Quantum Dots Made of Cadmium Selenide (CdSe): Formation and Characterization
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II–VI semiconductors such as ZnSe, ZnS, and CdSe have wider band gap and larger exciton binding energies as compared with III–V semiconductors. Excitons of II–VI semiconductors still exist at room temperature. Therefore II–VI semiconductor devices indicate a possibility of new devices using excitons, such as optical modulators and self-electro-optic effect devices. Furthermore, in the field of optical data transmission, green laser is suitable for short- and medium-range communication purposes using plastic optical fibers with polymethyl methacrylate cores, which have an advantage of lower costs than silica fibers. Therefore II–VI semiconductors are also optimum candidates for light source of optical data transmission through plastic optical fibers.
Semiconductor quantum structures have been intensively investigated since Esaki and Tsu proposed novel artificial superlattice structures. Recently, quantum dots (QDs) have attracted much attention for the optoelectronic device applications and fundamental physics because they provide zero-dimensional structures with δ-function density of states, which dramatically improve performances of optoelectronic devices such as semiconductor lasers. To fabricate lower-dimensional structures, great efforts have been made using various methods, such as selective epitaxial growth, lithography, etching, etc. However, these techniques give film damages, such as defects and contamination. On the other hand, self-organized QDs have an advantage of fabrication of high-density and high-quality QDs. Especially, InAs QDs of III–V materials have been known to be formed on GaAs surfaces, which are followed by the two-dimensional growth of InAs wetting layer, because InAs has a larger lattice constant by 7% than GaAs.
CdSe/ZnSe system is expected to naturally form QDs because of large lattice mismatch of about 7% between ZnSe and CdSe. In this session, we have investigated the formation and optical properties of self-organized CdSe QDs on ZnSe (001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM) measurements. Moreover, applying the QD system to optoelectronical devices to understand carrier dynamics and energy structures of QDs is very important. For example, relaxation mechanism of InAs QDs has been reported by PL excitation measurements. As compared with InAs QDs, CdSe QDs have stronger electron–phonon interactions and much larger band-gap energy. Then, the carrier relaxation mechanisms in self-organized CdSe QDs are more interesting subjects. In this study, we have also investigated optical properties of self-organized CdSe QDs by selectively excited PL measurements.