A technique is presented to create porous silicon (PS) thin films with long-term optical stability when exposed to normal room conditions. This technique requires the thin films to undergo a rapid thermal annealing treatment at a relatively low temperature of 600 degrees C in N-2 atmosphere for 6 min. Annealing at such a low temperature enables the surface passivation of PS via nitridation without causing the excessive roughness and sintering widely observed at higher annealing temperatures. A mechanism similar to the Haber process is proposed to explain the surface nitridation, in which the nitrogen atmosphere interacts with the as-anodized hydrogen surface species. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3489075] All rights reserved.