Dependence of the doping efficiency on material composition in n-type a-SiO_x:H

Amorphous hydrogenated silicon suboxides (a-SiO_x:H) deposited by plasma enhanced chemical vapour deposition (PECVD) have a band gap which can be tuned from 1.9 to 3.0 eV by varying the oxygen content from 0 to 50 at.%. n- and p-type doping is realised by adding PH₃ and B₂H₆, respectively, to the source gases SiH₄, H₂ and CO₂. Alloying with increasing amounts of oxygen reduces the mean co-ordination number 〈r〉 from a value close to 4 (a-Si:H) to approximately 2.7, which gradually approaches the ideal value of 〈r〉=2.4 for network glasses. Thus the incorporation of dopant atoms into electrically active, fourfold co-ordinated sites becomes more unlikely with increasing [O]. As a consequence the conductivity, defect density and doping efficiency in phosphorus doped n-type SiO_x undergo drastic changes and show increasingly intrinsic character for higher oxygen concentrations. The dependence of the doping efficiency on average co-ordination 〈r〉 is examined in a quantitative manner.