Metallic impurities, incorporation surface

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. 

Careful specification of purity, unallowable impvuities, fabrication method, post-fabrication treatments, packaging, etc. of the source materials purchased can be important in obtaining a reproducible process. Using inexpensive material or material of unknown origin often creates problems. Often impurities such as O, N, C, and H are not specified by the supplier and they may be present in significant quantities. Examples of unspecified impurities are oxidized surfaces of reactive metals, hydrogen incorporated in electrorefined chromium, carbon monoxide in nickel purified by the carbonyl process, and helium in natural quartz. Generally it is better to specify vacuum-melted materials from the supplier when possible. 

Metallic impurities (colloids and particle) are of special concern because of their ability to be readily incorporated in exposed areas by substitution in the crystal lattice, or by being chemisorbed to active surfaces. Wafer surfaces are especially vulnerable to chemisorption during the etching and cleaning processes. Acids utilized for these processes continually generate chemically active and reactive surfaces. 

Flaws in the anodic oxide film are usually the primary source of electronic conduction. These flaws are either stmctural or chemical in nature. The stmctural flaws include thermal crystalline oxide, nitrides, carbides, inclusion of foreign phases, and oxide recrystaUi2ed by an appHed electric field. The roughness of the tantalum surface affects the electronic conduction and should be classified as a stmctural flaw (58) the correlation between electronic conduction and roughness, however, was not observed (59). Chemical impurities arise from metals alloyed with the tantalum, inclusions in the oxide of material from the formation electrolyte, and impurities on the surface of the tantalum substrate that are incorporated in the oxide during formation. 

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