The optical transition in porous Si: The effects of quantum confinement, surface states and hydrogen passivation

We present a theoretical study of two infinite wires of Si with a different lateral size. The analysis is based on the linear muffin tin orbitals method in the atomic sphere approximation (LMTO-ASA). We consider free, partially and totally H-covered[001] Si quantum wires with rectangular cross-section. The results of this investigation prove the quantum wire nature of porous Si and interpret many of its physical features. In particular we show that a) as expected quantum confinement originates the opening of the LDA gap; b) the gap opening effect is asymmetric: 1/3 of the widening is in the valence band, while 2/3 in the conduction band; c) the near band gap states originate from Si atoms located at the center of the wire; d) the confinement is enhanced in the case of free surfaces; e) the imaginary part of the dielectric function shows a low-energy side structure strongly anisotropic, identified as responsible of the luminescence transition; f) the presence of dangling bonds destroys the luminescence properties; g) in spite of feature c), all Si atoms ape collectively involved in the luminescence transition; h) the shift detected by the Si L(2,3) VV Anger signal is due to H-interaction effect and is not a measure of the quantum confinement effect; i) the Si atoms probed by the Si L(2,3) VV Auger are bonded with H and H-2.