The 2-dimensional angular correlation of positron annihilation radiation method was described, and it was shown that it could be advantageously used to study the electronic structure of defects; in addition to the use of standard positron lifetime and Doppler broadening measurements. By using annihilation fractions which had been determined by means of lifetime measurements, 2-dimensional angular correlation of positron annihilation radiation distributions were separated for negatively charged and neutral As vacancies in n-type material. In the case of electron-irradiated semi-insulating material, 2-dimensional angular correlation of positron annihilation results were presented for the negatively charged Ga vacancy and for positron Rydberg states which were introduced by Ga antisites. The 2-dimensional angular correlation of positron annihilation for delocalized positrons, which was required for the separation process, was obtained from measurements which were performed on as-grown semi-insulating material. As a result, the electronic structures of the vacancies were found to be well-described by recent molecular dynamics calculations. The present method also provided information concerning atomic relaxations around vacancy sites. Those were also in agreement with calculated relaxations.

A.A.Manuel, R.Ambigapathy, P.Hautojärvi, K.Saarinen, C.Corbel: Journal de Physique III, 1995, 5[C1], 73-80