Excitation Mechanisms of RE Ions in Semiconductors

Abstract

This chapter presents an overview of the mechanisms responsible for the excitation of optically active rare-earth (RE) ions in semiconductors. Besides resonant excitation of the RE 4f shell, several non-resonant processes can take place in which the host is excited first. These indirect mechanisms involve nonradiative transfer of the recombination energy of electrons and holes to nearby RE ions. Distinct excitation processes arise because of the various conditions under which the electron may recombine with a hole. The different possibilities are presented and discussed in the first part of this chapter. Carriers of opposite charge bind to each other to form either a free exciton or a trapped (bound) exciton. In the latter case, the trapping can arise from the incorporation of RE ions which induces distortions of the host lattice. The exciton trapping can also be due to an impurity, a local defect or even an extended defect. Other possible mechanisms involve the capture of an electron by the 5d shell changing the valence state from trivalent to divalent with the subsequent capture of a hole. Finally, the role of impurities associated with donor-acceptor pairs in the recombination of electrons and holes with energy transfer to RE ions is discussed. In the second part of this chapter the specific case of RE-doped GaN is considered. Results are presented to show that local defects play a major role in the excitation process by binding excitons with a subsequent energy transfer to RE ions. A general modelling of the RE excitation mechanism mediated by bound excitons (BE) is presented and discussed. Finally, experiments using two excitation sources are shown to give valuable information concerning the RE-related defects.