Construction of wide gap semiconductor nano-Quantum Dots

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The ultimate goal of this research is to establish the fundamental materials technology for the short-wavelength optoelectronics. To this end, we have set the following research objectives : (1) development of widegap II-VI compound semiconductors which have bandgaps ranging from the visible to uv wavelength region ; (2) development of self-organizing growth processes for those semiconductors to fabricate qunantum dots with a nano-meter size. The materials system to be investigated includes a ZnO-based materials system, ZnSe/ZnSe-based heterostructure system, and ZnCdSe/ZnSe-based heterostructure system. We have selected those material systems because of the following reasons : (1) They can cover the wavelength region from the visible to uv range ; (2) The exciton binding energies of ZnO and ZnSe/ZnSe quantum wells have large binding energies of 60 meV and 40 meV, respectively, which are much larger than the thermal energy at room temperature thereby making excitons survive at room tem … More perature or even at higher temperatures ; (3) We have been extensively working on blue-green light emitting devices of ZnCdSe/ZnSe heterostructures. I would like to mention that we have already established MBE techniques to grow those II-VI compounds, which is quite essential to lead the research project to a success.The achievements obtained in this research project can be summarized as follows : (1) We have established the molecular beam epitaxy technique for the growth high-quality widegap II-VI compound semiconductors. In addition to the already established MBE growth techniques for ZnSe, ZnSe, and ZnCdSe, we have developed (a) the oxygen-plasma assisted MBE growth technique for the growth of high-quality ZnO-based materials, (b) the fabrication techniques for heterostructures of ZnSe/ZnS, ZnCdSe/ZnSe, and ZnMgO/ZnO, and (c) the technique to control the lattice polarity of ZnO layers. Those techniques are crucial to the fabrication of well-controlled nano-scale semiconductor quantum dots. (2) We have established the self-organized fabrication processes for ZnO quantum pyramids, ZnSe/ZnS nano-scale quantum dots, and ZnCdSe/ZnSe nano-scale quantum dots. In particluar, the fabrication of ZnO quantum pyramids are the first achievements in oxide semiconductors. (3) We have discovered various novel optical properties unique to those material systems and nano-scale quantum dots. In particular, (a) The effects of localization in ZnCdSe/ZnSe quantum dots on optical properties have been elucidated, which can be utilized to enhance emisison probabilities thereby leading to the fabrication to high-bright light emitting devices. (b) The realization of induced emission from ZnO and ZnO/ZnMgO quantum structures based on excitonic mechanisms, which may open up a excitonic optical devices. (c) The first observation of spectral diffusion in ZnCdSe/ZnSe quantum dots, which may be utilized to fabricate novel optical memory devices. We hope that those achievements will contribute to the establishment of short-wavelength optoelectronics. Less