Native and Chemical Oxides

An oxide layer of about 1 nm thickness is present on a silicon wafer as received from the supplier. This oxide is called a native oxide and forms on every bare silicon surface exposed to ambient air. A bare silicon surface can be generated, for example, by cleaving a silicon crystal in high vacuum. 

The formation of a chemical oxide in pure DI water was found to depend critically on the DOC of the water [Gr3, Mol, Li9]. In DI water of very low DOC ( 0.004 ppm), no native oxide forms. Furthermore the reverse reaction is observed for elevated temperatures (80 °C) and long etching times (60 min) thin native or chemical oxides are removed and a hydrogen-terminated surface is established [Wa2]. 

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Under ambient atmospheric conditions a native oxide is formed on cleaved Si surfaces. The properties of native and chemical oxides are discussed in Section 5.2. The well-defined surface conditions produced by wet processes like rinsing and cleaning procedures will be discussed below. 

To understand the electrochemical behavior of silicon, however, the formation and the properties of anodic oxides are important The formation of an anodic oxide on silicon electrodes in HF and HF-free electrolytes will therefore be discussed in detail in this chapter. The formation of native and chemical oxides is closely related to the electrochemical formation process and will be reviewed briefly. The anodic oxidation of porous silicon layers is closely related to the morphology and the luminescent properties of this material and is therefore discussed in Section 7.6. 

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