Direct thermal atomic layer deposition of high-<i>κ</i> dielectrics on monolayer MoS₂: nucleation and growth

High dielectric constant (high-<i>κ</i>) 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 MoS₂) is a promising candidate for optoelectronic applications and is commonly synthesised <i>via</i> chemical vapour deposition (CVD) to enable large-area device production. The growth of uniform high-<i>κ</i> dielectrics on bulk materials is routinely achieved <i>via</i> thermal atomic layer deposition (ALD), but continuous deposition on MoS₂ is notoriously challenging due to the absence of dangling bonds on the basal plane. The resulting unique nucleation and growth characteristics of high-<i>κ</i> dielectrics on 1L MoS₂ are not fully understood, particularly on large-area CVD-1L MoS₂. In this work, we investigate the nucleation and growth of aluminium oxide (Al₂O₃) and hafnium dioxide (HfO₂) on CVD-1L MoS₂ films <i>via</i> direct thermal ALD at 200 °C. We vary the number of ALD cycles and monitor the morphology of the deposited high-<i>κ</i> layer <i>via</i> atomic force microscopy, observing ALD-Al₂O₃ and ALD-HfO₂ films on CVD-1L MoS₂ to exhibit island features for all cycle numbers investigated (up to 200 cycles). We reveal the development of Al₂O₃ on CVD-1L MoS₂ proceeds <i>via</i> 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 HfO₂ on CVD-1L MoS₂ 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-Al₂O₃ and ALD-HfO₂ films on the Raman and photoluminescence character of CVD-1L MoS₂, and quantify changes in electron density. Our growth study offers valuable insights into the nucleation and development of high-<i>κ</i> dielectric films on CVD-1L MoS₂, enhancing the understanding of dielectric integration for MoS₂-based devices.