Exciton isoelectronic bound

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. 

Edge emission is due to exciton recombination (Sect. 3.3.1). Usually this emission is due to bound excitons, i.e. an exciton of which either the electron or the hole is trapped at an imperfection in the lattice. The elucidation of the nature of this imperfection is often a difficult task. As an example of such an emission we can mention the exciton emission of GaP N. Nitrogen is an isoelectronic dopant (on phosphorous sites). The exciton is bound to this nitrogen impurity before it decays. The emission is situated at about 0.02 eV below Ej,. Another semiconductor for which exciton emission has been thoroughly studied is CdS. 

In semiconductors containing isoelectronic centres with an attracting potential for electrons or holes mentioned in Sect. 1.3.2, free excitons can be trapped because of the preferential interaction of these centres with the electron (or hole) part of the exciton. The hole (resp. electron) part of the exciton is then comparable to a hole (resp. electron) bound to a neg-... 

The value of the Ag (D) level deduced from E 0 is in very good agreement with the value Ev + 0.34 eV, obtained from DLTS measurements [12]. Lines A, B and C have also been observed in PL experiments and Ag-isotope effects have been identified it has been pointed out that the decay time of these lines is consistent with their attribution to an exciton bound to an isoelectronic... 

This seems to be also valid for excitons bound to deep neutral centres not necessarily isoelectronic, giving pseudo-donors or pseudo-acceptors. 

Fig. 7.20. Spectrum of the recombination of the exciton bound to the isoelectronic (Bes, Be ) centre in silicon showing line A, B, and B. The inset shows a schematic energy diagram for the IBE (see text). The additional magnetic-field-induced B line is indicated by a broken line (after [94]). Reproduced with permission from the...

TABLE 6 Parameters Relating to Excitons Bound to Isoelectronic Impurities in Silver Halides3... 

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