Characterization of amorphous semiconducting silicon-boron alloys prepared by plasma decomposition

Abstract

Amorphous semiconducting films of the Si-B binary system have been prepared by rf plasma decomposition of silane-diborane gas mixtures. The infrared vibrational modes, optical absorption, conductivity, spin resonance, and hydrogen content of these films have been studied as a function of preparation conditions. These alloy films differ substantially from chalcogenide glasses on the one hand and from group-IV amorphous semiconductors on the other. They contain between 10- and 45-at.% hydrogen and between ${10}^{18}$ and ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ unpaired spins. The spin-resonance linewidth is inhomogeneously broadened because of a large spectrum of bond distances and bond strengths. This spectrum yields a few very broad infrared absorption bands and, in the boron-rich films, an unusually broad and slowly varying absorption edge. This large spectrum of bond strengths results from the electron-deficient nature of boron which can associate with the environment in two-center and three-center bonds involving hydrogen, silicon, and other boron atoms. The slope of the conductivity curves decreases slowly without distinct exponential regions and without showing Mott's law of variable-range hopping near 77 K. Except for the two end compositions of the binary system, the Fermi level is close to the valence band, presumably because tetrahedrally coordinated boron atoms act as acceptors and produce a self-doping effect.