Photoleitung Und Lumineszenz In Kristallen Der ZnS-gruppe

Abstract

Synopsis Photoconductivity and luminescence are closely related to the problem of the behavior of electrons and holes in crystals. On the one hand, photoconductivity gives information about the concentration of excited electrons in the conduction band and/or the holes in the valence band. On the other hand, luminescence gives information as to the recombination of electrons and holes at those defects which are referred to as activators (because here, the recombination coefficient of radiating transitions is greater than that of radiationless transitions). At the same time the efficiency of the luminescence gives information about the radiationless transitions at the other lattice defects. An understanding of the phenomena of luminescence may be obtained from a simplified crystal model, which assumes the presence of two kinds of lattice defects — one having levels below the Fermi level (activators) and the other having levels above (traps). The corresponding reaction equations are established and discussed. A sufficiently accurate solution is possible for the stationary state with the aid of the neutrality condition. This establishes the dependence of the radiation efficiency on the activating intensity, the width of the forbidden zone, and the energy levels of the various lattice defects and their concentrations, in agreement with experiment. For a spacecharge-free photoconductor, the same dependence is found for the electron concentration in the conduction band, and consequently for the photocurrent also. (The contribution of the holes to the photoconductivity in sulphides is apparently negligible, on account of their small mobility). The non-linear terms and the saturation phenomena which give rise to the observed non-linearities of the efficiency and the photocurrent are shown to be essential. The buildup can be discussed qualitatively with the aid of the reaction equations, even when more than one type of lattice defect is present. It turns out that the photocurrent and luminescence show a quite different behavior. For the decay, more general relationships are obtained by taking into account the contribution of the holes, especially the possibility of a hyperbolic decay of the luminescence. An estimation of the behavior of crystals activated by corpuscular, X- or gamma rays shows that one cannot necessarily assume that the activation is uniform as is desirable for photoconductivity experiments. Finally the magnitude of the recombination coefficient (capture cross-section) is examined. The limits for the validity of the present model are pointed out.