It was recalled that the photo-refractive effect in materials such as the above depended upon the photo-ionization of deep defect levels which were inadvertently present rather than those which were introduced in a controlled manner. By using thermally stimulated conductivity measurements, a preliminary attempt was made to associate specific levels with a particular sillenite member and impurity dopant. It was found that, although many features were universal, significant changes occurred when Ge was substituted for Si and when p-type (Al) and n-type (P) impurities were used as dopants.

D.E.Davies, J.J.Larkin: Applied Physics Letters, 1995, 67[2], 188-90