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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

Since the beginnings of the semiconductor revolution, device engineering and material development have been deeply interconnected. Innovations in one invariably spurred development in the other. One of the most important current challenges are developing techniques which enable deposition of materials on arbitrary substrates and the design constraints of devices fabricated via these techniques. This thesis focuses on furthering the materials development-device design cycle for three techniques: (i) epitaxial lift-off, (ii) nanowire growth via the vapor-liquid-solid growth mode, and (iii) the thin-film vapor-liquid-solid growth technique. The physics of devices made via (i) and (ii) are discussed in chapters two and three, the material science of (iii) is discussed in chapter four, and a method to engineer the physical and electronic properties of individual nanowires made via (ii) is discussed in chapter

Chapter two deals with the device physics and performance of ultra-thin compound semiconductor on insulator field effect transistors. Due to its excellent electron transport properties, InAs is used as the material of choice for the transistors, termed XOI FETs. These devices are fabricated utilizing an epitaxial layer transfer technique, enabling highly lattice mismatched single-crystalline layers of InAs to be deposited on Si wafers. Allowing devices which combine the excellent transport properties of compound-semiconductors and the established processing infrastructure for silicon. Chapter three discusses the design constraints and guidelines for nanopillar photovoltaics, specifically those fabricated utilizing the CdS/CdTe material system. Critically, the materials parameters that are favorable to non-planar cells are discussed, and the performance expectations for CdS/CdTe nanopillar photovoltaics are discussed.

Chapter four focuses on a growth technique developed to enable growth of ultra-large grain III-V semiconductors on non-epitaxial substrates, termed thin-film vapor-liquid-solid growth. This growth technique takes advantage of the vapor liquid solid growth mode to enable films with thickness down to 500 nm and grain sizes greater than 50 m. This represents a significant advance over all conventional growth techniques, which produce films with grain sizes 10-100x smaller. Furthermore, the optoelectronic quality of these materials is within a few percent of single crystal, as determined by quantitative luminescence efficiency measurements. Finally, Chapter five discusses the use of nanowires as physical templates, allowing engineering of Van der Waals forces to fabricate self-selective electrical connectors. By utilizing nanowire forests as starting templates, successive layers of material are deposited, each playing a specific role in the final structure, enabling composite material properties otherwise not possible.

    Cover page: Material Science and Device Physics of Semiconductors on Arbitrary Substrates
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    © 2015 American Chemical Society. The role of TiO2 thin films deposited by atomic layer deposition on p-InP photocathodes used for solar hydrogen generation was examined. It was found that, in addition to its previously reported corrosion protection role, the large valence band offset between TiO2 and InP creates an energy barrier for holes reaching the surface. Also, the conduction band of TiO2 is well-aligned with that of InP. The combination of these two effects creates an electronselective contact with low interface recombination. Under simulated solar illumination in HClO4 aqueous electrolyte, an onset potential of >800 mV vs RHE was achieved, which is the highest yet reported for an InP photocathode.
      Cover page: Role of TiO2 Surface Passivation on Improving the Performance of p‑InP Photocathodes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2011)
      Nanoscale size-effects drastically alter the fundamental properties of semiconductors. Here, we investigate the dominant role of quantum confinement in the field-effect device properties of free-standing InAs nanomembranes with varied thicknesses of 5-50 nm. First, optical absorption studies are performed by transferring InAs "quantum membranes" (QMs) onto transparent substrates, from which the quantized sub-bands are directly visualized. These sub-bands determine the contact resistance of the system with the experimental values consistent with the expected number of quantum transport modes available for a given thickness. Finally, the effective electron mobility of InAs QMs is shown to exhibit anomalous field- and thickness-dependences that are in distinct contrast to the conventional MOSFET models, arising from the strong quantum confinement of carriers. The results provide an important advance towards establishing the fundamental device physics of 2-D semiconductors.
        Cover page: Highly Quantum-Confined InAs Nanoscale Membranes
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2013)
        III-V photovoltaics (PVs) have demonstrated the highest power conversion efficiencies for both single- and multi-junction cells. However, expensive epitaxial growth substrates, low precursor utilization rates, long growth times, and large equipment investments restrict applications to concentrated and space photovoltaics (PVs). Here, we demonstrate the first vapor-liquid-solid (VLS) growth of high-quality III-V thin-films on metal foils as a promising platform for large-area terrestrial PVs overcoming the above obstacles. We demonstrate 1-3 μm thick InP thin-films on Mo foils with ultra-large grain size up to 100 μm, which is ~100 times larger than those obtained by conventional growth processes. The films exhibit electron mobilities as high as 500 cm²/V-s and minority carrier lifetimes as long as 2.5 ns. Furthermore, under 1-sun equivalent illumination, photoluminescence efficiency measurements indicate that an open circuit voltage of up to 930 mV can be achieved, only 40 mV lower than measured on a single crystal reference wafer.
          Cover page: A direct thin-film path towards low-cost large-area III-V photovoltaics.
          • 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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