Evidence for multiple polytypes of semiconducting boron carbide (C₂B₁₀) from electronic structure

Abstract

Boron carbides fabricated via plasma enhanced chemical vapour deposition from different isomeric source compounds with the same C2B10H12 closo-icosahedral structure result in materials with very different direct (optical) band gaps. This provides compelling evidence for the existence of multiple polytypes of C2B10 boron carbide and is consistent with electron diffraction results.

The local environment of Co in B5CHx

“…6 Figures 2 and 3 and the underlying calculations show that a single icosahedra having cobalt within the cage does have a larger band gap ͑8.5 eV͒ than the dimeric icosahedra cage structure ͑7 eV͒. The addition of a boroncarbide matrix about the cluster, with the removal of many of the terminating hydrogen atoms, further reduces the band gap, as confirmed by larger cluster calculations.…”

“…[2][3][4][5] These boron carbides have the approximate stoichiometry of B 5 CH x and exhibit a range of p-or n-type electronic properties, presumably as a result of differing polytypes. 5,6 Because these semiconductor materials have the icosahedra as the basic "building block," the icosahedra have the composition C 2 B 10 H x ͑x ϳ 1͒, with the 12 main group atoms providing the basic structural framework ͑of the icosahedra͒. Successful n-type doping of ␤-rhombohedral boron has been accomplished with transition metal dopants such as iron, [7][8][9][10][11] vanadium, 10,11 chromium, 10,11 nickel, 11 while cobalt, 10,11 and zirconium 10 are more likely p-type dopants.…”

The coadsorption and interaction of molecular icosahedra with mercury

“…Models for increasing donor and acceptor state densities are complicated by the obser vation that there are several polytypes of similar composition boron carbides (studies in some detail for the nomi nally C 2 B 10 semiconducting boron car bides [4,25,26]) and both n-type and p-type semiconducting boron carbides are possible without transition metal doping.…”

“…In the absence of transition metal doping, it is clear that to some ex tent the behavior of molecular carbo rane fi lms does provide some indica tion of the properties of dehydrogenated C 2 B 10 icosahedra that are the building blocks of semiconducting boron car bide [4], in spite of the vastly larger HOMO-LUMO band gap of the mo lecular fi lms when compared to the direct band gap of the various semi conducting boron carbides [25]. We show evidence here that neither the molecular analogs, nor the associated hydrogenated molecular species (in cluding the main group source com pounds for fabricating semiconducting boron carbide) provide a good indica tor of the metal doped semiconduct ing boron carbide properties.…”

The all boron carbide diode neutron detector: Comparison with theory

“…In looking at the two semiconducting boron carbides formed from the PECVD decomposition of the different closodicarbadodecaboranes we note that, although the direct band gaps are very similar [32], their contribution to the depletion region of a diode is slightly different, as indicated in Fig. 2.…”