Electron donor impurity centres

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 presence of the region of weak dependence of the conductivity of alloyed semiconductors on temperature can be explained by tunneling of electrons from one impurity centre to another, unoccupied centre. The necessary condition of the impurity conductivity is the partial filling of the impurity levels. At low temperatures this conduction can be maintained only by semiconductor compensation, i.e. by the simultaneous presence of donor and acceptor impurities. In the case, for instance, of the n-type semiconduc-... 

Fig. 22. Energy levels for the compensated semiconductor of the n-type. Charge transfer is carried out by means of tunneling from an occupied donor level to a vacant donor level. The presence or the absence of the point designated "e indicates the presence or absence of the electron on the impurity centre.

Extrinsic semiconductors are materials containing foreign atoms (FAs) or atomic impurity centres that can release electrons in the CB or trap an electron from the VB with energies smaller than Eg (from neutrality conservation, trapping an electron from the VB is equivalent to the release of a positive hole in the otherwise filled band). These centres can be inadvertently present in the material or introduced deliberately by doping, and, as intrinsic, the term extrinsic refers to the electrical conductivity of such materials. The electron-releasing entities are called donors and the electron-accepting ones acceptors. When a majority of the impurities or dopants in a material is of... 

In a semiconductor, substitutional FAs from the same column of the periodic table as the one of the crystal atom they replace are usually electrically inactive and they are called isoelectronic with respect to the semiconductor. It can occur, however, that for some isoelectronic impurities or electrically-inactive complexes, the combination of the atomic potential at the impurity centre with the potential produced by the local lattice distortion produces an overall electron- or hole-attractive potential in a given semiconductor. This potential can bind an electron or a hole to the centre with energies much larger than those for shallow electrically-active acceptors or donors. The interaction of these isoelectronic impurities traps the free excitons producing isoelectronic bound excitons which display pseudo-donor or pseudo-acceptor properties. This is discussed later in this chapter in connection with the bound excitons, and examples of these centres are given in Chaps. 6 and 7. 

It was mentioned in Sect. 1.3.2 that in semiconductors, isoelectronic impurity centres could present a relatively strong attracting potential for electrons or holes. Excitons can be trapped by or created at these isoelectronic centres to form an isoelectronic bound exciton (IBE). The electron (hole) of this exci-ton is also more strongly bound to the isoelectronic centre than in classical excitons and the second constituent of the exciton, hole (electron) can be considered to be bound to a compound negative or positive ion. These structures are similar to those of neutral donors or acceptors and they are called isoelectronic donors or acceptors [104]. When formed by near band-gap or above band-gap laser illumination, the long lifetimes of these IBEs result in sharp PL lines, and this has for some time aroused interest for these centres as potential near IR radiation emitters. 

If the film contains a high density of impurity states that act as electron donors or acceptors, the space-charge region may become thinner than the film itself. A more general expression for the photocurrent coversion efficiency that takes this possibility into account can be derived if the simplifying assumption is made that the diffusion length of minority carriers is likely to be small in polycrystalline or amorphous films that contain high densities of recombination centres... 

Impurity conduction can also be studied in compensated semiconductors, i.e. materials containing acceptors as well as donors, the majority carriers (or the other way round). For such materials, even at low concentrations, activated hopping conduction can occur (Chapter 1, Section 15), some of the donors being unoccupied so that an electron can move from an occupied to an empty centre. Here too a metal-insulator transition can be observed, which is certainly of Anderson type, the insulating state being essentially a result of disorder. 

The compensation of impurities is an equilibrium process resulting from the minimization of the electronic energy in the crystals. Thus, under equilibrium conditions at low temperature, donors or acceptors can be either neutral (D° or A0) or ionized (I) 1 or A ). In weakly-compensated materials, the out-of-equilibrium partial photoionization of donors in n-type materials or of acceptors in p-type materials produces photoelectrons or photoholes. At very low temperature, these photocarriers can then be trapped by neutral donors or acceptors to produce D or A+ ions. These centres are equivalents of the H ion and they are introduced in Sect. 1.3.3. 

Most impurity and defect states in SiC can be considered as deep levels. Both capacitance and admittance spectroscopy provide data on these deep levels which can act as donor or acceptor traps. Bulk 6H-SiC contains intrinsic defects which are thermally stable, up to 1700 °C. In epitaxial films of 6H-SiC a deep acceptor level is seen in boron-implanted samples but not when other impurities are implanted. Other centres, acting as electron traps, are also seen in p-n junction and Schottky barrier structures. Irradiation of 6H-SiC produces 6 deep levels, reducing to 2 after annealing. Only limited studies have been carried out on the 3C-SiC polytype, in the form of epitaxial films on silicon substrates. No levels were seen in thick films but electron traps were seen in thin n-type films and a hole trap (structural defect) was found to be a mobility killer. Neutron irradiation produces defects most of which can be removed by annealing. Two levels were found in Al-implanted 4H-SiC. 

---