Investigation of static electron irradiation effects in bulk Si and thin Si films at energies far below threshold

Abstract

This paper investigates the defect production in Si reported to occur at electron irradiation energies far below the theoretically predicted 170‐keV bulk‐damage threshold. In contrast to published results, our experiments show no permanent resistance changes in bulk samples at fluences as high as 2×10¹⁸ 20‐keV e/cm² (78 K). Furthermore, fundamental damage studies using EPR techniques and optical spectroscopy confirm that fewer than 10⁻⁴−10⁻⁵ atomic displacements occur per 20‐keV electron. These results disagree with previous studies of irradiation‐induced resistance changes in bulk samples and epitaxial films from which it was indirectly determined that subthreshold atomic displacements appear following ionization of a Si K‐shell electron. However, we do find large resistance decreases in p‐type Si/spinel films at fluences ≳10¹⁵ 20‐keV e/cm² (300 K) that appear similar to anomalous results in the literature. But the energy and fluence dependences of the resistance change in our experiments are correctly predicted by theory for an irradiation‐induced charge accumulation layer localized within K‐shell hypothesis is clearly ruled out since the resistance effect remains at energies well below the K‐shell ionization potential. We do not find a corresponding irradiation effect in n‐type Si/spinel films which is consistent with the fact that nearly all the previously reported subthreshold irradiation effects have appeared in p‐type Si. Our results clarify some puzzling aspects of previously reported low‐energy electron irradiation effects in Si and also indicate that the unambiguous determination of defect introduction or existing defect modification by subthreshold irradiation of semiconductors requires fundamental techniques that can be rendered immune to surface phenomena.