Direct thermal atomic layer deposition of high- dielectrics on monolayer MoS: nucleation and growth.

High dielectric constant (high-) materials must be successfully integrated with single-layer transition metal dichalcogenides for future nanoscale device technologies. With high carrier mobility and relatively strong visible light emission, monolayer molybdenum disulfide (1L MoS) is a promising candidate for optoelectronic applications and is commonly synthesised chemical vapour deposition (CVD) to enable large-area device production. The growth of uniform high- dielectrics on bulk materials is routinely achieved thermal atomic layer deposition (ALD), but continuous deposition on MoS is notoriously challenging due to the absence of dangling bonds on the basal plane. The resulting unique nucleation and growth characteristics of high- dielectrics on 1L MoS are not fully understood, particularly on large-area CVD-1L MoS. In this work, we investigate the nucleation and growth of aluminium oxide (AlO) and hafnium dioxide (HfO) on CVD-1L MoS films direct thermal ALD at 200 °C. We vary the number of ALD cycles and monitor the morphology of the deposited high- layer atomic force microscopy, observing ALD-AlO and ALD-HfO films on CVD-1L MoS to exhibit island features for all cycle numbers investigated (up to 200 cycles). We reveal the development of AlO on CVD-1L MoS proceeds a three-dimensional growth mode, and we estimate the vertical and lateral growth rates to be 0.09 ± 0.01 nm per cycle and 0.06 ± 0.01 nm per cycle, respectively. In contrast, we find direct ALD of HfO on CVD-1L MoS exhibits negligible lateral growth, with a vertical growth rate of 0.14 ± 0.01 nm per cycle. We also investigate the thickness-dependent effects of ALD-AlO and ALD-HfO films on the Raman and photoluminescence character of CVD-1L MoS, and quantify changes in electron density. Our growth study offers valuable insights into the nucleation and development of high- dielectric films on CVD-1L MoS, enhancing the understanding of dielectric integration for MoS-based devices.