Reduced Graphene Oxide/Amorphous Carbon P-N Junctions: Nanosecond Laser Patterning

The device integration of graphene and reduced graphene oxide (rGO) is impeded by scalability and high temperature (>2000 K) treatment required for effective reduction into high-quality rGO. In this article, we present a novel approach for direct laser writing of heavily reduced graphene oxide films by nanosecond laser melting of amorphous carbon on silicon (001) substrates under ambient conditions. Ultrafast quenching from the undercooled melt state above the melting threshold energy density (E_d) of 0.4 J/cm² leads to the formation of large-area rGO films. The first-order phase transformation of liquid carbon into graphene is triggered by low undercooling at the C melt/silicon interface. The laser-irradiated rGO films exhibit electron mobility of 12.56 cm²/V s and charge carrier concentration of -1.2 × 10²¹/cm³ at 300 K. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest low disorder and charge transport via 2D Mott variable range hopping between the graphene islands for rGO films. The localization length corresponding to the size of these graphitic domains is 3 nm. The ultrafast regrowth of rGO creates an atomically sharp interface between n-type rGO and p-type amorphous carbon, resulting in p-n junction heterojunction diodes with a turn-on voltage of 0.3 V, rectification ratio of 110@±1.5 V, and activation energy of 0.13 eV under reverse bias. This unique laser processing method solves the problems of traps and defects associated with equilibrium-based rGO fabrication methods, enabling high conductivity and mobility, providing insights into the fundamental mechanism driving laser writing of graphene-based materials on silicon.