Ellipsometric and Rutherford backscattering characterization of low-energy hydrogen-, helium-, neon-, and argon-bombarded silicon

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Abstract
Ellipsometry and Rutherford backscattering/channeling were used to study silicon surfaces modified by low-energy hydrogen, helium, neon, and argon ion bombardment. Ellipsometry was found to be a very sensitive technique for characterization of these samples, able in some cases to simultaneously distinguish the ion identity, incident energy, and total dose experienced by the material. A comparison of the ellipsometric results for the different cases indicates qualitative differences in the physical modifications induced in the silicon surface region under bombardment by different ions. Comparison of the ellipsometric results with Rutherford backscattering/channeling data yielded estimates of the complex refractive index of the silicon layer modified by hydrogen and helium bombardment, showing a general decrease in the refractive index and an increase in the optical extinction coefficient upon bombardment. It was possible to explain these changes, at least qualitatively, by using the Bruggeman effective medium approximation to estimate the volume composition of this layer subject to certain simplifying assumptions. One result of this was the development of the exact solution for the three-component Bruggeman analysis; these equations are derived in the Appendix.