Reactions of Disilane on Cu(111): Direct Observation of Competitive Dissociation, Disproportionate, and Thin Film Growth Processes

We report a detailed study using reflection absorption infrared (RAIR), temperature-programmed reaction (TPR), Auger electron (AES) spectroscopies, and low-energy electron diffraction (LEED) of the interaction and thermolytic reactions of disilane on a Cu(111) surface. Disilane adsorbs dissociatively on Cu(111) at temperatures as low as 90 K. At low coverages Si-Si and Si-H bond scissions yield two adsorbed fragments which are identified as being the SiH fragment and adsorbed H atoms, respectively. Low fluxes of disilane (≤ 5 × 10¹² molecules/s) favor the formation of these dissociative adsorption products. Using higher fluxes, the exposures lead to the concomittant formation of SiH₂ and SiH₃ moieties. The yields of these later species depend very sensitively on both the absolute and relative surface coverages of Si and H. The decomposition processes of adsorbed SiH₃ and SiH₂ are characterized strongly by coverage dependent kinetics. The SiH₃ species is stable over a limited temperature range (T < 150 K); upon heating it undergoes sequential Si-H bond cleavages to form a surface bound monohydnde The dihydride is stable to ∼180 K. The monohydride decomposes at higher temperatures (T > 250 K), leaving behind surface bound Si. The recombinative desorption of dihydrogen occurs at ∼300 K. This bimolecular process competes with another associative reaction which leads to the formation and desorption of silane (T ∼ 230 K) from the surface. The amount of Si deposited on the surface depends sensitively on the surface temperature and the magnitude of the disilane exposure. A high coverage suicide surface phase is readily formed above the dihydrogen desorption temperature. This thin film is characterized by an ordered (√3 × √3)R30° overlaver structure which is thermally stable over a wide range of temperatures. At higher temperatures, where atomic mobilities are higher, the growth of multilayer intermetallic thin films can be effected.