event
Thursday 13 Oct 2011: Probing size-dependent light-matter interactions and structural phase change properties with nanowires
Professor Ritesh Agarwal - University of Pennsylvania
Harrison 170 (3D Visualisation Suite) 14:00-15:00
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. Our efforts towards coupling semiconductor nanowires with nanocavity
plasmons and the effect of exciton-plasmon interaction on emission properties will be discussed.
In the second part of my talk I will discuss our efforts to study 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 etchinduced
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 nonvolatile
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.
