Vacancies in highly doped silicon studied by positron annihilation spectroscopy

Vacancy defects and their effect on electrical deactivation in highly doped silicon have been studied using positron annihilation spectroscopy. The dominant vacancy-impurity complexes are identified. The results explain the formation of compensating vacancy-impurity complexes by thermal processes during the growth. The migration kinetics leading to stable defects complexes is studied in detail with the help of electron irradiation. These results are further applied to understand the defect formation during molecular beam epitaxy under ionimplantation or low temperature growth. In case of As doping the electron irradiation induced vacancy-arsenic pairs (V-As1) become mobile at 450 K and migrate until stopped by substitutional As to form V-As2 complexes. Subsequently the V-As2 complexes start to diffuse at 700 K and create stable V-As3 complexes by migration. The recovery of V-As3 defects takes place after 1100 K annealing. Similar defects were observed also in P and Sb doped Si. The formation of V-As3 defects at 700 K and their annealing at 1100 K are in perfect agreement with the well-known properties of the electrical compensation in highly doped Si. The V-As3 defect is also the dominant compensating vacancy defect in heavily As-doped MBE grown Si. Larger vacancy complexes, tentatively identified as V2-As5, are also formed at high concentrations. The V-As3 and V2-As5 complexes are removed by annealings at 800 and 900°C, respectively. However, they are likely to reconstruct during the cooling down by subsequent migrations of V, V-As and V-As2. In highly Sb-doped Si grown by molecular beam epitaxy at low temperatures the open volume defects are neighbored by 1 - 2 Sb atoms, and their concentration is large enough to be important for the electrical deactivation. Annealing experiments show that vacancy defects are unstable already at 400 - 500 K and form larger vacancy-Sb complexes, most likely by the migration of V-Sb pairs. The formation of thermal vacancies in highly n-type Si starts already at 700 K and at high temperatures the vacancies are mainly isolated from impurities. Upon cooling down the vacancies are quenched to stable vacancy-impurity complexes such as V-As3 and V-P3, which act as electrically compensating defects.reviewe