The Condensed Matter & Surface Sciences Colloquium series presents Dr. Nikolas Podraza on “Utilizing Optical Characterization for Materials and Device Development” on Thursday, April 10, at 4:10 p.m. in Walter Lecture Hall 245.
Dr. Nikolas Podraza
Podraza is Assistant Professor of Physics at the University of Toledo. “Current research interests include the physical mechanisms of film growth that control the electromagnetic, optical, and infrared-vibrational properties of thin film materials used in opto-electronic device applications. These areas involve studying film growth mechanisms and structure to identify nucleation and growth behavior as well as phase changes in thin films; the optical properties pertaining to solid state physical properties including the electronic band structure of materials, as well as the phonon or vibrational modes in solids; electromagnetic and optical behavior and how it can be manipulated by structure such as in metamaterials and plasmonics; and finally the total impact that all of these areas upon device physics and functionality,” says his website.
Abstract: The interactions between light and matter surround us constantly, but require a fundamental understanding of electricity and magnetism, optics, quantum mechanics, and condensed matter physics to be properly utilized. In essence, the characteristics of a material influence the response of incoming electromagnetic radiation, specifically the electric field of incident light, as described by the spectroscopic complex index of refraction (N = n + ik) or dielectric function (ɛ = ɛ1 + iɛ2). Such material characteristic may include the chemistry, order (crystal phase or amorphous nature), composition (voids, composites, etc.), and structure (thin film thicknesses, multiple layers stacks, spatial non-uniformity, general morphology). All of these properties influence the behavior of a thin film in opto-electronic devices (such as solar cells), as well as the device’s ultimate performance. In turn, materials and structures may be modified to control this behavior as in plasmonic, multiple layer optic, and polarization generation / detection applications or in devices exploiting one or more of these effects. Non-destructive optical metrology techniques such as spectroscopic ellipsometry have been of interest for the semiconductor industry and researchers developing thin film technologies as no additional sample preparation is required and layers in the device configuration can be probed. Over the near infrared to ultraviolet (currently 1700 to 190 nm or 0.75 to 6.5 eV) ellipsometric spectra can be acquired quickly (~50 ms), allowing for monitoring changes occurring during processing or quickly scanning samples after processing for quality control and assessing uniformity. Extension of ellipsometric measurements further into the infrared (currently to 33 mm or 0.04 eV) provide additional sensitivity to the vibrational modes and / or conduction by free carriers of each component layer, which can be connected to bonding configurations and electrical response, respectively. This characterization capability allows for the material properties, N or ɛ, and microstructure to be probed, and then used to guide the design of optical and electrical devices. The ability to tailor the microstructure of materials to control the optical response will be discussed in terms of variations in N or ɛ specifically with respect to anisotropic structurally engineered thin films; the relationship of N or ɛ to microstructure and electrical properties of materials used in thin film (CdTe, CIGS, Si:H) photovoltaics; and how material and microstructural variations impact the performance of photovoltaic devices for enhanced light collection and improved efficiency.
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