The internal surface of cavities produced by high dose He implantation in Si are known to effectively act as gettering centres. A buried layer of cavities can thus be used to trap detrimental metal impurities before they can interact with voluntarily introduced dopants [1, 2]. In this year we used the perturbed angular correlation technique (PAC) to study the creation and development of such He-induced cavities. In metals it has been found that the PAC probe 111In is trapped in cavities where it occupies substitutional terrace sites in the inner walls of the cavities [3].
In order to investigate the interaction of In with cavities in Si the Bonn isotope separator was used to implant overlapping profiles of He (10 keV) and radioactive 111In (160 keV) into undoped FZ-silicon. To get insight into the cavity formation mechanism samples were prepared with various He-doses (6·1015, 2·1016 and 6·1016 ions/cm2). Further, different implantation and annealing sequences were used. Typically the samples were measured directly after implantation and after different annealing steps (thold = 10 min, T = 500° − 1100°C).
At higher doses (2·1016 and 6·1016 ions/cm16) we find a large fraction of 111In probe atoms subjected to an electric field gradient (EFG) corresponding to a quadrupole interaction frequency (QIF) of νQ = 411(6) MHz with ν=0.25(4). The corresponding defect configuration is formed most effectively after He implantation into well annealed, 111In doped Si. This and the affinity of In to vacancies leads us to the assumption that, similarly to the situation in metals, the Indium atoms act as nucleation centres for vacancy clusters (cavities) and are situated on the inner walls of the cavities. The close similarity of the observed QIF with the QIF reported by Krausch et al. [4] for 111In on external (100) Si surfaces supports this assumption.
The observation that In is trapped in He induced cavities in Si opens various possibilities to study the formation process of cavities as well as their effectiveness as getters for metals, since it can be expected that an incorporation of metals into the cavities would most probably change the QIF experienced by the In probe.
[1] S. M. Myers et al.: J. Appl. Phys. 80 (1996) 3717
[2] V. Raineri et al.: J. Appl. Phys. 78 (1995) 3727
[3] R. Schumacher and R. Vianden: Phys. Rev. B36 (1987) 8259
[4] G. Krausch et al.: Surface Science 285 (1993) 81