Impurity-Induced Self-Trapping of Holes and Minority-Ion Percolation in TlCl–TlBr Mixed Crystals

Abstract

Electronic state of TlCl–TlBr mixed crystals is investigated optically by focusing a particular attention on a role of bromine ions. In Cl-rich crystals, a Stokes-shifted emission band due to bromine is observed. The dependence of its peak energy on exciting photon energy and the time decay of its peak intensity are explained in terms of hole relaxation to the self-trapped states at bromine clusters. In the bromine concentration range of 0.3–0.4 mole fraction, the emission spectrum changes abruptly and above this concentration, only an edge emission is observed. This shows a transition from the impurity-induced self-trapped hole state to a free hole state and it is discussed in relation to the percolation of bromine ions. The presence of a small but abrupt deviation of lattice constant from Vegard’s law at this concentration is also pointed out.