Scattering in an Amorphous Layer Measured by Dechanneling

Channeling experiments by proton backscattering have been made along the 〈111〉 axis of silicon crystals with aluminum and gold layers to investigate the depth dependence of the dechanneled fraction on crystal temperature (80-300 °K), beam energy (0.6-1.5 MeV), and film thickness (100-4000 Å). The dechanneled fractions $\chi \mathrm{}\left(z\right)\mathrm{}$ have been compared with calculated values obtained by two different procedures. (i) The changes with depth in the transverse energy of a channeled particle due to crystal scattering have been described by the steady-increase approximation and a maximum allowed transverse energy, related to the experimental critical angle, was assumed for a channeled particle. The adopted angular distribution of beam particles just beneath the crystal surface accounted for both the scattering in the amorphous surface layer and through the crystal surface. (ii) Alternatively, $\chi \mathrm{}\left(z\right)\mathrm{}$ has been obtained by convolution of the angular distribution of particles scattered only in the amorphous surface layer with the experimental channeling angular-yield profile measured in an uncovered crystal. Both procedures give good agreement with experiments, thus supporting the steady-increase approximation for the crystal scattering and Meyer's treatment for the amorphous scattering. As a prelude to measurements in heavily damaged crystals, these approaches have been reversed, so that the distribution of scattered particles and then the thickness of the amorphous layer have been obtained by dechanneled-fraction measurements. The extension of the method to the case of small defect concentration in a nearly perfect crystal is briefly discussed.