Reactive pulsed laser deposition of CN_x films

Carbon nitride (CNx) films were prepared by reactive pulsed laser deposition at nitrogen partial pressure PN2varying from 0 to 300 mTorr. It is found that the atomic fraction of nitrogen f in the films first increases with increasing PN2, reaches a maximum of 0.32 at PN2=100 mTorr, and then decreases to a saturated value of 0.26 at PN2≳200 mTorr. Because of the absence of energetic particles in reactive pulsed laser deposition, the limited nitrogen content cannot be attributed to preferential sputtering of nitrogen that is generally observed in particle-assisted deposition of CNx films. Infrared absorption experiments show the existence of C≡N bonds and graphitic sp2 bonds. The sp2 bonds become IR active because of symmetry breaking of graphitic rings as a consequence of nitrogen incorporation. CNx films deposited at low PN2 (e.g., 5 mTorr) are more graphitic than the diamondlike pure carbon sample deposited at PN2=0, so have a slightly narrower electron band gap Eopt and a significantly higher room-temperature electrical conductivity σR. At PN2≳200 mTorr, nitrogenation of the films is very pronounced, leading to a wide band gap (Eopt≳1.5 eV), long electron band tail (E0≳0.7 eV), and extremely low σR(<1×10−13 Ω−1 cm−1). In addition, both the hardness and Young’s modulus are greatly reduced, for example, from 41.3 and 285 GPa for the pure carbon sample to 1.2 and 23.8 GPa, respectively, at f=0.32.