On the dynamical conductivity in icosahedral boron-rich solids

Abstract

The FIR spectra of boron-rich solids measured down to 10 cm −1 exhibit strongly increasing reflectivities towards lower frequencies. These spectra are well described when a superposition of Drude-type and hopping-type dynamical conductivity is assumed. The corresponding fits were simultaneously made for the real and the imaginary part of the dielectric function to achieve unambiguous parameters. The results obtained can be described consistently, for example, with DC conductivity measurements, and moreover in the case of β-rhombohedral boron, with the known energy band scheme. The bipolaron hopping model of the electronic transport in boron carbide is proved to be inappropriate.

Evidence of the Superposition of Drude Type and Hopping Type Transport in Boron-Rich Solids

“…The parameters determined by fitting the optical spectra (for the complete set see (19)) are to be compared with other experimental results. At first, the squared plasma frequency , which is proportional to the density of Drude-type-free carriers, is compared in Fig.…”

Electrical transport in amorphous semiconducting AlMgB14 films

“…However, XPS measurements also indicated significant amounts of O and Fe in the films, which was discussed elsewhere. 13 XPS also indicated that there is no appreciable difference in composition between room-temperature-and 573 K-deposited AlMgB 14 films. The microstructure of AlMgB 14 films, which was examined by transmission electron microscopy, 12 remains amorphous regardless of substrate temperature, and no evidence of conducting inhomogeneities or crystalline domains was observed.…”

“…Compared with the electrical resistivity of other boron-rich boride films, which typically cover a wide range of values from ϳ10 3 to 10 9 ⍀ cm, the electrical resistivity of room-temperature-and 573 K-deposited AlMgB 14 films is approximately three to eight orders of a)…”

“…A well-defined linear behavior is evident, which is somewhat unusual given an amorphous semiconductor with complex composition, as such a "clean" doping behavior suggests that a single donor level is providing the conduction electrons. The activation energies are 0.13 eV for the roomtemperature-deposited AlMgB 14 film and 0.17 eV for the 573 K-deposited AlMgB 14 film, indicating that the Al, Mg, and/or Fe introduce a donor level below the conduction-band edge. Due to the presence of minor amounts of Si ͑Ͻ1 at.…”

“…Figure 3 shows the Tauc plots of AlMgB 14 films, from which the optical band gaps are determined to be ϳ0.5 eV, and it appears that the deposition temperature does not have a strong effect on the optical gaps. Therefore, the different carrier concentrations in AlMgB 14 films can only be attributed to the trap states of different characters, i.e., an increase in deposition temperature leads to a higher density of well-formed B 12 icosahedra, 12 which, in turn, gives rise to more electron trap states in the gap. As the density of these trap states increases, a decrease in carrier concentration is expected, which is indeed observed in this study.…”