Substitutional and interstitial doping of amorphous silicon nitride

Abstract

Doped films of amorphous silicon nitride have been prepared by the glow-discharge technique over a wide range of ammonia-silane mixtures. Phosphorus- and boron-doped specimens were produced by admixing measured amounts of phosphine or diborane during deposition. Interstitial doping of specimens was attempted with sodium by ion-beam implantation, and with lithium by evaporation and in-diffusion. The electrical conductivity and the optical gap of these specimens have been determined as a function of the volume ratio of ammonia to silane. All four dopants raise the conductivity of low-nitrogen-content specimens to about 10^-2ω^-1cm^-1 at room temperature, without changing the optical gap. With increasing nitrogen content the effect of phosphorus, boron and sodium doping decreases rapidly. In contrast, incorporation of lithium significantly increases the room-temperature conductivity of even high-nitrogen-content films. By careful control of the preparation conditions, near-stoichiometric, optically transparent specimens have been produced in which the room-temperature conductivity has been increased by fourteen orders of magnitude through lithium doping.