First principles simulations of nanoelectronic devices

December 2, 2011 Ph.D. Thesis Presentation

Jesse Maassen

(Supervisor : Prof. Hong Guo)

Department of Physics, McGill University,

Montreal, QC Canada

Why first principles theory?

Line of ~ 50 atoms

2012 22 nm

Year Channel length

2015 16 nm

2018 11 nm ?(Source: ITRS 2010)

Why first principles theory?

Science Engineering

Atomic structure :

surfaces, chemical bonding, interfaces, dissimilar materials, charge transfer, roughness, variability, …

tunneling, conductance quantization, spin-transport, …

Quantum effects :First principles

How to calculate transport properties?

Taylor et al., PRB 63, 245407 (2001)Waldron et al., PRL 97, 226802 (2006)Maassen et al., IEEE (submitted)

Applications.

Graphene-metal interface

Localized doping in Si nano-transistors

Dephasing in nano-scale systems

Maassen et al., Appl. Phys. Lett. 97, 142105 (2010); Maassen et al., Nano. Lett. 11,151 (2011)

Applications.

Maassen and Guo, preprint to be submitted

Applications.

Maassen et al., PRB 80, 125423 (2009)

Applications.

Maassen et al., PRB 80, 125423 (2009)

Application : Graphene-metal interface

Motivation :

Graphene has interesting properties (i.e., 2D material, zero gap, linear dispersion bands, …).

For electronics, all graphene sheets must be contacted via metal electrodes (source/drain).

How does the graphene/metal interface affect the response of a device?

Theoretical studies exclude accurate treatment of electrodes.

Transport properties :

Atomic structure :

Cu, Ni and Co (111) have in-place lattice constants that almost match that of graphene.

Equilibrium interface structure determined from atomic relaxations.

MetalMetal

Maassen et al., Appl. Phys. Lett. 97, 142105 (2010); Maassen et al., Nano. Lett. 11,151 (2011)

Ni(111) contact :

Linear dispersion bands near Fermi level.

Zero band gap.

States only in the vicinity of K.

Ni(111) contact :

Strong hybridization with metal

Band gap opening

Graphene is spin-polarized

Maassen et al., Nano. Lett. 11, 151 (2011)

: Top-site C(pz): Hollow-site C(pz): Ni(dZ

Ni(111) contact :

CHANNEL

Application : Localized doping in Si nano-transistors

Motivation :

Leakage current accounts for 60% of energy in transistors.

Two sources : (i) gate tunneling and (ii) source/drain tunneling.

How can highly controlled doping profiles affect leakage current ?

Structure: n-p-n and p-n-p. Channel doping: B or P. L = 6.5 nm 15.2 nm Si band gap = 1.11 eV

Technical details regarding random doping, large-scale modeling and predicting accurate semiconductor band gaps can be found in the thesis.

GMAX / GMIN ~ 50.

Lowest G with doping in the middle of the channel.

G decreases with L.

Variations in G increase dramatically with L.

G decreases with L.

Variations in G increase dramatically with L.

Summary

First principles transport theory is a valuable tool for quantitative predictions of nanoelectronics, where atomic/quantum effects are important.

I determined that the effect of metallic contacts (Cu, Ni, Co) can significantly influence device characteristics. I found that the atomic structure of the graphene/metal interface is crucial for a accurate treatment.

My simulations on localized doping profiles demonstrated how leakage current can be substantially reduced in addition to alleviating device variations.

Thank you!

Questions ?

First principles simulations of nanoelectronic devices

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