Scanning tunneling spectroscopic studies revealed the quantum-confinement effects in Ge nanocrystals formed with ultrahigh density (>1012cm−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 ∼1.4eV with decreasing size. This size dependence can be explained by the quantum-confinement effect in Ge nanocrystals.

A scheme of photoabsorption spectroscopy (PAS) based on scanning tunneling microscopy (STM) applicable to semiconductors has been devised to circumvent experimental difficulties inherent in STM-PAS measurements so far conducted in the conventional lock-in detection scheme. The present scheme uses multiplexed lights from an interferometer for spectroscopic lights and the modulated current flowing through the STM tip for the signal from which the photoabsorption spectrum is deduced by Fourier transformation. The technique under an optimized condition has been applied to Si samples deposited with epitaxially grown Ge1−xSnx nanodots, which proved the spatial resolution of a few nanometers.