Laplace deep-level transient spectroscopy studies of the divacancy in alpha-particle irradiated silicon

Abstract

Defects in semiconductors are usually detrimental to the device operation. Particularly in space, the high levels of radiation induce defects that damage electronics in satellites and space craft. However, in some devices defects are crucial to device operation and they are purposely introduced into the semiconductor during manufacture. In both cases it is important to be able to characterize these defects in order to find ways to remove defects that are detrimental and to introduce or keep those defects that are useful.

During this research, the deep-level transient spectroscopy (DLTS) technique was performed on Schottky diodes fabricated on n-type silicon, which were irradiated by alpha particles from an Am241 source. In contrast to gamma and electron irradiation, which practically induce only point defects, alpha particles produce some defect clusters as well, especially in the region just before coming to rest. In particular, the two charged states of the divacancy were investigated.

These investigations included the determination of the DLTS signature (the ionization enthalpy and apparent capture cross-section) of the observed defects. The depth profile and introduction rate of the defects were also determined. This was then compared to previously done research on electron irradiated silicon to determine if any other unknown defects arose from alpha-particle irradiation.

The conventional deep-level transient spectroscopy spectrum showed three discrete peaks at 90 K, 125 K and 225 K, when recorded at a rate window of 80 s-1. By comparison with literature, it was determined that the peak at 90 K was due to both the CiCs defect and the VOi defect, while the peak at 125 K was due to V2 (=/-) defect level and the peak at 225 K was due to the PV defect and the V2 (-/0) defect level.

The annealing profile of both charge states of the peaks due to the divacancy showed annealing in the range 350 K to 400 K, which was not observed in electron-irradiated diodes. We suggest that this is due to defect clusters annealing out, releasing interstitials that combine with the divacancies thereby converting them to highly mobile vacancies. It was also observed that, in the region 550 K to 620 K, where the divacancy anneals, the two peaks annealed by different amounts. This is not the case in electron-irradiated material. We therefore suggest that the V2 (=/-) charge transition level is suppressed by cluster effects.