Synthesis and characterization of silicon clathrates of type I Na8Si46 and type II NaxSi136 by thermal decomposition

Type I (Na8Si46) or type II (Nax?24Si136) silicon clathrates films with a large 15 × 45 mm2 surface have been synthesized from p-type and intrinsic c-Si (001) wafers using two-step process without the need of any glove box. Conditions to selectively obtain either phase finely tuned. Optical absorption coefficients are found much larger in Si than diamond visible light range. provide direct bandgap around 1.9 eV which is supporting high coefficient observed. Photovoltaic response has confirmed Surface Photovoltage. As prepared show many defects, cracks inhomogeneities drastically reduced thanks pressure annealing treatment.