| 1. |
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| 2. |
- Guerriero, E., et al.
(författare)
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High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency f(max), cutoff frequency f(T), ratio f(max)/f(T), forward transmission coefficient S-21, and open-circuit voltage gain A(v). All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained f(max)/f(T) > 3, A(v) > 30 dB, and S-21 = 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance similar to 600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.
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| 3. |
- 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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| 4. |
- Khosa, Rabia Y., et al.
(författare)
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Electrical characterization of amorphous Al2O3 dielectric films on n-type 4H-SiC
- 2018
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 8:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on the electrical properties of Al 2 O 3 films grown on 4H-SiC by successive thermal oxidation of thin Al layers at low temperatures (200°C - 300°C). MOS capacitors made using these films contain lower density of interface traps, are more immune to electron injection and exhibit higher breakdown field (5MV/cm) than Al 2 O 3 films grown by atomic layer deposition (ALD) or rapid thermal processing (RTP). Furthermore, the interface state density is significantly lower than in MOS capacitors with nitrided thermal silicon dioxide, grown in N 2 O, serving as the gate dielectric. Deposition of an additional SiO 2 film on the top of the Al 2 O 3 layer increases the breakdown voltage of the MOS capacitors while maintaining low density of interface traps. We examine the origin of negative charges frequently encountered in Al 2 O 3 films grown on SiC and find that these charges consist of trapped electrons which can be released from the Al 2 O 3 layer by depletion bias stress and ultraviolet light exposure. This electron trapping needs to be reduced if Al 2 O 3 is to be used as a gate dielectric in SiC MOS technology.
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| 5. |
- Khosa, Rabia Y., et al.
(författare)
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Electrical characterization of high k-dielectrics for 4H-SiC MIS devices
- 2019
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Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 98, s. 55-58
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Tidskriftsartikel (refereegranskat)abstract
- We report promising results regarding the possible use of AlN or Al 2 O 3 as a gate dielectric in 4H-SiC MISFETs. The crystalline AlN films are grown by hot wall metal organic chemical vapor deposition (MOCVD) at 1100 °C. The amorphous Al 2 O 3 films are grown by repeated deposition and subsequent low temperature (200 °C) oxidation of thin Al layers using a hot plate. Our investigation shows a very low density of interface traps at the AlN/4H-SiC and the Al 2 O 3 /4H-SiC interface estimated from capacitance-voltage (CV) analysis of MIS capacitors. Current-voltage (IV) analysis shows that the breakdown electric field across the AlN or Al 2 O 3 is ∼ 3 MV/cm or ∼ 5 MV/cm respectively. By depositing an additional SiO 2 layer by plasma enhanced chemical vapor deposition at 300 °C on top of the AlN or Al 2 O 3 layers, it is possible to increase the breakdown voltage of the MIS capacitors significantly without having pronounced impact on the quality of the AlN/SiC or Al 2 O 3 /SiC interfaces.
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| 6. |
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| 7. |
- Melios, C., et al.
(författare)
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Tuning epitaxial graphene sensitivity to water by hydrogen intercalation
- 2017
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 9:10, s. 3440-3448
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Tidskriftsartikel (refereegranskat)abstract
- The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.
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| 8. |
- ul-Hassan, Jawad, et al.
(författare)
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Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0001) layers
- 2015
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 82:C, s. 12-23
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Tidskriftsartikel (refereegranskat)abstract
- Quasi-free-standing monolayer and bilayer graphene is grown on homoepitaxial layers of 4H-SiC. The SiC epilayers themselves are grown on the Si-face of nominally on-axis semi-insulating substrates using a conventional SiC hot-wall chemical vapor deposition reactor. The epilayers were confirmed to consist entirely of the 4H polytype by low temperature photoluminescence. The doping of the SiC epilayers may be modified allowing for graphene to be grown on a conducing substrate. Graphene growth was performed via thermal decomposition of the surface of the SiC epilayers under Si background pressure in order to achieve control on thickness uniformity over large area. Monolayer and bilayer samples were prepared through the conversion of a carbon buffer layer and monolayer graphene respectively using hydrogen intercalation process. Micro-Raman and reflectance mappings confirmed predominantly quasi-free-standing monolayer and bilayer graphene on samples grown under optimized growth conditions. Measurements of the Hall properties of Van der Pauw structures fabricated on these layers show high charge carrier mobility (> 2000 cm(2)/Vs) and low carrier density (
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| 9. |
- Westlund, Andreas, 1985, et al.
(författare)
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Graphene self-switching diodes as zero-bias microwave detectors
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:9, s. 093116-
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Tidskriftsartikel (refereegranskat)abstract
- Self-switching diodes (SSDs) were fabricated on as-grown and hydrogen-intercalated epitaxial graphene on SiC. The SSDs were characterized as zero-bias detectors with on-wafer measurements from 1 to 67 GHz. The lowest noise-equivalent power (NEP) was observed in SSDs on the hydrogen-intercalated sample, where a flat NEP of 2.2 nW/Hz½ and responsivity of 3.9 V/W were measured across the band. The measured NEP demonstrates the potential of graphene SSDs as zero-bias microwave detectors.
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| 10. |
- 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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| 11. |
- 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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| 12. |
- Winters, Michael, 1986, et al.
(författare)
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Assessment of H-intercalated graphene for microwave FETs through material characterization and electron transport studies
- 2015
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 81:1, s. 96-104
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene is grown on semi-insulating (SI) 4H-SiC in a hot wall CVD reactor by graphitization and in-situ intercalation with (H)ydrogen. A holistic material characterization is performed in order to ascertain the number of layers, layer uniformity, and electron transport properties of the epi-layers via electronic test structures and Raman spectroscopy. Bilayer graphene field effect transistors (GFETs) are fabricated using a full electron beam lithography (EBL) process which is optimized for low contact resistances of r(c)
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| 13. |
- 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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| 14. |
- Winters, Michael, 1986, et al.
(författare)
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Characterization and physical modeling of MOS capacitors in epitaxial graphene monolayers and bilayers on 6H-SiC
- 2016
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Ingår i: AIP Advances. - : AMER INST PHYSICS. - 2158-3226. ; 6:8
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Tidskriftsartikel (refereegranskat)abstract
- Capacitance voltage (CV) measurements are performed on planar MOS capacitors with an Al2O3 dielectric fabricated in hydrogen intercalated monolayer and bilayer graphene grown on 6H-SiC as a function of frequency and temperature. Quantitative models of the CV data are presented in conjunction with the measurements in order to facilitate a physical understanding of graphene MOS systems. An interface state density of order 2 . 10(12)eV(-1)cm(-2) is found in both material systems. Surface potential fluctuations of order 80-90meV are also assessed in the context of measured data. In bilayer material, a narrow bandgap of 260meV is observed consequent to the spontaneous polarization in the substrate. Supporting measurements of material anisotropy and temperature dependent hysteresis are also presented in the context of the CV data and provide valuable insight into measured and modeled data. The methods outlined in this work should be applicable to most graphene MOS systems. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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| 15. |
- Winters, Michael, 1986
(författare)
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Electron Transport and Charge Control in Epitaxial Graphene
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene monolayers and bilayers have attracted research interest in both the physics and electronic materials communities owing to their unique band structures. In a pristine monolayer, carriers travel at the Fermi velocity v_f = 1e8 cm/s and exhibit linear dispersion. However, the lack of an energy gap in monolayer material makes applications in active devices challenging. In bilayers, carriers behave classically with an effective mass of m^* = 0.035m_e and exhibit parabolic dispersion along with adequate transport properties. Graphene bilayers are attractive as a narrow energy gap may be opened in the presence of a symmetry breaking potential. Native and intercalated graphene produced by epitaxy on semi-insulating 4H(6H)-SiC substrates are particularly relevant as highly crystalline material may be prepared on large area substrates.In this work, the low field and high field transport properties of native and intercalated epitaxial layers are investigated in electron devices. The results are then considered within the contextual framework provided by fundamental principles. The influence of charged impurity, neutral impurity, and acoustic phonon scattering on conductivity and carrier mobility is considered alongside low field transport data. High field measurements are performed in order to estimate a saturated electron velocity of 2e7 cm/s in epitaxial graphene on SiC. Mobility and saturation velocity influence transconductance and frequency performance in active devices. In both native and intercalated devices, velocity saturation occurs as a consequence of scattering with surface optical modes in the SiC substrate. In this respect, SiC is an ideal substrate for graphene as it demonstrates a high energy surface optical mode (115meV) relative to SiO2 (55meV).Charge control is also investigated via low temperature capacitance voltage measurements in large area metal oxide graphene (GMOS) capacitors with a low temperature Al2O3 dielectric. Results are then correlated with current voltage characteristics in active devices abricated in graphene monolayers and bilayers in which bias dependent instabilities and hysteresis are frequently observed. The CV results highlight the influence of surface potential fluctuations (90meV) and interface states (2e12 eV^{-1}cm^{-2}) on the transfer characteristics of active devices. The density of interface states is found to be relatively high in graphene MOS relative to the 5e10 eV^{-1}cm^{-2} achieved in thermally oxidized silicon MOS. Collecting results, it is possible to gain insight into the physical properties of the material while simultaneously outlining perspectives and challenges for technological applications. The electron transport results motivate further investigations into substrate engineering in graphene MOS in order to improve the saturation velocity and transport characteristics. Meanwhile, the charge control experiments demonstrate a need for improved dielectric films in graphene MOS structures. Together, the electron transport and charge control results provide a thorough description of the electronic properties of MOS devices in epitaxial graphene.
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| 16. |
- 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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| 17. |
- Winters, Michael, 1986, et al.
(författare)
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High frequency electromagnetic detection by nonlinear conduction modulation in graphene nanowire diodes
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:14
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Tidskriftsartikel (refereegranskat)abstract
- We present graphene nanowires implemented as dispersion free self switched microwave diode detectors. The microwave properties of the detectors are investigated using vector corrected large signal measurements in order to determine the detector responsivity and noise equivalent power (NEP) as a function of frequency, input power, and device geometry. We identify two distinct conductance nonlinearities which generate detector responsivity: an edge effect nonlinearity near zero bias due to lateral gating of the nanowire structures, and a velocity saturation nonlinearity which generates current compression at high power levels. The scaling study shows that detector responsivity obeys an exponential scaling law with respect to nanowire width, and a peak responsivity (NEP) of 250 V/W (50 pW/ Hz) is observed in detectors of the smallest width. The results are promising as the devices exhibit responsivities which are comparable to state of the art self switched detectors in semiconductor technologies.
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| 18. |
- Winters, Michael, 1986, et al.
(författare)
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Hysteresis modeling in graphene field effect transistors
- 2015
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 117:7, s. Art. no, 074501-
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Tidskriftsartikel (refereegranskat)abstract
- Graphene field effect transistors with an Al2O3 gate dielectric are fabricated on H-intercalated bilayer graphene grown on semi-insulating 4H-SiC by chemical vapour deposition. DC measurements of the gate voltage nu(g) versus the drain current i(d) reveal a severe hysteresis of clockwise orientation. A capacitive model is used to derive the relationship between the applied gate voltage and the Fermi energy. The electron transport equations are then used to calculate the drain current for a given applied gate voltage. The hysteresis in measured data is then modeled via a modified Preisach kernel.
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