Transcript Title:

Undergraduate Nanoelectronics Laboratory
at the University at Buffalo and Demonstration
V. Mitin
Electrical Engineering Department
University at Buffalo, Buffalo, NY 14260-1920, USA
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Nanoelectronics: the future of Electronics
As transistor’s size becomes much smaller than micron, Microelectronics becomes
Nanoelectronics (www.itrs.net/Common/2003ITRS/ExecSum2003.pdf )
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Challenges and Solutions
• Challenge: preparation of a new generation of workers
with solid skills in Nanoelectronics and Nanotechnology,
overall
• General approach to solution: acquiring the practical
skills in Nanoelectronics through hands-on experience
• Our specific solution: Interdisciplinary Nanoelectronics
Laboratory for the Engineering/Science Undergraduate
Curriculum
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Scanning tunneling microscopy
This novel technique yields surface topographies in real space and work
function profiles on an atomic scale directly in real space.
We know that the removal of an electron from the conduction band of a solid,
requires a certain amount of energy called the affinity. For a metal or a doped
semiconductor, when the conduction band is partially filled, the energy to
remove an electron is lower and it is called the work function. Let us consider
two conducting solids separated by a space. In terms of classical physics, a
transfer process of an electron from one solid into another can be thought of
as an electron transfer over a vacuum barrier. The process requires additional
energy and because of this it has a small probability. According to quantum
mechanics, a particle can penetrate in classically forbidden spatial region
under a potential barrier. This phenomenon was called tunnelling. Thus,
electron transfer between two solids can occur as a tunnelling process
through (under) the vacuum barrier.
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Different tunneling experiments have been performed, for example, by using
two metal films separated by vacuum or a solid-state insulator (a sandwich
structure). Each of the metal films can be considered as an electrode and
when a voltage bias is applied to these electrodes a so-called tunneling
electric current is produced. This current can give information on electronic
properties, but obviously the information will be averaged over the area of the
metal film surface. By appropriate shaping of one of the electrodes spatial
resolution of far smaller scales than that of sandwich structures can be
achieved. Since vacuum is conceptually a simple tunnel barrier, such
experiments pertain directly to the properties of the electrodes and their bare
surfaces. Clearly, vacuum tunneling offers fascinating and challenging
possibilities to study surface physics and many other related areas.
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The tunnel current, JT, is a sensitive function of the gap between the tip and the surface, s,
i.e.,
JT  VT exp(-Aφ1/2 s)
where φ is the average barrier height; the numerical value of A is equal to unity if φ is
measured in eV and s in Å.
The control unit, applies a DC voltage, Vz, to the
piezodrive, Pz, such that JT remains constant when
piezodevices Px and Py, move the tip over the
surface of the sample. At constant function φ,
Pz
Vz(x,y) yields the topography of the surface, that is
Vz
Px z(x,y), directly, as illustrated at a surface step in
the figure.
δ
Py
V
Control
Unit
T
B
A
C
Δs
s
IT
Fig. 1. The principles of
operation of the Scanning
Tunnelling Microscope.
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Tunnel tips used nowadays are typically made of tungsten or molybdenum
wires with the tips of overall radii of < 1 μm. However, the rough macroscopic
grinding process creates many rather sharp minitips. The tunnel current is
extremely sensitive to the vacuum gap, s; this is why the minitip closest to the
sample defines the whole current through the tip.
Actually, the lateral resolution is given by the width of the tunnel channel, which
is extremely narrow. Additionally, focusing of the tunneling current (in addition
to the geometrical one) occurs due to a local lowering of the tunnel barrier
height at the apex of the tip. At present, the resolution of the scanning tunneling
microscopy reaches 0.05 Å vertically and well below 2 Å laterally.
Scanning tunneling microscopy is subject to some restrictions in application:
only conductive samples can be investigated, and measurements usually have
to be performed in ultra-high vacuum.
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Cross-sectional scanning tunneling microscopy can be performed
at the cleaved edge to study buried structures.
(b)
(a)
(c)
Fig. 2. Cross-sectional scanning tunneling microscopy: (a) STM image of a stack of InAs islands in GaAs;
(b) comparison between a measured and simulated height profile for a similar sample; (c) lattice
parameter in growth direction in an InAs island; experimental data are obtained from cross-sectional
STM, solid line is obtained from a simulation assuming an In content increasing from island base to island
apex. [From J. Stangl, V. Holý, et. al., Structural properties of self-organized semiconductor
nanostructures, Figs. 25 and 26, Reviews of Modern Physics, v. 76, pp. 725-783 (2004).]
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Apart from structural information, low-temperature scanning tunnelling
spectroscopy has been used for wavefunction mapping of single electron states
in nanostructures. Being applied to the InAs dots (islands) the STM methods
directly reveal s-, p-, d-, and even f-type states as made visible by an asymmetry
of the electronic structure, attributed to a shape asymmetry of the islands.
Simulation of the electron ground state and first excited state of an InAs island
corresponds well with the STM image, showing that the wavefunctions in such
islands are indeed atom-like.
(d)
Fig. 3 Cross-sectional scanning tunneling microscopy: (d) the electronic wavefunction measured at two
different tip biases, compared to simulations for the ground and the first excited states.
Two measurements were performed at different voltages at the STM tip: at a low bias of 0.69 V, only s
electrons contribute, and at a larger bias of 0.82 V, both s and p electrons contribute to the STM image.
[From J. Stangl, V. Holý, et. al., Structural properties of self-organized semiconductor nanostructures, Figs.
25 and 26, Reviews of Modern Physics, v. 76, pp. 725-783 (2004).]
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Atomic force microscopy
An atomic force microscope measures the force between the sample surface
and a very fine tip. The force is measured either by the bending of a cantilever
on which the tip is mounted – the contact mode – or by measuring the change
in resonance frequency due to the force – the tapping mode. A typical
resolution is several nanometers laterally and several angstroms vertically.
Fig. 4. AFM in the contact mode.
The size of the tip at the end is
about 30-50 nm.
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The top surface of PbSe/ PbEuTe multilayers is shown. Both materials are
semiconductors. From Figure 5 (a), one can see that PbSe forms triangular
pyramids with [001] side facets.
(a) (b)
PbSe dots
nm
[010]
Fig. 5. PbSe islands with [001]-type facets: (a) the AFM image of the top surface of a
PbSe/PbEuTe island multilayer; (b) AFM image 3×3 μm2 of the top surface of a PbSe/PbEuTe
island multilayer. Islands are arranged in a regular hexagonal array up to the sixth-nearest
neighbor. [From J. Stangl, V. Holý, et. al., Structural properties of self-organized
semiconductor nanostructures, Figs. 25 and 26, Reviews of Modern Physics, v. 76, pp. 725783 (2004).]
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EE342 Lab Course Review
Equipment: STM EasyScan-2
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Nanosurf EasyScan 2 STM
500 nm lateral range
200 nm Z-range, 3 pm Z-resolution
Nanosurf EasyScan2 STM unit
7.6 pm lateral resolution.
Maximum 10 mm diameter sample size.
Tips are simply cut from a Pt/Ir wire without any etching in
hazardous substances
• Current set point 0.1 - 100 nA in 25 pA steps
• Tip voltage ± 10 V in 5mV steps
• Imaging modes: Constant Current (Topography), Constant
Height (Current)
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Equipment: AFM EasyScan-2
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Nanosurf EasyScan 2 AFM
70 micron lateral range
14 micron Z-range
1.1 nm lateral resolution
Nanosurf EasyScan2 AFM unit
0.21 nm Z-resolution
Virtually unlimited sample size.
Sample observation optics Dual lens system (top/side
view)
• Optical magnification Top 12 x / Side 10 x
• View field Top 4 x 4 mm / Side 5 x 3 mm
• Imaging modes Static Force (Contact), Const.Force
(Topography), Const.Height (Deflection)
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Lab 1: Introduction to Scanning Tunneling
Microscopy
• Objectives:
– Introduce student to what STM is, how important STM is
for the understanding and characterizing the nano world
– Student understand basic principles of STM and the
operation of Nanosurf EasyScan2 STM
– Learn how to use the EasyScan 2 software
– Obtain the atomic structure images of Highly Oriented
Pyrolytic Graphite
– Be able to obtain good images at atomic scale of Highly
Oriented Pyrolytic Graphite
– Analyze and present the results obtained
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Lab 1: Introduction to Scanning Tunneling
Microscopy
• Some of the results obtained by our students working with
Highly Oriented Pyrolytic Graphite sample
4 nm scan
8 nm scan
• Sample control question for write-up:
What is the distance between two bright ‘hills’ of the graphite layer?
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Lab 4: Introduction To Atomic Force Microscopy
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Objectives:
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Introduce to the basic principles of AFM and the
operation of Nanosurf EasyScan2 AFM
Understand the basic principles of the two most popular
operation modes, contact mode and non-contact mode
Understand the advantages and disadvantages of each
operation mode and when to use them
Sample control question for write-up:
List up to four possible experiments when one should use
the contact and the non-contact modes
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Lab 5: AFM Images Data Acquisition
• Objectives:
– Reinforce the understanding of
AFM and its operation
– Practice operating the AFM with
contact mode
– Obtain the images of the
semiconductor microstructures
– Analyze the obtained data
Microstructure sample
• Sample control question for write-up:
Describe the microstructure from the images that you obtained
(periodicity, height, width of the structure etc.)
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EE342 Lab Course Summary
• EE342 Undergraduate Nanoelectronics Lab has been
established with three Nanosurf EasyScan STMs and one
AFM due to NSF CCLI Program support
• The lab is targeted for Electrical Engineering sophomores
and juniors
• The goal of the lab is to give students opportunity to see and
analyze the nanoscale structures
• Students have two labs with the STM and two labs with the
AFM at EE Department and 5 additional lab experiments on
quantum phenomena at Physics Department
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Acknowledgements
• NSF, for the support of this project through NSF NUE and CCLI programs
• School of Applied Sciences and Engineering of UB, for providing us with various
kinds of support during this project
• Nanoscience Instruments Inc. for technical support
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