Impure Material with Dopant

When CaCl2 is added to NaCl, as an example, Ca ions go into Na sites following the equation 

with the addition of CaCl2, [Vjqg and [C/ ] increase and and [IVa ] decrease according to the equation for Schottky defect formation (Eq. (12.4)) and the equations for cation and anion vacancy formation (Eqs (13.14) and 

In discussing the defect formation and distribution so far, we assumed, for simplicity, that the interface acts as a perfect source and sink of vacancies and atoms, and provides an infinite number of sites for ions with a charge opposite to the excess charge in the space charge region. However, this assumption is too simple to quantitatively describe the real phenomena. In reality, the number of sites for ions at the interface is limited and the ions at the interface interact with 

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Semiconductors (qv) are materials with resistivities between those of conductors and those of insulators (between 10 and 10 H-cm). The electrical properties of a semiconductor determine the hmctional performance of the device. Important electrical properties of semiconductors are resistivity and dielectric constant. The resistivity of a semiconductor can be varied by introducing small amounts of material impurities or dopants. Through proper material doping, electron movement can be precisely controlled, producing hmctions such as rectification, switching, detection, and modulation. 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

These detectors require a material with an impurity dopant ionization energy corresponding to the wavelength to be detected to satisfy condition 1. Conditions 2 and 3 are given by (4.43) and (4.44), except that here there is only one type of carrier affecting detector performance, and in (4.43) represents the... 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

The upshot of all this research since 1954 is rather modest, with the exception of the GE research, which indicates that techniques and individual materials have to be married up an approach which is crucial for one material may not be very productive for another. This is of course not to say that this 40-year programme of research was wasted. The initial presumption of the potential value of ultra-pure metals was reasonable it is the obverse of the well-established principle that minor impurities and dopants can have major effects on the properties of metals. 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

The activation energy Ea - defined as Ec - Ey for the conduction band (and analogously for the valence band), can be used to assess the presence of impurities. Due to their presence, either intentional (B or P dopant atoms) or unintentional (O or N), the Fermi level shifts several tenths of an electron volt towards the conduction or the valence band. The activation energy is determined from plots of logafT) versus 1/7, with 50 < 7 < 160°C. For undoped material Ea is about 0.8 eV. The Fermi level is at midgap position, as typically Eg is around 1.6 eV. 

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