Influence of Hydrogenation of Defects at the Microcrystallite‐Glass Interface on the Nonlinear Optics of CdSe Microcrystallites

Abstract

BYThe nonlinear optical properties of 3D-confined semiconductor microcrystallites (MC) embedded in a glass matrix have been the subject of intense research in the last years. Having small MCs with nearly identical sizes a characteristic MC absorption line can be observed in the linear spectrum up to room temperature. Its halfwidth is essentially influenced by the MC size distribution because the MCenergy states depend sensitively on the MC size. In e.g. /1 to 31 the bleaching of the MC-absorption line at laser excitation was studied. For MC sizes in the strong confinement limit the achievable bleaching value is distinctly smaller than expected for a simple two-level model. Different explanations were taken into consideration : traps in the MC, in the glass, and at the interface / 4 , 5 1 , Auger recombination / 5 to 7 1 , two-photon or free-carrier absorption into continuum states / 5 , 81. All these processes result in a partly deoccupation of the excited MC level. On the other hand, at resonant laser excitation of the MC simultaneously a darkening is observed, too, that means that after laser exposure a smaller bleaching value as well as a faster recovery of the bleaching of the MC line are obtained in comparison with the behaviour found in "fresh" samples /e.g. 8, 2 / . This darkening is irreversible and increases with exposure time. In some cases after long (MC-resonant or UV) laser irradiation a darkened area on the glass is visible, too. The darkening can be made reversible only by heating the sample over 450 O C . From our point of view at resonant MC excitation an energy transfer of excited carriers into the glass matrix occurs which is advanced by the MC-glass interface states and, as a consequence, (i) the observable bleaching of the MC line is smaller than expected and (ii) a darkening effect is obtained related with the properties of the glass matrix. The latter may be due to the charge state changes of network modifier ions or by structural changes in the glass. As e.g. discussed in / 9 / to explain permanent light-induced gratings in Eu-doped silicate glasses the ) Invalidenstr. 110, DDR-1040 Berlin, GDR.

Effect of Hydrogenation on Photodarkening in CdS-Doped Glass

“…This indicates that the number of defects in our sample are few before irradiation and are created by irradiation, since the ESR signal is very weak before irradiation and increases after irradiation. On the other hand, the sample used by Woggon et al 13) may have contained many defects before irradiation, therefore they observed effects of hydrogenation in the sample before irradiation. These results indicate that glass composition plays an important role in the creation and annihilation of defects.…”

“…The effect of hydrogenation was investigated for CdSe-doped borosilicate glass before laser irradiation by Woggon et al 13) They measured the kinetics of induced transmission of the glass. They observed that the time constant of the fast component of the induced transmission for the hydrogenated sample is about twice that as before hydrogenation.…”

Dynamics of optical excitations in quantum dots of wide-gap semiconductors

“…In glasses, experiments are rare which give a direct comparison o f nonlinearoptics before and after a controlled interface modification. Changes o f the optical nonlinearity after controlled surface passivation by hydrogenation have been established for CdSe quantum dots in borosilicate glass matrix [48,49]. Over-barrier transitions o felectrons out ofthe quantum dot into the glass have been proven by measuring the thermo-stimulated luminescence o f semiconductor-doped glasses [50].…”

“…The models presentedto explain the recombination dynamics of II-VI QDs embedded in organics or glasses can be classified in (i) three-or four-level systems without taking into account photodarkening [13,42] and (ii) modified three level systems including photodarkening by either the intensity-dependent activation o f a new trap state [43,44] or two-step absorption to excited states followed by the transfer into the glass matrix [45,48,49]. In addition, Auger processes have been considered in [47,50,52].…”

“…11 -could be achieved only by changing the cross-section~r (increasing induced absorption) and the capture time rcap without any modification in the electron-hole recombination time T0 (for more details see [49]). Experimentscarried out by modifying the interface properties [48] reflected the predicted behaviour.…”