The ML of II – VIsemiconductors during the movement of charged dislocations takes place in thefollowing steps 13: (i) The plastic deformation causes movement of dislocations. (ii) the electric field of the charged dislocationscauses bending of the valance band and conduction band as well as dislocationbands, (iii) subsequently, the electrons from the electron trap tunnel to the conduction band, (iv) therecombination of electrons with the holes gives rise to the light emissioncharacteristic of the activator centres. In the case of Mn doped II – VI semiconductors,the impact of accelerated electrons in presence of electric field ofdislocations, with the Mn2+ centres may cause the excitation of Mn2+centres and the subsequent de-excitation gives rise to the light emission characteristicof Mn2+ ions.
Alternately, the light produced during theelectron-hole recombination may excite the Mn2 + centres and thesubsequent de-excitation may give rise to the luminescence characteristic of Mn2 + ions. Thecharged dislocation detrapping model is applicable to the ML induced by slowdeformation of ZnS : Mn crystals, in which continuous ML are pulsed ML produced13. This model cannot explain the following characteristics of the ML ofZnS:Mn crystals : (i) For low stresses the ML intensity produced during suddenrelease of applied pressure is equal to that obtained during the application ofpressure, because the dislocation cannot traverse the path that is done duringthe application of pressure, (ii) In ZnS :Mn crystals, for low value of appliedpressure the ML emission of the same intensity takes place during thesuccessive number of applications of pressure, (iii) in the crystals such as Xor ?- irradiated alkali halides the ML is produced by the dislocationdetrapping, the ML intensity becomes negligible after certain number ofapplications of pressure. However, in ZnS:Mn crystals the ML appears for largenumber of pressings where initially the ML intensity decreases with successivenumber of applications of the pressure and for larger number of pressing the MLintensity attains nearly a constant value (iv) whereas ZnS:Mn crystals exhibitML during their elastic deformation, ZnS :Cu, ZnS:Ag, ZnS: Au crystals do notshow ML during their elastic deformation. This results indicate that the changein local structure near the ML ions in ZnS: Mn is responsible for ML andtherefore piezoelectrically induced detrapping model is applicable.
(a) Piezoelectrically-induceddetrapping model of ML(i) The application of pressure produceslocal piezoelectric field in the crystals, whereby the piezoelectric field nearactivator ions may be high due to the change in the local structure. (ii) Thelocal piezoelectric field may reduce the trap-depth of the carriers or it maycause the band bending. (iii) In the case of decrease in trap-depth of the carriers, thermal detrappingof the carriers may be produced. In the case of band bending, the trappedcharge carriersmaytunnel to the respective band.(iv) In the case of Mn doped II – VI semiconductors,the impact of accelerated electrons in presence of piezoelectric field, withthe Mn2+ centres may cause the excitation of Mn2+ centresand the subsequent de-excitation may give rise to the light emissioncharacteristic of Mn2+ ions.
Alternately, the light produced duringthe electron-hole recombination may excite the Mn2+ centres and thesubsequent de-excitation may give rise to the luminescence characteristic of Mn2+ ions. Inthis case, the detrapping rate is proportional to v0 and thepiezoelectric field is also proportional to v0, and therefore, theML intensity is proportional to v02,which is in accordance with the experimental results. Thus, at high pressingrate the piezoelectrically-induced detrapping model of ML is applicable to ZnS:Mn.
Itseems that, for large strain rate the pining time of dislocation becomes verysmall and no significant number of detrappedelectrons are produced by the dislocations. Whereas, in the case ofpiezoelectrically induced detrapping process all the filled electron traps mayinteract with the piezoelectric field and number of detrapped electron mayincrease with increasing value of the applied pressure.