5 - Substitutional doping

Summary

In 1975 Spear and LeComber reported that a-Si:H could be doped by the addition of boron or phosphorus; their conductivity data are reproduced in Fig. 5.1. This first observation of electronic doping in an amorphous semiconductor set the stage for the subsequent development of a-Si: H electronic technology. The addition of small quantities of phosphine or diborane to the deposition gas results in changes in the room temperature conductivity by more than a factor 10⁸. The activation energy decreases from 0.7 –0.8 eV in undoped material to about 0.15 eV with phosphorus doping and 0.3 eV for boron. Subsequent experiments confirmed that the conductivity change was due to a shift of the Fermi energy, and that n-type and p-type conduction was occurring (Spear and LeComber 1977). The explanation of the results in terms of substitutional doping has never been doubted.

Examples of the conductivity temperature dependence o(T) of ntype and p-type a-Si: H are shown in Fig. 5.2 (Beyer and Overhof 1984). The thermally activated G(T) implies that the Fermi energy always remains in localized states and there is never metallic conductivity. EF is prevented from reaching the conducting states above the mobility edge by the high density of band tail localized states and also by a low doping efficiency. The conductivity is lower in p-type samples than ntype, primarily because the wider valence band tail keeps EF farther from the mobility edge.