On the diffusion of free carriers in β-rhombohedral boron

To determine the diffusion of untrapped carriers in β-rhombohedral boron, we constructed a feedback pico-ammeter based on pulse integration technique. This enabled measuring deviations from the bias in a 109 Ω sample in the order of 1 nA with 0.7 ms time resolution. For the first time, we obtained the drift velocity of optically generated untrapped electron–hole pairs 106(20) cm s−1 yielding for the band-determined diffusion coefficient image and for the carrier mobility image. Fitting Fickʹs second law to the measured trap-determined dispersion of carriers yields the ambipolar diffusion coefficient image and 0.28(10) cm2 s−1 at 260 and 340 K, respectively. The thermal activation energy of 0.18 eV agrees with the well-known trapping levels in β-rhombohedral boron.