Basic Research for the Fabrication of Light Emitting Devices Utilizing Quantum Confined Exciton Molecules

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The objective of this project is to pursue basic research on radiative and nonradiative recombination processes of excitons and exciton molecules in widegap semiconductors. This was facilitated by making detailed characterization on the correlation between microscopic structures and macroscopic optical properties in both ZnSe-based II-Vl semiconductors and GaN-based III-V semiconductors by means of employing time-resolved photoluminescence (TRPL) spectroscopy. The results obtained in the period of 2000 to 2001 can be summarized as follows.(1) Time-space resolved luminescence apparatus was developed by combining optical microscope with ultraviolet optics, pico-second pulsed lasers and streak camera. PL dynamics in self-formed CdSe quantum dots (QDs) grown on cleaved ZnSe (110) substrates was characterized using this system. Large fluctuation of both wavelengths and lifetimes was observed in PL spectra. It can be understood as size of potential confinement differs with QDs, so that trans … More ition energy as well as oscillator strength of excitons is modulated accordingly.(2) Dynamical behavior of optical gain formation has been assessed at room temperature (RT) in the InGaN multi quantum well (MQW) lased Baser diodes (LDs) by employing pump ＆ probe (P&P) spectroscopy under the pulse width of 150 fs. The LDs are composed of (a) In_<0.1>Ga_<0.9>N-In_<0.02>Ga_<0.98>N MQW and (b) In_<0.3>Ga_<0.7>N-In_<0.05>Ga_<0.95>N MQW, whose stimulated emissions correspond to near ultraviolet (390 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to LQL by 500 meV for the sample (b). It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to LQL, and then relaxed to the localized tail within the time-scale of about 5 ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of LQL under higher photo-excitation. Less