Electronic Structure of Solids Measured by X-Ray Emission and Photoluminescence Under XUV Excitation

The excitation spectra of porous silicon samples in the photon energy range of 99-111 eV have been measured. The visible light efficiency peaks around the Si L _{2,3} edge and dips at 106 and 108 eV, which indicates that the oxides of Si are not the major luminescent material and that Si particles are. A model using measured absorption coefficients of Si and its oxides to describe the spectra has been developed. The results show that there is typically a non-luminescent Si overlayer on the surface of our porous silicon samples. The Si L_{2,3} edge of the luminescent particles in the bulk porous silicon is shifted towards higher energy by 0.20 eV compared with the bulk value. The data analysis indicates that at most only one third of the electron-hole pairs created in a Si particle diffuse away from it, and that only Si particles within a narrow range of sizes are luminescent even though a wide range of particle sizes is present in the sample. Observed radiation damage effects was observed, especially for samples being exposed to soft X rays for the first time. The change in excitation spectral shape is believed due to damage in terms of physically reasonable changes in the values of the parameters in our model. The X-ray satellites on the high energy sides of the K emission edges of Li and Be and the L2,3 emission edges of Na, Mg, and Al have been measured. The satellites have been identified as the radiative decay of double core ionization states. Fitting the emission edges and the satellites with a model, both the phonon broadening in the main band and the satellites are evaluated for the first time. A considerable difference in the phonon broadenings between the single core decay and the double core decay was observed beyond experimental and other physical variables' uncertainties. The difference in the broadenings is interpreted as a direct evidence of non-linear phonon coupling.