Recombination Processes for Deep Impurity States Degenerate with the Conduction Band

Abstract

Lattice relaxation phenomena are of current technological and scientific interest as evidenced by the large number of review papers published recently [1-4]. They lead to such effects as metastability or bistability even in covalent crystals, e.g. EL2 defect in GaAs, DX centers in GaAlAs [4], thermal donors in silicon [3] etc. In this paper we study the influence of lattice relaxation on the autoionization and recombination processes from the excited states of Eu 2 + in Cax Cd1xF2 . The Eu2 + (4f 7) ion forms a relatively shallow donor with 0.33 eV thermal ionization energy in CdF2 and a very deep center (with the energy level located more than 4 eV below the conduction band) in CaF2. Depending on the composition x the 4 f 6 5d1 (e) excited states of Eu2+, extending from 3 eV to 3.7 eV above the ground state, are totally or partly degenerate with the continuum of conduction band (CB) states, or are located within the forbidden gap. The emphasis of this paper is put on the correlation between the recombination mechanism and the position of the excited states in respect to the CB states. A unique property of CdF2 is the extremely large lattice relaxation concomitant with the change of the charge state of a deep center [5]. For Eu 2+ the lattice relaxation energy is estimated to be about 2.4 eV. In consequence, the 4f 7 4 f 6 5d1 intra-ion absorption is superimposed on a very weak photoionization background and well resolved (Fig. 1). The resonant nature of the excited states was proved by a unique photoEPR experiment discussed in a previous paper [6]. The appropriate configuration coordinate diagram is shown in Fig. 2a.