Effects of gallium doping on the properties of amorphous-SiC:H films prepared by magnetron cosputtering

Amorphous SiC:H films have been doped with gallium. The films were prepared by magnetron cosputtering of silicon and gallium in an atmosphere of Ar and CH4. The effects of gallium concentration (z) on the structural, optical, electrical, and optoelectronic properties of the films were investigated. Doping at even very low Ga concentrations below Z=10−3 results in a reduction of both the dark and the photoconductivity, which were found to be decreased by more than two orders of magnitude, and the activation energy of the dark conductivity is increased by about 30%. An increase of z up to 10−2 leads to further reduction of the photoconductivity without any significant change of its activation energy, while the dark conductivity slightly increases and the corresponding activation energy drops to about 50% of its value observed for z<10−3. The optical band gap remains unchanged at such low Ga concentrations. These results indicate that the n-type conduction of the undoped film confirmed by a thermoelectric power experiment is compensated by the gallium doping; the type of conduction shifts from n to weak p with increasing z. At higher Ga concentrations z>10−2, infrared absorption measurements indicate a strong reduction of the H concentration in the films, which was taken as evidence for a distortion of the amorphous network by excessively incorporated Ga atoms, which suppress the attachment of H to dangling Si bonds. In this fluence regime the optical band gap shrinks by about a factor of 2 and the photoconductivity becomes quenched, while the dark conductivity increases by about six orders of magnitude.