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We have studied the evolution of valence-band spectra during Si layer thinning in a silicon-on-insulator (SOI) substrate by X-ray photoelectron spectroscopy (XPS) in order to observe the quantum-confinement effect in two-dimensional Si. It was clearly observed that the valence-band maximum (VBM) shifts towards higher binding energies with decreasing Si thickness (< ∼10 nm). The VBM shifts were ascribed to vertical confinement of heavy holes in the quantum-well structure of a vacuum/single-crystalline Si/buried amorphous SiO2.

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