Borkovska L. Electronic and ionic processes in light-emitting semiconductor materials based on II-VI and III-V compounds

The dissertation is aimed at elucidation of the mechanisms of defect rearrangement under electric field, thermal annealing, and irradiation as well as at clarification of the influence of this rearrangement on electron and ion processes in bulk and nanoscale light-emitting semiconductor materials based on II-VI and III-V compounds. Four classes of nanoscale objects that are relevant for application in highly efficient, energy saving and cheap light-emitting devices of visible and near-infrared spectral ranges have been investigated: thin films, heterostructures with quantum wells (QW) and self-organized quantum dots (QDs), colloidal QDs and nanocomposites on their basis. In addition, the single crystals have been studied as the model objects. It is shown that in the single crystals and thin films based on II-VI semiconductors the rearrangement of ionized point defects under internal electric fields in the region of p-n junctions and the fields created by electric charges localized on the surface plays a significant role in the reconstruction of defect system. Using the drift of ionized defects, it is shown that the interstitial zinc, Zni, that is a shallow donor in ZnO, is a part of defect complex responsible for green luminescence band. The model of the Tb3+ luminescence center in ZnO including the substitutional terbium, Tb3+Zn, and interstitial oxygen is proposed. In the nanoscale heterostructures based on II-VI compounds, the negative impact of cation vacancies on QD self-organization is revealed for the first time. It is shown that in CdSe/ZnSe QD heterostructures with a large number of cation vacancies, a wetting layer with an ensemble of QDs with shallow exciton localization potential are formed due to enhancing interdiffusion processes. Similarly, insertion of 1 monolayer of CdTe in the Cd0.4Zn0.6Te/ZnTe QW promotes Cd/Zn interdiffusion and formation of the QW with shallow exciton localization potential that causes an increase in 8 times of the photoluminescence (PL) intensity. The process of thermal generation of cation vacancies in the near surface layer of CdSe/ZnSe QD heterostructures is revealed and the influence of cation vacancies on the degradation of the QD structures under thermal annealing is demonstrated. Higher thermal stability of the QD structures in comparison with the QWs is confirmed and ascribed to effective localization of carriers in the QDs that prevents their diffusion into other parts of heterostructure, in which radiation-stimulated strain relaxation could occur. The mechanisms of thermal quenching of exciton PL in CdSe/ZnSe and InAs/InGaAs/GaAs heterostructures with self-organized QDs were established and the luminescent methods for control of defect distribution in these materials were proposed. An analytical solution of the system of coupled equations in the stationary case for the model of independent capture of charge carriers in the CdSe/ZnSe QD heterostructure is obtained. It is shown that this model satisfactorily describes thermal quenching of quantum dot PL intensity. In the undoped and doped with metal impurities colloidal QDs based on II-VI and I-III-VI compounds, the processes of surface defect reconstruction under light irradiation, interaction with heavy metal ions and conjugation with biomolecules were identified and their mechanisms were established. It is found that the changes in the PL characteristics of CdSe(Te)/ZnS QDs occurred upon drying the QD solution on a solid substrate are caused by QD oxidation resulted in the reduction of QD’s core diameter and rearrangement of defects on its surface. It is shown for the first time that QD conjugation with biomolecules promotes QD oxidation. In the QD composites, the effects of photo- and thermally stimulated enhancement and degradation of the QD luminescence caused by rearrangement of polymer functional groups at the QD/gelatin and QD/polyvinyl alcohol interfaces which affects surface defect passivation were identified. The mechanisms of the effects were proposed. Higher stability of the gelatin-based composites was demonstrated. The method for the increase of the PL intensity of InGaAsN/GaAs QW heterostructures by adding about 1% Sb acting as surfactant is proposed. The possibility of application of colloidal QDs based on I-III-VI compounds for heavy metal ion detection has been demonstrated. A simple method for detection of QD bioconjugate is proposed. The method is based on registration of the changes that occur in the spectral position of the PL band of CdSe(Te)/ZnS QDs due to conjugation with bio-molecules upon drying on a solid substrate.