Apr 18, 2012
FIB patterning controls position and dimensions of quantum dots
Lateral patterning of InAs/GaAs quantum dots (QDs) using an in vacuo focused ion beam (FIB) provides a unique way to control the position, dimensions and areal density of QDs. For unpatterned, self-assembled QDs grown by molecular beam epitaxy, the position, dimensions and areal density of the QDs are primarily controlled by the particular growth conditions (for example, temperature, deposition rate and the amount of InAs deposited). FIB can be used to laterally pattern small holes in the substrate, providing preferred nucleation sites where the QDs will form. Control over the lateral position, areal density and the dimensions of the QDs could particularly benefit applications such as quantum computing.
The work done at the University of Michigan focuses on the effects of lateral FIB patterning and layering on multilayer InAs/GaAs(001) QD structures. Patterning by in vacuo FIB is unique from other patterning techniques because it eliminates exposure of the sample to air between patterning and dot growth. The team found that the size of the FIB-patterned holes and the lateral pattern spacing provide a way to control QD position and dimensions, giving a means of tailoring QD areal density and size.
The patterning conditions could be adjusted so that QDs formed at 100% of the FIB-patterned holes, and did not nucleate away from the holes. Additionally, analysis of the change in QD size as a function of pattern spacing provided a means of estimating the maximum average adatom surface diffusion length for the given growth conditions, a characteristic that can be difficult to measure.
These findings are significant because they provide an understanding of how to use the patterning conditions to achieve a desired QD size and areal density without altering other growth conditions. This is important because the QD dimensions and areal density affect the electronic and optical properties of the QDs as well as their ability to be used for specific applications where these properties are critical. For example, quantum-computing applications may require specific placement of QDs at a low density, while laser or solar-cell applications may desire a very high areal density of QDs with uniform size distribution.
Further details can be found in the journal Nanotechnology.
About the author
This work was conducted by researchers in the Materials Science and Engineering department and the Physics department at the University of Michigan. The work is a part of the Center for Solar and Thermal Energy Conversion, which is an Energy Research Frontier Center at U of M, funded by the US Department of Energy. Andrew Martin is a PhD student in Prof. Joanna Millunchick’s group in the MSE department and is studying multilayer QD structures with type-I and type-II band offsets for use in quantum dot intermediate band solar cells. Timothy Saucer is a PhD student in Prof. Vanessa Sih’s group in physics and is studying exciton properties in III-V semiconductor heterostructures. Garrett Rodriguez was a visiting summer student from Alma College working with Prof. Sih’s group. The authors also have three recent publications on the effects of patterning on the optical properties of InAs/GaAs QDs and on single-dot emission from patterned QDs.