Thermal oxidation of porous silicon:: Study on reaction kinetics

Porous silicon (PS) samples obtained by dark etching of p(+)-type silicon wafers are oxidized in dry air, at various temperatures from 200 degreesC up to 800 degreesC for 1-20 h durations, to determine the kinetics of the reaction. The extent of oxidation calculated from mass gains is plotted as a function of oxidation time and temperature. By fitting the general reaction kinetic solutions of different-order reactions, one finds that the function valid for first-order kinetics gives the best matches. From the obtained reaction rates, the pre-exponential factors and activation energies of the oxidation process are determined using Arrhenius plots. It is found that, depending on the temperature range of the oxidation process, either one or two chemical reactions take place. At lower temperatures (200-400 degreesC), a reaction with an activation energy E-a,E-1 approximate to 7 kJ(.)mol(-1) and a pre-exponential factor A(1) approximate to 0.2 h(-1) dominates. When the temperature is increased (400-800 degreesC), a second reaction starts, showing E-a,E-2 approximate to 50 kJ(.)mol(-1) and A(2) approximate to 100 h(-1).