Modulated Photoconductivity of High-Purity and Carbon-Doped beta-Rhombohedral Boron

Frequency-modulated photoconductivity spectra of high-purity and up to 1.1 at.% carbondoped beta-rhombohedral boron were measured between about 96 and 380 K to get information on electronic transport with small relaxation times. At T<190 K steps in the photoconductivity spectra are correlated with the occupied well-known electron traps. Between about 250 and 350 K the low-energy photoconductivity shows peaks at about 0.23 and 0.46 eV, which can be attributed to carbon donor states in the band gap. For T>170 K the interband recombination is fast enough to become appreciable in the dark phase of the modulation, and the interband photoconductivity becomes prevailing. The interband transition energies known from absorption measurements are confirmed. The thermal activation energy of the electron mobility is about 14 meV, probably due to a shallow level off the conduction band.