Defects can introduce electronic states in the gap of semiconductors. These mid gap states modify the usual valence band ↔ conduction band carrier transitions, and induce alternative generation and recombination pathways. From a macroscopic point of view, these new pathways are responsible for the leakage currents of devices, the changes in the effective doping concentration, the reduction of the charge collection efficiency, or the appearance of new photoluminescence bands.
Using ab initio simulations we have analyzed how the band diagram of crystalline Si is modified by the presence of defects, and how the induced mid-gap defect states can affect the recombination of carriers. We have also studied the charge localization trapping in amorphous silicon and crystalline/amorphous interfaces, identifying the relevant atomic structures responsible for the trapping of carriers. In particular, we have found that strained amorphous regions in Si act as hole traps and prevent from efficient p-type doping.
(left) Band diagram and (right) localization of electronic states around a particular configuration of a tri-interstitial cluster in silicon (orange atoms): states at the bottom of the conduction band (red), and states at the bands induced by the defect (blue). The overlap shown will help charge recombination.
Evaluation of the atomic structure of relevant defects and complexes
Modeling of the relevant energies that govern the stability and diffusion of defects
Calculation of the electronic levels of defects to correlate with their macroscopic effects
Simulation of dynamics of defects based on fundamental properties