High temperature trapping at defects

At temperatures above 100 K, the defect recombination mechanism changes gradually from tunneling to direct capture of a mobile electron or hole at a defect. The capture rate defines the capture cross-section such that the free carrier lifetime is given by 

Furthermore, the free carrier mobility and lifetime are related to the drift mobility and total lifetime for deep trapping, x, by 

Combining these expressions gives for the two mechanisms of capture (Street 1984), 

The value of UpT is obtained from the time-of-flight experiment which measures charge transported by the drift of optically excited carriers across a sample (see Section 3.2.1). Deep trapping causes the 

Measurements of for doped and undoped samples of different defect densities are shown in Fig. 8.23 (Street, Zesch and Thompson 1983). PdT is inversely proportional to the defect density, so that a single value of the capture cross-section can be inferred from Eq. (8.58) or Eq. (8.59). There are four possible trapping transitions of electron and holes into dangling bond defects, and these are illustrated in Fig. 

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