Si31tracer studies of the oxidation of Si, CoSi2, and PtSi

Abstract

Radioactive $^{31}\mathrm{Si}$ (${T}_{\frac{1}{2}}=2.62$ hours) was used to study the mechanism of atmospheric steam oxidation of silicon, Co${\mathrm{Si}}_{2}$, and PtSi. Measurement of the radioactivity profile in the Si${\mathrm{O}}_{2}$ layer formed on Si shows that oxide growth takes place at the Si-Si${\mathrm{O}}_{2}$ interface by oxygen diffusion through the growing Si${\mathrm{O}}_{2}$ layer. The sharpness of the measured activity profile also indicates that Si is very immobile in Si${\mathrm{O}}_{2}$ at the oxidation temperature of 1000\ifmmode^\circ\else\textdegree\fi{}C. The oxidation studies on Co${\mathrm{Si}}_{2}$ and PtSi also show that Si${\mathrm{O}}_{2}$ formation takes place at the silicide-Si${\mathrm{O}}_{2}$ interface and that oxygen is the diffusing species through the Si${\mathrm{O}}_{2}$ layer. By marking these silicides with $^{31}\mathrm{Si}$ and measuring the radioactivity left in the silicide layer as a function of Si${\mathrm{O}}_{2}$ thickness, it was shown that silicon diffuses through the silicide by a substitutional (vacancy) diffusion mechanism. Nonradioactive silicon supplied from the underlying silicon substrate mixes completely with the radioactive silicon in the silicide layer, owing to the high silicon self-diffusion coefficients in both Co${\mathrm{Si}}_{2}$ and PtSi at the oxidation temperatures used.