Semiconductor heterostructures are essential components of most electronic devices including transistors, photovoltaics and light-emitting diodes. Their properties, and hence function, depend on the electronic structure at the interface between layers which is intrinsically difficult to measure directly. Following the recent rise in interest in two-dimensional materials (2DMs), 2D heterostructures formed by simply stacking layers of 2DMs are showing great promise for new devices and new phenomena.
The electronic properties of 2D semiconductor heterostructures are determined by the electronic structure of their constituent layers, band alignments at their interfaces, and interaction effects between layers. Understanding each of these is critical to the development of new 2D electronic devices and for understanding the new phenomena that have been observed, such as interlayer excitons with bound electron and hole localized on neighboring layers. Although there are many theoretical predictions of the electronic structure of 2DMs in stacked heterostructures, few experimental measurements have been made. Angle resolved photoemission spectroscopy (ARPES) has demonstrated capability for determining the full momentum-resolved electronic structure of 2DMs. But conventional ARPES requires macroscopic single crystal samples whilst stacked 2DM heterostructures are typically only a few micrometres across. To resolve this problem, we have applied spatially resolved ARPES with sub-micrometre resolution (micro-ARPES) at the Spectromicroscopy beamline at Elettra to directly measure electronic structure in stacked layers of 2DMs. Micro-ARPES can be applied as a microscopy (scanning photoemission microscopy, SPEM) and a spectroscopy. SPEM gives spatial maps with sub-micrometre resolution which can be used to probe heterogeneity across a sample and to identify regions of interest. Holding the beam in a fixed position, full Fermi surface maps can be acquired from regions as small as a micrometer.
Using microARPES, we measured electronic structure in stacks of graphene with layers of MoSe2 and WSe2. These transition metal dichalcogenides (MX2) are the prototypical 2D semiconductors, with indirect band gaps in their bulk form but direct gaps in the monolayers. Through careful sample design, we were able to directly measure this layer dependent electronic structure with high spatial, momentum and energy resolution (see Fig.1). This allows key electronic structure parameters to be extracted, such as the effective mass, spin-orbit coupling, and band widths, as well as band alignments relative to graphene. However, the central object of our study was the measurement of band alignments and interaction effects in semiconductor heterobilayers.
Figure 1. microARPES analysis of the layer dependent electronic structure of WSe2. Samples were made by the dry transfer of exfoliated WSe2 flakes, resulting in WSe2 encapsulated between graphene on top and graphite underneath, placed on a silicon substrate. (a) Optical image of the resultant stack. The WSe2 is outlined in red. (b) Angle integrated spectra from the different WSe2 regions as labelled. The peak emission shifts to lower energy as the number of layers increases. (c) SPEM map, showing the energy of peak emission and demonstrating the uniformity across the different regions. Full E-k spectra were acquired on each region. (d) Slices in the Γ-K direction on regions as labelled, with DFT predictions of the band structure overlaid. The experimental data has been doubly differentiated with respect to energy to enhance contrast.
To this end, stacks of MoSe2 on WSe2 were investigated, with intriguing results (see Fig. 2). A type II (staggered gap) band alignment between the layers is observed, with a valence band offset between layers of ΔVBO=0.30±0.03 eV. This explains the formation of an interlayer exciton and, combined with photoluminescence measurements, gives a binding energy of ~0.2 eV which is an order of magnitude larger than the binding energy of spatially indirect excitons in GaAs/AlGaAs double quantum wells. The effect of interlayer hybridisation is also apparent in the band structure measurements and is dependent on the relative orientation of the layers. When the MoSe2 and WSe2 are crystallographically aligned, evidence is found for the formation of a commensurate phase not present in misaligned samples. But in both cases the VBM remains at the Brillouin zone corner, K, so that a direct gap is retained.
Figure 2. Bands in a 2D heterostructure. (a) Optical image showing monolayer MoSe2 and WSe2 sheets which overlap, with the MoSe2 on top, in an aligned heterobilayer region (H). Their boundaries are indicated with color-coded dotted lines. Scale bar: 5 µm. (b) Angle-integrated spectra in each of the three regions. (c) Map of the energy of maximum emission which identifies the different regions and shows the uniformity within them. (d) Momentum slices along Γ-K in the three regions. The superposed dashed lines are DFT calculations for MoSe2 monolayer (green), WSe2 monolayer (red), and commensurate heterobilayer (blue). The graphene valence band is indicated by a white dotted line. (e) A momentum slice through Γin another heterobilayer intentionally misaligned by about 30°. Here only two bands are seen, indicating that the third band near Γin the aligned heterobilayer, (d), arises from commensurate domains.
These results establish the key electronic parameters of MoSe2-WSe2 heterobilayers, explaining the formation of interlayer excitons and demonstrating the potential for band engineering through hybridisation between layers. Moreover, they demonstrate that microARPES combined with careful sample design provides invaluable information for realizing the potential of 2D semiconductor heterostructures. The measurement principles derived here are generically applicable to the study of stacked layers of 2D materials, and we expect microARPES will play a significant role in the determination of the local electronic structure and chemical potential in a wide range of 2D materials and devices.
N.R. Wilson1, P.V. Nguyen1, K. Seyler2, P. Rivera2, A. J. Marsden1, Zachary P.L. Laker1, Gabriel C. Constantinescu4, V. Kandyba5, A. Barinov5, N.D.M. Hine, X. Xu3, D. H. Cobden2
1 Department of Physics, University of Warwick, Coventry, UK
2 Department of Physics, University of Washington, Seattle, Washington, USA
3 Department of PhysicsandDepartment of Material Science and Engineering, University of Washington, Seattle, Washington, USA
4 TCM Group, Cavendish Laboratory, University of Cambridge, Cambridge, UK
5 Elettra - Sincrotrone Trieste SCpA, Trieste, Italy
Neil R. Wilson e-mail: firstname.lastname@example.org
N. R. Wilson, P.V. Nguyen, K. Seyler, P. Rivera, A. J. Marsden,Zachary P.L. Laker,Gabriel C. Constantinescu,V. Kandyba, A. Barinov, N. D.M. Hine, X. Xu, D. H. Cobden, "Determination of band offsets, hybridization and exciton binding in 2D semiconductor heterostructures", Science Advances, Vol. 3, no. 2, e1601832, DOI: 10.1126/sciadv.1601832