Search for quantum confinement effects in ultrathin layers of hydrogenated amorphous silicon

The possibility of observing a step-like structure caused by quantum confinement in the differential transmission or absorption spectra of ultrathin (50 Angstrom or less) films of hydrogenated amorphous silicon (a-Si:H) was reinvestigated experimentally and theoretically. This step-like structure was initially reported by Hattori et al. (1988, Appl. Phys. Lett., 53, 2170 and 1988, Phys. Rev. Lett., 60, 825) and later by Chen et al. (1991, Chinese Phys. Lett., 8, 432). Despite an improved signal-to-noise ratio no such structure was detected either by photothermal modulation or by differentiation of the transmission, reflection and absorption spectra of a-Si : H films prepared under various conditions. Computer simulations show that the step-like structure should essentially be unobservable because the presence of tail slates eliminates any sharp features in the joint density of states of electrons and holes. The presence of tail states is proved by photoluminescence and photothermal deflection spectroscopy measurements in ultrathin a-Si:H films. It is argued that quantum confinement of holes should be absent and that of electrons marginal for thicknesses larger than 10-20 Angstrom because of their low mobilities.