Defects thermal emission energies

Defect level spectroscopy - thermal emission energies... 

The DLTS measurements so far describe only electronic states in the upper half of the band gap. The lower half of the gap is harder to study because of the difficulty of making a stable junction to p type a-Si H. One technique uses light illumination of n-type samples to probe the lower half of the gap (Lang et al. 1984). The optical absorption populates the defect states with holes which are removed by thermal excitation to the valence band. This experiment measures the thermal emission energy because the information comes from the thermal emission step rather than from the initial optical excitation. The data for the lower half of the gap in Fig. 4.17 are derived from this type of experiment. The results are consistent with the usual DLTS where the two results overlap, but there are various new peaks seen in the spectra, with no obvious correlation with the sample growth properties. The addition of the illumination makes the analysis much harder and it is difficult to judge whether all the extra structure is real. 

In measurements of the defect energies, it is essential to distinguish between the thermal emission and optical transition energies, to account properly for lattice relaxation effects (see Section... 

The energies of thermal emission, observed by DLTS and by optical absorption and luminescence agree to within 0.1 eV. The Stokes shift is therefore very small and unable to account for the capture cross-section within the multiphonon model. The mechanism of non-radiative capture at defects remains puzzling. 

It was suspected that the low energy emission band results from keto defects that were introduced either during synthesis or by photo-oxidation during service. Experiments with poly(9,9-dioctylfluorene-co-fluo-renone) with 1% fluorenone as a model compound demonstrated that flu-orenone defects are generated by photo-oxidation and by thermal-oxidation. " Moreover, the formation of these defects is catalyzed by the metals with a low work function that are used as cathode materials in light-emitting diodes. 

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