When 3:30 PM - 5:00 PM Feb 11, 2011
Where 1670 CSE
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Ritesh Agarwal, University of Pennsylvania, Department of Materials Science and Engineering

Probing size-dependent light-matter interactions and structural phase change properties with nanowires

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 smaller than the waveguided light. Due to the tight photonic confinement, interesting size-dependent dispersion properties of various propagating optical modes are observed.  In addition, tight optical confinement leads to very strong light-matter coupling in nanowires (polaritons), and we have obtained one of the highest couplings reported for optical cavities. 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, Ge2Sb2Te5), 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 data storage. 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. Our efforts towards assembling multi-state memory switching devices, studying the effect of mechanical stress on electrical properties and direct observation of the switching process via in situ TEM techniques will be discussed.  Our studies demonstrate that phase-change nanowires hold great promise as building blocks for miniaturized memory devices and as model systems for in-depth understanding of size-dependent phase transitions in confined geometries in self-assembled, defect-free nanostructures. 

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