Quantum Dot article

Gina Miller (nanogirl@halcyon.com)
Wed, 15 Sep 1999 13:22:16 -0700

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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