P. Kühne

Hole-channel conductivity in epitaxial graphene determined by terahertz optical-Hall effect and midinfrared ellipsometry

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.

THz dielectric anisotropy of metal slanted columnar thin films

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.

Invited Article: An integrated mid-infrared, far-infrared, and terahertz optical Hall effect instrument

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 × 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.

Terahertz optical-Hall effect characterization of two-dimensional electron gas properties in AlGaN/GaN high electron mobility transistor structures

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)m0. 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.

Temperature dependent effective mass in AlGaN/GaN high electron mobility transistor structures

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.

Electron effective mass in Al0.72Ga0.28N alloys determined by mid-infrared optical Hall effect

The effective electron mass parameter in Si-doped Al

Optical Hall effect-model description: tutorial.

_{0.72}

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

Ga

Decoupling and ordering of multilayer graphene on C-face 3C-SiC(111)

_{0.28}

Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect

N is determined to be