Session R17: 2D Semiconductor Physics I

2D semiconductor optoelectronics

The advent of graphene and related 2D materials has recently led to a new technology: heterostructures based on these atomically thin crystals. The paradigm proved itself extremely versatile and led to rapid demonstration of tunnelling diodes with negative differential resistance, tunnelling transistors, photovoltaic devices, etc. By taking the complexity and functionality of such van der Waals heterostructures to the next level we introduce quantum wells engineered with one atomic plane precision. Light emission from such quantum wells, quantum dots and polaritonic effects will be discussed.   

Tuning the trion photoluminescence polarization in monolayer WS$_{2}$

Monolayer transition metal dichalcogenides (TMDs) such as MoS$_{2}$ or WS$_{2}$ are semiconductors with degenerate, yet inequivalent k-points labeled K and K’ that define the direct gap. The valence band maximum in each valley has only one spin state of opposite sense for K and K’. Consequently, one can selectively populate each valley independently with circularly polarized light, and determine the valley populations via the polarization of emitted light. Optical emission is dominated by neutral and charged exciton (trion) features, and changes in emitted polarization provide insight into the fundamental processes of intervalley scattering. We prepare single-layer WS2 films such that the photoluminescence is from the negatively charged trion and observe a room temperature optical polarization in excess of 40% for the trion. Using an applied gate voltage, we can modulate the electron density and subsequently the polarization continuously from 20-40%. Both the polarization and the emission energy monotonically track the gate voltage with the emission energy increasing by 45 meV. We discuss the role electron capture of the trion has on suppressing the intervalley scattering process.   

High circular polarization in a MoSe$_{2}$ light-emitting transistor

The exclusive coupling between the valley degree of freedom and the optical helicity is a unique phenomenon in transition metal dichalcogenides (TMDs), and thus the circularly polarized luminescence is one of the main research topics in these materials. MoSe$_{2}$, however, is known to exhibit exceptionally low polarization in photoluminescence (PL). Here, we report electroluminescence (EL) properties of MoSe$_{2}$ demonstrating electrical switching of the optical helicity in the same manner as WSe$_{2}$ [1]. More importantly, the observed polarization in EL is one order of magnitude higher than that in PL. The present results reveal that the mechanism of EL polarization possesses the intrinsic robustness against intervalley scattering. [1] Y. J. Zhang, et.al., Science 344, 725 (2014).   

Single Quantum Emitters in Monolayer Tungsten Diselenide

Single quantum emitters are essential for developing photonic quantum technologies, providing single photon sources as well as stationary quantum bits. While they have been realized in a variety of solid state systems including single quantum dots and color centers in diamond, their three dimensional bulk matrix will be difficult to integrate with emerging nanoscale devices. We present single quantum emitters in a two-dimensional semiconductor, in the form of excitons localized to defects within atomically thin Tungsten Diselenide monolayers. These localized excitons show strong photoluminescence with 130 \textmu eV emission lines from two non-degenerate, cross-polarized transitions. Second-order correlation measurements show strong photon anti-bunching, establishing that these localized excitons are single photon emitters. Magneto-optical measurements reveal an exciton g-factor of 8.7, significantly larger than that of delocalized excitons. In addition to potential advantages such as efficient photon extraction and in-situ control of local environment, the two-dimensional matrix can be incorporated into more complex van-der-Waals heterostructure devices. This enables external control of emitters in the semiconductor, while integrating seamlessly with nanoscale device architectures.   

Quantm confined stark effect of single photon emitters in atomically thin semiconductors

The optical properties of semiconducting monolayer materials have been widely studied since the isolation of monolayer transition metal dichalcogenides (TMDCs). They have rich opto-electronic properties owing to their large direct bandgap, the interplay between the spin and the valley degree of freedom of charge carriers, and the recently discovered localized excitonic states giving rise to single photon emission. We study quantum confined Stark shift from these localized emitters present on the edges of monolayer tungsten diselenide. We employ a vertically stacked van der Waal\textsc{\char13}s heterostructure to fabricate a field effect device using hexagonal boron nitride as the tunnel barrier on either side of the TMDC and few layer graphene as top and bottom electrical contacts. We report the Stark shift of different defect centers to have linear or both linear and quadratic behavior with electric field. Further, evaluation of the spectral shift in the photoluminescence signal as a function of the applied voltage enables us to extract the polarizability as well as information on the dipole moment of an individual defect center.   

Optical studies of dual gated WSe2 transistors

Recent advances in the development of atomically thin layers of transition metal dichalcogenides (TMDs) have opened up new possibilities for the exploration of novel 2D physics as well as materials for applications. The atomic thickness of these materials allows effective control of their optical and electronic properties by electrostatic gates. In this work, we fabricate dual-gate transistors of monolayer WSe2 and investigate the optical and electronic properties as a function of doping and fields by the absorption and photoluminescence spectroscopy. The combination of the top and bottom gates allows us to independently vary the electric field and doping density in the monolayer over a large range. As a function of doping density, we observe the evolution of the electronic excitations from locally bound excitons, to excitons and charged excitons, to the Fermi edge singularity. We will discuss the effects of external fields on these excitations and the effects of strong Coulomb interactions in 2D semiconductors.~   

Photocurrent measurements in Coupled Quantum Well van der Waals Heterostructures made of 2D Transition Metal Dichalcogenides.

, Luis A. Jauregui, Alex A. High, Alan Dibos, Elgin Gulpinar, Kateryna Pistunova, Hongkun Park, Philip Kim Harvard University, Physics Department -abstract- Single layer transition metal dichalcogenides (TMDC) are 2-dimensional (2D) semiconductors van der Waals (vdW) characterized by a direct optical bandgap in the visible wavelength (\textasciitilde 2 eV). Characterization of the band alignment between TMDC and the barrier is important for the fabrication of tunneling devices. Here, we fabricate coupled quantum well (CQW) heterostructures made of 2D TMDCs with hexagonal Boron nitride (hBN) as an atomically thin barrier and gate dielectric and with top and bottom metal (or graphite) as gate electrodes. We observe a clear dependence of the photo-generated current with varying hBN thickness, electrode workfunctions, electric field, laser excitation power, excitation wavelength, and temperature. We will discuss the implication of photocurrent in relation to quantum transport process across the vdW interfaces.   

Multi-terminal Monolayer WSe2 devices

Two-dimensional transition-metal dichalcogenide (TMD) semiconductors are promising materials for next-generation electronic and optoelectronic devices. WSe2 in particular has shown excellent optical properties, but it has proven difficult to make reliable electrical contacts to this material. We use a new chemical vapor deposition technique to grow monolayer single crystal WSe2 reliably on a large scale with edges up to 15 microns long. We then fabricate these crystals into multi-terminal devices encapsulated in boron nitride using dry transfer techniques. We achieve sufficiently good electrical contacts reproducibly to allow comprehensive study of the intrinsic optical and electrical properties of gated WSe2 monolayers as a function of temperature and magnetic field.   

Gate-defined Single Electron Transistor in a Graphene-MoS2 van der Waals Heterostructure

We report experimental demonstration of fabrication of laterally confined single electron transistor (SET) on MoS2 transition metal dichalcogenide (TMDC) semiconductor. A few atomic layers of MoS2 single crystals are encapsulated in hBN layers in order to improve mobility of 2-dimensional (2D) electron channel. Graphene layers are employed to provide Ohmic contact to the TMDC channels. The laterally confined quantum dots are formed by electrostatically depleting the near-by 2D channel employing local gate fabricated by electron lithography. Typical SET transport signatures such as gate-tunable Coulomb blockade have been observed. We have demonstrated the quantum confinement can be sensitively tuned to adjust the dot-reservoir coupling. The work paves way for more complicated device structure such as valley-spin filter and vertically coupled quantum dots in Coulomb drag devices.   

Probing interlayer interactions in WS2-graphene van der Waals heterostructures

Two-dimensional crystals based van der Waals coupled heterostructures are of interest owing to their potential applications for flexible and transparent electronics and optoelectronics. The interaction between the 2D layered crystals at the interfaces of these heterostructures is crucial in determining the overall performance and is strongly affected by contamination and interfacial strain. We have fabricated heterostructures consisting of atomically thin exfoliated WS2 and chemical-vapor-deposited (CVD) graphene, and studied the interaction and coupling between the WS2 and graphene using atomic force microscopy (AFM), Raman spectroscopy and femtosecond transient absorption measurement (TAM). Information from Raman-active phonon modes allows us to estimate charge doping in graphene and interfacial strain on the crystals. Spatial imaging probed by TAM can be correlated to the heterostructure surface morphology measured by AFM and Raman maps of graphene and WS2, showing how the interlayer coupling alters exciton decay dynamics quantitatively.   

Coherently stacked MoS$_{2}$/WSe$_{2}$ heterostructures: Moir\'{e} pattern and its effect on interlayer couplings

Vertically stacked heterojunctions (HJs) of transition metal dichalcogenides (TMDs) have been proposed as fundamental building blocks for several novel electronic and photonic devices. Although such HJs can be easily achieved by sequential transferring of different TMDs, this approach is not scalable, and the orientation relationship between the layers is difficult to control. A much more desirable approach is to directly grow one kind of TMD on top of the other. In addition to being a scalable platform, the epitaxial approach also results in a well-defined orientation relationship. A very important question to ask is ``What is the role of the interlayer coupling on the electronic structures of such a bilayer stack?'' By using scanning tunneling microscopy/spectroscopy (STM/S) and first-principles calculations, we investigate the MoS$_{2}$/WSe$_{2}$ vertical heterojunctions formed by direct epitaxial growth. The different lateral lattice constants between MoS$_{2}$ and WSe$_{2}$ lead to the formation of a well-ordered Moire pattern with a superlattice constant of \textasciitilde 8.5 nm. This superlattice reflects the variation of the lateral alignment between the MoS$_{2}$ and WSe$_{2}$ lattices. STS shows very large variations of interlayer coupling, as a function of the lateral alignment. More interestingly, depending on the location in the BZ, the interlayer coupling has very different consequences on the electronic structures.   

Interface exciton at lateral heterojunction of monolayer semiconductors

Heterostructures based on 2D transition metal dichalcogenides (TMDs) have attracted extensive research interest recently due to the appealing physical properties of TMDs and new geometries for forming heterostructures. One such heterostructure is the lateral heterojunctions seamlessly formed in a monolayer crystal between two different types of TMDs, e.g. WSe2 and MoSe2. Such heterojunction exhibits a type II band alignment, with electrons (holes) having lower energy on the MoSe2 (WSe2) region. Here we present the study of an interface exciton at the 1D lateral junction of monolayer TMDs. With the distance dependent screening, we find that the interface exciton can have strong binding even though the electron-hole separation is much larger compare to the 2D excitons in TMDs. Neutral excitons are studied using two different approaches: the solution based on a real-space tight binding model, and the perturbation expansion in a hydrogen-like basis in an effective mass model. We have also used the latter method to study charged excitons at a MoSe2-WSe2-MoSe2 nanoscale junction.   

Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS$_{\mathrm{2}}$ and Metal/WSe$_{\mathrm{2}}$ Heterojunctions

Transition metal dichalcogenides (TMDCs), such as MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, are free of dangling bonds, therefore make more `ideal' Schottky junctions than bulk semiconductors, which produce recombination centers at the interface with metals, inhibiting charge transfer. Here, we observe a more than 10X enhancement in the indirect band gap PL of TMDCs deposited on various metals, while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$, and subsequently recombine radiatively with minority holes. Since the conduction band at the K-point is 0.5eV higher than at the $\Sigma $-point, a lower Schottky barrier of the $\Sigma $-point band makes electron injection more favorable. Also, the $\Sigma $ band consists of the sulfur $p_{\mathrm{z}}$ orbital, which overlaps more significantly with the electron wavefunctions in the metal. This enhancement only occurs for thick flakes, and is absent in monolayer and few-layer flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures. Reference: DOI: 10.1021/acs.nanolett.5b00885