Impurities and defects

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

Common teniiinology used to characterize impurities and defects in semiconductors includes point and line defects, complexes, precipitates and extended defects. These teniis are somewhat loosely defined, and examples follow. 

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

Nondestructive qualitative and quantitative analysis of impurities and defects, and their distributions in luminescent materials... 

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

Photoluminescence is a well-established and widely practiced tool for materials analysis. In the context of surface and microanalysis, PL is applied mostly qualitatively or semiquantitatively to exploit the correlation between the structure and composition of a material system and its electronic states and their lifetimes, and to identify the presence and type of trace chemicals, impurities, and defects. 

E J. Dean. Prog. Crystal Growth Charact. 5i 89, 1982. A review of PL as a diagnosdc probe of impurities and defects in semiconductors by an important progenitor of the technique. 

These new systems are not accurately encompassed by the true definition of CVD, but will be considered here as they are an important step in the evolution of thin-fllm deposition techniques. Using these new delivery methods the stability of the precursor, and its reproducible behavior with respect to the method of delivery, is fundamentally more important than its volatility. Moreover, the clean deposition of the precursor and the subsequent low levels of impurities and defects within the final material can now be considered of greater importance than volatility when choosing or designing a precursor compound. 

Hydrogen Passivation of Deep Level Impurities and Defects 372... 

A. Mandelis, A. Budiman and M. Vargas, Photothermal Deep-Level Transient Spectroscopy of Impurities and Defects in Semiconductors... 

For a simple system such as 2-propanol, which decomposes to acetone, with no appreciable chain reactions, the QY reaches a plateau at very low light intensity.74) The precise value depends sensitively upon the characteristics of the particular film, probably depending upon the presence of trace impurities and defects, which could act as recombination sites. For a system in which there are radical chain reactions, such as acetaldehyde, QY can continue to increase, even at very low lnorm values, and can in principle exceed unity.75)... 

Large grains, low amounts of impurities and defects and solid solutions Large, needle like grains, composites with SiC, or refractory silicides Large, needle-like grains, composites with SiC, or refractory silicides... 

INFLUENCE OF IMPURITIES AND DEFECTS ON ELECTRONIC STRUCTURE OF CARBON NANOTUBES... 

The investigation of impurities and defects, their energetic and interactions is seemed to be very important because appearance even one single defect can change not only value but also the type of nanotubes conductivity. 

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