Fabrication and Characterization of Nanoscale Pillar Arrays With Engineered Phononic and Photonic Properties

This dissertation reports the results of the investigation of nanoscale pillar structures with engineered phononic and photonic properties. It has been proven that phonon states in semiconductors can be tuned by external boundaries, either as a result of phonon confinement effects in individual nanostructures or as a result of artificially induced external periodicity, e.g., phononic crystals. The possibility of engineering acoustic phonon spectrum is of ultimate importance from scientific and application perspectives since they are the main heat carriers in nonmagnetic semiconductor and insulator materials. The change in acoustic phonon dispersion would affect the heat transport properties, alter the electron-phonon interactions, and affect the optical properties of the material system. It has been recently suggested that periodic structures with properly tuned dimensions can act simultaneously as phononic and photonic crystals, strongly affecting the light-matter interactions. In this dissertation research, I prepared nanoscale pillar structure samples and investigated their properties. The “pillar with hat” structures were fabricated using electron beam lithography on silicon substrates followed by inductively-coupled plasma cryogenic dry etching. The hats of the pillars were created with a unique design to have exactly the same orientation plane as the substrate. The structures were inspected with scanning electron microscopy. I used Brillouin-Mandelstam spectroscopy to measure the dispersion of acoustic phonons with energies in the range from 2 GHz up to 20 GHz through the entire second and higher order Brillouin zones. I analyzed the spectral signatures resulting from the surface ripple mechanism which dominates the light scattering in these specific samples. I found clear signatures of the phonon spectrum modification in the appearance of localized phonon sub-bands at energies between 2˗20 GHz. The variable angle ellipsometry measurements indicated modification of light scattering due to nanostructuring. The experimental data confirmed the dual functionality of the structure with engineered phononic-photonic properties. The results obtained in this dissertation research have important implications for the next generation of photonic and optoelectronic device technologies