FABRICATION OF RADIAL P-N GERMANIUM NANOWIRES FOR THERMOPHOTOVOLTAIC APPLICATIONS

Open Access
Author:
Evey, Jeffrey
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 31, 2009
Committee Members:
  • Joan Marie Redwing, Thesis Advisor/Co-Advisor
Keywords:
  • germanium
  • photovoltaic
  • thermophotovoltaic
  • template
  • porous
  • gold-seeded
  • nanowire
  • metalorganic
  • chemical vapor deposition
Abstract:
Interest in thermophotovoltaic systems began with Dr. Henry Kolm and Dr. Aigrain, doing research at MIT in the late 1950’s and early 1960’s.1 Since that time, research has focused for the most part on system components: the development for instance of the emitters, reflectors, recyclers, the thermophotovoltaic cell, and the filters. Interest remains in improving the efficiency and reducing the cost of the photovoltaic module in a thermophotovoltaic system. This research is focused on the fabrication of radial p-n junction germanium nanowires for thermophotovoltaic cells. Germanium nanowire photovoltaic cells have potential advantages over other possible candidates: germanium has a bandgap suitable to the spectrum of emitters being developed, and germanium has a long history of fundamental and technological research. Additionally, axially aligned, epitaxially grown nanowires allow for the vertical (axial) absorption of light, with radial carrier transport, potentially allowing for a higher tolerance for bulk recombination rate, due to the shorter carrier extraction lengths. The research herein has focused on achieving two distinct goals: germanium nanowire core growth and p-type doping, and germanium thin film deposition and n-type doping. Germanium nanowire growth has been performed with the goal of achieving p-type nanowire cores grown inside gold-seeded porous anodized-aluminum-oxide-on-glass substrates. In order to extract correlations between growth process variables and structural properties for germanium nanowire core growths, germanium wafers were also used as a substrate. Scanning electron microscopy and transmission electron microscopy were used for structural characterization. High quality epitaxial nanowire growth was obtained on germanium wafers, with some tapering due to simultaneous gas phase decomposition of germane. The thin film deposited by the simultaneous gas phase decomposition was single crystal if germane only was flowing. If a dopant source (diborane) is present, there is an increase in the rate of thin film deposition. Tapering was found to increase substantially at diborane to germane flow ratios of greater than 10-4. The n-type germanium thin film coating was deposited on sapphire for growth rate measurements and electrical characterization, and on germanium planar wafers and the same covered in epitaxial germanium nanowires for device measurements. The film was characterized by four-point probe and Hall measurements, and high electron concentrations were obtained using phosphine to germane ratios greater than approximately 2.1 * 10-3. This highly doped n-type germanium thin film, when coating single crystal nanowires, was clearly polycrystalline. P-n junctions were fabricated and characterized to determine the current-voltage characteristics. N-type thin films deposited on planar p-type germanium wafers with or without germanium nanowires were measured. Planar samples exhibited clear rectification and were compared to the diode equation with series resistance and ideality factor to extract diode performance measures. Recombination, as manifest in the faster current turn-on from the thicker of two thin films, may limit the planar diode quality, placing limits on the thickness of the films. The nanowire junctions were not rectifying. Rectification might be strengthened by cleaning the nanowire surface by an oxidation-etch procedure performed on nanowires prior to thin film shell deposition. Future research in epitaxial n-type thin film deposition on the nanowire core, or improving the doping character of the nanowire core, are likely to be among the most beneficial avenues for improving device performance.