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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2019)

Silicon nanostructures serve as the backbone of modern electronics and photonics. Particularly, silicon nanoresonator became an increasingly important building block for the advanced photonics applications, including sensing, wavefront engineering and integrated photonics. Its crystallinity engineering is critical for enabling the manufacturability as well as fueling the continuous innovation on the device functionalities. Meanwhile, the scalable patterning of such nanostructures could offer cost-effective and highly controllable manufacturing routes beyond the conventional complementary metal-oxide-semiconductor (CMOS) fabrication. Owing to the controllable energy delivery and capability to interact with nanostructures, pulsed laser processing could significantly shed light on the above challenges and opportunities.

The pulsed laser processing of semiconductor material has been intensely investigated in the last century providing insights into fundamental light-material interaction. In early this century, thin-film laser processing became the main research topic as the complex coupling of heat transfer and phase transformation has enabled the technical innovation towards thin-film transistors in the display industry. However, when the lateral dimensions of the semiconductor material further shrunk down to the order of 100nm, the governing laws of the multiphysics interaction had to incite a significant paradigm shift in the study of the optical coupling, heat transfer and phase transformations.

This dissertation focus on understanding new coupling mechanism of pulsed laser interaction with silicon nanostructures, and exploiting them for scalable patterning and crystal engineering of the functional resonator arrays. Based on the unique near-field enhanced optical absorption, we first demonstrate the optically directed assembly of silicon nanoresonators. Spherical silicon nanoresonators can be fabricated with programmable numbers and combinations. Based on the ultrafast quenching rate and high adhesion work of silicon, we then demonstrate the deformation-free reversible phase transformation of silicon resonators that enables the active modulation of visible wavelength metasurfaces. Our in situ reflection probing of a single silicon nanodisk coupled with comprehensive simulations helps to reveal the mechanism behind the amorphization. We believe the experimental probing results lends support to the hypotheses silicon relaxation time scale on the silicon glass transformation. Lastly, through femtosecond laser irradiation, we observed the ultrafast field enhanced selective material removal within nanodisks. The detailed dynamics of the field enhancement, carrier excitation, non-thermal melting and subsequent cold ablation are studied ex-situ and in-situ. The mechanism can be used for scalable manufacturing of bowtie sensing structures and laser printing optical metasurfaces.

    Cover page: Pulsed Laser-induced Light Coupling and Phase Transformation of Silicon Nanostructures
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Transition metal dichalcogenides (TMDCs) have shown exceptional optoelectronic properties that can potentially substitute conventional silicon-based devices and be utilized in sensors and energy devices. To exploit their wide array of potential applications, it is necessary to develop methods capable of on demand, location selective, and tunable formation of structures of arbitrary shape. Here, we demonstrated high-speed direct writing of MoSe2 by laser-induced selenization process in vacuum or ambient environment. Laser irradiation on predeposited Se/Mo multilayer promptly forms polycrystalline MoSe2 in a site-selective manner without use of photomask and toxic gas. In situ reflectance measurement and temperature simulation identified distinct characteristic processing stages, revealing that the time required for synthesizing â20 nm-thick MoSe2 polycrystal film is of the order of 10-3s, which is significantly faster than the conventional selenization process using furnace annealing. We believe that the laser synthesis of MoSe2 demonstrated in this study is of general applicability and therefore offers a simple and straightforward route for obtaining arbitrary patterns in TMDCs.
      Cover page: High-Speed Direct Writing of MoSe2 by Maskless and Gas-Free Laser-Assisted Selenization ProcessCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      In this study, we observe and study the early evolution of cavitation bubbles generated during pulsed laser ablation of titanium targets in different liquid environments utilizing a high-resolution stroboscopic shadowgraphy system. A hydrodynamic model is proposed to calculate the early pressure changes within the bubble and in the surrounding fluid. Our results show that the cavitation bubble is a low-pressure region that is bounded by a high-pressure fluid lamina after the incipient stage, and its evolution is primarily affected by the liquid density. Moreover, the initial bubble pressure increases substantially in high viscosity liquids. This work illuminates how the liquid properties affect the early bubble dynamics and is a step towards a deeper understanding of laser-materials interactions in liquid environments.
        Cover page: Early dynamics of cavitation bubbles generated during ns laser ablation of submerged targets.
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2022)
        Chemical doping can be used to control the charge-carrier polarity and concentration in two-dimensional van der Waals materials. However, conventional methods based on substitutional doping or surface functionalization result in the degradation of electrical mobility due to structural disorder, and the maximum doping density is set by the solubility limit of dopants. Here we show that a reversible laser-assisted chlorination process can be used to create high doping concentrations (above 3 × 1013 cm−2) in graphene monolayers with minimal drops in mobility. The approach uses two lasers—with distinct photon energies and geometric configurations—that are designed for chlorination and subsequent chlorine removal, allowing highly doped patterns to be written and erased without damaging the graphene. To illustrate the capabilities of our approach, we use it to create rewritable photoactive junctions for graphene-based photodetectors.
          Cover page: A laser-assisted chlorination process for reversible writing of doping patterns in grapheneCreative Commons 'BY-NC' version 4.0 license
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2019)
          Organic semiconductors (OSCs) offer a new avenue to the next-generation electronics, but the lack of a scalable and inexpensive nanoscale patterning/deposition technique still limits their use in electronic applications. Recently, a new lithographic etching technique has been introduced that uses molecular dopants to reduce semiconducting polymer solubility in solvents and a direct-write laser to remove dopants locally, enabling rapid OSC etching with diffraction limited resolution. Previous publications postulated that the reaction that enables patterning is a photochemical reaction between photoexcited dopants with neutral solvent molecules. In this work, we analyze the photoinduced dissolution kinetics of F4TCNQ doped P3HT films using time-resolved in situ optical probing. We find two competing mechanisms that control de-doping and dissolution: the first is the photochemical reaction posited in the literature, and the second involves direct heating of the polymer by the laser, inducing increased solubility for both the polymer and dopant. We show that the wavelength-specific photochemical effect is dominant in low photon doses while the photothermal effect is dominant with high excitation rates regardless of laser wavelength. With sufficiently high optical intensity input, the photothermal mechanism can in principle achieve a high writing speed up to 1 m/s. Our findings bring new insights into the mechanisms behind laser direct writing of OSCs based on dopant induced solubility control and enable ultraprecise fabrications of various device configurations in large-scale manufacturing.
            Cover page: High-Speed Photothermal Patterning of Doped Polymer Films
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2018)
            Micro-electromechanical (MEM) switches, with advantages such as quasi-zero leakage current, emerge as attractive candidates for overcoming the physical limits of complementary metal-oxide semiconductor (CMOS) devices. To practically integrate MEM switches into CMOS circuits, two major challenges must be addressed: sub 1 V operating voltage to match the voltage levels in current circuit systems and being able to deliver at least millions of operating cycles. However, existing sub 1 V mechanical switches are mostly subject to significant body bias and/or limited lifetimes, thus failing to meet both limitations simultaneously. Here 0.2 V MEM switching devices with ≳106 safe operating cycles in ambient air are reported, which achieve the lowest operating voltage in mechanical switches without body bias reported to date. The ultralow operating voltage is mainly enabled by the abrupt phase transition of nanolayered vanadium dioxide (VO2 ) slightly above room temperature. The phase-transition MEM switches open possibilities for sub 1 V hybrid integrated devices/circuits/systems, as well as ultralow power consumption sensors for Internet of Things applications.
              Cover page: A 0.2 V Micro‐Electromechanical Switch Enabled by a Phase Transition
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