Session C32: 2D Atomic Layer Hetero-devices

Interlayer electron-hole pair multiplication by hot carriers in atomic layer semiconductor heterostructures

Two-dimensional heterostructures composed of atomically thin transition metal dichalcogenides~provide the opportunity to design novel devices for the study of electron-hole pair multiplication. We report on highly efficient multiplication of interlayer electron-hole pairs~at the interface of a tungsten diselenide / molybdenum diselenide heterostructure. Electronic transport measurements of the interlayer current-voltage characteristics indicate that layer-indirect~electron-hole pairs are generated by hot electron impact excitation. Our findings, which demonstrate an efficient energy relaxation pathway that competes with electron thermalization losses, make 2D semiconductor heterostructures viable for a new class of hot-carrier energy harvesting devices that exploit layer-indirect electron-hole excitations.   

2D Heterocrystals: Vertical stacking of rotationally aligned electronically, structurally and chemically dissimilar 2D materials

We present a new species of 2D materials called `Heterocrystals' (HCs), which are layered stacks of chemically, structurally, and electronically dissimilar 2D materials that grow with perfect rotational alignment and long-range order, despite substantial lattice mismatch. We have successfully grown a family of 2D HCs using chemical vapor deposition. Our investigations reveal a novel lattice matching such that n unit lengths of one lattice approximately matches m lengths of the other lattice, e.g. 3x3 unit cells of Bi2Se3 match nearly perfectly with 4x4 unit cells of MoS2, forming the larger HC unit cell. The HC exhibits a variety of electronic and optical properties different from its parent 2D crystals. Such large changes in properties are in sharp contrast with those seen from randomly stacked 2D materials, or those grown with high crystallographically commensurate structures. We will present a range of novel, tunable and reconfigurable optical and electronic properties of these new systems, including transition from direct to indirect band gap, tunable and reversible photoluminescences, electronic transport, and Raman spectroscopic results. Our experimental results are in good agreement with theoretical results.   

Tunneling in BP-MoS$_{\mathrm{2}}$ heterostructure

Tunnel field effect transistor (TFET) is considered to be a leading option for achieving SS \textless 60 mV/dec. In this work, black phosphorus (BP) and molybdenum disulfide (MoS$_{\mathrm{2}})$ heterojunction devices are fabricated. We find that thin BP flake and MoS$_{\mathrm{2}}$ form normal p-n junctions, tunneling phenomena can be observed when BP thickness increases to certain level. PEO:CsClO$_{\mathrm{4}}$ is applied on the surface of the device together with a side gate electrode patterned together with source and drain electrodes. The Fermi level of MoS$_{\mathrm{2}}$ on top of BP layer can be modulated by the side gating, and this enables to vary the MoS$_{\mathrm{2}}$-BP tunnel diode property from off-state to on-state. Since tunneling is the working mechanism of MoS$_{\mathrm{2}}$-BP junction, and PEO:CsClO$_{\mathrm{4\thinspace }}$possesses ultra high dielectric constant and small equivalent oxide thickness (EOT), a low SS of 55 mV/dec is obtained from MoS$_{\mathrm{2}}$-BP TFET.   

Electronic structure and charge injection across transition metal dichalcogenide heterojunctions: theory and experiment.

We combine {\em ab initio} density functional electronic structure calculations for an NbSe$_2$/WSe$_2$ bilayer with quantum transport measurements of the corresponding heterojunction between a few-layer WSe$_2$ semiconductor and a metallic NbSe$_2$ layer. Our theoretical results suggest that, besides a rigid band shift associated with charge transfer, the presence of NbSe$_2$ does not modify the electronic structure of WSe$_2$. Since the two transition metal dichalcogenides are structurally similar and display only a small lattice mismatch, their heterojunction can efficiently transfer charge across the interface. These findings are supported by transport measurements for WSe$_2$ field-effect transistors with NbSe$_2$ contacts, which exhibit nearly ohmic behavior and phonon-limited mobility in the hole channel, indicating that the contacts to WSe$_2$ are highly transparent.   

Improving device quality in graphene heterostructures

Improving the quality of van der Waals heterostructures remains an important issue for the fabrication of novel electronic devices and the study of the interesting physics. In particular, heterostructures based on graphene have been shown to exhibit ballistic behavior at length scales of tens of microns yet observation of many fractional Quantum Hall states has been less clear than expected. To elucidate this issue, we characterize disorder in various graphene devices through analysis of low-field Shubnikov-de Haas oscillations. Our results demonstrate that the quantum lifetime is improved by an order of magnitude through the encapsulation of graphene by hexagonal boron nitride. Comparison with the mean scattering lifetime measured at zero magnetic field gives some clues about the origin of the remnant disorder in these devices. Implications and prospects for future device improvements will be discussed.   

Vertical transport in graphene/h-BN/graphene structures: multiple negative differential conductance regions and phonon assisted tunneling

Van der Waals (vdW) structures - formed by stacking different layers of two-dimensional crystals, which are held together via interlayer vdW interactions - have emerged in recent years as a route to create systems with tailored properties. Among the many possibilities, graphene/hexagonal boron nitride (h-BN)/graphene structures have received considerable attention, having been shown to operate as transistors and to display negative differential conductance. In this work, we study in detail the effect of the rotational alignment between the h-BN spacer and the graphene layers in the vertical current of a graphene/h-BN/graphene device. We show that for small rotation angles, the transference of momentum by the h-BN crystal lattice to the tunneling electrons leads to multiple peaks in the I-V curve of the device, with associated multiple negative differential conductance regions. We also study the effect of scattering by phonons in the vertical current and see how the spontaneous emission of optical phonons opens up new inelastic tunneling channels, which give origin to sharp peaks in the second derivative of the current with respect to the bias.   

Dual-Gated MoTe$_{2}$/MoS$_{2}$ van der Waals Heterojunction p-n Diode

2D materials are promising for future electronic and optoelectronic applications. In this regard, it is important to realize p-n diodes, the most fundamental building block of all modern semiconductor devices, based on these 2D materials. While it is challenging to achieve homojunction diodes in 2D semiconductors due to lack of reliable selective doping techniques, it is relatively easier to achieve diode-like behavior in van der Waals (vdW) heterostructures comprising different 2D semiconductors. Here, we demonstrate dual-gated vdW heterojunction p-n diodes based on p-type MoTe$_{2}$ and n-type MoS$_{2}$, with hBN as the top and bottom gate dielectric. The heterostructure stack is assembled using a polymer-based `dry-transfer' technique. Pt contact is used for hole injection in MoTe$_{2}$, whereas Ag is used for electron injection in MoS$_{2}$. The dual-gates allow for independent electrostatic tuning of the carriers in MoTe$_{2}$ and MoS$_{2}$. Room temperature interlayer current-voltage characteristics reveal a strong gate-tunable rectification behavior. At low temperatures, the diode turn-on voltage increases, whereas the reverse saturation current decreases, in accordance with conventional p-n diode behavior.   

Investigation of factors that influence the quality of van der Waals heterostructures made by direct pick-up

Two dimensional material van der Waals heterostructures, such as twisted bilayer graphene, can be assembled by the prevailing pick-up procedure.\footnote{K. Kim et al. \textbf{Nano Lett.} 16.3:1998 (2016)} \footnote{Y. Cao et al. \textbf{Phys. Rev. Lett.} 117.11:116804(2016)} However, the size and the quality of the devices are limited by defects such as ``Bubbles'' in the samples. We investigate sources that can impair the quality of van der Waals heterostructures using optical, atomic force microscope, and scanning electron microscope imaging. Choices of polymer layers for picking up, schemes of stacking heterostructures, and influences of pre- and post- furnace annealing for stack fabrication are studied. In conjunction with transport measurements, we refine the procedure for making high-quality twisted bilayer graphene and other heterostructure devices.   

2d Heterojunctions From Non-Local Manipulation of the Interactions: Single and Two-Particle Properties

Semiconductors play a major role in modern optoelectronics. Especially heterojunctions are central building blocks of various applications, which commonly rely on interfaces of different materials. Here, we propose a novel scheme to induce heterojunctions within a single \emph{homogeneous} layer of a two dimensional (2d) material based on Coulomb-interaction effects. Therefore we make use of the fact that in 2d semiconductors the Coulomb interaction can modify band gaps on an eV scale and can be drastically manipulated by external screening. This allows to spatially control the band gap by structured dielectric surroundings. We provide a proof of principle by combining a real-space tight-binding description with a many-body formalism for a model system emulating transitionmetal dichalcogenides. We find sizable spatial band-gap modulations yielding type-II heterojunctions as needed for solar cells or quantum dots and present detailed insights into their excitation-induced two-particle properties. Utilizing the Bethe-Salpeter equation we show that Rydberg-like higher excitonic states can be strongly tuned by the dielectric surroundings. This effect may be used for efficient trapping of these excitonic states upon tailoring of the environment.   

Band gap renormalization and work function tuning in MoSe$_{\mathrm{2}}$/hBN/Ru(0001) heterostructures

The van der Waals interaction in vertical heterostructures made of two-dimensional (2D) materials relaxes the requirement of lattice matching, therefore enabling great design flexibility to tailor novel 2D electronic systems. Here we report the successful growth of MoSe$_{\mathrm{2}}$ on single-layer hexagonal boron nitride (hBN) on the Ru(0001) substrate using molecular beam epitaxy. Using scanning tunneling microscopy and spectroscopy, we found that the quasi-particle bandgap of MoSe$_{\mathrm{2}}$ on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru which causes residual metallic screening from the substrate. In addition, the electronic structure and the work function of MoSe$_{\mathrm{2}}$ are modulated electrostatically with an amplitude of about 0.13 eV. Most interestingly, this electrostatic modulation is spatially in phase with the Moiré pattern of hBN on Ru(0001) whose surface also exhibits a work function modulation of the same amplitude.   

Band Offsets Engineering for van der Waals Heterostructure Devices

Two-dimensional crystals (2D) and their stacks in van der Waals heterostructures became prospective for novel devices and physics. To surmount commensurability limitations within first-principles investigations, the coincidence lattice method is developed\footnote[1]{D. S. Koda, F. Bechstedt, M. Marques, and L. K. Teles, \textit{J. Phys. Chem. C} \textbf{120} (2016) 10895.}, enabling studies on interlayer twist\footnote[2]{D. S. Koda, F. Bechstedt, M. Marques, and L. K. Teles, \textit{J. Electron. Mater.} (2016).} and quasiparticle corrections despite limited computational resources. Interesting properties are observed within stacked systems, such as structural deformation on contact, strong orbital hybridization, and tunable band offsets by application of pressure and vertical electric fields\footnote[3]{D. S. Koda, F. Bechstedt, M. Marques, and L. K. Teles, submitted to the \textit{J. Phys. Chem. Lett.} (2016).}. These studies could help to develop versatile electronic and optoelectronic devices and unravel new physics within 2D interfaces.   

Electronic Structure Modeling for 2D Layered Material Stacks

The van der Waals heterostructures have promising electronic applications and offer access to two-dimensional theoretical physics. It is crucial to have efficient approaches to model these heterostructures with desired properties by harnessing different flavors of the layered materials and the twist angle in between. In our work, we construct the ab-initio tight binding models by employing Wannier transformation of density functional theory calculation for both intra- and inter-layer couplings. Our models are further applied to the comparison with magneto-transport experiments, incommensurate stacking crystals and the strain effects.