Positron probing of gamma-irradiated Ge doped with P, As, Sb, and Bi:Changes in atomic structures of defects due to n→p conversion

摘要

The emission of the high-momentum annihilation radiation from the subvalent ion core shells and electron density around a positron localized at a vacancy-group-V-impurity atom complexes produced in oxygen-lean Ge doped with P, As, Sb, and Bi by irradiation with ~(60)Co gamma-rays at room temperature have been investigated with the help of the angular correlation of annihilation radiation (ACAR) before and after n-p conversion. The probability of positron annihilation in the subvalent shells of atoms incorporated in dominant radiation centers was found to be dependent on the ratio of the ion core radii r_i(P~(5+), As~(5+))/r_i(Ge~(4+))<1 and r_i(Sb~(5+), Bi~(5+))/r_i(Ge~(4+))>1, respectively. In passing from P to As impurity atoms the activation energy DEe of electron emission to be detected by DLTS measurements is increased by ～(+0.017 eV) vs. the increase of the electron density parameter to be reconstructed by ACAR data, Δr'_s=r'_s(As)-r'_s(P)≈0.029 a.u. On the contrary, in passing from Sb to Bi impurity atoms, DEe value is decreased by≈(-0.028 eV) whereas the electron density parameter rises by Δr's=r'_s(Bi)-r'_s(Sb)≈0.04 a.u. After n-p conversion a marked decrease in both the electron density and the number of ion cores around the positron points to the fact that the radiation-produced complexes with group-V-impurity atoms (P, As, Sb, Bi) are of a multi-vacancy character. The deep acceptor states in the forbidden gap (E_v+0.1), (E_v+0.12), (E_v+0.16) eV to be attributed to the P-, As-, Sb-,and Bi-containing multi-vacancy centers, respectively, were found to contribute to lessening the electron density around the trapped positron. It is argued that a close similarity of the As~(5+) and Ge~(4+) ion cores results in a small (but marked) augmentation in the electron density around the positron in As-containing multi-vacancy centers after n→p conversion. A trend for inward relaxation of the ion cores is observed in all radiation-produced centers studied.