From: Gina Miller (nanogirl@halcyon.com)
Date: Wed Sep 15 1999 - 14:22:16 MDT
If you do not already have a subscription to SCIENCE you may want to pick up
issue (3.Sept. issue) to read an article on a new formation mechanism of
quantum dots (page 1551). Here is some information regarding this, sent to
me by: Thomas Dekorsy. (Pictures were included in this data, but was not
transferrable)
Gina "Nanogirl" Miller
Atomic sandblasting creates regular array of quantum dots
Thomas Dekorsy and Stefan Facsko
You ever walked along a beach and wondered about the regular patterns of the
wet send? Similar pattern like these regular sand ripples can be found in
completely different systems: the formation of sand dunes in the dessert, on
the water under certain wind conditions, in the erosion formation on hills,
but also on completely different length scales like the regular pattern of a
cauliflower or the growth of ice crystals. Such pattern formation process
are amazing and fascinate mathematicians, physicists and chaos scientists
for the last years. It appears that a very unique description underlies all
these intriguing phenomena. One of the important questions is of course who
can we make use of these self-ordering phenomena.
Scientist from the Technical University of Aachen in cooperation with the
Technical University of Darmstadt now discovered a new pattern formation
mechanism, which allows to produce regular nanometer structures on a
semiconductor surface (S. Facsko et al., Science, 3. September 1999). This
mechanism is be a new pathway for generating nanometer structures for future
electronic devices.
As so often in science, the discovery happened by accidence: A well known
analysis tool for semiconductors is so called sputtering. A semiconductor
surface is bombarded with ions which sputter off atoms from the surface,
these secondary ions can be mass analyzed giving the composition of the
semiconductor. This technique can be compared to sand blasting, where the
atoms take over the function of the sand grains. The longer you sputter the
material will be removed atomic layer by layer giving a depth resolution of
the semiconductors composition. These technique is particular important for
the development of new electronic and optoelectronic devices which consist
of nanometer layers of different material. Such sputtering based depth
analysis was performed by the scientist in Aachen in the group of Prof.
Heinrich Kurz on semiconductor heterostructures provided by the Technical
University of Darmstadt (Prof. H Hartnagel) in order to study their
composition. These samples grown by molecular beam epitaxy (MBE) were
atomically flat before the investigation. All the time the scientists were
confronted with the problem that they did not get a depth resolution in the
analysis. Almost frustrated Stefan Facsko and Thomas Dekorsy started to look
at these samples with techniques which allow the investigation the surface
morphology. The first pictures they got with a scanning tunneling microscope
(STM) and a scanning electron microscope (SEM) set them in unbelief. They
expected the surface to be roughened by the sputtering process. Such
roughening is known to reduce the depth resolution drastically. Instead of
looking onto a rough surface the surface was completely covered with a
regular array of nanostructures with a diameter and distance in the range of
some 10 nm (see large picture on page 1). Immediately further samples were
sputtered and old samples investigated: all exhibited the same regular
patterns. It became clear that a new mechanism for producing quantum dots
has been found.
Quantum dots are believed to be the keystones for the next generation
electronics like single electron transistors or quantum computing devices.
If an electron is fixed within a dot of some nanometer diameter it behaves
according to the laws of quantum mechanism which is very different from the
behavior of a bunch of millions of electrons in a conventional CMOS
transistor.
Following this observation several experiments were performed in order to
understand the formation mechanism of the dots. It is a self-organization
process which occurs during the sputtering. The bombarding ions roughen the
surface but a diffusion process tends to smoothen the surface again. Under
the right conditions the interplay between roughening and smoothing produces
the observed regular pattern. When the ions hit the surface under off-normal
incidence ripples are formed, very similar to the above mentioned sand
ripples. However under normal incidence (90 degrees) as used in the
experiments a regular array of quantum dots is generated.
Further investigations showed that the diameter of the dots can be well
controlled by the sputtering time. Depending on whether 40 seconds or 2
minutes are sputtered the dot diameter increases from 20 nm to 40 nm. While
the first observation were made on the semiconductor Gallium-Antimonide
recent experiments showed that also Indium-Antimonide and Germanium can be
structured this way. These observations suggest that the mechanism is
universal and can be applied to several different semiconductors and other
materials if the right parameters are chosen. Of great technical importance
is the fact that the once formed pattern array can be transferred into
underlying material. This allows the fabrication of structures which give a
confinement of electrons in all three spatial dimensions, so that the
quantum mechanical electron levels can be controlled which is of importance
for making quantum electronic devices.
In comparison to other techniques for the fabrication of quantum dots the
discovered method seems to be the fastest and most cost effective. Within a
minute 100 billion of quantum dots are produced on a square centimeter with
a commercial sputtering system. The area which is structured in one
technological step is only limited by the sputter equipment, which was
presently some square centimeters. However there exist other sputtering
machines which operate on several inch wafers or even on square meters. Such
machines would allow to modify even large surfaces with nanometer structures
which strongly change the optical, mechanical and thermal properties of the
surface.
The other most important technique for producing quantum dots are presently
the direct patterning of semiconductors by lithographic methods. However, in
order to produce structures in the range of 10 nm expensive the
electron-beam lithography has to be employed. Another important approach is
the self-organized growth of quantum dots by MBE machines. Here the number
of materials is limited for which the desired growth modes occurs. The new
„Facsko-Dekorsy“ way discovered opens a third possibility in addition to the
established methods.
Point of contact: Dr. Thomas Dekorsy, Stefan Facsko
Institute for Semiconductor Electronics
RWTH Aachen
Sommerfeldstr. 24
D-52056 Aachen, Germany
Tel. +49-241-807806
FAX +49-241-8888246
e-mail: dekorsy@iht-ii.rwth-aachen.de, facsko@iht-ii.rwth.-aachen.de
Gina "Nanogirl" Miller
Nanotechnology Industries
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http://www.nanoindustries.com
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nanogirl@halcyon.com
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