Large-Area, Pulsed Laser Deposition of MoS₂/a-BN Heterostructures for Back-Gate Field Effect Transistors Applications

Transition metal dichalcogenides (TMDCs) have emerged as alternative 2D materials for field effect transistor fabrication. Molybdenum disulfide (MoS₂) is a TMDC with semiconducting properties with monolayer (1ML) MoS₂ having a 1.8 eV direct bandgap and multilayer (&gt;2ML) MoS₂ a 1.2 eV indirect bandgap [1]. Sulphur vacancies tend to form on the top sulphur layer which gives MoS₂ a natural n-type doping. However, exposure to ambient oxygen and humidity oxidizes the surface of MoS₂ and, along with increased number of defects from the sulphur vacancies, MoS₂-based devices can lose performance. Therefore, encapsulation with different materials has been suggested as a solution for improving the device performance [2]. Hexagonal boron nitride (h-BN) is a 2D material with a 6.1 eV band gap [3] which can be utilised as an encapsulation material and also an insulator. The weak basal plane van der Waals forces of h-BN will not modify the surface of MoS₂ and hence improve the device performance.<br/><br/>One of challenges of 2D materials is the lack of scalable and reproducible large area 2D material growth. We propose to utilise pulsed laser deposition (PLD) as an alternative to high-quality, large-area growth of 2D materials capable of depositing uniform, large-area, thin-films with good control of the deposition parameters. High quality MoS₂ thin films can be grown with PLD [4], on the other hand, h-BN is difficult to grow with PLD because it needs temperatures above 1000°C. Therefore, amorphous boron nitride (a-BN) is grown as encapsulation, featuring large dielectric strength and low refractive index [5].<br/><br/>In this work, PLD was used to deposit a-BN thin-films on SiO₂/Si substrates. Subsequently, PLD was used to deposit the MoS₂/a-BN heterostructure on SiO₂/n++ Si substrates with optimized deposition parameters for MoS₂ [4] and a-BN, varying the thickness of MoS₂ (1ML, 2ML, 4ML, 10ML, 15 ML). Back-gated field effect transistors (FETs) were fabricated on the MoS₂/a-BN heterostructure samples using standard lithography techniques, followed by e-beam evaporation of Ti/Au contacts and rapid thermal annealing to reduce the contact resistance. The MoS₂/a-BN back-gated FETs were electrically characterized, resulting in mobilities increasing one order of magnitude when compared to back-gated MoS₂ FETs without encapsulation. The devices can turn on and off, although their leakage current is still significant (~10² - 10³ nA), which can be attributed to the large voltages needed to control the gate (&gt;50 V). Density functional theory (DFT) studies to understand the material interface will also be conducted and the results will be presented. DFT simulations of MoS₂-based back-to-back Schottky devices will be shown to compare the measured output characteristics with the calculated ones. The effect of h-BN encapsulation on the back-to-back Schottky devices was also calculated.<br/>References<br/>[1] J. Gao, D. Nandi, and M. Gupta, “Density functional theory—projected local density of states—based estimation of Schottky barrier for monolayer MoS2,” <i>J. Appl. Phys.</i>, vol. 124, no. 1, p. 14502, Jul. 2018.<br/>[2] S. Roy <i>et al.</i>, “Structure, Properties and Applications of Two-Dimensional Hexagonal Boron Nitride,” <i>Adv. Mater.</i>, vol. 33, no. 44, p. 2101589, Nov. 2021.<br/>[3] M. J. Molaei, M. Younas, and M. Rezakazemi, “A Comprehensive Review on Recent Advances in Two-Dimensional (2D) Hexagonal Boron Nitride,” <i>ACS Appl. Electron. Mater.</i>, vol. 3, no. 12, pp. 5165–5187, Dec. 2021.<br/>[4] D. Gudi, P. Sen, A. A. Forero Pico, D. Nandi, and M. Gupta, “Optimization of growth parameters to obtain epitaxial large area growth of molybdenum disulfide using pulsed laser deposition,” <i>AIP Adv.</i>, vol. 12, no. 6, p. 065027, 2022.<br/>[5] S. Hong <i>et al.</i>, “Ultralow-dielectric-constant amorphous boron nitride,” <i>Nature</i>, vol. 582, no. 7813, pp. 511–514, 2020.