AIP Advances 7, 045307 (2017); https://doi.org/10.1063/1.4982068 7, 045307
© 2017 Author(s).
Characteristics of layered tin disulfide
deposited by atomic layer deposition with H2S annealing
Cite as: AIP Advances 7, 045307 (2017); https://doi.org/10.1063/1.4982068
Submitted: 16 January 2017 . Accepted: 12 April 2017 . Published Online: 19 April 2017
Seungjin Lee, Seokyoon Shin, Giyul Ham, Juhyun Lee , Hyeongsu Choi, Hyunwoo Park, and Hyeongtag Jeon
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Characteristics of layered tin disulfide deposited by atomic layer deposition with H
2S annealing
Seungjin Lee, Seokyoon Shin, Giyul Ham, Juhyun Lee, Hyeongsu Choi, Hyunwoo Park, and Hyeongtag Jeona
Division of Materials Science and Engineering, Hanyang University, Seoul, South Korea (Received 16 January 2017; accepted 12 April 2017; published online 19 April 2017)
Tin disulfide (SnS2) has attracted much attention as a two-dimensional (2D) material.
A high-quality, low-temperature process for producing 2D materials is required for future electronic devices. Here, we investigate tin disulfide (SnS2) layers deposited via atomic layer deposition (ALD) using tetrakis(dimethylamino)tin (TDMASn) as a Sn precursor and H2S gas as a sulfur source at low temperature (150◦ C). The crystallinity of SnS2 was improved by H2S gas annealing. We carried out H2S gas annealing at various conditions (250◦ C, 300◦ C, 350◦ C, and using a three- step method). Angle-resolved X-ray photoelectron spectroscopy (ARXPS) results revealed the valence state corresponding to Sn4+ and S2- in the SnS2 annealed with H2S gas. The SnS2 annealed with H2S gas had a hexagonal structure, as measured via X-ray diffraction (XRD) and the clearly out-of-plane (A1g) mode in Raman spectroscopy. The crystallinity of SnS2 was improved after H2S annealing and was confirmed using the XRD full-width at half-maximum (FWHM). In addi- tion, high-resolution transmission electron microscopy (HR-TEM) images indicated a clear layered structure. ©2017 Author(s). All article content, except where oth- erwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4982068]
INTRODUCTION
Since the separation of graphene from graphite was reported in 2004, there has been increased interest in two-dimensional (2D) materials.1 Due to its high mobility, transmittance, mechanical strength, and flexibility, graphene has been researched for use in future electronic devices such as flexible and wearable devices.2–4However, the lack of a band gap in pristine graphene leads to poor transistor performance. This limitation has triggered much research to explore other 2D semicon- ducting materials as analogues of graphene. Recently, transition metal dichalcogenides (TMDs) have attracted considerable attention as alternatives to graphene.5–7 TMDs have stoichiometry of MX2, which describes a transition metal (M = Mo, W, or Nb) sandwiched between two layers of chalco- gen atoms (X = S, Se, or Te), with strong in-plane covalent bonding between metal-chalcogen and weak out-of-plane van der Waals bonding between layers. These TMD materials have a hexagonal structure similar to graphene and a suitable band gap for fabricating thin film transistors (TFT).8,9 One of the most researched TMDs is molybdenum disulfide (MoS2). According to previous reports, field effect transistors using MoS2as a channel material had an adequate on/off ratio and electrical carrier mobility.9,10
Most TMDs that possess good electrical properties are formed by mechanical exfoliation because high-quality crystalline TMDs can be obtained by this method.9–11However, this method has low productivity and is not compatible with current device manufacturing processes due to scale limi- tations involving uniform synthesis. Another method to obtain TMDs is chemical vapor deposition (CVD), which uniformly produces large, high-quality TMDs.12–16 To realize this CVD method,
aAuthor to whom correspondence should be addressed; electronic mail:[email protected]
2158-3226/2017/7(4)/045307/7 7, 045307-1 © Author(s) 2017
045307-2 Leeet al. AIP Advances7, 045307 (2017)
many precursors have been investigated for deposition of TMDs. Although this method can produce mono- or few-layer TMDs with large areas, the process is generally carried out at high temperatures, which is not an appropriate condition for flexible, wearable devices. Thus, a low-temperature pro- cess and material with high crystallinity are essential for using 2D materials in flexible, wearable devices.
Tin disulfide (SnS2) is an n-type semiconducting layered material that has a hexagonal CdI2- type structure. Similar to other TMDs materials, SnS2composed of tin is sandwiched between two sulfur layers with strong covalent bonding, whereas each monolayer is connected with weak van der Waals bonding. According to a previous report,17a transistor using SnS2 as a channel material fabricated by a mechanical exfoliated method showed outstanding properties, with a high electrical carrier mobility of∼230cm2V-1s-1and an on/off ratio>106. In addition, it was reported that SnS2has been synthesized by a chemical vapor transport method at 620-680◦C.18However, in order to apply SnS2 in future electronic devices, especially flexible, wearable ones, it must be synthesized over a large area with high crystallinity at low temperatures. Low-temperature processes for the synthesis of TMDs such as SnS2have not yet been thoroughly researched.
In this study, we investigated the characteristics of layered SnS2 by atomic layer deposition (ALD). The ALD method involves self-limiting reactions on the substrate surface with precise control of film thickness, high conformality, and high film crystallinity.19Thus, ALD is considered to be a good candidate for synthesizing 2D materials. In addition, we conducted post-deposition annealing to improve the crystallinity of SnS2 with an H2S and Ar gas mixture. As previously reported,20 the crystallinity of 2D materials was improved by post deposition annealing with sulfur powder.
However, the annealing process with sulfur powder has fundamental issues because the temperature at which the sulfur powder can be vaporized is over 120◦ C. Moreover, most sulfur vapor has a S8ring structure below 500◦C, which results in a non-uniform reaction.21,22 H2S gas has a simple structure compared to sulfur vapor and can produce a uniform reaction during the annealing process.
Additionally, using highly reactive H2S gas produces a more favorable chemical reaction than sulfur powder when synthesizing 2D materials.23While ALD MoS2annealed with H2S gas was previously reported,24there have been no studies about post deposition annealing of SnS2with H2S gas. Thus, we carried out H2S gas annealing at various conditions and analyzed the physical and chemical characteristics with varying heat treatment conditions.
EXPERIMENTAL
Layered SnS2 was deposited by thermal ALD using tetrakis(dimethylamino)tin (TDMASn, [(CH3)2N]4Sn) as a Sn source and hydrogen sulfide (H2S) as a sulfur source. Two types of substrate were prepared for this process: A 285-nm-thick thermally-oxidized silicon dioxide (SiO2) layer on a p-type silicon (Si) and p-type Si. The 285-nm-thick SiO2layer on p-type Si substrates were cleaned by ultra-sonication in acetone, methanol, and deionized water for 15 min each. Si substrates were also cleaned by hydrofluoric acid for 1 minute. The layered SnS2ALD process consists of four steps:
TDMASn injection 1s - Ar purge 40s - H2S gas injection 3s - Ar purge 50s. During the ALD process, the process pressure was maintained at 1.2 torr with flowing Ar for 200 sccm. Substrate temperature was fixed at 150◦ C, and the Sn precursor and H2S gas were heated to 50◦ C. Ar gas was used as a bubbler gas to pulse Sn precursor into the chamber for 30 sccm. The layered SnS2 was deposited during 48 ALD cycles, and its thickness was about 3.6 nm.
In order to improve the crystalline quality of as-deposited SnS2, the SnS2was annealed at various temperatures with H2S gas in quartz tube furnaces. The annealing conditions are 250, 300, and 350◦C for 1 hour. In addition, in order to improve the crystalline quality and stabilize the SnS2surface, we developed a three-step annealing method. Three-step annealing was continuously performed at 250, 300, and 350◦C for 1 hour, respectively. Before the annealing process was initiated, the furnace was pumped down to 1x10-2 torr using a vacuum rotary pump and purged with high-purity Ar gas to atmospheric pressure. Then, the furnace was flushed with a mixed gas composed of H2S (4%) and Ar (96%), and the furnace temperature gradually increased to the annealing temperature (250, 300, and 350◦C) at a rate of 8.3◦C/min. During the annealing process, the chamber pressure was maintained at atmospheric pressure.
The crystal structure and crystalline quality of SnS2deposited via ALD were measured using both X-ray diffraction (XRD, Rigaku DMAX-2500) with Cu Kα radiation (λ=0.15418nm) and Raman spectroscopy with a 532 nm Nd:Yag laser. Angle-reserved X-ray photoelectron spectroscopy (ARXPS) with a Thermo Fisher Scientific Theta probe and an Al KαX-ray source was used to deter- mine the chemical properties of SnS2. The thickness and layered structure of SnS2 were observed using high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 F20).
RESULTS AND DISCUSSION
ARXPS analyses were conducted to investigate the chemical bonding state of SnS2 annealed with H2S gas. The photoelectron emission angles ranged from 25◦to 75◦, revealing the bonding state of SnS2at all locations; this is because the surface sensitivity can be increased by varying the angle at which photoelectrons are detected. As the emission angle increases, a larger number of photoelectrons near the surface of SnS2 are detected compared to that at the interface. The ARXPS spectra of Sn 3d and S 2p were calibrated to the C-C bond peak in the C1s spectra at 284.5 eV. We analyzed the ARXPS spectra of SnS2as a function of annealing temperature at an emission angle of 75◦. In earlier studies, the binding energies of Sn 2d5/2corresponding to SnS and SnS2were 485.7 and 486.6 eV, respectively.25Also, the binding energies of S 2p corresponding to SnS and SnS2 were assigned to 161.0 and 161.6 eV, respectively. As shown in Figs.1aandb, the Sn 3d5/2and S 2p3/2peaks of SnS2
annealed with H2S gas are all located near 486.6 eV and 161.6 eV, respectively, exhibiting the valance states of Sn4+and S2-. These results show that Sn and S atoms are fully bound to SnS2 under all annealing conditions. However, in the case of as-deposited SnS2, the binding energies of Sn 3d5/2and S 2p3/2are slightly shifted to lower values, which indicates that small amounts of Sn2+are present in the as-deposited SnS2. In the case of as-deposited SnS2, the presence of Sn2+is attributed to the low temperature ALD process which is not sufficient to perfectly form SnS2corresponding to valance state of Sn4+. Figures1canddpresent the ARXPS spectra of the step-annealed SnS2at the Sn 3d and S 2p core levels, respectively, as a function of emission angle. As shown in Fig.1c, the Sn 3d5/2peak
FIG. 1. ARXPS spectra of SnS2of a) Sn 3d and b) S 2p peaks depending on the annealing conditions at an emission angle of 75◦. ARXPS spectra of SnS2annealed with a three-step method depending on emission angle for c) Sn 3d and d) S 2p peaks.
045307-4 Leeet al. AIP Advances7, 045307 (2017)
FIG. 2. a) XRD spectra of the as-deposited and H2S gas-anneal SnS2. b) Trends of FWHM and grain size of as-deposited and H2S gas-annealed SnS2.
is located at 486.6 eV for all emission angles, which can be clearly identified as the valence state of Sn4+. In addition, Fig.1dshows the binding energy of S 2p3/2to be 161.6 eV at all emission angles, corresponding to S2-; this indicates that S binds with Sn to form SnS2. These results show that SnS2
is evenly formed inside the film.
Figure2ashows the XRD spectra comparing the various annealing temperatures. As shown in Fig.2a, the diffraction patterns of as-deposited and annealed SnS2are observed at about 2θ= 14.9◦, corresponding to the hexagonal SnS2(001) plane (JCPDS No. 23-0677), in all samples. Furthermore, the diffraction peak of SnS2 that was annealed with H2S gas is significantly larger than that of as- deposited SnS2, indicating that SnS2 annealed with H2S gas can improve the crystallinity of the hexagonal SnS2(001) plane. Additionally, the crystalline quality of SnS2can be evaluated using the width and intensity of the diffraction peak. As shown in Fig. 2b, the full-width at half-maximum (FWHM) of the peak corresponding to annealed SnS2 is smaller than that of as-deposited SnS2, indicating that the crystallinity is improved by H2S gas annealing. The FWHM value of SnS2annealed by the three-step method is the smallest at 1.55 degrees, which indicates that the crystalline quality is better than that of any other annealed SnS2. This is because the three-step annealing method can reduce surface defect of SnS2. According to a previous report,26the multi-step annealing can reduce surface defect in the films, which lead to improve the crystallinity of films. In addition, the FWHM value of SnS2annealed at 350◦C is larger than that at 300◦C, which is attributed to re-evaporation or reduction at the SnS2 surface.20Thus, it is assumed that initiating the annealing process at a low temperature (250◦C) can stabilize the surface of SnS2 to prevent re-evaporation or reduction. The
FIG. 3. Raman spectra of SnS2according to annealing conditions.
average crystalline size calculated via the Scherrer equation is also shown in Fig.2b. The crystalline size tends to increase with decreasing FWHM.
Raman spectroscopy was used to analyze the crystalline quality of layered SnS2. In the Raman spectra of SnS2, two phonon modes that respectively relate to the in-plane vibration of Sn and S atoms (E2g) and the out-of-plane vibration of S atoms (A1g) of SnS2are observed. These two peaks are located at 313cm-1for the A1gmode and 208cm-1 for the E2gmode. However, the E2gpeak is not observed because of either the weak rejection of Rayleigh radiation or the selection rules for scattering geometry.27 Figure3 shows the Raman spectra for various annealing temperatures. As shown in Fig.3, the Raman peak of as-deposited SnS2located at 313cm-1exhibits a weak intensity and broad spectra. After H2S gas annealing, the A1gpeak intensity significantly increased compared to that of as-deposited SnS2, which indicates that the crystallinity is improved. In addition, the Raman intensity of the A1g peak was reduced when it was annealed at 350◦ C due to re-evaporation or reduction (as previously mentioned), and the Raman intensity of SnS2 annealed via the three-step method is the highest. These results show that re-evaporation or reduction of the SnS2surface can be inhibited by starting annealing at a low temperature.
In order to confirm the thickness and two-dimensional layered structure of layered SnS2, cross- sectional TEM analyses were performed. Cross-sectional TEM samples were prepared by a precision ion polishing system (PIPS). Figure4ashows the cross-sectional TEM image of as-deposited SnS2,
FIG. 4. Cross-sectional HR-TEM images of a) as-deposited SnS2and SnS2annealed with b) three-step method and c) at 300◦C, respectively. d) Intensity profile of SnS2annealed with the three-step method showing the interlayer spacing.
045307-6 Leeet al. AIP Advances7, 045307 (2017)
and Figures4bandcshow the cross-sectional TEM images of SnS2 annealed at 300◦C and using the three-step method, respectively. As shown in Fig.4a, as-deposited SnS2is not fully crystallized because the deposition temperature of our ALD process is too low for crystallization to occur. How- ever, as shown in Figs.4bandc, the orientation of the layer parallel to the substrate was observed in SnS2annealed both at 300◦C as well as with the three-step method. In other words, a two-dimensional layered structure was formed in both samples, indicating that the crystallinity of SnS2is improved by H2S gas annealing. This result is in good agreement with the XRD and Raman data. The thickness of SnS2 is about 3.64 nm, which is equivalent to six layers of SnS2. Figure4dshows the intensity profile of SnS2 annealed with the three-step method. As shown in Fig.4d, the interlayer spacing is measured to be about 0.62 nm, which is consistent with the interplanar spacing of SnS2in a previous report.17As mentioned above, when SnS2is annealed at 350◦C, re-evaporation or reduction occurs at the surface of SnS2. As shown in Fig.4bhowever, using the three-step method, re-evaporation or reduction do not occur. Thus, it is reasonable to conclude that initiating an annealing process at low temperatures (250◦C) can stabilize the surface of SnS2.
CONCLUSION
In this study, the effect of H2S gas annealing on layered SnS2 using ALD at low temperatures (150◦ C) was investigated. Our results show that the crystallinity of SnS2 is improved with H2S gas annealing. Moreover, starting the annealing process at low temperatures (or using the three-step method) can more effectively stabilize the surface of SnS2. The XRD FWHM value of SnS2annealed with the three-step method was the lowest at 1.55 degrees, and the grain size was the largest at 5.4 nm among the SnS2samples. Raman data also show that the crystallinity is improved after H2S gas annealing. ARXPS analysis reveals the valance states of Sn4+and S2-in all the SnS2 samples annealed with H2S gas, showing that the SnS2 annealed using the three-step method was evenly formed inside the film. The two-dimensional layered structure of SnS2annealed at 300◦ C as well as with the three-step method was confirmed, and it is reasonable to conclude that re-evaporation or reduction at the SnS2surface were inhibited by three-step annealing, as can be seen in the HR-TEM images. In the field of 2D materials, ALD is a promising technique for creating high crystalline materials via annealing with H2S gas. This study broadens the research base regarding 2D materials as graphene analogues.
ACKNOWLEDGMENTS
This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (NRF-2014M3A7B4049367).
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