Abstract-Multi-scale investigations of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC

• C. Bouhafs^a,, 
• A.A. Zakharov^b, 
• I.G. Ivanov^c, 
• F. Giannazzo^d, 
• J. Eriksson^c, 
• V. Stanishev^a, 
• P. Kühne^a,
• T. Iakimov^c, 
• T. Hofmann^e, 
• M. Schubert^e, 
• F. Roccaforte^d, 
• R. Yakimova^c, 
• V. Darakcheiva^a

• ^a Terahertz Materials Analysis Center, Department of Physics, Chemistry and Biology, IFM, Linköping University, Linköping, S-58183 SE, Sweden
• ^b MaxLab, Lund University, S-22100 Lund, Sweden
• ^c Department of Physics, Chemistry and Biology, IFM, Linköping University, Linköping, S-58183 SE, Sweden
• ^d CNR-IMM, Strada VIII, 5, 95121 Catania, Italy
• ^e Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
• 
  

http://www.sciencedirect.com/science/article/pii/S0008622317301549

In this work, we report a muti-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 × 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiO_xlayer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO₂ and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation.

Abstract-Screening effects in metal sculptured thin films studied with terahertz Mueller matrix ellipsometry

• T. Hofmann^{a, b, c, ,} , 
• S. Knight^b, 
• D. Sekora^b, 
• D. Schmidt^b, 
• C.M. Herzinger^d, 
• J.A. Woollam^d, 
• E. Schubert^b, 
• M. Schubert^b, c

• ^a Department of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, U.S.A
• ^b Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, U.S.A.
• ^c Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden
• ^d J.A. Woollam Co. Inc. 645 M Street, Suite 102, Lincoln, NE 68508, U.S.A.
• 
  

http://www.sciencedirect.com/science/article/pii/S0169433216329324

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 before and after the slanted columnar thin film was passivated by a conformal alumina coating. A simple effective medium dielectric function homogenization approach which describes isolated, electrically conductive columns rendering the thin film biaxial (orthorhombic) is used to model the observed optical responses. Upon passivating the slanted columnar thin film with a 3 nm thick alumina film an increase of both the real and the imaginary part of the dielectric function for all major polarizability directions is found and attributed to screening effects within the spatially coherent metal nanocolumns.

Abstract-Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies

S. Knight, S. Schöche, V. Darakchieva, P. Kühne, J.-F. Carlin, N. Grandjean, C. M. Herzinger, M. Schubert, and T. Hofmann
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-40-12-2688

The effect of a tunable, externally coupled Fabry–Perot cavity to resonantly enhance the optical Hall effect signatures at terahertz frequencies produced by a traditional Drude-like two-dimensional electron gas is shown and discussed in this Letter. As a result, the detection of optical Hall effect signatures at conveniently obtainable magnetic fields, for example, by neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high-electron mobility transistor structure grown on a sapphire substrate is used for the experiment. The optical Hall effect signatures and their dispersions, which are governed by the frequency and the reflectance minima and maxima of the externally coupled Fabry–Perot cavity, are presented and discussed. Tuning the externally coupled Fabry–Perot cavity strongly modifies the optical Hall effect signatures, which provides a new degree of freedom for optical Hall effect experiments in addition to frequency, angle of incidence, and magnetic field direction and strength.

Abstract-An integrated mid-infrared, far-infrared and terahertz optical Hall effect instrument

P. Kühne, C. M. Herzinger, M. Schubert, J.A. Woollam, T. Hofmann

(Submitted on 21 Jan 2014)

http://arxiv-web3.library.cornell.edu/abs/1401.5372

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 3×3 block of the normalized 4×4Mueller 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.