Mittuniversitetet

The formation energies and relaxed structures of the neutral vacancies in InP are calculated by planewave density functional theory, using supercells of 8, 64, 216 and 512 atoms together with finite size scaling. The effects of electrostatic defect-defect interactions and the truncation of elastic relaxations by the finite supercell are examined. The unrelaxed formation energies are 4.95+/-0.10 and 3.00+/-0.10eV for V-In(+0) and V-P(+0) respectively, and the relaxed formation energies are 4.20+/-0.05 and 2.35+/-0.15 eV. When relaxed, V-In(+0) remains very nearly symmetric with a 40% volume reduction, V-P(+0) on the other hand develops a double dimerized structure, with an similar to12% Jahn-Teller distortion and a 45% volume reduction. It is shown that finite size scaling sometimes reveals that energies are not converged even in the 512 atom supercell, and can thus be important tool in theoretical studies of defects in semiconductors, if sufficient care is taken with strongly Jahn Teller active defects.