The possible IR- and Raman-active phonons of α-boron. β-boron, and boron carbide have been obtained by group theoretical methods. Reflectivity and absorption measurements in the spectral range from 10 to 1450 cm
The optical absorption spectrum of &,C, exhibits an indirect interband transition with onset at A UJ = (0.48k0.03) eV. I n the subsequent region of lower absorption (a = 400 cm-*) traps of great density are essentially effective for the optical properties. The lattice vibration spectrum starting a t 4 pm has oscillations a t 6.55, 9.25, 10.5, 11.9, 14.4, 17.3, and 25.4 pm. A monotonic increase of the reflectivity occurring at wavelengths t > 30 pm can be assigned t o the strongly damped plasma resonance of free holes. The comparison with the properties of P-rhombohedra1 boron indicates distinct similarities.Das optische Absorptionsspektrum von B&, zeigt einen bei fi w = (0,48* 0,03) eV einsetzenden indirekten Interbandiibergang. I n dem a n die Kante anschlieDenden Bereich geringerer Absorption (a = 400 cm-l) wirken sich Haftstellen groSer Zustandsdichte stark auf die optischen Eigenschaften aus. Bei 4 pm setzen die Gitterschwingungen ein, welche im Reflexionsspektrum zu Resonanzstellen bei 6,55; 9,25; 10,5; 11,9; 14,4; 17,3 und 25,4 K r n fuhren. Ein monotoner Anstieg des Reflexionsverrnogens fur I > 30 pm wird durch die stark gedampfte Plasmaschwingung freier Locher erkliirt. Ein Vergleich mit den Eigenschaften des P-rhomboedrischen Bors zeigt deutliche Parallelen auf.
By use of an etching solution of 70% by weight H2O, 22% K3[Fe(CN)6] and 8% KOH largely plane sample surfaces can be obtained, the quality of which corresponds to electrolytically polished metal surfaces. A possibly existing crystalline surface layer does not exceed a thickness of 25 Å. The method is suitable for preparing extremely thin samples, which are transparent for visible light and high energy electrons. X-ray investigations and electron micrographs indicate lattice defects of great concentration in the available monocrystals.
I n the intrinsic range the anisotropy of the reflectivity amounts to about 2% while the dichroism is very small at high photon energies and can only be measured when a & 2 x X lo3 cm-l. Especially distinct is the anisotropy of the lattice vibrations. Similar to polycrystalline boron two indirect interband transitions occur; the energy gaps are 1.561 and 1.39 eV with E 11 c and 1.541 and 1.34 eV with E 1 c. The edge tail is dominated by an absorption band, which is possibly caused by C or Si impurities.Im Bereich der Eigenabsorption betragt die Anisotropie des Reflexionsvermogens etwa 2%, wahrend der Dichroismus im Bereich hoherer Photonenenergien sehr klein ist und erst bei Absorptionswerten a 2 x lo3 cm-l meBbar wird. Die Anisotropie der Gitterschwingungen ist jedoch sehr ausgepragt. Wie in polykristallinem Bor treten zwei indirekte Interbandiibergange auf; die Bandabsttnde betragen fur E 11 c 1,561 und 1,39 eV, fur E 1 c 1,541 und 1,34 eV. Der Kantenauslaufer wird von einer Absorptionsbande beherrscht, die wahrscheinlich auf Coder Si-Verunreinigungen zuruckzufuhren ist.
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