Hybrid 2D/3D Graphitic Carbon Nitride‐Based High‐Temperature Position‐Sensitive Detector

Ultraviolet position‐sensitive detectors (PSDs) are expected to undergo harsh environments, such as high temperatures, for a wide variety of applications in military, civilian, and aerospace. However, no report on relevant PSDs operating at high temperatures can be found up to now. Herein, we design a new 2D/3D graphitic carbon nitride (g‐C3N4)/gallium nitride (GaN) hybrid heterojunction to construct the ultraviolet high‐temperature‐resistant PSD. The g‐C3N4/GaN PSD exhibits a high position sensitivity of 355 mV mm−1, a rise/fall response time of 1.7/2.3 ms, and a nonlinearity of 0.5% at room temperature. The ultralow formation energy of −0.917 eV atom−1 has been obtained via the thermodynamic phase stability calculations, which endows g‐C3N4 with robust stability against heat. By merits of the strong built‐in electric field of the 2D/3D hybrid heterojunction and robust thermo‐stability of g‐C3N4, the g‐C3N4/GaN PSD delivers an excellent position sensitivity and angle detection nonlinearity of 315 mV mm−1 and 1.4%, respectively, with high repeatability at a high temperature up to 700 K, outperforming most of the other counterparts and even commercial silicon‐based devices. This work unveils the high‐temperature PSD, and pioneers a new path to constructing g‐C3N4‐based harsh‐environment‐tolerant optoelectronic devices.

temperatures, especially in the scenarios of UV light detection such as aerospace, missile tracking, fire alarm, corona detection, etc. [27][28][29][30] Therefore, UV PSDs operating at high temperature is urgently needed.Previous studies have demonstrated that the aromatic heterocyclic structure of g-C 3 N 4 makes it highly stable even at 800 K. [31,32] Hence, the wafer-scale, large bandgap, good homogeneity, and high-temperature resistance g-C 3 N 4 films are ideal candidates for UV detectors working at high temperatures.
In this work, PSDs operating at high temperatures above 400 K are demonstrated for the first time, which are fabricated from a 2D/3D g-C 3 N 4 /gallium nitride (GaN) heterojunction.These devices exhibit a response region of 300-430 nm, a high position sensitivity of 355 mV mm −1 , a rise/fall response rate of 1.7/2.3ms, and a nonlinearity of 0.5% at room temperature.By merits of the strong built-in electric field of the 2D/3D hybrid heterojunction and robust thermostability of g-C 3 N 4 , the g-C 3 N 4 -based PSD can operate under a high-temperature environment up to 700 K.At 700 K, the PSD exhibits a high position sensitivity of 315 mV mm −1 , accurate angle detection with a low nonlinearity of 1.4% and high repeatability with no LPV decrease for 30 cycles, even having never been demonstrated in the commercially available silicon-based devices.This work realizes the high-temperature UV PSDs for the first time and paves the way for research on optoelectronic devices for harsh environments.

Results and Discussion
Since the nucleation and subsequent film growth during the VTC process is rather substrate-tolerant, g-C 3 N 4 films (2.7 eV) can be deposited on various wide-bandgap semiconductors, such as GaN (3.4 eV), diamond (5.5 eV), and β-Ga 2 O 3 (4.8eV) (see Figure S1).Among these semiconductors, GaN has a wide bandgap, high thermal conductivity, high-temperature stability, and high carrier drift rate. [33,34]Therefore, g-C 3 N 4 /GaN heterojunction is preferentially considered to construct UV-light-sensitive high-temperature PSDs.As shown in Figure 1a, g-C 3 N 4 with a tri-s-triazine structure is grown on substrates by a VTC method utilizing the melamine source.The g-C 3 N 4 with the tri-striazine structure is recognized as the most stable phase of carbon nitride among all their allotropes, and its interlayers are stacked via the weak van der Waals force. [35,36]The micrograph of the g-C 3 N 4 grown on a GaN substrate is presented in Figure 1b.The middle area is bare GaN caused by scratching the continuous g-C 3 N 4 films via a tweezer.The AFM image shows that the thickness of the g-C 3 N 4 film grown on the GaN substrate is about 70 nm in Figure 1c.The photoluminescence (PL) mapping image of g-C 3 N 4 deposited on GaN is acquired in Figure 1d, ranging from 450 to 600 nm under 325 nm excitation.The pixel intensity contrast of the mapping image is no more than 0.7%, which indicates the high uniformity of the as-grown g-C 3 N 4 .
The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HR-TEM) images of g-C 3 N 4 are shown in Figure 1e,f, respectively.Figure 1e indicates that the g-C 3 N 4 has a typical layered structure.The lattice fringes of 0.32 nm observed in Figure 1f are in accordance with the interlayer spacing of graphitephase carbon nitride structure, implying the high-degree crystallization of g-C 3 N 4 .The high-resolution X-ray photoelectron spectroscopy (XPS) spectra of the C 1 s and N 1 s are shown in Figure 1g,h, respectively.The high-resolution XPS spectrum of C 1 s contains three peaks located at 284.6, 286.6, and 288.3 eV, which can be assigned to surface contamination (C=C), sp 2 -hybridized carbon (C=N), and sp 2 -bonded carbon in the N-containing aromatic ring (N-C=N), respectively. [37]The XPS of N 1 s possesses three peaks at 398.8, 400.2, and 401.2 eV, which can be attributed to sp 2 -hybridized nitrogen involved in the tris-triazine rings (C-N=C), tertiary nitrogen N-(C) 3 groups, and terminal amino groups on the surface (C-N-H), respectively. [14,38]In Figure 1i, the UV-visible absorption spectra of the g-C 3 N 4 /GaN and GaN have cut-off wavelengths of 450 and 385 nm, respectively.And the inset of Figure 1i presents ultrahigh visible transparency and uniformity of the g-C 3 N 4 film on the GaN substrate.
Before beginning the research of position-sensitive performance, the photoelectric properties of g-C 3 N 4 /GaN heterojunction are investigated.As given in Figure 2a, Au and Ni/Au electrodes are deposited on the surface of the g-C 3 N 4 and GaN films, respectively, to form an ohmic contact. [14,39,40][43] The dark I-V curve of g-C 3 N 4 /GaN heterojunction possesses a rejection ratio of 50 at AE2 V in Figure 2b.Under 405-nm illumination with an intensity of 26 mW cm −2 , the heterojunction has an on/off ratio of 2 × 103 .Figure 2c illustrates the response spectra of heterojunctions ranging from 300 to 430 nm under different reverse biases.The peaks observed at 340 and 405 nm, as marked by the arrows in Figure 2c, come from the optical absorption edge of GaN and g-C 3 N 4 (Figure S2), respectively.The responsivity as a function of reverse biases is given in Figure 2d.The responsivity of 340 and 405 nm increases with the biases, and the maximum responsivity of 68 mA W −1 is obtained at a bias of −4 V.
On basis of the benign UV-light responsivity and selectivity, we have further explored the PSD device performance of this g-C 3 N 4 /GaN heterojunction.The schematic diagram of the g-C 3 N 4 /GaN PSD has been shown in Figure 3a.And the corresponding device fabrication process has been described in detail in the schematic diagram of Figure S3.46] The inversion layer is formed by which minority carriers in g-C 3 N 4 and GaN gather in the interface of the g-C 3 N 4 /GaN heterojunction. [17]Previously, many researchers have reported that position sensitivity shows a significant dependence on the thickness (d) of the semiconductor film and the electrode distances (2 L) as shown in the inset of Figure 3b. [23,47,48]The transverse I-V curves of PSD with different g-C 3 N 4 film thicknesses are also given in Figure 3b, which shows that the heterojunction resistance increases with the thickness of g-C 3 N 4 .
When a light spot with a diameter of 0.05 mm illuminates on the g-C 3 N 4 film, the excited electrons will diffuse from the illuminated regions toward the non-illuminated ones due to the electric field gradient, which results in LPV between the two electrodes.In Figure 3c, the position-sensitive characteristic of PSD with different g-C 3 N 4 film thicknesses is provided.The LPV shows an approximately linear dependence when the laser spot moves between the two electrodes on the g-C 3 N 4 surface.The linear relationship can be explained according to onedimensional diffusion theory. [49]And the corresponding equation for LPV is as follows: [47,50] where K is the proportionality coefficient, N 0 is the number of the separated electron-hole pairs per second at light position x, L is the half-distance between the two electrodes, and l is the electron diffusion length in the g-C 3 N 4 .
Position sensitivity is a significant parameter for evaluating the performance of PSD, which can be deduced by: [51] Position sensitivity The relationship of position sensitivity versus the thickness of g-C 3 N 4 films is depicted in Figure 3d.The position sensitivity increases dramatically at first with the increasing thickness of g-C 3 N 4 when the thickness is <70 nm.This phenomenon results from the gradually enhanced built-in field of g-C 3 N 4 /GaN heterojunction with the increasing thickness of g-C 3 N 4 . [52,53]And when the thickness of g-C 3 N 4 films is over 70 nm, the decreasing sensitivity can be ascribed to the greatly increased recombination probability for longer longitudinal diffusion in the g-C 3 N 4 layer. [54]The large response path length is also an important parameter for PSD.Thus the LPV dependence of position under different contact distances ranging from 1 to 8 mm is provided in Figure 3e.As shown in Figure 3f, the position sensitivity diminishes gradually with the electrode distance, which is in accord with Equation (2)   and can be ascribed to the exponential decay of photon-generated carriers when they diffuse toward the electrodes. [55]ore in-depth research of the g-C 3 N 4 /GaN PSD with an electrode distance of 2 mm and a thickness of 70 nm is carried out.The nonlinearity is another important merit of PSDs, which characterizes the error of detection and can be expressed as: [56] Nonlinearity ¼ where X i and X T i are the measured and actual positions, respectively; N represents the measured number of positions, and L is the distance between the two electrodes.As shown in Figure 4a, the LPV exhibits a monotonic linear response with the laser spot position between the two electrodes.And the PSD possesses a position sensitivity of 355 mV mm −1 and nonlinearity of 0.5%.Especially, the nonlinearity of 0.5% is much superior to the commercial requirements (<15%). [56,57]he position-dependent LPV with different light intensities is shown in Figure 4b.All of these curves possess fine linearity.And the light power is also a key factor affecting the position sensitivity apart from the thickness of g-C 3 N 4 films and electrode spacing.As shown in Fig- ure 4c, the position sensitivity increases markedly with light power and then tends to saturate at higher light power, due to increased recombination rates in the carrier. [58,59]Figure 4d shows the response speed of the PSD at a light intensity of 26 mW cm −2 with the light spot on the device at a position of (x = 0.5 mm) in the device.And a fast rise/decay time of 1.7/ 2.3 ms can be obtained.
The working mechanism of g-C 3 N 4 /GaN PSD is provided in Figure S4.As shown in Figure S4a, when a UV light is illuminated on g-C 3 N 4 , the UV light will be absorbed by g-C 3 N 4 and GaN.Photogenerated electrons and holes are generated in g-C 3 N 4 and GaN and then separated by the built-in electric field at the junction. [17]The photogenerated electrons and holes are transmitted to the g-C 3 N 4 and the GaN layer via the built-in electric field, separately. [23]Next, as shown in Figure S4b, the electric-field gradient between the illuminated and the nonilluminated zones results in excess photogenerated electrons diffused from the illuminated spot toward the electrodes, which results in LPVs in the two terminal electrodes. [60]oreover, as a shortboard against the influence of high temperature, the thermodynamic stability of g-C 3 N 4 has been also appraised by the first-principles calculations.Energy Environ.Mater.2024, 7, e12515 α = β = 90°, γ = 120°).The ELF values show there is covalent bonding between C and N atoms.The thermodynamic phase stability calculations (Figure 5b) indicate that g-C 3 N 4 can keep robust stability owing to its ultralow formation energy of −0.917 eV per atom with respect to decomposition into graphite C and nitrogen N 2 .To examine the thermal stability of g-C 3 N 4 carbon at high temperatures, we have performed first-principles molecular dynamics simulations with the canonical (NVT) ensemble at the temperature of 700 K. Figure 5c shows the fluctuations of potential energy as a function of simulation time.After heating at a high temperature (700 K) for 3 ps with a time step of 1 fs, no structure reconstruction is found, except for slight fluctuations of the total energy, which strongly suggests that the g-C 3 N 4 can exist even at high temperatures up to 700 K.
To verify the suitability of g-C 3 N 4 film at the experiment level, the PL of g-C 3 N 4 has been conducted in the temperature range of 300-700 K in Figure 5d.And then the relative PL intensity at each temperature point is plotted in Figure 5e, where relative intensity gradually decreases with increasing temperature due to the temperature-induced fluorescence quenching. [61]As shown in Figure 5f, the original PL performance remains nearly constant in successive 30 heating/cooling cycles, indicating the g-C 3 N 4 has great thermostability.
Due to the excellent thermal stability of g-C 3 N 4 and outstanding device performance in high-temperature environments, the photoelectric properties of g-C 3 N 4 -based PSD under high-temperature conditions are worth exploring.First of all, the high-temperature stability of light detection from the 2D/3D heterojunction is studied in Figure S5.The UV-light responsivity of g-C 3 N 4 /GaN heterojunction increases with temperature, while the on/off ratio decreases with temperature.When the temperature is 700 K, the on/off ratio is still 26, which indicates this device can still be operated normally in this high-temperature environment.In addition, after 200 cycles of testing, the photocurrent of the g-C 3 N 4 /GaN heterojunction device has hardly changed at the temperature of 700 K (Figure S6).As a proof of principle, the g-C 3 N 4based UV PSDs have been applied to a harsh environment, which can be evaluated from the temperature-dependent photoelectric measurement.
In Figure 6a, the curves of LPV are depicted as a function of light spot position under variable temperature conditions ranging from 300 to 700 K.All the curves show a similar linear change, signifying the high adaptability of these g-C 3 N 4 -based UV PSDs in extreme temperatures.To investigate their performance at elevated temperatures, the extracted sensitivity and nonlinearity of the PSD versus temperature are given in Figure 6b.Here, it is worth mentioning that the sensitivity of PSD generally increases up to 460 mV mm −1 with temperature until the temperature reaches 500 K.The enhanced sensitivity originates from the decreased Fermi energy levels of p-GaN with rising temperature. [62,63]The decreased Fermi energy levels of p-GaN give rise to the enhancement of the built-in electric field, which is of positive correlation with the LPV. [15,64]The slightly decreased sensitivity can be attributed to the accelerated recombination rate of carriers when the temperature is above 500 K. [28] Astonishingly, when the device temperature is raised to 700 K, the position sensitivity can maintain 89% of that at room temperature with the value of 315 mV mm −1 .To the best of our knowledge, the position sensitivity of 460/315 mV mm −1 obtained under high-temperature environments of 500/700 K is one of the highest values among previous reports, such as SiC, 2D materials, and emerging perovskite materials, even comparable with the commercially available silicon device (Table S1). [57,65,66]In particular, our PSD device is the only one that can be operated in a high-temperature environment beyond 420 K. Furthermore, the light on/off switching character of PSD in different temperature environments is shown in Figure 6c.It is observed that the response performance of PSD under a wide temperature range from 300 to 700 K is stable and repeatable.Even after 30 cycles of light switching under a high temperature of 700 K, the LPV of the device can still keep 99% of the initial value, determining the extremely stable response performance of the g-C 3 N 4based PSD (Figure 5d).
The angle detection of non-contact sensing based on PSD is a significant application field.The implementation of noncontact UV optical sensing under high-temperature environments is present via an application of precise angle measurement, as illustrated in Figure 5e.The 405 nm light spot is shot onto the mirror and then reflected toward the PSD.When the mirror rotates at a tiny angle of Δθ, a small displacement (ΔL) of the reflected light on the PSD will be generated.And the relationship between Δθ and ΔL follows Equation ( 4): In Figure 5f, Δθ with a range from −1 to 1 degree is measured and the Δθ exhibits a superb linear dependence on ΔL even under the high temperature of 700 K.According to Equation ( 3), the nonlinearity of Δθ under 700 K is only 1.4%, which indicates the PSD can accurately accomplish the angle measurement.And the better nonlinearity of 0.6% can be obtained when the temperature is lower than 500 K in Figure S7.The stable angular detection ability under high temperatures demonstrates that the PSD is significant potential for applications in severe working conditions such as those of astronomy, flame detection, and engine monitoring. [67,68]

Conclusion
In summary, a UV PSD device has been reported with the g-C 3 N 4 / GaN heterojunction, whose response ranges from 300 to 450 nm, high position sensitivity of 355 mV mm −1 , rise/fall response rate of 1.7/2.3ms, and nonlinearity of 0.5% at room temperature.The unique 2D/3D hybrid heterojunction can produce a strong built-in electric field to effectively separate photogenerated electrons and holes transferred from the illuminated spot toward the electrodes.Meanwhile, the thermodynamic calculations from first principles stability demonstrate that g-C 3 N 4 can keep robust stability against heat owing to its ultralow formation energy of −0.917 eV atom −1 .Thus, the thermostable PSD devices can be obtained with the g-C 3 N 4 /GaN heterojunction, exhibiting an ultra-high position sensitivity of 460/ 315 mV mm −1 with high temperatures of 500/700 K, respectively.And the application of accurate angle detection of this PSD under such a high-temperature environment of 700 K can be also demonstrated with a low nonlinearity of only 1.4%.Our work possesses significant implications for the research of optoelectronic devices in harsh environments.

Experimental Section
Fabrication of g-C 3 N 4 /GaN heterojunction photodetectors: The g-C 3 N 4 is directly deposited on p-type GaN by a VTC process in a double-zone tube furnace, which controls the sublimation, transport, and condensation of melamine.
The melamine precursor is placed in the low-temperature zone of 280 °C, while the GaN substrate is placed in the high-temperature zone of 550 °C.Melamine vapor will be transported to the high-temperature zone, and then deposited onto the GaN substrate via the condensation reaction when the double zones reach the target temperature.The argon is used as carrier gas with a flow of 200 sccm.By adjusting the growth time, the thickness of g-C 3 N 4 films can be adjusted.After the growing process is complete, the tube furnace is cooled by natural cooling.The PSD based on g-C 3 N 4 /GaN is prepared by UV photolithography and magnetron sputtering technology. [69,70]aterial characterization and photoelectrical measurements: The surface morphology characteristics of the g-C 3 N 4 are characterized by optical microscopy (BX51M, Olympus), transmission electron microscopy (TEM, JEM-G20, JEOL), and atomic force microscopy (AFM, SmartSPM, AIST).The absorption and transmission spectra are investigated by a UV-Vis spectrophotometer (UH4150, Hitachi).The photoluminescence of the g-C 3 N 4 is carried out by the F-7000 fluorescence spectrophotometer (Hitachi).And the XPS measurement of g-C 3 N 4 is carried by Thermo (250xi, ESCALAB).The photoelectric performances of the PSD are investigated using a semiconductor parameter analyzer (4200-SCS, Keithley), and a laser (405 nm) with a lens is utilized as the light source.In addition, the light source is mounted on a two-axis micropositioner to precisely locate the laser spot on the devices.The spectral response of the PSD was measured using a photoresponse testing system consisting of a 150 W Xe arc lamp, monochromator, chopper, and lock-in amplifier.The oscilloscope (DPO 2024, Tektronix) is utilized to measure the response rate and the heating plate (HP202DN, Strider Instruments) is utilized to provide high-temperature environments.
Theoretical calculations: All first-principles calculations were carried out by utilizing the plane-wave pseudopotential approach within the framework of density functional theory as implemented in the Vienna Ab initio Simulation Package.The projector augmented wave method is adopted with 2s 2 2p 2 for C and 2s 2 2p 3 for N treated as valence electrons.The electronic wave functions were expanded in plane-wave basis sets with the kinetic energy of 520 eV and the Brillouin zone k-point meshes with 2π × 0.02 Å−1 density was used to ensure the energy convergence.The convergence criteria employed for both electronic self-consistent relaxation and ionic relaxation, are set to 10 −5 eV and 10 −2 eV Å−1 for energy and force, respectively.The first-principles molecular dynamics simulations are performed in the canonical (NVT) ensemble with the Nos thermostat with a time step of 1 fs at 700 K and lasting for 3 ps.

Figure 1 .
Figure 1.a) Schematic diagram of 2D g-C 3 N 4 grown on GaN substrates.b) Optical micrograph of the g-C 3 N 4 on the GaN substrate.c) AFM image of the g-C 3 N 4 film on the GaN substrate.d) PL mapping of the g-C 3 N 4 on GaN substrate.e) TEM and f) HR-TEM images of the layered g-C 3 N 4 .High-resolution XPS spectra of g) C 1 s and h) N 1 s of the g-C 3 N 4 .i) UV-visible absorption spectra of a GaN substrate and the g-C 3 N 4 film on a GaN substrate; inset is a photograph of the transparent g-C 3 N 4 /GaN structure.

Figure 2 .
Figure 2. a) Schematic diagram of the g-C 3 N 4 /GaN heterojunction, inset is the corresponding energy band diagram.b) I-V curve of heterojunction under dark and illumination.c) Response spectra of g-C 3 N 4 /GaN heterojunction with different reverse biases.d) The responsivity as a function of reverse bias.

Figure 3 .
Figure 3. a) Schematic diagram of the g-C 3 N 4 /GaN PSD.b) Transverse I-V curves of the PSD, inset is the sectional diagram of PSD.c) Position-dependent LPV for g-C 3 N 4 /GaN PSD with different g-C 3 N 4 thicknesses.d) Position sensitivity versus thicknesses of g-C 3 N 4 .e) LPV as a function of position with different electrode distances.f) Position sensitivity versus electrode distances.
Figure 5a shows the crystal structures and ELF of 2D g-C 3 N 4 .The 2D g-C 3 N 4 can be crystallized in orthorhombic crystal system with the space group of Cmcm (a = 7.13 Å, b = 12.35 Å, c = 7.13 Å, and

Figure 4 .
Figure 4. a) Position-dependent LPV for PSD with a g-C 3 N 4 thickness of 70 nm and an electrode distance of 2 mm.b) Dependence of the LPV on incident power with a 405-nm laser.c) Position sensitivity of the PSD as a function of laser power.d) The time-dependent LPV.

Figure 5 .
Figure 5. a) Electron localization function (ELF) of g-C 3 N 4 .b) The phase stability of g-C 3 N 4 with respect to decomposition into graphite C and nitrogen N 2 .c) The energy fluctuations of g-C 3 N 4 as a function of the molecular dynamic simulation step at 700 K, and the inset shows the snapshot of the g-C 3 N 4 structure.d) Thermal stability measurement of g-C 3 N 4 in the temperature range 300-700 K. e) The PL relative intensity as a function of temperature.f) Thirty heating/cooling cycling tests of the composites at two representative temperature points (300 and 700 K).

Figure 6 .
Figure 6.a) Position-sensitive characteristics under different temperature environments.b) The position sensitivity and nonlinearity as a function of temperature.c) Lateral photovoltaic response under different temperature environments.d) Lateral photovoltaic response with 30 cycles under 700 K. e) Experimental setup for angle detection under high temperature.f) The angle detection of PSD under 700 K.