Energy band gaps and excited states in Si QD/SiO _x /R _yO _z (R  =  Si, Al, Zr) suboxide superlattices

X-ray and optical spectroscopies were applied in order to study the band structure and electronic excitations of the SiO _x /R _yO _z (R  =  Si, Al, Zr) suboxide superlattices. The complementary x-ray emission and absorption measurements allow for the band gap values for the SiO _x layers to be established, which are found to have almost no dependency on the cation type R. It is determined that, after annealing, the stoichiometric factor x remains near 1.8 in all the systems under study, implying that the silicon quantum dot synthesis reaction is not fully completed. It is shown that the SiO _x /Al₂O₃ multilayer contains octahedral structural motifs (SiO₆) usually found in stishovite, whereas SiO _x /SiO₂ and SiO _x /ZrO₂ demonstrate an electronic structure similar to conventional silica. The intrinsic electronic excited states are examined by means of synchrotron-excited photoluminescence spectroscopy. Low-energy UV-excited luminescence of SiO _x layers is found to have the same spectrum in all of the studied structures, while VUV-excited spectra strongly depend on the cation R. In these measurements, manifestations of 'slow' exciton-mediated and 'fast' defect-related luminescence are distinguished using nanosecond time resolution. It is shown that both mobile and bounded excitons appear in the suboxide layer under 6.2 eV and 5.8 eV irradiation and then relax radiatively through the triplet-singlet transition of the neighbouring oxygen-deficient centers. The complete picture of the optical excitation and relaxation processes in these materials is illustrated in a general diagram depicting electronic states.