Tensile Strain: a Reliable Tool for Nanostructure Self-assembly and a Simple Route to Infrared Optoelectronics
Jul 11, 2013
from 03:00 PM to 04:00 PM
|Where||ENGR. IV Bldg., Tesla Room 53-125|
|Contact Name||Dr. Eric Diebold|
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Paul J. Simmonds
UCLA – NanoMaterials Laboratory
Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable novel 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 recent experiments in which we have demonstrated a 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 InAlAs(110) and InAlAs(111)B surfaces. 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 and photoluminescence at energies lower than that of bulk GaAs. This method can be extended to other zinc blende and diamond cubic materials to form novel IR optoelectronic devices based on tensile SAQDs.
Dr. Paul Simmonds completed his PhD in semiconductor physics at the University of Cambridge, UK where he worked with Profs. 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 Prof. Christopher Palmstrøm at the University of Minnesota / UCSB and then, from early 2009, at Yale University with Prof. 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. Since September 2011, Paul has managed the Integrated NanoMaterials Laboratory in CNSI at UCLA. Working with Prof. Diana Huffaker and Dr. Baolai Liang, Paul oversees research on two interconnected MBE tools configured to grow a range of different semiconductor materials for electronic and photonic applications, including long-wave IR superlattices and SAQDs for next generation solar cells.