Role of hydrogen atoms in anodized porous silicon

Hydrogen atoms chemisorbed in anodized porous silicon (PS) during anodization in a HF solution have been investigated by using both experimental techniques and a semi-empirical calculation method. The results show important roles of chemisorbed H atoms in PS on anodization mechanisms and a slight expansion of Si-Si bond length, as in the case of the structural change and low-temperature oxidation process of PS films previously reported. Fine structures observed in the infrared absorption band of the Si-H stretching vibrations can be related to charge redistributions of H-chemisorbed Si atoms which were calculated for various clusters with Si_nH_m using the AM1 method. The calculation results on the Si-Si bond length in the clusters are also consistently explained in relation to slight increases in the lattice constant of PS: the origin comes from Si charges attracted by chemisorbed H atoms on the pore walls.