Intercalation of hafnium oxide between epitaxially-grown monolayer graphene and Ir(111)substrate

Intercalation of insulating materials between epitaxial graphene and the metal substrates is highly demanded to restore the intrinsic properties of graphene,and thus essential for the graphene-based devices.Here we demonstrate a successful solution for the intercalation of hafnium oxide into the interface between full-layer graphene and Ir(111)substrate.We first intercalate hafnium atoms beneath the epitaxial graphene.The intercalation of the hafnium atoms leads to the variation of the graphene moire superstructure periodicity,which is characterized by low-energy electron diffraction(LEED)and lowtemperature scanning tunneling microscopy(LT-STM).Subsequently,we introduce oxygen into the interface,resulting in oxidization of the intercalated hafnium.STM and Raman's characterizations reveal that the intercalated hafnium oxide layer could effectively decouple the graphene from the metallic substrate,while the graphene maintains its high quality.Our work suggests a high-k dielectric layer has been successfully intercalated between high-quality epitaxial graphene and metal substrate,providing a platform for applications of large-scale,high-quality graphene for electronic devices.

Er intercalation and its impact on transport properties of epitaxial graphene

Intercalation of atomic species is a practicable method for epitaxial graphene to adjust the electronic band structure and to tune the coupling between graphene and Si C substrate. In this work, atomically flat epitaxial graphene is prepared on 4H-SiC(0001) using the flash heating method in an ultrahigh vacuum system. Scanning tunneling microscopy, Raman spectroscopy and electrical transport measurements are utilized to investigate surface morphological structures and transport properties of pristine and Er-intercalated epitaxial graphene. It is found that Er atoms are intercalated underneath the graphene layer after annealing at 900℃, and the intercalation sites of Er atoms are located mainly at the bufferlayer/monolayer-graphene interface in monolayer domains. We also report the different behaviors of Er intercalation in monolayer and bilayer regions, and the experimental results show that the diffusion barrier for Er intercalated atoms in the buffer-layer/monolayer interface is at least 0.2 eV higher than that in the first/second graphene-layer interface. The appearance of Er atoms is found to have distinct impacts on the electronic transports of epitaxial graphene on SiC(0001).

Bimodal growth of Fe islands on graphene

Magnetic metals deposited on graphene hold the key to applications in spintronics. Here, we present the results of Fe islands grown on graphene/Si C(0001) by molecular beam epitaxy, which are investigated by scanning tunneling microscopy. The two types of islands distinguished by flat or round tops are revealed, indicating bimodal growth of Fe. The atomic structures on the top surfaces of flat islands are also clearly resolved. Our results may improve the understanding of the mechanisms of metals deposited on graphene and pave the way for future spintronic applications of Fe/graphene systems.

A self-powered sensitive ultraviolet photodetector based on epitaxial graphene on silicon carbide

A self-powered graphene-based photodetector with high performance is particularly useful for device miniaturization and to save energy.Here,we report a graphene/silicon carbide（SiC）-based self-powered ultraviolet photodetector that exhibits a current responsivity of 7.4 m A/W with a response frequency of over a megahertz under 325-nm laser irradiation.The built-in photovoltage of the photodetector is about four orders of magnitude higher than previously reported results for similar devices.These favorable properties are ascribed to the ingenious device design using the combined advantages of graphene and SiC,two terminal electrodes,and asymmetric light irradiation on one of the electrodes.Importantly,the photon energy is larger than the band gap of SiC.This self-powered photodetector is compatible with modern semiconductor technology and shows potential for applications in ultraviolet imaging and graphene-based integrated circuits.

Spectroscopic and scanning probe analysis on large-area epitaxial graphene grown under pressure of 4 mbar on 4H-SiC(0001)substrates

We produced epitaxial graphene under a moderate pressure of 4 mbar （about 400 Pa） at temperature 1600 ℃. Raman spectroscopy and optical microscopy were used to confirm that epitaxial graphene has taken shape continually with slight thickness variations and regularly with a centimeter order of magnitude on 4H-SiC （0001） substrates. Then using X-ray photoelectron spectroscopy and Auger electron spectroscopy, we analyzed the chemical compositions and estimated the layer number of epitaxial graphene. Finally, an atomic force microscope and a scanning force microscope were used to characterize the morphological structure. Our results showed that under 4-mbar pressure, epitaxial graphene could be produced on a SiC substrate with a large area, uniform thickness but a limited morphological property. We hope our work will be of benefit to understanding the formation process of epitaxial graphene on SiC substrate in detail.

Temperature dependence of the thickness and morphology of epitaxial graphene grown on SiC (0001) wafers

Epitaxiai graphene is synthesized by silicon sublimation from the Si-terminated 6H SiC substrate. The effects of graphitization temperature on the thickness and surface morphology of epitaxial graphene are investigated. X-ray photoelectron spectroscopy spectra and atomic force microscopy images reveal that the epitaxial graphene thickness increases and the epitaxial graphene roughness decreases with the increase in graphitization temperature. This means that the thickness and roughness of epitaxial graphene films can be modulated by varying the graphitization temperature. In addition, the electrical properties of epitaxial graphene film are also investigated by Hall effect measurement.

Raman analysis of epitaxial graphene grown on 4H-SiC (0001) substrate under low pressure condition

In this paper, we report a feasible route of growing epitaxial graphene on 4H-SiC （0001） substrate in a low pressure of 4 mbar （1 bar=105 Pa） with an argon flux of 2 standard liters per minute at 1200, 1300, 1400, and 1500 ℃ in a commercial chemical vapour deposition SiC reactor. Using Raman spectroscopy and scanning electron microscopy, we confirm that epitaxial graphene evidently forms on SiC surface above 1300 ℃ with a size of several microns. By fitting the 2D band of Raman data with two-Lorentzian function, and comparing with the published reports, we conclude that epitaxial graphene grown at 1300 ℃ is four-layer graphene.

Comparison of the formation epitaxial graphenes on Si- and process and properties of C-face 6H-SiC substrates

In this paper, the epitaxial graphene layers grown on Si- and C-face 6H-SiC substrates are investigated under a low pressure of 400 Pa at 1600℃ By using atomic force microscopy and Raman spectroscopy, we find that there are distinct differences in the formation and the properties between the epitaxial graphene layers grown on the Si-face and the C-face substrates, including the hydrogen etching process, the stacking type, and the number of layers. Hopefully, our results will be useful for improving the quality of the epitaxial graphene on SiC substrate.

Significant photoelectrical response of epitaxial graphene grown on Si-terminated 6H-SiC

Photoelectrical response characteristics of epitaxial graphene （EG） films on Si- and C-terminated 6H-SiC, and transferred chemical vapor deposition （CVD） graphene films on Si-terminated 6H-SiC have been investigated. The results show that upon illumination by a xenon lamp, the photocurrent of EG grown on Si-terminated SiC significantly increases by 147.6%, while the photocurrents of EG grown on C-terminated SiC, and transferred CVD graphene on Si-terminated SiC slightly decrease by 0.5% and 2.7%, respectively. The interfacial buffer layer between EG and Si-terminated 6H-SiC is responsible for the significant photoelectrical response of EG. Its strong photoelectrical response makes it promising for optoelectronic applications.

High temperature characteristics of bilayer epitaxial graphene field-effect transistors on SiC Substrate

In this paper,high temperature direct current（DC） performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25℃ to 200℃.At a gate voltage of-8 V（far from Dirac point）,the drainsource current decreases obviously with increasing temperature,but it has little change at a gate bias of ＋8 V（near Dirac point）.The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies.Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures.The devices exhibit almost unchanged DC performances after high temperature measurements at 200℃ for 5 hours in air ambience,demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices.

Effect of 6H-SiC (1120) substrate on epitaxial graphene revealed by Raman scattering

A nonpolar SiC（1120） substrate has been used to fabricate epitaxial graphene （EG）. Two EGs with layer numbers of 8-10 （referred to as MLG） and 2-3 （referred to as FLG） were used as representative to study the substrate effect on EG through temperature-dependent Raman scattering. It is found that Raman lineshifts of G and 2D peaks of the MLG with temperature are consistent with that of free graphene, as predicted by theory calculation, indicating that the substrate influence on the MLG is undetectable. While Raman G peak lineshifts of the FLG to that of the free graphene are obvious, however, its lineshift rate （-0.016 cm-1/K） is almost one third of that （-0.043 cm-1/K） of an EG on 6H-SiC （0001） in the temperature range from 300 K to 400 K, indicating a weak substrate effect from SiC （1120） on the FLG. This renders the FLG with a high mobility around 1812 cm2.V-1 .s -1 at room temperature even with a very high carrier concentration about 2.95 × 10 ^13 cm-2 （p-type）. These suggest SiC （1120） is more suitable for fabricating EG with high performance.

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors （GFETs） fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density I DS, improved transconductance g m, reduced sheet resistance lion, and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Spectroscopic Evidence for Electron Correlations in Epitaxial Bilayer Graphene with Interface-Reconstructed Superlattice Potentials

Superlattice potentials are theoretically predicted to modify the single-particle electronic structures. The resulting Coulomb-interaction-dominated low-energy physics would generate highly novel many-body phenomena. Here,by in situ tunneling spectroscopy, we show the signatures of superstructure-modulated correlated electron states in epitaxial bilayer graphene(BLG) on 6H-Si C(0001). As the carrier density is locally quasi-‘tuned’ by the superlattice potentials of a 6 × 6 interface reconstruction phase, the spectral-weight transfer occurs between the two broad peaks flanking the charge-neutral point. Such a detected non-rigid band shift beyond the single-particle band description implies the existence of correlation effects, probably attributed to the modified interlayer coupling in epitaxial BLG by the 6×6 reconstruction as in magic-angle BLG by the moiré potentials. Quantitative analysis suggests that the intrinsic interface reconstruction shows a high carrier tunability of ~1/2 filling range, equivalent to the back gating by a voltage of ~70 V in a typical gated BLG/SiO_(2)/Si device. The finding in interfacemodulated epitaxial BLG with reconstruction phase extends the BLG platform with electron correlations beyond the magic-angle situation, and may stimulate further investigations on correlated states in graphene systems and other van der Waals materials.

Ytterbium Intercalation of Epitaxial Graphene Grown on Si-Face SiC

The rare-earth metal, ytterbium (Yb), was deposited on graphene grown on Si-face SiC, kept at room temperature. Yb was not found to intercalate, destroy or dope the graphene layer before subsequent heating, unlike alkali metals such as Li and Na. Our photoemission results reveal that heating to 300oC promotes Yb intercalation into the graphene-substrate interface. Real-time low energy electron microscopy (LEEM) measurements indicated intercalation to start at a sample temperature of around 220oC. In the intercalation process, Yb penetrates through the graphene and buffer layer and forms bonds to the silicon in the topmost SiC substrate bilayer, as indicated by the shifted components observed in the C 1s, Si 2p, and Yb 4f spectra. The Yb intercalation decouples the buffer layer from the substrate and transforms it into another graphene layer as manifested by the absence of buffer layer spots in the μ-LEED pattern and the appearance of an additional π band in the ARPES spectra, respectively. Moreover, the observed shift of the Dirac point down from the Fermi level by 1.9 eV indicates electron doping of the graphene layer upon Yb intercalation. The Yb intercalated graphene sample was found to be thermodynamically stable up to temperatures around 700oC.

Observation of Electron-Phonon Couplings and Fano Re-sonances in Epitaxial Bilayer Graphene

The results of optical investigation of hydrogenated epitaxial bilayer graphene are presented. A softening and an increase of the intensity of the in-plane anti-symmetric phonon mode are observed at 0.2eV. It is suggested that they both originate from coupling of the optically active phonon mode to virtual electronic transitions, which is related to the band structure of bilayer graphene and leads to the “charged phonon” effect. In addition, it is noted that optically active phonon peaks have pronounced Fano shapelines. It is argued that the Fano shapeline is attributed to the interaction of the phonon mode with a continuum of electronic transitions in valence bands of hydrogenated bilayer graphene.

Improvement of Metal-Graphene Ohmic Contact Resistance in Bilayer Epitaxial Graphene Devices

We report on an improved metal-graphene ohmic contact in bilayer epitaxial graphene on a SiC substrate with contact resistance below 0.1 Ω.mm. Monolayer and bilayer epitaxial graphenes are prepared on a 4HoSiC substrate in this work. Their contact resistances are measured by a transfer length method. An improved photoresist-free device fabrication method is used and is compared with the conventional device fabrication method. Compared with the monolayer graphene, the contact resistance Rc of bilayer graphene improves from an average of 0.24Ω·mm to 0. 1 Ωmm. Ohmic contact formation mechanism analysis by Landauer＇s approach reveals that the obtained low ohmic contact resistance in bilayer epitaxial graphene is due to their high carrier density high carrier transmission probability, and p-type doping introduced by contact metal Au.

Fabrication and properties of silicene and silicene–graphene layered structures on Ir(111)

Silicene, a two-dimensional(2D) honeycomb structure similar to graphene, has been successfully fabricated on various substrates. This work will mainly review the syntheses and the corresponding prope√rties o√f silicene and√ silice√ne–graphene layered structures on Ir(111) substrates. For silicene on Ir(111), the buckled(3 ×3) silicene/(7 ×7)Ir(111) configuration and its electronic structure are fully discussed. For silicene–graphene layered structures, silicene layer can be constructed underneath graphene layer by an intercalation method. These results indicate the possibility of integrating silicene with graphene and may link up with potential applications in nanoelectronics and related areas.

Dual-wavelength ultraviolet photodetector based on vertical(Al,Ga)N nanowires and graphene

Due to the wide application of UV-A(320 nm–400 nm)and UV-C(200 nm–280 nm)photodetectors,dual-wavelength(UV-A/UV-C)photodetectors are promising for future markets.A dual-wavelength UV photodetector based on vertical(Al,Ga)N nanowires and graphene has been demonstrated successfully,in which graphene is used as a transparent electrode.Both UV-A and UV-C responses can be clearly detected by the device,and the rejection ratio(R254 nm/R450 nm)exceeds35 times at an applied bias of-2 V.The short response time of the device is less than 20 ms.Furthermore,the underlying mechanism of double ultraviolet responses has also been analyzed systematically.The dual-wavelength detections could mainly result from the appropriate ratio of the thicknesses and the enough energy band difference of(Al,Ga)N and Ga N sections.

形成过程的比较和 Si 脸和 C 脸 6HSiC 底层上的取向附生的 graphenes 的性质

In this paper,the epitaxial graphene layers grown on Si-and C-face 6H-SiC substrates are investigated under a low pressure of 400 Pa at 1600 C.By using atomic force microscopy and Raman spectroscopy,we find that there are distinct differences in the formation and the properties between the epitaxial graphene layers grown on the Si-face and the C-face substrates,including the hydrogen etching process,the stacking type,and the number of layers.Hopefully,our results will be useful for improving the quality of the epitaxial graphene on SiC substrate.

Transfer-free chemical vapor deposition graphene for nitride epitaxy: challenges, current status and future outlook

Graphene, a two-dimensional material with outstanding electrical and mechanical properties, has attracted considerable attention in the field of semiconductor technologies due to its potential use as a buffer layer for the epitaxial Ⅲ-nitride growth. In recent years, significant progress has been made in the chemical vapor deposition growth of graphene on various insulating substrates for the nitride epitaxy, which offers a facile, inexpensive, and easily scalable methodology. However, certain challenges are still present in the form of producing high-quality graphene and achieving optimal interface compatibility with Ⅲ-nitride materials.In this review, we provide an overview of the bottlenecks associated with the transferred graphene fabrication techniques and the state-of-the-art techniques for the transfer-free graphene growth. The present contribution highlights the current progress in the transfer-free graphene growth on different insulating substrates, including sapphire, quartz, SiO_(2)/Si, and discusses the potential applications of transfer-free graphene in the Ⅲ-nitride epitaxy. Finally, it includes the prospects of the transfer-free graphene growth for the Ⅲ-nitride epitaxy and the challenges that should be overcome to realize its full potential in this field.