Research Grant Report
Studies on New Light Emitting Materials Using Environmentally Friendly Semiconductor Nanocrystals
2004 Volume 12 Pages 36-41
Details
2004 Volume 12 Pages 36-41
Recently, optical properties of wide band-gap semiconductors have been attracted much attention because they have potential applications for future optoelectronic devises in blue-UV wavelength region. Zinc-oxide (ZnO) is one of the wide band-gap semiconductors, and it is an environmentally friendly material. Furthremore, it has rather large exciton binding energy (60 meV). Due to the large exciton binding energy, the exciton states are stable even at room temperature. Therefore, it is a desirable material for future exciton-related optoelectronic devices. Recently, many researchers have studied on fabrication and optical properties of ZnO nanostructures since optically excited laser action was reported in 1997.
Because the exciton states are stable in ZnO, it provides to study exciton state confined in nanometer space. We have fabricated ZnO nanocrystals dispersed in glass films and studied their optical properties. The samples were fabricated by a sol-gel method. Using this method we can vary nanocrystal density very easily. By optimizing the fabrication conditions, we were able to construct transparent glass films enough to be used in optical measurements. Clear excitonic absorption peaks were observed in absorption spectra at 20 K. The observed absorption peaks were due to the A, B and C excitons in wurtzite ZnO nanocrystals, and C exciton absorption was the strongest due to the largest oscillator strength. Three PL peaks were observed in band-edge region at 20 K. These PL peaks seemed to be due to bound excitons and their phonon replicas. However, the temperature dependence of these PL peaks was different in each other. Furthermore, when we decrease the nanocrystal density in glass films, only one PL peak remained. These experimental results suggest that the remained PL is due to an excited state in independent individual nanocrystals whereas the vanished PL peaks are due to excited states in the aggregated nanocrystals.
