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![Conductive atomic force microscopy (C-AFM) local current mapping through an Al2O3 thin film deposited onto EG on axis 4H-SiC(0001). (a) Morphology of the probed sample area, including both uniform Al2O3 on 1 L EG and Al2O3 on a 2 L EG patch. (b) Current map collected on this area with a tip bias Vtip = 6 V. (c) Two representative local current-voltage characteristics collected by the C-AFM probe on Al2O3 in the 1 L and 2 L EG regions. Images adapted with permission from Ref. [21], copyright Wiley 2019.](publication/340416225/figure/fig2/AS:877854921330688@1586308331605/Conductive-atomic-force-microscopy-C-AFM-local-current-mapping-through-an-Al2O3-thin.png)
Conductive atomic force microscopy (C-AFM) local current mapping through an Al2O3 thin film deposited onto EG on axis 4H-SiC(0001). (a) Morphology of the probed sample area, including both uniform Al2O3 on 1 L EG and Al2O3 on a 2 L EG patch. (b) Current map collected on this area with a tip bias Vtip = 6 V. (c) Two representative local current-voltage characteristics collected by the C-AFM probe on Al2O3 in the 1 L and 2 L EG regions. Images adapted with permission from Ref. [21], copyright Wiley 2019.
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Due to its excellent physical properties and availability directly on a semiconductor substrate, epitaxial graphene (EG) grown on the (0001) face of hexagonal silicon carbide is a material of choice for advanced applications in electronics, metrology and sensing. The deposition of ultrathin high-k insulators on its surface is a key requirement for...
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Context 1
... electrical quality of the insulating layer was also evaluated by conductive atomic force microscopy (C-AFM) for current mapping and local current-voltage (I-V) analyses [21]. A morphology map of the scanned area is reported in Figure 6a, which includes both uniform Al2O3 on 1 L EG and Al2O3 on a 2 L EG patch. Figure 6b shows a current map collected on this area with a positive value of the tip bias Vtip = 6 V with respect to EG. ...
Context 2
... morphology map of the scanned area is reported in Figure 6a, which includes both uniform Al2O3 on 1 L EG and Al2O3 on a 2 L EG patch. Figure 6b shows a current map collected on this area with a positive value of the tip bias Vtip = 6 V with respect to EG. While uniform low current values were detected through the 12 nm Al2O3 film onto 1 L EG, the presence of high current spots was observed in the 2 L EG region. ...
Context 3
... uniform low current values were detected through the 12 nm Al2O3 film onto 1 L EG, the presence of high current spots was observed in the 2 L EG region. Figure 6c illustrates two representative local current-voltage characteristics collected by the C-AFM probe on Al2O3 in the 1 L and 2 L EG regions. While current smoothly increased with the bias for Al2O3 on 1 L EG, an abrupt rise of current was observed for Vtip > 6 V in the case of Al2O3 on 2 L EG. ...
Context 4
... electrical quality of the insulating layer was also evaluated by conductive atomic force microscopy (C-AFM) for current mapping and local current-voltage (I-V) analyses [21]. A morphology map of the scanned area is reported in Figure 6a The superior homogeneity of Al 2 O 3 deposition on 1 L EG areas indicates a higher reactivity of 1 L EG with respect to 2 L or few layers EG, which was explained in terms of the higher n-type doping and strain caused by the interfacial BL on a single graphene overlayer with respect to 2 L [21]. Recent experimental and theoretical investigations demonstrated that the interaction of polar water molecules with graphene depends on the Fermi level of graphene, i.e., its doping [53]. ...
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Citations
... The lack of out-of-plane bonds or surface groups on Graphene typically represents the main obstacle to film nucleation and consequently, uniform coverage is impossible to achieve (Kim et al. 2009). To overcome these nucleation issues, various approaches have been proposed to provide a functional group on the inert surface of Graphene (Giannazzo et al. 2020;Jeong et al. 2016), such as the functionalization of the Graphene surface by depositing metal or polymer seed layers (Jeong et al. 2016) or the deposition of an intermediate layer of HfO 2 by CVD to improve the nucleation of HfO 2 by ALD which follows (Lukosius et al. 2015). ...
In this paper, we present a comparative analysis of the Raman spectra of two heterostructures: Si/SiO2/HfO2/Graphene and Si/SiO2/Graphene/HfO2. In the former configuration, the Graphene layer is transferred onto the 5 nm-thick HfO2 layer, while the latter configuration involves the growth of à 5 nm-thick HfO2 layer by atomic layer deposition (ALD) on the Graphene layer. The analysis of the Raman spectra has demonstrated significant differences between the interfaces Graphene/HfO2 and HfO2/Graphene. The distinct Raman signatures observed emphasize the critical role played by the specific order and arrangement of the Graphene and HfO2 layers within the heterostructure, which impacts the charge transfer between these two materials and the presence or absence of strains. The Raman spectra corresponding to different thickness of the HfO2 layer (5, 10, 15, and 20 nm) in Si/SiO2/Graphene/HfO2 heterostructure shows the appearance of a small shoulder on the low-frequency side of the G band around 1523 cm⁻¹. Furthermore, it was observed that the intensity of this shoulder diminishes as the thickness of the HfO2 layer increases. The observed discrepancies between spectra, related to the experimental conditions employed for sample preparation, are attributed to the interplay of two phenomena: the transfer of charges among Graphene, HfO2, and SiO2, as well as the emergence of constraints.
... Reported methods for graphene passivation with Al 2 O 3 include electron beam evaporation, pulsed laser deposition (PLD), radio frequency (RF) sputtering [2] and atomic layer deposition (ALD) [3]. The ALD synthesis of Al 2 O 3 from trimethylaluminum (TMA) and deionized water is favored for its noninvasive character, lack of energetic or reactive species, subnanometer precision and layer uniformity [4]. However, the initial chemisorption of hydroxyl groups, as any other precursor molecules, is hindered by graphene's sp 2 -hybridized structure providing no out-of-plane bonds [4][5][6]. ...
... The ALD synthesis of Al 2 O 3 from trimethylaluminum (TMA) and deionized water is favored for its noninvasive character, lack of energetic or reactive species, subnanometer precision and layer uniformity [4]. However, the initial chemisorption of hydroxyl groups, as any other precursor molecules, is hindered by graphene's sp 2 -hybridized structure providing no out-of-plane bonds [4][5][6]. Unless chemisorption on graphene is enabled by reactive prefunctionalization with ozone [7], fluorine [8], nitrogen [9], oxygen [10] or hydrogen [5], the first half-cycle deposition must depend on physisorption. ambient atmosphere. ...
In this report, we introduce a novel method based on low-frequency noise analysis for the assessment of quality and pattern of inhomogeneity in intentionally-aged Hall effect sensors featuring hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene mesa on semi-insulating high-purity on-axis 4H-SiC(0001), all passivated with a 100-nm-thick atomic-layer-deposited Al2O3 layer. Inferring from the comparison of the measured noise and one calculated for a homogeneous sensor, we hypothesize about possible unintentional contamination of the sensors’ active regions. Following in-depth structural characterization based on Nomarski interference contrast optical imaging, confocal micro-Raman spectroscopy, high-resolution Transmission Electron Microscopy and Secondary Ion Mass Spectrometry, we find out that the graphene’s quasi-free-standing character and p-type conductance make the Al2O3/graphene interface exceptionally vulnerable to uncontrolled contamination and its unrestrained lateral migration throughout the entire graphene mesa, eventually leading to the blistering of Al2O3. Thus, we prove the method’s suitability for the detection of these contaminants’ presence and location, and infer on its applicability to the investigation of any contamination-induced inhomogeneity in two-dimensional systems. https://authors.elsevier.com/a/1eyP64xMlkIhhc
... While the state of the art of ALD on 2D materials has been the object of different comprehensive review papers in recent years [28][29][30], a dedicated review article on the direct ALD growth of high dielectrics driven by the 2D material and substrate interaction is still missing. The aim of the present work is to provide an overview in this research topic. ...
Atomic layer deposition (ALD) of high-κ dielectrics on two-dimensional (2D) materials (including graphene and transition metal dichalcogenides) still represents a challenge due to the lack of out-of-plane bonds on the pristine surfaces of 2D materials, thus making the nucleation process highly disadvantaged. The typical methods to promote the nucleation (i.e., the predeposition of seed layers or the surface activation via chemical treatments) certainly improve the ALD growth but can affect, to some extent, the electronic properties of 2D materials and the interface with high-κ dielectrics. Hence, direct ALD on 2D materials without seed and functionalization layers remains highly desirable. In this context, a crucial role can be played by the interaction with the substrate supporting the 2D membrane. In particular, metallic substrates such as copper or gold have been found to enhance the ALD nucleation of Al2O3 and HfO2 both on monolayer (1 L) graphene and MoS2. Similarly, uniform ALD growth of Al2O3 on the surface of 1 L epitaxial graphene (EG) on SiC (0001) has been ascribed to the peculiar EG/SiC interface properties. This review provides a detailed discussion of the substrate-driven ALD growth of high-κ dielectrics on 2D materials, mainly on graphene and MoS2. The nucleation mechanism and the influence of the ALD parameters (namely the ALD temperature and cycle number) on the coverage as well as the structural and electrical properties of the deposited high-κ thin films are described. Finally, the open challenges for applications are discussed.
... The direct ALD growth of Al 2 O 3 on largearea 1L MoS 2 induced by the Au underlayer can be of wide interest for electronic applications. [13][14][15] As an example, Park et al. [10] systematically investigated thermal ALD of Al 2 O 3 , using trimethyl-aluminum (TMA) as the Al precursor and water (H 2 O) as coreactant, on the surface of different TMDs, that is, MoS 2 , WS 2 , and WSe 2 multilayers. The deposition temperature (T dep ) and the TMA adsorption energy (E ads ) on the TMDs surface were demonstrated to play a crucial role on the uniformity of the deposited Al 2 O 3 (≈10 nm) films. ...
In this paper, the authors demonstrate the atomic layer deposition (ALD) of highly homogeneous and ultrathin (≈3.6 nm) Al2O3 films with very good insulating properties (breakdown field of ≈10–12 MV cm−1) directly onto monolayer (1L) MoS2 exfoliated on gold. Differently than in the case of 1L MoS2 supported by a common insulating substrate (Al2O3/Si), a better nucleation process of the high‐k film is observed on the 1L MoS2/Au system since the ALD early stages. Atomic force microscopy analyses show a ≈50% Al2O3 surface coverage just after 10 ALD cycles, its increase to >90% (after 40 cycles), and a uniform ≈3.6 nm film (after 80 cycles). The Al2O3 density on bilayer MoS2 is found to be significantly reduced with respect to 1L MoS2/Au, suggesting a role of screened interface charges with the metal substrate on the adsorption of ALD precursors. Finally, Raman and photoluminescence spectroscopy show a p‐type doping and tensile strain of 1L MoS2 induced by the Au substrate, providing an insight on the evolution of vibrational and optical properties after the Al2O3 deposition. The direct ALD growth of Al2O3 on large‐area 1L MoS2 induced by the Au underlayer can be of wide interest for electronic applications. The direct atomic layer deposition of Al2O3 on monolayer (1L) MoS2 supported by gold and by a common insulating substrate (Al2O3/Si) is investigated. The growth of compact and ultrathin (≈3.6 nm) Al2O3 film is obtained on 1L MoS2/Au, and it is discussed in terms of the peculiar interaction with the metal substrate.
... Hence, several strategies have been investigated in the last few years to improve the ALD growth on TMDs, by tailoring the process conditions and/or by appropriate pre-functionalization treatments of the surface. Many of these approaches were inspired by ALD growth experiments performed on graphene [13,14,15]. As an example, Park et al. ...
In this paper we demonstrated the thermal Atomic Layer Deposition (ALD) growth at 250 {\deg}C of highly homogeneous and ultra-thin ($\approx$ 3.6 nm) $Al_2O_3$ films with excellent insulating properties directly onto a monolayer (1L) $MoS_2$ membrane exfoliated on gold. Differently than in the case of 1L $MoS_2$ supported by a common insulating substrate ($Al_2O_3/Si$), a better nucleation process of the high-k film was observed on the 1L $MoS_2/Au$ system since the ALD early stages. Atomic force microscopy analyses showed a $\approx 50\%$ $Al_2O_3$ surface coverage just after 10 ALD cycles, its increasing up to $>90\%$ (after 40 cycles), and an uniform $\approx$ 3.6 nm film, after 80 cycles. The coverage percentage was found to be significantly reduced in the case of 2L $MoS_2/Au$, indicating a crucial role of the interfacial interaction between the aluminum precursor and $MoS_2/Au$ surface. Finally, Raman spectroscopy and PL analyses provided an insight about the role played by the tensile strain and p-type doping of 1L $MoS_2$ induced by the gold substrate on the enhanced high-k nucleation of $Al_2O_3$ thin films. The presently shown high quality ALD growth of high-k $Al_2O_3$ dielectrics on large area 1L $MoS_2$ induced by the Au underlayer can be considered of wide interest for potential device applications based on this material system.
... In this context, the integration of epitaxial graphene with insulators can bring this material closer to real electronic applications. In a series of three papers [44,59,60], we presented and critically discussed the atomic layer deposition technique as a promising approach to grow high-quality Al 2 O 3 layers of different thicknesses. It was revealed that for a deposition temperature of 250 • C, the transition from island-based growth to layerby-layer growth mode occurs after 40 ALD cycles that correspond to~2.4 nm ( Figure 13). ...
... In this context, the integration of epitaxial graphene with insulators can bring this material closer to real electronic applications. In a series of three papers [44,59,60], we presented and critically discussed the atomic layer deposition technique as a promising approach to grow high-quality Al2O3 layers of different thicknesses. It was revealed that for a deposition temperature of 250 °C, the transition from islandbased growth to layer-by-layer growth mode occurs after 40 ALD cycles that correspond to ~2.4 nm ( Figure 13). ...
Material growth on a dangling-bond-free interface such as graphene is a challenging technological task, which usually requires additional surface pre-treatment steps (functionalization, seed layer formation) to provide enough reactive sites. Being one of the most promising and adaptable graphene-family materials, epitaxial graphene on SiC, due to its internal features (substrate-induced n-doping, compressive strain, terrace-stepped morphology, bilayer graphene nano-inclusions), may provide pre-conditions for the enhanced binding affinity of environmental species, precursor molecules, and metal atoms on the topmost graphene layer. It makes it possible to use untreated pristine epitaxial graphene as a versatile platform for the deposition of metals and insulators. This mini-review encompasses relevant aspects of magnetron sputtering and electrodeposition of selected metals (Au, Ag, Pb, Hg, Cu, Li) and atomic layer deposition of insulating Al2O3 layers on epitaxial graphene on 4H-SiC, focusing on understanding growth mechanisms. Special deliberation has been given to the effect of the deposited materials on the epitaxial graphene quality. The generalization of the experimental and theoretical results presented here is hopefully an important step towards new electronic devices (chemiresistors, Schottky diodes, field-effect transistors) for environmental sensing, nano-plasmonics, and biomedical applications.
... Fabrication of graphene FETs involves the challenges of selecting an appropriate gate insulator and controlling its thickness [150]. Generally, the oxides such as SiO 2 , HfO 2 , Al 2 O 3 , and Y 2 O 3 are used as the gate dielectrics, but these tend to become amorphous with scaling and interactions with the graphene can occur, which makes the high-quality defect-free interface formation difficult [150][151][152]. Hexagonal boron nitride (h-BN) forms a weak van der Waals interaction with the graphene (lattice mismatch only 2% [153]) and has been utilized as an ideal insulator for the graphene FET formation [151]. ...
... Generally, the oxides such as SiO 2 , HfO 2 , Al 2 O 3 , and Y 2 O 3 are used as the gate dielectrics, but these tend to become amorphous with scaling and interactions with the graphene can occur, which makes the high-quality defect-free interface formation difficult [150][151][152]. Hexagonal boron nitride (h-BN) forms a weak van der Waals interaction with the graphene (lattice mismatch only 2% [153]) and has been utilized as an ideal insulator for the graphene FET formation [151]. Due to the lower value of a dielectric constant (k~4), h-BN is usually combined with a high k-dielectric layer such as Y 2 O 3 formed via atomic layer deposition on h-BN [154]. ...
This book is a compilation of recent studies by recognized experts in the field of epitaxial graphene working towards a deep comprehension of growth mechanisms, property engineering, and device processing. The results of investigations published within this book develop cumulative knowledge on matters related to device-quality epaxial graphene on SiC, bringing this material closer to realistic applications.
This is a reprint of articles from the Special Issue published online in the open access journal Applied Sciences (ISSN 2076-3417) (available at: https://www.mdpi.com/journal/applsci/special
issues/epitaxial graphene on sic).
... Kaushik et al. [3] reported on a principal possibility to tune structural and electronic properties of epitaxial graphene through nitrogen ion implantation. Fundamental knowledge on both copper electrodeposition and atomic layer deposition of high-k insulators on epitaxial graphene/SiC is provided by the authors of [4,5]. These results suggest that epitaxial graphene is a stable support for metal and metal oxides, which is important in the context of metal contacts, gating, etc. Concomitantly, the interaction between epitaxial graphene and its environment, including metal contacts may limit, to some extent carrier transport in epitaxial graphene and therefore needs to be considered in detail. ...
... At the same time, Giannazzo et al. [5] in their work gave an overview of the recent results on the growth of high-k insulators on epitaxial graphene on SiC, focusing on atomic layer deposition of Al 2 O 3 thin layers, which are important for fabrication of epitaxial graphene-based devices. It was argued that the monolayer epitaxial graphene uniformity is a key factor to achieve a homogeneous Al 2 O 3 coverage via direct deposition, the latter has not been successful before in other studies. ...
The aim of this Special Issue is to provide a scientific platform for recognized experts in the field of epitaxial graphene on SiC to present their recent studies towards a deeper comprehension of growth mechanisms, property engineering and device processing. This Special Issue gives readers the possibility to gain new insights into the nature of buffer layer formation, control of electronic properties of graphene and usage of epitaxial graphene as a substrate for deposition of different substances, including metals and insulators. We believe that the papers published within the current Special Issue develop cumulative knowledge on matters related to device-quality epaxial graphene on SiC, bringing this material closer to realistic practical applications
... Clearly, the direct ALD growth of Al2O3 on 1L EG can have important implications, especially for advanced electronic applications requiring the integration of ultra-thin insulating layers on top of graphene. However, the deposition of relatively thick (> 10 nm) Al2O3 films by direct ALD on EG has been considered so far [33], and a clear understanding of the nucleation and growth mechanisms of the Al2O3 films on the EG/SiC system in the early stages of the ALD process is still lacking. ...
In this work, the nucleation and growth mechanism of aluminum oxide (Al2O3) in the early stages of the direct atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by atomic force microscopy (AFM) and Raman spectroscopy. Contrary to what is typically observed 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 (2.4 nm thick) after only 40 ALD cycles due to the islands coalescence, and subsequent layer-by-layer growth. Raman spectroscopy analyses showed low impact of the ALD process on the defects density and doping 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 the integration of ultra-thin high-k insulators.
... Fabrication of graphene FETs involves the challenges of selecting an appropriate gate insulator and controlling its thickness [150]. Generally, the oxides such as SiO2, HfO2, Al2O3, and Y2O3 are used as the gate dielectrics, but these tend to become amorphous with scaling and interactions with the graphene can occur, which makes the high-quality defect-free interface formation difficult [150][151][152]. Hexagonal boron nitride (h-BN) forms a weak van der Waals interaction with the graphene (lattice mismatch only 2% [153]) and has been utilized as an ideal insulator for the graphene FET formation [151]. ...
... Generally, the oxides such as SiO2, HfO2, Al2O3, and Y2O3 are used as the gate dielectrics, but these tend to become amorphous with scaling and interactions with the graphene can occur, which makes the high-quality defect-free interface formation difficult [150][151][152]. Hexagonal boron nitride (h-BN) forms a weak van der Waals interaction with the graphene (lattice mismatch only 2% [153]) and has been utilized as an ideal insulator for the graphene FET formation [151]. Due to the lower value of a dielectric constant (k~4), h-BN is usually combined with a high kdielectric layer such as Y2O3 formed via atomic layer deposition on h-BN [154]. ...
The electronic and transport properties of epitaxial graphene are dominated by the interactions the material makes with its surroundings. Based on the transport properties of epitaxial graphene on SiC and 3C-SiC/Si substrates reported in the literature, we emphasize that the graphene interfaces formed between the active material and its environment are of paramount importance, and how interface modifications enable the fine-tuning of the transport properties of graphene. This review provides a renewed attention on the understanding and engineering of epitaxial graphene interfaces for integrated electronics and photonics applications.
