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Register a new userCharge transport and electron-hole asymmetry in low-mobility graphene/hexagonal boron nitride heterostructures
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Hongqi Xu Professor
Publication date
2018
fields
Physics
and research's language is
English
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Graphene/hexagonal boron nitride (G/$h$-BN) heterostructures offer an excellent platform for developing nanoelectronic devices and for exploring correlated states in graphene under modulation by a periodic superlattice potential. Here, we report on transport measurements of nearly $0^{circ}$-twisted G/$h$-BN heterostructures. The heterostructures investigated are prepared by dry transfer and thermally annealing processes and are in the low mobility regime (approximately $3000~mathrm{cm}^{2}mathrm{V}^{-1}mathrm{s}^{-1}$ at 1.9 K). The replica Dirac spectra and Hofstadter butterfly spectra are observed on the hole transport side, but not on the electron transport side, of the heterostructures. We associate the observed electron-hole asymmetry to the presences of a large difference between the opened gaps in the conduction and valence bands and a strong enhancement in the interband contribution to the conductivity on the electron transport side in the low-mobility G/$h$-BN heterostructures. We also show that the gaps opened at the central Dirac point and the hole-branch secondary Dirac point are large, suggesting the presence of strong graphene-substrate interaction and electron-electron interaction in our G/$h$-BN heterostructures. Our results provide additional helpful insight into the transport mechanism in G/$h$-BN heterostructures.
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The current research in graphene spintronics strives for achieving a long spin lifetime, and efficient spin injection and detection in graphene. In this article, we review how hexagonal boron nitride (hBN) has evolved as a crucial substrate, as an encapsulation layer, and as a tunnel barrier for manipulation and control of spin lifetimes and spin injection/detection polarizations in graphene spin valve devices. First, we give an overview of the challenges due to conventional SiO$_2$ substrate for spin transport in graphene followed by the progress made in hBN based graphene heterostructures. Then we discuss in detail the shortcomings and developments in using conventional oxide tunnel barriers for spin injection into graphene followed by introducing the recent advancements in using the crystalline single/bi/tri-layer hBN tunnel barriers for an improved spin injection and detection which also can facilitate two-terminal spin valve and Hanle measurements, at room temperature, and are of technological importance. A special case of bias induced spin polarization of contacts with exfoliated and chemical vapour deposition (CVD) grown hBN tunnel barriers is also discussed. Further, we give our perspectives on utilizing graphene-hBN heterostructures for future developments in graphene spintronics.
Among two-dimensional atomic crystals, hexagonal boron nitride (hBN) is one of the most remarkable materials to fabricate heterostructures revealing unusual properties. We perform first-principles calculations to determine whether intercalated metal atoms and vacancies can mediate interfacial coupling and influence the structural and electronic properties of the graphene/hBN heterostructure. Metal impurity atoms (Li, K, Cr, Mn, Co, and Cu) as extrinsic defects between the graphene and hBN sheets produce $n$-doped graphene. We also consider intrinsic vacancy defects and find that a boron monovacancy in hBN act as a magnetic dopant for graphene whereas a nitrogen monovacancy in hBN serves as a nonmagnetic dopant for graphene. In contrast, smallest triangular vacancy defects in hBN are unlikely to result in significant changes in the electronic transport of graphene. Our findings reveal that the hBN layer with some vacancies or metal impurities enhance the interlayer coupling in the graphene/hBN heterostructure with respect to charge doping and electron scattering.
The design of stacks of layered materials in which adjacent layers interact by van der Waals forces[1] has enabled the combination of various two-dimensional crystals with different electrical, optical and mechanical properties, and the emergence of novel physical phenomena and device functionality[2-8]. Here we report photo-induced doping in van der Waals heterostructures (VDHs) consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photo-induced doping maintains the high carrier mobility of the graphene-boron nitride (G/BN) heterostructure, which resembles the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially-varying doping profiles such as p-n junctions. We show that this photo-induced doping arises from microscopically coupled optical and electrical responses of G/BN heterostructures, which includes optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.
Hexagonal boron nitride (BN), one of the very few layered insulators, plays a crucial role in 2D materials research. In particular, BN grown with a high pressure technique has proven to be an excellent substrate material for graphene and related 2D materials, but at the same time very hard to replace. Here we report on a method of growth at atmospheric pressure as a true alternative for producing BN for high quality graphene/BN heterostructures. The process is not only more scalable, but also allows to grow isotopically purified BN crystals. We employ Raman spectroscopy, cathodoluminescence, and electronic transport measurements to show the high-quality of such monoisotopic BN and its potential for graphene-based heterostructures. The excellent electronic performance of our heterostructures is demonstrated by well developed fractional quantum Hall states, ballistic transport over distances around $10,mathrm{mu m}$ at low temperatures and electron-phonon scattering limited transport at room temperature.
We study fully hexagonal boron nitride (hBN)-encapsulated graphene spin valve devices at room temperature. The device consists of a graphene channel encapsulated between two crystalline hBN flakes; thick-hBN flake as a bottom gate dielectric substrate which masks the charge impurities from SiO2/Si substrate and single-layer thin-hBN flake as a tunnel barrier. Full encapsulation prevents the graphene from coming in contact with any polymer/chemical during the lithography and thus gives homogeneous charge and spin transport properties across different regions of the encapsulated graphene. Further, even with the multiple electrodes in between the injection and the detection electrodes which are in conductivity mismatch regime, we observe spin transport over 12.5 um long distance under the thin-hBN encapsulated graphene channel, demonstrating the clean interface and the pin-hole free nature of the thin-hBN as an efficient tunnel barrier.
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