Nanowires: Size-dependent Optical and Structural Phase Change Properties
University of Pennsylvania
About the Talk
Semiconductor nanowires offer a unique approach for the bottom up assembly of electronic and photonic devices with the potential of integrating photonics with existing technologies. The unique geometry and mesoscopic lengthscales of nanowires also makes them very interesting systems to study a variety size-dependent phenomenon where finite-size effects become important. I will discuss two different phenomena where interesting size-dependent properties originate at 100 nm lengthscales. In the first part of my talk I will discuss propagation of light in subwavelength nanowire optical cavities where the diameter of nanowires is much smaller than the waveguided light. Due to the tight photonic confinement, interesting size-dependent dispersion properties of various propagating optical modes are observed. The effect of size-dependent mode-dispersion on fabrication of subwavelength photonic devices for the generation, waveguiding, and detection of light at the nanoscale will be discussed.
In the second part of my talk I will discuss our efforts in studying reversible crystalline to amorphous phase transitions in chalcogenide nanowires (GeTe, Ge 2Sb 2Te 5), which are becoming important materials for Phase Change Memory (PCM) devices. Of the different memory device concepts being currently explored, PCM devices based on Ge-Sb-Te alloys are very promising for scalable device size, high-speed operation with nonvolatile random accessing capability. However, the top-down nature of thin-film device fabrication and etching-induced material damage leads to scalability problems at sub-100 nm size. Therefore, there is great interest in developing new materials and processing techniques to overcome this barrier. Self-assembled nanowires are particularly promising owing to their sub-lithographic size that is free of etch-induced damage. Reversible phase transitions in single-crystalline nanowire devices scaled down to 20 nm sizes are observed with dramatic reduction in switching currents and power consumption. Size-dependent spontaneous recrystallization kinetics is studied systematically and activation energies are obtained and our results demonstrate non-volatile data retention capabilities at 20 nm length scales. High-resolution TEM results clearly show that recrystallization occurs via nucleaction dominant mechanism, which follow the classic Avrami type kinetics even at sub-30 nm sizes. Quantitative modeling of the nucleation rates can only be achieved by using the heterogeneous nucleation theory which shows that the increase in surface-to-volume ratio with decreasing nanowire size provides efficient sites for nuclei generation. Our efforts towards assembling multi-state memory switching devices utilizing the different size-dependent electronic and thermal properties of GeTe and Ge 2Sb 2Te 5 materials in core-shell heterostructured nanaowires will also be discussed. Our studies suggest that phase-change nanowires hold great promise as building blocks for miniaturized memory devices and for in-depth understanding of size-dependent phase transitions in confined geometries in self-assembled, defect-free nanostructures.
About the Speaker
Ritesh Agarwal earned his undergraduate degree from the Indian Institute of Technology, Kanpur in 1996, and a master’s degree in chemistry from the University of Chicago. He received his PhD in physical chemistry from the University of California at Berkeley in 2001. After completing his PhD., Ritesh was a postdoctoral fellow at Harvard where he studied the optical and photonic properties of semiconductor nanowires. His work led to the development of electrically-driven single nanowire lasers and avalanche photodiodes. Ritesh is currently an assistant professor in the Department of Materials Science and Engineering at the University of Pennsylvania. His research interests include quantum confined optics and electronics in nanowire heterostructures, and studying phase transitions and electronic memory switching at the nanoscale. Ritesh is the recipient of the NSF CAREER award in 2007 and the NIH Director’s New Innovator Award in 2010.