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Roles of gas and solid components in the direct nitridation of silicon

The factors influencing the direct nitridation of silicon, including the effects
of the native oxide layer covering the surface of silicon, the effects of hydrogen
contained in the nitridation gas and the catalytic effects of metals added to the raw
material silicon, were investigated, using a tubular flow reactor and a fluidized-bed
reactor operated at temperatures ranging from 1150��C to 1390��C in a stream of
nitrogen containing 10% hydrogen.
The nitridation of silicon is not initiated until the native oxide is removed by
an assistance of hydrogen contained in argon during the pretreatment or in the
nitridation gas mixture. An induction period is observed before the initiation of the
nitridation and depends on the nitridation temperature as well as the pretreatment
time, which is associated with the removal of the oxide layer.
The presence of hydrogen in the nitridation atmosphere is crucial for the
nitridation of silicon. When pretreated silicon grains are exposed to nitrogen without
hydrogen for a time period as short as 5 minutes, the subsequent nitridation, even
with hydrogen, becomes extremely slow. The concentration of hydrogen as low as
0.3% is effective for sustaining the reactivity of silicon for the nitridation. The
results suggest the formation of a protective layer on the surface of silicon when
silicon grains are exposed to nitrogen without hydrogen. The protective film is
suspected to be silicon oxynitride, or a mixture of silicon oxynitride and silicon
dioxide or silicon nitride formed from the reaction of silicon with oxygen and
nitrogen, depending on the temperature of its formation. However, the protective
film does not form on the native oxide layer, and the reactivity of silicon is resumed
upon the removal of the native oxide.
An addition of calcium (as low as 0.125%) or yttrium (1.0-2.0%) to silicon
suppresses the formation of ��-silicon nitride while iron enhances the formation of
silicon nitride. Copper promotes not only the nitridation but also the formation of ��-silicon
nitride at 1200��C, but enhances the ��-phase formation at higher temperatures.
The role of liquid phases on the formation of ��-/��-silicon nitride was also discussed
based on the nitridation of silicon impregnated with copper, calcium, silver,
chromium and tungsten. / Graduation date: 2003
Date12 June 2002
CreatorsPavarajarn, Varong
ContributorsKimura, Shoichi
Source SetsOregon State University
Detected LanguageEnglish

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