Optical Properties of Ultrathin Crystalline and Amorphous Silicon Films

The complex dielectric functions (2<hν<4eV) of ultrathin crystalline silicon (c-Si) and amorphous silicon (a-Si:H) films consisting of isolated clusters have been measured at 250 ^∘{}C by real time spectroellipsometry. For c-Si cluster films ~12 thick, a well-defined absorption onset near 3 eV is observed that blueshifts with decreasing thickness, consistent with quantum confinement of electrons. A much broader absorption onset is observed for a-Si:H cluster films of similar thickness and is attributed to an electron mean free path less than the cluster size, which limits the observation of confinement effects.