
semimetals and semiconductors.

From Weisman 2004 [465].

P. E. Russell: russllp@appstate.edu

Nanotube Chirality

metalic

metallic

Semiconducting*

Semiconducting*

*or semimetallic

P. E. Russell: russllp@appstate.edu

The Nanotube transistor

Cncept

AFM of actual nanotube transistor

P. E. Russell: russllp@appstate.edu

P. E. Russell: russllp@appstate.edu

NANOELECTRONICS


What is the basic Physics of nanoelectronics?


The aim of Nanoelectronics is to process, transmit and store
information by taking advantage of properties of matter that are
distinctly different from macroscopic properties.



The relevant length scale depends on the phenomena
investigated: it is a few nm for molecules that act like transistors
or memory devices, and it can be 999 nm (~1 micron) for
quantum dots where the spin of the electron is being used to
process information.



Microelectronics, even if the gate size of the transistor is 50 nm,
is

not

an implementation of nanoelectronics, as no new
qualitative physical property related to reduction in size are
being exploited.

P. E. Russell: russllp@appstate.edu

The need for nano electronics


The last few decades has seen an exponential growth
in microchip capabilities due primarily to a decrease
in the minimum feature sizes. The resulting doubling
of processor speed every 18 months (known as
Moores Law) is, however, expected to break down for
conventional microelectronics in about 2015 for both
fundamental and economic reasons


[Nature 406,
1027 (2000)].


15 years corresponds to only 3 generations of
graduate students (2y MS, 3y Ph.D.)! The search is on,
therefore, for new properties, paradigms and
architectures to create a novel nanoelectronics.

P. E. Russell: russllp@appstate.edu

What is Nanoelectronics?


Semiconductor electronics have seen a sustained
exponential decrease in size and cost and a similar
increase in performance and level of integration over
the last thirty years (known as Moore's Law). The
Silicon Roadmap is laid out for about the next ten
years. After that, either economical or physical
barriers will pose a huge challenge.

P. E. Russell: russllp@appstate.edu

Moore’s Law


Moore first posited his exponentially
increasing transistor idea in a 1965 magazine
article in the 35th anniversary edition of
"Electronic Magazine" when the integrated
circuit was still in its teething phase (with
only a handful of components per chip).

P. E. Russell: russllp@appstate.edu

Moore’s Law


Moore's first prediction was based upon the
progress of the integrated chip up until that
point, which showed that since the
introduction of the first planar transistor in
1959, there had been a doubling of
components contained on a single chip every
year.


Moore's wasn't a particularly rigorous line of
scientific enquiry, but then history is full of
brilliant ideas derived from the intuitive
reasoning of geniuses.



How did Gordon Moore come up with Moore’s
Law?

P. E. Russell: russllp@appstate.edu

Moore’s Law


"I took that first few points, up to 60 components on a
chip in 1965 and blindly extrapolated for about 10
years and said okay, in 1975 we

ll have about 60
thousand components on a chip," recalls Moore.



"I had no idea this was going to be an accurate
prediction. And one of my friends, Dr. Carver Mead, a
Professor at Cal Tech, dubbed this Moore

猠L慷."



So, originally, Moore's prediction was a doubling of
complexity annually. But in 1975, after missing a vital
factor contributing to the chip's rate of "remarkable"
progress, Moore revised this prediction to the one we
currently: every two years.

P. E. Russell: russllp@appstate.edu

Moore’s Law


Moore's Law

P. E. Russell: russllp@appstate.edu

Moore’s Law


See the Intel technology timeline at:


http://www.intel.com/technology/timeline.pdf


The Revolution Begins

Invented 60 years ago, the transistor is a key building block of
today’s digital world.

Perhaps the most important invention of the 20th century,
transistors are found in many devices and are the building
blocks of computer chips.

Intel, the largest manufacturer of computer chips, continues to
innovate to help PCs and laptops become smaller, faster, sleeker
and more energy effcient.

Many new applications and inventions powered by transistors have
impacted all of our lives over the past 60 years.


But this is not nantechnology

P. E. Russell: russllp@appstate.edu

What is Nanoelectronics?


Moore's Law is related to the difficulty of making a profit in view of the
exorbitant costs of building the necessary manufacturing capabilities if
present day technologies are extrapolated.



The Silicon Roadmap is a direct consequence of the shrinking device
size, leading to physical phenomena impeding the operation of current
devices
-

relying on fairly straight forward improvements in engineering
and technology
-

not new science!


Solid State Physics, however, falls apart at the nanometer scale when
the number of surface (and/or interface states) begins to play a
dominate role and hence cannot be ignored.



For nanoelectronics, new science and new technology (Nanoscience
and nanotechnology) must come into play!



What are some of the issues we face in moving to nanotechnology?


P. E. Russell: russllp@appstate.edu

What are the some of the new effects?


Quantum and coherence effects, high electric fields
creating avalanche dielectric breakdowns, heat
dissipation problems in closely packed structures as
well as the non
-
uniformity of dopant atoms and the
relevance of single atom defects are all roadblocks
along the current road of miniaturization


[Nature 406, 1023 (2000)].



These phenomena are characteristic for structures a
few nanometers in size and, instead of being viewed
as an obstacle to future progress, might form the
basis of post
-
silicon information processing
technologies.

P. E. Russell: russllp@appstate.edu

More than just
electron
ics


It is even far from clear that
electrons
will be the method of
choice for signal processing or computation in the long term
-

quantum computing, spin electronics, optics or even computing
based on nanomechanics are actively being discussed.



Nanoelectronics
thus needs to be understood as a general field
of research aimed at developing an understanding of the
phenomena characteristic of nanometer sized objects with the
aim of exploiting them for information processing purposes.



Specifically, by electronics we mean the handling of complicated
electrical wave forms for
communicating information

(as in
cellular phones),
probing

(as in radar) and
data processing
(as in
computers) [Landauer, Science (1968)].

P. E. Russell: russllp@appstate.edu


Concepts at the fundamental research level are being
persued world
-
wide to find nano
-
solutions to these
three characteristic applications of electronics.


One can group these concepts into several main
categories:

1. Molecular electronics:
Electronic effects (e.g.
electrical conductance of C60)

2. Synthesis (DNA computing as a buzz word)

3. Quantum Electronics, Spintronics
(e.g. quantum dots,
magnetic effects)

4. Quantum computing

P. E. Russell: russllp@appstate.edu

Active fields related to nanoelectronics


Currently the most active field of research is the
fabrication and characterization of individual
components that could replace the macroscopic
silicon components with nanoscale systems.


Examples are molecular diodes , single atom
switches or the increasingly better control and
understanding of the transport of electrons in
quantum dot structures.


A second field with substantial activity is the
investigation of potential interconnects. Here, mostly
carbon nanotubes and self
-
assembled metallic or
organic structures are being investigated.

P. E. Russell: russllp@appstate.edu

Nanoelectronics Architectures


Very little work is being performed on architecture
(notable exceptions are HP's Teramac project [Heath
et al., Science 280
,
1716 (1998)] or IBM's selfhealing
Blue Gene project).



Furthermore,
modeling

with predictive power is in a
very early stage of development. This understanding
is necessary to develop engineering rules of thumb to
design complex systems. One needs to appreciate
that currently the best calculations of the
conductance of a simple molecule such as C60 are
off
by a factor of more than 30!



This has to do with the difficult to model, but non
-
trivial influence of the electronic contact leads.



P. E. Russell: russllp@appstate.edu

Nanoelectronics Architectures


This has to do with the difficult to model, but non
-
trivial influence of the electronic contact leads
-

just
the ability to have input and output of signals with
nanoelectronic devices is a major technical barrier.




The situation in quantum computing is somewhat
different. The main activities are on theoretical
development of core concepts and algorithms.
Experimental implementations are only starting.



An exception is the field of cryptography (information
transportation), where entangled photon states
propagating in a conventional optical fiber have been
demonstrated experimentally.

P. E. Russell: russllp@appstate.edu

What needs to be done ?


First, nanoelctronics is a wide open field with vast potential for
breakthroughs coming from fundamental research. Some of the
major issues that need to be addressed are the following:


1. Understand nanoscale transport! (closed loop between theory
and experiment necessary).


2. Develop/understand self
-
assembly techniques to do
conventional things cheaper. This has the future potential to
displace a large fraction of conventional semiconductor
applications.


3. Find new ways of doing electronics and find ways of
implementing them (e.g. quantum computing; electronics
modeled after living systems; hybrid Si
-
biological systems;
cellular automata).


Do not try and duplicate a transistor, but instead investigate new
electronics paradigms! Do research as a graduate student in this
field and lay the foundation for the ‘Intel’ of the New Millenium.

P. E. Russell: russllp@appstate.edu

What should today’s Physics student do?


1. Understand nanoscale transport! (closed loop between theory and
experiment is necessary). Most experiments and modeling concentrate
on DC properties, AC properties at THz frequencies are however
expected to be relevant.



2. Develop/understand self
-
assembly techniques to do conventional
things cheaper. This has the future potential to displace a large fraction
of conventional semiconductor applications. One needs to solve the
interconnect problem and find a replacement of the transistor. If this
can be done by self
-
assembly, a major cost advantage compared to
conventional silicon technology would result.



3. Find new ways of doing electronics and find ways of implementing
them (e.g. quantum computing; electronics modeled after living
systems; hybrid Si
-
biological systems; cellular automata). Do not try
and duplicate a transistor, but instead investigate new electronics
paradigms! Do research as a graduate student in this field and lay the
foundation for the Intel of the New Millenium.

P. E. Russell: russllp@appstate.edu

http://domino.research.ibm.com/comm/pr.nsf/pages/news.200
60324_carbonnanotube.html


IBM Milestone Advances Effort to Enhance
Semiconductors Through Nanotechnology

P. E. Russell: russllp@appstate.edu

IBM Milestone Advances Effort to Enhance
Semiconductors Through Nanotechnology


Yorktown Heights, NY, March 24, 2006

--

IBM announced
that its researchers have built the first complete
electronic integrated circuit around a single carbon
nanotube molecule, a new material that shows
promise for providing enhanced performance over
today’s standard silicon semiconductors.






The achievement is significant because the circuit
was built using standard semiconductor processes and
used a single molecule (C nanotube) as the base for
all components in the circuit, rather than linking
together individually
-
constructed components. This
can simplify manufacturing and provide the
consistency needed to more thoroughly test and
adjust the material for use in these applications.

P. E. Russell: russllp@appstate.edu

http://domino.research.ibm.com/comm/pr.nsf/pages/news.200
60324_carbonnanotube.html


IBM Milestone
Advances Effort to
Enhance
Semiconductors
Through
Nanotechnology

SEM image of hair

P. E. Russell: russllp@appstate.edu

When will nanoelectronics be ‘standard’


At least a decade away, by most peoples guess


Sometime between 2015 and 2025?



The nano
-
science

is rapidly evolving



The nano
-
engineering

to make actual production
scale nanoelectronics, with very high yield and
throughput, is not yet in sight
-

it is very difficult to do
the engineering unless we first understand the
Physics!

P. E. Russell: russllp@appstate.edu

Recall the areas of critical need


Energy


Information processing


Clean air and water

P. E. Russell: russllp@appstate.edu

Review of basic semiconductor Physics


When considering the Physics and Engineering of
traditional semiconductors, we are mainly concerned
with ‘bulk’ properties; and consider surfaces as
defects.


Thus approach is the opposite of the nanotechnology
approach, where the goal is to use the special
properties of materials and devices at the nm scale,
where surface atoms control properties.

P. E. Russell: russllp@appstate.edu

Recall the Si

band structure

P. E. Russell: russllp@appstate.edu

Simplified 2 dim Si showing 4 bonds per atom

P. E. Russell: russllp@appstate.edu

Electron excitation across the forbidden gap

P. E. Russell: russllp@appstate.edu

Thermal carrier (electron
-
hole pair) generation

P. E. Russell: russllp@appstate.edu

Concept of hole motion in valence band and annihilation

P. E. Russell: russllp@appstate.edu

Both electrons and holes

can drift in an electric field

P. E. Russell: russllp@appstate.edu

P. E. Russell: russllp@appstate.edu

Mass action law np=n
i
2

Mass action law np=n
i
2
= N
c
N
v
exp(
-
E
gap
/kT)

Where N
c
is the density of states near conduction band edge

N
v
is the density of states near valence band edge



P. E. Russell: russllp@appstate.edu

Adding impurities or dopnats

P. E. Russell: russllp@appstate.edu

Dopant levels for As impurities (valence V, n
-
type))

Each as
provides an
extra loosely
bound electron

P. E. Russell: russllp@appstate.edu

Boron as an example of n type doping

P. E. Russell: russllp@appstate.edu

Band diagram for b doped (p
-
type) Si

Small ionization
energy allows
thermal ionization
at room
temperature

P. E. Russell: russllp@appstate.edu

Band diagram for b doped (p
-
type) Si

Small ionization energy
allows thermal
ionization at room
temperature

Note that the electron
from the valance band
becomes
fixed charge


(in energy and position)

Dopants are far apart

P. E. Russell: russllp@appstate.edu

For an n
-
type
semiconductor (e.g.
Arsenic doped)
only the electrons
are mobile. The
ionized dopant
atoms do not move.

Thus we have
added both
‘mobile” and
‘fixed’ or
‘localized’
charge!

P. E. Russell: russllp@appstate.edu

we have

both ‘mobile’ and
‘fixed’ or ‘localized’
charge!

For the Case of an extrinsic (doped) semiconductor:

this is a very Important Concept!!!

Fixed charge: the ionized dopant atoms

Mobile charge: the extra electron or hole contributed by ionization

P. E. Russell: russllp@appstate.edu

Example of n
-
type material versus temperature

P. E. Russell: russllp@appstate.edu

Temperature dependency of conductivity: critical concept!

From this data we can measure:

Band gap energy E
gap
, and

Dopant ionization energy
D
E
ionization

P. E. Russell: russllp@appstate.edu

Conductivity of a Semiconductor


= conductivity,
e

= electronic charge,

n

= electron concentration in the CB,

e

= electron drift mobility,

p

= hole concentration in the VB,

h

= hole drift mobility



= en

e

+ ep

h

Note: this expression is valid for both intrinsic and extrinsic
semiconductors!


We now see that n≠p for extrinsic semiconductors.

P. E. Russell: russllp@appstate.edu

(The intrinsic
case is shown
here.)

We can determine
n and p (the
electron and hole
concentrations) if
we know g(E) and
f(E) and…