Chubarov Mikhail, Choudhury Tanushree H, Hickey Danielle Reifsnyder, Bachu Saiphaneendra, Zhang Tianyi, Sebastian Amritanand, Bansal Anushka, Zhu Haoyue, Trainor Nicholas, Das Saptarshi, Terrones Mauricio, Alem Nasim, Redwing Joan M
2D Crystal Consortium-Materials Innovation Platform (2DCC-MIP), Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
ACS Nano. 2021 Feb 23;15(2):2532-2541. doi: 10.1021/acsnano.0c06750. Epub 2021 Jan 15.
Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS monolayers on 2 in. diameter -plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS monolayers are largely free of IDBs but instead have translational boundaries that arise when WS domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS field effect transistors exhibited an / of ∼10 and mobility of 16 cm/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.
实现诸如WS等过渡金属二硫属化物（TMD）的晶圆级单晶薄膜需要取向畴的外延生长和合并以形成连续的单层。这些畴必须在衬底上沿相同的晶体学方向取向，以抑制反演畴界（IDB）的形成，而反演畴界是层状硫属化物的一个常见特征。在这里，我们通过金属有机化学气相沉积，使用多步生长工艺在直径2英寸的平面蓝宝石上展示了完全合并的单向WS单层，以实现具有低面内旋转扭曲（0.09°）的外延WS单层。透射电子显微镜分析表明，WS单层基本上没有IDB，而是具有平移边界，当晶格略有偏移的WS畴合并在一起时就会出现这种边界。通过调节单层生长速率，平移边界的密度和双层覆盖率显著降低。畴的单向取向归因于蓝宝石表面台阶的存在以及促进表面扩散、横向畴生长和合并同时保持对齐畴结构的生长条件。转移的WS单层在80 K时显示出中性和带电激子发射，与缺陷相关的发光可忽略不计。背栅WS场效应晶体管表现出约10的I/和16 cm²/（V·s）的迁移率。结果表明，实现无反演畴的晶圆级TMD单层具有接近剥离薄片性能的潜力。
