Characteristics of Fracto-Stimulated Luminescence in II-VI Compounds

摘要

The II-VI compounds are piezoelectric and moreover, they also possess charged dislocations. Thus, there are possibilities that the Mechanoluminescence(ML) in II-VI compounds may be caused by the piezoelectrification as well as by the movement of charged dislocations. The electric field produced due to the piezoelectrification of the newly created surfaces is sufficient to cause dielectric breakdown in the crystals or phosphors. In this electric field, tunneling of trapped electrons to conduction band may take place whose recombination with the luminescence center may give rise to the luminescence. The accelerated electrons in the conduction band may also stimulate luminescence center during their collisions with the luminescence centers. According to the charged dislocation model, the dislocation motion stimulates the electron trapped centers in the vicinity of dislocation core, at distance of the order of r_(int). The electrons tunnel into the conduction band, where they stay until they recombine with the luminescence center. Later on, the recombination of electrons with luminescence center give rise to the light emission. In such a mechanism we can also expect radiative recombination near the dislocation i.e. in the strong electric field produce by the dislocations. It has been found that the electric field due to the charged dislocations is one order higher as compared to the strength of piezoelectric field. Thus, it seems that the major contributions to the ML in II-VI compounds is from the moving charged dislocations. The expressions for I, I_m, t_m and I_T are derived The ML intensity should be optimum for a particular value of time. The experimental observations related to the dependence of ML intensity on deformation time are an accord with the theoretical prediction. The dependence of the peak of ML intensity I_m on the impact velocity v_o. The plot of graph between log (I_m/v_o) versus (1/v_o) is a straight line with a negative slope. This fact supports the theoretical prediction. The plot of log(I_T) versus (1/v_o) is a straight line with a negative slop. The dependence of the time t_m corresponding to the peak of the ML intensity versus time curve of the crystals on impact velocity of the piston. The graph plotted between t_m and log(1/v_o) is a straight line with positive slop. Such result is obtained experimentally.