Observation of the quantum-confinement effect in individual Ge nanocrystals on oxidized Si substrates using scanning tunneling spectroscopy

Scanning tunneling spectroscopic studies revealed the quantum-confinement effects in Ge nanocrystals formed with ultrahigh density (> 10¹² cm^-2) by Ge deposition on ultrathin Si oxide films. With decreasing crystal size, the conduction band maximum upshifted and the valence band minimum downshifted. The energy shift in both cases was about 0.7 eV with the size change from 7 to 2 nm. This shows that the energy band gaps of Ge nanocrystals increased to ∼1.4 eV with decreasing size. This size dependence can be explained by the quantum-confinement effect in Ge nanocrystals.