Tensile Strain: A Reliable Tool for Nanostructure Self-Assembly and a Simple Route to Infrared Optoelectronics

Dr. Paul Simmonds
Department of Physics
Boise State University 

Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable new devices by taking advantage of the strong band gap reduction induced by tensile strain. Tensile SAQDs with low optical transition energies could find application in the technologically important area of mid-infrared (IR) optoelectronics. In the case of Ge, biaxial tension can even cause a highly desirable crossover from an indirect- to a direct-gap band structure. However, the inability to grow tensile SAQDs without dislocations has impeded progress in these directions.

In this talk I will discuss experiments in which we have developed a novel method to grow dislocation-free, tensile SAQDs by employing the unique strain relief mechanisms of (110) and (111)-oriented surfaces. As a model system, I will show that tensile GaAs SAQDs form spontaneously, controllably, and without dislocations on both the (110) and (111) surfaces of InAlAs. The tensile strain reduces the band gap in the GaAs SAQDs by 40%, leading to robust quantum confinement. In contrast with traditional compressively strained SAQD systems, we observe photoluminescence at photon energies lower than that of bulk GaAs. I will present recent data showing that, as a result of the symmetry of the (111) surface, tensile SAQDs show great promise as entangled photon emitters. Tensile strained self-assembly is a versatile general approach that can be readily extended to other zinc-blende and diamond-cubic semiconductors to create new devices and explore new physics.


Macintosh HD:Users:Paul J Simmonds:Pictures:Paul.jpgBio: Paul Simmonds completed his PhD in semiconductor physics at the University of Cambridge where he worked with David Ritchie and Michael Pepper. His research focused on the growth of thin III-V semiconductor films by molecular beam epitaxy (MBE) for studies of electron transport in low-dimensional, high-mobility materials. Paul moved to the US in 2007 to work as a postdoc, first with Christopher PalmstrÝm at the University of Minnesota / UCSB and then, from early 2009, at Yale University with Minjoo Larry Lee. Paul’s research at Yale centered on his discovery that by using tensile strain it is possible to create III-V quantum dots on (110) and (111) surfaces, with potential significance for the fields of quantum computing and spintronics. In 2011, Paul began managing the Integrated NanoMaterials Laboratory at UCLA. Working with Diana Huffaker, Paul oversaw research on a wide range of different semiconductor materials for electronic and photonic applications. In September 2014, Paul became an Assistant Professor at Boise State University with joint appointment in Physics and Materials Science.