Optical absorption and transport mechanisms of dual ion-beam-deposited boron-rich boron nitride films

The optical absorption and electrical conductivity of amorphous B-rich boron nitride (BNx) films with N contents varying from 2.2 to 40.2 at.% were investigated. The density of states (DOS) of the films was extracted from the observed optical absorption spectra. All types of electron transitions from occupied initial states to empty final states were considered, including those between two localized states. Furthermore, a Gaussian-shaped energy band of localized states was added to the center of the pseudo-gap in order to obtain good fits for most of the spectra. The major conclusions are as follows. (1) The activation energy deduced from the temperature dependence of the electrical conductivity is low compared to the optical band gap, suggesting that hopping of charge carriers near the Fermi level dominates the electrical transport properties of the films. (2) With increasing N content, the hopping distance of charge carriers increases and the spread of the electron wavefunction narrows. This leads to a more than seven orders of magnitude drop in the room temperature electrical conductivity.