domingo, 21 de marzo de 2010

defectos

samer elatrache v-17810600 estudiante de CRF


The picture of a perfect crystal structure repeating a particular geometric
pattern of atoms without interruption or mistake is somewhat exaggerated.
Although there are materials-carefully grown silicon single crystals, for
example-that have virtually perfect crystallographic structures extending over
macroscopic dimensions, this is not generally true in bulk materials. In thin
crystalline films the presence of defects not only serves to disrupt the geometric
regularity of the lattice on a microscopic level, it also significantly
influences many film properties, such as chemical reactivity, electrical conduction,
and mechanical behavior. The structural defects briefly considered in this
section are grain boundaries, dislocations, and vacancies.
Grain Boundaries
Grain boundaries are surface or area defects that constitute the interface
between two single-crystal grains of different crystallographic orientation. The
atomic bonding, in particular grains, terminates at the grain boundary where
more loosely bound atoms prevail. Like atoms on surfaces, they are necessarily
more energetic than those within the grain interior. This causes the grain
boundary to be a heterogeneous region where various atomic reactions and
processes, such as solid-state diffusion and phase transformation, precipitation,
corrosion, impurity segregation, and mechanical relaxation, are favored or
accelerated. In addition, electronic transport in metals is impeded through
increased scattering at grain boundaries, which also serve as charge recombination
centers in semiconductors. Grain sizes in films are typically from 0.01 to
1.0 pm in dimension and are smaller, by a factor of more than 100, than
common grain sizes in bulk materials. For this reason, thin films tend to be
more reactive than their bulk counterparts. The fraction of atoms associated
with grain boundaries is approximately 2 a / I , where a is the atomic dimension
and 1 is the grain size. For 1 = loo0 A, this corresponds to about 5 in 1OOO.
Grain morphology and orientation in addition to size control are not only
important objectives in bulk materials but are quite important in thin-film
technology. Indeed a major goal in microelectronic applications is to eliminate
grain boundaries altogether through epitaxial growth of single-crystal semiconductor
films onto oriented single-crystal substrates. Many special techniques
involving physical and chemical vapor deposition methods are employed in this
effort, which continues to be a major focus of activity in semiconductor
technology.

Dislocations
Dislocations are line defects that bear a definite crystallographic relationship to
the lattice. The two fundamental types of dislocations-the edge and the screw
-are shown in Fig. 1-6 and are represented by the symbol I . The edge
dislocation results from wedging in an extra row of atoms; the screw dislocation
requires cutting followed by shearing of the perfect crystal lattice. The
geometry of a crystal containing a dislocation is such that when a simple closed
traverse is attempted about the crystal axis in the surrounding lattice, there is a
closure failure; i.e., one finally amves at a lattice site displaced from the
starting position by a lattice vector, the so-called Burgers vector b. The
individual cubic cells representing the original undeformed crystal lattice are
now distorted somewhat in the presence of dislocations. Therefore, even
without application of external forces on the crystal, a state of internal stress
exists around each dislocation. Furthermore, the stresses differ around edge
and screw dislocations because the lattice distortions differ. Close to the
dislocation axis the stresses are high, but they fall off with distance ( r )
according to a 1 / r dependence.
Dislocations are important because they have provided a model to help
explain a great variety of mechanical phenomena and properties in all classes
of crystalline solids. An early application was the important process of plastic
deformation, which occurs after a material is loaded beyond its limit of elastic
response. In the plastic range, specific planes shear in specific directions
relative to each other much as a deck of cards shear from a rectangular prism
to a parallelepiped. Rather than have rows of atoms undergo a rigid group
displacement to produce the slip offset step at the surface, the same amount of
plastic deformation can be achieved with less energy expenditure. This alternative
mechanism requires that dislocations undulate through the crystal, making
and breaking bonds on the slip plane until a slip step is produced, as shown in
Fig. 1-7a. Dislocations thus help explain why metals are weak and can be
deformed at low stress levels. Paradoxically, dislocations can also explain why
metals work-harden or get stronger when they are deformed. These explanations
require the presence of dislocations in great profusion. In fact, a density
of as many 10l2 dislocation lines threading 1 cm2 of surface area has been
observed in highly deformed metals. Many deposited polycrystalline metal thin
films also have high dislocation densities. Some dislocations are stacked
vertically, giving rise to so-called small-angle grain boundaries (Fig. 1-7b).
The superposition of externally applied forces and internal stress fields of
individual or groups of dislocations, arrayed in a complex three-dimensional
network, sometimes makes it more difficult for them to move and for the
lattice to deform easily.
The role dislocations play in thin films is varied. As an example, consider
the deposition of atoms onto a single-crystal substrate in order to grow an
epitaxial single-crystal film. If the lattice parameter in the film and substrate
differ, then some geometric accommodation in bonding may be required at the
interface, resulting in the formation of interfacial dislocations. The latter are
unwelcome defects particularly if films of high crystalline perfection are
required. For this reason, a good match of lattice parameters is sought for
epitaxial growth. Substrate steps and dislocations should also be eliminated
where possible prior to growth. If the substrate has screw dislocations emerging
normal to the surface, depositing atoms may perpetuate the extension of the
dislocation spiral into the growing film. Like grain boundaries in semiconductors,
dislocations can be sites of charge recombination or generation as a result
of uncompensated "dangling bonds. " Film stress, thermally induced mechanical
relaxation processes, and diffusion in films are all influenced by dislocations.

Vacancies
The last type of defect considered is the vacancy. Vacancies are point defects
that simply arise when lattice sites are unoccupied by atoms. Vacancies form
because the energy required to remove atoms from interior sites and place
them on the surface is not particularly high. This low energy, coupled with the
increase in the statistical entropy of mixing vacancies among lattice sites, gives
rise to a thermodynamic probability that an appreciable number of vacancies
will exist, at least at elevated temperature. The fraction f of total sites that will
be unoccupied as a function of temperature T is predicted to be approximately
reflecting the statistical thermodynamic nature of vacancy formation. Noting
that k is the gas constant and is typically 1 eV/atom gives f = lop5 at
loo0 K.
Vacancies are to be contrasted with dislocations, which are not thermodynamic
defects. Because dislocation lines are oriented along specific crystallographic
directions, their statistical entropy is low. Coupled with a high
formation energy due to the many atoms involved, thermodynamics would
predict a dislocation content of less than one per crystal. Thus, although it is
possible to create a solid devoid of dislocations, it is impossible to eliminate
vacancies.
Vacancies play an important role in all processes related to solid-state
diffusion, including recrystallization, grain growth, sintering, and phase transformations.
In semiconductors, vacancies are electrically neutral as well as
charged and can be associated with dopant atoms. This leads to a variety of
normal and anomalous diffusional doping effects

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