Surface chemistry of PH 3, PF 3 and PCl 3 on Ru(0001)

Abstract

The adsorption, desorption and decomposition of PH 3, PF 3 and PCl 3 on Ru(0001) have been studied by soft X-ray photoelectron spectroscopy (SXPS) using synchrotron radiation. Due to large chemical shifts in the P 2p core levels, different phosphorus containing surface species can be identified. We find that PF 3 adsorbs molecularly on Ru(0001) at 80 and 300 K. At 80 K, PH 3 saturates the surface with one layer of atomic hydrogen, elemental phosphorus, subhydride (i.e., PH x (0 < x < 3)) and PH 3, with a total phosphorus coverage of 0.4 ML. At 300 K, PH 3 decomposes into atomic hydrogen and elemental phosphorus with a phosphorus coverage of 0.8 ML. At 80 K, PCl 3 adsorbs dissociatively into atomic chlorine, elemental phosphorus, PCl and possibly PCl 2 and PCl 3 in the first monolayer. Formation of multilayers of PCl 3 is observed at 80 K. At 300 K, PCl 3 adsorbs dissociatively as atomic chlorine and elemental phosphorus with a saturation phosphorus coverage of 0.1 ML. The variation in total phosphorus uptake at 300 K from PX 3 ( X = H, F and Cl) adsorption is a result of competition between site blocking by dissociation fragments and displacement reactions. Annealing surfaces with adsorbed phosphorus to 1000 K results in formation of Ru z P ( z = 1 or 2), which is manifested by the chemical shifts in the P2p core level, as well as the P LVV Auger transition. The recombination of adsorbed phosphorus and adsorbed X ( = H, F and Cl) from decomposition is also observed, but is a minor reaction channel on the surface. Thermochemical data are used to analyze the different stabilities of PX 3 at 300 K, namely, PF 3 adsorbs molecularly and PH 3 and PCl 3 dissociate completely. First, we compare the heat of molecular adsorption and the heat of dissociative adsorption of PX 3 on Ru(0001), using an enthalpy approach, and find results consistent with experimental observations. Second, we compare the total bond energy difference between molecular adsorption and complete dissociation of PX 3 on Ru(0001). In particular, we apply Shustorovich's bond-order conservation-Morse potential (BOC-MP) method to estimate the heat of adsorption for PH 3 and PCl 3 and the bond energies of the relaxed P-X bonds of the adsorbed PX 3 on the surface. The bond strength difference among the relaxed P-X bonds (i.e., the relaxed P-F bond ( 475 kJ mol ) is much stronger than either the relaxed P-H bond ( 287 kJ mol ) or the relaxed P-Cl bond ( 288 kJ mol )) suggests that PF 3 is more stable than PH 3 and PCl 3 on Ru(0001) at 300 K. These values are used to evaluate the total bond energy differences between molecular adsorption and complete dissociation for each of the PX 3, and the results agree with the experimental trends.