Silicon oxygen content

The low background pressure (10 mbar) together with the purity of the gases used ensures a low concentration of contaminants. Amorphous silicon films made in the intrinsic reactor have been analyzed by using ERD, which is available in our laboratory [114]. The determined oxygen content in these films typically is lower than 3 x lO cm - which is somewhat lower than the values required for obtaining device quality films reported by Morimoto et al. [167]. 

The corona discharges produces oxygen ions and ozone, which may react with the photoconductor [634], As a means to circumvent possible degradation of the surface layer, an extra, protective thin layer was proposed, with high carbon content [101, 635, 636]. This would reduce silicon-oxygen reactions at the surface. Excellent electrophotographic characteristics have been obtained with a thin device comprising a 0.1-/rm-thick n-type a-Si H layer, a 1.0-/rm intrinsic a-Si H layer, a 0.1-/irm undoped a-SiCo i H layer, and a 0.014-/xm undoped a-SiCoj H layer [101]. 

The crosslinking of such types of silicones can be described by means of a polymerization reaction. The reaction rate (rP) of this process is a function of the light intensity, the exposure time, the acrylate content, the molecular weight of the uncrosslinked silicone, the photoinitiator and also of the oxygen content of the system. A typical reaction rate/time profile is shown in the Fig. 1. 

Figure 2. Oxygen index of silicone/bisphenol-A polycarbonate block polymers of varying silicone block content.

Any silicate that forms thermally and chemically stable residual compounds as its oxygen content is reduced provides a suitable source of silicon for this reaction. A typical process consists of alternating aluminum, silica, and graphite plates separated by 2—4-cm thick graphite spacers stacked in a graphite-lined alumina, tube and heated to 1400°C for 12 h in a nitrogen atmosphere. After cooling for approximately 6 h the fibers are removed. 

The product is a partially crystalline, partially amorphous mixture of SiC, C and Si02. The silicon carbide content was shown to be highest for gels with the lowest relative oxygen content.19... 

The multifilament fiber (10-20 xm diameter) as commercially produced consists of a mixture of /3-SiC, free carbon and SiOj. The properties of this fiber are summarized in Table 6.5. The properties of Nicalon start to degrade at temperatures above about 600°C because of the thermodynamic instability of composition and microstructure. A ceramic grade of Nicalon, called Hi Nicalon, having low oxygen content is also available Yet another version of a multifilament silicon carbide fiber is Tyranno, produced by Ube Industries, Japan. This is made by pyrolysis of poly (titano carbosilanes) and contains between 1.5 and 4wt% titanium. 

A completely amorphous structure was found by X-ray diffraction on fibers which were pyrolysed at temperatures up to 1300 °C. A crystallization starts around 1400 °C and nanocrystalline silicon carbide is formed with a crystallite size of about 2 nm. Compared to an uncured sample the crystallization is retarded. A significant crystallite growth occurs around 1500 °C connected with a decreasing of the fiber properties. The oxygen content of these SiC fibers is less than 1 wt. % found by neutron activation... 

Many of the inorganic chars are thermally unstable at elevated temperatures, generally as a result of poor control of stoichiometry, that is, high oxygen content and excessively high car-bon/silicon ratios. 

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

The behaviour of quartz crucibles and the interaction with liquid silicon was examined thoroughly to control the oxygen content in Cz wafers [42]. Important factors are the dissolution of quartz in liquid silicon and the evaporation of SiO from the melt. This results in a transport process of Si02 via the silicon melt into the gas phase and respectively to crucible dissolution and silicon melt contamination. 

---