| 1. |
- Habibpour, Omid, 1979, et al.
(författare)
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High gain graphene field effect transistors for wideband amplifiers
- 2014
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Ingår i: 44th European Microwave Conference, EuMC 2014 - Held as Part of the 17th European Microwave Week, EuMW 2014; Fiera di RomaRome; Italy; 6 October 2014 through 9 October 2014. - 9782874870354 ; , s. 371-373
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Konferensbidrag (refereegranskat)abstract
- We demonstrate graphene field of transistors (G-FETs) providing power gain of > 7 dB in a 50 O system. The G-FETs have S21 > 0 dB up to 7 GHz. The result indicates the feasibility for G-FET based wideband amplifiers.
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| 2. |
- Winters, Michael, 1986, et al.
(författare)
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A DC Comparison Study Between H-Intercalated and Native epigraphenes on SiC substrates
- 2013
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Ingår i: Materials Science Forum. - : Trans Tech Publications Inc.. - 1662-9752 .- 0255-5476. ; 740-742, s. 129-132, s. 129-132
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Konferensbidrag (refereegranskat)abstract
- The aim of this study is to compare DC characteristics of 'as-grown' and hydrogen (H)-intercalated epitaxial graphenes on SiC substrates [1,2]. Epitaxial graphene is grown on SiC at 1400-1600C, and H-intercalation is performed via in-situ introduction of Hydrogen during the graphitization process [5]. The fabrication processing steps used to define test structures are identical for the two materials. Results on the DC behavior and uniformity issues with respect to both materials are reported. The as-grown material behaves as a linear resistance, while H-intercalated demonstrates a non-linear characteristic. Hysteresis effects and time dependent behaviors are also observed in both materials. Extensive Hall measurements are performed on both materials with the aim of providing a qualitative understanding of material uniformity in both epigraphenes.
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| 3. |
- Winters, Michael, 1986, et al.
(författare)
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A temperature dependent measurement of the carrier velocity vs. electric field characteristic for as-grown and H-intercalated epitaxial graphene on SiC
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:19
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Tidskriftsartikel (refereegranskat)abstract
- A technique for the measurement of the electron velocity versus electric field is demonstrated on as-grown and H-intercalated graphene. Van der Pauw, coplanar microbridge, and coplanar TLM structures are fabricated in order to assess the carrier mobility, carrier concentration, sheet resistance, and contact resistance of both epi-materials. These measurements are then combined with dynamic IV measurements to extract a velocity-field characteristic. The saturated electron velocity measurements indicate a value of 2.33 x 10(7)cm/s for the as-grown material and 1: 36 x 10(7)cm/s for the H-intercalated material at 300 K. Measurements are taken as a function of temperature from 100K to 325K in order to estimate the optical phonon energy E-so of 4H-SiC by assuming an impurity scattering model. The extracted values of E-so are 97 meV for the as-grown sample and 115 meV for the H-intercalated sample. The H-intercalated result correlates to the anticipated value of 116 meV for 4H-SiC, while the as-grown value is significantly below the expected value. Therefore, we hypothesize that the transport properties of epitaxial graphene on SiC are influenced both by intercalation and by remote phonon scattering with the SiC substrate.
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| 4. |
- Winters, Michael, 1986, et al.
(författare)
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Carrier Mobility as a Function of Temperature in as-Grown and H-intercalated Epitaxial Graphenes on 4H-SiC
- 2014
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Ingår i: Materials Science Forum. - : Trans Tech Publications. - 1662-9752 .- 0255-5476. ; 778-780, s. 1146-1149, s. 1146-1149
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Konferensbidrag (refereegranskat)abstract
- The carrier velocity is measured as a function of electric field in as-grown and H-intercalaed epitaxial graphene grown on semi-insulating 4H-SiC in order to estimate the low field carrier mobility as a function of temperature. The mobility is also measured on the same samples as a function of temperature in a liquid Helium (He) cooled cryostat. The two temperature dependent measurements are compared in order to deduce the dominant carrier scattering mechanisms in both materials. In as-grown material, acoustic phonon scattering and impurity scattering both contribute, while impurity scattering dominates in H-intercalated material.
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| 5. |
- Winters, Michael, 1986
(författare)
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Electron transport studies in epitaxial graphene on SiC
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Furthermore, the dispersion in graphene k is light-like for graphene monolayers implying that electron transport would behave relativistically. If vsat = vf could be achieved in the material, then it is theoretically possible to achieve THz performance in long channel devices. Despite the nice theoretical picture, Nature is hardly so forthcoming. The prime objective of this work is to measure vsat in both as-grown and H-intercalated epitaxial graphene on 4H-SiC and 6H-SiC substrates. Hall measurements indicate that electron transport in Hintercalated material is found to be limited by impurity scattering. In the impurity scattering limit, one can infer a speed limit on the saturated electron velocity of vsat 2 · 107cm/s in epitaxial H-intercalated monolayers. This figure is definitive, as it sets epitaxial graphene on the same level as other semiconductor materials regarding the potential for frequency performance. In as-grown material it is possible to achieve a slightly higher vsat, but the material is generally very non-uniform. Both materials also suffer from the absence of a band-gap making device design intractable. In order to understand vsat in epitaxial graphene, theoretical and experimental approaches are needed. The consolidated theoretical model of ideal graphene is presented in detail. Graphene’s phonon spectrum, electronic band structure, and vf are derived from first principles. Band structure in bilayer graphene is also addressed and compared to the monolayer case. A possible solution to the band-gap problem is provided in the description of a graphene bilayer with an applied vertical electric field. Useful calculations are also shown regarding the density of states in monolayers and bilayers. The low vsat and high carrier density in epitaxial graphene motivate a semi-classical picture of electron transport via the Boltzmann Transport Equation. Special attention is directed towards the temperature dependence of phonon and long range and scattering mechanisms. Also described is an experimental characterization of epitaxial graphene layers on SiC. Since graphene monolayers and bilayers demonstrate very different physical properties, a method to accurately determine the number of layers is needed. Layer characterization via Raman spectroscopy is described in the context of theoretical and experimental results. Hall measurements are also shown for as-grown and H-intercalated layers providing valuable information about the mobility μ, sheet resistivity sh, and carrier density nsh. Experimental results obtained from pulsed IV measurements are also shown eventually bringing the discussion back to vsat. The nature of velocity saturation is then described in the context of the temperature dependent transport and scattering processes. In order to perform electrical measurements on graphene, a robust and minimally invasive fabrication strategy has been developed using both photolithography and electron beam lithography. These processes are designed in such a way as to preserve the quality of the epitaxial layer while providing outstanding contact resistance c
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