P. Soukiassian

SiO2/6H-SiC(0001)3×3 initial interface formation by Si overlayer oxidation

We investigate the initial oxidation and SiO2/6H-SiC interface formation by core level photoemission spectroscopy using synchrotron radiation. The results indicate that the direct oxidation of the 6H–SiC(0001)3×3 surface leads to SiO2 formation at low temperatures (500 °C) with a nonabrupt interface having significant amounts of mixed (Si–O–C) and intermediate (Si3+,Si2+,Si+) oxidation products. In contrast, C-free and a much more abrupt SiO2/6H-SiC(0001) interface formation is achieved when predeposited Si overlayer is thermally oxidized at low oxygen exposures and low temperatures (500 °C).

Direct SiO2/β‐SiC(100)3×2 interface formation from 25 °C to 500 °C

We investigate the β‐SiC(100)3×2 surface oxidation by core level and valence band photoemission spectroscopies using synchrotron radiation. Low molecular O2 exposures on the (3×2) surface reconstruction leads to direct SiO2/β‐SiC(100)3×2 interface formation already at room temperature (RT). To our best knowledge, this is the first example of RT oxidation leading directly to dominant silicon dioxide growth by O2 chemisorption only. The amount of SiO2 is enhanced when the surface temperature is raised by few hundred degrees only (<500 °C) during O2 exposures leading to ‘‘bulk oxide’’ formation already at small thicknesses. These findings are also relevant in low‐temperature semiconductor oxide processing.

Low‐energy Ar+ ion bombardment‐induced modification of surface atomic bond lengths on InP(100) wafer

We report the first direct measurement of surface interatomic bond distance modifications due to ion bombardment. The experiments were performed using low energy Ar+ ion on a InP(100) surface by photoemission x‐ray absorption fine structure. The structural changes are sputtering time dependent and result in first (P–In) and second (P–P) surface bond distances relaxation approaching the bulk values. It suggests that, prior to bond breaking, the Ar+ ion beam first stretches atomic bond lengths during a precursor stage, with small energy and momentum transfer. This work brings new insights into the knowledge of ion sputtering various micromechanisms which is relevant for surface preparation.

Promoted oxidation of aluminum thin films using an alkali metal catalyst

We investigate the effect of sodium overlayers on the oxidation of aluminum thin films by means of core level and valence band photoemission spectroscopy using synchrotron radiation. The presence of Na enhances the oxidation rate of the aluminum by nearly 4 orders of magnitude with formation, at room temperature, of alumina likely as α‐Al2O3. The sodium catalyst is removed by thermal desorption at temperatures below 300 °C leading to the formation of an aluminum oxide having a larger binding energy than stoichiometric alumina. It indicates the formation at the surface of an aluminum oxide having higher oxidation states as Al2O3+x. Unlike catalytic oxidation of elemental semiconductors such as silicon, formation of Na related oxides are shown to play an important role in the alkali metal promoted aluminum oxidation. Interestingly, Na appears a more efficient catalyst for Al thin film than on single crystal surface. The micromechanisms of promotion are found to be significantly different from that of alkali...

Si/6H–SiC(0001): An unexpected cubic 4×3 Si phase overlayer

We investigate Si deposition on the 6H–SiC(0001) 3×3 surface reconstruction by atom-resolved scanning tunneling microscopy. Upon thermal annealing, the Si thin film forms an unexpected structure having dimer rows in a cubic 4×3 surface array. Such a 4×3 Si phase has a very open surface very likely being at the origin of the exceptionally high reactivity to oxygen of the Si/6H–SiC(0001) system. These findings are relevant in silicon carbide oxidation.

Sb‐induced interatomic bond distance stabilization on InP(100) surface

We have studied the formation of the Sb/InP(100) interface by photoemission extended x‐ray absorption fine structure. Our results indicate that a low Sb coverage stretches and subsequently weakens P—In surface bonds which appears as a precursor stage prior to In—Sb bond formation. Then, at increasing Sb coverages, the substrate surface reconstructs with bonds approaching the relaxed clean InP(100) surface values. This results from the breakdown of In clusters (formed during surface preparation by ion sputtering) which leads to the formation of an InSb interface layer. The surface is finally found to be stabilized at higher Sb coverages with no more change in the substrate first and second near neighbor bond distances.

Al2O3+x/Al interface formation by promoted oxidation using an alkali metal and removal of the catalyst

We report the effect of Na overlayers on the oxidation of Al thin films by core level photoemission spectroscopy using synchrotron radiation. The presence of a Na monolayer enhances the aluminum room temperature oxidation rate by 4 orders of magnitude with formation of alumina as Al2O3 and Na oxides, possibly as aluminates. Furthermore, the Na catalyst could be desorbed from the surface by a rapid thermal annealing at temperatures below 300 °C, leading to the growth of aluminum oxides having a binding energy larger than stoichiometric Al2O3. It indicates the formation at the surface of Al oxides having higher oxidation states.

Atomic cracks and (23×2×3)-R30° reconstruction at 6H-SiC(0001) surface

We investigate the Si-rich 3×3 to Si-terminated 3×3 phase transition of the 6H-SiC(0001) surface by atom-resolved scanning tunneling microscopy. We find a 23×23-R30° reconstruction, coexisting with few 3×3 domains. While a high-quality 3×3 surface preparation is achieved with a very low defect density (<2% of atomic defects), the (23×2×3)-R30° reconstruction instead exhibits long atomic cracks suggesting important stress relief during the phase transition. A structural model containing 13Si atoms per unit cell distributed in three layers above the Si terminated bulk SiC substrate is proposed.

INITIAL STEPS OF ALKALI-METAL-PROMOTED OXIDATION OF THE Al(111) SURFACE

We investigate the initial Na-promoted oxidation of an Al surface at room temperature by core-level photoemission spectroscopy using synchrotron radiation. We bring the first evidence that, in the initial state, adsorbed oxygen atoms break the Na-Al bonds. This leads to the formation of an intermediate oxidation state involving both Na and Al sites with oxygen atom in a bridge-type site. This bonding configuration is shown to result from the Na/Al intermixing and appears as a precursor step playing a key role in the observed promoted oxidation. It is also likely to explain the formation, upon annealing, of Al oxides having a higher oxygen content than Al2O3.

S‐polarization photoemission extended x‐ray absorption fine structure study of clean and adsorbate covered Si(100)2×1 surface

This article presents a structural study of the clean and 1 monolayer Na covered Si(100)2×1 surface by s‐polarization photoemission extended x‐ray absorption fine structure and by core level and valence band photoemission spectroscopy using synchrotron radiation. The Na–Si and Si–Si distances are obtained by analyzing the extended x‐ray absorption fine structure signal at the Na 2p and Si 2p core level lines. The results clearly indicate a single site of adsorption for the Na atoms, the cave, with a single layer chain at saturation coverage, and no Na–Na distance consistent with ‘‘double layer’’ models. The existence and growth of an additional overlayer are related to the presence of impurities which, even at very low levels, enhance the sticking coefficient of an alkali atom. We also measure the Si–Si dimer bond length for clean Si(100)2×1 and Na/Si(100)2×1 surfaces. The experimental Na–Si and Si–Si bond lengths are found to be in excellent agreement with the recent results obtained by ab initio total e...