Modulation of Schottky Barrier Height by Nitrogen Doping and Its Influence on Responsivity of Monolayer MoS2 Photodetector

Monolayer MoS2 flakes are prepared by low‐pressure chemical vapor deposition on p‐type and n‐type silicon substrates and post‐treated under nitrogen (N2)‐rich conditions to incorporate nitrogen atoms in sulfur vacancies. Ultraviolet photoelectron spectroscopy (UPS) shows an increase in work function value by 0.47 eV and 0.53 eV compared to undoped MoS2 when grown on p and n‐type substrates, respectively. Photodetection experiments conducted for doped and undoped MoS2 grown on p‐type substrate reveal a decrease in the value of photo responsivity for N2 doped MoS2 (191 A W−1) compared with undoped MoS2 (572 A W−1). Also, MoS2 crystals grown and doped on an n‐type substrate display an important enhancement of the photoresponsivity from 63 A W−1 for undoped to 606 A W−1 for N2 doped MoS2. The modulation of Schottky barrier height for N2 doped MoS2 on p type substrate decreased whereas for n type substrate the high electric field created due to the difference in the Fermi level allows for greater separation of photogenerated charge carriers. This modulation in the photoresponsivity due to the selection of the type of substrate opens up new avenues of research and engineering of atomically thin optoelectronic devices.


Introduction
Research in 2D metal chalcogenides (MXs) has increased due to their enticing carrier mobility, tunable bandgaps, as well as their unusual excitonic and light-matter interaction features. Among many 2D layered materials, MoS 2 (Molybdenum disulfide) has gained the greatest interest to be the most www.advmatinterfaces.de salts and sulfur containing polymers. [16][17][18][19] Dopants used in TMDs include N 2 , phthalocyanine (Pc), Nb, and Zn, among others. [20][21][22][23] Covalent doping of TMDs, where single-atom dopants are injected via chalcogen or metal substitution into the TMD lattice, is a possible technique for achieving stable and controlled doping. [24] An MoS 2 example of covalent p-type doping has been demonstrated by replacing Mo with Nb in the development phase. [25] In addition, substitutional doping can impart novel properties to TMDs, like enlargement interlaying gap and optical band gap tuning. It has been established that doping into the host structure increases the inherent conductivity of active materials by changing or altering chemical bonds or the local lattice structure. [26,27] Hu et al. reported the inherent sulfur vacancies to provide a prime chance for modifying the N 2 doping of monolayer MoS 2 work function on the basis of quick thermal annealing in a nitrogen atmosphere, and these films are doped with in-plane nitrogen atoms. [28] Benjamin et al. proposed Pc metal substitution in p and n-type TMDs to tune their work function and compared in TMDs FET applications. [29] Our findings are effectively doping MoS 2 with N 2 on both n and p-type Si substrates to modulate the work function and photodetector application for the first time to report.
In this work, monolayer MoS 2 flakes are gown using the chemical vapor deposition (CVD) technique and further doped using nitrogen (N 2 ) under a vacuum environment. The XPS analysis reveals that the sulfur vacancy sites are replaced with N 2 also the modulation in the work function was confirmed and studied using UPS studies which observed an increment in the work function value when compared to undoped MoS 2 on both n and p-type Si substrates. The n-type behavior of MoS 2 grown on both n and p-type substrates is suppressed by chemically doping with the N 2 . The Raman analysis observed symmetric blue shift in both in-plane and out-of-plane frequencies for nitrogen-doped MoS 2 with respect to undoped MoS 2, confirming significant p-type doping and it matches well with the literature. Photoluminescence (PL) measurements also indicate the highest peak in intensity for N 2 -doped MoS 2 when compared with undoped MoS 2 , suggesting that the sulfur vacancies occupied by N 2 doping improve the crystal quality of the MoS 2 monolayer. It is important to note that PL and work function of monolayer MoS 2 can also be modulated by other factors such as chemical environment, surface adsorption, chemical doping and strain, etc. [30] To mitigate the above discrepancy, Raman measurements were performed which showed no signatures for the above factors. To understand the effect of light interaction on suppressed n-type behavior of MoS 2 , photodetection measurements were performed, which suggest a decrement in the responsivity values of N 2 -doped MoS 2 compared to undoped MoS 2 when the substrate is p-type. The UPS analysis revealed a decrease in the value of the Schottky barrier height N 2 -doped MoS 2 /p-Si, explaining the low responsivity values compared with the undoped counterpart. It should be noted that the increased Schottky barrier height in N 2 -doped MoS 2 samples grown on n-type substrates increases the built-in potential reducing the surface recombination probability, thereby enhancing the photocurrent. The present study of doped MoS 2 with different substrates and studying the charge transport opens up new avenues of research for doped 2D materials for various device applications.

Synthesis of N 2 -Doped MoS 2 Using CVD
Molybdenum trioxide (MoO 3 ) and sulfur (S) powder, each of 140 mg and 200 mg, respectively, were taken at first and were kept in two separate aluminum oxide (Al 2 O 3 ) boats. Si wafer of both p-type and n-type were subjected to the RCA cleaning process and then the same Si wafer with dimension (1 cm × 2 cm) was placed facing down onto the Al 2 O 3 boat containing MoO 3 . Before the synthesis procedure started, a high vacuum was created in a quartz tube by using a mechanical pump. After creating a sufficiently high vacuum of 0.035 m bar, the quartz tube was purged with Argon gas and the flow rate of the carrier gas was 50 sccm. The complete synthesis procedure was carried out by heating respective zones simultaneously and then maintaining temperatures of the MoO 3 and S zones at 850 and 200 °C, respectively. The growth temperature was kept stable for 20 min and consequently, the quartz tube was cooled naturally to room temperature. Similarly, for N 2 doping, the sample with MoS 2 flakes was kept in a CVD chamber and heated up to 750 °C by creating a high vacuum and then N 2 was introduced for about 2 h at the flow rate of 40 sccm, followed by natural cooling.

Device Fabrication
Finally, contacts were taken using standard E-beam lithography. The complete synthesis and the final metal contact deposited digital image are depicted in Figure 1. Schematic showing the photodetection setup of the device for both p-Si and n-Si substrates is shown Figure S1, Supporting Information.

Characterization of MoS 2
FESEM was done to confirm the morphology for MoS 2 deposited on Si wafers; uniform triangular flakes with sizes ranging from 15-40 µm were observed, as shown in Figure 2a. Also, Raman analysis was done for monolayer MoS 2 as can be clearly seen in Figure 2b that the Raman spectroscopy had two distinct peaks centered at 384 cm −1 and 403 cm −1 , which corresponded to E 2g 1 and A 1g modes; where E 2g 1 signified in-plane vibration of M and S atoms and A 1g signified out-of-plane vibration and the frequency difference between in-plane and out-plane was observed to be 19 cm −1 confirming monolayer MoS 2 flakes. [31,32]

XPS Analysis of Undoped MoS 2 and N-Doped MoS 2
XPS analysis was carried out to know the chemical composition of as grown MoS 2 crystals. The XPS survey spectra reveal the presence of Mo and S elements. In addition, an oxygen peak was also noticed due to the SiO 2 layer on the Si wafer. The higher resolution of Mo 3d and S 2p XPS spectra of MoS 2 was presented in Figure 2c,d, where the Mo 3d region shows two distinct peaks situated at 231.95 eV for Mo 3d 3/2 and 229.4 eV for Mo 3d 5/2 respectively. The S 2p region exhibits two separate peaks at 163.56 eV (S 2p 1/2 ) and 162.41 eV (S 2p 3/2 ), respectively. Also S 2s signal with a binding energy of 227.2 eV corresponds to MoS 2 . The absence of an S 2p signal at binding energies between 167 and 169 eV shows that sulfur atoms in the sample have not been oxidized. [33] The location and structure of the Mo 3d and S 2p doublets demonstrate that MoS 2 exists only in the semiconducting 2H phase. [34] Also the XPS analysis for the surface electron state and chemical contents of N-doped MoS 2 crystals were investigated.   Figure 3b, which is deconvoluted in two major peaks centered around 229 and 232 eV subsequent to Mo +4 3d 3/2 and Mo +4 3d 5/2 another located around 226 eV is ascribed to the S 2s. [35] Figure 3c depicts the S 2p of N-doped MoS 2 and it is deconvoluted into doublet peaks located around 163.1 eV and 162 eV attributed to S −2 2p 1/2 and S −2 2p 3/2 , respectively. [31] Further, the high-resolution spectrum of N 1s is shown in Figure 3d. In the N 1S spectrum, the high-intensity peak located around 395 eV is assigned to the Mo 3p 3/2 and another characteristic peak positioned around 399 eV is attributed to the MoN bond. [36] Also, high resolution Mo 3p3/2 spectra of undoped MoS 2 is included in Figure S2, Supporting Information, and it shows the absence of MoN bonding. XPS results clearly reveal that N is doped into MoS 2 and MoN bond is present. [37,38] Figure 4a shows the Raman analysis of both undoped and N 2 -doped MoS 2 and the effect of nitrogen on both frequency and intensity for E 2g 1 and A 1g modes with respect to undoped MoS 2 . It was observed that doped MoS 2 indicates a symmetric shift of both the Raman peaks to a greater frequency rate and increased Raman intensity. Also, it was observed that E 2g 1 has shifted to the higher frequency of 387cm −1 , which was 384 cm −1 for undoped MoS 2 and A 1g to 406 cm −1 , which was 403 cm −1 for undoped MoS 2 . It was concluded that the blue shift in E 2g 1 occurs due to MoN bonding and the blue shift in A 1g is inconsistent with p-type nitrogen doping. The PL analysis was carried out on both undoped and N 2 -doped MoS 2, and the same is illustrated in Figure 4b, at the K point of the Brillouin zone, two major characteristics in the spectra excitons A and B are associated with direct optical transitions from the highest valence band to lowest conduction band are visible. Furthermore, it can be observed that the PL intensity considerably increases about eight times for N 2 -doped monolayer MoS 2 compared with the undoped samples originated by the healing of available sulfur vacancies in monolayer MoS 2 with nitrogen doping.
Further, the PL emission of both samples at low temperatures were studied. Figure 4c depicts the normalized PL spectra and peak decomposition of the undoped and nitrogen-doped MoS 2 single layers measured at 150K. Both spectra show the emission of A (X 0 ) and B excitons and the trion (X − ), however, a feature located around 1.73 eV rises only in the case of undoped samples. This band (X D ) can be attributed to excitons bound to defects probably originated by sulfur vacancies. The fact that this PL band was not found in nitrogen-doped samples suggests a suppression of defect-related non-radiative recombination channels by the incorporation of nitrogen atoms into de the MoS 2 lattice. Moreover, to further confirm the origins of the excitonic transitions in undoped monolayer MoS 2 temperature and power-dependent PL measurements (at 150 K) were performed as shown in Figures S3 and S4, Supporting Information. By employing the power-law expression: I α L k , where I is the integrated PL intensity and L is the laser power excitation, we found slope coefficients (k) of 1 for excitons, 1.5 for trions and 0.89 for defects confirming the nature of each excitonic process. [39,40] Figures 5a,b show the I-V graphs of both undoped MoS 2 and nitrogen-doped MoS 2 grown on the p-type substrate under visible light illumination (554 nm), with contact electrodes one on www.advmatinterfaces.de top of the MoS 2 flake and the other on p-type Si substrate. I-V characteristics are observed to be Schottky in nature which is evident from the non-linear curves. Considering the stability of the device as a key factor, extreme care was taken for 12 h before photodetection measurements to avoid interference of the device with external light. The photodetection experiments were performed by varying the light intensity of the 554 nm source. The active area of the device was calculated to be 12 × 10 −6 cm 2 .
The increase in the photocurrent as a function of the light intensity due to the conventional photogenerated electron (e)-hole (h + ) pairs and their separation due to the built-in electric field. Figure 5c,d depicts the temporal response of undoped MoS 2 and nitrogen-doped MoS 2 upon the same wavelength (554 nm) excitation with constant light intensities, wherein it is clear that when the light source is ON, there is an increase in device current, and when the light source is OFF the current falls to its initial value in step manner which is inconsistent with I-V characteristics data. Moreover, the photocurrent increases steadily when the light intensity periodically increases, demonstrating high reproducibility for both device configurations (Figure 5e,f). The response time is the most significant factor in the analysis and is defined as the measure of the ability of the fabricated device to respond to an external stimulus. [41] It is determined as the time that was taken by the fabricated device current to reach from 10% to 90% of the final value. Also, the fall time is defined as the time taken by the fabricated device to fall from 90% to 10% of the initial value. Figure 5g Similar photodetection experiments were also conducted for undoped MoS 2 and nitrogen-doped MoS 2 grown on an n-type substrate. Figure 6a,b shows the I-V graphs of together undoped MoS 2 and N 2 -doped MoS 2 grown on the n-type substrate under the same configuration and conditions described above. As expected, I-V characteristics also present Schottky behavior, and the photocurrent ends to increase with a constant and varying incident light intensity (Figure 6c In order to design any electronic and optoelectronic device, a major consideration is the work function, which is the minimum energy required to free electrons from the Fermi level to the vacuum level. [42] UPS analysis was performed to examine the band structures of both undoped MoS 2 and nitrogen-doped MoS 2 grown on p-type and n-type substrates. The calculated secondary electron cut-off energy of undoped  Figure 7a. Similarly, for nitrogen-doped MoS 2 grown on a p-type substrate, the cut-off energy was examined to be 15.40 eV and the work function was measured to be 4.78 eV, as shown in Figure 7b. For nitrogen-doped MoS 2 grown on the n-type substrate, the cut-off energy was measured to be 15.69 eV and the work function was measured to be 4.84 eV as shown in Figure 7c. Based on these results, the resulting energy band diagram of both undoped MoS 2 and N 2 -doped MoS 2 grown on p-type and n-type substrates were extracted, as shown in Figure 7d,e. From the band structure, it can be clearly observed that when compared to undoped MoS 2, the work function increases by 0.47 eV and 0.53 eV for N 2 -doped MoS 2 grown on p-type and n-type substrates, respectively. A similar UPS analysis was performed for n and p-type Si substrates to extract the real band diagram. The explanation and the UPS analysis for both n/p-Si substrates can be found in Figure S5, Supporting Information. The above analysis was performed for 2 h of nitrogen doping. Upon increasing the doping time and keeping the flow of nitrogen to be same (40 sccm), it was observed that the work function increases from 4.78 eV (2 h) to 4.86 eV (4 h) to 4.94 eV (6 h) and the procedure was followed  for three independent devices. Further, there was no significant differences observed in the PL emission at room temperature of MoS 2 monolayers under varying doping time. The graph for variation of the work function (UPS) and PL with doping time can be found in Figure S6, Supporting Information.
Responsivity, external quantum efficiency, and detectivity are three major criteria of the photodetector and are the qualitative measure of the photocurrent generated per unit area upon light incidence per unit power. [43] The responsivity, EQE, and detectivity are given mathematically as follows: [44,45] = × λ λ λ

R I A P
(1) where I λ , P λ , A, I dark , e, and λ are photogenerated current, illumination power, active area of the device, dark current, and charge of an electron, wavelength, respectively. Figure 8a-c shows the responsivity, EQE, and detectivity plots as a function of the of optical power intensity for undoped MoS 2 on p-type (blue) and n-type (red) substrates and nitrogen-doped MoS 2 on p-type (gray) and n-type (green) substrates. The responsivity values for undoped and doped MoS 2 samples on p-type substrates under an optical power of 0.17 mW cm −2 were 572 and 191 A W −1 , respectively, the estimated value for EQE is 1281% and 267% for undoped MoS 2 and nitrogen-doped MoS 2 , finally, the detectivity of the photodetector was calculated to be 7.32 × 10 12 Jones and 1.52 × 10 12 Jones for undoped MoS 2 and nitrogen-doped MoS 2. Interestingly, the situation is reversed for devices fabricated on n-type substrates. Here the calculated responsivities for the same power intensity (0.17 mW cm −2 ) are 63 A W −1 for undoped and 606 A W −1 for N 2 -doped devices, while the examined value for EQE is 142.1% and 1357.2% and the detectivity is 8.131 × 10 12 Jones and 7.761 × 10 12 Jones for undoped MoS 2 and nitrogen-doped MoS 2 , respectively.
The comparison study of the responsivity, EQE, and detectivity on various device configurations for undoped MoS 2 and N 2 -doped MoS 2 on both p-type and n-type substrates are graphically shown in Figures S7 and S8, Supporting Information. It was observed that the responsivity follows the following trend: The careful evaluation of the Schottky barrier heights also revealed a similar trend: When the MoS 2 /Si devices are under illumination (doped and undoped), the energy of the incoming photon is higher than For MoS 2 (doped and undoped) grown on p-Si, the responsivity values are less for doped when compared to undoped. This can be attributed to the fact that the Schottky barrier height lowers upon doping, decreasing the electric field and thereby reducing the separation of photogenerated carriers, which finally decreases the current and the responsivity. For MoS 2 (doped and undoped) grown on n-Si, the observed trend was different, wherein for the doped MoS 2 , the responsivity values are higher when compared to undoped. Similar to the previous discussion for p-Si, the reason for higher responsivity for doped MoS 2 originated from the increased Schottky barrier height, raising the electric field and, thereby, the photocurrent. Figure 9a represents the band diagram of both p-type Si substrate and n-type undoped MoS 2 when isolated. When both n-type undoped MoS 2 and p-type Si are brought together, the Fermi level aligns uniform throughout the device to attain equilibrium and so band bending happens toward the internal electric field, as can be seen in Figure 9b. For n-type nitrogen-doped MoS 2 , which has suppressed n-type behavior and p-type Si substrate and the band diagram, when isolated and contacted, was shown in Figure 9c,d. It can be observed that the barrier height decreased drastically for nitrogen doped MoS 2 when compared to undoped MoS 2 . In order to operate the n-MoS 2 (doped/ undoped) and p-Si junction in reverse bias, the positive terminal is connected to n-MoS 2 (doped/undoped) and the negative terminal is connected to p-Si. When connected in reverse bias, electrons are collected at the positive terminal connected to n-MoS 2 (doped /undoped) and holes are collected at the negative terminal connected to p-Si, as can be seen in Figure 9b-d. Further, a similar band diagram was studied for n-type Si substrate and n-type MoS 2 and it was observed that the Schottky barrier height has increased for nitrogen-doped MoS 2 when compared to undoped MoS 2, and the corresponding band diagram when isolated and contacted are shown in Figure 9e-h, when connected in reverse bias, n-Si is connected to the positive terminal and n-MoS 2 (doped/undoped) is connected to negative terminal accordingly the corresponding carriers are collected at respective terminals as seen in Figure 9f-h.  www.advmatinterfaces.de Figure 9. a,b) Schematic illustrating band diagram of p-type Si substrate and n-type undoped MoS 2 both when isolated and contacted. c,d) Band diagram of p-type Si substrate and n-type N 2 -doped MoS 2 both when isolated and contacted. e,f) Schematic illustrating band diagram of n-type Si substrate and n-type undoped MoS 2 both when isolated and contacted. g,h) Band diagram of n-type Si substrate and n-type N 2 -doped MoS 2 both when isolated and contacted. www.advmatinterfaces.de hydrothermal process on cellulose paper and noted responsivity to be 99.3 A W −1 . [41] Songyu et al. have performed photodetection using MoS 2 by CVD and obtained responsivity of 99.9 A W −1 . [47] Liu et al. reported a direct CVD approach was used to synthesize high-quality V-MoS2 nanosheet array films using the induced buffer layer TiO 2 and achieved 133 A W −1 photo responsivity. [48] Mingsheng et al. demonstrated the photodetection using PdSe 2 /MoS 2 with Si/SiO 2 substrate using exfoliation technique and noted responsivity to be 42.1 A W −1 . [49] Enping et al. demonstrated the photodetection using WS 2 /Si with Si/SiO 2 substrate using the thermal decomposition technique and noted responsivity to be 0.224 A W −1 . [50] Xiao et al. performed photodetection using CVD grown MoS 2 on Si/SiO 2 substrate and noted responsivity to be 9.3 A W −1 . [51] Lin-Bao et al. demonstrated the photodetection using PdSe 2 /Ge with Ge substrate using selenization of Pd thin films technique and noted responsivity to be 0.53 A W −1 . [52]

Conclusion
Monolayer MoS 2 flakes are grown on both p-type and n-type Si substrates with sulfur defects in high vacuum conditions by CVD. These sulfur vacancies are properly used to have access to variation in work function by covalent N 2 doping. It was observed that work function increases by 0.47 and 0.53 eV for N 2 -doped MoS 2 in comparison to undoped counterparts when grown on p-type and n-type substrates, respectively. Supporting that N 2 doping tends to vary the work function of monolayer MoS 2 photodetection experiments were carried out for N 2 -doped MoS 2 grown on a p-type substrate, wherein it was observed that there is a drastic fall in the value of photoresponsivity compared to undoped MoS 2 . A similar analysis was done for N 2 -doped MoS 2 grown on an n-type substrate, wherein it was observed to increase in photoresponsivity compared to undoped MoS 2 due to an increase in Schottky barrier height between the n-type substrate and N 2 -doped MoS 2 compared to undoped MoS 2 . The increased Schottky barrier height between n-type MoS 2 and n-type Si increases the electric field, thereby increasing the separation of the photogenerated carriers eventually increasing the responsivity.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.