Impurities on the Surface

By using a laser with less power and the beam spread over a larger area, it is possible to sample a surface. In this approach, after each laser shot, the laser is directed onto a new area of surface, a technique known as surface profiling (Figure 2.4c). At the low power used, only the top few nanometers of surface are removed, and the method is suited to investigate surface contamination. The normal surface yields characteristic ions but, where there are impurities on the surface, additional ions appear. 

Flaws in the anodic oxide film are usually the primary source of electronic conduction. These flaws are either stmctural or chemical in nature. The stmctural flaws include thermal crystalline oxide, nitrides, carbides, inclusion of foreign phases, and oxide recrystaUi2ed by an appHed electric field. The roughness of the tantalum surface affects the electronic conduction and should be classified as a stmctural flaw (58) the correlation between electronic conduction and roughness, however, was not observed (59). Chemical impurities arise from metals alloyed with the tantalum, inclusions in the oxide of material from the formation electrolyte, and impurities on the surface of the tantalum substrate that are incorporated in the oxide during formation. 

The impurities on the surface are contained in the resulting water droplet or moisture film, and are collected in situ for further investigation by scanning the surface with an auxiliary water droplet (e.g., 50 pi). The VPD residue is allowed to dry in the center of the wafer and subjected to TXRF analysis. A schematic of a VPD reactor is shown in F ure 3. 

A SSIMS spectrum, like any other mass spectrum, consists of a series of peaks of dif ferent intensity (i. e. ion current) occurring at certain mass numbers. The masses can be allocated on the basis of atomic or molecular mass-to-charge ratio. Many of the more prominent secondary ions from metal and semiconductor surfaces are singly charged atomic ions, which makes allocation of mass numbers slightly easier. Masses can be identified as arising either from the substrate material itself from deliberately introduced molecular or other species on the surface, or from contaminations and impurities on the surface. Complications in allocation often arise from isotopic effects. Although some elements have only one principal isotope, for many others the natural isotopic abundance can make identification difficult. 

Figure 3.5 shows the positive SSIMS spectrum from a silicon wafer, illustrating both the allocation of peaks and potential isobaric problems. SSIMS reveals many impurities on the surface, particularly hydrocarbons, for which it is especially sensitive. The spectrum also demonstrates reduction of isobaric interference by high-mass resolution. For reasons discussed in Sect. 3.1.3, the peak heights cannot be taken to be directly proportional to the concentrations on the surface, and standards must be used to quantify trace elements. 

The effect of an impurity on the surface energy is often discussed in terms of the surface activity of the impurity BJb, defined as the slope of the surface tension or energy versus composition at infinite dilution ... 

Occlusion. Adsorbed impurities on the surface of the crystal are trapped by subsequent strata during crystal accretion. 

Ref 6) and Clift Fedoroff (Ref 7) described prepn of MeTetryl from beta- and gamma-Trinitrotoluene isomers always present as impurities on the surface of crysts of etude aipha-TNT. The jS and y isomers can be removed by washing the crysts of crude a TNT with 8% aqueous Na sulfite soln (called Sellite). This converts the isomers into water-soluble dinitrotoluene sulfonates. Then, on their treatment with me thy famine the following reaction takes place ... 

So-called wet solar cells show promise, particularly because of their relative ease of fabrication. In this type of photovoltaic cell, the junction is formed, between a semiconductor and a liquid electrolyte. No doping is required because a junction forms spontaneously when a suitable semiconductor, such as GaAs, is contacted with a suitable electrolyte, Three knotty problems (accelerated oxidation of surface of semiconductor exchange of ions between semiconductor and electrolyte forming a blocking layer and deposition of ions of impurities on the surface of the semiconductor) all have been solved and thus the concept now appears technically viable,... 

The V(110) surface was cleaned by repeated cycles of Ne+ sputtering at 600 K (1.0 kV 3 pA) followed by annealing to 1200-1400 K, as described in detail elsewhere.10 The impurity levels of residual C and S were less than 2% and 1% of a monolayer (ML), respectively, based on the AES measurements. The residual O impurity on the surface was estimated to be less than 6% of a ML from the HREELS measurements.10 Thin vanadium carbide films were produced by exposing the clean V(110) surface to ethylene or 1,3-butadiene at 600 K. Details concerning the formation and thermal stability of the thin carbide films on V(110) can be found elsewhere.4,11... 

As seen in Fig. 3a, impurities on the surface (C, N, O) accompanies Pt deposition. The value of inter-planes distance for 200, d = 0.203 nm was established under investigations of Pt films with TEM. Thus the lattice constant for Pt on GaAs surface is a = 2 d = 0.406 nm. This value is higher as compared with that given in literature, 0.392 nm [7]. The larger value in our case may be explained by impurities likely implanted in the Pt lattice or by the interaction of Pt with GaAs surface. The Pt particles with average dimension 5 10 nm were obtained with our technique [8],... 

The absorbing sites have been discussed in terms of the cation-anion couples in low coordination on the surface, but no hypothesis has been made regarding the nature of the emitting sites. The emitting sites could be (i) extrinsic impurities on the surface, e.g., TMIS or organic materials (ii) point defects such as trapped electron or hole centers or (iii) sites identical or similar to the absorbing sites. These possibihties have been considered by Coluccia (13). 

While the effect of cation impurities on the surface chemistry of MgO has been investigated in detail, very little is known about anion substitution. Defect formation and excitation energies for S and Se -doped bulk MgO have been calculated [182,183] but there are no data for the surface. In the bulk it has been estimated that the presence of S or Se impurities result in a outward relaxation of the Mg neighbors of 6% and 8%, respectively [182]. A recent report of the 0 "-S exchange reaction on MgO has been reported [184]. The reaction involves adsorption of CS2 on MgO powders and the subsequent exchange reaction with formation of COS and of S ions probably located at the low coordinated sites. It has been found that the basicity of the MgO surface doped with sulfur ions is drastically modified with respect to that of pure MgO [184]. 

FIGURE 17.15. The effect of absorption of an impurity on the surface of a crystal. In this Ccise a carboxylic acid (cinnamic acid) is bound to the crystal face as an impurity on crystals of an amide (cinnamide). As a result, crystal growth in the b direction is stopped (compare with Figure 2.8, Chapter 2, for the consequences of this). (Reproduced from Reference 69 with the permission of G. R. Desiraju and Elsevier Science Publishers BV, Academic Publishing Division, Amsterdam, The Netherlands.)... 

In addition to structural (geometric) heterogeneity, characterized by different pore shapes and sizes, activated carbons like all porous solids may present surface energetic heterogeneity (SEH). This heterogeneity is mainly due to the presence of functional groups and strongly bound impurities on the surface of pores, whose characteristics depend on the type of row material used and the conditions of carbonization and activation processes,... 

This formula, due to Einstein, was experimentally verified by Jean Perrin 94) by direct microscopic observation of spherical colloidal mastic particles (with radius 6.5-10 cm), which contained. small enclosures of impurities on the surface, thus permitting their rotary motion to be directly followed. 

Catalytic activity is essentially a surface phenomenon -201) hence particularly sensitive to the presence of impurities on the surface, which are often responsible for poisoning or for modifications in the catalytic selectivity The presence of sulfur on a metallic catalyst may induce rearrangement in the surface structure resulting in modification of the electronic distiibution mostly by dative bond formation, and, for higher coverages, in strong adatom-adatom interaction. 

Physical changes may result from distortion of the polymeric material by sunlight due to photodegradation and photocrosslinking and can be photosensitized by the presence of impurities on the surface which efficiently absorb light. Energy transfer processes may play a considerable role. 

X-ray photoelectron spectral data are useful for study of the surface composition of these materials. The spectra show the presence of substantial amounts of impurities on the surface of powdered YBa2Cu307. Specifically, we observe high percentages of carbon and proportionately less oxygen and metals than expected (see Table 1). 

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