C. M. Herzinger

University of Nebraska at Lincoln, Lincoln, NE, United States

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Publications (82)103.54 Total impact

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    ABSTRACT: We report on the development of the first integrated mid-infrared, far-infrared and terahertz optical Hall effect instrument, covering an ultra wide spectral range from 3 cm$^{-1}$ to 7000 cm$^{-1}$ (0.1-210 THz or 0.4-870 meV). The instrument comprises four sub-systems, where the magneto-cryostat-transfer sub-system enables the usage of the magneto-cryostat sub-system with the mid-infrared ellipsometer sub-system, and the far-infrared/terahertz ellipsometer sub-system. Both ellipsometer sub-systems can be used as variable angle-of-incidence spectroscopic ellipsometers in reflection or transmission mode, and are equipped with multiple light sources and detectors. The ellipsometer sub-systems are operated in polarizer-sample-rotating-analyzer configuration granting access to the upper left $3\times 3$ block of the normalized $4\times 4$ Mueller matrix. The closed cycle magneto-cryostat sub-system provides sample temperatures between room temperature and 1.4 K and magnetic fields up to 8 T, enabling the detection of transverse and longitudinal magnetic field-induced birefringence. We discuss theoretical background and practical realization of the integrated mid-infrared, far-infrared and terahertz optical Hall effect instrument, as well as acquisition of optical Hall effect data and the corresponding model analysis procedures. Exemplarily, epitaxial graphene grown on 6H-SiC, a tellurium doped bulk GaAs sample and an AlGaN/GaN high electron mobility transistor structure are investigated. The selected experimental datasets display the full spectral, magnetic field and temperature range of the instrument and demonstrate data analysis strategies. Effects from free charge carriers in two dimensional confinement and in a volume material, as well as quantum mechanical effects (inter-Landau-level transitions) are observed and discussed exemplarily.
    The Review of scientific instruments. 01/2014; 85(7).
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    ABSTRACT: We report on the polarization selection rules of inter-Landau-level transitions using reflection-type optical Hall effect measurements from 600 to 4000 cm^{-1} on epitaxial graphene grown by thermal decomposition of silicon carbide. We observe symmetric and antisymmetric signatures in our data due to polarization preserving and polarization mixing inter-Landau-level transitions, respectively. From field-dependent measurements, we identify that transitions in coupled graphene monolayers are governed by polarization mixing selection rules, whereas transitions in decoupled graphene monolayers are governed by polarization preserving selection rules. The selection rules may find explanation by different coupling mechanisms of inter-Landau-level transitions with free charge carrier magneto-optic plasma oscillations.
    Physical Review Letters 08/2013; 111(7):077402. · 7.73 Impact Factor
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    ABSTRACT: The temperature-dependence of free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas in a AlGaN/GaN heterostructure deposited on SiC substrate is determined using the THz optical Hall effect in the spectral range from 0.22 to 0.32 THz for temperatures from 1.5 to 300 K. The THz optical Hall-effect measurements are combined with room temperature mid-infrared spectroscopic ellipsometry measurements to determine the layer thickness, phonon mode, and free-charge carrier parameters of the heterostructure constituents. An increase of the electron effective mass from (0.22±0.01)m0 at 1.5 K to (0.36±0.03)m0 at 300 K is observed, which is indicative for a reduction in spatial confinement of the two-dimensional electron gas at room temperature. The temperature-dependence of the mobility and the sheet density is in good agreement with electrical measurements reported in the literature.
    Applied Physics Letters 11/2012; · 3.52 Impact Factor
  • Mat. Res. Soc. Symp. Proc. 01/2012; 1409:cc13-31.
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    ABSTRACT: We present non-contact, optical measurements of free-charge carrier mobility, sheet density, and effective mass parameters of the 2DEG for different HfO2 passivated AlGaN/GaN high electron mobility transistor structures at room temperature. Spectroscopic ellipsometry (SE) in the spectral range from THz and Mid-IR to the VUV and THz optical Hall-effect (generalized ellipsometry in magnetic fields) (OHE) are employed. Changes in the HfO2 layer growth conditions are found to drastically influence the electron density of the channel. The sheet density and the carrier mobility obtained by the optical investigations are in excellent agreement with results from electrical Hall-effect measurements. The electron effective mass parameters determined here using the OHE corroborate previous SdH and cyclotron resonance studies. The surface sensitivity of VUV-SE in combination with OHE allows for correlation of surface passivation and changes in the 2DEG properties.
    03/2011;
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    ABSTRACT: The free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas are exemplarily determined in the spectral range from 640 GHz to 1 THz in a AlGaN/GaN heterostructure using the optical-Hall effect at room temperature. Complementary midinfrared spectroscopic ellipsometry measurements are performed for analysis of heterostructure constituents layer thickness, phonon mode, and free-charge carrier parameters. The electron effective mass is determined to be (0.22±0.04)m<sub>0</sub> . The high-frequency sheet density and carrier mobility parameters are in good agreement with results from dc electrical Hall effect measurements, indicative for frequency-independent carrier scattering mechanisms of the two-dimensional carrier distribution.
    Applied Physics Letters 03/2011; · 3.52 Impact Factor
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    ABSTRACT: We report noncontact, optical determination of free-charge carrier mobility, sheet density, and resistivity parameters in epitaxial graphene at room temperature using terahertz and midinfrared ellipsometry and optical-Hall effect measurements. The graphene layers are grown on Si- and C-terminated semi-insulating 6H silicon carbide polar surfaces. Data analysis using classical Drude functions and multilayer modeling render the existence of a p -type channel with different sheet densities and effective mass parameters for the two polar surfaces. The optically obtained parameters are in excellent agreement with results from electrical Hall effect measurements.
    Applied Physics Letters 02/2011; · 3.52 Impact Factor
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    ABSTRACT: Ellipsometry has been proven as an excellent tool for the precise and accurate determination of material optical properties in the spectral range from the far infrared to the vacuum ultraviolet. In the terahertz frequency domain, however, ellipsometry is still in its infancy. Here we report on our recent development of rotating optical element frequency domain terahertz ellipsometry using electron-beam based, quasi-optical light sources. We demonstrate that high power backward wave oscillator type sources are readily available for the use in spectroscopic ellipsometry instrumentation for the terahertz spectral range. We review recent results on the application of terahertz ellipsometry. Exemplarily, the contact-free optical determination of free-charge carrier properties for very small doping concentrations and doping profiles in iso- and aniso-type Si homojunctions will be discussed. Furthermore, terahertz optical-Hall effect measurements on high-mobility epitaxial graphene on SiC and very low-doped Si are presented.
    Thin Solid Films 01/2011; 519(9):2593-2600. · 1.87 Impact Factor
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    ABSTRACT: The authors have measured the dielectric functions of three Si{sub 1-y}C{sub y} alloys layers (y ⤠1.4%) grown pseudomorphically on Si (001) substrates using molecular beam epitaxy at low temperatures. From the numerical derivatives of the measured spectra, they determine the critical point energies Eââ² and E⁠as a function of y (y ⤠1.4%) using a comparison with analytical line shapes and analyze these energies in terms of the expected shifts and splittings due to negative hydrostatic pressure, shear stress, and alloying. Their data agree well with the calculated shifts for Eâ, but the Eââ² energies are lower than expected. They discuss their results in comparison with recent tight-binding molecular dynamics simulations by Demkov and Sankey predicting a total breakdown of the virtual-crystal approximation for such alloys.
    MRS Online Proceeding Library 01/2011; 379.
  • P Kühne, T Hofmann, C M Herzinger, M Schubert
    Thin Solid Films 01/2011; xx:xx. · 1.87 Impact Factor
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    ABSTRACT: The anisotropic optical dielectric functions of a metal (cobalt) slanted columnar thin film deposited by electron-beam glancing angle deposition are reported for the terahertz (THz) frequency domain using generalized spectroscopic ellipsometry. We employ a simple effective medium dielectric function homogenization approach to describe the observed optical response. The approach describes isolated, electrically conductive columns which render the thin film biaxial (orthorhombic). Our findings suggest controlled variability of dielectric polarizability and anisotropy in the THz spectral range by choice of geometry, material, and structure.
    Applied Physics Letters 01/2011; 99(8):081903. · 3.52 Impact Factor
  • P. Kühne, T. Hofmann, C. M. Herzinger, M. Schubert
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    ABSTRACT: The optical-Hall effect comprises generalized ellipsometry at long wavelengths on samples with free-charge carriers placed within external magnetic fields. Measurement of the anisotropic magneto-optic response allows for the determination of the free-charge carrier properties including spatial anisotropy. In this work we employ the optical-Hall effect at terahertz frequencies for analysis of free-charge carrier properties in multiple valley band materials, for which the optical free-charge carrier contributions originate from multiple Brillouin-zone conduction or valence band minima or maxima, respectively. We investigate exemplarily the room temperature optical-Hall effect in low phosphorous-doped n-type silicon where free electrons are located in six equivalent conduction-band minima near the X-point. We simultaneously determine their free-charge carrier concentration, mobility, and longitudinal and transverse effective mass parameters.
    Thin Solid Films 01/2011; 519(9):2613-2616. · 1.87 Impact Factor
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    ABSTRACT: Magneto-optical properties of ferromagnetic slanted columnar thin films from cobalt made by electron-beam glancing angle deposition are reported. Magneto-optic ellipsometry in the polar Kerr effect configuration was employed to determine the field- and wavelength-dependent complex magneto-optic polarizability parameter within the spectral range from 0.74 to 3.0 eV. Kerr effect measurements and calculations reveal a strong azimuthal dependence for the incident linear polarization with peak Kerr rotation one order of magnitude larger than what has been reported for solid Co thin films.
    Applied Physics Letters 04/2010; · 3.52 Impact Factor
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    ABSTRACT: We report an experimental setup for wavelength-tunable frequency-domain ellipsometric measurements in the terahertz spectral range from 0.2 to 1.5 THz employing a desktop-based backward wave oscillator source. The instrument allows for variable angles of incidence between 30 degrees and 90 degrees and operates in a polarizer-sample-rotating analyzer scheme. The backward wave oscillator source has a tunable base frequency of 107-177 GHz and is augmented with a set of Schottky diode frequency multipliers in order to extend the spectral range to 1.5 THz. We use an odd-bounce image rotation system in combination with a wire grid polarizer to prepare the input polarization state. A highly phosphorous-doped Si substrate serves as a first sample model system. We show that the ellipsometric data obtained with our novel terahertz ellipsometer can be well described within the classical Drude model, which at the same time is in perfect agreement with midinfrared ellipsometry data obtained from the same sample for comparison. The analysis of the terahertz ellipsometric data of a low phosphorous-doped n-type Si substrate demonstrates that ellipsometry in the terahertz spectral range allows the determination of free charge-carrier properties for electron concentrations as low as 8x10(14) cm(-3).
    The Review of scientific instruments 02/2010; 81(2):023101. · 1.52 Impact Factor
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    ABSTRACT: Noninvasive optical measurement of hole diffusion profiles in p-p+ silicon homojunction is reported by ellipsometry in the terahertz (0.2–1.5 THz) and midinfrared (9–50 THz) spectral regions. In the terahertz region a surface-guided wave resonance with transverse-electrical polarization is observed at the boundary of the p-p+ homojunction, and which is found to be extremely sensitive to the low-doped p-type carrier concentration as well as to the hole diffusion profile within the p-p+ homojunction. Effective mass approximations allow determination of homojunction hole concentrations as p = 2.9×1015 cm−3, p+ = 5.6×1018 cm−3, and diffusion time constant Dt = 7.7×10−3 μm2, in agreement with previous electrical investigations.
    Applied Physics Letters 07/2009; 95(3). · 3.52 Impact Factor
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    ABSTRACT: Mono-crystalline silicon is chemically textured and coated with a silicon nitride surface anti-reflection (AR) film to improve light trapping and device efficiency in photovoltaic applications. The thickness and optical properties of the AR coating determine the effective suppression of reflected light. However, optical characterization of films on chemically textured surfaces is challenging due to the low reflectance. We present new measurement geometries and modeling methodology using Spectroscopic Ellipsometry (SE) to determine film thickness and optical properties of thin AR coatings on textured mono-crystalline silicon. Special measurement geometries are used to collect specular reflected light from the etched silicon pyramid facets. Both apexial and lateral measurement geometries are demonstrated, where the latter requires a special sample stage to tilt and rotate the sample to detect the specular reflected light from the pyramid facets. Measurement considerations are discussed including probe-beam incident angle, sample tilt and rotational angles. Effects of pyramid surface-coverage are also discussed in relation to the proposed measurement geometries. Previous modeling attempts using Effective Medium Approximation (EMA) theory showed inconsistent results when comparing apexial and lateral measurements of the same sample surface. In this work, results from a scattering-corrected modeling approach provide improved consistency for a series of SiNx coatings on textured monocrystalline silicon for both lateral and apexial measurement geometries.
    Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
  • T Hofmann, C M Herzinger, J A Woollam, M Schubert
    MRS Online Proceeding Library 01/2009; 1163E:1163-K08-04.
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    ABSTRACT: For ZnO, the optical dielectric functions for polarizations parallel and perpendicular to the op-tical axis were determined in the photon energy range from 4.0 to 9.5 eV by using generalized spectroscopic ellipsometry and the band structure was calculated by means of the empirical pseu-dopotential method. From the band structure, a theoretical dielectric function was derived. The dielectric functions reveal features that were identified as band-to-band transitions. The energies of those transitions were assigned to band-to-band transition energies of critical points of the calculated band structure.
    Journal- Korean Physical Society 08/2008; 53:71-15. · 0.51 Impact Factor
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    ABSTRACT: Classic electrical Hall effect measurements are standard for electrical characterization of free charge carriers in semiconductor layer structures. We demonstrate that magnetooptic generalized ellipsometry at long wavelengths when applied to conducting or semiconducting multilayer or nanoscopically in-homogeneous structures can yield equivalent and even much increased information. We term this new method optical Hall effect, because it finds simple explanation within the model described by E. H. Hall for the occurrence of the transverse and longitudinal voltages augmented by non-locality of the charge response in time. Transverse and longitudinal birefringence cause magnificent anisotropic polarization responses unraveling rich information on free charge properties of complex-structured samples due to external magnetic fields and collective movement of bound and unbound charge carriers. We demonstrate that with our technique density, type, mobility, effective mass including anisotropy can be measured without any electrical contact in buried structures, and which may have been inaccessible to any true electrical evaluation thus far. We predict a realm of applications for the optical Hall effect in future materials research and engineering. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 03/2008; 205(4):779 - 783. · 1.53 Impact Factor
  • Blaine Johs, C. M. Herzinger
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    ABSTRACT: As the diversity and complexity of ellipsometric applications continue to increase, so do the requirements for ellipsometric data accuracy. In the previous ICSE-3 conference, Aspnes identified this issue, and suggested a 0.1% target for ellipsometric accuracy [1]. Unfortunately, there is no generally accepted method for characterizing or quantifying ellipsometric data accuracy. In this paper, a simple method and metric are proposed for quantifying the accuracy of an ellipsometer system. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 02/2008; 5(5):1031 - 1035.

Publication Stats

774 Citations
103.54 Total Impact Points

Institutions

  • 1994–2013
    • University of Nebraska at Lincoln
      • • Center for Nanohybrid Functional Materials
      • • Department of Electrical Engineering
      • • Nebraska Center for Materials and Nanoscience
      Lincoln, NE, United States
  • 1995–2006
    • University of Leipzig
      • Institut für Experimentelle Physik
      Leipzig, Saxony, Germany