Department of Chemistry, KU Leuven (University of Leuven), Leuven 3001, Belgium.
imec, Kapeldreef 75, 3001 Leuven, Belgium.
J Chem Phys. 2019 Mar 14;150(10):104703. doi: 10.1063/1.5048346.
Monolayer-thin WS with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF and HS at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous AlO starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the AlO starting surface forms predominantly monolayer-thin WS crystals and AlF crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS layer proceeds with the next WS layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the AlO surface as soon as the WS layer completely covers the AlO surface and that the WS crystal basal planes and AlF crystals have a low reactivity for WF adsorption at 450 °C. Nonetheless, a second layer of WS starts to form before the first WS layer completely covers the starting surface, albeit the surface coverage of the second layer is low (<20%, after 25 min of CVD reaction). During the steady-state growth regime, predominantly the WS crystals in the second monolayer continue to grow in lateral dimensions up to ∼40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF adsorption on the WS basal plane compared to AlO. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS surface, before being incorporated at the more reactive edges of the WS crystals in the second layer. Such a process proceeds slowly with only up to 40% surface coverage of the second WS layer after 150 min of CVD reaction. The CVD reaction is mediated by the starting surface: WF precursor preferentially adsorbs on AlO, whereas adsorption is not observed on SiO. Nevertheless, WS grows on SiO in close proximity to AlO in 90 nm pitch AlO/SiO line patterns. Hence, functionalization of the starting surface (e.g., SiO with AlO) can provide opportunities to grow monolayer-thin WS crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.
具有 (0002) 纹理的单层 WS 通过气相前体 WF 和 HS 在 450°C 的沉积温度下从气相前体在覆盖有非晶态 AlO 起始表面的 300mm Si 晶片上通过化学气相沉积 (CVD) 生长。我们通过分析 WS 晶体的形态来研究 CVD 过程中的生长和成核机制。CVD 过程由两个不同的生长阶段组成。在 (i) 初始生长阶段，CVD 前体与 AlO 起始表面的快速自限制反应主要形成单层 WS 晶体和完全覆盖起始表面的 AlF 晶体。在 (ii) 稳定态生长阶段，在底层，第一个 WS 层上进行的速度较慢的各向异性反应进行，下一个 WS 层优先在横向尺寸上生长。我们提出，当 WS 层完全覆盖 AlO 表面时，前体吸附反应速率大大降低，并且 WS 晶体基面和 AlF 晶体在 450°C 下对 WF 吸附的反应性很低。尽管如此，在第一个 WS 层完全覆盖起始表面之前，第二层 WS 开始形成，尽管第二层的表面覆盖率较低（<25 min 的 CVD 反应后为 20%）。在稳定态生长阶段，主要是第二层中的 WS 晶体继续在横向尺寸上生长至约 40nm。与第一层底部的晶体相比，这些晶体的横向尺寸较大，这是由于 WS 基面与 AlO 相比对 WF 吸附的反应性较低。据推测，它们通过吸附在 WS 表面上并扩散穿过 WS 表面的前体物种侧向生长，然后在第二层 WS 晶体的更活跃边缘被掺入。这种过程进行得非常缓慢，在 150 min 的 CVD 反应后，第二层 WS 的表面覆盖率仅达到 40%。CVD 反应受起始表面介导：WF 前体优先吸附在 AlO 上，而在 SiO 上没有观察到吸附。尽管如此，WS 在 AlO/SiO 90nm 间距线图案中紧邻 AlO 的 SiO 上生长。因此，通过选择性、横向生长和可调节的晶体尺寸，可以在预定位置生长单层 WS 晶体，甚至在低温下，功能化起始表面（例如，具有 AlO 的 SiO）也可以提供机会。
