Positron annihilation spectroscopy (PAS)

Positron annihilation spectroscopy (PAS)

Head:
Dr. sc.nat. Gerhard Brauer

  • Co-worker:
    Dipl.-Phys. Wolfgang Anwand

 Goal:

  • Investigation of vacancy-type defects in solids and surfaces

Research topics:

 

  • investigations of vacancy-type defects caused by ion implantation
  • investigations of precipitates.
  • investigations of radiation-induced damage.
  • investigations of thin films and interfaces.
  • methodical developments.

 

Results


Equipment:

  • conventional positron lifetime and Doppler broadening spectroscopy
    system 1 :combined conventional positron lifetime and Doppler broadening measuring system,sample temperatures ranging from 10 K to room temperature.system 2 :single lifetime system, sample at room temperature.

  • mono-energetic positron beam for Slow Positron Implantation Spectroscopy (SPIS)
    magnetically guided mono-energetic positron beam,beam spot diameter 4 mm, high vacuum conditions,
    accelerator voltage from 30 V to 36 kV, measurement of the Doppler broadening of the positron
    annihilation line in dependence on the incident positron energy using a Ge detector,
    resolution of the Ge detector (1.09 + 0.01) keV at 511 keV.

mono-energetic positron beam apparatus of the PAS laboratory at Rossendorf

Method

Slow Positron Implantation Spectroscopy (SPIS):

Positrons of predetermined energies E (30 eV – 36 keV) are implanted at depths of up to a few micrometer in the sample. The motion of positron-electron pairs prior to annihilation causes a Doppler broadening of the photopeak in the measured energy spectrum of the annihilation photons characterized  by the lineshape parameter S. S is higher for positrons trapped at and annihilated in open-volume defects. 
The W parameter (“wing” or core annihilation parameter) is taken in high-momentum region far from the center. The parameters S an W are calculated as the normalized area of the curve in a fixed energy interval - see sketch below.

 

Sketch of the annihilation line changes


Definition of lineshape parameters S and W
 

Experimental setup
 

1 positron source      2 Helmholtz coils     3 solenoids    4 accelerator    5 iris aperture 
6 beam movement coils    7 valve     8 Ge-detector     9 sample chamber 

Data

The figure shows an S parameter vs. positron energy E plot of a 6H-SiC sample implanted with Ge+ ions. Based on the S(E)-data, a numerical solution of the positron diffusion equation is possible using the software package VEPFIT [1]. Thereby a Makhovian profile for the distribution of the thermalized positrons in the sample is assumed.

Evaluation

The fit is based on the assumption of a certain number of box-shaped depth profiles of vacancy-type defects created by the Ge+ implantation. The width of a box represents the depth and the height of a box is connected with size and concentration of a given defect.

[1] A.van Veen et al., in Positron Beams for Solids and Surfaces, edited by P.J.Schulz et al.,
     AIP Conf. Proc. No. 218 (AIP New York 1990), p. 171