Impurity atom

MgO can dilute ions of similar size, as for instance Ni or Co forming NiO-MgO and CoO-MgO solid solutions with an infinite range of composition. 

The effect of progressively replacing Mg by Ni or similar cations (Co, Cu ) on the surface properties has been investigated both experimentally [111, 163,164] and theoretically [165,166]. The presence of Ni + cations diluted in the MgO matrix results in an efficient catalyst for nitrous oxide, N2O, decomposition probably because of the different bond strength of the Ni-O and Mg-O bonds at the surface [163]. Also NO, which on pure MgO is weakly bound [167], on Ni-doped MgO forms relatively strong bonds with the Ni + cations [168]. 

Less is known about anion substitution. A recent study of the exchange reaction on MgO has been reported [169]. The reaction involves adsorption of CS2 on MgO powders and the subsequent exchange reaction with formation of COS and ions located at the low-coordinated sites. The basicity of the MgO surface doped with sulfur ions is drastically modified with respect to that of pure MgO [169]. The substitution of low-coordinated O anions of the MgO surface with sulfur anions completely suppresses the chemical activity of the sample. 

Another mechanism to generate O radicals on the MgO surface implies the creation of color centers and then their bleaching with N2O  

Clearly, the presence of species at the surface of MgO is not disconnected from the existence of other defects, low-coordinated anions or O vacancies. In fact, the complex interconversion of one center into another one is one of the reasons for the difficult identification of defect centers on oxide surfaces. 

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Several factors detennine how efficient impurity atoms will be in altering the electronic properties of a semiconductor. For example, the size of the band gap, the shape of the energy bands near the gap and the ability of the valence electrons to screen the impurity atom are all important. The process of adding controlled impurity atoms to semiconductors is called doping. The ability to produce well defined doping levels in semiconductors is one reason for the revolutionary developments in the construction of solid-state electronic devices. 

After some typical time, r, the electron will scatter off a lattice imperfection. This imperfection might be a lattice vibration or an impurity atom. If one assumes that no memory of the event resides after the scattering... 

Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

It is often necessary to iatroduce dopant atoms iato the epitaxial (epi) layers. Typically, the dopant sources are hydrides (qv) of the impurity atoms. Common dopants are boron hydride, ie, diborane(6) [19287-45-7] 2 6 p-ty e dopiag, and arsiae [7784-42-17, AsH, and phosphoms hydrides for n-ty e dopiag (11). For example ... 

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 impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

Unfortunately, both EEEM and EIM microscopes require a conducting sample, usually metaUic, capable of being fashioned into a very tine point. The microscopes are used for study of crystal defects, purity, and, with EIM, the identification of single impurity atoms. 

Impurity atoms having an ionic radius greater than that of silicon cause lattice expansion. 

Instead of depending on the thermally generated carriers just described (intrinsic conduction), it is also possible to deUberately incorporate various impurity atoms into the sihcon lattice that ionize at relatively low temperatures and provide either free holes or electrons. In particular. Group 13 (IIIA) elements n-type dopants) supply electrons and Group 15 (VA) elements (p-type dopants) supply holes. Over the normal doping range, one impurity atom supphes one hole or one electron. Of these elements, boron (p-type), and phosphoms, arsenic, and antimony (n-type) are most commonly used. When... 

A hst of some impurity semiconductors is given in Table 5. Because impurity atoms introduce new localized energy levels for electrons that are intermediate between the valence and conduction bands, impurities strongly influence the properties of semiconductors. If the new energy levels are unoccupied and He close to the top of the valence band, electrons are easily excited out of the filled band into the new acceptor levels, leaving electron holes... 

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. 

Flere Cl is the concentration of impurity in the liquid and I is the zone length. All concentrations are given in units of impurity atoms per unit volume. 

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-... 

One of the most fascinating applications of channeling RBS is the study of lattice locations of impurity atoms. By measuring the angular dependence of the back-scattering yield of the impurity and host atoms around three independent channeling axes it is possible to calculate the position of the impurity. Details can be found elsewhere [3.122]. 

A second doping method is the substitution of an impurity atom with a different valence state for a carbon atom on the surface of a fullerene molecule. Because of the small carbon-carbon distance in fullerenes (1.44A), the only species that can be expected to substitute for a carbon atom in the cage is boron. There has also been some discussion of the possibility of nitrogen doping, which might be facilitated by the curvature of the fullerene shell. However, substitutional doping has not been widely used in practice [21]. 

Table I. shows the calculated results of dilate limit. It is obvious that the nea.rest-neighlror interactions are of major importance. The fundamental characteristic features of ordering systems such as PdV, NiV, and NiAl are explained by the large positive value of the NN IE s positive means repulsion between the two impurity atoms. The continuous solid solubilities of CuNi a.nd AgPd correspond to the small values of the IE s. ...

Result by R.Zeller and co workers, quoted in ref.2, referring to a calculation in which all charges remain fixed except those of the impurity atom. 

Similar calculations were carried out for the single impurity systems, niobium in Cu, vanadium in Cu, cobalt in Cu, titanium in Cu and nickel in Cu. In each of these systems the scattering parameters for the impurity atom (Nb, V, Co, Ti or Ni) were obtained from a self consistent calculation of pure Nb, pure V, pure Co, pure Ti or pure Ni respectively, each one of the impurities assumed on an fee lattice with the pure Cu lattice constant. The intersection between the calculated variation of Q(A) versus A (for each impurity system) with the one describing the charge Qi versus the shift SVi according to eqn.(l) estimates the charge flow from or towards the impurity cell.The results are presented in Table 2 and are compared with those from Ref.lc. A similar approach was also found succesful for the case of a substitutional Cu impurity in a Ni host as shown in Table 2. 

The hardness and strength of alloys can be explained in terms of bonding. The impurity atoms added may form localized and rigid bonds. These tend to prevent the slippage of atoms past each other, which results in a loss of malleability and an increase in hardness. 

Graphite, the most important component of the lead of pencils, is a black, lustrous, electrically conducting solid that vaporizes at 1700°C. It consists of flat sheets of sp2 hybridized carbon atoms bonded covalently into hexagons like chicken wire (Fig. 5.22). There are also weak bonds between the sheets. In the commercially available forms of graphite, there are many impurity atoms trapped between the sheets these atoms weaken the already weak intersheet bonds and let... 

FIGURE 5.22 Graphite consists of layers of hexagonal rings of sp2 hybridized carbon atoms. The slipperiness of graphite results from the ease with which the layers can slide over one another when there are impurity atoms Iving between the planes. 

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