nextnano.NEGF - Software for
Quantum Transport

nextnano.QCL is a console application that is run from within the nextnanomat software (GUI). Alternatively, it can be executed from the command line. The input file specifies the device that shall be simulated.

nextnanomat is a convenient graphical user interface for nextnano.QCL. It can be downloaded from here. It can visualize 1D, 2D and 3D simulations results.

The input file specifies all properties of the device, such as geometry, material composition, temperature, grid,… Furthermore, it sets all parameters that are needed to define the program flow of nextnano.QCL. The keywords that can be used for this purpose are defined in the syntax of the input file.

nextnano.QCL exports its results to a directory. The output files are documented here.

The nextnano.QCL installation provides some example tutorials that can be run with nextnanomat, to get familiar with the program.

All material properties that are needed for simulation are described here.

The software can be obtained from here.

If you have further questions, see the FAQ or contact support@nextnano.com.

The code is based on the non-equilibrium Green's functions (NEGF) formalism (also known as the Keldysh, or Kadanoff-Baym formalism). This formalism allows to account for both quantum transport effects (i.e. coherent transport effects, such as resonant tunneling), as well as scattering mechanisms.

In the NEGF formalism, scattering processes are described in terms of self-energies. Self-energies and Green's functions are calculated in a self-consistent way, as both elastic and inelastic scattering processes are accounted within the the self-consistent Born approximation.

The code uses field-periodic boundary condition. In this way the simulation accounts for an infinite periodic structure, with a periodic electric field. Coherent transport between periods is accounted on a length set by <Coherence_length_in_Periods>.

In the beginning of the calculation, the single-band effective mass Schrödinger equation is solved in real space. The calculated energy levels and wave functions are then used as input to the NEGF algorithm. The wave functions are termed modes and the NEGF algorithm is written in terms of mode space and not real space to make it computationally more efficient. The number of QCL periods that are input to this Schrödinger equation are specified in <Number_of_lateral_periods_for_band_structure>. In general, the core of the NEGF algorithm should be rather independent of this number, e.g.

 <Number_of_lateral_periods_for_band_structure> 4 
 </Number_of_lateral_periods_for_band_structure>

should lead to very similar results compared to a value of 5 but the numerical values might differ slightly.

As a second step, the scattering coupling terms are calculated for each of the accounted mechanism (optical and acoustic phonons, charged impurities, interface roughness, alloy disorder).

Then, the main part of the calculation consists in the self-consistent NEGF solver. Starting from an initial guess of the Green's functions, the self-energies are calculated. The Green's functions are then calculated iteratively. Simultaneously, the mean-field electrostatic potential is calculted self-consistenly (Poisson's equation). Such iterations are made until convergence is reached for the Green's functions as well as for the calculted current.

From the Green's functions solution, the gain is then calculated (if requested in the input file).