The is done during the application of pressure, (ii)

            The ML of II – VI
semiconductors during the movement of charged dislocations takes place in the
following steps 13: (i) The plastic deformation causes movement of dislocations.  (ii) the electric field of the charged dislocations
causes bending of the valance band and conduction band as well as dislocation
bands, (iii) subsequently, the electrons from the electron trap  tunnel to the conduction band, (iv) the
recombination of electrons with the holes gives rise to the light emission
characteristic of the activator centres. In the case of Mn doped II – VI semiconductors,
the impact of accelerated electrons in presence of electric field of
dislocations, with the Mn2+ centres may cause the excitation of Mn2+
centres and the subsequent de-excitation gives rise to the light emission characteristic
of Mn2+ ions. Alternately, the light produced during the
electron-hole recombination may excite the Mn2 + centres and the
subsequent de-excitation may give rise to the luminescence characteristic of Mn
2 + ions.

The
charged dislocation detrapping model is applicable to the ML induced by slow
deformation of ZnS : Mn crystals, in which continuous ML are pulsed ML produced
13. This model cannot explain the following characteristics of the ML of
ZnS:Mn crystals : (i) For low stresses the ML intensity produced during sudden
release of applied pressure is equal to that obtained during the application of
pressure, because the dislocation cannot traverse the path that is done during
the application of pressure, (ii) In ZnS :Mn crystals, for low value of applied
pressure the ML emission of the same intensity takes place during the
successive number of applications of pressure, (iii) in the crystals such as X
or ?- irradiated alkali halides the ML is produced by the dislocation
detrapping, the ML intensity becomes negligible after certain number of
applications of pressure. However, in ZnS:Mn crystals the ML appears for large
number of pressings where initially the ML intensity decreases with successive
number of applications of the pressure and for larger number of pressing the ML
intensity attains nearly a constant value (iv) whereas ZnS:Mn crystals exhibit
ML during their elastic deformation, ZnS :Cu, ZnS:Ag, ZnS: Au crystals do not
show ML during their elastic deformation. This results indicate that the change
in local structure near the ML ions in ZnS: Mn is responsible for ML and
therefore piezoelectrically induced detrapping model is applicable. 

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(a)  
Piezoelectrically-induced
detrapping model of ML

(i) The application of pressure produces
local piezoelectric field in the crystals, whereby the piezoelectric field near
activator ions may be high due to the change in the local structure.

(ii) The
local piezoelectric field may reduce the trap-depth of the carriers or it may
cause the band bending.

(iii) In the case of decrease in trap-depth of the carriers, thermal detrapping
of the carriers may be produced. In the case of band bending, the trapped
charge carriers
may
tunnel to the respective  band.

(iv) In the case of Mn doped II – VI semiconductors,
the impact of accelerated electrons in presence of piezoelectric field, with
the Mn2+ centres may cause the excitation of Mn2+ centres
and the subsequent de-excitation may give rise to the light emission
characteristic of Mn2+ ions. Alternately, the light produced during
the electron-hole recombination may excite the Mn2+ centres and the
subsequent de-excitation may give rise to the luminescence characteristic of Mn2
+ ions.

In
this case, the detrapping rate is proportional to v0 and the
piezoelectric field is also proportional to v0, and therefore, the
ML intensity is  proportional to v02,
which is in accordance with the experimental results. Thus, at high pressing
rate the piezoelectrically-induced detrapping model of ML is applicable to  ZnS:Mn.    

It
seems that, for large strain rate the pining time of dislocation becomes very
small and no significant number of  detrapped
electrons are produced by the dislocations. Whereas, in the case of
piezoelectrically induced detrapping process all the filled electron traps may
interact with the piezoelectric field and number of detrapped electron may
increase with increasing value of the applied pressure.