Host material impurities, doped semiconductor

Semiconductors in nano-crystallized form exhibit markedly different electrical, optical and structural properties as compared to those in the bulk form [1-10]. Out of these, the ones suited as phosphor host material show considerable size dependent luminescence properties when an impurity is doped in a quantum-confined structure. The impurity incorporation transfers the dominant recombination route from the surface states to impurity states. If the impurity-induced transition can be localized as in the case of the transition metals or the rare earth elements, the radiative efficiency of the impurity- induced emission increases significantly. The emission and decay characteristics of the phosphors are, therefore, modified in nanocrystallized form. Also, the continuous shift of the absorption edge to higher energy due to quantum confinement effect, imparts these materials a degree of tailorability. Obviously, all these attributes of a doped nanocrystalline phosphor material are very attractive for optoelectronic device applications. 

Semiconductors may also be made from a material which is normally an insulator by introducing an impurity, a process known as doping. Figure 9.9 shows two ways in which an impurity may promote semiconducting properties. In Figure 9.9(a) the dopant has one more valence electron per atom than the host and contributes a band of filled impurity levels I close to the conduction band of the host. This characterizes an n-type semiconductor. An example is silicon (KL3s23p2) doped with phosphorus KL3s13pi), which reduces the band gap to about 0.05 eV Since kT at room temperature is about 0.025 cV, the phosphorus... 

Doping at these minute levels means that unwanted impurities in the host semiconductor must be reduced to very low levels 99.999 9 to 99.999 999% pure materials are often required. 

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