Electronic properties of substitutionally doped amorphous Si and Ge

Abstract

It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10⁻⁶ and 10⁻² parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 10¹⁹ cm⁻³, roughly equal to the number of states in the band tail. It is suggested that, in general, donor sites lie within the energy range of the electron tail states. The use of doped specimens in field effect measurements has considerably extended the range of the experimentally determined density of state function and new results are presented. It is concluded that the main material requirement for effective doping in an amorphous semiconductor is a very low overall density of gap states; in this respect evaporated or sputtered films are far less suitable than glow discharge specimens.