(Invited) Positron Annihilation Spectroscopy on Open-Volume Defects in Group IV Semiconductors
In recent years, we have studied the stability of the E-center in SiGe. In these studies it was observed that the E-center becomes thermally more stable when one or more of the Si atoms surrounding the donor-vacancy complex is replaced by a Ge atom. Furthermore, the increase in Ge around the E-center pulls down the second acceptor state of the E-center, observed in pure Ge, from the conduction band. [1,2,3]
In pure Ge, we have identified the divacancy in neutron irradiated Ge and characterized the monovacancy in in situ low temperature proton irradiated Ge. It was observed that the Frenkel pair in Ge anneals at approximately 100 K and the monovacancy at 200 K. Above 200 K, mobile neutral vacancies can pair and form divacancies. [4,5]
In Si, we have used PAS to study how vacancy engineering can be used to control the diffusion of dopants. By co-implantation of He and B we observed that He induced nanovoids trapped B and thus could be used to hinder the broadening of the B profile. 
A new and interesting field of research for PAS is the study of embedded nanoparticles. So far, we have studied Si-nanoparticles embedded in SiO2 and shown that PAS is able to characterize the interface states between the nanoparticles and the surrounding oxide.  Further studies on this subject are planned.
 Evidence of a second acceptor state of the E center in Si1-xGex, K. Kuitunen, F. Tuomisto and J. Slotte, Phys. Rev. B (BR) 76, 233202 (2007).
 Stabilization of Ge-rich defect complexes originating from E centers in Si1−xGex: P, S. Kilpeläinen, K. Kuitunen, F. Tuomisto, J. Slotte, H. H. Radamson, and A. Yu. Kuznetsov, Phys. Rev. B 81, 132103 (2010).
 Evolution of E-centers during annealing of Sb-doped Si0.8Ge0.2, S. Kilpeläinen, F. Tuomisto, J. Slotte, J. Lundsgaard Hansen and A. Nylandsted Larsen, Phys. Rev. B 83, 094115 (2011).
 Divacancy clustering in neutron irradiated and annealed n-type germanium, K. Kuitunen, F. Tuomisto, J. Slotte and I. Capan, Phys. Rev. B 78, 033202 (2008).
 Direct observations of the vacancy and its annealing in germanium, J. Slotte, S. Kilpeläinen, F. Tuomisto, J. Räisänen and A. Nylandsted Larsen, Phys. Rev. B 83, 235212 (2011).
 Vacancy engineering by He induced nanovoids in crystalline Si, S. Kilpeläinen, K. Kuitunen, F. Tuomisto, J. Slotte, E. Bruno, S. Mirabella and F. Priolo, Semiconductor Science and Technology 24, 015005 (2009).
 S. Kilpeläinen, J. Kujala, F. Tuomisto, J. Slotte, Y.-W Lu and A. Nylandsted Larsen, Si nanoparticle interfaces in Si/SiO2 solar cell materials, J. Appl. Phys. 114, 164316 (2013).