J. Prasad

Direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon

We report on micro‐Raman spectroscopy studies of porous silicon which show an amorphous silicon Raman line at 480 R cm−1 from regions that emit visible photoluminescence. A Raman line corresponding to microcrystalline silicon at 510 R cm−1 is also observed. X‐ray photoelectron spectroscopy data is presented which shows a high silicon‐dioxide content in porous silicon consistent with an amorphous silicon phase.

The interaction of H2O and Cl2 on Fe polycrystalline surfaces

Abstract Temperature-programmed desorption and Auger spectroscopy were used to study the interaction of H 2 O and Cl 2 on an Fe polycrystalline surface under UHV conditions over the temperature range of 90–900 K. The TPD spectra of H 2 O on a clean Fe(poly) surface show the desorption of H 2 O from two states at 175 and 205 K. There is no recombinant H 2 O desorption from the Fe(poly), in contrast to the Fe(100) surface. The adsorption and dissociation of H 2 O is significantly affected by the presence of Cl at the Fe(poly) surface. Pre-exposure of Cl 2 results in dissociatively adsorbed Cl (Cl a ) at the surface. Cl a acts as a site blocker, preventing H 2 O dissociation. Cl 2 adsorption on an Fe(poly) surface pre-exposed to H 2 O at 90 K leads to a Cl 2 -H 2 O reaction which results in the formation of an OH-Cl complex. This complex is stabilized to temperatures above 400 K in UHV and results in recombinant H 2 O desorption over a broad temperature range. Desorption of an FeOCl complex is observed at 240 K regardless of dosing sequence, but only at low adsorbate pre-coverages. These data demonstrate that Cl a inhibits hydroxylation of the Fe(poly) surface. The low temperature volatilization of the Fe surface via Fe oxychloride formation does not directly involve H 2 O (a) , but can only occur after H 2 O dissociation.

Site selective chemistry of S, Cl and H2O at the iron oxide surface

The interactions of Cl2, H2O, and H2S at an Fe oxide surface have been studied by X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum conditions. The Fe oxide contains both Fe2+ and Fe3+ sites. On the clean oxide surface, adsorption of H2S at 115 K yields both adsorbed sulfur (Sa) and SHa. On a surface preexposed to Cl2 at 115 K only SHa is observed. Exposure of H2S to a surface preexposed to H2O at 115 K results only in Sa. XPS measurements of the Fe(2p32) line show that Cl adsorbs only at Fe2+ sites, indicating that Cl and S compete for Fe2+ sites. Previous studies have shown that subsequent exposure of a sulfided oxide surface to Cl2 at 115 K results in desorption of Sa, with no effect on SHa. Annealing of the sulfided surface to 300 K prior to chlorine exposure at 115 K results in Sa being unaffected by the chlorine exposure, although SHa is converted to additional Sa during the annealing process. The annealing process also results in the conversion of Fe2+ to Fe3+ sites, as indicated by Fe(2p32) XPS spectra. These results indicate that there is significantly higher energy required for the conversion of Sa to FeS on the oxide surface than on metallic Fe.

The effects of chlorine on Fe and Fe2O3 surfaces in H2S environment

Abstract We have used X-ray photoelectron spectroscopy to study the effects of chlorine gas on Fe and Fe 2 O 3 surfaces pre-exposed to H 2 S at 105 K. H 2 S chemisorbs on the Fe 2 O 3 surface at 105 K to form adsorbed sulfur (S a ) and sulfylhydril (SH a ) intermediates. Subsequent exposure to Cl 2 vapor at 105 K results in preferential desorption of S a leaving the SH a intermediate intact. Contrasting behavior is observed for SH a and S a on metallic Fe. On the metallic Fe surface, exposure to chlorine vapor at 105 K leads to SH bond scission and thus conversion from SH a to S a . The sulfur chemisorption site on Fe 2 O 3 thus differs substantially from that on the metallic Fe surface.

Atomic hydrogen cleaning of a TiN surface

Abstract Atomic deuterium has been used to remove hydrocarbons and carbide species from a polycrystalline, partially oxidized TiN surface. The atomic deuterium was produced by flowing D 2 over a heated tungsten filament (2200 K). XPS studies indicate that the removal of hydrocarbons occurs readily while that of carbides occurs at a slower rate. No significant effects were observed for O and N. Some reduction of TiN to a metallic state is observed. These results indicate that atomic hydrogen can be used for removing contamination from TiN substrates without affecting the integrity of the substrate.

Effects of atomic hydrogen on Cu(II)bishexafluoroacetylacetonate interactions with a TiN surface

Abstract Cu(II)bishexafluoroacetylacetonate [Cu(II)(hfac)2] adsorbed onto a partially oxidized TiN substrate [TiN(O)] is reduced to Cu(0) by exposure to atomic deuterium at 450 K. Initial precursor interaction with the TiN(O) surface at 450 K results in an ad-layer consisting of Cu(I)hfac and hfac. Subsequent exposure to atomic deuterium at 450 K results in volatilization of organic species and reduction of Cu(I) to Cu(0). This reduction is concurrent with the removal of 50% of the Cu atoms from the surface. This indicates that the reduction involves a disproportionation of adsorbed Cu(I) species to yield adsorbed Cu(0) and a volatile Cu(II) species. The disproportionation is not observed in the absence of atomic deuterium and volatilization of adsorbed hfac. Atomic deuterium also removes adventitious carbon from a TiN(O) surface prior to precursor exposure. The removal of carbon is accompanied by substantial reduction of Ti(IV) to Ti(II) within the substrate surface region. Diffusion of Cu into the cleaned TiN(O) substrate begins above 450 K, but only becomes significant as the sample is heated above 750 K in ultra-high vacuum. These results indicate that atomic-hydrogen-assisted CVD is effective at inhibiting organic contamination at the Cu/TiN interface, and does not significantly degrade the diffusion barrier properties of the TiN substrate.

Surface Effects in the Mocvd of Copper

Recent findings concerning the surface chemistry of Cu(I)) and Cu(II) betadiketonate precursors on metal and TiN surfaces are reviewed. Interactions of Cu(I) hexafluoroacetylacetonate (L) (Cu(I)(hfac)(L); L=Lewis base) with the surface result in an adsorbed Cu(I)hfac intermediate. Cu(II)(hfac) 2 adsorbtion yields the Cu(I)hfac intermediate plus adsorbed hfac. Subsequent exposure to atomic hydrogen volatilizes the adsorbed hfac and results in the reduction of Cu(I) via disproportionation: 2Cu(I)(hfac) ⇒ Cu(0) + Cu(II)(hfac) 2 (desorbed). These results demonstrate that disproportionation can occur on contaminant free surfaces under UHV conditions, and that the mechanisms for Cu(I) and Cu(II)/H 2 film growth are similar. Implications for selectivity and low temperature deposition are explored. Potential shortcomings of TiN as an adhesion/diffusion barrier are also discussed.

Effects of sulfur upon Fe and Cr oxide thermal stability in 304 stainless steel

X-ray photoelectron spectroscopy measurements show that exposure of an Fe−Cr−Ni alloy to H2S and O2 under ultrahigh vacuum (UHV) conditions leads to the thermal instability of the Fe oxide without affecting the Cr oxide. An atomically clean metal surface, predominantly Fe in composition and free of internal sulfur, was exposed to H2S under controlled conditions in order to form a sulfided monolayer. Adsorbed S inhibited surface oxidation at temperatures between 325 K and 825 K. The presence of adsorbed S did not, however, inhibit the surface segregation of Cr for temperatures >600 K, compared to a S-free sample. Upon annealing of a sulfided sample to 900 K in UHV, the Fe oxide largely disappears, while no change is observed in the Cr oxide. The S-free sample shows no significant change in either the Fe or Cr oxides upon annealing. The results presented in this paper show that sulfidation of an alloy surface prior to oxidation and surface segregation can adversely affect the thermal stability of the Fe oxide without affecting the Cr oxide overlayer resulting from surface segregation and preferential oxidation.

Fe oxychloride desorption from an Fe polycrystalline surface

We report the formation of an Fe oxychloride species at the surface of an O- and Cl-covered Fe polycrystalline surface below 230 K under UHV conditions. Our data show that FeOCl (amu 107) desorbs at 230 K. For FeCl2 (amu 126) desorption, two desorption peaks are observed at 230 and 620 K in the presence of chemisorbed oxygen. In the absence of oxygen, only one FeCl2 desorption peak at 620 K is observed. Auger electron spectroscopy and temperature programmed desorption studies reveal that the reaction leading to Fe oxychloride formation involves dissociatively adsorbed O and Cl at the Fe surface. These results indicate that chemisorbed or low coordinate oxygen plays an important and previously unsuspected role in chloride-induced corrosion at ferrous surfaces.