Introduction

Currently, ultraviolet (UV) detection technology has become a highly significant research topic. The UV light in the UVC range (200–280 nm) is referred to as solar-blind UV[1], as it is absorbed by ozone, water vapor, and other substances. Consequently, solar-blind UV radiation is virtually absent on the Earth’s surface. The solar-blind UV detection technology has shown broad application prospects in numerous areas such as secure communication [2] and ultraviolet astronomy. AlGaN, a ternary alloy material composed of GaN and AlN [3], exhibits material characteristics that lie between the two [4, 5]. The bandgap width of AlGaN can be continuously adjusted between 3.4 and 6.2 eV by varying the proportion of Al composition. The absorption wavelength range covers the solar-blind UV spectrum. In addition, AlGaN possesses several advantages such as high temperature and radiation resistance [6, 7], making it an ideal material for solar-blind UV detectors. However, AlGaN-based photodetectors still face several limitations. For instance, AlGaN material suffers from high defect density and difficulties in p-type doping [8]. While great efforts have been dedicated to address these difficulties, other strategies should also be taken into account for enhancing performance of detectors.

MXene is a typical two-dimensional material composed of several atomic layers of transition metal carbides, nitrides, or carbonitrides, possessing excellent metallic conductivity, flexibility, and transparency [9]. Due to its tunable work function [10] and electronic band structure [11] through artificial design, MXene can be used as a Schottky electrode material [12]. In MXene, each atomic layer is bonded through in-plane covalent bonds, while the interlayer forces are relatively weak. In MXene, each atomic layer is composed of metal carbides or nitride atoms and connected to each other through covalent bonds in the plane. The formation of this covalent bond gives MXene good structural strength and stability [13,14,15]. Compared to intra layer covalent bonds, the interlayer forces of MXene are relatively weak. Interlayer forces are typically contributed by interactions such as van der Waals forces and hydrogen bonds. These interactions are relatively weaker than covalent bonds; therefore, layered structures are prone to phenomena such as detachment and insertion of ions under external forces. As a result, when forming heterostructures [16,17,18] with semiconductors [19, 20], MXene does not introduce defect states, thereby mitigating the Fermi level pinning effect [21]. In addition, the gas sensing mechanism can be modulated by Schottky junctions on two-dimensional materials and semiconductor surfaces [22, 23]. At present, the surface and heterojunction engineering of MXene has been widely used in sensors and other aspects. The rich surface functions of tunable electronic structures of two-dimensional materials have attracted extensive attention [24, 25].

In this work, based on aforementioned discussion, we propose an external modulation method by combining AlGaN with two-dimensional MXene (Ti3C2Tx) to construct Ti3C2Tx/AlGaN heterostructures. It has been shown that Ti3C2Tx undergoes a transition from a metallic material to a semiconductor depending on oxidation, and the performance of Ti3C2Tx/AlGaN detectors could be adjusted accordingly. The effect of oxidation process of Ti3C2Tx on Ti3C2Tx/AlGaN photodetectors has been investigated systematically. The optimized devices exhibited a rise time of 72 ms and a decay time 30 ms, achieving a responsivity of 3000 mA/W under 270 nm UV light irradiation.

Experimental

Preparation of Ti3C2Tx

Ti3C2Tx was prepared by etching Ti3AlC2 in a LiF/HCl system. 0.8 g LiF was dissolved in a 9 M HCl (10 mL) solution. The mixture was continuously stirred for 15 min. Then, 0.5 g of MAX phase Ti3AlC2 powder (98%, particle size 200 mesh, purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd.) was added to the reaction vessel. The mixture was continuously stirred and reacted for 48 h at 40 °C.

Next, the acidic mixture obtained from the reaction was added to dilute HCl (2 M, 10 mL) and centrifuged at 3500 rpm for 1 min for acid washing. This process was repeated to wash away unreacted LiF. The resulting mixture was then repeatedly washed with deionized water and centrifuged until the upper suspension becomes neutral. The bottom precipitate corresponds to the accordion-like multilayered Ti3C2Tx, which was collected by vacuum filtration. The Ti3C2Tx was ultrasonically cleaned for 15 min under an inert gas atmosphere and then centrifuged at 3500 rpm for 30 min.

Preparation of Ti3C2Tx/AlGaN heterojunction

The Ti3C2Tx/AlGaN heterojunction devices were fabricated using the drop-casting method. The n-AlGaN sample used in the experiment was grown on a (0001) sapphire substrate by metal–organic chemical vapor deposition (MOCVD). Firstly, the sapphire substrate was treated with hydrogen gas at 1100 °C. Then, a 500-nm AlN buffer layer was grown at 1225 °C. Subsequently, the GaN template layer was grown on the AlN layer, which included AlGaN/AlGaN strain superlattices (SLs) for dislocation filtering. The growth temperature was 1100 °C and the thickness was 140 nm. Finally, a 500-nm intrinsic AlGaN layer and a 500-nm silicon-doped n-AlGaN layer were grown at 1100 °C [26]. The AlGaN composition in terms of Al and Ga components is 0.55:0.45. A masked polyvinyl chloride (PVC) electrostatic film was applied to the surface of the AlGaN substrate. Then, the prepared Ti3C2Tx suspension was dropped into the gaps reserved by the mask and allowed to air dry naturally. Afterwards, the PVC mask was removed, and the sample was further dried before depositing silver electrodes on the top. We choose Ag as the contact electrode metal material with n-AlGaN for two reasons; first, Ag work function is 4.26 electron volts (eV), the contact surface of the two will form a barrier, under forward bias, Schottky barrier becomes thinner, promoting carrier (electron) injection from the metal into the N-type semiconductor. The injected electrons increase the concentration of electrons in the N-type semiconductor, thereby reducing the generation of dark current. Ag/AlGaN Schottky junction is used to prevent reverse leakage current and to reduce dark current generation by injecting carriers, which improves response speed and reduces noise level of the device. The energy band diagram of Ag/AlGaN junction is shown in Fig S8. On the other hand, Ag electrode is cheap, so we explore a low-cost method to modify AlGaN surface.

The structure of the Ti3C2Tx/AlGaN photodetector is shown in Fig. 1a. The size of the active region is 0.5 cm × 0.5 cm, and the area of MXene is 0.25 cm × 0.25 cm. Figure 1b shows the Scanning Electron Microscope (SEM) image of clay-like Ti3C2Tx, as depicted in the schematic. The particles observed within the layered structure represent partially oxidized TiO2 particles. As shown in Fig. 1c, the Transmission Electron Microscope (TEM) image and element mapping of the exfoliated layer Ti3C2Tx indicated that layered Ti3C2Tx was successfully prepared.

Fig. 1
figure 1

a Schematic diagram of Ti3C2Tx/AlGaN photodetector structure. b SEM image of multilayer Ti3C2Tx. c TEM image and elemental mapping of layered Ti3C2Tx after sonication exfoliation

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Results and discussion

The Ti3C2Tx colloidal solution was incubated and continuously stirred in a 60 ℃ environment. As the incubation and stirring process proceeded, oxidation of Ti3C2Tx occurred. Characterizations were performed on Ti3C2Tx samples with different oxidation times. As shown in TEM images in Fig. 2a–c, it can be observed that Ti3C2Tx exhibits a distinct layered structure when not subjected to incubation and oxidation. After partial oxidation, TiO2 nanoparticles present on the surface of the nanosheets. After 120-h oxidation, the nanosheet structure of Ti3C2Tx is completely transformed into TiO2 nanoparticles. This is because during the oxidation process, the surface Ti atoms first combine with O2 molecules in the environment, and then the intermediate layer Ti atoms diffuse to the outer layer and combine with O2, causing the layered structure to be completely distorted, resulting in the formation of particulate TiO2 and carbon substrates with C–C bonds [27, 28]. SEM images in Fig. 2d and e show the Ti3C2Tx coated on the surface of AlGaN before and after complete oxidation, respectively. Figures S1 a-b) depict atomic force microscopy images of Ti3C2Tx MXene drop casting on silicon wafer surfaces. Before drip casting, Ti3C2Tx is sonicated to form a clear layered structure in the image. Figures S2 a-b) show AFM images obtained after 42 h of oxidation with a significant increase in surface titanium dioxide particles. Figure. S3 a-b) show AFM images obtained after 120 h of oxidation where the lamellar structure has been completely lost and the surface appears granular. Different oxidation times of Ti3C2Tx were coated on glass substrates, and their crystal structure was characterized by X-ray diffractometer (XRD) in Fig. 2f. After 60-h incubation, peaks corresponding to the anatase and rutile phases of TiO2 are observed in the XRD spectrum, indicating partial oxidation. After 120-h incubation, there is a significant oxidation of MXene as evidenced by the disappearance of the (002) peak, while the peaks corresponding to the anatase and rutile phases of TiO2 remain. The TEM images and XRD patterns are highly correlated, providing evidence that as the oxidation of Ti3C2Tx progresses, the nanosheets gradually transform into TiO2 nanoparticles. This is also verified by the UV–Vis absorption spectra. Figure 2g displays the UV–Vis absorption spectra of Ti3C2Tx at various oxidation degrees, with the corresponding samples shown in the inset [29, 30]. The MXene colloid solution appears dark black without incubation and oxidation. After 60-h oxidation, partial oxidation of Ti3C2Tx occurs, resulting in a lighter color. After 120-h oxidation, most of the Ti3C2Tx has been oxidized, and the colloid solution turns gray. The optical bandgap of Ti3C2Tx is further analyzed using UV–Vis absorption spectroscopy, as shown in Fig. 2h. The optical bandgap of Ti3C2Tx was estimated to be highly consistent after moderate and complete oxidation, with Eg ~ 3.1 eV.

Fig. 2
figure 2

a–c TEM images of Ti3C2Tx MXene colloidal solution without oxidation, 60-h oxidation, and 120-h oxidation, respectively. d SEM image of the detector surface coated with Ti3C2Tx without oxidation. e SEM image of the detector surface coated with 120 h oxidized Ti3C2Tx. f XRD patterns of Ti3C2Tx MXene at different oxidation degrees. g UV–visible absorption spectra of Ti3C2Tx e at different oxidation degrees, and the inset is corresponding photos. h Optical bandgaps of Ti3C2Tx at different oxidation levels

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To validate the metallic properties of Ti3C2Tx, we spin-coated non-oxidized Ti3C2Tx onto a glass substrate and deposited Ag electrodes on one end. Linear sweep voltammetry (LSV) curves were measured, as shown in Fig. 3a. The LSV curve of Ti3C2Tx with Ag exhibits a clear linear relationship, indicating the metallic characteristics of non-oxidized MXene. The optoelectronic characteristics of the device were measured in both dark and under 270 nm ultraviolet light. The device exhibits distinct rectification characteristics, indicating that it is a well-performing diode-like structure. Under UV light irradiation, the I-V values of the device are higher than those under dark conditions, demonstrating its high sensitivity to UV light.

Fig. 3
figure 3

a The Ag/Ti3C2Tx Ohmic contact exhibits linear I-V characteristics. b The I-V curves of the photodetector under illumination at a wavelength of 270 nm and a light power of 3.5 mW/cm2 for oxidation times ranging from 0 to 120 h. c The photocurrent trend of the photodetector under the illumination of 3.5 mW/cm2 and 270 nm wavelength with 5 V bias voltage applied. d The I-V curves of the photodetector for the cases of 120-h oxidation. e The logarithmic I-V curves of the photodetector’s photocurrent for different oxidation times. f The photodetector’s photodetection response at different oxidation times

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In the Ti3C2Tx/AlGaN heterojunction, Ti3C2Tx not only forms a Schottky contact with AlGaN, but also serves as a transparent layer and a conductive layer [31, 32]. In order to investigate the effect of oxidation degree of Ti3C2Tx on the photodetection performance of the device, Ti3C2Tx with varying oxidation times was spin-coated onto the surface of AlGaN. Figure S4 shows the light–dark current curves of Ti3C2Tx MXene/n-AlGaN photodetectors with different oxidation times. Photocurrent variation trend of Ti3C2 Tx MXene/n-AlGaN photodetector with different oxidation time (logarithmic function coordinate) is shown in Figure S5. As shown in Fig. 3b and c, the photocurrent keeps increasing in the range of 0 to 42-h oxidation. After 42 h, both the photocurrent and dark current gradually decrease with further increase in oxidation time, ultimately reaching a stable state after 120 h without any further changes. This indicates that the resistance of the Ti3C2Tx/AlGaN device decreases initially and then increases as the oxidation degree of the Ti3C2Tx surface deepens. Figure 3d shows the photocurrent-to-dark current ratio of the device after 120 h of oxidation, with a light source of 270 nm ultraviolet light and an intensity of 3.5 mW/cm2. The responsivity curve of Ti3C2Tx MXene/n-AlGaN photodetectors after 120-h oxidation measured at different wavelengths is shown in Figure S5. Figure 3e depicts the I-V curves of the device at different oxidation times plotted on a logarithmic scale. As shown in Fig. 3f, the photodetection response of the device also initially increases and then decreases with oxidation time, reaching its peak at 42 h.

Since 42-h oxidization achieved the best performance, the Ti3C2Tx/AlGaN photodiode fabricated based on 42 h oxidized Ti3C2Tx is used for further investigation. The I-T curves of the Ti3C2Tx/AlGaN photodiode under different UV light powers at a bias voltage of 2 V are shown in Fig. 4a. When the device is subjected to UV light, the photoelectric current rapidly increases and reaches its maximum. When the UV light is turned off, the current drops sharply. Response time is also an important parameter in the performance of a photodetector. The rise time Tr of the photodetector device is defined as the time required for the photoelectric current to rise from 10% of the dark current to 90% of the photoelectric current. Conversely, the recovery time Td is defined as the time required for the photoelectric current to drop from 90 to 10% of the dark current. As shown in Fig. 4b, by enlarging one response cycle in the I-T curve of the device, we can determine the rise/recovery time of the device. It was found that the rise time is 72 ms and the recovery time is 30 ms. As shown in Fig. 4c, after 100 cycles, the photodetection performance remained stable.

Fig. 4
figure 4

a The i-t characteristics of the photodetector under different light power densities and 2 V bias. b The single cycle of the i-t curve. c The i-t curve of 100 cycles

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An explanation for the potential mechanism of the Ti3C2Tx/AlGaN photodetector is proposed. The energy band diagram of the Ti3C2Tx/AlGaN interface is shown in Fig. 5. As a metal carbide, Ti3C2Tx can form a Schottky contact with n-AlGaN. AlGaN material is a ternary alloy of GaN and AlN, and its bandgap varies with the change of Al composition. The bandgap can be estimated by the following formula [26]:

Fig. 5
figure 5

Band diagrams of the Ti3C2Tx/AlGaN device. a, b Band diagrams of Ti3C2Tx/n-AlGaN Schottky junction in equilibrium and under UV light illumination. c, d Band diagrams of TiO2/n-AlGaN heterojunction in equilibrium and under applied forward bias voltage

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$${\text{E}}_{\text{g}}{}^{\text{AlGaN}}=x{\text{E}}_{\text{g}}{}^{\text{AlN}}+\text{(}1-x{\text{)E}}_{\text{g}}{}^{\text{GaN}}-bx\text{(}1-x\text{)}$$
(1)

Here, x represents the Al composition in AlGaN. EgAlGaN, EgGaN, and EgAlN represent the bandgap widths of AlGaN, GaN, and AlN, respectively. b is the curvature factor, which was experimentally determined to be 1 eV. In this experiment, n-doped Al0.55Ga0.45N was used, with a bandgap of 4.59 eV. When Ti3C2Tx is not oxidized, its work function is 5.35 eV [33]. Experimental calculations indicate that after complete oxidation, Ti3C2Tx exhibits semiconductor properties with a bandgap of 3.1 eV.

When the Ti3C2Tx/n-AlGaN heterojunction structure is in equilibrium, the band diagram of the Ti3C2Tx/n-AlGaN Schottky junction is shown in Fig. 5a. The work function of Ti3C2Tx is 5.35 eV. In the Ti3C2Tx/n-AlGaN heterojunction structure, electrons will flow from heavily doped n-AlGaN to Ti3C2Tx, generating a depletion layer and establishing an internal built-in potential V1. When the device is under ultraviolet illumination, as shown in Fig. 5b, electrons absorb the energy from photons and transition to the conduction band, resulting in an increase in the Fermi level and the generation of the built-in potential V0. Consequently, the internal potential V1 is weakened by the photogenerated potential V0, resulting in a difference of q(V1-V0) between the Fermi levels of Ti3C2Tx and n-AlGaN under ultraviolet illumination. Therefore, this would reduce Vbi and would facilitate electron current from AlGaN to MXene. This photoexcitation process leads to an increase in the electron concentration in the conduction band, thereby enhancing the responsiveness of the photodetector[34].

Figure 5c illustrates the band diagrams of n-AlGaN and TiO2 under equilibrium conditions and under forward bias. TiO2 exhibits non-stoichiometric defects where the chemical formula can be written as TiO2-x. In this case, Ti3+ replaces Ti4+ resulting in oxygen vacancies, causing the electron concentration to be higher than the hole concentration in the charge carriers [35]. The schematic diagram of the energy bands before and after contact between TiO2 and n-AlGaN, as well as after applying bias voltage, is shown in Fig. S7. This macroscopically manifests as n-type doping, leading to the formation of a heterojunction with n-AlGaN. After complete oxidation of Ti3C2Tx to TiO2, the conduction band and valence band of TiO2 are located at − 4.40 eV and − 7.50 eV, respectively. The n-type behavior of both materials has been confirmed through Hall effect measurements. The Hall effect measurement of n-AlGaN indicates a charge carrier density on the order of 1017/cm3, while the TiO2 film formed from the oxidation of Ti3C2Tx exhibits a charge carrier density on the order of 1011/cm3. Due to the higher electron density on the n-AlGaN side compared to the TiO2 side, electrons will diffuse from the AlGaN thin film to the TiO2 side until the Fermi level aligns. This results in the formation of an electron depletion region on the AlGaN side and an electron accumulation region on the TiO2 side. The neutral depletion width formed by the electron depletion region and electron accumulation region contributes to the formation of the built-in potential V1 for charge carrier transport, as depicted in Fig. 5d. When a forward bias is applied to both ends of the n–n heterojunction relative to TiO2, electrons are injected from the TiO2 side to the AlGaN side due to drift current. As a result, the height of the barrier decreases by eV0. This causes a decrease in the height of the barrier by eV0.

Ti3C2Tx is a material with metallic properties, and its work function is approximately − 5.35 eV. When Ti3C2Tx undergoes partial oxidation, the resulting TiO2 has an increased work function. As a result, the built-in potential of the Ti3C2Tx/AlGaN interface increases, causing the depletion layer to widen. This widening of the depletion layer is beneficial for electron transport [36,37,38]. Simultaneously, compared to the unoxidized and fully layered Ti3C2Tx, the presence of TiO2 causes an expansion of the interlayer spacing in the Ti3C2Tx film, making the film surface more uniform and increasing the specific surface area. The TiO2/Ti3C2Tx structure provides channels for ion/electron transport, resulting in increased current in the device during partial oxidation of Ti3C2Tx. As the oxidation time increases, Ti3C2Tx is completely oxidized, with most of its two-dimensional structure replaced by TiO2 nanoparticles. Its metallic properties are completely lost, and the single-layer structure deteriorates, leading to a decrease in conductivity and a reduction in current in the device.

Conclusion

In summary, we have prepared the layered Ti3C2Tx material using an etching method, and a Ti3C2Tx/n-AlGaN photodiode structure for photodetection was fabricated using a simple drop-casting method. The Schottky junction formed in this structure effectively enhances the device’s photoresponsiveness. As Ti3C2Tx gradually oxidizes to TiO2 in the air, the photodetector exhibits an initial increase followed by a decrease in its photoresponse. However, when Ti3C2Tx is highly oxidized, it almost completely loses its layered structure and undergoes a transition from metal to semiconductor properties, resulting in decreased conductivity and degraded device performance. This work provides a certain experimental foundation for the subsequent construction of TiO2/Ti3C2Tx hybrid heterojunction photodetectors. The research findings suggest that the oxidation of Ti3C2Tx improves its energy level alignment with AlGaN, leading to suppressed carrier recombination. However, deep oxidation results in the loss of its layered structure, leading to a gradual decrease in the device’s photoresponse. This work provides a foundation for future development of TiO2/Ti3C2Tx hybrid heterostructure photodetectors. By controlling the degree of oxidation of MXene, it is possible to maximize the advantages of two-dimensional layered materials for designing more efficient day-blind photodetectors.