EVIDENCE OF AN OFF-CENTER POSITION OF THULIUM IN GAAS AND EXCITONIC EXCITATION OF THE TM(3+) EMISSION

By photoluminescence and by Zeeman spectroscopy we study the characteristic 4f luminescence transition H-3(5) --> H-3(6) at 1.0 eV of thulium in gallium arsenide which has been reported recently. It turns out that optically active Tm3+, which is present in mainly one specific type of center, does not occupy a simple substitutional lattice site. The results show a considerable tetragonal crystal field. The excitation mechanism of the 1.0-eV luminescence is investigated by photoluminescence excitation. The H-3(5) --> H-3(6) is pumped most efficiently by trapping of free excitons.