Optical absorption property of oxidized free-standing porous silicon films

We have systematically studied the evolution of the optical absorption of f reestanding PS films during thermal oxidation at 200 degrees C in air. Our experiment results show the evolution of transmission curve is quite compli cated, which red-shifts first and then blueshifts during thermal oxidation. At the same time, the transmission at the low energy decreases first and t hen increases. We propose an explanation as follows: (1) the energy gap ass ociated with each crystallite should increase during thermal oxidation proc ess, due to the quantum confinement effect; (2) the energy gap should decre ase with an increase in oxygen termination atoms. Both the increasing of th e gap due to the quantum confinement effect and the decreasing of the gap d ue to the Si-O bond formation cause a complicated evolution of optical abso rption.