| Summary: | 博士 === 國立交通大學 === 電子物理系所 === 97 === This dissertation is devoted to study the ripening dynamics of the self-assembled II-VI semiconductor quantum dots (QDs). The self-assembled CdSe/ZnSe and ZnTe/ZnSe QDs, which were grown on GaAs (001) substrate by molecular beam epitaxy (MBE), were employed to study various effects on the ripening dynamics. In the CdSe/ZnSe QDs system, the optimum growth temperature was found to be 260 oC. The Stranski–Krastanow (SK) growth mode was confirmed clearly by atomic force microscopy (AFM) images. The thickness of the wetting layer of the CdSe QDs is about 2.5 mono-layers (MLs). As the coverage increases, a complete transfer of the QD growth mode from the Frank van der Merve (FM) mode to the SK mode, followed by the ripened mode, was observed. A schematic diagram of the growth mechanism of self-assembled CdSe QDs was presented.
The effects of ZnSe partial capping and atomic oxygen on the ripening dynamics were investigated in the CdSe QDs system. AFM images show that the ripening of QDs is dramatically accelerated by the deposit of a ZnSe partial capping layer. The driving force of ripening enhancement is attributed to the increasing strain energy with capping thickness. For a ZnSe partial capping layer of below 3 MLs, photoluminescence (PL) exhibits a clear red-shift with increasing ZnSe MLs. It is attributed to the increasing size of the CdSe QDs with the ZnSe partial capping, in a manner that is consistent with the results of the AFM study. On the other hand, the ripening of CdSe QDs can be significantly enhanced by introducing atomic oxygen and was confirmed by a PL study. The atomic oxygens on the surface of ZnSe buffer layer probably change the local chemistry of the ZnSe surface and play the role of the nucleation sites. Therefore, the incorporations of atomic oxygen cause the increasing density of QDs and the decreasing size of CdSe QDs when the CdSe coverage thickness is kept the same.
Moreover, we investigated the effect of rapid thermal annealing (RTA) on ripening and band-bending effect for the type-II ZnTe/ZnSe QDs with 3.0 MLs. The self-assembled ZnTe/ZnSe type-II QDs were grown with the SK mode. The PL spectra of samples that were annealed at temperatures of over 400 oC reveal a strong red-shift in the peak energy. This significant red-shift is understood by the QD ripening, the increasing of the dot size is caused by the migration of atoms from the neighboring smaller QDs, activated by RTA process. In a time-resolved PL study, the fast recombination channel in annealed QDs is suppressed, implying that RTA reduces band-bending effect of ZnTe/ZnSe QDs. Finally, studies of the dependences of excitation power demonstrate that the observed reduction in the band-bending effect is attributed to the increase in the dot size upon the RTA process.
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