The results of several surface-sensitive techniques applied to the investigation of ion beam-treated a-Si:H/crystalline silicon structures, such as deep-level transient spectroscopy (DLTS), photoluminescence at 6 K and room temperature, and X-ray diffraction at grazing incidence (XRDGI) are presented. Three important results follow from this contribution. (i) Two groups of gap states with thermal activation energies of 0.71 and 0.84 eV were identified and found to be sensitive to illumination, this property exhibiting metastable character; we suppose effects similar to those observed in the porous silicon/silicon and a-Si:H/silicon structures. (ii) Broader luminescence peaks were identified optically with the energies lying in the range of 0.7 to 0.95 eV, the most distinct one being at 0.85 eV. (iii) X-ray reflection at 2 theta similar to 28 degrees has been found as the reflection suitable for tracing the structural properties of a-Si:H layer.