H.-S. Tao

STRUCTURE OF ULTRATHIN SIO2/SI(111) INTERFACES STUDIED BY PHOTOELECTRON SPECTROSCOPY

Device-grade ultrathin (9–22 A) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4+ (with respect to Si0) are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I1, I2, and I3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ∼2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N2 plasma. We find that annealing produces more abrup...

FACETING INDUCED BY ULTRATHIN METAL FILMS ON W(111) AND Mo(111): STRUCTURE, REACTIVITY, AND ELECTRONIC PROPERTIES

We have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger spectroscopy, LEED, thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films ≥ 1 monolayer thick of certain metals, i.e. they form faceted structures. For example, using a UHV STM to study Pd/W(111), we find that the Pd-covered W(111) surface becomes completely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage θc. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, oxygen or sulfur. In contrast, monolayer films of Ti, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or faceting on W(111) and Mo(111) surfaces. The faceting appears to be thermodynamically driven but kinetically limited: faceting is caused by an increased anisotropy in surface free energy that occurs for the film-co...

Growth, interfacial alloying, and oxidation of ultra-thin Al films on Ru(0001)

The growth and oxidation of ultra-thin aluminum films on Ru(0001) have been studied by low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) using both Mg K α and synchrotron soft X-ray radiation. For Al films of average thickness ∼ 15 A deposited at 300 K, LEIS demonstrates that the Ru substrate is completely covered. Upon annealing to ∼ 1000 K LEIS shows the reappearance of Ru at the surface. At the same time, the metallic Al 2p peak shifts to lower binding energy and a low binding energy shoulder appears on the Ru 3d peak, suggesting AlRu interfacial alloying. Annealing Al films to ∼ 1000 K in 1 × 10−4 Torr oxygen produces an oxidized surface layer that completely covers the Ru substrate; the resultant aluminum oxide films are stoichiometric.

Decomposition of P(CH3)3 on Ru(0001): comparison with PH3 and PCl3

Abstract The decomposition of P(CH3)3 adsorbed on Ru(0001) at 80 K is studied by soft X-ray photoelectron spectroscopy using synchrotron radiation. Using the chemical shifts in the P 2p core levels, we are able to identify various phosphorus-containing surface reaction products and follow their reactions on Ru(0001). It is found that P(CH3)3 undergoes a step-wise demethylation on Ru(0001), P(CH3)3 → P(CH3)2 → P(CH3) → P, which is complete around ∼450 K. These results are compared with the decomposition of isostructural PH3 and PCl3 on Ru(0001). The decomposition of PH3 involves a stable intermediate, labeled as PHx, and follows a reaction of: PH3 → PHx → P, which is complete around ∼190 K. The conversion of chemisorbed phosphorus to ruthenium phosphide is observed and is complete around ∼700 K on Ru(0001). PCl3 also follows a step-wise decomposition reaction, PCl3 → PCl2 → PCl → P, which is complete around ∼300 K. The energetics of the adsorption and the step-wise decomposition reactions of PH3, PCl3 and P(CH3)3 are estimated using the bond order conservation Morse potential (BOCMP) method. The energetics calculated using the BOCMP method agree qualitatively with the experimental data.

Faceting of W(111) induced by Pd and Pt adlayers — a photoemission study

High resolution photoelectron spectroscopy using synchrotron radiation is used to probe indirectly the substrate electronic response of faceting of W(111) induced by adsorbed Pd and Pt monolayers. The W4f 7/2 core level is measured in this study. For clean W(111), there are three components observed in W 4f 7/2 photoemission due to surface, subsurface and bulk atoms, respectively ; the core level shift between surface and bulk atoms is 430 meV. Upon adsorption of Pd or Pt at 300 K, only a bulk-like W 4f 7/2 peak is observed. The interface peak associated with W atoms at the Pd-W or Pt-W interface of the W(111) face coincides with the W bulk peak. Upon annealing of the metal-covered W(111) above ∼750 K, the entire surface undergoes a transition from planar to faceted ; the W 4f 7/2 core level associated with Pd- and Pt-covered W(211) facets broadens and its centroid shifts to the lower binding energy side of the W bulk peak. The centroid shift and peak broadening are attributed to W atoms at the Pd-W or Pt-W(211) interface of the facets. The relation between the interfacial energy and the binding energy difference of the interface W 4f 7/2 peak and the surface W 4f 7/2 peak is discussed.

W(111)-based bimetallic systems : core-level photoelectron spectroscopy studies

Abstract Ultrathin metal overlayers on W(111) are studied by high-resolution photoelectron spectroscopy using synchrotron radiation. The bimetallic systems are prepared by adsorbing approximately one monolayer of K, Co, Ni, Pd, Pt, Cu, Ag and Au on W(111) at 100 K. The W 4 f 7 2 core levels from the W(111) substrate are measured. It is found that the core-level W 4 f 7 2 binding energy shifts originating from W atoms at the interface correlate well with the heats of adsorption for metal films on W(111).

Surface chemistry of PH3, PF3 and PCl3 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 , 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 Shustorovichs 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.

Radiation‐induced decomposition of PF3 on Ru(0001)

Soft x‐ray photoelectron spectroscopy using synchrotron radiation has been employed to study the decomposition of PF3 on Ru(0001) at 80 K induced by energetic electrons. Due to the large binding energy shifts in the P 2p core levels, the phosphorus‐containing surface intermediates produced from the electron‐induced decomposition of PF3 can be identified and their evolution can be followed as the fluence of electrons is changed. A stepwise decomposition of PF3 induced by energetic electrons, i.e., PF3→PF2→PF→P, is observed. The dissociation cross section for PF3→PF2 induced by 550 eV electrons is measured to be on the order of ∼1×10−16 cm2. The main channels that lead to PF3 dissociation involve the excitation of F 2s as well as valence states of PF3.

The influence of preadsorbed K on the adsorption of PF3 on Ru(0001) studied by soft x‐ray photoelectron spectroscopy

Soft‐x‐ray photoelectron spectroscopy (SXPS) is utilized to study the coadsorption of K and PF3 on Ru(0001) at 90 and 300 K. In the absence of K, PF3 adsorbs molecularly at both temperatures. In the presence of a fractional monolayer of K, initially PF3 completely dissociates resulting in the formation of adsorbed KF and P species. As the surface is further exposed to PF3, some of the PF3 molecules adsorb via partial dissociation, resulting in the formation of PF and PF2. This process continues until all the K has reacted. At 300 K, a fraction of the incoming PF3 molecules react with the adsorbed KF and form a species which is tentatively identified as KPF6. The data show that surface chemistry is different at the two temperatures, as some of the chemical reaction channels occurring at 300 K are blocked at 90 K. The reduced surface mobility of the incident PF3 molecules at 90 K adversely affects the probability of PF3 and KF interactions, which, in turn, causes the concentration of adsorbed PF3 relative t...