Impurity: also donor

We first note that an isolated atom with an odd number of electrons will necessarily have a magnetic moment. In this book we discuss mainly moments on impurity centres (donors) in semiconductors, which carry one electron, and also the d-shells of transitional-metal ions in compounds, which often carry several In the latter case coupling by Hund s rule will line up all the spins parallel to one another, unless prevented from doing so by crystal-field splitting. Hund s-rule coupling arises because, if a pair of electrons in different orbital states have an antisymmetrical orbital wave function, this wave function vanishes where r12=0 and so the positive contribution to the energy from the term e2/r12 is less than for the symmetrical state. The antisymmetrical orbital state implies a symmetrical spin state, and thus parallel spins and a spin triplet. The two-electron orbital functions of electrons in states with one-electron wave functions a(x) and b(x) are, to first order,... 

The spectacular success of the semiconductor industry is based on the production of materials selectively designed for specialized applications in electronic and optical devices. By carefully controlled doping of semiconductors with selected impurities—electron donors or electron acceptors—the conductivity and other properties can be modulated with great precision. Fig. 12.8 shows schematically how doped semiconductors work. In an intrinsic semiconductor (a), conducting electron-hole pairs can only by produced by thermal or photoexcitation across the band gap. In (b), addition of a small concentration of an electron donor creates an impurity band just below the conduction band. Electrons can then Jump across a much-reduced gap to the conduction band and act as negatively-charged current carriers. This produces a n-type semiconductor. In (c), an electron acceptor creates an empty impurity band just above the valence band. In this case electrons can jump from the valence band to leave positive holes. These can also conduct electricity, since electrons falling into positive holes create new holes, a sequence... 

Ab-initio calculations were made on combinations of Ge and As, with As in the bulk and with As on the surface. The local softness, both for accepting electrons and donating electrons, was calculated. It was found that arsenic, a soft impurity (shallow donor) did congregate at the surface. A hard impurity (deep donor) was predicted not to. Also it was predicted that gallium (a soft acceptor) would not segregate at the grain boundaries. The PMH was obeyed for each kind of behavior. 

Acceptor An impurity (dopant) that decreases the number of free electrons in the material. See also Donor. 

Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

Molinari and Parravano (30) have also noted that the incorporation of a donor impurity (Al, Ga) into ZnO specimens promotes the exchange reaction, while an acceptor impurity (Li) slows it down. 

The growth of the catalytic activity of Si02 with respect to the hydrogen-deuterium exchange reaction upon addition of a donor impurity to specimens has also been observed by Taylor and his colloborators (31). 

Thus, Romero-Rossi and Stone (11) have found that the effect is enhanced on ZnO when an acceptor impurity (Li) is introduced into the specimen. The increase of the effect on Cu20 upon the introduction of acceptor impurities (S and Sb) has also been observed by Ritchey and Calvert (58). The addition of a donor (Cr) to ZnO, as reported (11), lowers the magnitude of the effect. 

At the same time Markham and Laidler (70) and also Veselovsky and Shub (71, 72) have shown that the photocatalytic activity of zinc oxide diminishes as a result of the calcination of specimens at high temperatures (around 1000°C) in the reduced atmosphere (such pretreatment results in an increase of the concentration of superstoichiometric zinc in the specimen). In other words, a donor impurity (zinc in excess of stoichiometry) retarded the reaction. 

Patel et al., 1974), but no effect is observed for neutral (Group IV) impurities in Ge or Si. Also, impurities that are electron-donors soften both Ge and Si at temperatures above about 450 °C whereas accepter type impurities soften Ge, but not Si. Another important point is that small impurity concentrations have little effect. The effects begin at concentrations of about 1018/cc. Since the atomic volume of Si is 20 A3, the critical ratio of impurity to Si atom is about 2 x 10 5. Therefore, the average lineal distance between impurity atoms is about one every 270 A. 

As an alternative to QDs, silicon can be doped with single atom impurities, in particular phosphorus, which acts as an electron donor. Donors can be implanted individually with a precision of about 10 nm. Either the 31P nuclear spin or the unpaired electron can be used as qubits [63, 64]. An advantage of silicon is its widespread use in current electronics, meaning that QC might profit from methods and technologies already developed for their classical cousins . Also, spins in silicon can attain extremely high coherence times experiments on 28 Si-enriched silicon show spin coherence times T2 exceeding 10 s [65]. The read-out and coherent manipulation of individual spin qubits in silicon have been recently achieved [66]. 

Most of the other metal-related deep levels in Si are also passivated by reaction with hydrogen (Pearton, 1985). Silver, for example, gives rise in general to a donor level at Ee + 0.54 eV and an acceptor level at Ec - 0.54 e V (Chen and Milnes, 1980 Milnes, 1973). These levels are very similar to those shown by Au, Co and Rh and raise the question of whether Au might actually be introduced into all of the reported samples or a contaminant, or whether as discussed by several authors there is a similar core to these impurity centers giving rise to similar electronic properties (Mesli et al., 1987 Lang et al., 1980). This problem has not been adequately decided at this time. It has been... 

Sulphur, selenium and tellurium can be incorporated into Si in a variety of forms (Grimmeiss et al., 1981 Wagner et al., 1984). As isolated ions, they are all double donors, with levels around 260 and 550 meV from the conduction band. These impurities may also be introduced as pairs, which also act as a double donors (Pensl et al., 1986). Depending on the thermal history of the Si during diffusion of S, Se and Te, they may also be incorporated as higher-order impurity complexes (Grimmeiss et al., 1981 Wagner et al., 1984). 

Impurity substitution that is effectively neutral, that is, neither donor nor acceptor, can also lead to significant changes in properties that are utilized in NTC thermistors. For example, the replacement of Ga3+ in the spinel MgGa204 by Mn3+ involves no apparent donor or acceptor action. The conductivity in the system MgGa2 Jt.MnJt.04 evolves from insulating (conductivity about 10-9 0 1m 1) for the parent phase with x = 0, to a conductivity approximately equal to that of germanium (10 10-1 m-1) in the compound MgGaMn04, in which x = 1. The resistivity decreases markedly with temperature and the compounds display typical NCT behavior. 

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