THE FORMATION OF SELF-ASSEMBLED CdSe QUANTUM DOTS

Semiconductor quantum dot structures, with atomic-like discrete energy states, are expected to improvement novel application in optical and electronic device, due to the three dimensional confinement on the carriers or excitons. Compared to lithographic methods, the Stranski-Krastanow mode (S-K mode) is a promising method, which doesn need complicated fabrication techniques and which achieves high optical quality due to low interface defects. However, the formation mechanism of self-assembled quantum dots (SAQD) is not very clear until now especially for lI-VT quantum dots such as CdSe/ZnSe. Hence, it is very difficult to control the growth of SAQD. Although there have some researches on how to control the size, density of SAQD, the results are not satisfied. Moreover, the formation mechanism of SAQD has attracted our interest not only for fabricating top- quality quantum dots to novel devices but also from the point of view of basic science. To study the formation mechanism of SAQD, the best way is to observe the formation process directly. Atomic force microscopy (AFM) is an effective instrumentation to get direct information of the surface morphology. However, AFM can be used as in-situ instrumentation. Hence, AFM usually be used to characterize the morphology or the ripening process of quantum dots. In this thesis, we observed the formation process of CdSe SAQD and studied the formation mechanism of CdSe SAQD systematically by AFM. The following is the major results: 1. High quality Self-assembled CdSe quantum dots were fabricated by low- pressure metalorganic chemical vapor deposition (LP-MOCVD) system. The formation process of SAQD was observed by AFM for the first time. The formation of CdSe SAQD under critical thickness was due to the effect of surface diffusion and strain release. The slow process of surface diffusion can compensate for the time that is needed for AFM. These results make it possible to directly observe the process of release strain and to obtain the actual information on the formation process of self- assemb led quantum dots by AFM. 2. The formation process of CdSe SAQD was investigated systematically by AFM. It revealed that the relaxation of misfit strain is completed by two competing processes. One is the formation of quantum dots assisted by surface diffusion; another is the formation of misfit dislocations. The surface roughness can allow easy to nucleation of misfit dislocations. This competitive mechanism can explain many discussing issues existed in the formation of CdSe SAQD. 3. The optical properties of CdSe SAQD and CdSe ultrathin layer were studied. The formation of SAQD was verified by the research of optical properties. The interface diffusion can release part of misfit strain and go against the formation of SAQD.