Undergraduate Nanoelectronics Laboratory

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Undergraduate Nanoelectronics Laboratory at the
University at Buffalo and Demonstration V.
Mitin Electrical Engineering Department Universi
ty at Buffalo, Buffalo, NY 14260-1920, USA
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Nanoelectronics the future of Electronics
As transistors 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(-Af1/2 s) where f is
the average barrier height the numerical value
of A is equal to unity if f 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
f, Vz(x,y) yields the topography of the surface,
that is z(x,y), directly, as illustrated at a
surface step in the figure.
Fig. 1. The principles of operation of the
Scanning Tunnelling Microscope.
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• This principal scheme of the tunnelling
  microscopy provides
• stability of a vacuum gap in the sub-Å range
• a lateral resolution in the Å range.
• This requires excellent vibration damping and
  very sharp tunnel tips. The first requirement is
  met by using high-developed and clever mechanical
  means. In fact, it is possible to use bungee
  cords if they are properly placed and have the
  desired elastic properties!

Hidden
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Tunnel tips used nowadays are typically made of
tungsten or molybdenum wires with the tips of
overall radii of lt 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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On the other hand, the tunnel current is
sensitive to material composition and strain.
Atomic resolution in both, lateral and vertical
directions makes STM an ideal tool for the
investigation of growing surfaces and facets at
this scale, which can give insight into growth
mechanisms. STM systems attached to a growth
chamber allow for measurements without breaking
the vacuum after growth. The tunnel current in
STM is sensitive only to a thin layer at a sample
surface, and therefore it might seem that buried
structures are beyond the scope of STM studies.
However, buried structures can be studied by STM.
Indeed, after cleaving samples STM can be
performed at the cleavage edge. Such a
cross-sectional STM can reveal details on the
inner structure of buried nano-objects.
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In Fig. 2.a, the measured and simulated
cross-sectional STM profiles for a stack of InAs
islands on GaAs substrate are shown. It is seen
that the buried InAs islands have the shape of
truncated pyramids. A compositional intermixing
in the islands was found with the GaAs
composition decreasing linearly from 0.4 at the
base to 0 at the top of the islands. The
corresponding lattice parameter distribution in
the growth direction is shown in Fig. 2.c. This
indicates directly an increase of compressive
strain in the GaAs matrix above and below the
islands.
Hidden
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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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For large scan sizes up to 100 100 µm2, the
lateral arrangement can also be obtained. With
AFM, any surface can be investigated almost no
sample preparation is required. A drawback of AFM
is that only structures on a surface can be
investigated. Furthermore, most semiconductor
materials oxidize under ambient conditions, so
that, strictly speaking, the AFM images usually
show the surface of this oxide. When obtaining
quantitative data such as structure lateral sizes
and height, this has to be kept in mind, as well
as the fact that the image is actually a
convolution of the samples surface morphology
with the shape of the microscope tip.
Figure 4 is a schematic view of a contact mode
AFM. Essentially, a micrometer-size cantilever
has an extremely sharp tip attached to it, which
is sharpened to about 30-50 nm at the end. A
low-power probe laser beam is reflected off of
the top of the cantilever, and into a
four-quadrant photodetector, which records the
position of the reflected beam. Note that the
probe beam need not to be perfectly aligned (as
long as some part of the beam is reflected into
the detector, and the surface does not reflect
too heavily into the detector), and need not even
to be smaller than the detector (since the
difference between the quadrant signals allows
the determination of the beam position). The
photodetector measures the position of the
reflected beam, which in turn gives information
about the position of the cantilever and hence
the tip. If the whole apparatus is raster-scanned
across the surface (or the sample is scanned
under the microscope), then an image of the
surface relief can be generated.
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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
101
110
010
nm
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 33 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. 725-783 (2004).
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EE342 Lab Course ReviewEquipment STM EasyScan-2

• Nanosurf EasyScan 2 STM
• 500 nm lateral range
• 200 nm Z-range, 3 pm Z-resolution
• 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)

Nanosurf EasyScan2 STM unit
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Equipment AFM EasyScan-2

• Nanosurf EasyScan 2 AFM
• 70 micron lateral range
• 14 micron Z-range
• 1.1 nm lateral resolution
• 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)

Nanosurf EasyScan2 AFM unit
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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

8 nm scan
4 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

• Objectives
• 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