Spontaneous growth of perfectly circular domains of MoS2 monolayers using chemical vapour deposition technique

Abstract: Very large-scale integration of devices in a circular pattern has several advantages over the commonly used rectangular grid layout. For the development of such integrated circuits on a 2D semiconductor platform, spontaneous growth of the material in the form of circular islands is desirable. Here, we report the natural formation of 1L-MoS2 circular islands of diameter as large as a few hundreds of micrometer on SiO2/Si substrates by chemical vapor deposition (CVD) technique without the use of any seeding layer. The size of the circles is found to increase with the amount of sulphur used during growth. The study reveals that these circular islands are formed with a less-defective interior and a more-defective outer part that is dominated by a large density of grain boundaries and twists. Due to the lower defect density, the interior region yields much higher photoluminescence than the peripheral part. Field effect transistors (FETs) are fabricated on inner and outer portions of a circle to estimate the mobility and concentration of the background carriers in the two regions. The study shows that the maximum mobility is more than double in the interior than the outer part. While the carrier concentration remains practically unchanged in the two regions. The natural tendency to minimize the strain energy resulting from the mismatch between the thermal expansion coefficients of the monolayer and the substrate as well as the edge energy that originates from the boundary tension are thought to be the driving forces behind the formation of these circular domains.