Semiconducting two-dimensional (2D) materials are studied intensively because of their promising performance in diverse applications from electronics to energy storage and catalysis. Recently, HfS2 and ZrS2 have emerged as potential rivals for the commonly studied 2D semiconductors such as MoS2 and WSe2, but their use is hindered by the difficulty of producing continuous films. Herein, we report the first atomic layer deposition (ALD) processes for HfS2 and ZrS2 using HfCl4 and ZrCl4 with H2S as the precursors. We demonstrate the deposition of uniform and continuous films on a range of substrates with accurately controlled thicknesses ranging from a few monolayers to tens of nanometers. The use of semiconductor industry-compatible precursors and temperatures (approximately 400 °C) enables facile upscaling of the process. The deposited HfS2 and ZrS2 films are crystalline, smooth, and stoichiometric with oxygen as the main impurity. As an important step toward applications of HfS2 and ZrS2, we show that their sensitivity toward oxidation can be overcome by minimizing the impurities in the reactor and by depositing a protective Al x Si y O z layer on the top without a vacuum break. Finally, we demonstrate HfS2 and ZrS2 photodetectors exhibiting good performance and stable operation in ambient conditions. Photoresponsivity comparable to thin films or even single flakes of HfS2 or ZrS2 deposited at much higher temperatures is achieved, although the response speed seems to be limited by photogating, as is common for 2D photodetectors. We expect the first ALD processes for HfS2 and ZrS2 to enable further exploration of these materials for various semiconductor applications.

Compared to the most well-known 2D material, graphene, which is a semi-metal, the semiconducting 2H phase of MoS 2 is advantageous in having a band gap suitable for electronic applications. In bulk form, MoS 2 has an indirect band gap of 1.3 eV, which increases as a function of decreasing film thickness. In monolayer MoS 2 (thickness ≈0.6 nm), the band gap becomes direct with a width of 1.8 eV. [1] Importantly, to meet the requirements of different applications, properties of MoS 2 and other TMDCs can be tuned by controlling the thickness, [1] doping and alloying, [5][6][7][8] surface modification and functionalization, [9][10][11] strain, [12,13] and by creating heterostructures with other 2D materials. [6,[14][15][16] The appealing properties of TMDCs have led to a wide range of proposed applications. MoS 2 has been extensively studied as a channel material in conventional field-effect transistors, [17][18][19][20][21] as well as phototransistors and other optoelectronic devices. [16,21,22] The 2D structure of TMDCs plays a crucial role in possible applications relying on more exotic quantum phenomena, such as valleytronics. [23,24] MoS 2 has also shown promise in, for example, catalysis, [25] batteries, [26] photovoltaics, [27] sensors, [28] and medicine. [29] The production of high-quality, large-area MoS 2 films with a thickness controllable down to a monolayer, as required in many of the aforementioned applications, still remains a major challenge. Additionally, in many cases, the processing temperature should be kept as low as possible in order to avoid damaging sensitive substrates, such as polymers or nanostructures. Initially, flakes of monolayer MoS 2 were produced from natural MoS 2 crystals using micromechanical exfoliation, a topdown method capable of producing high-quality monolayers, albeit with poor throughput as well as limited control over flake thickness and dimensions. [4,30,31] Liquid-phase exfoliation of bulk crystals, on the other hand, offers good scalability, but often suffers from limited flake size, poor crystallinity, or contamination. [4,31,32] Bottom-up methods offer a more controllable way to produce MoS 2 films. High-quality MoS 2 thin films are most commonly deposited by chemical vapor deposition (CVD) or sulfurization of metal or metal oxide thin films. The most common Molybdenum disulfide (MoS 2 ) is a semiconducting 2D material, which has evoked wide interest due to its unique properties. However, the lack of controlled and scalable methods for the production of MoS 2 films at low temperatures remains a major hindrance on its way to applications. In this work, atomic layer deposition (ALD) is used to deposit crystalline MoS 2 thin films at a relatively low temperature of 300 °C. A new molybdenum precursor, Mo(thd) 3 (thd = 2,2,6,6-tetramethylheptane-3,5-dionato), is synthesized, characterized, and used for film deposition with H 2 S as the sulfur precursor. Self-limiting growth with a low growth rate of ≈0.025 Å cycle −1 , straightforward thickness control, and large-area uni...

Aluminum fluoride thin films have potential in both optic and lithium-ion battery applications. AlF 3 thin films have mostly been deposited using physical vapor deposition methods. In this study, we present a new atomic layer deposition process for AlF 3 . Our method makes use of a halide−halide exchange reaction with AlCl 3 and TiF 4 as the precursors. With this new chemistry, thin films of AlF 3 can be deposited at a temperature range of 160−340 °C. The films have been studied by UV−vis spectroscopy, field emission scanning electron microscopy, X-ray diffraction, X-ray reflectance, atomic force microscopy, timeof-flight elastic recoil detection analysis (ToF-ERDA), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. At 220 °C, the growth rate of the films is approximately 1.1 Å per cycle, and the refractive index is 1.36 (at 580 nm). The films show only small amounts of Cl and Ti impurities when deposited at high temperatures, as determined by ToF-ERDA. Surface oxidation of the films due to moisture in ambient air is observed.