Semiconductor Fabrics and Single Crystal Fibers for Optoelectronic Applications

Restricted (Penn State Only)
Author:
Ji, Xiaoyu
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 20, 2017
Committee Members:
  • Venkatraman Gopalan, Dissertation Advisor
  • Venkatraman Gopalan, Committee Chair
  • Suzanne E. Mohney, Committee Member
  • Joan M. Redwing, Committee Member
  • Zhi-Wen Liu, Outside Member
  • John V. Badding, Committee Member
Keywords:
  • Optical fiber
  • Silicon Photonics
  • Chemical vapor deposition
  • Laser crystallization
  • Crystal growth
  • Optoelectronics
  • Hydrogenated amorphous silicon
  • Photovoltaics
  • Flexible electronics
Abstract:
Semiconductor core, glass cladding optical fibers are a class of unique fiber structures that are specifically designed for infrared applications. Semiconductors, such as silicon and germanium, have wide transmission windows extending into the mid- and even the far-infrared. They possess higher refractive indices and lower intrinsic absorption than silica within this wavelength range, making such materials ideal waveguiding media for infrared light when forming fiber waveguides with a silica glass cladding. Semiconductors can be precisely doped so they give superior optoelectronic responses, adding new functions to optical fibers. However, the performance of the fibers fabricated by existing methods is limited by the high optical losses due to the existence of grain boundaries inside the cores. These boundary defects scatter photons, resulting in increased optical losses, and trap carriers, degrading the optoelectronic responses. In this dissertation work, a scanning laser crystallization method for fibers is developed and utilized to overcome the drawbacks mentioned above. The pure silicon and germanium fibers are first deposited inside silica capillaries as amorphous states using high pressure chemical vapor deposition. Then a 488 nm continuous wave argon ion laser is employed as the heating source for crystallizing the semiconductor cores. Different from conventional thermal annealing, in which nucleation randomly initiates along the fiber, the focused laser heats up and melts the semiconductors locally, and the molten zone is moved directionally along the fiber by precisely moving the fiber. The fibers produced this way possess a single crystalline nature over a length on the scale of centimeters (~10,000 x improvement compared to thermal annealing). With small core diameters (< 6 μm), the length to diameter aspect ratio is increased to ~1600, making these the largest of any single crystal fibers produced by established methods. Because of the short laser irradiation time (1ms-10ms), which can be controlled by the fiber moving speed, the core-cladding interface remains both structurally and chemically smooth, and there is no significant oxygen diffusion from the glass cladding into the core. Because of these attributes, the fibers perform as low-loss waveguides. The removal of the grain boundaries also contributes to a higher photo-responsivity (the ability of generating currents under external electric field in response to light absorption). In order to understand the laser heating process and find a way to control the crystal growth, we perform finite element modeling to investigate how the fiber moving speed and the laser irradiation power affects the temperature profile induced inside the fiber cores. A processing diagram in the “laser power – scanning speed” space is proposed, and by comparing with the experimental results, we find that there exists an optimum range for the laser power and scanning speed, within which single crystal growth is preferred. After the laser processing, there is observable residual thermal stress in the semiconductor core. This thermal stress can be relaxed by etching away the surrounding glass cladding, which indicates that the thermal stress originated from the core-cladding interface. By using X-ray Laue diffraction, the full strain/stress tensor of the single crystal silicon microwire embedded in the glass capillary is determined. An unusual triaxial tensile strain is revealed, which has not been reported in silicon. We performed density functional theory (DFT) calculations using the experimental strain components as the input and predicted the electronic band structure change of the silicon microwire under such strain conditions. Strain can be utilized as a tool to tune material properties such as the optical band gap, carrier mobility and light emission efficiency. The fibers could also be used in infrared imaging as a way to spectroscopically distinguish different tissue components. Lastly, the fabrication of hydrogenated amorphous silicon and germanium based, conformally coated fabrics using the high pressure chemical vapor deposition is summarized, and preliminary Schottky junction solar cell behavior is demonstrated. These fabrics could be utilized as the platform for potential flexible electronic and optoelectronic devices, such as wearable solar collection textiles.