Aluminium-related point defects in wurtzite-type ZnS

Infrared absorption (350-520 cm -1 ) due to localised vibrational modes has been studied in heavily Al-doped self-compensated ZnS crystals with the wurtzite structure. Measurements were made in polarised light on crystallographically oriented samples for three different configurations of the vectors k and E of the incident radiation and the c axis of the crystal. Using the polarisation selection rules, the following defects were identified in the absorption spectra obtained: (i) unassociated Al Zn impurities with C 3v symmetry; (ii) Al Zn -V Zn next-nearest-neighbour pairs oriented perpendicularly to the c axis of the crystal, with C 1 symmetry; and (iii) Al Zn -V Zn next-nearest-neighbour pairs oriented at an acute angle to the c axis of the crystal, with C s symmetry. It was found that below 600K most of the Al impurities form pairs with Zn vacancies.