Genetic Algorithms for Positron Lifetime Data

Abstract

Recently, genetic algorithms have been applied for ultrafast optical spectrometry in systems with several convoluted lifetimes. We apply these algorithms and compare the results with POSFIT (by Kirkegaard and Eldrup) and LT programme (by Kansy). The analysis was applied to three types of samples: molybdenum monocrystals, Czochralski-grown silicon with oxygen precipitates, Si with under-surface cavities obtained by He + H ion co-implantation. In all three tests, the genetic algorithm performs very well, in particular for short lifetimes. Further developments to model the resolution function in genetic algorithms are needed.

Testing an Ortec Lifetime System

“…The analysis of lifetime spectra was performed with the LT package by J. Kansy [6]. In our previous paper [4] we have tested extensively that package vs. the genetic algorithms [7] and the POSFIT software. The LT package is quite flexible, in particular it allows the use of the trapping model apart from a straightforward multicomponent analysis.…”

“…On the other hand, the spread of existing experimental results and in some cases -like pure metals -their still poor agreement with the theory [2] make new measurements (and analysis packages [3]) still useful. Additionally, time resolution in lifetime beam experiments is hardly better than 250 ps [4,5] so mutual testing of different techniques is needed in order to obtain a kind of normalization, at least for the most simple systems, like pure, defect-free metals or Si.…”

Toward a European Network of Positron Laboratories

“…The fastest techniques show intrinsic resolutions not better than 160-180 ps, see, for example, [9]. In order to extract components of lifetime spectra that are characteristic for a given type of defect, numerical deconvolution procedures should be applied and results are subject to careful validations, see, for example, [10]. Precise measurements allow to distinguish closely positioned lifetime components, such as 221 ps in pure silicon and 250 ps in oxygen (or arsenic) decorated vacancies in Si [11,12].…”