On the breakdown of Friedel's law in electron backscattering channelling patterns

The theory for electron channelling patterns has been investigated. Calculations based on existing intensity expressions have been found inadequate for explaining recently observed deviations from Friedel's law. This is found to be the case even if the calculations are based on the full non-Hermitian eigenvalue matrix. The theory has hence been reinvestigated and a new intensity expression has been derived which includes the inter Bloch-wave coupling terms and is valid for non-centrosymmetric polyatomic crystals. The expression explains the observed asymmetries for GaSb which make it possible to determine unambiguously the correct non-centrosymmetric point group for this crystal. It is further found that the same effects in GaAs should be very weak in accordance with the non-observed deviation from Friedel's law in this case.