L2,3threshold spectra of doped silicon and silicon compounds

Abstract

Optical absorption in the neighborhood of the Si $L_{2,3}$ edge (∼ 100 eV) has been measured with high resolution in Si crystals with free-carrier concentration ranging from ∼ ${10}^{14}$ to ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$, as well as in amorphous samples of Si, Si${\mathrm{O}}_2$, and ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$. In all crystalline samples a steep steplike threshold is observed, whose shape and height are insensitive to free-carrier concentration across the insulator-metal transition and in the whole range explored in the present investigation. A slightly different edge line shape is observed in amorphous Si films. Sharp threshold spikes are also present in amorphous Si${\mathrm{O}}_2$ and ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$. These effects reflect a more localized final state. The results for crystalline Si are briefly discussed in terms of the similarity of the excess electronic charge distribution around the deep hole throughout the range of free-carrier densities investigated.