Ultrafast Laser-Material Interactions with Wide Bandgap Semiconductors and High Entropy Oxide Based Memristors

Abstract

Point defects in the materials used to fabricate electronic devices must be precisely controlled to achieve the desired properties because point defects are commonly considered the origin of device failure. Here, we explored several methods to control point defects. The unique material interactions with ultrafast (femtosecond, 10-15 s) pulsed lasers can induce point defects by inducing an extreme non-equilibrium state of matter, providing an ability to modify a material’s electrical properties with less damage. Oxygen vacancies in oxide-based memristors, which govern the device characteristics, can be controlled by forming high-entropy systems and tuning the growth conditions. In Part I of this dissertation, ultrafast laser material processing is explored in wide-bandgap semiconductors—specifically silicon carbide (4H-SiC) and gallium oxide (β-Ga2O3)—in order to overcome the processing challenges with these thermally and chemically-robust materials. Irradiation experiments with a fundamental and frequency-doubled pulse of a Ti:sapphire laser (780 nm, 150 fs pulse width) were conducted on 4H-SiC and β-Ga2O3 to investigate ultrafast laser-induced surface features and their threshold fluences. In β-Ga2O3, straight cracks aligned along the (001) crystal orientation, and submicron geometric recrystallization features were observed for the first time. After multiple pulse exposures, high spatial frequency laser-induced periodic surface structures and micro/nanostructures were observed. In 4H-SiC, frequency-doubled pulse irradiation created highly-aligned sub-100 nm ripple structures, which are promising in photonics and plasmonics applications. Ultrafast laser irradiation of β-Ga2O3 rendered the surface hydrophobic, as confirmed by contact angle measurements. XPS and Raman spectroscopy of the ultrafast laser-irradiated 4H-SiC confirmed oxidation and amorphization, respectively. In contrast, the laser irradiation of β-Ga2O3 did not change its crystallinity, as confirmed by electron backscatter diffraction and Raman spectroscopy. Electrical modification via ultrafast laser irradiation was demonstrated by rastering and subsequent metallization of 4H-SiC, which exhibited a 103 times increase in its lateral conductance with minimal surface damage. The clear photocurrent observed in the ultrafast laser-treated 4H-SiC Schottky diode suggests point defect generation after laser irradiation. All of these experimental results offer new opportunities for ultrafast laser processing of wide-bandgap semiconductors, as well as a fundamental understanding. In Part II of this dissertation, high entropy oxide based memristors were produced for the first time. High-entropy materials are composed of more than five metal elements stabilized by increased mixing entropy. A (Zr, Hf, Nb, Ta, Mo, W) high-entropy oxide was proposed to combine the strengths of Hf, Ta, and W-based memristors by the ‘cocktail’ effect. High-entropy oxide thin films were deposited by pulsed laser deposition with a uniform element distribution and low surface roughness. Various deposition conditions, including target hardness, oxygen partial pressure, substrate temperature, and target aging were carefully controlled to achieve material properties suitable for memristors. The optical characteristics obtained from the Tauc plots and chemical analysis from the XPS spectra confirmed the existence of intermediate states originating from oxygen vacancies from Mo and W suboxides. Electrical characterization of the high entropy oxide based memristor demonstrated superior performance in various aspects, including good variability, perfect retention, nonlinear characteristics, gradual conductance increase, and forming-free behavior, which is promising for neuromorphic applications. Competitive results were obtained, including forming free operation with a low SET voltage (<1 V), gradual conductance update, low device-to-device variability (σ <0.1 V), and 20 hours of retention at 100 ℃.