Haodong Zhang1 2 Thomas van Pelt2 Yashwanth Balaji2 Ankit Nalin Mehta2 Matty Caymax2 Iuliana Radu2 Wilfried Vandervorst2 Annelies Delabie1 2

1, KU Leuven, Leuven, , Belgium
2, Imec, Leuven, , Belgium

Two dimensional (2D) semiconductors are promising materials for application in ultrascaled electronic devices.[1] Tin sulfides are interesting layered semiconductors as they exist in different phases which exhibit different types of conduction. SnS2 is a n-type semiconductor with hexagonal crystal structure while SnS is a p-type semiconductor with orthorhombic crystal structure.[2] Chemical vapor deposition (CVD) is an alternative to grow 2D materials with monolayer control and large grain size (μm) over wafer scale.[3] The CVD of tin sulfides using SnCl4 and H2S has been reported.[4] However, the phases control of SnS2 vs SnS was not understood and the layers were μm thick with uncontrolled grain orientations, resulting in insulating rather than semiconducting properties.[4]
Insight on the phases control of SnS2 and SnS is required to obtain pure phase while the nucleation and growth mechanisms need to be studied for the monolayer thickness scaling and grain orientation control. Therefore, we first investigate the process window of the tin sulfides CVD using SnCl4 and H2S on thermally grown SiO2 at atmospheric pressure. For a H2S/SnCl4 flow ratio of 10, SnS is formed at temperatures higher than 450°C, while SnS2 is formed at lower temperatures. Interestingly, for a H2S/SnCl4 flow ratio of 20, SnS is formed at temperatures higher than 350°C with SnS2 formed at lower temperatures. This mechanism is attributed to the catalytic decomposition of H2S upon SnS2, whereby H2 and sulfur are generated. We propose that SnS2 is reduced to SnS by the generated H2 if large enough H2S/SnCl4 flow ratio and sufficiently high temperature are used, which provide sufficient reductant and thermal budget for the reduction of Sn4+, i.e. chemisorbed SnCl4 surface species and/or possible intermediate SnS2. Then, the n-type behavior of SnS2 and p-type behavior of SnS are verified using back-gated FETs.
Secondly, similar nucleation and growth behaviors of SnS2 and SnS on SiO2 are observed. A low nuclei density (105/cm2) and a large domain size of several μm for SnS is obtained at 400°C because of the large surface diffusion length of adatoms. Isolated 2D crystals of several monolayers thick are formed with basal plane parallel to the substrate. These 2D crystals preferentially grow laterally due to a higher reactivity of the crystal edges while the vertical growth can hardly be observed due to the relative inertness of the basal plane. Interestingly, the thickness of 2D SnS on SiO2 can be scaled by decreasing the deposition temperature. We propose that the thickness of 2D SnS is determined by the extent of agglomeration, which is impacted by temperature and hydrophilicity of substrate. This insight may guide the SnS2 and SnS CVD towards monolayer growth control.
[1] M. Chhowalla et al., Nat. Rev. Mater. 2016, 1, 16052
[2] J. Ahn et al., Nano Lett. 2015, 15, 3703-3708
[3] Kang et al., Nature 2015, 520, 656-660
[4] L. Price et al., Chem. Mater. 1999, 11, 1792-1799