The E center is probably the most studied point defect in elemental semiconductors. It consists of a vacancy and a group V donor impurity. It is a well known fact in Si that the E center has an acceptor level at approximately EC-0.45 eV [1], recently also a donor level has been found [2]. In Ge two acceptor levels have been reported [3]. Silicon-germanium (SiGe) has gained a lot of interest in recent years thanks to its promising properties in semiconductor technology. Also the E center has been the focus of numerous studies in SiGe. Kringhøj et al. has reported that distance of the E center acceptor level to the conduction band edge is independent of the Ge content in SiGe [4].
We have studied proton irradiated and annealed n-type SiGe with a P dopant concentration in the 1×1018 cm-3 range by utilizing the positron annihilation technique (PAS) which is sensitive to vacancy defects. The Ge content in the samples was 10, 20 and 30%. The results show that by annealing the irradiated samples at temperatures 275-325 oC, the Ge content around the E center increases. Furthermore, a step like behavior in the vacancy parameter S is observed when the PAS Doppler broadening technique is used as a function of temperature. This indicates that a charge transition takes place in the upper half of the band gap. According to previous studies, the acceptor level of the E center should be below mid gap in 30 % SiGe and thus this charge transition should be undetectable with PAS in n-type samples. Hence, we infer that an increase in Ge content around the E center pulls down the localized second acceptor state, found in Ge, into the SiGe band gap.
[1] G. D. Watkins and J.W. Corbett, Phys. Rev. 134, A1359 (1964).
[2] A. Nylandsted Larsen et al., Phys. Rev. Lett. 97, 106402 (2006).
[3] J. Fage-Pedersen, A. Nylandsted Larsen, and A. Mesli, Phys. Rev. B 62, 10116 (2000).
[4] P. Kringhøj and A. Nylandsted Larsen, Phys. Rev. B 52, 16333 (1995)
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