THE EFFECTS OF SUB-BANDGAP LIGHT ON QSSPC MEASUREMENT OF LIFETIME AND TRAP DENSITY: WHAT IS THE CAUSE OF TRAPPING?

摘要

The trapping signal from Quasi-Steady-State Photoconductance (QSSPC) measurements has generally been interpreted using the Hornbeck-Haynes model which attributes the signal to extra majority carriers balancing electrons in shallow traps. Investigation of possible explanations for differences in trapping as measured by various techniques leads to the possibility of an alternative explanation. A large part of this trapping signal in a QSSPC measurement could actually be due to "gating" of the majority-carrier conductance at grain boundaries. The grain boundaries present potential barriers to majority-carrier conduction. These potential barriers are reduced by illumination. Therefore, much of the trapping signal in a QSSPC measurement could be seen as the increase in majority-carrier lateral conductance (towards single-crystal behavior) at low illumination levels. QSSPC may be indicative of grain-boundary density, type and process history. Evidence to support these possibilities is shown for wafer and brick samples.