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Electrically modulated light-emitting diodes driven by resonant and antiresonant tunneling between Cr$_2$Ge$_2$Te$_6$ electrodes

Natalia Zawadzka, Kristina Vaklinova, Tomasz Woźniak, Mihai I. Sturza, Holger Kohlmann, Kenji Watanabe, Takashi Taniguchi, Adam Babiński, Maciej Koperski, Maciej R. Molas

TL;DR

The paper demonstrates electrically driven EL in a van der Waals tunneling LED assembled entirely from gapped materials: Cr$_2$Ge$_2$Te$_6$ electrodes, hBN barriers, and a monolayer WSe$_2$ emitter. Electroluminescence exhibits a nonmonotonic dependence on tunneling bias, driven by resonant and antiresonant tunneling as the DOS in the Cr$_2$Ge$_2$Te$_6$ electrodes aligns with final states in the tunneling process. DFT-calculated DOS of CGT correlates with EL intensity, particularly on positive bias, while negative-bias EL is dominated by nonradiative processes in WSe$_2$, indicating asymmetric charge injection. This architecture enables room-temperature, electrically modulated emission in ultrathin LEDs and offers a new way to probe electrode DOS via EL measurements.

Abstract

Exploring the electron tunneling mechanisms in diverse materials systems constitutes a versatile strategy for tailoring the properties of optoelectronic devices. In this domain, bipolar vertical tunneling junctions composed of van der Waals materials with vastly different electronic band structures enable simultaneous injection of electrons and holes into an optically active material, providing a universal blueprint for light-emitting diodes (LEDs). Efficient modulation of the injection efficiency has previously been demonstrated by creating resonant states within the energy barrier formed by the luminescent material. Here, we present an alternative approach towards resonant tunneling conditions by fabricating tunneling junctions composed entirely from gapped materials: Cr$_2$Ge$_2$Te$_6$ as electrodes, hBN as a tunneling barrier, and monolayer WSe$_2$ as a luminescent medium. The characterization of such LEDs revealed a nonmonotonous evolution of the electroluminescence intensity with the tunneling bias. The dominant role driving the characteristics of the electron tunneling was associated with the relative alignment of the density of states in Cr$_2$Ge$_2$Te$_6$ electrodes. The unique device architecture introduced here presents a universal pathway towards LEDs operating at room temperature with electrically modulated emission intensity.

Electrically modulated light-emitting diodes driven by resonant and antiresonant tunneling between Cr$_2$Ge$_2$Te$_6$ electrodes

TL;DR

The paper demonstrates electrically driven EL in a van der Waals tunneling LED assembled entirely from gapped materials: CrGeTe electrodes, hBN barriers, and a monolayer WSe emitter. Electroluminescence exhibits a nonmonotonic dependence on tunneling bias, driven by resonant and antiresonant tunneling as the DOS in the CrGeTe electrodes aligns with final states in the tunneling process. DFT-calculated DOS of CGT correlates with EL intensity, particularly on positive bias, while negative-bias EL is dominated by nonradiative processes in WSe, indicating asymmetric charge injection. This architecture enables room-temperature, electrically modulated emission in ultrathin LEDs and offers a new way to probe electrode DOS via EL measurements.

Abstract

Exploring the electron tunneling mechanisms in diverse materials systems constitutes a versatile strategy for tailoring the properties of optoelectronic devices. In this domain, bipolar vertical tunneling junctions composed of van der Waals materials with vastly different electronic band structures enable simultaneous injection of electrons and holes into an optically active material, providing a universal blueprint for light-emitting diodes (LEDs). Efficient modulation of the injection efficiency has previously been demonstrated by creating resonant states within the energy barrier formed by the luminescent material. Here, we present an alternative approach towards resonant tunneling conditions by fabricating tunneling junctions composed entirely from gapped materials: CrGeTe as electrodes, hBN as a tunneling barrier, and monolayer WSe as a luminescent medium. The characterization of such LEDs revealed a nonmonotonous evolution of the electroluminescence intensity with the tunneling bias. The dominant role driving the characteristics of the electron tunneling was associated with the relative alignment of the density of states in CrGeTe electrodes. The unique device architecture introduced here presents a universal pathway towards LEDs operating at room temperature with electrically modulated emission intensity.

Paper Structure

This paper contains 14 sections, 10 figures.

Table of Contents

  1. Introduction
  2. Experimental results
  3. Conclusions
  4. Methods
  5. Sample
  6. Crystal growth
  7. PL and EL measurements
  8. Theoretical calculations
  9. Acknowledgment
  10. Low-temperature photoluminescence
  11. Dependence of the charged and neutral exciton intensities on excitation power and bias voltage
  12. Current-voltage curves
  13. Density of states of CGT and graphene
  14. Calculated electronic structure of CGT

Figures (10)

  • Figure 1: The structure of the light-emitting diode (LED) based on van der Waals (vdW) tunneling junction. (a) Microscope optical image of the active part of the Cr$_2$Ge$_2$Te$_6$/hBN/WSe$_2$/hBN/Cr$_2$Ge$_2$Te$_6$ vdW LED. The $\text{WSe}_{2}$ monolayer and Cr$_2$Ge$_2$Te$_6$ contacts are highlighted by the outlines. (b) Schematic drawing of the vdW stack used in the LED architecture encapsulated with hBN films and deposited onto Si/SiO$_2$ wafer.
  • Figure 2: The room temperature ($\mathrm{T~=~300~K}$) photoluminescence (PL) and electroluminescence (EL) spectra of the Cr$_2$Ge$_2$Te$_6$/hBN/WSe$_2$/hBN/Cr$_2$Ge$_2$Te$_6$ light-emitting diode measured in a microscopic configuration. The PL spectrum was obtained at zero bias voltage and the EL spectra were detected at tunnelling bias voltages $V_T = \pm 30,~60,~90~V$. The EL spectra are multiplied by scaling factors shown in the figure for ease of comparison. The spectra are vertically shifted for clarity. Two Lorentz functions are fitted to the PL spectrum for the identified excitonic complexes, $i.e.$, X$^0$ and T$^-$.
  • Figure 3: (a) False-color map presenting the electroluminescence spectrum as a function of the applied voltage in the range of -90 V to 90 V. (b) Voltage-dependent integrated intensity of the EL signal with the corresponding X$^0$ and T$^-$ intensities in the same bias range as in panel (a). (c) Calculated density of states of Cr$_2$Ge$_2$Te$_6$ resolved by the atomic contributions from Cr, Ge, and Te. The energy scale in panel (c) is chosen to facilitate the comparison with the results shown in panel (b).
  • Figure 4: (a) Tunneling current–voltage characteristics of the studied light-emitting diode device, shown on a logarithmic scale. (b) Schematic illustration of the type-II band alignment in the Cr$_2$Ge$_2$Te$_6$/hBN/WSe$_2$/hBN/Cr$_2$Ge$_2$Te$_6$ heterostructure. The occupied states in Cr$_2$Ge$_2$Te$_6$ are shown in orange, while the occupied states in WSe$_2$ are shown in blue. The hBN barriers are represented by gray rectangles. (c) Applied intermediate bias below the electroluminescence threshold (from –19 V to 7 V). In this regime, electrons can tunnel through the hBN barrier into WSe$_2$, while holes cannot tunnel due to the absence of available empty states. (d) At higher bias voltages, both electrons and holes can tunnel into WSe$_2$ and form excitons, which recombine radiatively, resulting in electroluminescence. Alternatively, they may tunnel further into the opposite Cr$_2$Ge$_2$Te$_6$ contact without radiative recombination.
  • Figure S5: PL spectrum of WSe$_2$ monolayer measured at 5 K under 2.21 eV and 70 $\mu$W laser excitation.
  • ...and 5 more figures