INTERACTION OF SILICON CLUSTER IONS WITH AMMONIA - ANNEALING, EQUILIBRIA, HIGH-TEMPERATURE KINETICS, AND SATURATION STUDIES

The results of extensive studies of the chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) with ammonia are described. At room temperature all clusters react at close to the collision rate and collisional annealing of the clusters does not influence their reactivity. At temperatures slightly above room temperature (approximately 400 K) it is possible to establish an equilibrium. Binding energies of ammonia to the silicon clusters of approximately 1 eV were determined from measurements of the equilibrium constants as a function of temperature. These small binding energies indicate that molecular adsorption occurs at close to room temperature. Saturation experiments reveal that ammonia only binds molecularly to a small number of sites on the clusters. In contrast, on bulk silicon surfaces at room temperature, rapid dissociative chemisorption occurs until all the surface dangling bonds are saturated. At temperatures above approximately 470 K another process, probably dissociative chemisorption, becomes important. Absolute rate constants were measured for clusters with 30-70 atoms at a temperature of 700 K where the dissociative chemisorption process dominates. The sticking probabilities at this temperature are between 10(-3) and 10(-5), two to four orders of magnitude smaller than on bulk silicon at 700 K.