Visible photoluminescence from nanocrystallite Ge embedded in a glassy ${\mathrm{SiO}}_2$ matrix: Evidence in support of the quantum-confinement mechanism

Nanocrystallite Ge (nc-Ge) embedded in a glassy ${\mathrm{SiO}}_2$ matrix is fabricated and examined by x-ray photoelectron spectrometry, Raman spectrometry, and high-resolution transmission-electron microscopy. The precipitation and growth of nc-Ge are found to be related to a thermodynamical reduction of ${\mathrm{GeO}}_2$, the diffusion of Si atoms from the Si substrate into the glassy matrix, and an aggregation of small-sized nc-Ge. The size inhomogeneity can be precisely controlled by a double annealing, and the average size can be changed in the range of 2–6 nm. Broadband photoluminescence (PL) spectra are observed in the visible wavelength range at room temperature, and they exhibit pronounced blueshifts of the peak energies and broadening of the PL spectra, which can be correlated to the change in the size. The PL excitation spectra show a Stokes energy smaller than 0.1 eV and dependence on the measurement energy. The visible PL also shows a strong correlation to the presence and actual condition (i.e., size) of nc-Ge. Possible origins of the visible PL such as a quantum-confinement model in quantum dots, the presence of luminescent centers (Ge:E’) in silica glass, or a structural transition of nc-Ge are discussed. The present experimental data are concluded to be more consistent with a quantum-confinement model than with the other models.