On excitons and other gap states in boron carbide

Abstract

The excitons in boron carbide recently independently proved to be present by luminescence measurements (by Schmechel and co-workers) and by x-ray Raman scattering investigations in connection with ab initio calculations (by Feng and co-workers) are discussed, taking the actual structure of boron carbide within the homogeneity range (B4.3C–B∼11C) into account. The excitonic levels at 1.560 and 1.5695 eV obtained from the luminescence spectrum are attributed to the central B atom in the C–B–C and the C–B–B chains respectively. Considering the transition energies related to the different gap states obtained by optical and electrical measurements for energies below the interband transition of 2.09 eV, an actualized energy band scheme of boron carbide is presented.

Raman effect in icosahedral boron-rich solids

“…The homogeneity range of the chemical composition extends from B 4.3 C at the carbon-rich to about B ∼11 C at the boron-rich limit (see [40]). The structure is a b The resulting crystalline structure can be described using the formula (B 12 ) n (B 11 C) 1−n (CBC) p (CBB) q (B B) 1−p−q determined from the IR and Raman-active phonon spectra and other experimental results [41][42][43][44][45]. X-ray and nuclear magnetic resonance methods, which average over large sample volumes, fail in such cases.…”

“…Figure 6 shows the fundamental problem of the Raman spectroscopy of boron carbide: the spectra are qualitatively different when excited by different laser energies. The problem is related to the extremely high absorption coefficient of boron carbide in the range of fundamental absorption [49,50]. For these energies, the penetration depth of the exciting laser radiation (typically the 2.546 eV line of an Ar ion laser) is very shallow, and the bulk phonons do not contribute much to Raman scattering.…”

Electronic structures and mechanical properties of boron and boron-rich crystals (Part I)

“…For β-rhombohedral boron, the energy gap is estimated at 1.56 eV, but a considerable density of gap states covers a wide range of the gaps [103]. For boron carbides, an indirect gap was once identified as 0.48 eV by optical measurements [104].…”

“…For boron carbides, an indirect gap was once identified as 0.48 eV by optical measurements [104]. Since then, the presence of high-density of gap states and strong excitons has been recognized [103]. According to recent experiments [103], the band gap has been modified as 2.09 eV.…”

“…Since then, the presence of high-density of gap states and strong excitons has been recognized [103]. According to recent experiments [103], the band gap has been modified as 2.09 eV. On the other hand, all band calculations so far published show gaps as large as 3 eV [74,[105][106][107][108][109][110].…”