Reflection Electron Energy Loss Spectroscopy of Alkali Metals on Silicon

In an extended introduction studies of alkali adsorption on clean silicon surfaces are shortly reviewed and recent achievements are outlined. The main part describes our investigations of lithium and potassium adsorption by reflection electron energy loss spectroscopy (REELS). Beside (electronic) valence electron excitations also core losses of both silicon and these alkali metals were observed in the ionization loss spectroscopy mode (ILS). Additional information has been obtained by Auger electron spectroscopy (AES) and measurements of work function changes (WFC). The spectra of Li and K adsorption on the Si(100) plane show considerable differences which will be discussed in detail. By simulation of the REEL potassium spectra within the density of states (DOS) model using the Si(100)2 × 1 K DOS by R. Ramirez the origin of the 8.4 and 12.5 eV loss peaks can be traced to band structure features. In addition the known K peaks at 18.1, 20 and 22 eV are confirmed and their behaviour with increasing primary energy E$\text{}_{p}$, was investigated. While the K 3p-K 4s transition corresponding to the 18.1 eV loss increases with E$\text{}_{p}$, the dipole-forbidden K 3p-K 4p transition (20 eV) decreases. Shifts in the loss energy of these peaks as well as in the core loss energies at 295 and 296.8 eV are observed and explained by charge transfer.