A paper recently published in the journal Materials reviewed graphene/hexagonal boron nitride (hBN) heterostructure devices for the charge control and electrical tuning of color centers that form the foundation for photonic quantum technology devices.
(a )Atomic force microscopy (AFM) of a GaN QD that is uncovered and a cross-sectional TEM of a covered GaN/AlN QD. (b) PL spectra of a GaN/AlN QD obtained at different excitation powers. Image Credit: https://www.mdpi.com/1996-1944/17/16/4122
Background
hBN, a graphene-like two-dimensional (2D) material, is receiving significant attention as a photonic quantum technology platform. In its large bandgap, hBN can host defects that act as single-photon emitters (SPEs) at room temperature.
Specifically, hBN crystal defects have recently been recognized as color centers. A number of these defects are spin-active, which makes them suitable as spin qubits and SPEs. SPEs are the key building blocks for realizing several quantum technologies, including quantum communications, quantum metrology, and quantum computation.
In this paper, researchers reviewed graphene/hBN-based devices that tune the hBN SPEs' emission energies and control their charge states.
hBN SPEs' Electrical Charge Control
Electrically driven SPE fabrication is a critical step in scaling up photonic quantum technologies based on hBN. On-demand charge control of defects is essential for such devices. Studies have shown that when hBN is deposited on graphene, most of the SPEs are quenched due to a combination of energy and charge transfer.
hBN can be interfaced with graphene to exploit the emitters' charge control. This is realizable as studies have shown that changing the graphene layer's Fermi level leads to the defect being discharged and charged, resulting in the defect turning off/on.
The emitters' quenching due to graphene has also been utilized to localize emitters spatially within hBN. A host hBN layer with argon annealing activated emitters was embedded within a trilayer hBN structure, which was protected from the environment by the encapsulating hBN layer.
Then, this trilayer stack was placed on top of a graphene layer. The graphene layer windows were patterned using the electron beam lithography technique. Quenching was observed only in those regions surrounding windows where the graphene was in contact with hBN. The emitters above the window remained active.
Thus, vertical localization was realized by activating only the emitters in the stack's host layer, while lateral localization was achieved using graphene.
hBN SPEs’ Stark Tuning
hBN-based device fabrication for controlling SPE properties has gained momentum with the increasing understanding of hBN defects. The assembling of graphene/hBN heterostructure devices has become a common approach to establishing electrical control of hBN SPEs.
Emission lines can be tuned by applying an electric field across an SPE to induce a Stark shift in its zero-phonon line (ZPL) emission energy. In early research on the emitters' Stark shift in hBN performed using a graphene/hBN/graphene heterostructure, no significant ZPL emission energy shift was observed due to the application of a low electric field.
Although a ZPL shift bigger than the linewidth is necessary for an efficient tuning of the Stark effect, such a shift was not possible in that structure at room temperature. Yet, tuning was observed when measurements were done at 10 K temperature as the linewidth's thermal broadening was decreased.
The hBN's low out-of-plane dielectric constant, which leads to the device shorting at extremely low voltages, has been the primary cause of the limitation over applied voltages. V-shaped, quadratic, and linear Stark shifts were observed, with the linear Stark shift occurrences supporting the defects' existence with an out-of-plane dipole.
Additionally, SPEs displaying quadratic Stark shifts had substantial misalignment between the emission and excitation dipoles, as they possessed several excitation pathways for relevant electronic transitions. The defects demonstrating quadratic Stark shifts had a ∼150 Å3 polarizability, while the permanent dipole of the defects displaying linear Stark shift existed in the range from −0.9 D to 0.9 D.
Applying larger electric fields can induce larger shifts without triggering device breakdown. A study realized this by depositing hBN on 285 nm of silicon dioxide on silicon, followed by a graphene top gate on the hBN. Then, the silicon chip was back-gated to allow the application of larger electric fields.
Large Stark shifts of 24 meV per V/nm were achieved in this study at 15 K. This is the highest reported Stark shift to date owing to an electric field perpendicular to the host hBN's plane. A 100 nm hBN flake was exfoliated on two gold electrodes in a lateral device to ensure that an electric field between the gold electrodes would lead to an in-plane electric field.
In the device, the Stark shift caused by an in-plane electric field was 200 times greater compared to a vertical electric field-induced shift. Other works have demonstrated giant Stark shifts of 43 meV/(V/nm) at room temperature using a nanoscale four-electrode device for performing angle-resolved Stark shifts.
To summarize, this paper reviewed the advances in graphene/hBN heterostructure devices for the electrical control of hBN SPEs.
Journal Reference
Prasad, M. K., Taverne, M. P., Huang, C., Mar, J. D., Ho, Y. (2024). Hexagonal Boron Nitride Based Photonic Quantum Technologies. Materials, 17(16), 4122. DOI: 10.3390/ma17164122, https://www.mdpi.com/1996-1944/17/16/4122
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.