4.1. Overview

The nextnano.NEGF tool is designed for accurate quantum simulations of semiconducting devices using the Non-Equilibrium Green Functions (NEGF) formalism. It has been originally developed to simulate electron dynamics and gain in quantum cascade lasers (QCLs) and superlattices. It can also be apply to infrared detectors such as quantum well infrared detectors (QWIPs) and quantum cascade detectors (QCDs), as well as transport in resonant tunneling diodes (RTDs). It is progressively extended to further applications such as interband devices.

This simulation tool relies on the NEGF formalism, which has been shown to be a powerful framework to model quantum transport in presence of scattering processes.

The following scattering mechanisms are included:

  • Longitudinal polar-optical phonon scattering (polar LO phonon scattering)

  • Acoustic phonon scattering

  • Charged impurity scattering

  • Interface roughness scattering

  • Alloy scattering

  • Electron-electron scattering

A possible application is illustrated below with the calculation of current-voltage characteristics in a QCL.

Animated density of states
Current-voltage characteristics

Attention

To be added: emitted power as a function of potential drop

See also this video: nextnanoNEGF_video_ITQW_2017_1080p.mp4

NEGF simulation of a Quantum Cascade Laser

Figure 4.1.1 This video (mpg4, 23 MB) shows how a QCL works. The first 10 seconds summarize the features of our software, the remaining 20 seconds show the animated simulation results. At 20 seconds, the QCL starts to lase: The electron density (figure on the right) shows the population inversion, i.e. the higher state has a larger electron population than the lower lasing state.

Note

We think it makes sense to

  1. first get familiar with the nextnanomat user interface (using the nextnano++ tool) and

  2. then have a look at nextnano.NEGF.

You can solve the Schrödinger equation for a QCL heterostructure subject to an electric field using the nextnano++ tool.

You can find simple QCL wave function calculation (including intersubband transition matrix elements) examples here:

C:\Program Files\nextnano\2020_12_09\Sample files\nextnano++ sample files\

Start with these input files:

Once you have gained some familiarity with the nextnanomat user interface, you can eventually start using nextnano.NEGF.

Note

  1. Install the nextnano software.

    Installation

  2. Once done, install nextnano.NEGF.

You can download nextnano.NEGF from here:

Download nextnano.NEGF

Unzip the file, store it to a useful location on your hard disk, and set the path to the nextnano.NEGF.exe exectuable in nextnanomat ==> Tools ==> Options ==> Simulation ==> nextnano.NEGF executable file.

Please make sure to check in Windows File Explorer ==> View ==> File name extensions. If you don’t check this option, you will only see:

  • nextnano.NEGF (without file extension .exe.) and not

  • nextnano.NEGF.exe

Set location of executable

Figure 4.1.5 Set the path to the nextnano.NEGF.exe executable.

Sample input files can be found in the \nextnano.NEGF sample files\ folder. We recommend starting with this example: Open THz_QCL_GaAs_AlGaAs_Fathololoumi_OptExpress2012_10K-FAST.xml and press the run button. This is only a test file that runs fast (typically 10 minutes). For reliable calculations, please use the corresponding file THz_QCL_GaAs_AlGaAs_Fathololoumi_OptExpress2012-MEDIUM.xml which takes more CPU time (~12 hours) than *FAST.xml but produces much more accurate results. If the calculations take too long, your CPU might not be very fast or you do not have sufficient RAM. In that case, please contact us. You need at least 16 GB RAM, 32 GB are better. Please note that you should run the larger input files on a computer having 32 GB RAM. 16 GB might be not enough and require a change in the settings for the number of parallel threads used for the gain calculation. We recommend a Windows PC with 64 GB RAM, and a recent i7 CPU (e.g. a 6-core i7-8700). The whole PC is then around USD 1,000.

If the tool does not run, you might have to specify the Material_Database.xml file in Tools ==> Options ==> Material database.

We hope you enjoy using nextnano.NEGF. The technical part of nextnano.NEGF documentation is currently being updated and migrated to here from: https://www.nextnano.com/nextnano.NEGF/

Please let us know if you encounter any difficulty or notice any strange simulation results: Contact nextnano

Note also these slides on the implemented physics: Tutorial_IQCLSW_2018_Grange