Self-powered MoTe₂ homojunction photodetector with ultrafast response via h-BN encapsulation and doping regulation

Kangwei Shao^a, Haiyan Nan*^a, Renxian Qi^a, Chenxin Jiang^b, Haomin Wang^b, Zhengjin Weng^a, Jialing Jian^a, Shaoqing Xiao^a and Xiaofeng Gu^a
aEngineering Research Center of IoT Technology Applications (Ministry of Education), School of Integrated Circuits, Jiangnan University, Wuxi 214122, China. E-mail: jnanhaiyan@jiangnan.edu.cn
^bState Key Laboratory of Materials for Integrated Circuits, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China

First published on 15th August 2025

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1. Introduction

Two-dimensional (2D) materials are being considered for future photodetector technologies due to their atomic-scale thickness, strong light–matter interactions, and tunable bandgap properties.^1–4 van der Waals junctions made from 2D materials can create electric fields that help separate and transfer photogenerated charge carriers efficiently,^5–9 leading to improved optoelectronic device performance. These junctions exhibit exceptional performance with ultra-low noise and low power consumption under zero drain bias,^10–13 making them ideal for high-performance photodetectors, solar cells, and photovoltaic devices. This advancement in optoelectronic applications is promising for the development of novel technologies.

Lattice mismatch and transfer processes during fabrication can cause interfacial contamination and defects in heterojunctions, leading to degraded device performance.^14–18 Homojunction devices offer simpler architectures and better fabrication processes compared to van der Waals heterojunctions. Doping modulation is an effective method for constructing homojunctions, allowing for spatial variation of carrier type or density by introducing impurities with different types or concentrations, forming stable p–n junctions.^19–21 This approach preserves lattice continuity, minimizes interfacial defects, and provides excellent controllability and process compatibility for large-area fabrication and integration applications.

The three main doping strategies for 2D materials are surface charge transfer, substitutional doping, and intercalation doping.²² Substitutional doping can cause structural instability and defects,^23–25 while intercalation doping may lead to lattice damage and increased complexity.^26–29 In contrast, surface charge transfer is a simple and effective method, where charge exchange occurs between adsorbed atoms or molecules and the material surface.^30,31 A number of studies have demonstrated the effectiveness of this approach. Nan et al. used defect engineering combined with oxygen chemical adsorption to enhance the photoluminescence of monolayer MoS₂, and obtained the conclusion that oxygen-induced p-type doping and reduced nonradiative recombination at defect sites are responsible for the significant PL enhancement.³² Tongay et al. employed thermal annealing and molecular physisorption of O₂ and H₂O to modulate the photoluminescence of monolayer MoS₂, and concluded that electron depletion caused by charge transfer stabilizes neutral excitons and significantly increases PL intensity.³³ Similarly, Jürgen Ristein, through a comprehensive analysis of surface transfer doping techniques, explored their impact on the electronic properties of semiconductor materials and concluded that surface transfer doping is an effective strategy for precise control of electrical conductivity.³⁴ Deshun Qu et al. demonstrated that controlled oxygen-pressure thermal annealing (RTA) at 250 °C enables continuous carrier-type modulation in MoTe₂ from n-dominant ambipolar to unipolar p-type, where oxygen preferentially passivates tellurium vacancies via Mo–O bond formation, with XPS confirming chemical potential downshift – establishing a defect-engineering paradigm for 2D semiconductor polarity control.³⁵

A partially encapsulated lateral homojunction photodetector was successfully fabricated in this study. The MoTe₂ flake was partially encapsulated with h-BN and then annealed at high temperature in air. The exposed region of MoTe₂ became p-type doped due to adsorption of H₂O and O₂ molecules from the air, while the h-BN-encapsulated region maintained its intrinsic ambipolar behavior. The doping-induced Fermi level difference between the two regions created an internal electric field across the junction interface, enhancing the dissociation of photo-induced carriers and resulting in a high responsivity of 21 mA W⁻¹, a specific detectivity of 1 × 10¹¹ Jones, and a fast response time of 54.6 μs for self-powered photodetection.

2. Results and discussion

2.1. Structural configuration of the MoTe₂ homojunction

A MoTe₂ homojunction photodetector was constructed as shown in Fig. 1a. Contacts were placed on both the exposed and protected parts of the MoTe₂ layer, with the underlying SiO₂/Si serving as the back gate. The fabrication process is illustrated in Fig. S1. A h-BN flake was dry-transferred onto a clean SiO₂/Si substrate, followed by the dry transfer of a MoTe₂ flake onto the h-BN, thus partially encapsulating the MoTe₂ from the bottom. After spin-coating, photolithography was performed using a UV maskless lithography machine (TuoTuo Technology (Suzhou) Co., Ltd), followed by metal deposition and lift-off. A 50 nm-thick Au layer was deposited on both ends of the MoTe₂ flake as metal electrodes. A top h-BN flake was dry-transferred onto the MoTe₂ to complete the top encapsulation. The device was annealed in air at 250 °C for 15 minutes, causing the exposed MoTe₂ region to adsorb H₂O and O₂ molecules, resulting in p-type doping of that region. Fig. S2 demonstrates that MoTe₂ is most sensitive to the annealing temperature of 250 °C, and gradually transitions to p-type conductivity with increasing annealing time. Fig. 1b shows an optical microscopy image of the homojunction channel, with the blue area representing the bottom h-BN, the white region corresponding to MoTe₂, and the yellow area representing the top h-BN. Fig. 1c presents AFM image of the device, providing its height profile and contour. The measured thicknesses of the bottom h-BN, MoTe₂, and top h-BN are 17 nm, 40 nm, and 60 nm, respectively. The AFM topography clearly visualizes the interface region, highlighting the high quality of the semi-encapsulated MoTe₂ homojunction. The Raman spectrum is a powerful tool for evaluating the lattice integrity and defect density of two-dimensional materials. In Fig. 1d, the three main vibrational modes (A_1g at 174.63 cm⁻¹, E^2g₁ at 237.87 cm⁻¹, and B^2g₁ at 291.97 cm⁻¹) from both the encapsulated and exposed regions show highly consistent peak positions, full widths at half maximum (FWHM), and relative intensities, which also match well with the characteristic features reported in ref. 36. If lattice damage had occurred during the annealing or surface adsorption doping processes, it would typically lead to peak shifts, broadening, or the emergence of defect-related peaks. However, no such changes were observed in our measurements. Therefore, the two comparable Raman spectra not only demonstrate the high crystalline quality of the MoTe₂ film but also further confirm that the surface adsorption doping did not cause any structural damage to the crystal. The XPS characterization of MoTe₂ before and after annealing is shown in Fig. S3. The Mo 3d_5/2 peak is located at approximately 228.17 eV, the Te 3d_5/2 peak at around 572.23 eV, and the O 1s peak remains stable near 532.9 eV. These values closely match those reported for pure 2H-phase MoTe₂ in the literature. After annealing, the Mo 3d_5/2 peak shifts toward higher binding energy accompanied by a broadening of the FWHM, indicating partial oxidation of Mo. Meanwhile, the Te 3d_5/2 peak shifts from 572.2 eV to 572.85 eV with a similar increase in FWHM, suggesting partial oxidation of Te to TeO₂.

2.2. Electrical performance of the device

The output characteristics of the encapsulated and exposed regions of MoTe₂ after annealing are shown in Fig. 2a. Encapsulated MoTe₂ retains its original characteristics, forming ohmic contact with the gold electrodes. The current in the exposed MoTe₂ region increases by more than 10 times after annealing. Fig. 2b shows the corresponding transfer characteristics. Encapsulated MoTe₂ maintains its intrinsic bipolar behavior, while the exposed MoTe₂ exhibits degenerate p-type doping with significantly increased hole concentration. By placing the electrodes on the encapsulated and exposed regions of MoTe₂ respectively, a p–n homojunction was successfully fabricated. Fig. S4 presents the transfer and output characteristics, as well as the weak photoresponse of the device before annealing, confirming that no junction was formed prior to annealing and thus no self-powered photodetection capability was observed. Output characteristics under different gate voltages were measured to evaluate the performance of this homojunction, as shown in Fig. 2c. The homojunction exhibits pronounced rectifying behavior under positive gate bias, with a rectification ratio as high as 2.2 × 10³ at V_gs = 40 V. This rectification behavior confirms the formation of a built-in electric field, critical for achieving excellent self-powered optoelectronic performance. Fig. S5 re-evaluates the photoresponse characteristics of the device after three months of storage, revealing no significant degradation in photocurrent under the same illumination intensity. Moreover, over 100 cycles of photoresponse testing were conducted, further demonstrating the excellent stability of the device.

2.3. Optoelectronic performance of the device

Fig. 3a shows the output characteristics of the homojunction device under 637 nm laser illumination at different power levels. The corresponding relationship between photocurrent and applied bias voltage is extracted and presented in Fig. 3b. The curves exhibit a clear shift from the origin, with a high photocurrent of up to 20 μA even at zero bias. In which, the photocurrent I_ph is calculated using the following equation:

Iph = Ilight − Idark	(1)

here, I_light represents the current measured under illumination, while I_dark denotes the dark current. Fig. 3c displays the open-circuit voltage and short-circuit current under 637 nm laser illumination with varying power levels. Both the open-circuit voltage and short-circuit current increase with increasing optical power, further demonstrating the excellent self-powered optoelectronic performance of the device. The optical response curves of different power under 637 nm laser irradiation are shown in Fig. 3d, where the photocurrent increases rapidly with increasing light intensity. Moreover, photoresponsivity (R) serves as a crucial and essential parameter for assessing the photoresponse performance of photodetectors. R is calculated using the following equation:

	(2)

Fig. 3e further presents the self-powered photocurrent and responsivity under 637 nm laser illumination (V_gs = 0 V, V_ds = 0 V) at different optical power levels. The responsivity initially increases with rising optical power and then saturates, reaching a stable value of 16 mA W⁻¹ at a power of 1.13 μW. Fig. 3f shows the variation of specific detectivity with optical power under illumination of different wavelengths (V_gs = 0 V, V_ds = 0 V). The specific detectivity initially increases with increasing optical power and then decreases, reaching a maximum value of 1 × 10¹¹ Jones at an optical power of 1.13 μW. Specific detectivity (D*) is a key parameter used to assess a device's sensitivity to weak optical signals. Considering that the dominant source of background noise originates from the dark current, D* can be calculated using the following expression:

	(3)

In which, e is the charge of an electron, A is the area of the laser spot, and R is the photoresponsivity.³⁷ Fig. 3g presents the photoresponse curves under 447 nm, 637 nm, and 940 nm lasers illumination, all with an optical power of 5 μW. The photocurrents under 447 nm and 637 nm illumination are approximately 80 times higher than that under 940 nm, indicating that the device has a strong detection capability for visible light. In addition, the correlation between I_ph and P_in is described by the power-law function:

Iph ∝ Pαin	(4)

In an ideal photodiode, the photocurrent is linearly proportional to the incident power.³⁸ The fitted α values under different wavelength illuminations are shown in Fig. 3h. Under 447 nm and 637 nm illumination, α is slightly less than 1, while under 940 nm illumination, α reaches 1.2. For visible light, the device exhibits nearly linear photoresponse, indicating minimal carrier recombination and efficient charge separation at the interface, which can be caused by the inherent electric field across the interface. Under 940 nm infrared light illumination, the observed superlinear response (α = 1.2) may result from mechanisms such as trap filling and thermal excitation. Fig. 3i shows the maximum self-powered responsivity and specific detectivity under different wavelength illuminations (V_gs = 0 V, V_ds = 0 V). The device achieves a maximum self-powered responsivity of 21 mA W⁻¹ under 447 nm illumination and a maximum detectivity of 1 × 10¹¹ Jones under 637 nm illumination. The device demonstrates high responsivity and specific detectivity in the visible range, while both metrics drop significantly under near-infrared illumination. This is because visible photons possess enough energy to drive interband transitions across the bandgap of MoTe₂, whereas 940 nm photons (∼1.32 eV) have energies close to or even lower than the indirect bandgap of multilayer MoTe₂, resulting in lower excitation efficiency. The light output curves at different wavelengths, along with the corresponding photocurrent and responsivity, are provided in Fig. S6.

The response time of a photodetector is a key parameter that determines its bandwidth, temporal resolution, and high-speed performance. As shown in Fig. 4a and b, when V_ds = 0 V, the rise time is 54.6 μs and the decay time is 55.2 μs. Fig. 4c displays the device's response under different switching frequencies, further demonstrating its capability to detect ultrafast optical signals and maintain stable operation under high-frequency conditions. Fig. 4d and e show the photocurrent response of the homojunction device under 532 nm laser illumination at modulation frequencies of 2 kHz and 9 kHz, respectively. The device exhibits fast and highly repeatable switching behavior, indicating excellent stability and reliability when processing optical signals at varying frequencies. The cutoff frequency refers to the frequency at which the output signal amplitude drops to 3 dB below its initial value as the pulse laser modulation frequency increases. As shown in Fig. 4f, the photocurrent response as a function of modulation frequency reveals a cutoff frequency of 10 kHz for the device. Table 1 compares the performance of this work with that of state-of-the-art photodetectors, showing that our device demonstrates overall superior performance.

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As illustrated in Fig. 5(a), the optical microscope image features red coordinate axes aligned with the reference system used in the photocurrent mapping analysis. Fig. 5(b) presents the spatially resolved photocurrent mapping image acquired under a 1 V bias voltage. The photocurrent signal exhibits pronounced localization along the periphery of the top h-BN encapsulation layer, as evidenced by the intense red-orange contrast in this region. This spatial distribution suggests the existence of a robust built-in electric field at the top h-BN/MoTe₂ interface, which facilitates efficient separation of photogenerated electron–hole pairs. In stark contrast, the bottom h-BN-encapsulated boundary displays negligible photocurrent response, appearing as a uniform blue-green background in the false-color map. This asymmetry strongly corroborates that adsorbed H₂O and O₂ molecules on the exposed MoTe₂ surface induce Fermi level pinning, resulting in p-type doping. Consequently, a vertical p–n junction forms between the environmentally doped exposed region and the protected h-BN-encapsulated area. The absence of measurable photocurrent at the bottom interface further excludes potential artifacts from substrate-induced carrier scattering or interfacial strain effects, confirming the intrinsic origin of the observed photovoltaic behavior.

2.4. Formation mechanism of the MoTe₂ homojunction

Fig. 6a schematically illustrates the pre-annealing configuration of a partially h-BN encapsulated MoTe₂ device, with its equilibrium energy band diagram shown in Fig. 6c. In this pristine state, the exposed MoTe₂ surface remains free of molecular adsorbates, preserving the material's intrinsic ambipolar transport properties and resulting in flat energy bands (ΔΦ = 0) across the MoTe₂/h-BN heterostructure. Following atmospheric annealing (Fig. 6b), H₂O and O₂ molecules chemisorb onto the exposed MoTe₂ surface, acting as electron acceptors via the redox reaction MoTe₂ + O₂/H₂O → MoTe₂⁺ + e⁻, which lowers the Fermi level, inducing degenerate p-type doping in the exposed region. In contrast, the h-BN-encapsulated area retains intrinsic ambipolarity with E_F near mid-gap. This spatial carrier polarity modulation creates a type-II band alignment at the lateral heterojunction (Fig. 6d), where a Fermi level discontinuity drives band bending, generating a built-in electric field. Under illumination, this field enables efficient photogenerated carrier separation, reduces Schottky barrier asymmetry, and enhances minority carrier extraction.

3. Conclusion

We used a dry transfer technique to partially encapsulate MoTe₂. After annealing in air, H₂O and O₂ molecules were adsorbed onto the unencapsulated region, causing it to exhibit p-type behavior. The encapsulated part of MoTe₂ retained its intrinsic ambipolar characteristics, creating a p–n homojunction. This allowed for the separation of photogenerated carriers and resulted in a device with high responsivity, detectivity, and ultrafast response speed under zero bias. This approach offers a promising method for constructing high-performance 2D homojunction devices through interfacial molecular modulation, with potential applications in flexible optoelectronics, image sensors, imaging systems, and environmental sensing platforms.

4. Experimental section

MoTe₂ and h-BN were both obtained via mechanical exfoliation. MoTe₂ was then transferred onto h-BN using a dry transfer technique. After spin coating, photolithography, metal deposition, and lift-off processes, 50 nm of Au was deposited on both ends of the MoTe₂ as metal electrodes. A maskless UV lithography system (produced by Tuotuo Technology Co., Ltd, Suzhou) was used for the photolithography process. The thickness of the samples was measured using a scanning probe microscope equipped with an AFM module. Raman spectra were acquired using a micro-Raman spectrometer with a 532 nm laser. The photoelectric performance was measured using a Keithley 2634B source meter.

Author contributions

Kangwei Shao: investigation, data curation, writing original draft. Haiyan Nan: investigation, methodology, and formal analysis. Renxian Qi: validation, formal analysis. Chenxin Jiang and Haomin Wang: investigation (characterization), data curation, writing – review & editing. Zhengjin Weng and Jialing Jian: supervision, writing – review & editing. Shaoqing Xiao and Xiaofeng Gu: funding acquisition, supervision.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the SI. Please refer to the supporting information for details on device fabrication, the effect of annealing time on electrical properties, XPS analysis of MoTe₂ before and after annealing, electrical characteristics and self-powered photoresponse of the unannealed device, stability tests, and photoresponse under lasers of different wavelengths. See DOI: https://doi.org/10.1039/d5tc02340h

Acknowledgements

This work is supported by the National Science Foundation under Grants 62074070, 52203356, 62364003 and 62104084, the Natural Science Foundation of Jiangsu Province, China under Grants BK20221534 and BK20221065, the Natural Science Foundation of Jiangxi Province, China under Grants 20224BAB202035, Open Research Fund of State Key Laboratory of Materials for Integrated Circuits (No. SKLJC-K2024-X03).

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