Early stage donor-vacancy clusters in germanium Jose ´ Coutinho Æ Vitor J. B. Torres Æ Sven O ¨ berg Æ Alexandra Carvalho Æ Colin Janke Æ Robert Jones Æ Patrick R. Briddon Received: 5 September 2006 / Accepted: 26 October 2006 / Published online: 7 December 2006 Ó Springer Science+Business Media, LLC 2006 Abstract There is considerable experimental evi- dence that vacancies in Ge dominate several solid state reactions that range from self-diffusivity to metal and dopant transport. It is therefore vital that we fully understand how vacancies interact with other point defects in Ge. Here we have a look at the properties of small donor-vacancy (Sb n V m with m,n £ 2) complexes in Ge by ab-initio density functional modeling. Partic- ular attention has been payed to binding energies and to the electronic activity of the complexes. We found that all aggregates may contribute to the n fi p type conversion that is typically observed under prolonged MeV irradiation conditions. In general, Sb n V m defects are double acceptors. It is also suggested that sponta- neous formation of Sb 3 V complexes may limit the activation level of donors introduced by ion implanta- tion. 1 Introduction There has been a renewed interest in germanium as a material for microelectronics. This trend is mainly driven by its higher low-field carrier mobilities when compared to Si [1–3]. In the past, utilization of Ge in a CMOS technology has been hampered by the absence of a stable oxide with an acceptable interfacial quality, but with the recent advances in the field of high-j dielectrics Ge presents itself as a serious contender to be applied in sub-45 nm node technologies. In very low scale integration (VLSI) shallow junc- tions, dopants have to be accurately introduced into the material by ion implantation. Full activation of boron implants with [B s ] > 10 20 cm –3 has been successful and reported in the literature [4]. On the other hand, for n dopants, particularly phosphorous, a considerable gap between the activation level (~ 5 · 10 19 cm –3 ) and the equilibrium solid solubility of P in Ge at 500 °C (2 · 10 20 cm –3 ) has been observed. A large fraction of the P atoms remain inactive, perhaps in the form of vacancy- donor clusters, or residing in large circular shaped defects detected by transmission-electron microscopy [4]. Recent deep level transient spectroscopy studies by Kovacc ¸evic ´ et al. [5] in neutron and electron irradiated Sb-doped Ge report the annealing behavior of several radiation-induced levels, in particular divacancy and vacancy-donor related traps [6]. It is then established that SbV(= /–) (E c –0.34 eV) is stable up to about 170 °C, whereas V 2 (E c –0.29 eV) has been detected after anneals of up to 250 °C. Interestingly, a band corresponding to a hole trap with an emission enthalpy around 0.25–0.30 eV (labeled H2) was detected upon anneals of up to 300 °C, and this signal was J. Coutinho (&)  V. J. B. Torres Department of Physics, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal e-mail: coutinho@fis.ua.pt S. O ¨ berg Department of Mathematics, Lulea ˚ University of Technology, Lulea ˚ S-97187, Sweden A. Carvalho  C. Janke  R. Jones School of Physics, University of Exeter, Exeter EX4 4QL, UK P. R. Briddon School of Natural Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK J Mater Sci: Mater Electron (2007) 18:769–773 DOI 10.1007/s10854-006-9069-4 123