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Free charge carrier properties in group III nitrides and graphene studied by THz-to-MIR ellipsometry and optical Hall effect
Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
2019 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Development of silicon based electronics have revolutionized our every day life during the last five decades. Nowadays silicon 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, silicon cannot offer the required properties. Development of materials capable of providing high current densities, carrier mobilities and high breakdown fields is crucial for further progress in state of the art electronics.

Epitaxial graphene grown on semi-insulating silicon carbide substrates has a high potential to be integrated in 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.

Group III-nitrides have been extensively studied and already have proven their high efficiency as light emitting diodes for short wavelengths. High carrier mobilities and breakdown electric fields were demonstrated for group III-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 such as free charge carrier properties at high temperatures and wavefunction hybridization in AlGaN/GaN heterostructures.

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 junctions within the structure. Moreover, the use of 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 birefringence of conductive layers due to the free charge carriers interaction with long-wavelength electromagnetic waves under the influence of the Lorentz force. 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. The optical Hall effect measurements can provide quantitative information about free charge carrier type, concentration, mobility and effective mass parameters at temperatures ranging from few kelvins to room temperature and above. It further allows to differentiate the free charge carrier properties of individual layers in multilayer samples. The employment of a backside cavity for transparent samples can enhance the optical Hall effect and allows to access free charge carrier properties at relatively low magnetic fields using permanent magnet.

The optical Hall effect measurements at mid-infrared spectral range can be used to probe quantum mechanical phenomena such as Landau levels in graphene. The magnetic field dependence of the inter-Landau level transition energies and optical polarization selection rules provide information about coupling properties between graphene layers and the electronic band structure.

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 spectra by varying physically significant model parameters. Analysis of the generalized ellipsometry data at long wavelengths for samples containing free charge carriers by optical models based on the classical Drude formulation, augmented with an external magnetic field contribution, allows to extract carrier concentration, mobility and effective mass parameters.

The development of the integrated FIR and THz frequency-domain ellipsometer at the Terahertz Materials Analysis Center in Linköping University was part of the graduate studies presented in this dissertation. The THz ellipsometer capabilities are demonstrated by determination of Si and sapphire optical constants, and free charge carrier properties of two-dimensional electron gas in GaN-based high electron mobility transistor structures. The THz ellipsometry is further shown to be capable of determining free charge carrier properties and following their changes upon variation of ambient conditions in atomically thin layers with an example of epitaxial graphene.

A potential of the THz OHE with the cavity enhancement (THz-CE-OHE) for determination of the free charge carrier properties in atomically thin layers were demonstrated by the measurements of the carrier properties in monolayer and multilayer epitaxial graphene on Si-face 4H-SiC. 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 cm2V-1s-1. For the multilayer graphene, n-type doping with a carrier density in the low 1013 cm-2 range, a mobility of 470 cm2V-1s-1 and an effective mass of (0.14 ± 0.03)m0 were extracted. Different type of doping among monolayer and multilayer graphene is explained as a result of different hydrophobicity among samples.

Further, we have employed THz-CE-OHE to determine for the first time anisotropic mobility parameter in quasi-free-standing bilayer epitaxial graphene induced by step-like surface morphology of 4H-SiC. Correlation of atomic force microscopy, Raman scattering spectroscopy, scanning probe Kelvin probe microscopy, low energy electron microscopy and diffraction analysis allows us to investigate the possible scattering mechanisms and suggests that anisotropic mobility is induced by varying local mobility parameter due to interaction between graphene and underlaying substrate.

The origin of the layers decoupling in multilayer graphene on C-face 4H-SiC was studied by MIR-OHE, transmission electron microscopy and electron energy loss spectroscopy. The results revealed the decoupling of the layers induced by the increased interlayer spacing which is attributed to the Si atoms trapped between graphene layers.

MIR ellipsometry and MIR-OHE measurements were employed to determine the electron effective mass in a wurtzite In0.33Ga0.67N epitaxial layer. The data analysis revealed the effective mass parameters parallel and perpendicular to the c-axis which can be considered as equal within sensitivity of our measurements. The determined effective mass is consistent with linear dependence on the In content.

Analysis of the free charge carrier properties in AlGaN/GaN high electron mobility structures with modified interfaces showed that AlGaN/GaN interface structure has a significant effect on the mobility parameter. A sample with a sharp interface layers exhibits a record mobility of 2332 ± 73 cm2V-1s-1. The determined effective mass parameters showed an increase compared to the bulk GaN value, which is attributed to the penetration of the electron wavefunction into the AlGaN barrier layer.

Temperature dependence of free charge carrier properties in GaN-based high electron mobility transistor structures with AlGaN and InAlN barrier layers were measured by terahertz optical Hall effect technique in a temperature range from 7.2 K to 398 K. The results revealed strong changes in the effective mass and mobility parameters. At temperatures below 57 K very high carrier mobility parameters above 20000 cm2V-1s-1 for AlGaN-barrier sample and much lower mobilities of ~ 5000 cm2V-1s-1 for InAlN-barrier sample were obtained. At low temperatures the effective mass parameters for both samples are very similar to bulk GaN value, while at temperatures above 131 K effective mass shows a strong increase with temperature. The effective masses of 0.344 m0 (@370 K) and 0.439 m0 (@398 K) were obtained for AlGaN- and InAlN-barrier samples, respectively. We discussed the possible origins of effective mass enhancement in high electron mobility transistor structures.  

sted, utgiver, år, opplag, sider
Linköping: Linköping University Electronic Press, 2019. , s. 41
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1976
HSV kategori
Identifikatorer
URN: urn:nbn:se:liu:diva-154921DOI: 10.3384/diss.diva-154921ISBN: 9789176851326 (tryckt)OAI: oai:DiVA.org:liu-154921DiVA, id: diva2:1293885
Disputas
2019-03-27, Planck, Fysikhuset, Campus Valla, Linköping, 13:15 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2019-03-05 Laget: 2019-03-05 Sist oppdatert: 2019-03-05bibliografisk kontrollert
Delarbeid
1. Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications
Åpne denne publikasjonen i ny fane eller vindu >>Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications
Vise andre…
2018 (engelsk)Inngår i: IEEE Transactions on Terahertz Science and Technology, ISSN 2156-342X, Vol. 8, nr 3, s. 257-270Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry, coating, and modulation. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm−1 or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

sted, utgiver, år, opplag, sider
Institute of Electrical and Electronics Engineers (IEEE), 2018
Emneord
Ellipsometry, Frequency-domain analysis, Instruments, Measurement by laser beam, Coherence, Dielectrics, Laser beams
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-147883 (URN)10.1109/TTHZ.2018.2814347 (DOI)000431446900001 ()2-s2.0-85045191738 (Scopus ID)
Merknad

Funding agencies: Swedish Foundation for Strategic Research (SSF) [FFL12-0181, RIF14-055]; AForsk [13-318]; Swedish Research Council (VR) [2013-5580, 2016-00889]; Swedish Governmental Agency for Innovation Systems (VINNOVA Grant) [2011-03486]; Swedish Government Strategic 

Tilgjengelig fra: 2018-05-18 Laget: 2018-05-18 Sist oppdatert: 2019-03-05bibliografisk kontrollert
2. Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
Åpne denne publikasjonen i ny fane eller vindu >>Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies
Vise andre…
2017 (engelsk)Inngår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, s. 357-360Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
Elsevier, 2017
Emneord
THz optical Hall effect, Epitaxial graphene, Free charge carrier properties
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-132407 (URN)10.1016/j.apsusc.2016.10.023 (DOI)000408756700015 ()
Merknad

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]

Tilgjengelig fra: 2016-11-09 Laget: 2016-11-09 Sist oppdatert: 2019-03-05bibliografisk kontrollert
3. Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect
Åpne denne publikasjonen i ny fane eller vindu >>Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect
Vise andre…
2018 (engelsk)Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, nr 8, artikkel-id 082103Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Mid-infrared optical Hall effect measurements are used to determine the free charge carrier parameters of an unintentionally doped wurtzite-structure c-plane oriented In0.33Ga0.67N epitaxial layer. Room temperature electron effective mass parameters of m(perpendicular to)* = (0.205 +/- 0.013) m(0) and m(parallel to)* = (0.204 +/- 0.016) m(0) for polarization perpendicular and parallel to the c-axis, respectively, were determined. The free electron concentration was obtained as (1.7 +/- 0.2) x 10(19) cm(-3). Within our uncertainty limits, we detect no anisotropy for the electron effective mass parameter and we estimate the upper limit of the possible effective mass anisotropy as 7%. We discuss the influence of conduction band nonparabolicity on the electron effective mass parameter as a function of In content. The effective mass parameter is consistent with a linear interpolation scheme between the conduction band mass parameters in GaN and InN when the strong nonparabolicity in InN is included. The In0.33Ga0.67N electron mobility parameter was found to be anisotropic, supporting previous experimental findings for wurtzite-structure GaN, InN, and AlxGa1-xN epitaxial layers with c-plane growth orientation. Published by AIP Publishing.

sted, utgiver, år, opplag, sider
AMER INST PHYSICS, 2018
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-145763 (URN)10.1063/1.5018247 (DOI)000425977500010 ()
Merknad

Funding Agencies|Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486]; Swedish Research Council (VR) [2016-00889]; Competence Center Program [2016-05190]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFO Mat LiU [2009-00971]; University of Nebraska-Lincoln; Swedish Foundation for Strategic Research (SSF) [FL12-0181, RIF14-055, EM16-0024]; J. A. Woollam Foundation; J. A. Woollam Co., Inc.; National Science Foundation [MRSEC DMR 1420645, CMMI 1337856, EAR 1521428]

Tilgjengelig fra: 2018-03-22 Laget: 2018-03-22 Sist oppdatert: 2019-03-05
4. Properties of two-dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity-enhanced THz optical Hall effect
Åpne denne publikasjonen i ny fane eller vindu >>Properties of two-dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity-enhanced THz optical Hall effect
Vise andre…
2016 (engelsk)Inngår i: Physica Status Solidi C-Current Topics in Solid State Physics, Vol 13 No 5-6, Wiley-VCH Verlagsgesellschaft, 2016, Vol. 13, nr 5-6, s. 369-373Konferansepaper, Publicerat paper (Fagfellevurdert)
Abstract [en]

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.

sted, utgiver, år, opplag, sider
Wiley-VCH Verlagsgesellschaft, 2016
Serie
Physica Status Solidi C-Current Topics in Solid State Physics, ISSN 1862-6351
Emneord
AlGaN/GaN HEMTs, THz ellipsometry, 2DEG properties, THz optical Hall effect
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-133135 (URN)10.1002/pssc.201510214 (DOI)000387957200045 ()
Konferanse
11th International Conference on Nitride Semiconductors (ICNS), Beijing, China, August 30-September 4. 2015
Tilgjengelig fra: 2016-12-12 Laget: 2016-12-09 Sist oppdatert: 2019-03-05bibliografisk kontrollert

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