Atomic Layer Deposition of Emerging 2D Semiconductors, HfS₂ and ZrS₂, for Optoelectronics

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_xSi_yO_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.