Positron probing of disordered regions in neutron-irradiated silicon

The vacancy-rich disordered regions (DR) playing a key role in improving the thermoelectric figure-of-merit of silicon thermoelectric generators by reducing (by ∼90%) the thermal conductivity, have been probed with positrons. The DR were created by irradiating n-Cz-Si(P) material with the fast reactor neutrons. The parameter of the electron density r_s′ ≈ 2.18 a.u. contacting positrons in DR has been reconstructed using the data of the angular correlation of the annihilation radiation (ACAR); the amendments to the r_s′ value associated with the ion core electrons were taken into account. It is argued that the ion cores of atoms of silicon as well as the ones of the as-grown impurities (O, C) are involved in the open vacancy volume to be probed with positrons: a relaxation of the ion cores directed inward toward the vacancy volume seems to take place. These positron traps are formed beyond the vacancy-rich area of the disordered region. In the course of isochronal annealing, the traps are stable up to T_anneal. ≈ 520 °C when a recovery of ACAR parameters begins and then it continues up to ∼1050 °C. A vacancy center containing positrons in the open vacancy volume beyond the vacancy-rich area of the disordered region is shown schematically. The emission of electron–positron annihilation gamma-quanta dominates from within these positron traps. The values of both the electron–positron ion core radius (r_m) and electron density (parameter r_s′) suggest a relaxation directed toward inwards the open vacancy volume: the impurity atoms of oxygen and carbon are, perhaps, involved in the center.