Carbon. Auger spectrum

Fig. 2.24. Examples of the effect of different chemical states on the KLL Auger spectrum of carbon [2.130] (SiC and graphite denote Ar -bombarded surfaces ofSiCand graphite, respectively).

The state of bonding of carbon on the surface can easily be detected by AES. The carbon Auger peak shows a different fine structure for carbidic and graphitic carbon, as shown in Fig. 5a and b, respectively (32). Carbidic carbon characteristically shows three sharp peaks near 270 V the graphitic form shows a more rounded spectrum. It is therefore possible to differentiate the effects of carbidic and graphitic carbon on surface reactivity. 

The mechanism we believe is responsible for the large SiOj-to-Si etch-rate ratios which have been obtained in fluorine-deficient discharges is based on several experimental observations. First of all, it has been shown that there are several ways in which carbon can be deposited on surfaces exposed to CF, plasmas. One way is to subject the surface to bombardment with CF ions which are the dominant positive ionic species in a CF plasma. The extent to which this can occur is shown by the Auger spectra in Fig. 3.3. Curve (a) is the Auger spectrum of a clean silicon surface and curve (b) is the Auger spectrum of the same surface after bombardment with 500 eV CFj" ions. Note that the silicon peak at 92 eV is no longer visible after the CFj bombardment indicating the presence of at least two or three monolayers of carbon. Another way in which carbon can be deposited on surfaces is by dissociative chemisorption of CFj or other fluorocarbon radicals. 

The XPS/Auger spectrum of a membrane prior to reaction revealed the presence of sulfur uniformly distributed throughout the palladium as an impurity (see Figure 5a). Spectra of the membrane after reaction at 200°C for forty hours indicated that both surfaces of the membrane had been preferentially enriched with traces of sulfur (see Figure 5b). Depthprofiling experiments revealed the formation of a trace sulfur surface layer which is approximately 20 A thick. Spectraof both the raw and usedfoils showed carbon on the surface, however the concentration was too low to differentiate from possible sample contamination. 

Figure 4. Fragment of Auger-spectrum corresponding to the carbon line. The spectrum was produced for cathodic product of electrolysis (a) without etching and (b) at the etching with Ar ions over the course of 40 min.

Auger electron spectroscopy (AES) was used in combination with secondary ion mass spectrometry (SIMS) to distinguish between four types of carbonaceous deposits, on metal foils (rhodium, iridium and platinum). The foils were coked by exposing to ethylene at low pressure. Auger spectroscopy can distinguish between molecular or carbidic on the one hand, and graphitic or amorphous carbon on the other. The Auger spectrum of carbonaceous deposits on a metal is... 

Fig. 35. The Auger spectrum of TbNis after use as a methanation catalyst. The Ni signal at 830V is weak before sputtering it intensifies with sputtering. The Carbon signal at —270 V behaves in an opposite way (Wallace, 1978).

Chemical effects are quite commonly observed in Auger spectra, but are difficult to interpret compared with those in XPS, because additional core levels are involved in the Auger process. Some examples of the changes to be seen in the KLL spectrum of carbon in different chemical environments are given in Fig. 2.24 [2.130]. Such spectra are typical components of data matrices (see Sect. 2.1.4.2) derived from AES depth profiles (see below). 

The XPS survey spectrum of a 75 nm thick film of plasma polymerized acetylene that was deposited onto a polished steel substrate is shown in Fig. 18 [22]. This film consisted mostly of carbon and a small amount of oxygen. Thus, the main peaks in the spectrum were attributed to C(ls) electrons near 284.6 eV and 0(ls) electrons near 533.2 eV. Additional weak peaks due to X-ray-induced O(KVV) and C(KLL) Auger electrons were also observed. High-resolution C(ls) and 0(ls) spectra are shown in Fig. 19. The C(ls) peak was highly symmetric. 

Figure 3.24 The spectrum of the carbon KVV Auger transition contains chemical information and can be used as a fingerprint of the state of the carbon.

Figure 1. Auger electron spectrum of the surface of a Ru electrode before and after deactivation by reduction of carbon dioxide at higher temperatures ( 90 °C in 0.2 M Na2SC>4 at pH 4 and -0.545 V vs SCE).

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