Electroluminescence Induced by Minority Carrier Injection

In solid state devices such as p-n junctions, luminescence is created by forward polarization in the dark. In such a minority carrier device, electrons move across the p-n interface into the p-type and holes into the n-type regions, where they recombine with the corresponding majority carriers. As already pointed out in Section 2.3, this kind of luminescence has not been found with semiconductor-metal junctions (Schottky junctions), as nobody has succeeded in producing a minority carrier device because of Fermi level pinning. Since the latter problem usually does not occur with semi- 

Electroluminescence was first observed with n-GaP electrodes using hole donors such as [Fe(CN)(,] in alkaline or S2O8 in acid solutions [112]. In these two cases the corresponding standard potentials occur at or even below the valence band edge (see Table in Appendix). In the case of [Fe(CN)(s] no luminescence was found in acid solutions although the current-potential curve indicates that the redox species is reduced. The differences between alkaline and acid solutions can be explained by the pH-dependence of the position of the energy bands at the surface, as shown in Fig. 7.62. Since is far below Ep.redox at pH 1 no charge transfer between the redox couple and the valence band is possible anymore, and the cathodic current is only due to an electron transfer via the conduction band. 

Meanwhile electroluminescence has been observed with several other semiconductors such as GaAs and InP. Various authors have also studied the potential dependence of the emission in more detail. One example is the hole injection from Ce ions into n-GaAs (Fig. 7.63) [85]. The emission sets in at the same potential where the interfacial current also occurs. The current becomes diffusion-limited with increasing cathodic potentials. The emission shows, however, a peak around -0.5 V. If the potential scan 

7 Charge Transfer Processes at the Semiconductor-Liquid Interface 

So far, we have described the emission due to hole injection into an n-type semiconductor electrode. The question arises concerning whether the counterpart, i.e, the injection of electrons into p-type electrodes, can also be realized. The only example reported in the literature, is the oxidation of [Cr(CN)ft] at p-InP electrodes [86]. This is a redox couple with a very negative standard potential (t/ = -1.4 V (SCE)) with which electron injection into the conduction band of p-InP was possible (Fig. 7.64). Corresponding emission was observed. The same type of experiments with p-GaAs did not lead to any emission because the energy bands of GaAs are higher than those of InP. 

These are very good examples from the fundamental point of view which could never be realized with pure solid state devices. In addition, it should be emphasized that electroluminescence is a very useful in situ tool for the detection of minority carrier injection. In principle, the same type of information can be obtained by using the thin slice method (see Section 4.3). However, the latter technique is much more difficult to realize in practice. One example which shows very nicely the possibilities of the electroluminescence method is the investigation of the anodic decomposition of p-InP. In this case, luminescence has been observed upon the anodic polarization of this electrode in solutions free from any redox system. 

Since the emission yield (intensity divided by the interfacial current) remained constant, it was concluded that some of the decomposition steps occur via the conduction band [87]. Further details will be discussed in Chapter 8. 

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