SnS2 thin film with in-situ and controllable Sb doping via atomic layer deposition for optoelectronic applications

SnS2 stands out as a highly promising two-dimensional material with significant potential for applications in the field of electronics. Numerous attempts have been undertaken to modulate the physical properties of SnS2 by doping with various metal ions. Here, we deposited a series of Sb-doped SnS2 via atomic layer deposition (ALD) super-cycle process and compared its crystallinity, composition, and optical properties to those of pristine SnS2. We found that the increase in the concentration of Sb is accompanied by a gradual reduction in the Sn and S binding energies. The work function is increased upon Sb doping from 4.32 eV (SnS2) to 4.75 eV (Sb-doped SnS2 with 9:1 ratio). When integrated into photodetectors, the Sb-doped SnS2 showed improved performances, demonstrating increased peak photoresponsivity values from 19.5 A/W to 27.8 A/W at 405 nm, accompanied by an improvement in response speed. These results offer valuable insights into next-generation optoelectronic applications based on SnS2.


Introduction
Two-dimensional (2D) materials have recently garnered significant attention from researchers as potential candidates for next-generation electronics.These materials are characterized by strong covalent bonds within each atom forming one layer in the plane and relatively weak van der Waals bonds between layers cross plane.This unique feature allows 2D materials to be readily separated from bulk forms into monolayers or a few layers.Since the exfoliated monolayer graphene was introduced by Andre Geim and Konstantin Novoselov, [1] enormous research field has been opened about 2D material.In addition to graphene, numerous other 2D materials have been discovered and investigated such as hexagonal boron nitride (h-BN), black phosphorus (BP), and transition metal dichalcogenides (TMDCs) etc. [2][3][4][5][6][7] Among these, TMDC materials with the chemical formula MX2 (M = Mo, W, Sn, V etc, and X = S, Se, and Te), have been vigorously studied due to its favorable band gap properties, excellent physical, and electrical characteristics. [8,9]2][13] As one of the 2D TMDCs semiconductors, SnS2 has exhibited excellent performance in various applications, including lithium-ion batteries, photocatalysts, field-effect transistors, photodetectors, and more. [14,15][18][19] The measured field effect mobility in the SnS2 thin film transistor is reported to be ~ 50 cm 2 /Vs, with on/off current ratios reaching ~10 6 . [20,21]The self-driven photodetector devices based on SnS2/Si heterostructure exhibited a high responsivity of 0.12 A/W and detectivity of 9.35×10 10 Jones. [19]In addition, there are many related reports to tailor its electrical and magnetic performances by chemical doping such as Cu, Cr, Fe, In, Zn, Y, etc. [22][23][24][25][26][27] For instance, the introduction of indium (In) doping in monolayer SnS2 successfully modified its carrier type from n-type to p-type.
Additionally, the incorporation of ethylenediaminetetraacetic acid into SnS2 led to clear improvements in both mobility and the on/off ratio of the transistors. [28,29]Also, Sb-doped SnS2 has been reported because of the merit of similarity of the ionic radius between Sb (Sb 3+ : 0.90 Å and Sb 5+ : 0.74 Å) and of Sn 4+ (0.83 Å). [25,[30][31][32] Such similarity could result in enhanced lattice matching, thereby bolstering electronic transport performance.However, these approaches are limited either to theoretical simulations or are relatively impractical due to challenges related to low-temperature processing, scalability, and precise thickness controllability.
ALD offers advantages such as low-temperature synthesis and the ability to produce uniform and fully covered thin films, which sets it apart from other synthesis methods. [33,34]The separated precursor introduction and self-limiting process offer not only precise thickness control but also controllable doping by super-cycle during synthesis. [35]We analyzed the crystallinity, chemical bonding state, and optical properties according to Sb concentration in SnS2.After the introduction of Sb doping, systematic changes in material characteristics were observed without compromising the intrinsic nature of SnS2.In addition, Sb-SnS2 were successfully implemented as photodetector devices.The introduction of Sb doping in SnS2 resulted in the creation of a sub-bandgap feature.This sub-bandgap state promoted the recombination rate of photocarriers, which shortened their lifetime and, consequently, enhanced both photoresponsivity and response speed in the device.

Experimental Section
Film preparation: Before deposition, substrates (n-type silicon (Si), thermally grown silicon dioxide (SiO2), and glass) were cleaned with acetone, isopropanol, and deionized water sonication for 10 minutes each and dried using a nitrogen gun.UV-O3 treatment for 10 minutes was followed to remove organic contaminants.The pristine SnS2 and Sb-SnS2 thin films were deposited using a homemade hot-wall ALD reactor.Tetrakis(dimethylamido)tin (IV) (TDMASn), H2S (3 % in argon), and tris(dimethylamido)antimony (III) (TDMASb) were used as Sn, S, and Sb precursor sources, respectively.Purified nitrogen gas was used as carrier gas.The reactor temperature was maintained at 85 ℃ during deposition.TDMASn and TDMASb were kept at 50 ℃ and 40 ℃, respectively.The schematic of one complete ALD cycle of the SnS2 process is shown in Figure 1a.One ALD cycle of SnS2 consists of the TDMASn pulse 1.5 s, exposure 15 s, purge 15 s and H2S pulse 0.5 s, exposure 15 s, purge 15 s.The precursor introduction steps are separated by N2 purge so that the self-limiting reactions can be produced.A GPC of 1.8 Å /cycle at deposition temperature of 85 ℃ was obtained for SnS2 in amorphous phase.Figure 1b shows the thin film schematic of pristine SnS2 and Sb-SnS2 thin films before and after introducing super-cycle process.S1 in Supporting Information.Since all of the as-deposited thin films were amorphous, postannealing process at 300 ℃ for 90 minutes under sulfur ambient was performed to improve crystallinity.
Characterization: Thin film thickness was measured using an ellipsometer (Sentech Instrument GmbH).The crystallinity was analyzed using Grazing incidence X-ray diffraction (GI-XRD, with Co Κα radiation, D8 advance, Bruker) and Raman spectroscopy using 532 nm laser excitation (T64000, HORIBA).Surface morphologies were obtained using field emission scanning electron microscopy (Sigma 300-ZEISS FESEM).The chemical compositions and bonding states were characterized using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi by Thermo Scientific).XPS measurements were carried out using an XR6 monochromated Al Kα source (hν = 1486.6eV) and a pass energy of 20 eV.Ultraviolet photoemission spectroscopy (UPS, ESCALAB 250Xi by Thermo Scientific) was used to analyze the work function and valence band energy.As for the optical properties, ultraviolet-visible (UV-Vis) spectroscopy was used on thin films deposited on glass substrates (U-3900 spectrometer).

Photodetector fabrication:
The pristine SnS2 and Sb-SnS2 thin films were deposited onto a SiO2 (100 nm)/Si substrate.Subsequently, a standard lithography process (Laser writer μPG 101 Heidelberg Instruments GmbH and Sputter coater TORR CRC622) was employed to fabricate Cr (10 nm)/Au (90 nm) electrodes.The electrical characteristics of the photodetector device were measured using a Keithley 2450 source meter and probe station under various light wavelengths.
The response speed of the devices was assessed using an oscilloscope (Tektronix MDO3102).
Theoretical Calculations: Theoretical calculations were conducted using the Vienna Ab initio Simulation Package (VASP), employing the principles of density functional theory (DFT).
The Perdew−Burke−Ernzerhof formulation within the generalized gradient approximation was employed to account for the exchange-correlation potential.Interactions between core and valence electrons were modeled using the projector-augmented wave (PAW) method with a plane-wave basis set truncated at a cutoff energy of 500 eV.Convergence criteria for electronic relaxation were set to 10 −5 eV for energies and 0.01 eV/Å for forces.Brillouin zone sampling was performed using a 5 × 5 × 6 Monkhorst-Pack k-point mesh, centered at the Γ-point, for both structural optimization and subsequent energy evaluations.

Results and Discussion
Figure 2a shows GI-XRD patterns of pristine SnS2 and Sb-SnS2 thin films.A single peak appeared at 2θ = 17.2° for all samples which can be assigned as a 2H-SnS2 (001) plane hexagonal structure (JCPDS: 23-677).There were no traces of new phases that suggest Sb atoms were well incorporated into the SnS2 main phase without the formation of any Sb-based compounds.The intensity of Sb-SnS2 (001) peak was reduced relative to that of pristine SnS2, indicating that the replacement of Sn by Sb resulted in lower crystallinity than pristine SnS2. [26]Figure 2b shows Raman spectra of all samples with one strong peak near 315 cm -1 characteristic for SnS2 and assigned to its Sn-S out-of-plane vibration mode with A1g symmetry.After introducing Sb atom into SnS2, shoulder features appeared at lower frequencies adjacent to the main A1g peak, and the latter showed a slight softening, which is more apparent in the samples with higher content of Sb (SnS2:Sb = 19:1 and 9:1).These changes can be attributed to a decrease of the local symmetry caused by the Sb replacing the Sn sites and higher mass of Sb. [25] Similar observations have been made in other 2D alloy studies. [36,37]Figure 2c shows the SEM image of the surface morphology of 9:1 Sb-SnS2.The surface morphology for all the samples (Figure S1, Supporting Information) were platelet-like grains similar to the reported SnS2 thin film. [38]o investigate the chemical state of the element within pristine SnS2 and Sb-SnS2 thin films, the XPS was performed (Figure 2d-2f and Figure S2).The C 1s state (284.8eV) was used as a reference to calibrate the binding energy for all presented XPS spectra.Figure 2d shows Sn 3d state of each sample.For pristine SnS2, Sn 3d3/2 and 3d5/2 peaks were located at 495.6 eV and 487.2 eV, respectively.As the concentration of Sb dopant increased in the SnS2 samples, the binding energies of Sn 3d3/2 and 3d5/2 peaks exhibited a shift to lower binding energies.These values changed from 495.5 eV and 487.1 eV in the 99:1 ratio to 495.2 eV and 486.8 eV in the 9:1 ratio, with stepwise decrements of -0.1 eV, remaining the distance between two peaks at 8.4 eV for all samples.These values are in good agreement with the previously reported SnS2 compound. [39,40]Figure 2e shows S 2p state of each sample.Likewise, a decrease in the binding energies of the S 2p doublet was observed as the Sb concentration increased.This shift can be interpreted by considering a shift in the Fermi level of the samples, which decreased as the Sb was introduced into the SnS2 matrix. [41]gure 2f shows Sb 3d state in SnS2 matrix.For pristine SnS2, there was no observable Sb peak.
However, in the Sb-SnS2 thin films, the intensity of Sb 3d3/2 and 3d5/2 peaks gradually increased as the Sb concentration increased from 99:1 to 9:1.The peak positions are located in the range of 539.5 eV -539.9 eV for Sb 3d3/2 and 530.2 eV -530.6 eV for Sb 3d5/2, indicating the existence of the Sb 3+ chemical state.It is generally considered that Sb 3+ ion replaces Sn 4+ lattice site forming substitution doping.Thus, the replacement of Sn 4+ by Sb 3+ introduced an acceptor energy level near the valence band maximum, which was derived from a vacancy adjacent to an anion.The observed shift in the Sn and S binding energy peaks is a consequence of the increased concentration of acceptor impurities resulting from the formation of acceptor levels. [42,43]

Figure 3. (a) UPS spectra (left) and close-up of highest occupied molecular orbital (HOMO) region (right). (b) UV-Vis transmittance spectra. (c) Tauc plot for optical band gap energy of pristine SnS2 and Sb-SnS2 in different ratios (The inset shows close-up on photon energy intercept).
To characterize the band properties of ALD grown pristine SnS2 and Sb-SnS2 thin films, UPS and UV-Vis spectroscopy were carried out.Figure 3a  extracted by Tauc plot using the following equation: where α is an absorption coefficient, A is a constant and Eg is the optical band gap.SnS2 is an indirect band gap semiconductor and has no indirect to direct transition, so n = ½ was used which implies indirect allowed transition. [44]The extracted optical band gap for pristine SnS2 was 2.17 [47] Comparatively, the indirect bandgap energy for Sb-doped SnS2 with a 9:1 ratio was 2.11 eV, which represented the minimum bandgap energy observed in the Sb-SnS2 thin films.The optoelectronic properties of the pristine SnS2 and 9:1 Sb-SnS2 thin films were also investigated, as shown in Figure 4. Figure 4a shows the device schematic of the photodetector.
Figure 4b and 4c shows the current-voltage (I-V) characteristics of fabricated photodetector devices as a function of light density (LD) ranging from dark to 83.76 mW/cm 2 at 405 nm wavelength.The photocurrent consistently increased with increasing light power and showed significant enhancement for Sb-SnS2 device.The responsivity (R) of the photodetector can be calculated according to the following equation: [48]  =  ph  •  (3)   where  ph is the photocurrent, P is the incident light density and A is the effective illuminated area, which is about 0.3 mm 2 in this work.Figure 4d shows the obtained responsivity of devices with increasing light power under a bias voltage of 10 V at 405 nm.The peak R value of 19.5 A/W was obtained for the pristine SnS2 photodetector device.After Sb-doping employed, the R value was obviously improved to 27.8 A/W.It can be seen that R decreased progressively with the increasing light power density.This could be attributed to an excess photogeneration of carriers at highincident power levels, possibly leading to an increased rate of the Auger recombination process and therefore a reduction in the photocurrent. [49]Meanwhile, both devices could be ideally switched between "ON" and "OFF" states by periodically activating and deactivating the light (Figure 4e and 4f).The photocurrent of a pristine SnS2 device at 405 nm with a light power density of 83.76 mW/cm 2 was 58.02 μA (Figure 4e).In contrast, a high photocurrent of 85.23 μA was achieved at the same light power density for the Sb-SnS2 photodetector (Figure 4f).
The photoresponse speed of the photodetector devices is shown in Figure 4g.The rise (trise) and fall times (tfall) were computed from the time taken for the photocurrent to increase from 10% to 90% of the peak value and vice versa.The extracted trise and tfall were 322 and 302 μs for pristine SnS2 and 242 and 233 μs for Sb-SnS2, respectively.The faster response speed in Sb-SnS2 device benefited from enhanced charge electric efficiency by Sb as a sub-bandgap in a vertical direction. [50]bsequently, the photoresponse behaviors at different wavelengths were carried out from 405 nm to 980 nm.Both devices exhibited stable, fast, and distinct switching behaviors, suggesting rapid electron-hole pair generation and recombination activities.The Sb-SnS2 photodetector device exhibited higher photocurrent than that of pristine SnS2 (Figures 4h-4i).For example, the photocurrents of 23.97 and 41.24 μA were obtained at 450 nm with a light power density of 35.81 mW/cm 2 for pristine SnS2 and Sb-SnS2 devices, respectively.The band structures of pristine SnS2 and 9:1 Sb-SnS2 were simulated using Density Functional Theory (DFT), as shown in Figure 5a and 5b, respectively.The relative sizes the green, orange, and blue circles represent the atomic contributions from Sn, S, and Sb, respectively.
The calculated indirect bandgap for SnS2 is 2.18 eV, and for Sb-SnS2, it is 2.09 eV, which is consistent with the results obtained from Tauc plot calculation (Figure 3c).The composite band structure results originate from multiple factors.It is a result of the overlap of the individual energy bands of the main phase of SnS2 and the Sb doping.Additionally, subtle structural effects, such as charge redistribution among the different atomic constituents, contribute to the overall band structure of the material.These complex interactions led to the observed band structure in the composite material.The diagram illustrating the photosensing behavior of SnS2-based photodetectors is presented in Figure 5c.Due to the doping of Sb, the intrinsic Sn vacancies in SnS2 were filled with Sb atoms, which can introduce a variety of defects and potentially create trap states. [51]These localized defect states aided in trapping electrons, enabling Sb-SnS2 to absorb photons with lower energy than the intrinsic compound.Therefore, the wavelength corresponding to the peak photocurrent extended from 532 nm, characteristic of pristine SnS2, to 635 nm for the Sb-SnS2 device (Figure 4h-i). [52]Additionally, the doping of Sb in SnS2 has introduced a subbandgap state.Usually, the extended lifetime of photocarriers hinders the response speed because of residual carriers persisting when the light illumination is toggled.However, the Sb-doping resulted in sub-bandgap states within the intrinsic bandgap of SnS2, leading to an increased recombination rate of photocarriers and, consequently, a reduction in photocarrier lifetime.This reduction in photocarrier lifetime led to faster response rates in Sb-SnS2 sample compared to pristine SnS2.

Conclusion
In this work, Sb-doped SnS2 thin films deposited by ALD were studied.The SbSx one cycle introduction into different numbers of n (n = 99, 49, 19, and 9) cycles of SnS2 ALD process showed systematic doping effect and resulted in work function, Fermi level, and optical band gap modulation.Even after Sb doping into the SnS2, the main phase of SnS2 remained without forming any new Sb based compound phase.As the Sb concentration increased, the shift to lower chemical binding energies was observed, implying the Fermi level lowering due to Sb 3+ which is a wellknown p-type dopant.The improved optoelectronic performances were studied by fabricating photodetector using 9:1 Sb-doped SnS2, which showed the lowest band gap value compared to pristine SnS2.The photocurrent was doubled up and the highest photoresponsivity was increased from 19.5 A/W to 27.8 A/W at 405 nm, possessing fast photoresponse speed 242 μs of trise and 233 μs of tfall owing to sub-band formation after Sb doping.These results confirm Sb-doped SnS2 can be a great candidate for optoelectronic applications.

Figure
Figure 1c depicts the step recipe of ALD super-cycle.For the purpose of depositing Sb-SnS2 thin

Figure 1 .
Figure 1.(a) ALD process schematic for one complete SnS2 cycle.(b) Sb-SnS2 thin film schematic before and after super-cycle process.(c) Step recipe of super-cycle of SnS2 and SbSx.
shows UPS spectra (left) and close-up of the highest occupied molecular orbital (HOMO) region (right) results.The 0 eV in the binding energy equals the Fermi energy level.The work function of pristine SnS2 and Sb-SnS2 can be calculated using the following equation:  = ℎ − |  −   | (1) where  is the work function, ℎ is the photon energy of helium source 21.22 eV and Ecutoff can be extracted by extrapolation of UPS spectra in high binding energy.Therefore, due to the reduction of the Fermi level, the calculated work functions using the Ecutoff values for the pristine SnS2 and Sb-SnS2 in the 99:1, 49:1, 19:1, and 9:1 ratio samples were determined to be 4.32, 4.79, 4.84, 4.59 and 4.75 eV, respectively.In a similar manner, the valence band edge energy of each sample can be extracted with Figure 3a right represented as EHOMO.As the Sb concentration increased, the valence band edge energy gradually decreased to 1.79 eV, 1.37 eV, 1.30 eV, 1.25 eV, and 1.24 eV, implying that the Fermi level approached to valence band edge.These UPS results suggest a trend consistent with the p-doping effect of Sb shown in XPS results presented in Figure 2d-2f.Figure 3b shows the transmittance versus wavelength of each sample measured by UV-Vis.The transmittance systematically decreased with an increase in Sb concentration.In addition, an absorption band near 750 nm was also observed in 19:1 and 9:1 Sb-SnS2 samples due to the Sb dopant state.Figure 3c represents the optical band gap of pristine SnS2 and Sb-SnS2 thin films

Figure 5 .
Figure 5. DFT simulation and photogeneration mechanism of SnS2 based photodetectors.Energy band structure of (a) pristine SnS2 and (b) 9:1 Sb-SnS2 thin films.showing the relative contribution of each element.(c) The operating mechanism of photoexcitation for SnS2 photodetectors before and after Sb doping.