Luminescence and photoluminescence

Certain semiconductors, the best known being GaP, are observed to luminesce under conditions in which hole injection (n-type) or electron excitation (p-type) occurs. Luminescence may arise from bulk or surface states in p-GaP, for example, there is a strong (photo)luminescence peak at ca. 1.8 eV due to transition from a defect level associated with O to that associated with the Zn acceptor sites, as shown in Fig. 88 [163]. This transition may also be observed through electron injection into the CB although this is not electrochemically possible in aqueous electrolyte, owing to the fact that the CB lies above the stability range of water, it can be detected in non-aqueous solvents [163, 167], 

Similarly, for n-GaP, photoluminescence is observed at ca. 1.3 eV. However, for n-GaP in the presence of K3[Fe(CN)6] in the dark, weak electroluminescence was observed over a considerable spectral range, as shown in Fig. 89 [167] and a second peak at 1.6 eV appears not to be due to bulk but to surface processes. 

The most serious problem with luminescence studies is the difficulty of distinguishing between luminescence processes due to transitions to or from the level. If we assume that there is just one level, then the luminescence peak at 1.6 eV in Fig. 89 may be due to either of the processes shown in Fig. 

The process shown in Fig. 90(b) is favoured by the known position of the band edges in alkali, the similarity with the electroluminescence induced by persulphate, and the energy of the [Fe(CN)6]3, 4 redox couple. Either mechanism would, however, be consistent with the observation that the onset of electroluminescence coincides, approximately, with the flat-band potential, as shown in Fig. 91. Interestingly, the photoluminescence reaches a maximum at more negative potentials, a result not predicted by either model, as shown in Fig. 91 [167], 

Electroluminescence at p-GaAs in the presence of K2S208 has been observed [168, 198]. Unlike n-GaAs, which shows strong electroluminescence in the presence of peroxydisulphate, p-GaAs should show only photolumine- 

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