----------------------------------------------------------------- SEMINAR OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS Institute for Computational Sciences and Informatics CSI 898-Sec 001 (http://www.csi.gmu.edu/lcdm/seminar/schedule.html) ----------------------------------------------------------------- Floating bonds and gap states in a-Si and a-Si:H from first principle calculations. Marco Fornari George Mason University and Complex Systems Theory Branch, Naval Research Laboratory By means of ab-initio calculations we study the electronic properties of pure and hydrogenated amorphous silicon, starting from some selected configurations previously generated by other authors using molecular dynamics. The atomic-projected density of states and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of five-fold coordinated ($T_5$) defects in originating gap states through their nearest neighbors. Moreover, we show that the effect of hydrogen in the reduction of the gap density of states can be explained in terms of annihilation of $T_5$ defects, and not necessarily only with the well known mechanism of saturation of dangling bonds. Therefore, all the experimental evidencies, which in some cases seem partially favoring one type of coordination defect in contrast to the other, can be reconciled and fully explained in a unique picture including both of them. Monday , November 9 1998 4:30 pm Room 206, Science & Tech. I -----------------------------------------------------------------