Effect of substrate bias on the properties of a-C:H films by direct current saddle-field plasma-enhanced chemical-vapor deposition

Abstract

Hydrogenated amorphous-carbon (a-C:H) films were deposited on p-type Si(100) at room temperature by dc saddle-field plasma-enhanced chemical-vapor deposition using pure methane gas. The effect of substrate bias (Vs) from 0 to 400 V on deposition rate, hydrogen content, and chemical bonding of the films have been investigated. Fourier transform infrared spectrum shows that a-C:H films consist of sp2 and sp3 bondings and the relative intensity ratio of sp2 and sp3 [I(sp2)/I(sp3)] decreases from 0.28 to 0.19 as the substrate bias changes from 0 to 400 V. Deposition rates of the films rapidly decrease with increasing Vs up to 200 V, and then slightly decrease over Vs=200 V. The hydrogen content of the films increases as Vs goes higher and the number of C–H bondings of a-C:H shows same trends. Only in the range of Vs=0 V and Vs=100 V, the Raman band of graphite and disorder is observed. The emission of white photoluminescence (PL) light from the films is observed with the naked eye even at room temperature. The intensity of the PL has a maximum at Vs=200 V, then decreases as the Vs changes from 200 to 400 V. The optical band gap (Eg) determined by the Tauc relation is shifted from 1.48 to 2.9 eV with increasing Vs, while the peak position of the PL spectra is shifted slightly to higher energy.