| 1. |
- Armakavicius, Nerijus, et al.
(författare)
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Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
- 2017
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Ingår i: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 421, s. 357-360
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Tidskriftsartikel (refereegranskat)abstract
- Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012 cmￜￜￜ2 range and a free hole mobility parameter as high as 1550 cm2/Vs. We also find that 6 ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470 cm2/Vs and an order of magnitude higher free electron density in the low 1013 cmￜￜￜ2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.
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| 2. |
- Armakavicius, Nerijus, et al.
(författare)
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Properties of two-dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity-enhanced THz optical Hall effect
- 2016
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Ingår i: Physica Status Solidi C-Current Topics in Solid State Physics, Vol 13 No 5-6. - : Wiley-VCH Verlagsgesellschaft. ; , s. 369-373
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Konferensbidrag (refereegranskat)abstract
- In this work we employ terahertz (THz) ellipsometry to determine two-dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H-SiC substrates. The effect of the GaN interface exposure to low-flow-rate trimethylaluminum (TMA) on the 2DEG properties is studied. The 2DEG effective mass and sheet density are determined tobe in the range of 0.30-0.32m0 and 4.3-5.5×1012 cm–2, respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm2/Vs to values above 2200 cm2/Vs. A record mobility of 2332±61 cm2/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG.
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| 3. |
- Boosalis, Alexander, et al.
(författare)
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Spectroscopic Mapping Ellipsometry of Graphene Grown on 3C SiC
- 2012
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Ingår i: MRS Proceedings Volume 1407. - : Springer Science and Business Media LLC. ; , s. aa20-43
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Konferensbidrag (refereegranskat)abstract
- Spectroscopic mapping ellipsometry measurements in the visible spectrum (1.25 to 5.35 eV) are performed to determine the lateral variations of epitaxial graphene properties as grown on 3C SiC. Data taken in the visible spectrum is sensitive to both the Drude absorption of free charge carriers and the characteristic exciton enhanced van Hove singularity at 5 eV. Subsequent analysis with simple oscillator models allows the determination of physical parameters such as free charge carrier scattering time and local graphene thickness with a lateral resolution of 50 microns.
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| 4. |
- Bouhafs, Chamseddine, et al.
(författare)
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Decoupling and ordering of multilayer graphene on C-face 3C-SiC(111)
- 2016
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 109:20
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Tidskriftsartikel (refereegranskat)abstract
- We show experimentally that few layer graphene (FLG) grown on the carbon terminated surface (C-face) of 3C-SiC(111) is composed of decoupled graphene sheets. Landau level spectroscopy on FLG graphene is performed using the infrared optical Hall effect. We find that Landau level transitions in the FLG exhibit polarization preserving selection rules and the transition energies obey a square-root dependence on the magnetic field strength. These results show that FLG on C-face 3C-SiC(111) behave effectively as a single layer graphene with linearly dispersing bands (Dirac cones) at the graphene K point. We estimate from the Landau level spectroscopy an upper limit of the Fermi energy of about 60 meV in the FLG, which corresponds to a carrier density below 2.5 x 10(11) cm(-2). Low-energy electron diffraction mu-LEED) reveals the presence of azimuthally rotated graphene domains with a typical size of amp;lt;= 200 nm.mu-LEED mapping suggests that the azimuth rotation occurs between adjacent domains within the same sheet rather than vertically in the stack. Published by AIP Publishing.
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| 5. |
- Bouhafs, Chamseddine, et al.
(författare)
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Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC
- 2017
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Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 116, s. 722-732
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we report a multi-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 x 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 SiOx layer 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. (C) 2017 Elsevier Ltd. All rights reserved.
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| 6. |
- Briley, Chad, et al.
(författare)
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Effects of annealing and conformal alumina passivation on anisotropy and hysteresis of magneto-optical properties of cobalt slanted columnar thin films
- 2017
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Ingår i: Applied Surface Science. - : ELSEVIER SCIENCE BV. - 0169-4332 .- 1873-5584. ; 421, s. 320-324
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Tidskriftsartikel (refereegranskat)abstract
- We present magneto-optical dielectric tensor data of cobalt and cobalt oxide slanted columnar thin films obtained by vector magneto-optical generalized ellipsometry. Room-temperature hysteresis magnetization measurements were performed in longitudinal and polar Kerr geometries on samples prior to and after a heat treatment process with and without a conformal Al2O3 passivation coating. The samples have been characterized by generalized ellipsometry, scanning electron microscopy, and Raman spectroscopy in conjuncture with density functional theory. We observe strongly anisotropic hysteresis behaviors, which depend on the nanocolumn and magnetizing field orientations. We find that deposited cobalt films that have been exposed to heat treatment and subsequent atmospheric oxidation into Co3O4, when not conformally passivated, reveal no measurable magneto-optical properties while cobalt films with passivation coatings retain highly anisotropic magneto-optical properties (C) 2016 Published by Elsevier B.V.
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| 7. |
- Fullager, Daniel B., et al.
(författare)
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Broadband optical properties of aluminium zinc oxide thin films prepared by spatial atomic layer deposition
- 2018
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Ingår i: Thin Solid Films. - : ELSEVIER SCIENCE SA. - 0040-6090 .- 1879-2731. ; 653, s. 267-273
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Tidskriftsartikel (refereegranskat)abstract
- In this work, a broadband ellipsometric investigation of aluminium zinc oxide (AZO) from the infrared to the ultraviolet spectral range is presented. Aluminium zinc oxide samples were fabricated using spatial atomic layer deposition (SALD) having a range of percent aluminium incorporation. The AZO was deposited using a coflow of diethyl zinc (DEZ) and dimethyl aluminium isopropoxide (DMAI), where the amount of aluminium incorporation was controlled by varying the DMAI partial pressure. The broadband permittivity is reported as a function of the aluminium content of these aluminium zinc oxide thin films. A model dielectric function, which includes the contribution of recently predicted broad shallow donor states, is presented and discussed. We find that an increase in the aluminium content of the aluminium zinc oxide thin films results in a monotonically decreasing scattering time while the carrier density increases with diminishing efficiency. To demonstrate the necessity of broadband ellipsometric measurements for the accurate determination of the dielectric function of aluminium zinc oxide, a comparison with the classical Drude model and a semi-empirical extended Drude dispersion model, which are frequently employed in the literature to describe the optical response of aluminium zinc oxide, is given.
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| 8. |
- Fullager, Daniel B., et al.
(författare)
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Infrared dielectric response of nanoscribe IP-dip and IP-L monomers after polymerization from 250 cm(-1) to 6000 cm(-1)
- 2017
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Ingår i: Optical Materials Express. - : OPTICAL SOC AMER. - 2159-3930 .- 2159-3930. ; 7:3, s. 888-894
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Tidskriftsartikel (refereegranskat)abstract
- Direct laser writing via two photon polymerization has enabled previously unavailable degrees of freedom in the additive fabrication of micro-to-meso scale structures. The structures produced by these techniques are ideally suited to create optical devices which operate from the THz regime to the near infrared spectrum into the visible spectral range. Here we report on the infrared dielectric response of two monomers IP-dip and IP-L after polymerization which are frequently employed in commercial two photon lithography tools from nanoscribe over the spectral range of 250 cm(-1) to 6000 cm(-1). A parameterized dielectric function model is presented and discussed. (C) 2017 Optical Society of America
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| 9. |
- Fullager, Daniel B., et al.
(författare)
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Metalized Poly-methacrylate Off-Axis Parabolic Mirrors for Terahertz Imaging Fabricated by Additive Manufacturing
- 2019
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Ingår i: Journal of Infrared, Millimeter and Terahertz Waves. - : SPRINGER. - 1866-6892 .- 1866-6906. ; 40:3, s. 269-275
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Tidskriftsartikel (refereegranskat)abstract
- Terahertz radiation sources are currently one of the most widely used non-ionizing illumination mechanisms for security applications and also find increasing utilization in quality control of commercial products. Presently, a majority of these applications rely on scanning rather than direct imaging and implicitly suffer from temporal latency due to post processing. The monetary and temporal cost associated with procuring commercially manufactured optics that are suitable for imaging leads to fundamental limitations in the ability to rapidly develop application-specific imaging modalities using terahertz sources. Herein, we show a novel method for the rapid prototyping of metallic coated poly-methacrylate parabolic reflectors fabricated by stereolithographic 3D printing. Images comparing the performance of a commercially available off-axis parabolic reflector to our metalized poly-methacrylate prototype, which was designed to be identical to the commercially available mirror, are subsequently presented. The images show that at 530 GHz it is possible to produce a metalized poly-methacrylate off-axis paraboloid whose spatial beam profile is nearly identical to that of a commercially available equivalent.
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| 10. |
- Hofmann, Tino, et al.
(författare)
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Screening effects in metal sculptured thin films studied with terahertz Mueller matrix ellipsometry
- 2017
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Ingår i: Applied Surface Science. - : ELSEVIER SCIENCE BV. - 0169-4332 .- 1873-5584. ; 421, s. 513-517
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Tidskriftsartikel (refereegranskat)abstract
- The anisotropic optical dielectric functions of a metal (cobalt) slanted columnar thin film deposited by electronbeam 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. (C) 2016 Elsevier B.V. All rights reserved.
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