PHASE-CHANGE THIN FILMS FOR OPTICAL AND ELECTRICAL APPLICATIONS

Abstract

Reversible, nonvolatile, and pronounced refractive index modulation is an unprecedented combination of properties enabled by chalcogenide phase-change materials (PCMs). This combination of properties makes PCMs a fast-growing platform for active, low-energy nanophotonics, including tunability to otherwise passive thin-film optical coatings. However, compared with their electronic counterparts, optical PCMs remain relatively less mature. Accordingly, this thesis investigates sputtered chalcogenide PCMs as a thin-film platform for reconfigurable photonics, with an emphasis on optical device integration. In parallel, we explore an electrically motivated direction in multicomponent composition screening using the same sputtering-based thin-film platform.

In Chapter 2, we integrate the PCM Sb2Se3 into a novel four-layer thin-film optical coating that exploits photonic Fano resonances to achieve tunable structural colors in both reflection and transmission. We show, in contrast to traditional coatings, that Fano-resonant optical coatings (FROCs) allow for achieving transmissive and reflective structures with narrowband peaks at the same resonant wavelength. Moreover, we demonstrate asymmetric optical response in reflection, where Fano resonance and narrow-band filtering are observed depending upon the light incidence side. Finally, we use a multi-objective inverse design via machine learning (ML) to provide a wide range of solution sets with optimized structures while providing information on the performance limitations of the PCM-based FROCs. Adding tunability to the newly introduced Fano-resonant optical coatings enables applications in spectral and beam splitting, and simultaneous reflective and transmissive displays, diffractive objects, holograms, and more.

In Chapter 3, we develop a multilayer platform to enable direct laser writing of Sb2Se3 for arbitrary patterning, with the longer-term goal of enabling rewritable in-plane waveguides, including escalators for vertical coupling. We optimize and stack multiple layers of Sb2Se3 with thick oxide spacers, and improve the deposition conditions to reduce the surface roughness. Using a thick-oxide spacer within our Sb2Se3 stack with the ALD Al2O3 protection and capping layers, we demonstrate layer-selective laser writing, where patterns can be written at different depths by refocusing the laser. We also examine laser-induced crystallinity as a function of exposure time and power, as well as scan direction. We observe that the apparent grain orientation generally follows the writing direction while the grain size increases with writing time within a specific range. Finally, we explore the feasibility of using direct laser writing on PCMs in contact with 2D magnetic materials towards a reconfigurable dielectric screening platform.

In Chapter 4, we explore new high-entropy chalcogenide PCMs inspired by entropy-stabilization concepts. Starting from a Si-Sb2Se3 baseline and extending to a five-element Si-Ge2Sb2Se4Te system, we observed a promising preliminary result in which the annealed sample exhibited relatively homogeneous optical contrast and Raman features not observed in either Sb2Se3 or Ge2Sb2Se4Te, which are suggestive of a potentially single phase signature.However, this behavior could not be reproduced and is therefore treated as a tentative lead rather than a confirmed single phase result. Using SiSe2 as an alternative Si source for co-sputtering with Ge2Sb2Se4Te, we systematically varied annealing temperature and time. Under the current processing window, phase segregation (nanocomposite) dominated, identifying phase stability against segregation as a key bottleneck for high-entropy PCM discovery in this material system.

Collectively, this thesis establishes a sputtering-based process and integration framework for phase-change thin films. It demonstrates reconfigurable photonic functionality by embedding PCMs into engineered optical thin-film structures and investigates laser-induced crystallinity and microstructure control to support photonic patterning and integration.