Department of Chemistry, KU Leuven, Leuven 3001, Belgium.
IMEC, Kapeldreef 75, 3001 Leuven, Belgium.
J Chem Phys. 2017 Feb 7;146(5):052810. doi: 10.1063/1.4967406.
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of AlO (using Al(CH)/HO ALD) on MoS whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS, we exclude the impact of surface contamination from a transfer process by direct AlO deposition on synthetic MoS layers obtained by a high temperature sulfurization process. It is shown that AlO ALD on the MoS surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS top surface. During further deposition, the as-formed AlO nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous AlO film is obtained by lateral growth over the MoS crystal basal plane, with the point of layer closure determined by the grain size at the MoS top surface and the lateral growth rate. The created AlO/MoS interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the AlO layers. To conclude, our growth study reveals that the inherent reactivity of the MoS basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.
二维（2D）过渡金属二卤族化合物（TMDs）在纳米电子器件的应用中具有重要意义。它们的掺入需要在 2D TMD 上沉积 nm 厚的连续高 k 介电层。高 k 介电层的原子层沉积（ALD）在 Si 表面上已经得到很好的建立：高成核密度对于快速层闭合的重要性是众所周知的，并且成核机制已经得到了彻底的研究。相比之下，ALD 在 2D TMD 表面上的成核理解得较少，并且缺乏对沉积过程的定量分析。因此，在这项工作中，我们研究了 AlO（使用 Al(CH)/HO ALD）在 MoS 上的生长，我们试图通过几种互补的表征技术，包括 X 射线光电子能谱、弹性反冲探测分析、扫描电子显微镜和飞行时间二次离子质谱，提供对该过程的全面了解。为了揭示 MoS 的固有反应性，我们通过直接在高温硫化过程中获得的合成 MoS 层上沉积 AlO，排除了来自转移过程的表面污染的影响。结果表明，在 125°C 和 300°C 之间的温度下，MoS 表面上的 AlO ALD 受到强烈抑制，MoS 晶体基面没有生长，只有在 MoS 表面的线缺陷或晶界处选择性成核。在进一步的沉积过程中，形成的 AlO 纳米带在垂直和水平方向上都生长。最终，通过 MoS 晶体基面的横向生长获得连续的 AlO 薄膜，层闭合的点由 MoS 表面的晶粒尺寸和横向生长速率决定。所形成的 AlO/MoS 界面主要由范德华相互作用组成。通过在低温下沉积并结合短吹扫时间，可逆吸附在 MoS 基面上的贡献提高了成核。虽然这导致了更二维的生长，但在 AlO 层中也掺入了额外的 H 和 C 杂质。总之，我们的生长研究表明，MoS 基面对于 ALD 的固有反应性极低，这证实了需要对 TMD 表面进行功能化方法以实现 ALD 成核。
