Formation of a vertical SnSe/SnSe 2 p-n heterojunction by NH 3 plasma-induced phase transformation.

Layered van der Waals crystals exhibit unique properties making them attractive for applications in nanoelectronics, optoelectronics, and sensing. The integration of two-dimensional materials with complementary metal-oxide-semiconductor (CMOS) technology requires controllable n- and p-type doping. In this work, we demonstrate the fabrication of vertical p-n heterojunctions made of p-type tin monoselenide (SnSe) and n-type tin diselenide (SnSe 2 ). The p-n heterojunction is created in a single flake by the NH 3 -plasma-assisted phase transformation from SnSe 2 to SnSe. We show that the transformation rate and crystal quality strongly depend on plasma parameters like plasma power, temperature, partial pressure, NH 3 flow, and duration of plasma treatment. With optimal plasma parameters, the full transformation of SnSe 2 flakes into SnSe is achieved within a few seconds. The crystal quality and the topography of the fabricated SnSe-SnSe 2 heterostructures are investigated using micro-Raman spectroscopy and cross-sectional transmission electron microscopy. The formation of a p-n junction is verified by current-voltage measurements.