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.