Light helicity detector based on 2D magnetic semiconductor CrI3

Two-dimensional (2D) magnetic semiconductors, discovered in 2017, offer a unique platform for investigating light-matter interactions and magneto-optical/electrical phenomena at the atomically thin limit. These materials hold immense promise for applications in magneto-optoelectronic devices. Monolayer CrI3 exhibits ferromagnetic (FM) behavior, while multilayer CrI3 displays layered antiferromagnetic (AFM) characteristics with an easy magnetization axis perpendicular to the layers. Previous studies have explored CrI3's helical luminescence, tunneling magnetoresistance, and electrostatic doping, demonstrating the coupling between its optical/electrical and magnetic properties. This research aims to deepen our understanding of this interplay by fabricating and characterizing light helicity detectors based on graphene-CrI3-graphene van der Waals heterostructures. The study focuses on investigating the helicity-selective photoresponse properties through magneto-optoelectronic measurements of both monolayer and multilayer CrI3 devices, under varying magnetic fields and temperatures. This detailed investigation is crucial for advancing the development of novel spin-optoelectronic devices based on 2D magnetic semiconductors.

The discovery of intrinsic ferromagnetism in 2D van der Waals crystals, specifically Cr2Ge2Te6 and CrI3, in 2017 sparked significant research interest in 2D magnetic materials. These materials offer a unique opportunity to study light-matter interactions and magneto-optical/electrical phenomena at the atomic limit. Subsequent research on CrI3 has unveiled its layer-dependent magnetic properties, with monolayer CrI3 demonstrating ferromagnetism and multilayer CrI3 exhibiting layered antiferromagnetism. Studies have reported various phenomena, including helical luminescence, tunneling magnetoresistance, and the influence of electrostatic doping on CrI3's optical and electrical characteristics. This prior work establishes a foundation for exploring the interplay between the magnetic and optoelectronic properties of CrI3, which is crucial for developing magneto-optoelectronic devices. The current study builds on this foundation by exploring the helicity-dependent photoresponse in graphene-CrI3-graphene heterostructures.

The researchers fabricated light helicity detectors using graphene-CrI3-graphene van der Waals heterostructures. Few-layer graphene, CrI3, and hexagonal boron nitride (hBN) flakes were mechanically exfoliated and transferred onto a Si/SiO2 substrate. The hBN encapsulated the graphene-CrI3-graphene heterostructure, with the graphene layers acting as electrodes. All fabrication steps were performed in an inert atmosphere glove box. The devices were characterized using magneto-optoelectronic measurements in a dry cryostat equipped with a 9 T superconducting magnet. Circularly polarized light excited currents and reflective magnetic circular dichroism (RMCD) were measured under various magnetic fields and temperatures. The excitation light and magnetic field were perpendicular to the 2D layers and parallel to CrI3's easy magnetization axis. The beam diameter of the excitation light was approximately 1 µm. For photoresponse and RMCD measurements, the temperature was typically 2 K, and the excitation power was 10 µW, unless otherwise specified. Detailed measurements included light-on currents under right and left circularly polarized light, excitation power-dependent current-voltage (I-V) curves, and magnetic field-dependent photocurrents. The photoresponsivity, polarization, and RMCD were calculated and analyzed to determine the helicity-dependent photoresponse. The temperature dependence of the helicity-dependent photocurrent was also investigated. The researchers explored the origin of the helicity-dependent photocurrent by analyzing the split energy band structure in fully magnetized CrI3. Additionally, they investigated abnormal negative photocurrents observed at higher bias in both monolayer and multilayer CrI3 devices, analyzing the tunneling current mechanisms (direct and Fowler-Nordheim tunneling) and their temperature dependence to propose a possible explanation.

The study revealed a clear helicity-selective photoresponse in both monolayer and multilayer CrI3 devices. In monolayer CrI3, the photoresponsivity polarization (ρ) switched between approximately -6% and +6% as the magnetic field reversed, vanishing above 40 K (near the Curie temperature). In multilayer CrI3, ρ exhibited multiple plateaus with magnetic field sweeps, saturating at approximately ±4.5% at high fields, reflecting the layered AFM nature. The observed helicity-dependent photocurrent is consistent with the RMCD signals, confirming the magneto-optical nature of the phenomenon. A key finding was the observation of abnormal negative photocurrents at higher bias voltages in both monolayer and multilayer devices. Analysis suggests that this negative photocurrent is related to the reduction of the tunneling current under light illumination, especially prominent in the Fowler-Nordheim tunneling regime at higher bias voltages. This reduction is temperature-dependent, disappearing above 60 K, possibly due to light-induced defects in CrI3. The temperature dependence of the negative photocurrent is consistent with the behavior of the dark tunneling current, further supporting this explanation. The experiments provided valuable data for understanding the complex interplay between light, magnetism, and charge transport in 2D magnetic semiconductor materials. The observed helicity-dependent photocurrent response is a strong indication that the devices are functional helicity detectors. The results also shed light on the underlying mechanisms of charge transport in these materials at different bias levels.

The findings address the research question by demonstrating a clear correlation between the helicity of incident light, the magnetic state of CrI3, and the resulting photocurrent. The helicity-selective photoresponse originates from the spin-dependent band structure of CrI3, leading to different absorption of right and left circularly polarized light. The abnormal negative photocurrent at higher bias is explained by light-induced changes in the tunneling current. The significance of these results lies in demonstrating the potential of CrI3-based heterostructures for developing efficient light helicity detectors and advancing spin-optoelectronic devices. The observation of temperature-dependent behavior provides further insights into the underlying mechanisms governing the interaction between light, magnetism, and charge transport in these materials. This comprehensive investigation of both monolayer and multilayer CrI3 expands our fundamental understanding of 2D magnetic semiconductors and their potential applications.

This research successfully fabricated and characterized light helicity detectors based on graphene-CrI3-graphene van der Waals heterostructures. The devices exhibited a clear helicity-selective photoresponse directly related to CrI3's magnetic state. The observation of abnormal negative photocurrents at higher bias provides valuable insights into the charge transport mechanisms. These findings demonstrate the potential of CrI3-based heterostructures for spin-optoelectronic applications. Future research could focus on optimizing device performance, exploring different materials and heterostructures, and investigating the dynamics of light-matter interactions in these systems at a more fundamental level to further our understanding of this promising material.

While the study provides valuable insights, some limitations exist. The observed negative photocurrent phenomenon requires further investigation to definitively confirm the proposed mechanism. Additional studies could involve more sophisticated techniques to characterize light-induced defects and their impact on charge transport. Furthermore, the study primarily focused on a specific wavelength of excitation light (633 nm). Investigating the helicity-dependent response over a broader wavelength range could provide a more complete understanding of the material's optoelectronic properties. The use of relatively low-power laser for excitation limits the scope of the findings in higher energy density applications. The influence of defects on the long-term stability of the device is yet to be thoroughly explored.