A new beamline for positron annihilation lifetime spectroscopy

Abstract

The major aspect of this research is the development of a variable-energy PALS beamline based on a Surko-type buffer gas positron trap, the first of its kind in the world. A significant amount of work has gone into optimization of the timing and intensity characteristics of positron pulse. Successful implantation of 'Mills ideal function' bunching method has led to positron pulses of 800ps, allowing depth selective studies across a range of defect sizes, from point defects with lifetimes as low as 200ps, to large voids (~10 nm) resulting in lifetimes >50 ns. In addition to experimental development aspects, the materials investigations presented here focus on defects in semiconductor materials produced by ion implantation and focused laser pulses.

The PALS beamline was used to demonstrate the suppression of void formation due to the Si/Si0{u2082} interface in Helium ion implanted silicon-on-insulator (SOl). Vacancy clustering in self-ion-implanted Germanium has also been studied using both the PALS beamline and Rutherford Backscattering Spectroscopy (RBS). The evolution of the ion implantation induced damage during annealing was investigated with results indicating that vacancy cluster occurs in both crystalline and amorphous states. In gallium antimonide, positrons were used to probe void formation in swift heavy ion implantation at various fluences, leading to a better understanding of the transition between isolated channels and heavily porous states as the ion implantation fluence increases. Finally, the positron beamline was used to study the large voids produced by focused femtosecond laser pulses in fused silica, where lifetimes associated with o-Ps annihilation in 0{u2082} filled voids were observed. By combining the PALS measurements with TEM images, the residual 0{u2082} pressure inside the voids after isochronal annealing was estimated and compared with expected values from Monte-Carlo simulations.