AVS 61st International Symposium & Exhibition
    Spectroscopic Ellipsometry Focus Topic Thursday Sessions
       Session EL+AS+EM+EN+SS-ThM

Paper EL+AS+EM+EN+SS-ThM11
Infrared to Ultraviolet Optical Properties of Gadolinium Gallium Garnet (Gd3Ga5O12) and Bismuth Germanate (Bi4Ge3O12) Single Crystals

Thursday, November 13, 2014, 11:20 am, Room 304

Session: Spectroscopic Ellipsometry for Photovoltaics and Instrument Development
Presenter: Kiran Ghimire, University of Toledo
Authors: K. Ghimire, University of Toledo
H. Haneef, University of Toledo
N.J. Podraza, University of Toledo
Correspondent: Click to Email
The optical properties of commercially available oxide single crystals gadolinium gallium garnet (Gd3Ga5O12) and bismuth germanate (Bi4Ge3O12) have been studied over a maximum spectral range of 0.034 to 6.5 eV by multiple spectroscopic ellipsometry and transmittance measurements, via a multichannel ellipsometer from the near infrared to ultraviolet, a Fourier transform infrared (FTIR) ellipsometer, and a spectrophotometer. Spectroscopic measurements from each instrument and over the respective spectral ranges have been analyzed differently yet yield optical properties over the full measured range. Near infrared to ultraviolet ellipsometric spectra are analyzed using a divided spectral range procedure whereby information below and above the band gap are fit to models with separate physically realistic parameterizations of the complex dielectric function spectra (ε = ε1 + iε2) that share the same structural parameters—surface roughness thickness in these cases. The surface roughness thicknesses are then fixed and direct numerical inversion is used to determine ε over the continuous spectral range. Analysis of transmittance and FTIR ellipsometric spectra also relies upon fixing surface roughness from near infrared to ultraviolet spectroscopic ellipsometry analysis and either direct numerical inversion or parametric models to determine ε. In the vicinity of the band gap, the absorption coefficient (α) obtained from ε is then combined with low values of ε obtained from transmittance below the absorption edge, where ellipsometry lacks sensitivity. The combined α from transmission and ellipsometry is used to determine the band gap of the materials. Unlike Gd3Ga5O12, the band gap of the Bi4Ge3O12 is sufficiently within the measured spectral range so critical point analysis has been performed on Bi4Ge3O12 by extending the measured spectral range up to 6.5 eV, where the material was found to have additional critical points. FTIR ellipsometric spectra are analyzed with a parametric model combining Gaussian and Lorentzian broadened resonance features to represent modes attributed to chemical bonding and lattice vibrations. The results of these analysis procedures yield e from the infrared to ultraviolet, from which information on the band gap, electronic transitions, and vibrational modes are obtained.