Enhanced Optical Response of SnS/SnS2 Layered Heterostructure

The SnS/SnS2 heterostructure was fabricated by the chemical vapor deposition method. The crystal structure properties of SnS2 and SnS were characterized by X-ray diffraction (XRD) pattern, Raman spectroscopy, and field emission scanning electron microscopy (FESEM). The frequency dependence photoconductivity explores its carrier kinetic decay process. The SnS/SnS2 heterostructure shows that the ratio of short time constant decay process reaches 0.729 with a time constant of 4.3 × 10−4 s. The power-dependent photoresponsivity investigates the mechanism of electron–hole pair recombination. The results indicate that the photoresponsivity of the SnS/SnS2 heterostructure has been increased to 7.31 × 10−3 A/W, representing a significant enhancement of approximately 7 times that of the individual films. The results show the optical response speed has been improved by using the SnS/SnS2 heterostructure. These results indicate an application potential of the layered SnS/SnS2 heterostructure for photodetection. This research provides valuable insights into the preparation of the heterostructure composed of SnS and SnS2, and presents an approach for designing high-performance photodetection devices.


Introduction
Two-dimensional (2D) van der Waals materials have attracted a lot of attention due to their distinct optical and electrical properties [1][2][3][4][5]. Graphene has been the most investigated layered material in recent years due to its specific physical [6,7]. However, graphene is a zero band gap material, which limits its applications in electronic devices. Hence, 2D materials with semiconducting properties are in demand for next generation electronic devices [8]. The 2D layered materials with semiconductor behaviors can realize the thin and flexible requirements for the electronic devices' applications. 2D semiconducting materials such as transition metal dichalcogenides (TMDs) [9], black phosphorus (BP) [10,11], NbOI 2 [12], tellurene [13], SiAs [14], CuInP 2 S 6 [15], and TaNi 2 Te 3 [16] have been discovered as promising candidates for novel electronic devices in the future. Among these 2D layered materials, TMDs are the most studied materials due to their specific dimensional geometrics and distinct physical and optical properties [17,18]. To date, electronic devices fabricated by semiconducting 2D layered TMDs materials with layers such as p-n diode [19], field effect transistor [20], gigahertz frequencies FET [21], Fin shaped FET [22,23], and phototransistors [24] has been achieved. A 1-bit microprocessor with logical operations made by 2D TMDs have also been implemented [25]. SnS 2 is a member of the 2D semiconductor family, and has a hexagonal CdI 2 type crystal structure [26]. In recent studies, SnS 2 has shown an n-type nature and has been shown to be a good candidate for many applications with good performance, such as photodetectors [27][28][29], sensors [30,31], and field-effect transistors [32,33]. However, for the devices' applications, the block potential between n-and p-region heterostructures is an important issue [34], and hence studies on p-type dopants in SnS 2 [35] or p-type SnO thin layers on n-type SnS 2 nanosheets have also been reported [36]. It has been observed that devices made from layered materials using the exfoliation method tend to be smaller in size. The fabrication of large-sized optoelectronic devices is an important issue. SnS 2 and SnS have a high potential use in optoelectronic and photoconductive devices. There are a number of methods for the growth of SnS 2 and SnS films, such as dip coating [37], thermal evaporation [38], and chemical vapor deposition (CVD) [39]. Each method has its advantages and disadvantages; for example, the dip coating and spray pyrolysis methods provide low-cost thin semiconductor films. However, the film's quality and uniformity may not be good [37]. Films grown by thermal evaporation may need post-growth annealing [38]. CVD is a widely used technology for the growth of thin films in which the amount of gas-phase precursors can be controlled by carrier gas, and thin films are deposited on a heated substrate controlled at a stable temperature. CVD offers an advantage by relying on chemical reactions that enable researchers to obtain high-quality and pure phase semiconductor films. Furthermore, CVD does not require high-vacuum environments, making it a popular technology for mass production [39]. Mechanical exfoliation is known as a useful technique to fabricate a monolayer photodetector or heterojunction devices with good performance in nano-dimensions, but it has complications for mass production or applications in solar energy with a large area. The large-area SnS/SnS 2 heterostructure can be produced by CVD methods and paves the way for developing a solar panel on glass substrates which can be used for semi-transparent windows and also can provide power sources in buildings for the purpose of saving energy.
In this study, we adopt SnS as a p-type material and fabricate the SnS/SnS 2 pn heterostructure by chemical vapor deposition methods. Here, we have successfully fabricated a centimeter-sized SnS/SnS 2 pn heterostructure. The potential applications of SnS/SnS 2 pn heterostructures for photodetection are explored. The SnS/SnS 2 heterostructure was characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), and the phase properties were achieved by Raman spectroscopy. Tests of their optical responses, such as absorption and photoresponsivity investigations, were also performed. The results of SnS/SnS 2 heterostructure characterizations were determined and possible mechanisms were also discussed. The study proposes a method to enhance photodetection through the utilization of a SnS/SnS 2 heterostructure.

Materials and Methods
The growth of SnS 2 (CAS:1315-01-1), SnS (CAS:1314-95-0) thin films and the SnS/SnS 2 heterostructure was carried out on glass substrates by the CVD method. The band gap energies of SnS 2 and SnS cover the light spectra from the near-infrared to visible light range. At the same time, tin and sulfur are non-toxic and earth-abundant elements. Considering the impact on the environment and manufacturing cost, SnS 2 and SnS thin films become potential candidates for realizing cheap solar cells. Glass is an optical transparency substrate with strong mechanical strength. For the same reason, glass is a suitable substrate not only for new-generation solar cells, but also for photodetectors. Thus, in this study we chose microscope glass (Matsunami Glass S1111 White Slide Glass) as a substrate. Glass substrates are cleaned by acetone and methanol, then put into an ultrasonic bath for 30 min to remove any contaminants on the glass. After that, we use a nitrogen gun to blow away any dust or residuals on the substrates.
A three-zone horizontal furnace is equipped with three temperature controllers to control the zone temperatures separately. The source materials including tin (II), chloride (SnCl 2 , 1.5 g) (CAS:7772-99-8), and elemental sulfur (CAS:7704-34-9) powder (S, 3.5 g) were placed in crucibles arranged in the center of zone one. Two electric heaters and temperature controllers were used to control the crucibles' temperatures at 280 • C and 180 • C for SnCl 2 and elemental S, respectively, to provide tin atoms and sulfur atoms. The glass substrates were arranged in zone three, about 30 cm away from the source materials. The zone one heater was set at 100 • C, and the zone two and zone three were set at 350 • C and 300 • C for the growth of SnS 2 thin films, while for the growth of SnS thin film the zone temperatures were set at 150 • C and 100 • C, respectively. The carrier gas Ar (CAS:7440-37-1) flow rate was controlled by a mass flow controller and set at 70 sccm. When the zone temperatures reached stable values, the source materials were supplied constantly by heating the crucibles for 20 min. After film growth, the furnace was naturally cooled to room temperature under an Ar flow. For the growth of the SnS/SnS 2 heterostructure, the SnS 2 thin film was prepared in advance and then covered by a shadow mask for the growth of SnS thin films. Thus, the SnS 2 and SnS films were stacked vertically. All the samples were grown to the size of~10 × 10 mm 2 in this study.
The XRD patterns were investigated by using a Rigaku D/max-2200 PC X-ray diffractometer. The morphology and crystal structure were investigated by a Hitachi S-4800 field emission scanning electron microscope (FESEM). The elemental compositions of SnS 2 and SnS films were checked by energy-dispersive X-ray spectroscopy (EDS). The Raman spectroscopies were carried out on a Nanofinder 30 (Tokyo Instruments, Tokyo, Japan) three-dimensional laser Raman spectrometer equipped with a 532 nm laser. The laser power was operated at~1 mW to avoid heating effects. For the measurements of the absorption spectra of the SnS 2 and SnS thin film crystals and to determine their bandgaps, a 1/4 m monochromator (MKS, Irvine, CA, USA) was equipped with a Si photodetector with a sensing range of 1.1~3.1 eV. A 130 W halogen lamp was used to produce the light with a wide photon energy range and the monochromatic light was selected by the monochromator to illuminate the measured sample. The Si photodetector was used to receive the transmitted light.
Photoconductivity (PC) measurements can be taken at different illumination frequencies, illumination intensities, or bias voltages to examine the response-time constants and responsivity of the photosensitive materials. The system consisted of a diode laser (405 nm) controlled by a function generator (GW Instek AFG-2225) to provide an on/off light at different frequencies, and a lock-in amplifier (AMETEK Signal Recovery 7265 DSP Lock-in Amplifier) which was used to record the photo-induced current. A dc bias voltage was provided by a sourcemeter (Keithley 2400) and the photo-induced current was amplified by a low-noise current preamplifier (Stanford Research Systems SR 570) before feeding into the lock-in amplifier. A continuously variable neutral density filter wheel was employed to tune the illumination intensity of the laser diode for the PC measurements taken at different illumination intensities. A schematic optical measurement setup for the measurement of the photoconductivity at different bias voltages, illuminating frequencies or intensities, is shown in Figure 1.  The growth of SnS2 thin films have a preferred orientation in the (001) plane, because {001} plane of SnS2 has the lowest surface energy (g100 = 0.034 eV/Å 2 and g001 = 0.0065 eV/ [42]. In Figure 2a, the XRD pattern of SnS2 thin film shows that the intensities from (00 (100), and (110) are obviously higher than other planes. After forming the SnS2 nucle the new species favorably develop themselves along the [001], [010], and [110] directio due to the anisotropic nature of atomic bounding in SnS2 [43]. The small crystal may gr laterally before merging because of the selective reactions likely to happen at the crys edges. The coalescence of different domains generates grain boundaries composed of d locations. Subsequently, the growth changes into 3D mode, and develops vertically at grain boundaries due to their higher reactivity than in the basal planes. Finally, the ve cally oriented SnS2 nanosheet arrays with very high shape similarity are built which be observed in the SEM image of the SnS2 thin film. SnS crystal has an orthorhombic str ture. It consists of two layers stacked perpendicular to the c-axis, where 'Sn' and 'S' ato are tightly bound in each layer, while the bonding between the layers is of a weak van Waals type. The orthorhombic unit cell is similar to a nanobox, with three different ed lengths a, b, and c, which are at an angle of 90° to each other. The growth mechanism SnS thin film could be simply understood as a block-building task. The SnCl2 and S ato are constantly supplied from different quartz pipes. In the reaction zone, they are mix and chemically transformed into an SnS compound to form the SnS nucleus with a na   Table 1 lists the deduced lattice constants of SnS 2 and SnS from XRD. The deduced lattice constants of SnS 2 and SnS are in a reasonable agreement with the reported literature [40,41]. The growth of SnS 2 thin films have a preferred orientation in the (001) plane, because the {001} plane of SnS 2 has the lowest surface energy (g 100 = 0.034 eV/Å 2 and g 001 = 0.0065 eV/Å 2 ) [42]. In Figure 2a, the XRD pattern of SnS 2 thin film shows that the intensities from (001), (100), and (110) [43]. The small crystal may grow laterally before merging because of the selective reactions likely to happen at the crystal edges. The coalescence of different domains generates grain boundaries composed of dislocations. Subsequently, the growth changes into 3D mode, and develops vertically at the grain boundaries due to their higher reactivity than in the basal planes. Finally, the vertically oriented SnS 2 nanosheet arrays with very high shape similarity are built which can be observed in the SEM image of the SnS 2 thin film. SnS crystal has an orthorhombic structure. It consists of two layers stacked perpendicular to the c-axis, where 'Sn' and 'S' atoms are tightly bound in each layer, while the bonding between the layers is of a weak van der Waals type. The orthorhombic unit cell is similar to a nanobox, with three different edge lengths a, b, and c, which are at an angle of 90 • to each other. The growth mechanism of SnS thin film could be simply understood as a block-building task. The SnCl 2 and S atoms are constantly supplied from different quartz pipes. In the reaction zone, they are mixed and chemically transformed into an SnS compound to form the SnS nucleus with a nanobox shape. The carrier gas blows them toward the deposition zone, where they can be adsorbed on the glass substrate. These orthorhombic nanoboxes are stacked in a preferred orientation along (010) plane. The multiple peaks in the XRD pattern of SnS thin film shown in Figure 2b indicates that the film is polycrystalline and can be indexed based on an orthorhombic SnS (JCPDS 39-0354) cell. It is noteworthy to mention that XRD peaks show the strong intensity of the (111) plane because the lattice planes with lower surface energy tend to dominate the growth mechanism. Here, the more closely packed (111) crystallographic plane is preferred over the others [44].

Results and Discussion
Sensors 2023, 23, x FOR PEER REVIEW 5 of 12 orientation along (010) plane. The multiple peaks in the XRD pattern of SnS thin film shown in Figure 2b indicates that the film is polycrystalline and can be indexed based on an orthorhombic SnS (JCPDS 39-0354) cell. It is noteworthy to mention that XRD peaks show the strong intensity of the (111) plane because the lattice planes with lower surface energy tend to dominate the growth mechanism. Here, the more closely packed (111) crystallographic plane is preferred over the others [44].  Raman spectra of (a) SnS2 and (b) SnS films in the range of 50-500 cm −1 are shown in Figure 3. For SnS2 film, there are two peaks, which were identified as Eg and A1g active modes [45]. For SnS film, the Raman peaks at 181 cm −1 and 210 cm −1 are associated with the Ag modes, while the peak at 152 cm −1 is assigned to B3g mode [46]. An additional mode, at 306 cm −1 , might come from the Sn2S3 phase [47].   Raman spectra of (a) SnS 2 and (b) SnS films in the range of 50-500 cm −1 are shown in Figure 3. For SnS 2 film, there are two peaks, which were identified as E g and A 1g active modes [45]. For SnS film, the Raman peaks at 181 cm −1 and 210 cm −1 are associated with the A g modes, while the peak at 152 cm −1 is assigned to B 3g mode [46]. An additional mode, at 306 cm −1 , might come from the Sn 2 S 3 phase [47]. Figure 4a-c shows the FESEM images of (a) SnS 2 films, (b) SnS films, and (c) the SnS/SnS 2 heterostructure grown on the glass substrate by chemical vapor deposition. As shown in Figure 4a, the SnS 2 film displays a leaf-like shape morphology. The morphology exhibits a hexagonal shape which indicates the typical layered structure of SnS 2 film. Figure 4b shows the FESEM image of SnS film, where the morphology of cluster grains can be observed. The composition of SnS 2 and SnS films were also confirmed by EDS measurement. Figure 4d,e shows the EDS measurement results of SnS 2 and SnS films. In this study, the alloy elemental ratio determined by EDS of SnS 2 film is Sn:S = 35%:65% and for SnS film it is Sn:S = 47%:53%. From the obtained results, it can be shown that the composition of the grown film is in a reasonable agreement with the nominal composition. In Figure 4c, the morphology image at the interface of the SnS/SnS 2 heterostructure is displayed (the schematic diagram of SnS/SnS 2 heterostructure is shown in the inset). From the FESEM images combined with XRD and Raman characterization, we can confirm that the SnS/SnS 2 heterostructure was successfully fabricated.

SnS2
3.628 3.628 5.906 Raman spectra of (a) SnS2 and (b) SnS films in the range of 50-500 cm −1 are shown in Figure 3. For SnS2 film, there are two peaks, which were identified as Eg and A1g active modes [45]. For SnS film, the Raman peaks at 181 cm −1 and 210 cm −1 are associated with the Ag modes, while the peak at 152 cm −1 is assigned to B3g mode [46]. An additional mode, at 306 cm −1 , might come from the Sn2S3 phase [47].   shown in Figure 4a, the SnS2 film displays a leaf-like shape morphology. The morphology exhibits a hexagonal shape which indicates the typical layered structure of SnS2 film. Figure 4b shows the FESEM image of SnS film, where the morphology of cluster grains can be observed. The composition of SnS2 and SnS films were also confirmed by EDS measurement. Figure 4d,e shows the EDS measurement results of SnS2 and SnS films. In this study, the alloy elemental ratio determined by EDS of SnS2 film is Sn:S = 35%:65% and for SnS film it is Sn:S = 47%:53%. From the obtained results, it can be shown that the composition of the grown film is in a reasonable agreement with the nominal composition. In Figure 4c, the morphology image at the interface of the SnS/SnS2 heterostructure is displayed (the schematic diagram of SnS/SnS2 heterostructure is shown in the inset). From the FESEM images combined with XRD and Raman characterization, we can confirm that the SnS/SnS2 heterostructure was successfully fabricated.    Figure 5 shows the plot of the experimental absorption spectra of (a) SnS 2 and (b) SnS films at room temperature. The spectra show an indirect nature of SnS 2 and SnS films in this study. The band gap energies (E g ) for SnS 2 and SnS films can be determined from the plot of the square root of the absorption coefficient versus the photon energy [48]. The obtained band gap energy values of SnS 2 and SnS films at room temperature are 2.24 and 1.20 eV, respectively. The values of the determined band gap in this study are in a reasonable agreement with the reported literature [26,[49][50][51].
In order to study the optical response behavior, we performed frequency dependence photoconductivity experiments on SnS 2 , SnS, and their heterostructure in Figure 6. The frequency dependence of photoconductivity can be described by the relation [52,53]: where I ac is the ac component of the photocurrent, I dc represents the steady state photocurrent, k 1 and k 2 are the amplitude coefficients, and τ 1 and τ 2 are the carrier time constants of two decay processes. The determined parameters are listed in Table 2. The results indicate that the ratio (k 1 ) of the long-time constant decay process of SnS 2 and SnS is 0.31:0.34. However, the ratio (k 1 = 0.27) of the long-time constant decay process is improved in the SnS/SnS 2 heterostructure. The normalized photoconductivity decays to almost zero at frequency~200 Hz for SnS 2 . For SnS, the normalized photoconductivity is around 0.1 at frequency~200 Hz. This is due to the fact that the carrier time constants of the two decay processes of SnS are faster than SnS 2 . For the SnS/SnS 2 heterostructure, the determined carrier time constant is faster than for SnS and SnS 2 films. However, the carrier transport kinetics of the SnS/SnS 2 heterostructure is significantly enhanced, perhaps due to the improved additional trap state in the interface, which causes a shorter time-constant decay process. In order to study the optical response behavior, we performed frequency dependence photoconductivity experiments on SnS2, SnS, and their heterostructure in Figure 6. The frequency dependence of photoconductivity can be described by the relation [52,53]: where Iac is the ac component of the photocurrent, Idc represents the steady state photocurrent, k and k are the amplitude coefficients, and τ and τ are the carrier time constants of two decay processes. The determined parameters are listed in Table 2. The results indicate that the ratio (k ) of the long-time constant decay process of SnS2 and SnS is 0.31:0.34. However, the ratio (k = 0.27) of the long-time constant decay process is improved in the SnS/SnS2 heterostructure. The normalized photoconductivity decays to almost zero at frequency ~200 Hz for SnS2. For SnS, the normalized photoconductivity is around 0.1 at frequency ~200 Hz. This is due to the fact that the carrier time constants of the two decay processes of SnS are faster than SnS2. For the SnS/SnS2 heterostructure, the determined carrier time constant is faster than for SnS and SnS2 films. However, the carrier transport kinetics of the SnS/SnS2 heterostructure is significantly enhanced, perhaps due to the improved additional trap state in the interface, which causes a shorter time-constant decay process. Further studies of the photoelectrical response properties of (a) SnS 2 films, (b) SnS films, and (c) the SnS/SnS 2 heterostructure are shown in Figure 7. The bias voltage-dependent photoresponsivity experiments at the 405 nm excitation light source were measured. As seen in Figure 7, the photoresponsivity increased gradually with the increasing bias voltage. Under a high bias voltage, the electron-hole pairs generated by the excitation light source can be more efficiently separated with an increasing drift velocity, resulting in the high photoresponsivity. The observed linear photoresponsivity depends on the bias voltage, which indicates the ohmic contact between the SnS 2 and SnS films and the SnS/SnS 2 heterostructure. The photoresponsivity of SnS/SnS 2 heterostructure at 10 V is improved several times compared to SnS 2 and SnS films. This might due to the improved interface properties at the SnS/SnS 2 heterostructure. around 0.1 at frequency ~200 Hz. This is due to the fact that the carrier time constan the two decay processes of SnS are faster than SnS2. For the SnS/SnS2 heterostructur determined carrier time constant is faster than for SnS and SnS2 films. However, the ca transport kinetics of the SnS/SnS2 heterostructure is significantly enhanced, perhap to the improved additional trap state in the interface, which causes a shorter time-con decay process.   Further studies of the photoelectrical response properties of (a) Sn films, and (c) the SnS/SnS2 heterostructure are shown in Figure 7. The pendent photoresponsivity experiments at the 405 nm excitation light so ured. As seen in Figure 7, the photoresponsivity increased gradually w bias voltage. Under a high bias voltage, the electron-hole pairs generated light source can be more efficiently separated with an increasing drift v in the high photoresponsivity. The observed linear photoresponsivity de voltage, which indicates the ohmic contact between the SnS2 and Sn SnS/SnS2 heterostructure. The photoresponsivity of SnS/SnS2 heterostruc proved several times compared to SnS2 and SnS films. This might due interface properties at the SnS/SnS2 heterostructure. In Figure 8, the power-dependent photoresponsivity of the (a) SnS2 and the (c) SnS/SnS2 heterostructure at 405 nm under a bias voltage o formed. The responsivity decayed with the increasing illuminated light servation of the reduced responsivity in this study might come from the as defects and charged impurities, in the SnS2, SnS, and SnS/SnS2 hetero is known that the built-in electric field could easily enhance the carriers junction. The enhanced photoresponsivity was attributed to the existence tric field at the SnS/SnS2 heterostructure interface [54]. The stacking SnS/ ture not only improves the photoresponsivity but also benefits the ext sponse range from 1 to 3 eV covering the visible-NIR range. In the co In Figure 8, the power-dependent photoresponsivity of the (a) SnS 2 and (b) SnS films and the (c) SnS/SnS 2 heterostructure at 405 nm under a bias voltage of 10 V were performed. The responsivity decayed with the increasing illuminated light intensity. The observation of the reduced responsivity in this study might come from the trap states, such as defects and charged impurities, in the SnS 2 , SnS, and SnS/SnS 2 heterostructure films. It is known that the built-in electric field could easily enhance the carriers' transport at the junction. The enhanced photoresponsivity was attributed to the existence of a built-in electric field at the SnS/SnS 2 heterostructure interface [54]. The stacking SnS/SnS 2 heterostructure not only improves the photoresponsivity but also benefits the extending of the response range from 1 to 3 eV covering the visible-NIR range. In the condition with low illumination light intensity, the trap states could capture the photo-generated carriers, which avoid the electron-hole pair recombination. As the illumination light intensity becomes higher and higher, most of the electron-hole pair recombination becomes dominated as the number of photo-generated carriers capture by the trap states is limited. Thus, the power-dependent illumination shows the trend of decreased photoresponsivity correlating with the saturation of trap states under a higher light illumination intensity [55][56][57]. Table 3 summarizes the photoresponsivity measured in this work. From the table, we can observe that the photoresponsivity of SnS 2 and SnS are 0.14 × 10 −3 A/W and 0.09 × 10 −3 A/W, respectively, in this work, which is smaller than the reported values of SnS 2 (0.21 × 10 −3 A/W) [58] and SnS (4 × 10 −3 A/W) [59]. This might be due to the larger-scale sample size, which may have more defects, thus lowering the quality of the film. However, the performance could be efficiently improved by the SnS/SnS 2 heterostructure with the photoresponsivitỹ 7.31 × 10 −3 A/W, which might come from the extending broadband response range. The results showed that the photoresponsivity could be enhanced about 7 times by introducing the heterostructure in the present study. It is noticed here that detectivity is also an important characteristic for photodetection devices. However, in this work, our heterostructure is in the size of 10 × 10 mm 2 and the electrode spacing is 2 mm, which is about 100 to 1000 times that of the standard fabrication (2~20 um). The detectivity will be deteriorated due to the sample size dimension. Our main contribution in this work is to propose a largesize SnS/SnS 2 heterostructure fabricated by CVD; we discover that its photoresponsivity can be enhanced by introducing the heterostructure. The further investigation of detectivity characteristic improvement with the SnS/SnS 2 heterostructure could be another valuable study topic.
A/W, respectively, in this work, which is smaller than the reported values o 10 −3 A/W) [58] and SnS (4 × 10 −3 A/W) [59]. This might be due to the larger size, which may have more defects, thus lowering the quality of the film. performance could be efficiently improved by the SnS/SnS2 heterostructure toresponsivity ~7.31 × 10 −3 A/W, which might come from the extending b sponse range. The results showed that the photoresponsivity could be enha times by introducing the heterostructure in the present study. It is noticed tectivity is also an important characteristic for photodetection devices. Ho work, our heterostructure is in the size of 10 × 10 mm 2 and the electrode spa which is about 100 to 1000 times that of the standard fabrication (2~20 um). T will be deteriorated due to the sample size dimension. Our main contributio is to propose a large-size SnS/SnS2 heterostructure fabricated by CVD; we di photoresponsivity can be enhanced by introducing the heterostructure. The tigation of detectivity characteristic improvement with the SnS/SnS2 heterost be another valuable study topic.

Conclusions
In this study, we first demonstrated the growth of a large-area SnS/SnS ture on glass substrates by the chemical vapor deposition method. The film structure were characterized by XRD, Raman spectroscopy, and FESEM tec quency-dependence photoconductivity was used to analyse the carrier tran in SnS/SnS2 heterostructure. The SnS/SnS2 heterostructure demonstrated a constant decay process ratio of 0.729, accompanied by a time constant of 4. improvement of the carrier decay process might be due to the improved ad state in the interface, which causes a shorter time-constant decay process. T pendent photoresponsivity shows that a deteriorated response with increas

Conclusions
In this study, we first demonstrated the growth of a large-area SnS/SnS 2 heterostructure on glass substrates by the chemical vapor deposition method. The films and heterostructure were characterized by XRD, Raman spectroscopy, and FESEM techniques. Frequency-dependence photoconductivity was used to analyse the carrier transport kinetics in SnS/SnS 2 heterostructure. The SnS/SnS 2 heterostructure demonstrated a shorter timeconstant decay process ratio of 0.729, accompanied by a time constant of 4.3 × 10 −4 s. The improvement of the carrier decay process might be due to the improved additional trap state in the interface, which causes a shorter time-constant decay process. The power-dependent photoresponsivity shows that a deteriorated response with increasing illumination intensity can be correlated with the saturation of trap states under a higher light illumination intensity. The results reveal that the photoresponsivity of the SnS/SnS 2 heterostucture has been enhanced to 7.31 × 10 −3 A/W, which is about seven times larger than that of individual films. The findings of this study suggest that the SnS/SnS 2 pn heterostructure exhibits improved photodetection properties, indicating its potential for use in optoelectronic devices. The further improvement of the photoresponsivity performance of SnS/SnS 2 pn heterostructure could be a further research topic.

Data Availability Statement:
The data presented in this study are available in this article.