Green’s functions

Axial energy cut-off

Heterostructures consist of a large number of subbands, but only a few near the band gap are relevant for the electronics and optoelectronics (Figure 4.2.1 left). nextnano.NEGF therefore selects a limited number of subbands and construct the Green’s function basis (Figure 4.2.1 right).

Figure 4.2.1 Schematics of miniband selection in the “electron picture”, in which the valence band is regarded as an almost fully-occupied sea of electrons.

You can specify the cut-off either by the energy range measured from the ground states (EnergyRangeAxial and/or EnergyRangeAxialValence), or by the number of subbands (NumberOfConductionSubbands and/or NumberOfValenceSubbands). The number of subbands is considered to be without the spin degree of freedom.

Energy range of the Green’s functions

Per default, the energy range of the Green’s functions are set automatically considering the ground state energies of the electrons, axial and in-plane energy ranges, and the bias. However, if needed, this default energy range can be modified by setting a negative value for EminShift and/or positive value for EmaxShift. Please check the output of the spectral functions to know whether the default setting of the energy range of the Green’s functions is sufficient. Since September 2024, a warning is raised if the energy range may be insufficient.

Coherence length

Note

Model description to be added.