L. Tichý

Index of refraction and d. c. electrical conductivity in Ge40−xSbxS60 glasses

The spectral dependence of the index of refraction (n) in Ge40−xSbxS60 glasses has been described by a simple one-oscillator approximation using the Wemple-DiDomenico treatment (W-D). The possibility of application of the Moss formula or W-D in some amorphous materials for estimation of the optical gap (Eg) fromn or vice versa has been examined. From the results of optical and d.c. electrical conductivity measurements and bond statistics suggestions it can be concluded that many types of defect states are created in Ge40−xSbxS60 glasses, which pin the Fermi level in the vicinity ofEg/2.

Photostructural changes in some ternary Ge-Sb-S chalcogenide layers

The changes of optical transmission and of the index of refraction were studied in flash evaporated Ge-Sb-S layers. Photobleaching, photodarkening and photoinduced crystallization were observed depending on the composition of layers. The effects are qualitatively explained using the idea of photoinduced atoms and/or chemical bonds rearrangement.

Preparation and some physical properties of semiconducting GeSb2Te4 crystals

High purity single crystals of GeSb2Te3 were prepared. Values of the electrical conductivity, thermoelectric power, Hall constant, Nernst-Ettingshausen coefficient and of the IR reflectivity in the plasma oscillation region were obtained. Analysis of the results yielded the dielectric constant (ϵg = 39), index of refraction nλ→0 = 6.15, the character of scattering of the free carriers, the Fermi level (EF = 0.31 eV) the relaxation time of the free carriers (〈τ〉= 0.86×10−14s) and the value of Npmc. To interpret the results, three-valley model of the energy band was proposed and values of the Hall structure factor (β = 0.67), the Hall scattering factor (M1 = 0.7), the free-hole concentration (Np = 4.8 ×1020 cm−3) and of the conductivity effective mass m∗ = 0.55 and m1 = m3 = 1.34 m0, m2 = 0.36 m0 were calculated.

Free carrier scattering mechanism in Bi2Se3 crystals

The “single valley” model proposed byKöhler andLandwehr is used for the calculation of transport parameters corresponding to “pure” Bi2Se3 crystals. From the measurements of transmission, reflectivity and the Hall constant the effective masses are determined,m⊥c=0·13m0 andm∥c=0·56m0; using a simplification described in the paper the Fermi level is calculated to lie 0·14 eV above the bottom of the conduction band. This result allows us to conclude that a mixed mechanism of free carrier scattering exists inn-Bi2Se3 crystals, viz., the scattering by acoustic phonons prevails contributing about 75% and the scattering on ionized impurities contributes the remaining 25%. This result applies to “pure” Bi2Se3 crystals with free electron concentration 2×1019cm−3.

Lattice defects of Bi2Se3 doped with germanium

Homogeneous Bi2Se3 crystals with various germanium content were prepared by means of modified Bridgman technique. Reflectivity measurements in the region of plasma resonance frequency of the free carriers revealed a shift of the reflectivity minimum towards shorter wavelengths for samples with higher germanium content. Combining the reflectivity data and the results of the Hall constant measurement we determined the free-carrier concentration for individual Ge-doped Bi2Se3 samples. It is shown that the germanium atoms give rise to donor levels. The ionization energy of these levels was estimated at less than 0·026 eV. The donor levels are most probably connected with the existence of singly ionized germanium atoms in interstitial positions.