Quantum Size Effects in Ge Microcrystals Embedded in SiO₂ Thin Films

By applying an rf co-sputtering technique, we have succeeded in producing Ge microcrystals in SiO 2 thin films for which direct characterization by transmission electron microscopy is possible. As the size of the Ge microcrystals decreased from 6.5 to 2.7 nm, the optical extinction (absorption) spectra changed drastically. The structures corresponding to interband transitions in bulk Ge disappeared, and the onset of extinction shifted to higher energies. These spectral changes seem to arise from the three-dimensional confinement of electrons and holes in the microcrystals.