AVS2012 Session EL+TF+AS+EM+SS-TuP: Spectroscopic Ellipsometry Poster Session
Tuesday, October 30, 2012 6:00 PM in Central Hall
EL+TF+AS+EM+SS-TuP-1 Ellipsometric Characterization of Iron Pyrite (FeS2) and Samarium Sesquisulfide (Sm2S3) Thin Films
Amitabha Sarkar, Natale Ianno (University of Nebraska-Lincoln); Joseph Brewer (Rare Earth Solar)
Iron pyrite (FeS2) and samarium sesquisulfide (Sm2S3) are transition metal chalcogenides characterized as absorbing semiconductors with bandgaps of 0.95 eV and 1.8 eV respectively. Synthesis of both n-type and p-type samples have been reported in the form of single crystals and thin films for both materials. As a result of these properties they have received considerable interest as photovoltaic absorber materials. We present the characterization of FeS2 and Sm2S3 thin films using spectroscopic ellipsometry. FeS2 thin films were synthesized by sulfurizing DC magnetron sputtered iron films and reactive ion sputtered iron (III) oxide films in H2S / Ar atmosphere. Sm2S3 thin films were synthesized by reactive ion sputtering of Sm in an H2S / Ar atmosphere. This analysis gives the optical properties of chalcogenide films from near-UV (300 nm) to the mid-IR (20 μm). This can then be correlated to the structural and electronic properties as well. The analysis is corroborated with results obtained from Raman spectroscopy, scanning electron microscopy, profilometry, X-ray diffraction (XRD), and Van der Pauw measurements. The ellipsometric results can be used to access different processing methods for synthesizing FeS2 and Sm2S3, to determine the presence of different phases and intermediate products. This work will lay the foundation for employing in situ ellipsometry as a process monitor and quality control tool during manufacture of earth abundant chalcogenide thin films.
EL+TF+AS+EM+SS-TuP-2 Temperature Dependence of the Dielectric Function of Germanium by Spectroscopic Ellipsometry
Amber Medina, Lina Abdallah, Stefan Zollner (New Mexico State University)
Germanium has important applications in photovoltaics as a substrate for III/V triple-junction solar cells, especially in space vehicles and for terrestrial concentrator-based applications. Unfortunately, the optical properties of germanium (complex refractive index and absorption coefficient) and their temperature dependence (important to consider the effects of the space environment or the radiation-induced heating in concentrators) are not as well known as for silicon, which limits the accuracy of modeling for solar cells and Ge-based optical interconnects. In this work, we report precision measurements of the complex refractive index of germanium from 0.5 to 6.6 eV at room temperature using variable-angle spectroscopic ellipsometry. To improve accuracy, especially at photon energies below 2 eV, we used a Berek waveplate compensator. By cleaning a commercial Ge wafer in isopropanol followed by deionized water, we were able to reduce the native oxide thickness to 1.3 nm. Heating the wafer in UHV at 700 K did not reduce the oxide thickness further. (The oxide thickness can be determined with precision measurements of Δ below the band gap on a single-side polished wafer.) From the ellipsometric angles of the Ge wafer measured at three angles of incidence (65, 70, and 75° ), we calculated the dielectric function from 0.5 to 6.6 eV, by correcting for the effects of a native oxide.
Mounting our wafer in a compact UHV cryostat allowed temperature-dependent measurements from 80 to 700 K at 70° angle of incidence. Using similar methods as described above, we determined the dielectric function at different temperatures. We also determined the critical-point parameters (amplitude, energy, phase angle, and broadening) of the E0, E1, E1+Δ1, E0’, and E2 critical points as a function of temperature. To separate the non-resonant contributions from the critical-point line shapes, we calculated the second derivative of the dielectric function with respect to photon energy and fitted the result to analytical line shapes with two-dimensional critical points. In general, our results are in good agreement with those of Viña et al. However, our results cover a wider spectral range and are more accurate because of the use of a compensator. Work is in progress to form thermal oxides on Ge wafers by annealing in oxygen, which will allow a multi-wafer analysis for Ge similar to work on Si by Herzinger et al.
This work was supported by NSF (HRD-0803171 and DMR-11104934) and the New Mexico Louis Stokes Alliance for Minority Participation.
Reference: L. Viña, S. Logothetidis, M. Cardona Phys. Rev. B 30, 1979 (1984).