Resonant tunnelling in a twist controlled anisotropic van der Waals homostructure

Twisted trilayer structures of anisotropic black phosphorus can be used to create resonant tunnelling diodes in which twist angle between the layers dictates the vertical transport behaviour

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Twistronics in two dimensional layered materials has emerged as one of the most interesting and intriguing research area of contemporary condensed matter physics. It has provided an extra knob to tune the material’s properties. Observation of several exotic phenomena in twisted junctions has attracted more focus to twistronics based research these days. For instance, intralayer transport has been found to exhibit characteristics of a Mott insulator at half-filling, superconductivity, ferromagnetism etc. in graphene bilayers at smaller twist angles  [1-3]. Another intriguing research topic in twistronics could be the investigation on interlayer current-transport. Since, various new factors such as interlayer momentum mismatch created through rotated Fermi surfaces, any possible effects on interlayer coupling etc., could have a significant impact on the interlayer current-transport. As a consequence, new phenomena could emerge especially in anisotropic van der Waals homojunctions when relative twist angle between stacked layers is varied from small to large values.

We have tested this conjecture by measuring interlayer current-transport in high quality homostructures made of twisted anisotropic black phosphorus multilayers. The device consists of a trilayer structure in which a twisted thin non-degenerate black phosphorus layer is sandwiched between two thicker degenerate black phosphorus layers (Figure 1). We have demonstrated resonant tunneling through these structures, without the need of any physical tunnel barrier.

Figure 1. a, High resolution electron microscopic image of one of the twisted black phosphorus trilayer homostructures used in this study. b, Current-voltage (I-V) characteristics obtained on trilayer orthogonal heterojunction devices where middle black phosphorus layer with varying thicknesses (7L-14L) is θ = 90o twisted compared to top and bottom black phosphorus terminals. c, Similar I-V measurements on various devices where middle black phosphorus layer is twisted at various angles as labelled.

The main reason to use black phosphorus for this study is its (i) thickness dependent workfunction and (ii) strong dependence of interlayer coupling with twist angle [4-7]. Hence, at certain twist angles, interlayer coupling strength goes from strong to weak. In monolayer black phosphorus, each phosphorus atom has three-fold coordination via sp3 hybridization, leaving two electrons as a lone pair. For AB stacked black phosphorus, lone pair overlap is enough to have strong interlayer interaction. However, due to the directional dependent nature of the lone pairs, finite twist in bilayer black phosphorus leads to variable lone pair overlaps, which in turn has a substantial effect on the interlayer coupling. Effective modulation of interlayer coupling strength with twist has an important consequence on the interlayer transport, and it is through exploiting this effect that facilitates to engineer a quantum well within a twisted BP homostructure, which helps to observe resonant tunneling of carriers through well-resolved quantum well states within the non-degenerate black phosphorus. In such resonant tunnel diodes, tunneling is exclusively dictated by twist controlled interlayer coupling. 

Interestingly, resonant tunnel diodes based on twisted anisotropic black phosphorus homojunctions exhibit higher tunneling conductance (~1000 A/cm2) as compared to other resonant tunnel diodes based on vdW heterojunctions with physical barriers [8-9]. Moreover, twist controlled tunnel devices with high current density could further be employed to realize high speed electronic devices such as THz oscillators and ultrafast switches etc. Parasitic capacitance at higher frequencies (one of the limitations to achieve desired THz regime) will be greatly reduced due to absence of resistive physical barriers. Hence, such homojunctions could offer great potential to attain much-needed THz operation which is quite useful due to their superior resolution when compared to other safe wavelengths.

Right to left: Prof. Changgu Lee (Group leader), Dr. Yasir Hassan, Dr. Pawan Srivastava and Dr. Budhi Singh at Graphene Engineering Laboratory, School of Mechanical Engineering, Sungkyunkwan University.

Read our article in Nature Electronics at https://dx.doi.org/10.1038/s41928-021-00549-1.

References:

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  2. Cao,Y et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556,43-50(2018).
  3. Sharpe, A. L. et al. Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene. Science 365,605-608(2018).
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Pawan Srivastava

Postdoc, Sungkyunkwan University