Unique Laser Ion Source Produces Semiconductors
WARSAW, Poland, Oct. 21, 2011 — A unique laser ion source has been built that is equipped with a special system for accelerating ions to a chosen energy and for eliminating admixtures.
This device has been used to produce samples of a new generation of semiconductors: a layer of silica in which germanium nanocrystals have been formed.
For ion implantation — that is, “hammering” ions into the surface layer of the material — conventional ion accelerators commonly are used. Laser ion sources, though simpler, cheaper and more universal, emit wide-energy ions usually accompanied by some admixtures.
The work was carried out at Institute of Plasma Physics and Laser Microfusion (IPPL).
Laser ion sources are devices that produce ions from the interaction of a focused laser beam with a target placed in a vacuum vessel. Admixtures that happen to be in the target often cause problems — together with the proper ions, they can modify the sample. Moreover, the laser pulse pulls out atoms and debris from the target, depositing them on the irradiated sample and modifying its surface.
Jerzy Wolowski from IPPLM near the new laser ion source. (Photos: IPPLM)
“To prevent such effects, we have designed and built a device for ion implantation with a unique electric system for ion acceleration,” said Marcin Rosinski, a researcher at IPPLM.
Ion implementation is the process of embedding ions into the surface layer of the sample to change some mechanical or electrical properties of the material. Currently, ion accelerators are routinely used for this purpose.
Laser ion sources have a chance to improve those devices: They are smaller and simpler and can produce ions from materials with a high melting point such as tantalum or tungsten. The ion beam can easily be modified by changing the parameters and geometry of the laser-target–sample system. The released ions can be accelerated in the external electric field.
However, to use the laser ion sources in industry, some requirements must be fulfilled: The beam of ions cannot possess impurities, and the ions should have almost the same specific energy. To meet both criteria, a laser ion source with a special electrostatic system must be applied.
In the device built at IPPLM, the low-energy laser pulse lasts 3.5 ns. The laser pulse energy, in the first phase of laser-matter interaction, is transferred to free electrons, which subsequently ionize atoms of target material and admixtures. The main part of the laser pulse energy directly heats ionized matter (plasma), causing its quick expansion. A broad energy distribution of the ions expanding from plasma results from the nature of the process of plasma generation.
Simulation of the trajectory of ions in a laser ion source.
Some particles and debris extracted from the target by the laser pulse are electrically neutral, which is why they expand without deflection in the electric field and go straight onto the screen that is placed exactly on the axis of the system to shield the sample. At the same time, laser-produced ions that avoid the screen are accelerated and focused by the electric field onto the sample located on the axis behind the screen.
“We have selected the parameters of the field in such a way that only the chosen ions of the target reach the sample. The spot of the focused beam is 1 mm in diameter,” explained Rosinski.
The low-energy laser used in the experiment does not heat itself and is capable of generating 10,000 or more laser pulses within some 10 minutes. Those advantages make it possible for scientists to control precisely the number of ions that reach the sample.
The solution proposed by the IPPLM scientists has been used to explore the process of germanium ion implantation in a silica layer with a view to fabricating germanium nanocrystals within it. Thus, a modified semiconductor has been created whose prospective implementation in electronics is widely anticipated, for example in miniaturization of some memory chips or in elements for light emission.
To obtain germanium nanocrystals from the laser-produced ions, the implanted sample should be heated from 600 to 1200 °C. This process creates some germanium crystals ranging in size from a few nanometers to 20 nm.
“Our implanted samples, after heating, are examined with the use of various sophisticated, currently available measuring methods in the specialized laboratories, mainly at the universities in Messina and Catania in Sicily. We have analyzed both the results of ion implantation and the formation of nanocrystal structures in the samples,” said Rosinski.
The laser ion source built and tested at IPPLM is a prototype device expected to find applications in industry.
“In two years, we will have finished the work connected with optimization of our device regarding industrial usage, but we have already started looking for enterprises that are interested in implementing this technology,” said researcher Jerzy Wolowski.
For more information, visit: www.ifpilm.pl/ifpilm.pl/en
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