Vapor-Phase Synthesis of Two-Dimensional Metal Chalcogenide Semiconductors

Bergeron, Hadallia

doi:https://doi.org/10.21985/n2-wz2z-rs78

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Vapor-Phase Synthesis of Two-Dimensional Metal Chalcogenide Semiconductors

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As demonstrated by efforts in graphene commercialization, scalable synthesis and high-quality material availability are primary limiting factors for the realization of technologies based on two-dimensional (2D) materials. Thus, in considering the fate of emergent 2D materials such as the metal chalcogenides, the challenge of scalable synthesis is a highly relevant if not prohibitive one. Of the available synthesis methods, vapor-phase techniques are best suited for the wafer-scale fabrication of high-quality 2D materials. The development of vapor-phase synthesis methods for 2D metal chalcogenides is thus a vital effort for the realization of the novel and high-performance electronic and optoelectronic applications lauded by the 2D material literature. While more established 2D metal chalcogenides (e.g., MoS2) have several vapor-phase methods at their disposal, others are more difficult to grow (e.g., InSe) and call for systematic studies to devise a reliable synthesis method. Additionally, vapor-phase growth can be leveraged to assemble 2D materials into diverse heterostructures via a van der Waals (vdW) interface without the substrate lattice matching constraints that govern the growth of conventional three-dimensional (3D) materials. This capability will facilitate a crucial step in the technological implementation of 2D semiconductors – their integration with 3D materials. Hence, in addition to expanding the variety of 2D metal chalcogenides which can be synthesized via vapor-phase methods, further exploration into their vapor-phase integration with functional 3D materials is warranted. This thesis first focuses on the use of vapor-phase synthesis for the vdW-mediated assembly of monolayer MoS2 into a heterostructure with amorphous Al2O3, which is a bulk-like material with a high dielectric constant (κ). The presented work demonstrates the value of integrating a 2D semiconductor with an existing 3D technology. In particular, the use of high-κ dielectrics such as atomic layer deposition (ALD)-derived amorphous Al2O3 is ubiquitous for low-power electronics. Here, the hybrid 2D MoS2/3D Al2O3 heterostructure was achieved by the direct growth of monolayer MoS2 using chemical vapor deposition (CVD) onto 20-nm-thick Al2O3 grown using ALD. The resulting MoS2/Al2O3 heterostructures were fabricated into enhancement-mode field-effect transistors (FETs) which exhibit high performance in low- power electronics metrics. The tactic of direct growth of MoS2 on the dielectric, which avoids the deleterious doping effect of dielectric deposition onto MoS2, is a tactic not easily accessible to 3D semiconductors which struggle to grow on amorphous substrates. The presented work thus substantiates the prospect of scalable MoS2/high-κ structures for low-power electronics and illustrates the advantage of vdW-mediated vapor-phase growth in accessing new fabrication schemes. The latter part of the thesis presents the development of a vapor-phase synthesis method for large-area InSe films. So far, the complicated indium–selenium phase diagram has presented a significant hurdle to the growth of ultrathin films of InSe. This hurdle is one not easily overcome by the tedious trial-and-error experimental design which is characteristic of 2D material synthesis development, a process colloquially referred to as a “dark art” rather than a science. In this work, a synthesis method was rationally determined by elucidating the structural and compositional evolution of ultrathin InSe films deposited using pulsed laser deposition (PLD) and subsequently processed via vacuum thermal annealing. The method yielded thickness-tunable ultrathin InSe films with high crystallinity and no detectable impurity phases. The InSe films demonstrated high responsivity in phototransistors and were patterned for high-yield arrays of top-gated enhancement-mode InSe FETs. This work accomplishes the large-area device implementation of ultrathin InSe films and achieves a level of electronic uniformity yet to be demonstrated in 2D InSe synthesis. Moreover, it demonstrates the merit of phase exploration in 2D materials for the purpose of rational synthesis design and advocates for systematic studies into 2D material growth in the hope of enlightening a “dark art” with rationality and reproducibility.

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