| 1. |
- Roccaforte, F., et al.
(författare)
-
Towards vertical Schottky diodes on bulk cubic silicon carbide (3C-SiC)
- 2022
-
Ingår i: Applied Surface Science. - : ELSEVIER. - 0169-4332 .- 1873-5584. ; 606
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, we demonstrate the feasibility of fabricating vertical Schottky diodes on bulk cubic silicon carbide (3C-SiC) material obtained by combining sublimation epitaxy and chemical vapor deposition, starting from 4 degrees -off axis 4H-SiC. First, the good quality of the epilayers grown with this method was demonstrated by morphological and structural analyses. Then, fabricated vertical Pt/3C-SiC Schottky diodes exhibited an ideality factor of 1.21 and a barrier height of 0.6 eV, as determined by thermionic emission model. The temperature dependent forward current analysis indicated the formation of an inhomogeneous barrier, which has been related with the presence of conductive surface defects, detected by nanoscale local current measurements. On the other hand, the reverse leakage current could be described by thermionic field emission model, including image force lowering. These findings demonstrate the viability of the proposed approach for bulk 3C-SiC growth for device fabrication. The material quality and the feasibility of fabricating vertical diodes based on 3C-SiC with a low barrier pave the way for the application of this polytype for medium-voltage power devices.
|
|
| 2. |
- Schiliro, E., et al.
(författare)
-
Aluminum oxide nucleation in the early stages of atomic layer deposition on epitaxial graphene
- 2020
-
Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 169, s. 172-181
-
Tidskriftsartikel (refereegranskat)abstract
- The nucleation and growth mechanism of aluminum oxide (Al2O3) in the early stages of atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by atomic force microscopy (AFM), conductive-atomic force microscopy (C-AFM) and Raman spectroscopy. Differently than for other types of graphene, a large and uniform density of nucleation sites was observed in the case of EG and ascribed to the presence of the buffer layer at EG/SiC interface. The deposition process was characterized by Al2O3 island growth in the very early stages, followed by the formation of a continuous Al2O3 film (similar to 2.4 nm thick) after only 40 ALD cycles due to the islands coalescence, and subsequent layer-by-layer growth. The electrical insulating properties of the deposited ultrathin Al2O3 films were demonstrated by nanoscale current mapping with C-AFM. Raman spectroscopy analyses showed low impact of the ALD process on the defects density of EG. The EG strain was also almost unaffected by the deposition in the regime of island growth and coalescence, whereas a significant increase was observed after the formation of a compact Al2O3 film. The obtained results can have important implications for device applications of epitaxial graphene requiring ultra-thin high-k insulators. (C) 2020 Elsevier Ltd. All rights reserved.
|
|
| 3. |
- Bouhafs, Chamseddine, et al.
(författare)
-
Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC
- 2017
-
Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 116, s. 722-732
-
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.
|
|
| 4. |
- Giannazzo, F, et al.
(författare)
-
Electronic transport at monolayer-bilayer junctions in epitaxial graphene on SiC
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:23
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional maps of the electronic conductance in epitaxial graphene grown on SiC were obtained by calibrated conductive atomic force microscopy. The correlation between morphological and electrical maps revealed the local conductance degradation in epitaxial graphene over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at the 1L/2L junction is a purely quantum-mechanical effect independent on the interaction with the substrate. First-principles transport calculations indicate that the weak wave-function coupling between the 1L pi/pi* bands with the respective first bands of the 2L region gives rise to a strong suppression of the conductance for energies within +/- 0.48 eV from the Dirac point. Conductance degradation at 1L/2L junctions is therefore a general issue for large area graphene with a certain fraction of inhomogeneities in the layer number, including graphene grown by chemical vapor deposition on metals. DOI: 10.1103/PhysRevB.86.235422
|
|
| 5. |
- Giannazzo, F., et al.
(författare)
-
Graphene integration with nitride semiconductors for high power and high frequency electronics
- 2017
-
Ingår i: Physica Status Solidi (a) applications and materials science. - : WILEY-V C H VERLAG GMBH. - 1862-6300 .- 1862-6319. ; 214:4
-
Tidskriftsartikel (refereegranskat)abstract
- Group III nitride semiconductors (III-N), including GaN, AlN, InN, and their alloys, are currently the materials of choice for many applications in optoelectronics (light-emitting diodes, laser diodes), and high-power and high-frequency transistors. Due to its attractive electrical, optical, mechanical, and thermal properties, graphene (Gr) integration with III-N technology has been considered in the last few years, in order to address some of the major issues which still limit the performances of GaN-based devices. To date, most of the studies have been focused on the use of Gr as transparent conductive electrode (TCE) to improve current spreading from top electrodes and light extraction in GaN-LEDs. This paper will review recent works evaluating the benefits of Gr integration with III-N for high power and high frequency electronics. From the materials side, recent progresses in the growth of high quality GaN layers on Gr templates and in the deposition of Gr on III-N substrates and templates will be presented. From the applications side, strategies to use Gr for thermal management in high-power AlGaN/GaN transistors will be discussed. Finally, recent proposals of implementing new ultra-high-frequency (THz) transistors, such as the Gr base hot electron transistor (GBHET), by Gr integration with III-N will be highlighted. (C) 2016 WILEY-VCH Verlag GmbH amp; Co. KGaA, Weinheim
|
|
| 6. |
- Giannazzo, F., et al.
(författare)
-
Impact of substrate steps and of monolayer-bilayer junctions on the electronic transport in epitaxial graphene on 4H-SiC (0001)
- 2013
-
Ingår i: Silicon Carbide and Related Materials 2012. - : Trans Tech Publications Inc.. ; , s. 113-116
-
Konferensbidrag (refereegranskat)abstract
- Two dimensional maps of the electronic conductance in epitaxial graphene (EG) grown on SiC were obtained by conductive atomic force microscopy (CAFM). The correlation between morphological and electrical maps revealed the local conductance degradation in EG over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at 1L/2L junction is a purely quantum mechanical effect, due to the weak coupling between 1L and 2L electron wavefunctions.
|
|
| 7. |
- Yakimova, Rositsa, et al.
(författare)
-
Growth, defects and doping of 3C-SiC on hexagonal polytypes
- 2017
-
Ingår i: ECS Transactions. - : Electrochemical Society. - 9781607685395 ; , s. 107-115, s. 107-115
-
Konferensbidrag (refereegranskat)abstract
- Technologies for the growth of 3C-SiC with crystalline quality and crystal size similar to hexagonal counterparts (6H- or 4H-SiC) are still at the laboratory stage. There are several challenges in the control of polytype stability and formation of structural defects which have to be eliminated to reveal the full potential of this material. Nevertheless, 3C-SiC has been explored for various energy, environment and biomedical applications which significantly benefit from the intrinsic semiconductor properties of this material. The future of 3C-SiC and its applications depends on the advances which will be made in improving crystalline quality, enlarging crystal size and controlling doping levels which have not been entirely explored due to the lack of high quality 3C-SiC substrates. This paper reviews recent progress in growth and doping of thick 3C-SiC layers on hexagonal SiC substrates using sublimation epitaxy. It covers the growth process on off-axis substrates and defects occurrence, as well as the issue of obtaining high resistivity material.
|
|
| 8. |
- Giannazzo, F., et al.
(författare)
-
Probing the uniformity of hydrogen intercalation in quasi-free-standing epitaxial graphene on SiC by micro-Raman mapping and conductive atomic force microscopy
- 2019
-
Ingår i: Nanotechnology. - : IOP PUBLISHING LTD. - 0957-4484 .- 1361-6528. ; 30:28
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer graphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 degrees C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (Phi(B)). The Phi(B) values evaluated from local I-V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky-Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.
|
|
| 9. |
- Nicotra, G., et al.
(författare)
-
Interface disorder probed at the atomic scale for graphene grown on the C face of SiC
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:15, s. 155411-
-
Tidskriftsartikel (refereegranskat)abstract
- We use aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, atomic force microscopy, and the density functional theory to study the structural and electronic characteristics of graphene grown on the C face of SiC. We show that for high growth temperatures the graphene/SiC(000 (1) over bar) interface is dominated by a thin amorphous film which strongly suppresses the epitaxy of graphene on the SiC substrate. This film maintains an almost fixed thickness regardless of the number of the overlying graphene layers, while its chemical signature shows the presence of C, Si, and O. Structurally, the amorphous area is inhomogeneous, as its Si concentration gradually decreases while approaching the first graphene layer, which is purely sp(2) hybridized. Ab initio calculations show that the evaporation process and the creation of Si vacancies on the C face of SiC strongly enhance the surface disorder and designate defect areas as preferential sublimation sites. Based on these features, we discuss differences and similarities between the C-only buffer layer that forms on the Si face of SiC and the thicker C-Si-O amorphous film of the C face.
|
|
| 10. |
- Shtepliuk, Ivan, et al.
(författare)
-
Manipulation of epitaxial graphene towards novel properties and applications
- 2020
-
Ingår i: MATERIALS TODAY-PROCEEDINGS. - : ELSEVIER. - 2214-7853. ; , s. 37-45
-
Konferensbidrag (refereegranskat)abstract
- The integration of epitaxial graphene on 4H-SiC with different metals may allow tunability of electronic and optical properties of graphene, enabling novel high-performance devices. Here we present a Raman spectroscopy study on epitaxial graphene decorated with electrodeposited Pb and Li adatoms and with magnetron sputtered 5 nm-thick Ag nano-island films. We find that the presence of metals on the epitaxial graphene surface generates defects and induces n-type doping, which is evidenced by the observation of the defect related Raman modes (namely D, D and D + G) and systematic red-shift of the main characteristic modes of graphene. In-depth statistical analysis of the Raman data before and after metal deposition complemented by density functional theory (DFT) calculations allowed to link the interaction strength between the three selected metals and graphene with the metal-induced changes in the vibrational/electronic properties of graphene. Large-area uniform electron doping of epitaxial graphene and surface-enhanced Raman scattering (SERS) effect are reached by room temperature deposition of Ag nano-island films. Very promising results have been obtained from graphene subjected to electrochemical intercalation by Li, which can serve as prerequisites of the construction of Li batteries. The strong interaction between Li or Pb with graphene implies the possibility to exploit the epitaxial graphene as an efficient material for energy storage or for heavy metal sensing, while predominant van der Waals interaction between Ag and graphene favors the formation of extremely thin silver coatings towards two-dimensional metal systems. The present results give better understanding of the nature of epitaxial graphene response to metal deposition and can be useful to design high-performance energy storage devices, optical sensors and heavy metal detection systems. (C) 2019 Elsevier Ltd. All rights reserved.
|
|
| 11. |
|
|
| 12. |
- Sonde, S., et al.
(författare)
-
Effect of graphene/4H-SiC(0 0 0 1) interface on electrostatic properties in graphene
- 2012
-
Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : Elsevier. - 1386-9477 .- 1873-1759. ; 44:6, s. 993-996
-
Tidskriftsartikel (refereegranskat)abstract
- Electrostatic properties, quantum capacitance (Cq) and local density of states (LDOS) are evaluated for graphene on 4H-SiC(0 0 0 1) by measuring the local capacitance with Scanning Capacitance Spectroscopy (SCS). Two distinct samples were used for comparative study, viz., graphene exfoliated and deposited on 4H-SiC(0 0 0 1)—DG, and graphene grown epitaxially on 4H-SiC(0 0 0 1)—EG. We observed a distinctly lower screening length (rscr) and Cq while wider variations in the LDOS for EG. Such differences are attributed to the peculiar interface between EG/4H-SiC(0 0 0 1), which is known to be more or less defective having the presence of positive charges.
|
|
| 13. |
- Sonde, S, et al.
(författare)
-
Electrical properties of the graphene/4H-SiC (0001) interface probed by scanning current spectroscopy
- 2009
-
Ingår i: PHYSICAL REVIEW B. - 1098-0121. ; 80:24, s. 241406-
-
Tidskriftsartikel (refereegranskat)abstract
- The current transport across the graphene/4H-SiC interface has been investigated with nanometric lateral resolution by scanning current spectroscopy on both epitaxial graphene (EG) grown on (0001) 4H-SiC and graphene exfoliated from highly oriented pyrolytic graphite deposited on the same substrate [deposited graphene (DG)]. This study reveals that the Schottky barrier height (SBH) of EG/4H-SiC (0.36 +/- 0.1 eV) is similar to 0.49 eV lower than the SBH of DG/4H-SiC (0.85 +/- 0.06 eV). This result is discussed in terms of the Fermi-level pinning similar to 0.49 eV above the Dirac point in EG due to the presence of positively charged states at the interface between the Si face of 4H-SiC and the carbon-rich buffer layer, which is the precursor for EG formation.
|
|
| 14. |
- Sonde, S, et al.
(författare)
-
Role of graphene/substrate interface on the local transport properties of the two-dimensional electron gas
- 2010
-
Ingår i: APPLIED PHYSICS LETTERS. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 97:13, s. 132101-
-
Tidskriftsartikel (refereegranskat)abstract
- The electron mean free path (l(gr)) is "locally" evaluated by scanning capacitance spectroscopy on graphene obtained with different preparation methods and on different substrates, i.e., graphene exfoliated from highly oriented pyrolitic graphite (HOPG) and deposited (DG) on 4H-SiC(0001) and on SiO2 and epitaxial graphene grown on 4H-SiC (0001) (EG). l(gr) in DG on SiC was more than four times larger than in DG on SiO2. The improved mean free path is explained by the higher permittivity of SiC compared to SiO2, yielding a better dielectric screening of charged-impurities, and by the weaker coupling of graphene two-dimensional-electron-gas with surface polar phonons of SiC. On the other hand, l(gr) on EG is on average similar to 0.4 times that on DG-SiC and exhibits large variations from point to point, due to the presence of a laterally inhomogeneous positively charged layer at EG/SiC interface.
|
|
