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Spectroscopic Ellipsometry as a Versatile, Non-Contact Probe of Optical, Electrical, and Structural Properties in Thin Films: Applications in Photovoltaics
Junda, Maxwell M.

2017, Doctor of Philosophy, University of Toledo, Physics.

This dissertation describes a collection of studies that demonstrate and expand the many configurations in which spectroscopic ellipsometry (SE) can be applied to material characterization, primarily for thin films. The materials investigated each have relevance to photovoltaics, but the methods described herein can be applicable to the study of materials used in virtually any application. In aggregate, the measurement and data modeling techniques represent a broad set of tools that can be used to study the optoelectronic and structural characteristics of amorphous, polycrystalline, nanostructured, or inhomogeneous layers within thin film solar cell devices. The capabilities for SE to determine properties of a sample of interest well beyond simple optical response functions are demonstrated.


In particular, SE is used in a real-time, in situ configuration where a series of measurements are taken continually during the deposition of all layers in complete hydrogenated amorphous silicon (a-Si:H) solar cells. Thus, the application of real-time SE (RTSE) to the entire process of solar cell fabrication is realized. The optical response and thicknesses of each layer are obtained and are used to interpret variation in the measured electrical performance between different devices. The SE-derived results are then used as inputs to a simulation of the expected current generated by the devices, the results of which were successful in identifying damage to a transparent conducting layer resulting from exposure to plasma during sample fabrication as the source of performance losses.


The study of full a-Si:H-based solar cells revealed the presence of subtle optical property gradients within individual layers. The ability to characterize slight inhomogeneity using RTSE was further developed using measurements collected for a-Si:H films deposited under various conditions and on various substrates. In particular, this work systematically examines a range of modeling configurations in the virtual interface analysis (VIA) technique used to extract the results. Through testing the influence of two specific modeling parameters on the overall error associated with each model, the optimum analysis models are identified, enabling the extraction of the most accurate results. Ultimately, the evolution of an optical broadening parameter is extracted for a-Si:H films grown on various substrates and under various conditions. Limitations of the VIA technique are also identified and discussed.


Next, the effects of varying deposition and processing conditions on the optical response of resultant films is studied through ex situ SE measurements of a series of oxygenated cadmium sulfide (CdS:O) films. A custom parametric description of the optical response of these films applicable to polycrystalline semiconductors is developed that makes use of physically realistic descriptions of the optical features over the full measured spectral range. From the resulting optical properties, increasing oxygen presence during deposition is shown to suppress absorption in the films and modify the band gap energy for as-deposited films. Additionally, annealing is shown to revert all CdS:O band gap energies to that of pure cadmium sulfide (2.4 eV) and improve crystallographic order. Since CdS:O is used in high performance thin film photovoltaics, these optical results contribute to an explanation of what the role of this material is specifically in improved device performance, specifically the decreased optical absorption at ultraviolet photon energies.


Finally, the benefits of extending SE into the THz frequency regime is investigated. Since THz SE is an emerging subfield of ellipsometry unique THz-specific considerations are investigated and discussed. Limitations imposed by the instrument are identified and efforts to identify the minimum measurement resolution and range that will still produce acceptable results are presented as a means for decreasing total measurement time. Then, the results of THz SE applied to single bulk crystals is presented where sensitivity to carrier concentrations as low as 1014 cm-3 is demonstrated. Finally, a study of the effects of doping on a single walled carbon nanotube (SWCNT) thin film is reported. More specifically, the optical response of the SWCNT film over a wide spectral range spanning the THz to the ultraviolet and uniaxially anisotropic electrical properties are determined for the film in two doping states.

Nikolas Podraza, Dr. (Advisor)
Collins Robert, Dr. (Committee Member)
Yan Yanfa, Dr. (Committee Member)
Gao Bo, Dr. (Committee Member)
Hofmann Tino, Dr. (Committee Member)
153 p.

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Junda, M. (2017). Spectroscopic Ellipsometry as a Versatile, Non-Contact Probe of Optical, Electrical, and Structural Properties in Thin Films: Applications in Photovoltaics. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/

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Junda, Maxwell. "Spectroscopic Ellipsometry as a Versatile, Non-Contact Probe of Optical, Electrical, and Structural Properties in Thin Films: Applications in Photovoltaics." Electronic Thesis or Dissertation. University of Toledo, 2017. OhioLINK Electronic Theses and Dissertations Center. 15 Nov 2018.

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Junda, Maxwell "Spectroscopic Ellipsometry as a Versatile, Non-Contact Probe of Optical, Electrical, and Structural Properties in Thin Films: Applications in Photovoltaics." Electronic Thesis or Dissertation. University of Toledo, 2017. https://etd.ohiolink.edu/

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