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# Light helicity detector based on 2D magnetic semiconductor CrI3

## Abstract

Two-dimensional magnetic semiconductors provide a platform for studying physical phenomena at atomically thin limit, and promise magneto-optoelectronic devices application. Here, we report light helicity detectors based on graphene-CrI3-graphene vdW heterostructures. We investigate the circularly polarized light excited current and reflective magnetic circular dichroism (RMCD) under various magnetic fields in both monolayer and multilayer CrI3 devices. The devices exhibit clear helicity-selective photoresponse behavior determined by the magnetic state of CrI3. We also find abnormal negative photocurrents at higher bias in both monolayer and multilayer CrI3. A possible explanation is proposed for this phenomenon. Our work reveals the interplay between magnetic and optoelectronic properties in CrI3 and paves the way to developing spin-optoelectronic devices.

## Introduction

Since the first discovery of intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals of Cr2Ge2Te6 and CrI3 in 20171,2, 2D magnetic materials have aroused wide research interest. They provide a platform for studying light–matter interactions and magneto-optical/electrical phenomena at the atomically thin limit and also promise magneto-optoelectronic devices application3. Monolayer CrI3 has ferromagnetic (FM) nature, whereas multilayer CrI3 has a layered antiferromagnetic (AFM) nature with an easy magnetization axis perpendicular to the layers2,4,5,6. To date, various studies about CrI3 were reported, including helical luminescence7, tunneling magnetoresistance5,6,8 and electrostatic doping9, which demonstrated that the optical/electrical properties of CrI3 are coupled with its magnetic property. To further study 2D magnetic semiconductor materials and advance their application, understanding the interplay between their magnetic and optoelectronic properties and developing magneto-optoelectronic devices are indispensable.

In this work, we fabricate light helicity detectors based on graphene-CrI3-graphene vdW heterostructures and study their helical photoresponse properties via magneto-optoelectronic measurements. Devices with two representative thicknesses (monolayer and multilayer) of CrI3 are investigated. We investigate their circularly polarized light excited current and reflective magnetic circular dichroism (RMCD) under various magnetic fields μ0H and temperatures. The devices exhibit helicity-selective photoresponse behavior determined by the magnetic state of CrI3. Both the helicity-dependent photocurrent and illumination-dependent tunneling current are studied in detail. The calculated photoresponsivity polarization has a clear relation with magnetic field, consistent with the RMCD signals. For the monolayer CrI3 device, when the applied magnetic field switches the CrI3 between the up magnetized state (μ0H > 0.1 T) and the down magnetized state (μ0H < −0.1 T), the photoresponsivity polarization ($$\rho$$) changes between ~−6 and +6%, and the RMCD signal changes between ~+1 and −1%. The photoresponsivity polarization equals zero, i.e., the helicity-dependent photoresponse phenomenon vanishes, at temperature higher than 40 K, close to the Curie temperature (Tc) of monolayer CrI3 (45 K)2. For the multilayer (14 nm thick) CrI3 device, $$\rho$$ value displays multiple plateaus with magnetic field sweeping, and saturates at ~±4.5% when | μ0H | > 2.2 T, corresponding to different spin configurations in CrI3 enabled by spin-flip transitions at different magnetic fields. Specifically, for both monolayer and multilayer CrI3 devices, the light-on current decreases with increasing excitation power at a higher bias. Based on detailed discussions, we provide a possible explanation to this phenomenon. Our work reveals the interplay between magnetic and optoelectronic properties in CrI3, and paves the way to developing spin-optoelectronic devices.

## Results

### Device fabrication and characterization

The CrI3 based light helicity detector was fabricated by mechanically exfoliating and dry transferring few-layer graphene, CrI3, and hexagonal boron nitride (hBN) flakes from their bulk crystals to construct graphene-CrI3-graphene heterostructure. To avoid degradation8, the device was encapsulated by hBN flakes. The top and bottom graphene layers act as the electrodes. All the procedures were conducted in a glove box within an inert atmosphere (<0.1 ppm of water and oxygen) (see Methods). Figure 1a, b show the schematic diagram and optical image, respectively, of a monolayer CrI3 light helicity detector D1. We measured the circularly polarized light excited current and RMCD of the device under various magnetic fields and temperatures. Both the excitation light and magnetic field μ0H are perpendicular to the 2D layers and parallel to the easy magnetization axis of CrI3 (see Methods). Notably, in the experiment, the beam diameter of the excitation light is about 1 μm, less than the scale of vertical tunneling junction. For photoresponse and RMCD measurements, the temperature is 2 K and the excitation power is 10 μW, unless otherwise specified.

### Circularly excited light-on currents

The monolayer CrI3 device was first fully magnetized at |μ0H| > 0.3 T. Then its light-on currents (Ilight) were measured at μ0H = 0 T under both right circularly polarized light ($${\sigma }^{+}$$) and left one ($${\sigma }^{-}$$) excitations. Their difference (ΔIσ = I+ − I) is plotted as a function of applied bias (V) in Fig. 1c, where $${I}_{\pm }$$ are the light-on currents under $${\sigma }^{\pm }$$ excitations. The ΔIσV curves exhibit distinguishable difference between the Ilight under $${\sigma }^{+}$$ and $${\sigma }^{-}$$ excitations, with negative (positive) slope for the up (down) magnetized state of monolayer CrI3. These results show that the photocurrent in a fully magnetized CrI3 is helicity-dependent.

### Light Power dependent magneto-optoelectronic response

The excitation power dependent current versus bias relations (IV curves) of a monolayer CrI3 with down magnetized state under $${\sigma }^{+}$$ excitation at μ0H = 0 T (increased from –0.3 T) are shown in Fig. 2a. For identical bias, the Ilight increases monotonically with the excitation power. We calculate photoresponsivity $$R=\frac{{I}_{{{{{{\rm{ph}}}}}}}}{P}$$, where Iph is the photocurrent defined as Iph = Ilight – Idark, Idark is dark current (i.e., tunneling current in dark), and P is the excitation power. The excitation power dependent R+ (under $${\sigma }^{+}$$excitation) and R (under $${\sigma }^{-}$$ excitation) at V = 0.2 V are plotted in Fig. 2b. The clear difference between the R+ and R illustrates again that the photocurrent is helicity-dependent and the device can serve as a light helicity detector10. The key parameter for a helicity detector is the photoresponsivity polarization defined as $$\rho =\frac{{R}_{+}-{R}_{-}}{{R}_{+}+{R}_{-}}$$, where $${R}_{\pm }$$ are the $${\sigma }^{\pm }$$ photoresponsivities11. Based on the above results, we obtain $$\rho \approx -6$$% (6%) for the up (down) magnetized monolayer CrI3 at μ0H = 0 T, which, represented by red (blue) dots in Fig. 2c, is independent of excitation power.

### Magnetic order dependence of the helical photocurrents

In order to further investigate the relation between the helical photocurrent and the intrinsic magnetism of the monolayer CrI3, we directly measured the photocurrent Iph (V = 0.15 V) of D1 under various magnetic fields μ0H by the lock-in technique. The excitation laser is modulated to switch between $${\sigma }^{+}$$ and $${\sigma }^{-}$$ by a quarter wave plate equipped with a motorized rotation stage (see Methods). The circularly polarized photocurrent versus the phase of the quarter-wave plate relations for upstate (red dots) and downstate CrI3 (blue dots) are opposite (Fig. 2d), with Iph($${\sigma }^{+}$$) > Iph($${\sigma }^{-}$$) for the down state CrI3, and vice versa. When μ0H sweeps between ±0.3 T, the photocurrent behaviors are opposite for $${\sigma }^{+}$$ and $${\sigma }^{-}$$ excitation as shown in Fig. 2e. By curve fitting the data as shown in Fig. 2d with a cosine function, we can obtain the corresponding photoresponsivity polarization $$\rho$$. When μ0H sweeps between −0.3 and 0.3 T, $$\rho$$ changes between 6 and −6% (Fig. 2f), correspondingly, the RMCD signal changes between −1 and +1% (the inset in Fig. 2f). The monolayer CrI3 spin-flip transitions occur at ±0.1 T. While the device exhibits significant magnetic field-dependent helical photocurrent property, the dark tunneling current Idark is almost independent of the magnetic field as shown in Supplementary Fig. 1, because the resistances of the down- or up-magnetized CrI3 are equal. All the above results demonstrate that the magnetic field-dependent circularly polarized photocurrent can be used to detect the helicity of the incident light.

We also measured the photoresponsivity polarization $$\rho$$ of the helicity detector D1 under the zero-field-cooling condition (Fig. 3). When the temperature decreases from 80 to 2 K, $$\rho$$ increases from 0% to a saturation value of 6% (black dots). By curve fitting the data with $$\rho \left(T\right)={\rho }_{0}{(1-\frac{T}{{T}_{c}})}^{\beta }$$ (red line), where $${\rho }_{0}$$ is the critical amplitude, $$\beta$$ is the critical exponent, we obtain Tc = 40 K, close to the Curie temperature of monolayer CrI3 (45 K).

The origin of the helicity-dependent photocurrent can be understood by analyzing the split energy band structure in a fully magnetized CrI312,13 as shown in Supplementary Fig. 2a. The energy bands for electrons with different spins exhibit different band gaps. The narrower band gap corresponds to majority-spin electrons, and vice versa. Under the 633 nm light excitation, only the majority-spin electrons can be excited and form a few types of excitons7,14. These excitons obey the helicity-selective transition rule (Supplementary Fig. 2b), determined by the optical selection rules. From the frequency-dependent circularly polarized absorbance of ferromagnetic monolayer CrI3 at normal incidence14, we can see that, at the 633 nm (1.96 eV) excitation wavelength, $${\sigma }^{+}$$ and $${\sigma }^{-}$$ light absorbances are different, which are dominated by the B+ and B bright exciton absorbances, respectively, leading to the helicity-dependent photocurrent under applied bias. The helicity-dependent absorption is also demonstrated by the RMCD signals (the inset in Fig. 2f), which indicate the difference in reflectivity of the $${\sigma }^{+}$$ and $${\sigma }^{-}$$ light (see Methods).

### Helical photocurrents in multilayer CrI3 helicity detector

Figure 4a shows an optical image of a 14 nm thick CrI3 light helicity detector D2. The magnetoresistances measured in dark at V = 0.3 V under various magnetic fields are shown in Supplementary Fig. 3a. The magnetoresistance is significant with abrupt changes around ±1 and ±2 T, consistent with the RMCD result (Fig. 4b). The IdarkV curves at three representative magnetic field (0, 1.5, and 3 T) are shown in Supplementary Fig. 3b. We can see that, for the applied magnetic fields in our experiment, the tunneling currents in dark are negligible within V = ±0.19 V. To eliminate the influence of the tunneling current (to be discussed in the next section), herein, the photocurrents are measured at V = −0.15 V under various magnetic fields by the lock-in technique. Both the RMCD signal (Fig. 4b) and photoresponsivity polarization $$\rho$$ (Fig. 4c) show five plateaus when μ0H sweeps between −3.5 and +3.5 T, corresponding to five magnetic states enabled by the layered AFM nature of multilayer CrI3. The photoresponsivity polarization $$\rho$$ equals 0 at μ0H = 0 T, corresponding to the AFM ground state, and saturates at ±4.5% when | μ0H | > 2.2 T, corresponding to the fully spin-polarized states.

### The abnormal negative photocurrents at higher bias

The IV curves of a ~10 nm thick CrI3 light helicity detector D3 are measured at μ0H = 3 T under $${\sigma }^{-}$$ excitation with various powers (from 0 to 100 μW) (Fig. 5a). The IV curves are linear within V = ±0.19 V, but become nonlinear and cross to each other at higher biases (e.g., 0.25 V, the insets in Fig. 5a). Specifically, for identical bias within ±0.19 V, the Ilight increases monotonically with the excitation power increasing. However, it decreases first and then increases with the excitation power increasing at a higher bias, e.g., V = 0.25 V (Supplementary Fig. 4). Such intriguing phenomenon can be reproduced in D2 (Supplementary Fig. 5). We extract the photocurrents Iph by subtracting Idark from Ilight (Fig. 5a) as shown in Fig. 5b. We can see that, for all the excitation powers, Iph increases linearly with bias within V = ±0.19 V, but deviates from linear behavior and even decreases at higher bias, resulting in a negative photocurrent. The critical biases of ±0.19 V coincide with the bias at which Idark becomes significant (see the 0 μW excited IV curve in Fig. 5a).

In order to explore the abnormal negative photocurrent phenomenon in multilayer CrI3 devices, we consider that the Ilight includes the photocurrent Iph and the tunneling current in dark Idark. The Iph includes the photoconductive current Ipc and the variation of the tunneling current under light illumination ΔIT. Given a linear relationship between the Ipc and applied bias V, we extract the photoconductivity current Ipc by linear fitting the IphV curve (under 100 μW excitation) within V = ±0.15 V, and extending the result to the full bias range of ±0.25 V (red line in Fig. 5c). Next, we obtain the ΔIT (blue line in Fig. 5c) by subtracting Ipc (red line) from the photocurrent Iph (black line). We can see that, under light illumination, the tunneling current reduces at a higher bias (|V | > 0.19 V), and the reduction magnitude | ΔIT | increases monotonically with excitation power (Supplementary Fig. 6). If normalize the ΔITV and IdarkV curves by their respective current values at V = 0.25 V, we find that the two curves overlap well with each other (Supplementary Fig. 7), indicating that the ΔIT is proportional to the Idark, regardless of the applied bias. These results suggest that the abnormal negative photocurrent phenomenon at higher bias in multilayer CrI3 devices may result from tunneling current reduction under light illumination.

It is worth noting that the negative photocurrent phenomenon can also be observed in the monolayer CrI3 device at a higher bias (|V | > 0.25 V, Fig. 5d). We plot the dark tunneling current in a log(I/V2) vs. 1/V diagram15,16 to examine the tunneling mechanisms in both monolayer and multilayer CrI3. The log(I/V2)−1/V curve for monolayer device (Fig. 5e) shows negative (positive) slope at higher (lower) bias region, corresponding to Fowler–Nordheim (FN) tunneling (direct tunneling)17. Direct tunneling occurs in the lower bias region, i.e., the carriers directly tunnel through a trapezoidal barrier (the right inset in Fig. 5e) without entering into the conduction band of semiconductor18. FN tunneling occurs in a higher bias region (>0.25 V for monolayer CrI3 case), wherein the energy band bends more and the electrons tunnel into the conduction band of the semiconductor (the left inset in Fig. 5e)6,8. The log(I/V2)−1/V curve for the ~10-nm CrI3 device D3 is shown in Supplementary Fig. 8, where the FN tunneling occurs at V > 0.19 V. These results indicate that there exists a relationship between the abnormal photocurrent and the tunneling current with FN tunneling mechanism.

We further perform the temperature-dependent measurement in the ~10 nm thick CrI3 device D3. As temperature increases, the | ΔIT | (measured at V = −0.25 V) exhibits a monotonous decrease until zero at a temperature higher than the Curie temperature of CrI3 (Tc = 61 K4). This result indicates that the negative photocurrent phenomenon weakens as temperature increases, and vanishes at temperature higher than 60 K. The temperature dependence of the dark tunneling current Idark for the 10-nm CrI3 device is shown in Supplementary Fig. 9a. As the temperature increases from 2 K, the Idark first decreases and then increases. The turning point is 60 K. Similar result was reported previously for CrI3 tunneling device8. The IphV relations at various temperatures are shown in Supplementary Fig. 9b. At lower temperature, the IphV curves are nonlinear due to the negative photocurrent. As temperature increases, the IphV curves become linear, corresponding to the weakening negative photocurrent phenomenon.

One possible origin for the abnormal negative photocurrent phenomenon is the light-induced defects in CrI319. The defects may trap the electrons which tunnel into the conduction band of CrI3 through FN tunneling, resulting in reduced conductivity. At higher temperatures (kBT > activation energy of the trapped electron), the trapped electrons would release and contribute again to conductivity. Therefore, the absolute value of ΔIT decreases with increasing temperature as shown in Fig. 5f. Nevertheless, to fully verify this, further study of dynamic process of the interaction between photons and electrons is needed.

## Discussion

In conclusion, we fabricated light helicity detectors based on graphene-CrI3-graphene vdW heterostructures and measured their circularly polarized light excited current and RMCD under various magnetic fields. The helicity-dependent photocurrent has a clear relation with a magnetic field, consistent with the RMCD result. For the monolayer CrI3 device D1, the photoresponsivity polarization switches between ±6% as the magnetic field sweeps between ±0.3 T. The helicity-dependent photoresponse phenomenon vanishes at temperature higher than 40 K, close to the Curie temperature (Tc) of monolayer CrI3 (45 K). For the multilayer CrI3 device, the photoresponsivity polarization performs AFM properties that saturates at ±4.5% when | μ0H | > 2.2 T, and equals 0 at μ0H = 0 T. Moreover, we find abnormal negative photocurrent phenomenon at higher bias in both monolayer and multilayer CrI3. We try to unveil the origin of this phenomenon by investigating the tunneling current mechanism in CrI3, together with the temperature dependent measurement. Our work reveals the interplay between magnetic and optoelectronic properties in CrI3 and paves the way to developing spin-optoelectronic devices.

## Methods

### Device fabrication

First, few-layer graphene, CrI3, and hBN (10–30 nm) flakes were exfoliated onto polydimethylsiloxane (PDMS) substrates in an argon gas-filled glove box with <0.1 ppm concentration of oxygen and water. Then, the hBN encapsulated graphene-CrI3-graphene heterostructure was assembled layer-by-layer on a Si/SiO2 substrate in the same glove box with the dry transferring method using the PDMS substrate as stamp under the help of a microscope attached with a micro-manipulator. The two graphene electrodes were contacted to the pre-prepared Cr/Au (5/25 nm) electrodes for measuring purpose. The thicknesses of the flakes were estimated by optical contrast and confirmed by atomic force microscopy after device measurements.

### RMCD and magneto-optoelectronic measurements

Both RMCD and magneto-optoelectronic measurements were performed in a dry cryostat (attoDRY2100, base temperature of 1.7 K) equipped with a 9 T superconducting magnet and a home tailored microscopic setup in back-scattering geometry. The RMCD is defined as $$({R}_{\sigma +}-{R}_{\sigma -})/({R}_{\sigma +}+{R}_{\sigma -})$$, where $${R}_{\sigma \pm }$$ are the intensities of the $${\sigma }_{\pm }$$ reflected light. For RMCD measurement, the temperature was 2 K, and the device was illuminated by a 633 nm He-Ne laser with a power of 10 μW. The laser was modulated by a chopper and a photoelastic modulator (PEM100), and was focused on the device with a beam diameter ~1 μm by a low-temperature apochromatic objective (LT-APO/VISIR/0.82). The collected light from the sample via the same objective was sent to a photomultiplier tube (PMT1001) equipped with a lock-in amplifier (HF2LI). For magneto-optoelectronic measurement, circularly polarized light was realized by modulating the 633 nm He-Ne laser with a polarizer and a quarter wave plate equipped with a motorized rotation stage. A chopper was employed to switch the light between on and off states with a frequency of 73 Hz. The photocurrent was measured directly by the lock-in amplifier. The IV curves were measured with a Keithley 2636B source/measure unit. The temperature was 2 K and the excitation power was 10 μW, unless otherwise specified.

## Data availability

All relevant data are available from the corresponding author on request.

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## Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 61521004, 61874003, and 62174005, received by L.D.). W.J. acknowledges financial support from the National Natural Science Foundation of China (Grants No. 61761166009 and No. 11974422), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB30000000). Calculations were performed at the Physics Lab of High-Performance Computing of Renmin University of China, Shanghai Supercomputer Center. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (JPMJCR15F3), JST.

## Author information

Authors

### Contributions

L.D. and X.C. conceived the project. X.C. fabricated the devices in collaboration with Z.C. and M.L.; X.C. conducted the measurements; X.C. and L.D. performed data analysis. C.W. and W.J. helped with the theoretical analysis and calculation. P.G. helped with the magnetoresistance measurements. S.Y. grew the CrI3 crystals. Y.L. prepared the metal electrodes. T.T. and K.W. provided the hBN crystals. L.D. supervised this research. X.C. and L.D. wrote the manuscript. All authors contributed to discussions.

### Corresponding author

Correspondence to Lun Dai.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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Cheng, X., Cheng, Z., Wang, C. et al. Light helicity detector based on 2D magnetic semiconductor CrI3. Nat Commun 12, 6874 (2021). https://doi.org/10.1038/s41467-021-27218-3

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