ALD/ALE 2021 Session EM9: Ternary and Quaternary Materials

Two dimensional (2D) layered transition metal dichalcogenides (TMDCs) have attracted great attention owing to its excellent properties such as superior mechanical flexibility, transparency thermal stability, and compatibility to silicon CMOS processes. In particular, molybdenum disulfide (MoS₂) has been extensively studied in recent years. MoS₂ has been considered as attractive channel material for electronic switch device applications in the form of field-effect transistors (FETs) due to superior electrical properties such as thickness-dependent bandgap, high field-effect mobility, high current on/off ratio (>10⁸), nearly ideal subthreshold swing (SS). Despite all the advantages of MoS₂ as a promising candidate to replace silicon in such devices, large variations in the threshold voltage (V_th) of FET makes difficult it in practical application. The Vth of silicon-based semiconductor technology at the 22 nm technology node is 0.289 V for high performance logic and 0.413 V for low operating power logic. In contrast, values of Vth of back-gated MoS₂ FETs being reported vary from −30 V to 40 V. In the case of Si-based FET, V_th can be easily tuned by doping through ion implantation. However, this method cannot be applied to MoS₂ because energetic processes such as implantation, plasma easily cause damage to MoS₂. In this study, we developed one-step doping process during chemical vapor deposition (CVD) of MoS₂. electrical properties such as carrier density, bandgap and chemical composition tuned according to dopant concentration. In addition, V_th modulation of MoS₂ was carried out by using substitutional doping of W. V_th of MoS₂ FET has decreased by more than 10 V after doping.

References

[1]B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nat. Nanotechnol., vol. 6, no. 3, pp. 147–150, 2011, doi: 10.1038/nnano.2010.279.

[2] S. Kim et al., “High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals,” Nat. Commun., vol. 3, 2012, doi: 10.1038/ncomms2018.

Metal sulfides are of high importance for thin-film photovoltaics applications. Some of the most widely studied structures belong to the quaternary compounds group and play the role of solar cell absorber: CIGS (CuIn_(1-x)Ga_xSe₂) and CZTS (Cu₂ZnSnS₄). The ternary structure, Cu₂SnS₃ (CTS),^[1] has been reported in the literature as a simplified and potential absorber material. Ordinarily, methods different than ALD are used for their synthesis. In case of record CIGS^[2] (23.35% efficiency) and CZTS^[3] (12.6%) cells, DC sputtering and solution approaches were used, respectively.

ALD is dedicated to the preparation of uniform thin films. There are a few reports in the literature concerning ALD synthesis of technologically important ternary^[4–8] and quaternary^[9] sulfides. The ALD synthesis of sulfide-based absorbers is hindered by some limitations: precursor chemical compatibility, including reactivity with the sulfur precursor, parasitic reactions, and the limited volatility of precursors. These limitations are reflected by low growth per ALD cycle. The other aspects that limit ALD utility are high moisture and oxygen sensitivity, and high prices of commercially available metal precursors.

In this study, a new process of ALD deposition of ternary sulfide CuSn_xS_y is presented. The material has been prepared using affordable copper and tin precursors. This is contrasted with the reported ALD processes, which use expensive organometallic reactants as sources of Cu and Sn. The synthesis has been carried out at low temperature. Films of CuSn_xS_y varying in stoichiometry were synthesized. Film morphology, including cross-section imaging, was provided by SEM analysis. The elemental composition was determined by EDS. The results of optical (UV-vis spectroscopy) and electrical measurements (Hall effect, four-point probe) are presented and discussed in the context of the photovoltaic application of ALD synthesized CuSn_xS_y films.

The obtained results highlight the versatility of the ALD technique and provide an example of a cost-effective synthesis of thin-film absorber material.

[1] A. C. Lokhande et al., J. Mater. Chem. A 2019, 7, 17118–17182.

[2] M. Nakamura et al., IEEE J. Photovoltaics 2019, 9, 1863–1867.

[3] W. Wang et al., Adv. Energy Mater. 2014, 4, 1–5.

[4] N. Schneider et al., Nanotechnology 2015, 26, 54001.

[5] H. Le Tulzo et al., Sol. Energy Mater. Sol. Cells 2019, 200, 109965.

[6] H. Le Tulzo et al., Materials 2020, 13, 1–14.

[7] S. C. Riha et al., ACS Appl. Mater. Interfaces 2017, 9, 4667–4673.

[8] N. Schneider et al., Mater. Today Chem.2018, 10, 142–152.