Annealing effects on optical and photoluminescence properties of a-C: H films

In this study, optical and photoluminescence (PL) properties of amorphous hydrogenated carbon (a-C: H) films are reported along with thermal annealing effects. The layers were prepared by dc glow discharge decomposition of methane (CH4) at a frequency of 20 kHz. The optical Tauc gap (E-g) of as-deposited films can vary from 0.8 to 2.4 eV, depending mainly on reactor pressure. E-g increases by similar to0.1 eV for annealing temperatures, T-a less than or equal to 220degreesC, whereas in the range of T-a from 250degreesC to 300degreesC, a decrease of about 0.45 eV is observed, signalling the development of graphitic short range order as shown by dielectric losses measurements. Wide-gap (greater than or equal to 1.8 eV) a-C: H films exhibit visible PL at room temperature. Annealing of a-C: H with E-g = 1.8 eV, from 100degreesC to 220degreesC, improves the PL intensity significantly, indicating a decrease of defects in the material. On the other hand, the dominant PL peak is now located in the whole 1.8-2 eV energy range, rather than at 1.87 eV for non-annealed films. This is likely assigned to the slight increase of E-g by similar to0.1 eV.