Nitridation Kinetics and Thermodynamics of Silicon Powder Compacts

Abstract

The nitridation kinetics of Si powder compacts were studied by measuring the flow rate dependence of N 2 and N 2 –H 2 gas mixtures during slow heating of Si compacts while simultaneously monitoring the oxygen partial pressure of the egress gas. The reaction was found to occur in two distinct stages. In pure nitrogen the initial stage was interpreted in terms of devitrification of the native silica layer, catalyzed by Fe impurities, and the exposure of the underlying Si. The reaction sequence at that point has been unequivocally shown to be image followed by the formation of oxynitride according to image as evidenced by a large increase in the oxygen partial pressure simultaneously with the onset of the initial stage. In the absence of hydrogen, both reactions are rapidly suppressed as the oxygen liberated competes with the nitrogen for the exposed silicon surface and reoxidizes it. However, in the presence of hydrogen, the oxygen liberated reacts with the hydrogen and prevents the reoxidation of the Si. The net reaction in this case is image and/or image with the second reaction being thermodynamically favored. The main nitridation reaction occurring between 1300° and 1400°C appears to be diffusion controlled and depends on the nature of the passivating layer that forms during the initial stage.