Experimental Characterization of Deep Levels

Despite the success of purely optical methods such as luminescence and absorption in studying shallow energy levels, they are seldom used for characterization of deep states. This is mainly because the deeper levels of interest are usually nonradiative ones or killer centers for which such techniques are not applicable. In addition, the deep position within the energy gap means optical experiments must be performed in the infrared where 

Capture and emission processes at a deep center are usually studied by experiments that use either electrical bias or absorbed photons to disturb the free-carrier density. The subsequent thermally or optically induced trapping or emission of carriers is detected as a change in the current or capacitance of a given device, and one is able to deduce the trap parameters from a measurement of these changes. 

The rate equation for nT the density of traps occupied by an electron, is 

The time constant of the exponential is given by the sum of all the capture and emission rates  

The thermally activated emission rates are proportional to a Boltzmann factor, and by use of the principle of detailed balance can be related to the capture cross section (a )  

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