Germanium surface and interfaces (Invited Paper)

Germanium surface and interfaces are modeled based on the requirement that surface charge neutrality is satisfied. It is found that Ge interfaces have remarkable electronic properties stemming from the fact that the energy gap is low and the CNL is located very low in the gap close to the valence band. Because of this, acceptor defects (probably dangling bonds) are easily filled building a negative charge at the interface which easily inverts the surface of n-type Ge at no gate bias and for low doping ND and moderate to high interface state density Dit. This has important consequence in the electrical characteristics of Ge transistors. In p-channel FETs, an undesired positive threshold voltage VT of +0.2 to +0.5 V is predicted depending on ND, Dit and the equivalent oxide thickness. In n-channel FETs, inversion is inhibited and VT could become higher than 1 V if the Dit is well in excess of 1013 eV−1 cm−2.