MnBi₂Te₄ Thin-Film Photodetector with a Millisecond Response Speed and Long-Term Air Stability

Abstract

Topological materials with well-defined surfaces and edges have become a prominent research topic. As topological insulators, MnBi₂Te₄ thin films, with their unique surfaces, exhibit exceptional electron transport properties and good applicability in low-noise, high-sensitivity photoelectric detection. This paper reports a straightforward, efficient, and cost-effective thermal evaporation method for preparing quantum MnBi₂Te₄ thin films, along with an investigation into their photoelectric detection performance. These films can be used to fabricate array devices, with the resulting photodetectors achieving a response current of 97 mA W⁻¹ at room temperature and a response speed of <1 ms. Moreover, they demonstrate stability in the air for >30 d, with the photoelectric performance degrading by <15%. Our research introduces a new application for topological materials in photoelectric detection and establishes a strong foundation for the design and development of high-performance photodetectors in the future.

1. Introduction

Photodetectors are devices that convert incoming light into electrical signals, with applications in the aerospace industry, image remote sensing, signal transmission, and various other fields [1,2,3]. Over the past decade, two-dimensional (2D) layered materials have rapidly emerged as the most promising components for cutting-edge detectors owing to their unique crystal structures and material diversity [4,5,6]. Photodetectors based on materials such as graphene, black phosphorus, and 2D transition metal disulfide compounds have been extensively studied owing to their superior gate tunability and light responsiveness [7,8,9]. Additionally, topological insulators with outstanding surface conduction properties have attracted considerable attention in the fields of photoelectric detection and imaging [10,11].

Owing to their unique properties, topological insulators are crucial in the research and development of new electronic devices. To date, the most well-known three-dimensional (3D) topological insulators include bismuth telluride and selenide, among others, which have distinct physical and nonporous gaps [12,13], leading to superior optical absorption capabilities. Currently, the layered material MnBi₂Te₄, which is held together by van der Waals forces, has been theoretically and experimentally confirmed as a topological insulator with an interlayer antiferromagnetic sequence [14,15]. As a material where topological properties and magnetism coexist, MnBi₂Te₄ has a complex structure. This material has exhibited intriguing topological properties in numerous experiments, particularly in thin-film forms [16,17]. However, there have been relatively few reports on the optical properties of MnBi₂Te₄ materials.

Herein, we present a straightforward method for preparing MnBi₂Te₄ thin films using thermal evaporation and investigate their photoelectric detection performance. Testing revealed that the MnBi₂Te₄ thin-film sensor operates stably at room temperature, responds to light across the visible to near-infrared spectrum, exhibits strong air stability, and can potentially be used to prepare array devices. Our findings suggest that MnBi₂Te₄ film is a promising material for photoelectric detection, with potential applications in aerospace, broadband spectral detection, optical imaging, and communication.

2. Experiment

Materials: The MnBi₂Te₄ precursor powder used in this study is a 99.5% pure synthetic powder supplied by Nanjing Muke Nanotechnology Co., Ltd. ( Nanjing China).

Thin-film preparation: The MnBi₂Te₄ thin film in this experiment was prepared using thermal deposition. During the deposition process, the MnBi₂Te₄ thin film was deposited at a rate of 0.03 nm s⁻¹, with a vacuum level of 5 × 10⁻⁴ Pa at a temperature of 350 °C. The electrode, with a thickness of 30 nm, was also prepared using the thermal evaporation method. The channel size of the fabricated device was 200 μm × 200 μm.

Characterization of the thin film’s properties: X-ray diffraction (XRD) was performed using Rigaku SmartLab 9 (Zhongkebaice, Chengdu, China), Raman spectroscopy was conducted with HORIBA Jobin Yvon LabRAM HR Evolution, energy-dispersive X-ray spectroscopy (EDS) was conducted using an OXFORD X-Max, and scanning electron microscopy (SEM) imaging was performed with Thermo Fisher Scientific Apreo 2.

Photoelectric performance characterization: The photoelectric performances, including the photocurrent (I_ph) and current–voltage (I–V) curves, were measured using a Keithley 2636B at room temperature and atmospheric pressure.

3. Results and Discussion

3.1. Morphology and Composition Characterization of the MnBi₂Te₄ Thin Film

In this experiment, MnBi₂Te₄ thin film was prepared by thermally evaporating its precursor powder. To characterize the surface morphology of the MnBi₂Te₄ thin film, SEM was used (Figure 1a,b). Figure 1a, captured at 10,000× magnification, shows that the film’s surface is uniform and dense. For a more detailed analysis, Figure 1b, with a magnification of 100,000×, shows the thin film’s surface in greater detail, indicating a high-quality surface state that supports the successful fabrication of subsequent devices.

To further investigate the properties of the MnBi₂Te₄ thin film, XRD and Raman spectroscopy were performed (Figure 1c,d, respectively). The XRD pattern (Figure 1c) reveals characteristic peaks that correspond to the crystal structure and phase composition of the thin film. Owing to the specific layered structure of MnBi₂Te₄, its XRD pattern exhibits distinct diffraction peaks. The peak near 25 degrees is particularly notable because it is a typical diffraction feature of MnBi₂Te₄, closely associated with its crystal structure. Although Raman spectroscopy does not directly measure magnetism, the magnetic state of MnBi₂Te₄ can influence the vibrational patterns observed in the Raman spectrum. Changes in the magnetic state of MnBi₂Te₄ can alter the intensity, position, or shape of certain characteristic peaks in the Raman spectrum. The peak observed at 530 cm⁻¹ in Figure 1d is likely associated with these magnetic-induced changes in the material’s vibrational properties. The XRD and Raman tests confirm that our MnBi₂Te₄ thin film exhibits excellent crystallization properties, providing a solid foundation for the subsequent fabrication of related devices. The even formation of the film enhances its suitability for studying photoelectric performance. Consequently, we proceeded with the preparation of the device.

To further assess the quality of the MnBi₂Te₄ film, EDS mapping was used to analyze its elemental composition (Figure 2). Figure 2a presents the EDS mapping distribution of the MnBi₂Te₄ thin-film elements, highlighting the presence of Mn, Bi, and Te in the target region. Observed Si is attributed to the Si/SiO₂ substrate. Figure 2b shows the EDS mapping of the test area, confirming the elemental composition of the film. Figure 2c shows the simultaneous distribution of Mn, Bi, and Te in the target region. Figure 2d–f show the individual distributions of Mn, Bi, and Te, respectively. The proportions of Mn, Bi, and Te components are close to 1:2:4, which suggests that the quality of the MnBi₂Te₄ film prepared by the simple evaporation method is suitable for large-scale film production.

3.2. Photoelectric Performance Testing of MnBi₂Te₄ Thin-Film Device

To investigate the photoelectric conversion performance of the MnBi₂Te₄ thin film, linear gold electrodes were deposited onto the film through thermal evaporation (Figure 3a). The current and voltage of the MnBi₂Te₄ thin-film device were measured using a probe station, and the resulting I–V curve is depicted in Figure 3a. The device exhibited photoconductivity characteristics, with the current varying linearly in response to changes in voltage. To evaluate the optoelectronic conversion performance of the device, it was illuminated with a light source of varying wavelengths (450, 650, and 808 nm) controlled by a signal generator. The current changes across the device were measured to assess its photoelectric conversion performance at different wavelengths. Figure 3b–d show the current response curves of the device under 450, 650, and 808 nm light irradiation, respectively. The measurements were performed with a bias voltage of 10 mV and a light source intensity of approximately 0.171 mW cm⁻². The figure shows that under identical conditions, the photocurrents for the three wavelengths were 600, 300, and 200 nA, respectively. Notably, the photocurrent in the visible range is considerably higher than that in the near-infrared range, primarily owing to the material’s differential absorption properties.

For the MnBi₂Te₄ device, the photocurrent originates primarily from the intrinsic photoelectric effect of the MnBi₂Te₄ material. As a narrow bandgap semiconductor with a bandgap of approximately 0.1 eV [14,15], MnBi₂Te₄ is responsive to light in the range of 450–808 nm. When exposed to light, the photon energy of the incident light exceeds the material’s bandgap, exciting electrons from the valence band to the conduction band and creating electron–hole pairs. These charge carriers are then separated by an electric field, generating a photocurrent. Additionally, as a topological insulator, MnBi₂Te₄ exhibits unique topological surface states that can considerably influence its photoelectric properties. In the photoelectric effect, carriers from these surface states may contribute to the formation of the photocurrent [16]. When MnBi₂Te₄ is prepared as a thin film, the impact of its topological effects is enhanced, potentially leading to a greater contribution to the overall photocurrent.

3.3. Air Stability Testing and Photoelectric Conversion Performance Calculation of MnBi₂Te₄ Thin-Film Device

To thoroughly evaluate the stability of the device, we varied the voltage across the device while maintaining consistent conditions. As depicted in Figure 4a, we tested the device under illumination from light sources of different wavelengths (450, 650, and 808 nm), with the light density kept constant at 0.171 mW cm⁻². We observed that the photocurrent in all three wavelength ranges increased as the voltage applied across the device was increased. By calculating the average values and including error bars, we observed that the photocurrent variation in the device remained stable, with fluctuations of <5% across multiple tests. The photocurrent increase was positively correlated with the voltage applied across the device. The MnBi₂Te₄ device’s air stability is primarily attributed to the inherent stability of the material itself, which can be reliably maintained in the air over extended periods [17].

We then examined the device’s stability in air without any protective measures under standard temperature and pressure conditions (Figure 4b). The overall performance of the device remained stable over time, though a noticeable decline was observed on the 10th day. The maximum change in current across the two bands was 15%. Additionally, we demonstrated that the device’s performance was not considerably affected by exposure to air, establishing a solid foundation for future research.

Current responsivity (R_i) measures the optical-to-electrical conversion efficiency of a photodetector and is a key indicator of photoelectric device performance [18,19,20]. To evaluate this for the MnBi₂Te₄ thin-film device, we extensively analyzed its responsiveness. Figure 5a,b show the response curves of the device under various bias voltages and light power densities at different wavelengths. We calculated the R_i of the MnBi₂Te₄ thin-film device using the formula:

$$R_i={\displaystyle \frac{I_{\mathrm{p}\mathrm{h}}}{p}}$$

(1)

where I_ph is the photocurrent, and p is the incident laser power density [21,22,23].

The R_i curves of the photodetector were derived using the abovementioned formula. The maximum responsivity achieved was 97 mA W⁻¹ at 450 nm. Figure 4b shows that R_i decreases as the incident power density increases. This reduction in photocurrent response with higher light intensity is attributed to the photogating effect in the device.

Additionally, response time is a crucial indicator of the photoelectric conversion performance of optoelectronic devices [22,24]. Figure 6a,b show the response time curves of the MnBi₂Te₄ thin-film device at 650 and 808 nm. Notably, the rise (τ_on) and fall times (τ_off) for the device were less than 970 and 890 μs at 650 nm and less than 870 and 1120 μs at 808 nm.

To evaluate the performance of the MnBi₂Te₄ thin-film devices, we reviewed published literature on similar devices and compiled comparative results in

Table 1. Comparison of photoelectric performances between the devices in this work and previously reported devices.

Active Materials	Ri (mA/W)	Wavelength Range (nm)	τon/τoff (ms)	Ref.
Bi2Te3	133.2	1064–1550	/	[8]
Bi2O2Se	6500	808	3.2/4.6	[12]
Cd3As2 plate	5.9	532–10,600	/	[24]
Cd3As2 platelets	0.27	1064	/	[25]
Cd3As2 thin film	12.48	450–10,600	150/276	[26]
TaAs crystal	0.7	438–10,290	200/300	[27]
MoTe2	0.4	532–10,600	/	[28]
MoS2	50.7	445–2717	/	[29]
MnBi2Te4	97	450–808	0.9/1.1	This work

. This table contrasts the performance of our MnBi₂Te₄ thin-film device with that of previously reported devices. The comparison reveals that MnBi₂Te₄ thin-film photodetectors have advantages in several aspects, including R_i, wavelength range, and response times (τ_on/τ_off). These benefits underscore the potential of MnBi₂Te₄ thin films for use in array devices and other advanced applications.

4. Conclusions

Herein, we successfully prepared high-quality MnBi₂Te₄ thin films using a simple and cost-effective thermal evaporation method and examined their photoelectric detection performance. The MnBi₂Te₄ thin-film sensor demonstrated an impressive current responsivity of 97 mA W⁻¹ and an exceptionally fast response speed of approximately 1 ms at room temperature, making it a superior choice among similar materials. Additionally, the sensor exhibited excellent air stability, with a photoelectric performance degradation of <15% even after >30 d, ensuring robust long-term reliability for practical applications. The broad spectral response of the MnBi₂Te₄ thin film was further validated by testing the photoelectric conversion performance of the device under various conditions. This demonstrates the material’s versatility for applications in broadband spectrum detection, imaging, and communication. Additionally, the successful fabrication of 1 × 3 linear devices indicates that MnBi₂Te₄ thin films can be used to create array devices, establishing a solid foundation for their application in integrated optoelectronics.