Radiative recombination at defects

The defect recombination is predominantly, but not completely, non-radiative. The quenching of the band tail luminescence at high temperature or at high defect density is accompanied by the onset of a weak luminescence transition at lower energy. Some typical luminescence spectra are shown in Fig. 8.25 for doped and high defect density undoped material. The peak energy at low temperature is 

8 eV in n-type a-Si H and 0.9 eV in p-type and decreases slightly at elevated temperature. There is agreement that the transition is from the band edge to the defect, but considerable debate over which of the four possible transitions in Fig. 8.24 are radiative. The author has proposed that the luminescence is from transition A in n-type material and transition D in p-type (Street et al. 1984). Irrespective of which is the correct transition, the radiative process is only a minor component of the defect recombination. The defect luminescence has no more than 1 % of the quantum efficiency of the maximum band tail luminescence intensity. 

Defect recombination is therefore primarily non-radiative. The standard theories of non-radiative transitions are based on the multiphonon processes described in Section 8.1.2. A temperature-independent capture cross-section of about 10 cm is characteristic of a defect state with a strong electron-phonon coupling (see Fig. 8.6). 

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Fig. 3 depicts low-temperature PL spectra of the implanted samples. A broad band at 0.92 eV with small intensity is observed in the as-implanted sample. It can be attributed to radiative recombination at defects induced by ion implantation. 

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