Band Alignment Tunability of 2-Layer MoS₂/SiO₂ Interface with The Use of Inorganic Salts in Liquid-CVD Sulfurization

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), molybdenum disulfide (MoS₂), in the semiconducting 2H phase, is one of the most extensively studied, because of its stability, versatility, and applicability in several fields, such as electrochemistry, optoelectronics, and microelectronics [1, 2]. To achieve sustainable exploitation of 2D-MoS₂, a scalable synthesis over large macroscopic areas is required. In this respect, chemical vapor deposition (CVD) is a versatile and cost-effective method to address the deposition of high-quality few-to-single layers MoS₂ with controlled thickness, uniformity and defectivity up to the wafer scale and with satisfactory carrier mobility [3]. Having 2D-MoS₂ based microelectronic devices as target, the electron band alignment at MoS₂/SiO₂ interface is a key-parameter dictating the electrostatic properties of the stacks such as built-in voltages, transistor thresholds, as well as the tunneling barrier heights. Therefore, the electrostatic control of the MoS₂/SiO₂ interface, entailing the energy barrier height at the junction and the emergence of interface dipoles, is essential to overcome parasitic or undesired short-channel effects in ultra-scaled field-effect transistors (FETs) based on MoS₂[1], or to fabricate compliant stacks in ultra-steep sub-threshold slope tunneling FET [4].<br/>Despite previous attempts proved that the use of inorganic salts, such as NaCl, is effective in promoting selective extended 2D-MoS₂ growth [5], no attention has been put on the electrical properties at the MoS₂/SiO₂ interface. Here, we performed internal photoemission (IPE) spectroscopy, a method successfully applied to 2D layered materials interfacing [6], on large-area (cm²) bi-layers MoS₂ directly grown on SiO₂/Si substrates via liquid precursor CVD (L-CVD) sulfurization, where the ammonium heptamolybdate (AHM) Mo precursor, is provided via solution deposition with inorganic salts (NaCl, NaOH, KCl, KI) spun over the substrate, to assess any possible effect of these inorganic molecules on the MoS₂/SiO₂ interface properties, in particular to determine the energy band alignment of MoS₂ with SiO₂. We measured IPE thresholds from 3.9 to 4.2 eV, depending on the type of inorganic salt in solution, which in any case disagrees with the 3.6 eV value obtained in CVD grown MoS₂ from MoO₃ and S powders with the assistance of organic perylene-3,4,9,10-tetracarboxylic acid tetra-potassium salt (PTAS) [7]. To in-depth elucidate the origin of such discrepancy, we performed structural, morphological, and chemical analysis (SEM, AFM, Raman, XPS). For the case of NaOH, we revealed the presence of Na and OH residuals. Our findings are consistent with the emergence of Si-OH dipoles at the interface that individually dependent with each surface treatment in use, thus explaining the observed 0.6 eV energy widening in the IPE threshold.<br/><br/>Our study evidence the impact of the junction characteristics, and thus the need to consider them, on the real electrical behavior of devices integrating 2D materials, with critical implications on proposed models and reported values so far. We also report a relatively easy route to tune the band alignment at 2D-MoS₂/SiO₂ interface, a fundamental property to consider in the perspective of direct integration of 2D materials in the process flow for microelectronic devices.<br/><br/>This study was supported by the Government of Italy through Ministero dell’Università e della Ricerca (MUR) under the PRIN projects “aSTAR”, n. 2017RKWTMY and “PHOTO” n. 2020RPEPNH.<br/><br/><b>References </b><br/>[1] C. Martella et al., Nanomaterials 12, 4050 (2022)<br/>[2] M. Bhatnagar et al., Nanoscale 12, 2 24385 (2020)<br/>[3] C. Martella et al, Adv. Mater. Interf. 7, 2000791 (2020)<br/>[4] I. Shlyakhov et al., APL Mater 6, 026801 (2017)<br/>[5] L. Huang et al., J. Am. Chem. Soc. 142, 13130 (2020)<br/>[6] V. V. Afanas’ev et al., J. Phys.: Cond. Matter 32, 413002 (2020)<br/>[7] P.P. Tummala et al., Materials 13, 2786 (2020)