Oxide layers on SiC

Figure 7.12. Surface chemistry of SiC in high vacuum as a function of temperature. Wettability of SiC by non-reactive metals is mainly dictated by the oxidation layer on SiC at T T and by the graphite-rich layer formed by Si evaporation at T T. ...

Figure 1. Examples of oxidation layers on SiC, (a) Cross-section of B after 500h exposure at 850"C in atmospheric pressure of water vapour (w). (b) Oxidation layer about 100 nm thick in A after 500 h exposure at SSCC. (c) Oxidation layer about 500 nm thick in B after 500 h exposure at 850 C atmospheric pressure of water vapour (w).

Increase Temperature Stability Fiber-reinforced CMCs have been demonstrated to survive in the severe environment of a gas turbine engine for 2500 hours at temperatures up to 1200°C. The use of environmental barrier coatings (EBCs) such as oxide layers on SiC appears to help extend durability, but more research is needed to determine whether they present a long-term solution. 

Researchers have been developing SiC-CMCs in order to obtain an oxidation-resistant, tough thermostructural material. In general, a SiC-based material easily forms a protective oxide layer on its surface at high temperatures... 

However, carbon materials have a serious shortcoming. They are easily oxidized above 530°C in air. It is possible to protect graphite plates or carbon fibers with SiC coating by CVD or pyrolysis of polymer containing Si and C.16-18 SiC is known as an effective material to prevent oxidation and corrosion due to the strong covalent bond and the passive oxidation by forming a protective Si02 layer on SiC.19-24... 

Thermal oxidation of the two most common forms of single-crystal silicon carbide with potential for semiconductor electronics applications is discussed 3C-SiC formed by heteroepitaxial growth by chemical vapour deposition on silicon, and 6H-SiC wafers grown in bulk by vacuum sublimation or the Lely method. SiC is also an important ceramic ana abrasive that exists in many different forms. Its oxidation has been studied under a wide variety of conditions. Thermal oxidation of SiC for semiconductor electronic applications is discussed in the following section. Insulating layers on SiC, other than thermal oxide, are discussed in Section C, and the electrical properties of the thermal oxide and metal-oxide-semiconductor capacitors formed on SiC are discussed in Section D. 

Procedures for thermal oxidation of SiC have been developed and shown to produce oxide layers useful in the fabrication of planar SiC microelectronic devices. The SiC oxidation rate has been studied under conditions commonly used in integrated circuit fabrication. The oxidation rate constants derived in these studies are useful for predicting the oxide thickness formed on SiC under similar conditions. The metal-oxide-semiconductor capacitors formed by thermal oxide layers on both 3C- and 6H-SiC have been shown to have low interface charge densities, suitable for transistor applications. 

Surface Oxides The oxide layers on C- and Si-terminated faces of SiC differ in composition and structure [27,30]. The oxide layer is directly connected to the topmost SiC bilayer by a Si-C bond on (0001) whereas a linear Si-O-Si bridge makes the contact on (0001). The latter structure leads to a stronger bonding and a slower decomposition rate of the Si-face. Zinovev et al. [31] observed a thicker oxide layer on the Si-face compared to the C-face. X-ray photoelectron spectroscopy (XPS) analysis of SiC wafers detected SiO C, on the C-face and Si-face however, the oxygen content was higher on the C-face. Therefore, the thermal stability of the surface oxides at elevated temperatures (>1300°C) was reported to be higher on the Si-face compared to the C-face. 

Oxidation due to an overheated upper part of SiC blocks usually may be seen as white layers on SiC blocks, which may become cracked in the future. 

Chemical Resistance. The oxidation resistance of SiC is excellent due to the formation of a layer of Si02 on the surface. It is inert to most chemicals at room temperature. 

A reducing atmosphere with very low oxygen content may not lead to a significant active-oxidation reaction for SiC. Since the treatment for 30 min appears to be ineffective for forming a good SiC layer on MWCNTs, the SiC-coated samples prepared for 15 min were utilized to investigate the coating mechanism. 

Fig. 3.14 (a) XPS spectra at take-off angles of 0° and 60°, as measured from the surface normal from a silicon crystal with a thin layer of SiC>2 on top. The relative intensity of the oxide signal increases significantly at higher take-off angles, illustrating that the surface sensitivity ofXPS improves, (b) Plot of Si4+/Si 2p peak areas as a function of take-off angle. The solid line is a fit based on Eq. (3-10), and corresponds to an oxide thickness of 2.0 nm. (From [37]). 

Figure 7.8 shows a ZnO film deposited on 3C-SiC buffered Si(lll). The CVD-grown, only 3 nm thin 3C-SiC buffer layer does not improve the in-plane orientation of ZnO on Si. As already demonstrated in Fig. 7.7, polycrystalline ZnO without any preferential in-plane orientation was found. The HRTEM inset in Fig. 7.8 shows an amorphous layer at the ZnO/3C-SiC interface, which is formed by oxidation of the CVD grown 3C-SiC film. Interestingly, these ZnO films on SiC-buffered Si are c-axis textured and show slightly improved PL and CL characteristics compared to ZnO grown directly on silicon [50]. 

Flat AIN films were fabricated into gated cathode structures [15]. The films included thin AIN on SiC and a graded layer with a concentration graded from GaN to nearly 100% AIN at the surface. The structures employed an oxide spacer layer and an Al film on the oxide served as the grid electrode. The results for both structures were similar. Devices worked for a few minutes, but they exhibited grid currents ranging from 10 to 1000 times the collector current. These preliminary measurements suggest that further materials research on the surfaces of nitrides may lead to room temperature electron emitter structures. 

The corrosion resistance of structural ceramics consisting of oxides and non-oxides is presented in Table 4.93. The data presented in the table are of limited generality. The code letters A, B and C must be used for comparison of the materials. It is obvious from the data that both oxide-based and non-oxide-based ceramic materials are attacked at low and intermediate temperatures. The relatively poor corrosion resistance of SiC and Si3N4 may be due to the formation of Si02 layers on the ceramic components. More detailed information on the corrosion resistance of structural ceramic materials may be found in the literature.111... 

Thermal diffusivity of oxidation-resistant SiC/C compositionally graded graphite materials has been measured by using the laser flash method. In order to study the effect of the SiC/C graded layer on the diffusivity, the thickness of the graded layer and the SiC content were changed. In addition, the specific surface areas of the SiC/C materials have been measured. It is shown that the effect of the SiC/C graded layer on thermal diffusivity was small within SiC contents (0.27-8.52 mass%) used in this study. 

Regarding the nature of the active surface composition of the bare MCP s, detailed investigations have been made by Panitz et al. (13) and Siddiqui (20). These show that, although the MCP faces are coated with an electrode material (Ni, Cr, nichrome), there is a thin (100 A) top surface layer rich in potassium (20) that has been transported up from the underlying glass. The inner surfaces of the channels also have a surface layer which is rich in alkali (primarily K) metals (oxides), Si, and SiC>2 (13, 20). These surface layers, in addition to the composition of the bulk glass (PbO + Si02, predominantly), determine the QDE (19). Improvements in the QDE may, however, be obtained by the use of photocathode materials deposited on the MCP surface, which will be discussed later in this paper. 

Many examples of surface-surfactant interactions which promote self-assembly are known. Apart from gold-thiol monolayers which are formed because of the creation of the strong S—Au bond, other commonly studied monolayers include alkyltrichlorosilane layers on hydroxylated surfaces (such as SiC>2)6, fatty acids on metal oxide surfaces7 8 and alkyl phosphonate salts on zirconium9. 

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