VISIBLE AND INFRARED PHOTOLUMINESCENCE FROM DEPOSITED GERMANIUM-OXIDE CLUSTERS AND FROM Ge NANOCRYSTALS

摘要

Following Canham's report of visible photoluminescence (PL) from porous silicon, the optical and electronic properties of nano-structures made from silicon (Si) or germanium (Ge) have attracted much attention, because they open a new possibility for photonic applications by the use of group-IV elements. In particular, PL properties of Si nanocrystals (nc-Si) have been widely studied and the relationship between the size of nc-Si and the PL peak energy has been revealed experimentally for at least red and near-infrared (NIR) PL.~(2-4) According to these reports, nc-Si with about 4 nm in diameter exhibits a PL peak at about 1.4 eV. As the size decreases further, the PL peak shifts to higher energies and reaches the visible region for nc-Si smaller than 2 nm. In contrast to nc-Si, there have been few reports on the size dependence of the PL spectra for Ge nanocrystals (nc-Ge). The nc-Ge has been prepared by several methods and these samples exhibit strong visible PL at about 2.2 eV independent of the size of nc-Ge (2-15 nm) and the preparation methods.~(5-11) The purpose of this work is to experimentally reveal the size dependence of the PL spectra for clusters in gas phase and nanocrystals in matrices. We will report on: (1) the infrared HOMO-LUMO gap of Ge clusters having 1 nm diameter,~(12) (2) the optical properties of deposited germanium-oxide (Ge-O) prepared from vapor depositions of small Ge-O clusters,~(13) and (3) the PL of nc-Ge dispersed in SiO_2 films.~(14) The photoelectron spectroscopy (PES) of cluster anions in gas phase~(12,15) has been proven to be a powerful technique to study the electronic and geometric structures of atomic clusters in gas phase as a function of size,~(16,17) due to its ability to combine size selectivity with quantitative spectral sensitivity. Since photoelectron spectra deliver information about the final states, the anion PES should reveal the electronic properties of the neutral clusters. Together with our developed method of halogen atom (F, Cl) doping, the HOMO-LUMO gap of the neutral Ge_n and Ge_nO_n clusters has been measured. From these gas phase experiments, it has been found that the HOMO-LUMO gaps of Ge_nO_n are 2.9-1.7 eV for n=2-5. And the corresponding visible PL from the deposited Ge-O species on the substrate was observed. Furthermore, the PL properties of nc-Ge in SiO2, prepared by cosputtering of Ge and SiO_2 and post annealing, were studied, in which the PL spectra strongly depending on the size were observed in the NIR region.