Amorphous hydrogenated silicon suboxides $(a-{\mathrm{SiO}}_x:\mathrm{H})$ deposited by plasma enhanced chemical vapor deposition have a band gap which can be tuned from $1.9$ to 3.0 eV by varying the oxygen content [O] from $0$ to 50 at. %. n- and p-type doping is realized by adding ${\mathrm{PH}}_3$ and ${\mathrm{B}}_2{\mathrm{H}}_6,$ respectively, to the source gases ${\mathrm{SiH}}_4,$ ${\mathrm{H}}_2,$ and ${\mathrm{CO}}_2.$ Alloying with increasing amounts of oxygen reduces the average coordination number $〈r〉$ from a value close to $4$ $(a-\mathrm{Si}:\mathrm{H})$ to $\approx 2.7,$ which gradually approaches the ideal value of $〈r〉=2.4\mathrm{}$ for network glasses. This goes along with a softening of the amorphous ${\mathrm{SiO}}_x$ network, i.e., a reduction of the mechanical hardness of the material, which is also predicted by rigidity percolation theory. Also the incorporation of dopant atoms into electrically active, fourfold coordinated sites becomes more unlikely with increasing [O]. As a consequence, n- and p-type doped ${\mathrm{SiO}}_x$ shows increasingly intrinsic character for higher oxygen concentrations. Doping fails for values of $〈r〉<3\mathrm{}$ and the doping efficiency tends towards zero. Thus, an overall fourfold coordination was found to be a crucial requirement for efficient doping in amorphous semiconductors.

• Received 29 July 2003

DOI:https://doi.org/10.1103/PhysRevB.69.115206