Electron Injection Studies of Radiation Induced Positive Charge in MOS Devices

Avalanche injection and internal photo-emission techniques have been used to study the capture of electrons by positive charges introduced into the oxide layers of MOS capacitors. These two techniques have been used to study the positive charge in Al gate and poly-Si gate capacitors. The electron capture crosssection of this charge has been found to depend on the composition of the interface; positive charge at the Si interface tends to have a coulombic electron capture cross-section (~ 4×10-13cm2), while that at the alumninum interface has a non-coulombic electron capture crosssection (≤ 10-14cm2). The location of the positive charge induced by radiation under positive or negative bias has been determined in a completely non-destructive manner by photocurrent-voltage experiments. It has been found that under a positive irradiation bias, positive space charge accumulates within ~ 50 A of the Si-SiO2 interface, while under a negative bias, the space charge is within ~ 50 Å of the Al-SiO2 interface. In both cases there is evidence for some charge at the other interface introduced by the irradiation.