Corrosion and Passivation Studies

Note that the value of the redox potential is critical. If the redox potential lies in the band gap of the semiconductor, monitoring the reaction red ox -l-e at the tip will not help for lithography. As the tip comes closer to the surface the reaction stops (negative feedback). This behavior, characteristic of insulating electrodes. 

The passivation of III -V compounds remains a challenging problem for microelectronics but has still received no answer as efficient as the H-termination of Si. The increase of the photoluminescence intensity and lifetime after treatments in sulfide solutions have recently attracted much attention [67]. Dagata et al. [160] have compared, by STM/STS in the ambient atmosphere, the passivation of GaAs in (NH4)2S 

The role of the P2S5 in the passivating solution seems very important to the interfacial chemistry and to STM imaging. The dip in the P2S5/(NH4)2S mixture leads to the formation of a phase at the very surface of GaAs and an As Oj, phase 

This kind of problems seems to be less dramatic when imaging the surface in-situ as Fig. 32 b shows. The interesting point in this image is the anisotropic shape of structures which are parallel [110] and suggest a complete restructuring of the initial surface, due to etching in the sulfide solution. Moreover sequences of images show a preferential attack of structures at their narrowest extremity, which indicates a chemical dissolution and also that the surface is probably terminated by one short of atoms (probably Ga one bound to S species). Atomic resolution has, however, not been achieved yet on this surface. 

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In this review results from two surface science methods are presented. Electron Spectroscopy for Chemical Analysis (ESCA or XPS) is a widely used method for the study of organic and polymeric surfaces, metal corrosion and passivation studies and metallization of polymers (la). However, one major accent of our work has been the development of complementary ion beam methods for polymer surface analysis. Of the techniques deriving from ion beam interactions, Secondary Ion Mass Spectrometry (SIMS), used as a surface analytical method, has many advantages over electron spectroscopies. Such benefits include superior elemental sensitivity with a ppm to ppb detection limit, the ability to detect molecular secondary ions which are directly related to the molecular structure, surface compositional sensitivity due in part to the matrix sensitivity of secondary emission, and mass spectrometric isotopic sensitivity. The major difficulties which limit routine analysis with SIMS include sample damage due to sputtering, a poor understanding of the relationship between matrix dependent secondary emission and molecular surface composition, and difficulty in obtaining reproducible, accurate quantitative molecular information. Thus, we have worked to overcome the limitations for quantitation, and the present work will report the results of these studies. 

When the sweep rate is very low, in the range of v = (0.1—5) mVs , measurement is conducted under quasi-steady-state conditions. The sweep rate plays no role in this case, except that it must be slow enough to ensure that the reaction is effectively at steady state along the course of the sweep. This type of measurement is widely used in corrosion and passivation studies, as we shall see, and also in the study of some fuel cell reactions in stirred solutions. Reversing the direction of the sweep should have no effect on the current-potential relationship, if the sweep is slow enough. Deviations occur sometimes as a result of slow formation and/or reduction of surface oxides or passive layers. Because the sweep rate is slow, the potential is often swept only in one direction, and the experiment is then referred to as linear sweep voltammetry (LSV). 

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