Isoelectronic centres donors

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. 

The values of the hole binding energies of the (S,Cu) isoelectronic centre are 137 and 292meV for Sa and Sb, respectively. The ID ionization energies associated with this centre (65.28 and 66.21 meV) are significantly larger than those of the pseudo-donor (C,0) complexes associated with lines C and P, and this has been attributed to the (S,Cu) centre for a central-cell potential which is also attractive for electrons, but to a lesser extent than for holes [18]. 

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. 

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

It appears that no absorption measurements of (S,Cu) in the near IR has been reported, but absorption measurements of the isoelectronic donor centres associated with Sa and Sb have been performed at lower energies under continuous photoexcitation with a Nd-YAG laser operated at 1.06 or 1.32pm (1.17 or 0.939eV) by Beckett et al. [18]. At LHeT, the creation in the triplet state is predominant and the ground state for the EM spectra is therefore, the triplet states Sa° and Sb°. The photoinduced spectrum so obtained is displayed in Fig. 6.39. 

NO shows a wide variety of coordination geometries (linear, bent, doubly bridging, triply bridging and quadruply bridging — see p. 453) and sometimes adopts more than one mode within the same complex. NO has one more electron than CO and often acts as a 3-electron donor — this is well illustrated by the following isoelectronic series of compounds in which successive replacement of CO by NO is compensated by a matching decrease in atomic number of the metal centre ... 

The chemical shift value for [ CpMo(CO)2 2(fi-PMes )] at < p=687ppm is in accord with other similar compounds like [ Co(CO)3 2()t-PMes )] at < p=664ppm, and [ CpV(CO)2 2(p-PMes )] at < p=657ppm. The two major contributions to the downfleld shift are the three-membered metallacycle, and the 7r-donor interaction of the phosphorus lone pair with both metal centres. This is not surprising, as the PMes moiety is identical in all three compounds, and the metal fragments have 15 VE (Co and Mo) and 14 VE (V), respectively. They are isoelectronic with respect to the phosphorus moiety, and thus the vanadium centres are allowed to make up their formal electron deficiency by upgrading the metal-metal single bond to a double bond. 

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