Interband and Gap State Related Transitions in β‐Rhombohedral Boron

Abstract

The anisotropy of the electronic interband transitions of 0-rhombohedra1 boron is determined by optical absorption measurements. The band gaps (indirect allowed transitions) are 1.32(1) and 1.50(1) eV for E 11 c, respectively, 1.29(1) and 1.46(1) eV for E I c (extrapolated to T = 0 K). At low temperatures these gaps decrease, while the thermal equilibrium develops. The temperature dependence of the gap isattributed to the thermaldisorder caused byphonons of 121.9(E 11 c)and 94.1 meV(E I c) representing the most prominent intraicosahedral vibrations. Six intrinsic trapping levels for electrons in distances between 0.17 and 1.15 eV from the conduction band edge are determined, the closer ones in agreement with different photo effects. The trapping levels are attributed to distortion states of the icosahedra due to a dynamical Jahn-Teller effect. This conception allows the interpretation of other physical properties like internal friction, discontinuities of thermal expansion, and electrical conductivity, of the temperature dependence of the ESR linewidth, and of the density of paramagnetic centers (spin density and magnetic susceptibility).Die Anisotropy der elektronischen Interbandubergange des P-rhomboedrischen Bors wurde aus der optischen Absorption bestimmt. Die Bandabstande (indirekt erlaubte Ubergange) betragen, extrapoliert auf T = 0 K, 1,32(1) und 1,50(l)eV fur E 11 c bzw. 1,29(1) und 1,46(l)eV fur E l c. Bei tiefen Temperaturen nehmen die Bandabstande im Verlauf der Einstellung des thermischen Gleichgewichtes ab. Der Temperaturgang der Bandabstande wird der temperaturbedingten Fehlordnung des Gitters durch die thermische Anregung der starksten intraikosaedrischen Phononen bei 121.9 ( E 1) c) und 94,l meV ( E i c) zugeordnet. Sechs intrinsische Haftniveaus fur Elektronen in Ahtanden zwischen 0,17 und 1,15 eV vom Leitungsband werden nachgewiesen, die bandnahen sind in Ubereinstimmung mit verschiedenen Photoeffekten. Die Haftniveaus werden Verzerrungszustanden der Ikosaeder infolge eines dynamischen Jahn-Teller-Effektes zugeschrieben. Diese Konzeption erlaubt die Erklirung anderer physikalischer Eigenschaften wie der inneren Reibung, der Diskontinuitaten der thermischen Dilatation und der elektrischen Leitfahigkeit, der Temperaturabhangigkeit der ESR-Linienbreite sowie der Konzentration lokalisierter paramagnetischer Zentren (Spindichte und magnetische Suszeptibilitat).

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

“…While for B : V, which becomes n-type already at 0.5 at.%, the density increases monotonously with increasing metal content, for B : Co, which is p-type for all compounds investigated, a weak decrease is followed by a certain increase of the density. The results can be consistently interpreted within the band scheme of -rhombohedral boron (32). When the doping level of V inrhombohedral boron is close to the conduction band edge, the density of hopping sites within this level probably overlaps with intrinsic trapping states increases with increasing V content.…”

Boron Quasicrystals and Boron Nanotubes: Ab Initio Study of Various B96 Isomers

“…Such material properties are very useful in thermoelectrical and nuclear devices (4). Werheit et al (5) experimentally determined many of the properties of -rhombohedral boron and measured the related band gap to be 1.56 eV.…”

Nickel doping of boron carbide grown by plasma enhanced chemical vapor deposition

“…States pinned to one band edge have been proposed for boron carbide 2 and have been identified pinned to conduction band edge. 15,16 It may be that such states are occupied or filled through nickel doping. Due to the location of the gap states near the conduction band edge, much larger applied voltages should be needed to observe the negative differential resistance for diodes constructed with p-type Si͑111͒, as compared with diodes constructed with n-type Si͑111͒.…”