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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2017)

This dissertation is divided into two parts. The first will discuss a theory that was identified to narrow the size distribution of nanoparticles grown via ion beam synthesis (IBS). Specifically, a simple mathematical argument explains a processing route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained.

The second part presents a theory for nucleation in confined high-aspect ratio thin films. Specifically, classical nucleation theory is used to consider the solidification of a melt confined between two planar surfaces. The critical nuclei shapes and the associated nucleation energy barriers are computed as a function of the thickness of the film, and the film's relevant bulk and interface energies. The analysis is then repeated for the melting transition, and expressions for the depression and elevation of the melting temperature are found. A nucleus morphology diagram is constructed. This diagram presents the lowest energy morphology of the nuclei, as well as melting points, as a function of the system parameters. Using the nucleus morphology diagram, experimental and system parameters that allow for the desired nucleation behavior can be identified. Furthermore, the nucleus morphology diagram illustrates a region of parameter space where the film is predicted to solidify above its thermodynamic bulk melting temperature, a behavior termed presolidification. The theory is used to predict the temperature at which nucleation of the solid phase and liquid phase are expected for Ge between two glass substrates. Furthermore, a possible route for controlling the orientation of the film is identified as confinement of the film can lead to a large range of solid phase nucleation temperatures, which are a function of the system's interfacial free energies. By controlling the growth temperature, certain orientations may not be able to nucleate thereby reducing the possible number of orientations within a film.

    Cover page: A Theory of Nanoparticle Growth and a Theory of Thin Film Nucleation
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    The application of strain to semiconductors allows for controlled modification of their band structure. This principle is employed for the manufacturing of devices ranging from high-performance transistors to solid-state lasers. Traditionally, strain is typically achieved via growth on lattice-mismatched substrates. For two-dimensional (2D) semiconductors, this is not feasible as they typically do not interact epitaxially with the substrate. Here, we demonstrate controlled strain engineering of 2D semiconductors during synthesis by utilizing the thermal coefficient of expansion mismatch between the substrate and semiconductor. Using WSe2 as a model system, we demonstrate stable built-in strains ranging from 1% tensile to 0.2% compressive on substrates with different thermal coefficient of expansion. Consequently, we observe a dramatic modulation of the band structure, manifested by a strain-driven indirect-to-direct bandgap transition and brightening of the dark exciton in bilayer and monolayer WSe2, respectively. The growth method developed here should enable flexibility in design of more sophisticated devices based on 2D materials.Strain engineering is an essential tool for modifying local electronic properties in silicon-based electronics. Here, Ahn et al. demonstrate control of biaxial strain in two-dimensional materials based on the growth substrate, enabling more complex low-dimensional electronics.
      Cover page: Strain-engineered growth of two-dimensional materials
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2019)
      In recent years, there have been tremendous advancements in the growth of monolayer transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). However, obtaining high photoluminescence quantum yield (PL QY), which is the key figure of merit for optoelectronics, is still challenging in the grown monolayers. Specifically, the as-grown monolayers often exhibit lower PL QY than their mechanically exfoliated counterparts. In this work, we demonstrate synthetic tungsten diselenide (WSe2) monolayers with PL QY exceeding that of exfoliated crystals by over an order of magnitude. PL QY of ~60% is obtained in monolayer films grown by CVD, which is the highest reported value to date for WSe2 prepared by any technique. The high optoelectronic quality is enabled by the combination of optimizing growth conditions via tuning the halide promoter ratio, and introducing a simple substrate decoupling method via solvent evaporation, which also mechanically relaxes the grown films. The achievement of scalable WSe2 with high PL QY could potentially enable the emergence of technologically relevant devices at the atomically thin limit.
        Cover page: Synthetic WSe2 monolayers with high photoluminescence quantum yield
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2016)
        The III-V compound semiconductors exhibit superb electronic and optoelectronic properties. Traditionally, closely lattice-matched epitaxial substrates have been required for the growth of high-quality single-crystal III-V thin films and patterned microstructures. To remove this materials constraint, here we introduce a growth mode that enables direct writing of single-crystalline III-V's on amorphous substrates, thus further expanding their utility for various applications. The process utilizes templated liquid-phase crystal growth that results in user-tunable, patterned micro and nanostructures of single-crystalline III-V's of up to tens of micrometres in lateral dimensions. InP is chosen as a model material system owing to its technological importance. The patterned InP single crystals are configured as high-performance transistors and photodetectors directly on amorphous SiO2 growth substrates, with performance matching state-of-the-art epitaxially grown devices. The work presents an important advance towards universal integration of III-V's on application-specific substrates by direct growth.
          Cover page: Direct growth of single-crystalline III–V semiconductors on amorphous substrates
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