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Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
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2017 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, 357-360 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 421, 357-360 p.
Keyword [en]
THz optical Hall effect, Epitaxial graphene, Free charge carrier properties
National Category
Physical Sciences Condensed Matter Physics Atom and Molecular Physics and Optics Ceramics
Identifiers
URN: urn:nbn:se:liu:diva-132407DOI: 10.1016/j.apsusc.2016.10.023ISI: 000408756700015OAI: oai:DiVA.org:liu-132407DiVA: diva2:1045443
Note

Funding agencies: Swedish Research Council (VR) [2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486, 2014-04712]; Swedish foundation for strategic research (SSF) [FFL12-0181, RIF14-055]

Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2017-10-23Bibliographically approved
In thesis
1. Structural and Electronic Properties of Graphene on 4H- and 3C-SiC
Open this publication in new window or tab >>Structural and Electronic Properties of Graphene on 4H- and 3C-SiC
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Graphene is a one-atom-tick carbon layer arranged in a honeycomb lattice. Graphene was first experimentally demonstrated by Andre Geim and Konstantin Novoselov in 2004 using mechanical exfoliation of highly oriented pyrolytic graphite (exfoliated graphene flakes), for which they received the Nobel Prize in Physics in 2010. Exfoliated graphene flakes show outstanding electronic properties, e.g., very high free charge carrier mobility parameters and ballistic transport at room temperature. This makes graphene a suitable material for next generation radio-frequency and terahertz electronic devices. Such applications require fabrication methods of large-area graphene compatible with electronic industry. Graphene grown by sublimation on silicon carbide (SiC) offers a viable route towards production of large-area, electronic-grade material on semi-insulating substrate without the need of transfer. Despite the intense investigations in the field, uniform wafer-scale graphene with very high-quality that matches the properties of exfoliated graphene has not been achieved yet. The key point is to identify and control how the substrate affects graphene uniformity, thickness, layer stacking, structural and electronic properties. Of particular interest is to understand the effects of SiC surface polarity and polytype on graphene properties in order to achieve large-area material with tailored properties for electronic applications. The main objectives of this thesis are to address these issues by investigating the structural and electronic properties of epitaxial graphene grown on 4HSiC and 3C-SiC substrates with different surface polarities. The first part of the thesis includes a general description of the properties of graphene, bilayer graphene and graphite. Then, the properties of epitaxial graphene on SiC by sublimation are detailed. The experimental techniques used to characterize graphene are described. A summary of all papers and contribution to the field is presented at the end of Part I. Part II consists of seven papers.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 74 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1793
National Category
Condensed Matter Physics Materials Chemistry Textile, Rubber and Polymeric Materials Other Materials Engineering Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-132408 (URN)10.3384/diss.diva-132408 (DOI)9789176856789 (ISBN)
Public defence
2016-11-28, Planck, Fysikhuset, Campus Valla, Linköping, 11:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2013−5580VINNOVA, 2011−03486Swedish Foundation for Strategic Research , FFL12−0181Swedish Foundation for Strategic Research , RIF14−055Linköpings universitet, LiU No2009 00971
Note

Research Funders not listed under Research funders and strategic development areas: Marie Curie actions under the Project No.264613-NetFISiC, the centre of Nano Science and Nano technology (CeNano).

Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2016-11-09Bibliographically approved
2. Study of novel electronic materials by mid-infrared and terahertz optical Hall effect
Open this publication in new window or tab >>Study of novel electronic materials by mid-infrared and terahertz optical Hall effect
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Development of silicon based electronics have revolutionized our every day life during the last three decades. Nowadays Si based devices operate close to their theoretical limits that is becoming a bottleneck for further progress. In particular, for the growing field of high frequency and high power electronics, Si cannot offer the required properties. Development of materials capable of providing high current densities, carrier mobilities and high breakdown fields is crucial for a progress in state of the art electronics.

Epitaxial graphene grown on semi-insulating silicon carbide substrates has a high potential to be integrated in the current planar device technologies. High electron mobilities and sheet carrier densities make graphene extremely attractive for high frequency analog applications. One of the remaining challenges is the interaction of epitaxial graphene with the substrate. Typically, much lower free charge carrier mobilities, compared to free standing graphene, and doping, due to charge transfer from the substrate, is reported. Thus, a good understanding of the intrinsic free charge carriers properties and the factors affecting them is very important for further development of epitaxial graphene.

III-group nitrides have been extensively studied and already have proven their high efficiency as light sources for short wavelengths. High carrier mobilities and breakdown electric fields were demonstrated for III-group nitrides, making them attractive for high frequency and high power applications. Currently, In-rich InGaN alloys and AlGaN/GaN high electron mobility structures are of high interest for the research community due to open fundamental questions.

Electrical characterization techniques, commonly used for the determination of free charge carrier properties, require good ohmic and Schottky contacts, which in certain cases can be difficult to achieve. Access to electrical properties of buried conductive channels in multilayered structures requires modification of samples and good knowledge of the electrical properties of all electrical contact within the structure. Moreover, the use of electrical contacts to electrically characterize two-dimensional electronic materials, such as graphene, can alter their intrinsic properties. Furthermore, the determination of effective mass parameters commonly employs cyclotron resonance and Shubnikov-de Haas oscillations measurements, which require long scattering times of free charge carriers, high magnetic fields and low temperatures.

The optical Hall effect is an external magnetic field induced optical anisotropy in  conductive layers due to the motion of the free charge carriers under the influence of the Lorentz force, and is equivalent to the electrical Hall effect at optical frequencies. The optical Hall effect can be measured by generalized ellipsometry and provides a powerful method for the determination of free charge carrier properties in a non-destructive and contactless manner. In principle, a single optical Hall effect measurement can provide quantitative information about free charge carrier types, concentrations, mobilities and effective mass parameters at temperatures ranging from few kelvins to room temperature and above. Further, it was demonstrated that for transparent samples, a backside cavity can be employed to enhance the optical Hall effect.

Measurement of the optical Hall effect by generalized ellipsometry is an indirect technique requiring subsequent data analysis. Parameterized optical models are fitted to match experimentally measured ellipsometric data by varying physically significant parameters. Analysis of the optical response of samples, containing free charge carriers, employing optical models based on the classical Drude model, which is augmented with an external magnetic field contribution, provide access to the free charge carrier properties.

The main research results of the graduate studies presented in this licentiate thesis are summarized in the five scientific papers.

Paper I. Description of the custom-built terahertz frequency-domain spectroscopic ellipsometer at Linköping University. The terahertz ellipsometer capabilities are demonstrated by an accurate determination of the isotropic and anisotropic refractive indices of silicon and m-plane sapphire, respectively. Further, terahertz optical Hall effect measurements of an AlGaN/GaN high electron mobility structures were employed to extract the two-dimensional electron gas sheet density, mobility and effective mass parameters. Last, in-situ optical Hall effect measurement on epitaxial graphene in a gas cell with controllable environment, were used to study the effects of environmental doping on the mobility and carrier concentration.

Paper II. Presents terahertz cavity-enhanced optical Hall measurements of the monolayer and multilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed p-type doping for monolayer graphene with a carrier density in the low 1012 cm−2 range and a carrier mobility of 1550 cm2/V·s. For the multilayer epitaxial graphene, n-type doping with a carrier density in the low 1013 cm−2 range, a mobility of 470 cm2/V·s and an effective mass of (0.14 ± 0.03) m0 were extracted. The measurements demonstrate that cavity-enhanced optical Hall effect measurements can be applied to study electronic properties of two-dimensional materials.

Paper III. Terahertz cavity-enhanced optical Hall effect measurements are employed to study anisotropic transport in as-grown monolayer, quasi free-standing monolayer and quasi free-standing bilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed a strong anisotropy in the carrier mobilities of the quasi freestanding bilayer graphene. The anisotropy is demonstrated to be induced by carriers scattering at the step edges of the SiC, by showing that the mobility is higher along the step than across them. The scattering mechanism is discussed based on the results of the optical Hall effect, low-energy electron microscopy, low-energy electron diffraction and Raman measurements.

Paper IV. Mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect measurements are employed to determine the electron effective mass in an In0.33Ga0.67N epitaxial layer. The data analysis reveals slightly anisotropic effective mass and carrier mobility parameters together with the optical phonon frequencies and broadenings.

Paper V. Terahertz cavity-enhanced optical Hall measurements are employed to study the free charge carrier properties in a set of AlGaN/GaN high electron mobility structures with modified interfaces. The results show that the interface structure has a significant effect on the free charge carrier mobility and that the sample with a sharp interface between an AlGaN barrier and a GaN buffer layers exhibits a record mobility of 2332±73 cm2/V·s. The determined effective mass parameters showed an increase compared to the GaN value, that is attributed the the penetration of the electron wavefunction into the AlGaN barrier layer.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. 27 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1790
National Category
Condensed Matter Physics Atom and Molecular Physics and Optics Materials Chemistry Other Physics Topics Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-142220 (URN)10.3384/lic.diva-142220 (DOI)9789176854334 (ISBN)
Presentation
2017-11-03, Jordan-Fermi, J402, Fysikhuset ing 57, Campus Valla, Linköpings universitet, Linköping, 10:00 (English)
Opponent
Supervisors
Available from: 2017-10-23 Created: 2017-10-23 Last updated: 2017-10-31Bibliographically approved

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