Impurity atoms, diffusion

Chemical effects also occur in crystalline oxides that is, impurity atoms diffuse at varying rates through oxides. In all cases, cation diffusion is much faster than oxygen diffusion, but similar cations, such as Ca and Mg, can behave very differently in different oxides. This is due to differences in chemical potential and activity, and mobility differences. 

The new phases are characterized by considerable volume changes in relation to the basic material. The stresses occur on the interphase boundaries, and microcracks are formed. The example of such damage is metal embrittlement when forming hydride phases. The internal stresses also have an effect on kinetics of a new phase growth. Let us consider the residual stresses in a hollow cylinder. Maximal concentration of the impurity atoms occurs on the area boundary, where the new phase formation takes place. Its further growth is realized at the expense of impurity atoms diffusion. The task of defining kinetics of the new phase growth in the hollow cylinder is mathematically formulated as follows... 

It was concluded that Eu impurity atoms diffused predominantly via lattice sites. VA.Didik, E.A.Skoryatina, V.P.Usacheva, A.V.Golubkov, V.V.Kaminskil Technical Physics Letters, 2004, 30[9], 756-8... 

Theoretical studies of diffusion aim to predict the distribution profile of an exposed substrate given the known process parameters of concentration, temperature, crystal orientation, dopant properties, etc. On an atomic level, diffusion of a dopant in a siUcon crystal is caused by the movement of the introduced element that is allowed by the available vacancies or defects in the crystal. Both host atoms and impurity atoms can enter vacancies. Movement of a host atom from one lattice site to a vacancy is called self-diffusion. The same movement by a dopant is called impurity diffusion. If an atom does not form a covalent bond with siUcon, the atom can occupy in interstitial site and then subsequently displace a lattice-site atom. This latter movement is beheved to be the dominant mechanism for diffusion of the common dopant atoms, P, B, As, and Sb (26). 

The technology of silicon and germanium production has developed rapidly, and knowledge of die self-diffusion properties of diese elements, and of impurity atoms has become reasonably accurate despite die experimental difficulties associated widi die measurements. These arise from die chemical affinity of diese elements for oxygen, and from die low values of die diffusion coefficients. 

The heart of the energy-dispersive spectrometer is a diode made from a silicon crystal with lithium atoms diffiised, or drifted, from one end into the matrix. The lithium atoms are used to compensate the relatively low concentration of grown-in impurity atoms by neutralizing them. In the diffusion process, the central core of the silicon will become intrinsic, but the end away from the lithium will remain p-type and the lithium end will be n-type. The result is a p-i-n diode. (Both lithium-... 

UHV is necessary but not sufficient to ensure an uncontaminated surface. Certainly, the surface will not be contaminated by atoms arriving from the vacuum space, but such contamination as it had before the vacuum was formed has to be removed by bombardment with argon ions. This damages the surface structurally, and that has to be healed by in situ heat treatment. That, however, allows dissolved impurities to diffuse to the surface and cause contamination from below. This problem has to be dealt with by many cycles of bombardment and annealing, until the internal contaminants are exhausted. This is a convincing example of Murphy s Law in action one of the many corollaries of the Law is that new systems generate new problems . 

The diffusion of an impurity atom in a crystal, say K in NaCl, involves other considerations that influence diffusion. In such cases, the probability that the impurity will exchange with the vacancy will depend on factors such as the relative sizes of the impurity compared to the host atoms. In the case of ionic movement, the charge on the diffusing species will also play a part. These factors can also be included in a random-walk analysis by including jump probabilities of the host and impurity atoms and vacancies, all of which are likely to vary from one impurity to another and from one crystal structure to another. All of these alterations can be... 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

Diffusion in general, not only in the case of thin films, is a thermodynamically irreversible self-driven process. It is best defined in simple terms, such as the tendency of two gases to mix when separated by a porous partition. It drives toward an equilibrium maximum-entropy state of a system. It does so by eliminating concentration gradients of, for example, impurity atoms or vacancies in a solid or between physically connected thin films. In the case of two gases separated by a porous partition, it leads eventually to perfect mixing of the two. 

H is the activation energy for volume diffusion of the impurity atoms, P = CO1/CO2. 

Fig. 4.22 On the surface of a solid, there are a wide variety of atomic processes. A formation of a surface vacancy-adatom pair, or their recombination B association or dissociation of adatoms with an atomic cluster and cluster diffusion C diffusion of a surface vacancy, especially toward the lattice step D falling off a lattice step of an adatom E diffusion of a substitutional or interstitial impurity atom and its interaction with an adatom F diffusion of an adatom and its long range interactions with other adatoms G diffusion, dissociation and activation of a ledge atom H dissociation and activation of a kink atom into an adatom, a ledge atom, or an adatom on the layer above.

As an example of adatom-lattice defect interactions on a surface, we will describe here a study of the interaction between an adatom and an impurity atom in a surface layer. An impurity atom, like any other lattice defect, in the surface layer can perturb the periodicity of the surface both electronically and elastically. Such a perturbation will change the potential energy of a diffusing adatom on the surface. A study of adatom-... 

If adatom-impurity atom interaction is attractive, then the impurity atom can act as a trapping center. A diffusing adatom may be trapped. In heterogeneous catalysis, the reaction rate may be changed by the trapping effect of impurities as also by lattice defects and lattice steps and so on. 

In crystal growth by vapor condensation, an attractive impurity atom may act as a nucleation center by trapping diffusing adatoms when the number of trapped adatoms reaches a critical number. 

Despite a possible wide significance of such a topic, there is only one reported study of adatom-substitutional impurity atom interaction, where the interaction of a W adatom with substitutional Re atoms in a W lattice is studied by using a W-3% Re alloy as the substrate.182 The planes used in FIM studies of adatom behavior are usually quite small containing only a few hundred atoms. Thus a plane of a W-3% Re alloy is likely to contain a few Re substitutional atoms. The perturbation to the overall electronic and elastic properties of the substrate lattice should still be relatively small. Therefore the interaction of a single substitutional impurity atom with a diffusing adatom can be investigated. 

Energy levels corresponding to electrons localized near the surface may also be present. These are termed surface states. For example, adsorbed ions are one type of surface state they may be of the form of donors, such as hydrogen, which yield electrons to the material, or in the form of acceptors, such as oxygen, which accept, or trap electrons from the material. In Fig. 1, surface traps of the acceptor type are shown, and it is indicated that there are two possible levels present. Other possibilities are impurity atoms at the surface, which are introduced in the preparation of the sample or diffuse from the interior during heat treatment, or nonstoichiometry of the surface layers of the compound. Surface... 

Extrinsic Crystal Self-Diffusion. Charged point defects can be induced to form in an ionic solid by the addition of substitutional cations or anions with charges that differ from those in the host crystal. Electrical neutrality demands that each addition results in the formation of defects of opposite charge that can contribute to the diffusivity or electronic conductivity. The addition of aliovalent solute (impurity) atoms to an initially pure ionic solid therefore creates extrinsic defects.10... 

The curve separating displaced and undisplaced positions of a crystal, and thus at the center of a dislocation, is lermed the dislocation line. Within a distance of one or two lattice constants of Ihe dislocalion line the aloins are displaced by an amount more than can be represented fairly as a strain A screw location may have a substantial hole down the dislocation line, through which impurity atoms may diffuse. 

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