Excitation energy evolution of photoluminescence spectrum in amorphous hydrogenated silicon

Photoluminescence peak energy at 15 K in amorphous hydrogenated silicon (a-Si:H), prepared at substrate temperatures ( T s = ) 180–250 ∘C, increases with excitation energy in the excitation energy range lower than 1.52 eV in the Urbach tail region whilst for higher excitation energy range the peak energy is constant. The excitation energy at which the peak energy becomes constant is named the hopping-gap. The a-Si:H prepared at T s lower than 180 ∘C has higher hopping-gaps. Furthermore, it is shown that the rate of increase of the peak energy to the excitation energy agrees with that in photoluminescence of porous Si. The photoluminescence in a-Si:H is concluded to be due to the radiative recombination of electron–hole pairs quantum-confined in hydrogen-free Si nanostructures with a radius of 3–5 nm.