A COMPARISON OF THE OPTICAL-PROPERTIES OF ULTRATHIN AMORPHOUS AND CRYSTALLINE SILICON FILMS

The optical gaps of ultrathin amorphous and crystalline silicon films consisting of isolated clusters have been measured at 250 degrees C as continuous functions of film thickness by real time spectroscopic ell ipsometry. For c-Si cluster films similar to 1.2 nm thick, a well-defi ned optical gap and absorption onset is identified near 3 eV that blue -shifts and steepens with decreasing thickness. A broader absorption o nset leading to a much lower gap for a-Si:H cluster films of similar t hickness is attributed to an electron mean free path that is less than the cluster size.