Enhanced photosensitivity in a hybrid WSe2/2DEG heterojunction using a buried TiO2 photosensitive layer

In this study, we integrated the wide-bandgap material TiO2 as a photosensitive layer with the WSe2/2DEG heterostructure, creating a hybrid WSe2/TiO2/2DEG heterojunction. This hybrid structure significantly improves the device’s photosensitivity, exhibiting a high rectification effect and a switching ratio of 103. The photodetector shows excellent performance, with a responsivity of 0.61 A W−1 and a detectivity of up to 1.1×1011 Jones at 405 nm, along with a very fast photoresponse speed. The buried TiO2 channel allows photogenerated electrons to easily flow through the reduced barrier at the depleted region. This hybrid heterojunction holds promise for the development of high-performance photoelectric devices.


Introduction
Over the past decade, researchers have made significant breakthroughs in the study of two-dimensional (2D) materials, propelling this field to the forefront of materials science [1,2].Since the discovery of graphene in 2004, the family of 2D materials has grown to include graphene, black phosphorus, transition metal dichalcogenides (TMDs), and others [3][4][5].TMDs, in particular, have been found to exhibit excellent optical properties, offering extensive potential for applications in optoelectronics and optical devices [6,7].Photodetectors are among the devices derived from TMDs that convert light signals into electrical signals, finding widespread use in communication, imaging, security, and scientific research.These 2D-based photodetectors can achieve high sensitivity across a broad wavelength range while providing fast response times, low power consumption, high integration, and flexibility, making them a major focus of current optoelectronics research [4,8,9].For example, a wide-bandgap WS 2 /Ge heterojunction photodetector demonstrates a high responsivity of 634.5 mA W −1 and a detectivity of up to 4.3 × 10 11 Jones, with a response ranging from deep ultraviolet (200 nm) to mid-wave infrared (4.6 μm) at room temperature [10].
As research progresses, the heterostructures of two-dimensional materials, held together by van der Waals forces, have led to a range of intriguing phenomena and promising application opportunities.By skillfully combining different two-dimensional materials, it's possible to manipulate electronic structures, optical properties, and magnetic characteristics, resulting in a variety of novel properties that are hard to achieve with traditional materials [11,12].Additionally, stacking in the vertical direction can be accomplished through various preparation methods without the need for lattice matching.In the realm of photodetectors, a 2D stacking system with a wide bandgap and a p-n heterostructure can lead to high photoelectric performance in devices.For instance, a high photoresponsivity (3 A W −1 ) has been observed in the vertical WSe 2 /WS 2 heterostructure [13].Besides, a novel planar 2D heterojunction consisted of 2D TMDs and two-dimensional electron gas (2DEG) represents the perfect rectifying behavior and novel physical properties [14][15][16].However, the typical planar WSe 2 /2DEG heterojunction exhibits a persistent photoconductivity characteristic, which allows the existence of charge carriers with infinite lifespan at specific transition temperatures, achieving photoconversion and storage rather than photoinduction in the device [14].Returned to that band alignment in the depleted region, the p-n interface owns a large barrier and intrinsic unique intrinsic space charge region, which cause the existence of infinite-lifetime photocarriers.Therefore, we want to realize transient photoconductivity for the purpose of high photosensitive detection, we can adjust the heterogeneous structure and introduce a wide bandgap photosensitive layer in the vertical direction to achieve the purpose [17,18].
In this study, we designed and fabricated a hybrid heterojunction of WSe 2 /TiO 2 /2DEG to enhance photoconductivity.In this heterostructure, the horizontal p-n junction is composed of 2D p-WSe 2 and n-2DEG, while the vertical stack consists of a TiO 2 film and a p-n junction interface.By introducing the widebandgap TiO 2 photosensitive layer, we improved the light absorption and emission characteristics of the device due to its excellent transparency and refractive index.Additionally, with an appropriate barrier structure, we optimized the transport performance and photoresponse in the heterojunction.Experimental results show that, with the buried TiO 2 photosensitive layer, the device exhibits transient photoconductivity, with a response rate of 600 mA W −1 and a detectivity of 1.1 × 10 11 Jones, along with an ultra-fast response speed.The photodetector demonstrates broad-spectrum photoresponse from ultraviolet to infrared light at room temperature, with optimal response at a 405 nm wavelength.Moreover, its self-driven feature and low turn-on voltage contribute to ultralow power consumption, making it ideal for various applications.This study provides a novel approach for designing and enhancing high-performance photodetectors for future use.

Methods
The fabrication process for WSe 2 /TiO 2 /2DEG hybrid devices involves several techniques, including Pulsed Laser Deposition (PLD), Mechanical Exfoliation, Photolithography, and Magnetron Sputtering, as depicted in figure 1.The first step is depositing an epitaxial TiO 2 film with a thickness of 50 nm onto a (001)-oriented STO substrate using PLD.The deposition temperature and oxygen pressure are set at 680 °C and 13 Pa, respectively, with an excimer laser operating at a frequency of 3 Hz and a laser power of 1.5 J cm −2 .Next, WSe 2 flakes are mechanically exfoliated from a bulk crystal and transferred onto the newly deposited TiO 2 film.Afterward, metallic platinum (Pt) electrodes are sputtered onto the WSe 2 flakes following photolithography.Ar + ion bombardment is then applied to the other end of the WSe 2 , creating another conductive 2DEG electrode [19].Finally, the remaining photoresist is dissolved and removed in acetone.This process results in the successful fabrication of a hybrid heterojunction of WSe 2 /2DEG with a buried TiO 2 layer.
X-ray Diffraction (XRD) was performed using a Bruker D8 diffractometer to analyze the crystal structure and orientation of the TiO 2 film, scanning across a θ-2θ range from 20°to 80°.The device's planar configuration was visualized through metallographic microscopy with an OLYMPUS BX51, and the film's thickness was measured with an Atomic Force Microscope (AFM, Bruker Multimode 8).To examine the transport properties of the hybrid heterojunction, the electrodes at both ends (one with a Pt electrode and the other with 2DEG) were individually connected to electrical test platforms using a wedge bonder.Photoelectrical measurements were performed with a Keithley 6517b high-resistance meter.Laser sources with wavelengths ranging from 405 to 808 nm were used to measure photoresponse.

Results and discussion
X-ray diffraction (XRD) was conducted to confirm the structure of the TiO 2 epitaxial films.Figure 2(a) shows distinct diffraction peaks in the θ-2θ scan corresponding to TiO 2 (002) and substrate STO(00 l) (l = 1,2,3).No other impurity peaks were observed, confirming a high-quality, epitaxial-oriented rutile-phase TiO 2 on the STO substrate [20].The planar morphology of this hybrid heterostructure is displayed in figure 2(b), revealing a well-  Figure 3(a) shows typical diode behavior from the dark I-V characteristics, with a consistent threshold voltage at different biases.Note that the formation of the p-WSe 2 /n-2DEG heterojunction in our devices results in good rectification.Throughout the entire bias range, the dark leakage current at reverse bias is approximately in the picoampere range, making it negligible.However, the dark leakage current in forward bias increases with voltage, leading to a decrease in the rectification ratio under higher bias conditions [21].The optimal rectification ratio can reach nearly 10 3 at a bias of +4 V (as indicated in table 1), which is used for subsequent photoresponse measurements.To understand the significant role of the wide-bandgap TiO 2 photosensitive layer in device performance, a systematic study on the enhanced photoconductivity was conducted in this hybrid heterojunction.During the measurement, samples were illuminated with light sources of different wavelengths, ensuring the same number of photons was used by calculating and controlling various power densities.Figure 3(b) shows the I-V curves of a hybrid WSe 2 /TiO 2 /2DEG heterojunction under different wavelengths at room temperature, all demonstrating a typical rectification effect with visible light.Specifically, the highest photocurrent (around mA) and the lowest turn-on voltage (+0.6 V) were observed with 405 nm light.The inset reveals that the maximum photocurrent at the same bias is inversely proportional to wavelength, owing to complete absorption within the visible light spectrum in the buried TiO 2 layer [22].
To study the photoresponse of the heterojunction at room temperature, we examined the switching performance of the hybrid heterojunction under different lighting conditions.We analyzed the relationship between photoresponse and wavelength by using a uniform light power density of 100 mW cm −2 with a light ON/OFF interval of 100 s.Across the entire optical wavelength spectrum, the photocurrents increased quickly into a conductive state when exposed to light and reverted to the initial insulative state after the light was blocked, as shown in figure 3(c).The ON/OFF switching ratio, which is detailed in table 2, strengthened as the wavelength decreased, consistent with the earlier results showing maximum photocurrent.Additionally, the optimal switching performance at 405 nm light is due to the suitable bandgap (3.0 eV, rutile TiO 2 ) [23].The photoresponse in this hybrid heterojunction is 2-3 times higher than in planar WSe 2 /2DEG heterojunction, attributed to the added TiO 2 layer with a wider bandgap [24].Subsequently, the 405 nm laser, which is the most responsive, was used with varying power densities to study power density-dependent switching performance, as shown in figure 3(d).As the power density increased, both the photocurrent and the ON/OFF ratio grew larger, driven by the increased generation of photogenerated carriers at higher light intensity [25].This, combined with the previous results, indicates that introducing an epitaxial TiO 2 film as a wide-bandgap photosensitive layer  enhances the device's transparency and refractive index across the spectral range while optimizing light absorption and photocarrier emission in the heterojunction photodetector.Moreover, the buried layer can improve transport behavior and reduce leakage current in the heterojunction by forming a suitable barrier structure, as discussed later.
To understand the fundamental properties of a WSe 2 /2DEG heterojunction with a wide-bandgap photosensitive layer, we conducted a series of measurements and calculations to evaluate key detector parameters such as dark current, photocurrent, responsivity (R), and detectivity (D * ) at room temperature.Using a 405 nm light with a power intensity of 3.6 mW cm −2 , the dark and light I-V curves of the WSe 2 /TiO 2 /2DEG heterojunction detector are compared in figure 4(a).Rectification behavior is observed with and without illumination.Additionally, a non-zero photocurrent is detected at a bias voltage of 0 V, indicating the presence of an internal built-in electric field and the separation of electron-hole pairs at the WSe 2 /2DEG interface.This finding suggests the potential for self-powered and self-driven photoelectric detection in this heterojunction.R and D * are typically used to quantitatively characterize the photoelectric performance of a photodetector.Responsivity is defined as the ratio of photocurrent to incident power density.The formula for responsivity is given as [26]: where, I ph denotes the photocurrent, P in is the incident light power density, and S is the effective area of light illumination.In this case, the effective area of WSe 2 /TiO 2 /2DEG heterojunction is approximately 900 μm 2 .The sensitivity of the photodetector, D * can be determined using the formula [26]: * where e represents the elementary charge, and I dark corresponds to the dark current.According to the above formulas, the values of R and D * at different light intensities are calculated and plotted in figure 4(b).As the light power density increases, the photocurrent decreases, causing the R value to decline due to the reduced exciton recombination rate [27].At the minimum power density of 0.042 mW cm −2 , the device achieves its highest values for R and D * , reaching 0.61 A W −1 and 1.1 × 10 11 Jones, respectively.This indicates that these values are among the highest for photodetectors based on transition metal dichalcogenides (TMDs) [4,28].Response time (τ) indicates the speed at which the photodetector responds as the photocurrent rises or decays to a stable level [29].τ is typically divided into rise time (τ r ) and decay time (τ d ).By calculating the time it takes for the current to increase (or decrease) from 10% to 90% (or from 90% to 10%), the corresponding τ r and τ d can be obtained, as shown in figure 4(c).The stable value for τ r and τ d are determined to be 70 and 80 ms, respectively.Unlike the persistent photoconductivity observed in van der Waals heterojunctions (seen in figure 4(d)-(f)) [15], the transient photoconductivity in the hybrid heterojunction is due to the increased photogenerated electron-hole pairs from the buried TiO 2 layer, which accelerates their separation.This leads to a significant improvement in the rectification, photoelectric response, and photoresponse time of the hybrid heterojunction, as outlined in table 3. We aim to analyze the underlying mechanism behind the enhanced photoresponse in the hybrid heterojunction, based on specific energy-band diagrams.The role of the wide-bandgap photosensitive TiO 2 layer has been systematically investigated under zero bias and forward bias conditions.And Reactive ion etching of Ar + affects the barrier height in semiconductor devices, mainly due to changes in the surface stoichiometric ratio, particularly the decrease in oxygen concentration [30].Etching removes oxygen and introducs oxygen vacancies on the surface, leading to the formation of Ti 3+ and 2DEG on the surface of STO.This process results in n-type electronic conduction at the surface of the STO substrate.
At zero bias, the contact between p-WSe 2 and n-2DEG forms a depletion region in the hybrid heterojunction.Figure 5(a) shows the equilibrium band alignment of the hybrid heterojunction in the dark, indicating a type II band alignment.Under illumination, photogenerated electron-hole pairs in the depletion region can be separated by the built-in electric field.Subsequently, electrons are transferred to the p-WSe 2 region and collected by the Pt electrode, while holes flow into the n-2DEG region, as illustrated in figure 5(b).Thus, the energy-band configuration at zero bias in the hybrid heterojunction is similar to that in the planar heterojunction.
At forward bias, the buried TiO 2 film provides a tunneling channel in the vertical direction, forming a band alignment of p-WSe 2 /TiO 2 /n-2DEG, as illustrated in figure 5(c).Thanks to the medium work function of TiO 2 , the barrier height at the depletion region is further reduced (from 0.9 eV to 0.7 eV), allowing electrons and holes to diffuse more easily through this region.In the dark, the Schottky barriers at the WSe 2 /TiO 2 (0.70 eV) and TiO 2 /2DEG (0.71 eV) interfaces suppress the dark current, explaining the lower dark current observed in figure 4(a).Under light illumination, photogenerated electron-hole pairs in WSe 2 and TiO 2 are separated within the TiO 2 layer, where free electrons can more readily transfer to the p-WSe 2 region [31].As the power density increases, more photogenerated carriers are excited, leading to an increase in the forward current.This suggests that TiO 2 plays a crucial role in efficient charge carrier extraction.
Furthermore, compared to pure planar heterojunctions, the added TiO 2 layer provides better control of transparency and refractive index across the spectral range, impacting light absorption and transmission characteristics.This contributes to improved light absorption and photocarrier emission performance in the heterojunction photodetector [32].Additionally, incorporating TiO 2 as the photosensitive layer enhances carrier transport properties in the heterojunction, affecting the quantity of carriers and their mobility across the energy band [33,34].

Conclusion
In summary, we fabricated the WSe 2 /2DEG interface and included a photosensitive TiO 2 thin film to create a hybrid heterojunction.At room temperature, the device exhibits significant current rectification characteristics, with a peak value of 1.5 μA and a photo-dark current switching ratio of up to 10 2 .Regarding photodetection, the WSe 2 /TiO 2 /2DEG heterojunction displays excellent photoelectric performance at 405 nm, with a responsivity of 0.61 A W −1 , a detectivity of 1.1×10 11 Jones, and millisecond photoresponse times.The introduction of the wide-bandgap photosensitive TiO 2 thin film not only extends the wavelength range of heterojunction detection but also optimizes electron emission and carrier transport characteristics, leading to a significant enhancement in the device's photoelectric performance.This approach, which combines a wide-bandgap dielectric layer with a two-dimensional material-based heterojunction, offers a promising pathway for designing and improving high-performance photodetectors in the future.
Figure 2(c) depicts a current-voltage (I-V) curve for the WSe 2 /TiO 2 /2DEG heterojunction under ±4 V bias, demonstrating excellent rectification at room temperature, with a maximum photocurrent in the microampere range under forward bias and no current under reverse bias.The inset in figure 2(c) illustrates the photoresponse of the hybrid heterojunction under light illumination at room temperature, showing consistent on/off switching.

Figure 2 .
Figure 2. (a) XRD θ-2θ scanning image of TiO 2 film grown on STO(001) substrate.(b) The photographic image for a hybrid WSe 2 / TiO 2 /2DEG heterojunction.(c) I-V curve of the heterojunction at room temperature, the inset shows a typical photoresponse switching.(d) Bandgap fitting of the TiO 2 thin film.

Figure 3 .
Figure 3. (a) I-V curves of the heterojunction at +2 V-+10 V. (b) I-V curves of WSe 2 /TiO 2 /2DEG heterojunction at different optical wavelengths, the inset shows the variation of highest photocurrent at each wavelength.(c) Photoelectric response of WSe 2 /TiO 2 / 2DEG heterojunction at various wavelength with the same power density, at a bias voltage of +4 V. (d) Photoelectric response of WSe 2 /TiO 2 /2DEG heterojunction irradiated with different power densities at 405 nm light.

Figure 4 .
Figure 4. (a) Light and dark current curves of WSe 2 /TiO 2 /2DEG heterojunction at room temperature.(b) Response rate and detection rate of the heterojunction photodetector as a function of light intensity.(c) Decay and Rise response time of the heterojunction detector are calculated from a complete periodic signal.Comparison of the (d) I-V curves, (e) photoelectric response and (f) response time between WSe 2 /TiO 2 /2DEG and WSe 2 /2DEG.And the inset illustrates the variation of dark current in two heterojunctions.

Figure 5 .
Figure 5.At zero bias, energy-band diagram of a hybrid heterogeneous under (a) dark and (b) light conditions.(c) Forward biased energy-band diagram under light condition.

Table 1 .
Rectification ratio of heterojunction at a range of 2 V-10 V.

Table 2 .
The switching ratio of heterojunction under different wavelengths of light.

Table 3 .
Comparative results in the photosensitivity of two heterojunctions.