Dissertation Defense

Ultrafast Laser Material Interactions with Wide Band Gap Semiconductors and High Entropy Oxide Based Memristors

Minhyung Ahn
Minhyung Ahn


Ultrafast laser material processing is explored in wide band gap semiconductors, specifically silicon carbide (4H-SiC) and gallium oxide (β-Ga2O3), in order to overcome the processing challenges of these thermally and chemically robust materials. The ability of ultrafast lasers to induce a highly non equilibrium state of matter by nonlinear excitation processes, and subsequent electron-phonon relaxation processes, results in laser induced periodic surface structures (LIPSS) and point defect generation. In 4H-SiC, highly aligned sub 100 nm ripple structures were created, which is promising for photonics and plasmonics applications. Irradiation on heated 4H-SiC may suppress Coulomb explosion. Rastering experiments on 4H-SiC demonstrate 103 times increase of lateral conductance with low surface damage. In β-Ga2O3, straight cracks aligned to the (001) crystal directions and sub-micron geometric-shaped recrystallization features were observed for the first time. Ultrafast laser irradiation of β-Ga2O3 can convert the surface to hydrophobic, confirmed by contact angle experiments.

(Zr,Hf,Nb,Ta,Mo,W) high entropy oxides (HEO) thin films were successfully deposited by pulsed laser deposition for memristor. Optical characteristics and X-ray photoelectron spectroscopy (XPS) analysis identified Mo and W suboxides as the origin of oxygen vacancies, revealing the resistive switching mechanism. HEO based memristors demonstrated superior performance by forming free operation with low SET voltage (<1V), low device-to device variability (σ <0.1 V), and 20 hours of retention in 100 ℃, which are promising in neuromorphic applications.

Chair: Professor Jamie Phillips

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