Electrical control of interlayer exciton dynamics in atomically thin heterostructures (1812.08691v1)

Abstract: Excitons in semiconductors, bound pairs of excited electrons and holes, can form the basis for new classes of quantum optoelectronic devices. A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. Employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate the transport of neutral interlayer excitons across the whole sample that can be controlled by excitation power and gate electrodes. We also realize the drift motion of charged interlayer excitons using Ohmic-contacted devices. The electrical generation and control of excitons provides a new route for realizing quantum manipulation of bosonic composite particles with complete electrical tunability.

• The paper introduces a novel electrical control method for interlayer excitons in TMD heterostructures, achieving lifetimes up to 600 ns.
• It employs advanced device fabrication with hBN encapsulation and transparent gates to finely tune carrier densities and electric fields.
• Detailed photoluminescence measurements reveal controlled exciton diffusion over microns and directional drift under applied in-plane electric fields.

Electrical Control of Interlayer Exciton Dynamics in Atomically Thin Heterostructures

The paper under consideration demonstrates a comprehensive exploration of interlayer excitons in van der Waals (vdW) heterostructures, focusing specifically on semiconducting transition metal dichalcogenides (TMDs). These excitons, formed by electrons and holes in distinct layers, exhibit properties that can be exploited in the development of quantum optoelectronic devices. The research shows that the interlayer excitons possess substantial binding energies and extended lifetimes, positioning them as potential candidates for quantum information and many-body system applications.

Key Findings

1. Generation and Control Mechanisms: The paper describes the optical and electrical generation of both neutral and charged interlayer excitons using monolayer TMD heterostructures. Notably, the dynamics of these excitons can be modulated by manipulating excitation power and gate electrodes. This electrical control introduces a novel mechanism for manipulating such excitons, with potential applications in quantum systems.
2. Device Fabrication and Measurements: The research involved fabricating atomically thin optoelectronic devices from MoSe$_2$ and WSe$_2$ DQW systems, encapsulated within hexagonal boron nitride to create a protective environment. These devices feature optically transparent gates and Ohmic electrical contacts, allowing precise control of the carrier densities and the electric field across the heterostructures. Photoluminescence (PL) measurements confirmed various phenomena, including Stark shifts indicative of electric field effects on excitons.
3. Exciton Lifetime and Density: The paper reveals that interlayer excitons exhibit a unique electric field tunability, with lifetimes reaching approximately 600 nanoseconds for neutral excitons. Charged excitons displayed reduced lifetimes around 100 nanoseconds, correlating with increased doping levels. Exciton density manipulation was achieved by varying laser power, affecting PL emissions and transport characteristics across the heterostructure.
4. Transport and Diffusion Dynamics: Experimental data showed that neutral interlayer excitons could be transported over several microns across the heterostructure, facilitated by high exciton densities and long lifetimes. The diffusion constant was determined to fall within specific bounds influenced by the dipolar interactions between excitons. These dynamics underscore the potential for developing exciton-based transport mechanisms in solid-state systems.
5. Charged Exciton Drift: The paper also details how charged interlayer excitons can be influenced by in-plane electric fields, demonstrating drift motion across the sample. This capability further expands the functional versatility of such 2D TMD heterostructures in device applications where directional exciton flow is required.
6. Electrical Generation of Excitons: The authors accomplished electrical generation of interlayer excitons through carrier injection, leveraging the type-II band alignment of the heterostructure. This diode-like behavior enables the use of such systems as electrically driven near-infrared light sources, with tunable emission energies.

Implications and Future Directions

This research contributes significantly to the understanding and potential application of exciton dynamics in quantum and optoelectronic devices. The ability to electrically control and generate interlayer excitons paves the way for more complex, electrically tunable quantum devices. These could include exciton condensates if exciton densities can be sufficiently increased without optical excitation. The results also suggest avenues for novel optoelectronic devices that exploit the spin and valley degrees of freedom, which are naturally coupled in TMD materials.

For future developments, expanding the paper to include interactions with other types of heterostructures or exploring temperature dependencies could provide further insights into the robustness and versatility of these systems. Additionally, engaging with resonant excitation techniques may illuminate alternative mechanisms and facilitate clearer distinctions between potential excitation scenarios. As research continues to uncover the properties of these materials, the integration of TMD heterostructures into practical quantum technologies becomes increasingly feasible.