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Complex interfaces

The materials involved in novel semiconductor circuits are becoming more and more complex, which is not least evident in the latest generation of gate stacks involving high-k dielectrics. In addition to the pure material properties, the focus of research is now on modeling advanced junctions and interfaces between different materials. For this, a detailed description on the quantum-mechanical level is required, in order to calculate e.g. the leakage current or Schottky barrier. Moreover, since these interfaces are very thin, even on a nanoscale level, defects play a crucial role. Therefore, an atomistic description is needed to be able to predict how impurities, vacancies, etc influence the properties of the junction. Several academic researchers, including Prof. Nishi's group at Stanford, are using Atomistix ToolKit (ATK) from QuantumWise to model such structures.




The QuantumWise software platform, with the first-principles engine Atomistix ToolKit (ATK) and the graphical user interface Virtual NanoLab (VNL) allows researchers to focus on the relevant points for their projects, namely the investigation of the electrical and spin properties of novel device structures, rather than spending time writing their own quantum-mechanical codes, or struggling with data import/export to visualize the results.

The unique feature of ATK to calculate ballistic tunneling current in nanostructures enables users to

  • compute the current-voltage characteristics of a junction, and from this extract e.g.
    • the leakage current,
    • the Schottky barrier of a metal/semiconductor interface (this can also be a junction between two nanotubes, or a metal and a nanotube);
  • calculate spin transport properties in magnetic tunnel junctions (more information is available separately);
  • investigate the influence of defects such as impurities and vacancies on the transport properties;
  • study charge transfer across a junction

The calculations in ATK are based on first-principles methods, thus the software can be used to investigate novel materials, since there is no requirement for empirical input to the computational model. The types of systems that can be considered therefore spans a wide range of materials:

  • traditional metal/semiconductor interfaces
  • high-k dielectrics
  • magnetic tunnel junctions
  • metallic or semiconducting nanowires
  • nanotubes, including metal/nanotube contacts
  • metallic nanoclusters
Moreover, a local basis set expansion of the electron density is used, and ATK is therefore a more efficient and accurate tool for modeling local defects such as impurities or vacancies than plane wave codes. The algorithms used in ATK are also optimized to allow for simulations of large-scale systems, with up to 500-1,000 atoms.


Examples and references

Below we illustrate some of the applications for complex interfaces that users of Atomistix ToolKit have studied with the software. To view all references to papers studied with ATK, see the publication list.


TMR studies of Fe/MgO/Fe


M. Stilling, K. Stokbro & K. Flensberg, J. Comp.-Aid. Mater. Design 14, 141 (2007) and Mol. Sim. 33, 557 (2007)

Two studies of Fe/MgO/Fe MTJ structures. Among other things the influence of random perturbations to the atomic coordinates perpendicular to the Fe/MgO interface is found to lead to significant differences in the conductance. The authors also show how the conductance is lowered substantially if the Fe interfaces are oxidized (as also noticed in experiments), and discuss how inclusion of Au in the interface might reduce this problem.

Schottky barrier height tuning at NiSi2/Si interfaces


Li Geng et al., Chinese Physics Letters 26, 037306 (2009) and IEEE Electron Device Letters 29, 746 (2008)

These articles investigate the mechanism of Schottky barrier height (SBH) tuning at NiSi2/Si interfaces, by incorporation of Ti, Sc, V, and Y atoms segregated into the interface. The metal/semiconductor interface states within the gap region are greatly decreased when the Si dangling bonds are saturated, leading to a new pinning-free interfacial structure. The calculations can be used to explain previously reported experimental phenomena.

Spin-polarized transport in graphene nanoribbons with impurities


T. Kamiya et al., pss(a) 205, 1929 (2008)

Electronic structures and carrier transport properties for Au/ZnO/Au and Mg/ZnO/Mg two-probe models are studied. It was found that the electronic structures of the idealized models were a Schottky contact for the Au/ZnO/Au interface and an ohmic contact for the Mg/ZnO/Mg interface.

Spin-polarized transport in a CrAs/AlAs heterojunction

CrAs(001)/AlAs(001) transmission spectrum as a function of bias and energy

Y. Min et al., Journal of Magnetism and Magnetic Materials 321, 312 (2009)

Calculations of spin-dependent quantum transport in a CrAs(001)/AlAs(001) heterogeneous junction predicts a strong diode effect of charge and spin current. The minority spin current is inhibited when a bias voltage is applied to the terminals of both CrAs and AlAs. The majority spin current is inhibited when the bias voltage is applied to the terminal of CrAs and relaxed when the bias voltage is applied to the terminal of AlAs. A charge and spin current diode is interesting for reprogrammable logic applications in the field of spintronics.

Effects of dopants in Pt–SrTiO3–Pt heterostructures

SrTiO3 with doping

Z. Wang et. al., APL 94, 252103 (2009)

The electronic structure and quantum transport of Pt–SrTiO3–Pt heterostructures are investigated with special focus on effects of dopants. The intrinsically closed conductance channel in SrTiO3 opens up after doping substitutional atoms of higher valency for Sr or Ti, resulting in enhancement in electron transmission at Fermi level and a drastic increase in the current with bias. The switch of the channel is suggested to be due to the redistribution of density of states on orbitals of the channel atoms.

Si nanowires

Si nanowire

Man-Fai Ng et al., Nano Letters, 8, 3662 (2008)

The current-voltage characteristics of small-diameter hydrogenated and pristine silicon nanowires (SiNWs) are found to depend strongly on length, growth orientation, and surface modification of the SiNWs. In particular, a length of 3 nm is suggested for the nanowires to retrieve its intrinsic conducting properties from the influences of both the electrodes and metal/semiconductor mismatched surface contact; surface reconstruction would enhance the conductance in hydrogenated SiNW, which is explained by the extra conducting eigenchannel found in the transmission spectrum. Discussions with available experimental data are given.

Silver atomic switch

Silver atomic switch

Z. Wang et al., APL 93, 152106 (2008) and Nano Letters 7, 2688 (2007)

Two studies of an atomic switch based on an Ag-Ag2S-Ag junction. An atomic conductance channel made of Ag is generated in the Ag2S layer after structure optimization, resulting in large enhancement of the electron transmission coefficient at the Fermi level and metallic behavior of the system. Such spontaneous metallization has been observed experimentally, and is found to involve the precipitation of several Ag atoms. The results of these publications may be helpful in designing switches of smaller size and may even open up an avenue for clarifying the transport mechanism involved in other solid electrolyte atomic switches.

Cu/nanotube interface

(4,4) CNT on Cu 100 surface



S. Compernolle et al., Physical Review B 77, 193406 (2008)

Carbon nanotubes (CNTs) are a promising candidate to replace copper interconnects. An ab initio study is presented on the conductance of a closed-packed bundle of very narrow metallic (4,0) CNTs, which is vertically placed on a Cu (100) surface. While the intertube interactions have no significant impact on the conductance, which however is highly dependent on the exact geometry of the interface.

Schottky barrier height in a metal-CNT contact

Nanotube Schottky diode

P. Bai et al., Nanotechnology 19, 115203 (2008)

Semiconducting carbon nanotube Schottky diodes are modeled as a nanotube embedded in a metal electrode, to resemble the experimental set-up. The rectification behaviour of the diode is mainly due to the asymmetric electron transmission function distribution in the conduction and valence bands and can be improved by changing metal-SCNT contact geometries. The threshold voltage of the diode depends on the electron Schottky barrier height which can be tuned by altering the diameter of the SCNT. Contrary to the traditional perception, the metal-SCNT contact region exhibits better conductivity than the other parts of the diode.

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