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Pulsed positron beam for atomic and nanoscale characterization of thin films

Floris Reurings ,  Antti Laakso ,  Antti Pelli ,  Klaus Rytsölä ,  Sami Kivistö ,  Filip Tuomisto ,  Pekka Hautojärvi 

Helsinki University of Technology, Laboratory of Physics (FYSLAB/TKK), POB 1100, Otakaari 1 M, Espoo 02015-TKK, Finland


Positron annihilation spectroscopy is an experimental method to probe the local electron density and atomic structure at the site chosen by the electrostatic interaction of the positron. Information of the structure can be obtained from the time and energy spectra of the positron annihilation radiation. The positron lifetime is a measure of the electron density, whereas the Doppler broadening of the annihilation radiation reflects the electron momentum distribution. With these methods it is thus possible to investigate local structures embedded in the bulk of the material, such as missing atoms, clustering of atoms, superlattices, quantum dots, as well as free volumes and void sizes in polymers and biological materials. These imperfections often determine the crucial properties of new materials, such as mechanical properties, electrical conductivity, diffusivity or light emission. Hence combining positron annihilation with electrical and optical measurements leads to quantitative studies of electrical compensation, light absorption and emission in new and functional materials.

For studying thin layers, slow monoenergetic positrons are needed. These are produced with slow-positron beams, in which positrons emitted e.g. in β+-decay are moderated and accelerated before being implanted into the specimen. Positron beams suitable for Doppler broadening studies are technically simple and therefore very common. However, knowledge of the moment-of-birth of the positron is lost in the moderation process. In order to perform lifetime measurements, this problem must be overcome for example by pulsing the beam.

We have constructed a pulsed positron beam for lifetime measurements in thin layers. We present the details of the system and demonstrate its performance. The positron implantation energy can be varied between 3–30 keV corresponding to a mean implantation depth of 0–2 μm. For temperature-dependence studies, the sample temperature can be controlled in the range 20–800 K.


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Related papers

Presentation: Poster at E-MRS Fall Meeting 2007, Symposium J, by Floris Reurings
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-14 11:51
Revised:   2009-06-07 00:44