Silicon surface conditions

To date, numerous radical-induced hydrosilylations of terminal olefins or acetylenes have been reported for the H-terminated Si(l 11) surfaces. These reactions are mainly performed by using thermal conditions, UV irradiation, or electrochemistry. More recently, a very mild method was developed for the attachment of high-quality organic monolayers on crystalline silicon surfaces. 

It is very clear that silicon is one of the most important materials in modern technologies, especially in electronics. Silicon is also one of most common element on the earth. Silicon surface is readily oxidized under ambient condition. A silicon substrate is covered by a silica (SiO c) layer. This silica layer can be controlled easily by chemical reagents, heating, electrochemical treatment, and so on. 

A silicon surface, no mater how well it is prepared, is not perfectly flat at the atomic scale, but has surface defects such as surface vacancies, steps, kinks sites, and dopant atoms. The dissolution of the surface is thus not uniform but modulated at the atomic scale with higher rates at the defects and depressed sites. The micro roughness of the surface will increase with the amount of dissolution due to the sensitivity of the reactions to surface curvature associated with the micro depressed sites. These sites, due to the higher dissolution rates, will evolve into pits and eventually into pores. Depending on the condition, a certain amount of dissolution is required before the initiation of pores on all types of materials. 

The surface condition of a silicon crystal depends on the way the surface was prepared. Only a silicon crystal that is cleaved in ultra high vacuum (UHV) exhibits a surface free of other elements. However, on an atomistic scale this surface does not look like the surface of a diamond lattice as we might expect from macroscopic models. If such simple surfaces existed, each surface silicon atom would carry one or two free bonds. This high density of free bonds corresponds to a high surface energy and the surface relaxes to a thermodynamically more favorable state. Therefore, the surface of a real silicon crystal is either free of other elements but reconstructed, or a perfect crystal plane but passivated with other elements. The first case can be studied for silicon crystals cleaved in UHV [Sc4], while unreconstructed silicon (100) [Pi2, Ar5, Th9] or (111) [Hi9, Ha2, Bi5] surfaces have so far only been reported for a termination of surface bonds by hydrogen. 

Different chemical treatments for silicon can be categorized depending on the condition of the Si surface after the clean. The two basic surface conditions for a silicon surface are hydrophobic and hydrophilic. 

A hydrophilic surface condition has been related to the presence of a high density of silanol groups (Si-OH) or to a thin interfacial oxide film. Such an oxide can be produced chemically by hot HN03 or by solutions containing H202. The three most common cleaning solutions for silicon are based on the latter compound ... 

Measurements of mobility in PS suffer from the fact that the number of free charge carriers is usually small and very sensitive to illumination, temperature and PS surface condition. Hall measurements of meso PS formed on a highly doped substrate (1018 cm3, bulk electron mobility 310 cm2 V-1 s-1) indicated an electron mobility of 30 cm2 V 1 s 1 and a free electron density of about 1013 cm-3 [Si2]. Values reported for effective mobility of electron and hole space charges in micro PS are about five orders of magnitude smaller (10-3 to 10 4 cm2 V 1 s ) [PelO]. The latter values are much smaller than expected from theoretical investigations of square silicon nanowires [Sa9]. For in-depth information about carrier mobility in PS see [Si6]. 

Difference In surface concentrations required for achieving the shape-selectivity indicates fornatlon of silica with different surface conditions. The relationship between shape-selectivity and surface silicon concentration, however, does not largely depend on the Included cation, proton or sodium, but rather on the composition of zeolites. Strong dependence on the composition was confirmed on the dealumlnated mordenlte, since the behavior was not in agreement with those on the native species but with those expected from the composition. Therefore, growth of silica and pore size enclosure can be summarized. 

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