Michael Heuken, AIXTRON SE, Herzogenrath, Germany
Two-dimensional transition metal dichalcogenides (2D-TMDC) are being extensively explored by the scientific community as potential building blocks for (opto-)electronic components. 2D devices such as field-effect transistors (FET) based on MoS₂ or WSe₂ have shown promising performance and are even considered for integration into Si CMOS technology. To meet the requirement of commercialization, metal-organic chemical vapor deposition (MOCVD) has been widely accepted as a scalable and controllable technique to realize multi-wafer fabrication of 2D-TMDC with excellent quality, homogeneity and reproducibility. Nevertheless, the detailed mechanisms of 2D-MOCVD are not fully understood, and systematic efforts need to be devoted to study the impact of different growth parameters in detail.
In this review, we report on the latest progress using MOCVD for the fabrication of 2D-TMDC monolayers and the direct synthesis of their 2D-2D heterostructures. Commercial AIXTRON CCS reactors (7 x 2" or 1 x 300/200 mm setup) with advanced spectroscopic in-situ monitoring systems are employed as the experimental platform. Tungsten/molybdenum hexacarbonyl (WCO/MCO), di-tert-butyl sulfide (DTBS) and di-iso-propyl selenide (DiPSe) are used as precursors with H₂ as carrier gas. c-plane sapphire with a nominal 0.2° off-cut towards m-plane is chosen as substrate. An appropriate annealing step is carried out before each deposition. Si/SiO₂ wafer serves as substrate to investigate direct integration of TMDC in CMOS processes.
Temperature, as one of the most powerful MOCVD parameters, influences not only the critical nucleus size, but also the diffusion length of adatoms on the substrate surface. For different TMDCs from the (Mo,W)(S,Se)₂ family, the nucleation temperature is correspondingly adjusted in a range from 460 to 750 °C, in order to achieve a uniform distribution of the seeded TMDC nuclei on the wafer. In addition to temperature, other factors like substrate morphology (quality) and the chemical termination of the substrate surface are also found to play a vital role. For instance, by injecting DiPSe prior to WCO, the coverage and uniformity of WSe₂ nuclei on the Se-passivated sapphire surface are improved in comparison to nucleating on the pristine H₂-desorbed Al-rich sapphire surface. As additional building block in 2D electronics graphene and hBN synthesis is investigated on wafer up to 200 mm diameter with temperature control of up to 1400 °C and in situ monitoring tools for controlling other process conditions including the precursor flow. Methane gas is used as carbon source for CVD growth of graphene. For hBN, both CVD using borazine precursor and metal organic CVD route using triethylborane and ammonia have been explored. Material characterization reported here relies on spectroscopic in-situ reflectance measurements, SEM (scanning electron microscopy), AFM (atomic force microscopy), Raman, photoluminescence (PL) and X-ray photoemission spectroscopy (XPS).
Usually, during the growth step, closing a monolayer without premature bilayer formation is rather challenging. Bilayer nucleation inevitably sets on when size of monolayer domains already formed exceeds the metal adatom migration length (of the order of tens of nm) on top of these domains. Nevertheless, by smart MOCVD recipe design, for example by carefully tuning the parameters (incl. temperature) for the lateral-growth stage and ramping the flux of the growth-limiting metal species, coherent monolayers with sparse bilayer coverage (< 10%) can be achieved.
Based on the individual MOCVD recipes for TMDC monolayers, pioneering studies about the directly successive growth of 2D-2D heterostructures are presented. It is found that the growth of the second material is highly influenced by the first material underneath. Generally, a larger growth rate can be observed. Compared to the individual monolayers, the heterostructures show unique optical and structural properties, which is possibly caused by the interlayer coupling between layers. Electrical characterization about FETs built on 2D-material is also included in this work. Finally, an outlook will be given including strategies to improve layer properties based on MOCVD system adaptions (e. g. by additives promoting metal adatom migration), processing steps and finally device performance and early killer applications.