Large‐Area Growth of Ferromagnetic Q‐Carbon Thin Films

Plasma‐enhanced chemical vapor deposition (PECVD)‐assisted growth of undoped Q‐carbon films over a large area for wafer‐scale integration of diamond thin films and related structures is reported. A detailed mechanism is proposed to describe the formation of Q‐carbon via the low‐energy ion bombardment in the PECVD process. The energy of these ions is just adequate to generate a Frenkel pair, which facilitates the conversion of twofold coordinated sp² carbon units in the as‐deposited carbon layer to fourfold sp³‐bonded tetrahedral carbon units in Q‐carbon but does not induce damage to the formed structure. This enhances the sp³ content and the atomic number density due to the random packing of tetrahedral units in the Q‐carbon structure. The cluster of four tetrahedra leads to the formation of the diamond unit cell, which provides a nucleus for diamond growth. Thus, large‐area diamond films can be grown by seeding with Q‐carbon layers. Further, attributed to its unique structure, it is shown that Q‐carbon is ferromagnetism at room temperature, while the untreated amorphous carbon is diamagnetic. With the thickness of Q‐carbon less than 10 nm, epitaxial growth of diamond films can be achieved via domain‐matching epitaxy. Wafer‐scale integration of Q‐carbon opens up new avenues to develop diamond‐based heterostructures.