Sample illumination facility for slow positron beam studies and its application to the photoionization cross-section of the DX center in Al Ga As

We have built a sample illumination facility for slow positron beam studies. It enables the illumination of the sample with monochromatic light in visible and near-IR wavelength range with the maximum photon flux of 1013–1015 sy1 cmy2. Light is guided from the monochromator into the vacuum chamber by an optical fibre bundle and focused onto the sample with a mirror and a lens. We have applied the new illumination facility in studying the photoionization cross-section of the DX center in AlGaAs. The measurements were done using photon energies in the range of 1.0–1.6 eV and the cross-sections were calculated from the behaviour of the positron annihilation parameters as a function of the photon fluence. The resulting photoionization cross-sections were found to coincide well with the results from the photo-capacitance measurements. q1999 Elsevier Science B.V. All rights reserved