Amorphous carbon nitride (a-CNx) films were formed by supermagnetron sputter deposition using N-2 and/or Ar gases. Supplying rf power with a substrate-holding electrode (bias sputter) and lowering the gas pressure were found to be effective at decreasing the optical band gap and increasing the hardness. Nitrogen concentrations of bias sputtered films were about 32-35 mass% (30-100 mTorr). The a-CNx films deposited for electron field emission showed a low-threshold electric field (Em). With the decrease of gas pressure, admixture of Ar to N-2 or the use of pure Ar, and the use of bias sputter, the E-TH of a-CNx films largely decreased to 11 V/mu m (30 mTorr Ar/N-2 bias sputter). (C) 2009 Elsevier B.V. All rights reserved.