Simulation Tools

Transport in Mesoscopic Systems

What is the TIMES project?

Simulation  of nanoscale systems is a valuable tool for both scientific research and microelectronics design as transistor scaling has made the experimental development of new technologies difficult;. traditional trial-and-error design approaches are challenging, time-consuming and expensive.

The development of computational methods to describe charge carrier transport at the nanoscale is a central theme in atomic-scale modelling and device simulations over the last decade. Material properties such as effective masses and mobility are substantially different with respect to their bulk values when materials are structured on a few nanometre length scale: the determination of physical parameters requires an explicit treatment of the electronic structure in nanoscale devices. As well, the description of charge transport on this length scale requirs a fully quantum mechanical treatment.

 How often is it that you have come across a new idea or an application when studying charge transport in nanostructures that your computational platform can not support? Have you wished it were easier to switch between differrent electronic structure descriptions to analyse your data in a way that allows you to assess the impact of electronic structure approximations independently of the transport equations? It is the aim of this project to accelerate development, identify missing features and work towards delivering solutions for nanolectronic and molecular electronic design by sharing a common development platform that will allow simulation community to explore quantum charge transport for development of future nanoelectronics and nanotechnologies.

What are the tools?

The core engine of this initiative is the transport code TIMES (short for Transport In MEsoscopic Systems) which is supported by several utility and post-processing tools. Uniquely, this approach allows interfacing to various electronic structure platforms and data analysis tools. The TIMES compute engine calculates the scattering coefficients for "waves" propagating elastically across an interfacial region between idealised structures. "Waves" is the generic term used to describe electrons obeying quantum mechanics, acoustic or lattice vibrations, superconducting electron-hole pairs etc. Post-processing of the scattering coefficients may yield electrical or thermoelectric coefficients for electronics and heat conductivity for given applications. The current version of the TIMES platform we are sharing  focuses on the calculation of current-voltage characteristics for two terminal electronic devices; existing modifications to open source DFT codes also allow for the introduction of a cylindicrical, electrostatic gating field across a nanowire device. 

The initial simulation framework is based on treating materials and device modelling on the same footing within a single quantum mechanical treatment, i.e., the electronic Hamiltonian for the calculation of electric currents is derived from an atomic-scale electronic structure platform. The design principles of TIMES are:

-          Availability, that is, the source code needs to be freely accessible;

-          Portability derived from decoupling the electronic structure calculation from the transport simulation part

-          Reusability enabled by continuous support/development to user requirements

-          Scalability available through the development of new platforms and parallel transport algorithms

Using the Hamiltonian input from the electronic structure platform, the TIMES code employs the recursive Green's functions method and linear algebra routines to calculate the wave-mechanical scattering matrix that yields transition amplitudes between incoming/outgoing states. A self-consistency step has also been implemented to couple with Hamiltonians obtained from Density Functional Theory (DFT) in the Kohn-Sham approximation and is available upon request. In this case, the charge density matrix is calculated from a Kohn-Sham Hamiltonian including self-energies representing semi-infinite source and drain electrode to determine current. This density is used in a new DFT step to extract a new device region Hamiltonian matrix. This process iterates till the charge density converges. A quantum electronic transport scheme can be developed using this approach with any electronic structure treatment that uses a localised basis set description of the Hamiltonian.

What is available?

The source code is freely available to both co-developers and for application driven use. To date, the code has been used with DFTB+ and with the open source DFT code OpenMX. In addition to the source code, to enable benchmarking the code, a wide range of tests cases are available, with applications drawn from the following list of material systems: 

 

Atomic chains

 

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Metal Nanowires

 

 

Semiconductor Nanowires

 

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Carbon Nanotubes

 

How to obtain a copy

Simply contact Giorgos Fagas (Georgios.Fagas@tyndall.ie) with your request.

TIMES Publications: Development and Applications (Please cite as appropriate depending on your application)

  1. L. Ansari, G. Fagas, J.-P. Colinge and J.C. Greer, “Subthreshold Behaviour of Junctionless Silicon Nanowire Transistors from Atomic Scale Simulations”, Solid-State Electronics 71, 58-62 (2012)
  2. L. Ansari, B. Feldman, G. Fagas, J.-P. Colinge, and J.C. Greer: ‘Simulation of Junctionless Si Nanowire Transistors with 3 nm Gate Length’, Applied Physics Letters 97, 062105 (2010)
  3. G. Fagas and J. C. Greer: ‘Ballistic Conductance in Oxidised Si Nanowires’, Nano Letters, 9 1856-1860 (2009)
  4. G. Fagas, G. Tkachov, A. Pfund, and K. Richter: ‘Geometrical Enhancement of the Proximity Effect in Quantum Wires with Extended Superconducting Tunnel Contacts’, Phys. Rev. B 71, 224510 (2005)
  5. G. Fagas, A. Kambili, and M. Elstner: ‘Complex-band structure: a method to determine the off-resonant electron transport in oligomers’, Chem. Phys. Lett. 389, 268-273 (2004)
  6. R. Gutierrez, G. Fagas, K. Richter, F. Grossmann, and R. Schmidt: ‘Conductance of a molecular junction mediated by unconventional metal-induced gap states’, Europhys. Lett. 62, 90-96 (2003)
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