A study of charge-carrier recombination in intrinsic hydrogenated amorphous silicon ͑a-Si:H͒ is presented using pulsed electrically and pulsed optically detected magnetic-resonance spectroscopies in order to measure the influence of spin-dependent recombination on photoluminescence ͑PL͒ and photoconductivity ͑PC͒. The experiments show band tail state recombination influencing the PL but not the PC which constitutes geminate recombination of correlated charge carriers that do not contribute to charge transport. In contrast, nongeminate recombination through silicon dangling bonds is observed influencing both PL and PC. The experiments presented constitute a direct and unambiguous observation of geminate and nongeminate recombination in a-Si:H.

The optical and structural properties of amorphous sputtered films of Ge 2 Sb 2 Te 5 depend strongly on the preparation conditions. Films grown at higher growth rates exhibit greater local strains as indicated by the slope of the optical absorption in the exponential "band-tail" region, but these films also incorporate smaller densities of oxygen impurities. At slower growth rates the band-tail slopes are sharper (smaller local strains) but there is greater oxygen incorporation. We will discuss several experiments that suggest that the local strain relief in the films grown at slower growth rates is due to a greater ability of the atoms to rearrange on the growing surface and not to increased oxygen incorporation. Small angle x-ray scattering experiments show that the films exhibit small elliptical "voids" with long axes perpendicular to the growing surface. The approximate dimensions of these voids are 3 x 20 nm. These films can be switched optically with little change in surface topography as measured by atomic force microscopy. Electron spin resonance measurements indicate that paramagnetic defects exist in some films but are either absent or below the detection limit (~ 10 18 cm -3 ) in most films. The implications of these results for the switching mechanisms will be discussed.