Donor impurities and DX centers in the ionic semiconductor CdF2: Influenceof covalency

Analysis of the electronic structure of MF86- complexes (M = Al, Ga, In, Sc , Y) in the predominantly ionic Semiconductor CdF2 shows a significant incr ease of covalency of the chemical bond for Al, Ga, and In impurity centers as compared with a regular crystal, i.e., with a CdF86- complex, and an inc rease of the ionicity of the bond for Sc and Y. As a result, an essential n oncenterosymmetrical distortion of the center occurs possible for the forme r centers, when capturing an extra electron by the center. Two additional p rerequisites for the formation of such a (DX) state of the center are the p resence of both d-d interactions of the impurity ion and cations and of fil led atomic-type shells at the single-ionized impurity ion. The former condi tion is not realized for Al. For Sc and Y both breaking of the latter condi tion and increase of ionicity of the bond at doping inhibit formation of DX centers; these ions in CdF2 produce only the shallow donor levels.