Electronic properties, passive layers semiconductors

The famous Mott-Schottky relationship [25,26] in Eq. 5-21 represents a different potential-dependent surface capacitive case. This relationship was derived to express the electronic properties of passive capacitive films of constant thickness formed on metals. The methods based on the Mott-Schottky equation have been widely used as a valid tool to determine semiconductive character and dopant density of the surface films in the semiconductor industry and in corrosion studies. The change of the space-charge layer capacitance of the passive film (or space charge distribution) depends on the difference between the applied DC potential V and flat band potential V g characteristic of the surface film, where Np = concentration of donors (or acceptors) or "doping density" ( 10 - lO cm" ), and Cg = 1.6 KT C electron charge ... 

Up to this point this text has focused primarily on materials themselves and not how to produce them. A major aspect of materials science is the control of the kinetic and thermodynamic conditions under which materials are produced to yield specific properties. This chapter and the ones that follow describe some of the ways semiconductor electronic materials are created as thin films. For comparison, the most popular method of production of bulk materials was covered in Chapter 4. Bulk wafers are useful as substrates but are impractical for many applications, especially where alloys are needed. In current technology, thin films constitute most of the active and passive layers that are used in electronic devices. 

The introduction of QDs into aqueous media is usually accompanied by drastic decreases in the luminescence yields of the QDs. This effect presumably originates from the reaction of surface states with water, a process that yields surface traps for the conduction-band electrons [63]. As biorecognition events or biocat-alytic transformations require aqueous environments for their reaction medium, it is imperative to preserve the luminescence properties of QDs in aqueous systems. Methods to stabilize the fluorescence properties of semiconductor QDs in aqueous media (Figure 6.2) have included surface passivation with protective layers, such as proteins [64, 65], as well as the coating of QDs with protective silicon oxide films [66, 67] or polymer films [43, 68, 69). Alternatively, they can be coated with amphiphilic polymers, which have both a hydrophobic side chain that interacts with the organic capping layer of the QDs and a hydrophilic component, such as a poly(ethylene glycol) (PEG) backbone, for water solubility [70, 71). Such water-soluble QDs may retain up to 55% of their quantum yields upon transfer to an aqueous medium. 

Polyimides have been widely used in the advanced microelectronics industry such as passivation or stress-relief layers for high-density electronic packaging, interlayer dielectric layers for wafer-level semiconductor fabrication, or alignment layers for liquid crystals in advanced liquid crystal display devices (LCDs) owing to their outstanding thermal, mechanical and good insulation properties with low dielectric constant, good adhesion to common substrates and superior chemical... 

Other industrial processes require that materials undergo a chemical process called passivation, which is essentially the rendering of the surface of a material inert to chemical reaction through the formation of a thin coating layer of oxide, nitride, or some other suitable chemical form. With its ability to accurately measure the thickness and properties of thin films such as oxide layers on a surface, electron spectroscopy is uniquely appropriate to use in industries that rely on passivation or on the formation of thin layers with specific properties. One such industry is the semiconductor industry, upon which the computer and digital electronics fields have been buUt. AES and XPS are commonly used to monitor the quality and properties of thin layers of semiconductor materials used to construct computer chips and other integrated circuits. 

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