Electron statistics and cluster formation in CdF2 semiconductors with DX-centers

Equilibrium statistical and nonequilibrium photo-induced distributions of electrons over the levels of CdF2:DX were studied as function of the temperature. Optical, conductivity, as well as 113Cd and 19F nuclear spin-lattice relaxation data were taken into consideration. The heights of the tunneling barriers separating deep and shallow states of bistable DX-centers formed in CdF2 by Ga and In dopants, as well as the ionization energy of the deep states were determined for both dopants. CdF2:Ga semiconductors proved to have very high degree of compensation by interstitial F- ions, K>0.996. Most Ga ions are located in expanded ordered structures (clusters), and may form only shallow one-electron states. Features of these structures result in very narrow impurity band (<0.02 eV) at Ga concentrations up to ∼1020 cm-3, which is responsible for the CdF2:Ga 'free electron' conductivity. The remaining less than 1% of all Ga ions are placed into the 'cluster free' regions of the crystal, and form DX-centers where each center can bind either one, or two electrons. In CdF2:In, an increase of In content up to ∼1019 cm-3 and above results in cluster formation. In contrast with Ga, In ions in clusters form only deep, two-electron states. At high In-doping level (>1 mol%), the concentration of DX-centers (located in the 'cluster free' regions of the crystal) is very small