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Multi-million-atom simulations and polarisation enable QD metrology

Researchers have used a simulation of indium arsenide quantum dots, comprised an indium-rich central core surrounded by an indium-poor region. This model enables accurate reproduction of experimentally measured optical spectra

Growth dynamics in real quantum dot devices has been modelled by an international team from Tyndall National Institute, Ireland, the National Nanotechnology Laboratory-CNR, Italy, and Purdue University, Indiana.

The researchers used multi-million atom simulations, that are claimed to deliver unprecedented precision. The semiconductor device simulation tool, called NEMO 3D, was developed at NASA and Purdue. The NEMO 3D software can model structures containing up to 50 million atoms through parallelised computation on high performance supercomputing clusters.

By accurately mimicking indium-gallium intermixing and indium-segregation effects with an innovative two-layer composition model for quantum dots (QDs), the researchers can reproduce polarisation-dependent optical emission spectra of an MBE-grown InAs QD.

Figure: (a-d) QD geometries (f) Comparison of the polarisation dependent TE and TM optical spectra from the experimental measurements and the atomistic theoretical calculations: Black = Experiment, Red = Pure InAs QD, Blue = In0.7Ga0.3As QD, and Green = QD with two-layer composition model

QDs, often referred to as “artificial atoms”, exhibit unique electronic and optical properties that can be custom designed for a variety of applications. These include optical communications, quantum information technology and medical science.

However, the structural properties of self-assembled QDs are strongly affected by surface and growth conditions. Nanoscale phenomena such as indium-gallium intermixing and Indium-segregation effects during the capping and post-growth annealing processes strongly affect the actual shape, size, and composition profile of QDs. 

Despite significant advancements in characterisation techniques, mystery surrounds details of QD geometry and chemical composition.

Tyndall has proposed an innovative two-composition model for the InAs QDs, comprised of an indium-rich central core surrounded by an indium-poor region close to the edges of the QD. This model allowed the researchers to accurately reproduce the experimentally measured optical spectra.  

The modelling of growth dynamics is a challenging computational task as it requires simulation of realistic QD sizes with atomistic resolution.

By performing a systematic set of multi-million atom atomistic simulations, the researchers in Ireland found a correlation between their calculations and the experimentally measured data. The results quantitatively show the influence of indium-gallium intermixing and indium segregation effects on the polarisation properties of the QDs. 

The work based on this research has been published in the journal Nanotechnology.

“Detailed study of growth dynamics allows us to accurately explain the experimental evidence and resolve the existing discrepancy between theory and experiment.” explains Muhammad Usman, from the Tyndall National Institute and lead author of the paper.

“Furthermore, the understanding of strain fields in and around the quantum dots as a function of the indium-gallium intermixing developed through our systematic set of atomistic simulations will allow us to tune polarisation properties of quantum dots which is critical for several optoelectronic technologies”, he continues.

The theoretical modelling work was performed by Muhammad Usman who is currently working as a post-doc researcher at the Tyndall National Institute, Cork, Ireland in Eoin P. O’Reilly’s research group.

The NEMO 3D simulator was developed in Gerhard Klimeck‘s research group at NASA/JPL/Caltech and Purdue University, West Lafayette, Indiana, USA.

The quantum dot samples were grown by scientists Vittorianna Tasco and Adriana Passaseo and characterised by Maria Teresa Todaro and Milena De Giorgi at the National Nanotechnology Laboratory of CNR-Nanoscience, Lecce, Italy.

Further details of this work have been published in the paper, " The polarization response in InAs quantum dots: theoretical correlation between composition and electronic properties", by M. Usman et al, 2012 Nanotechnology, 23,165202. DOI:10.1088/0957-4484/23/16/165202.

The paper may be accessed via the link :  http://iopscience.iop.org/0957-4484/23/16/165202
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