Modeling of negatively charged states at the Ge surface and interfaces

Abstract

Modeling based on surface charge neutrality predicts that the Ge surface tends to be p-type, irrespective of the bulk conductivity. This is a consequence of the fact that the Ge band gap is small and the charge neutrality level lies low in the gap very close to the valence band, probably determined by low-lying unpassivated surface dangling bond acceptors or other defects. According to the model, the acceptor defects build negative charge, inverting the surface of n-type Ge at no gate bias for low doping concentration (<1016 cm−3) and moderate or high interface state densities (>5×1011 eV−1 cm−2). This is predicted to cause undesired positive threshold voltage shift in the range of 0.2–0.4 V in Ge p-channel field effect transistors. The model also predicts that inversion in n-channel field effect transistors is inhibited, which could be related to the observed poor performance of these devices.