Intersubband Transitions in Thin Bismuth Films

Abstract

The reflectivity and transmittivity of thin monocrystalline bismuth films on barium fluoride are measurcd in the spectral range of 80 to 4000 cm-'. The measurements are performed at room temperature and at 5 K. At 5 K, thickness dependent absorption maxima occur if the film thickness is less than 50 nm. These maxima are interpreted as transitions between subbands of valence and conduction band at the L-points of the Brillouin zone. The spectral position of the maxima is explained assuming an elevated gap energy due to lattice mismatch between films and substrates. Conclusions on the boundary condition of the electron wave function are supported by an analysis of the plasma frequencies of the films.