THEORETICAL DESCRIPTIONS OF POROUS SILICON

The current theoretical investigations of silicon crystallites are dis cussed with particular emphasis on porous silicon. First of all variou s calculations of the energy gap are compared with recent experimental data for crystallites. Then the radiative lifetime is determined in t erms of purely electronic transitions. This provides a useful scheme o nly for small crystallites, since for larger clusters, closer to the b ulk situation, phonon-assisted transitions will dominate. The relative importance of these two processes is estimated as well as their cross -over vs. size. The following part is concerned with the effect of def ects: dangling bonds at the surface and donor (or acceptor) impurities . Non-radiative capture by neutral dangling bonds dominates, so that t he presence of one such dangling bond is likely to kill the luminescen ce. Charged dangling bonds lead to radiative capture which can explain the IR luminescence. The effect of donor impurities is shown to be co mpletely different than in the bulk. The last part is devoted first to a full calculation of the excitonic exchange splitting, showing that the commonly used simplified model is only valid for strongly asymmetr ic crystallites. It is then shown that the electron-lattice interactio n produces a non-negligible Stokes shift which adds to the effect of t he exchange splitting. Finally Auger recombination is calculated and s hown to be very efficient, which is in line with experimental evidence . A general discussion is finally given, showing that a consistent pic ture is beginning to emerge.