Optische und photoelektrische Eigenschaften von β‐rhomboedrischem Bor. I. Optische Konstanten, Bandkante und Absorption freier Landungsträger

Abstract

The temperature dependence of the absorption spectrum from the edge to the inset of the lattice vibrations can be described by indirect band-band transitions and excitations from a thermally occupied trap level, the existence of which has been proved in the second part of this paper by illumination a t low teinperatures changing the absorption and photoconductivity spectrum inportantly. -Free carrier absorption superimposed on the lattice spectrum has been found a t high temperatures. Its increase proportional to suggests scattering by charged centres. Their density is about 5 x 1O20 em+ surpassing impurities by a factor of 102. The effective mass of holes is estimated at 8 m, corresponding to a conductivity by hopping processes. The complicated lattice vibration spectrum is nearly independent of temperature between 4 and 13 pm indicating that there are no combination bands.

Phase transition in β-rhombohedral boron near 550 K

“…In any case, taking the temperature-dependent structure investigations by Lundström et al [8] (see Fig. 2a), which are admittedly unsatisfactorily resolved, and the temperature-dependent phonon spectra by Jaumann and Werheit [22,23] up to w870 K into account, it seems evident that this phase transition is restricted to local changes of the b-rhombohedral boron structure, whose basic parameters largely persist.…”

Optical Anisotropy of β‐Rhombohedral Boron from 0.4 to 16 μm

“…It seems to be probable that it is caused by impurities, and the results of analysis in Table 1 point t o C and Si. I n the much purer polycrystalline boron of the same producer (C concentration 30 ppm) the corresponding absorption band has not been found [6], and also in the absorption measurements of Juryska on single crystals [12] it cannot be recognized. But the impurity level with the activation energy of 0.6 eV, which has been derived from elastoresistance measurements by Fuhs [23], can probably be identified with this level.…”

“…Contrary to polycrystalline boron [6] the edge tail in the absorption spectra of the single crystals ( Fig. 1) is dominated by an additional absorption process being distinctly different for both crystals and both crystal orientations.…”

“…The lattice vibration spectrum gives the strong anisotropy of the numerous bands which had been found in polycrystals [6]. Especially conspicious is the anisotropy in the range of the band a t 8 pm the oscillator strength of which with polarization E 11 c is comparable to the strongest bands a t longer wavelengths.…”

Interaction of Optically Excited Carriers with Intraicosahedral Phonons

“…This shift is insignificant with respect to its value. However, its sign proves that the effect discussed is not due to an increase of temperature by the optical excitation, which would cause an opposite shift (12,13) There seem to be rather broad bands in the difference spectra of -rhombohedral boron in Fig. 1 with maxima at about 100 and 500 cm\ (70 and 130 meV) evoked by the optical excitation.…”