Henn H. Soonpaa

Optical constants of bismuth tellurium sulfide from ellipsometric data

The problem of determining the optical constants of uniaxial anisotropic crystals by ellipsometry has been studied. In case the optical axis is perpendicular to the surface—true in most of the cleavable crystals and in films where the anisotropy is induced by a size effect—it is practically impossible to evaluate the component Nc = nc + ikc (electric vector parallel with the optical axis) unless the values of n and k are very small. Reasonable values of Na = na + ika can be obtained, regardless of the Nc value assumed.

Electrical conductivity in a conductor 5 atoms thick

Abstract A single crystal 5 atoms thick comes close to the approximation of a quasi-two-dimensional system. Still, it retains some characteristics of the bulk material as revealed by the virtual absence of any contact effects as long as the contacts consist of thicker areas of the same crystal. Electrical conductivity and electron mobility are strongly influenced by adsorption on the surface. The onset of an additional conduction process at high current density and low temperature is reported.

A precision measurement of film thicknesses

Abstract The method of “direct counting” thickness measurements has been extended to the analysis of the shapes of diffracted X-ray beams. The spacings of subsidiary maxima of a diffracted beam directly correlate with crystal thickness. When the crystal thickness can vary only in steps of several atomic spacings (9.8173 A or five atoms in Bi8Te7S5), the separation of subsidiary maxima can satisfy only one number of quintuple layers — hence the name “direct counting”.

Electrical conductivity in an extremely thin single crystal

Abstract Size effect quantization has been shown to cause a sharp deviation from Fuchs equation. Crystals of Bi 8 Te 7 S 5 have atomically smooth surfaces, and their thicknesses are known to the exact number of atoms. In these crystals the number of possible values for the Fermi momentum is limited.

Size effect semimetal to semiconductor transition

Abstract Semimetallic Bi 8 Te 7 S 5 changes into a semiconductor as a result of size effect quantization. In crystals of thicknesses 20 atoms or less an energy gap appears, and it increases with decreasing thickness. These energy gaps have been measured.

2-D conductance and magnetoconductance in a thin crystal of Bi14Te11S10.

Abstract In a crystal of Bi14Te11S10 the number of scattering centers is manipulated by allowing gases to adsorb/desorb on the surface. An increase in the number of scattering centers causes the temperature dependence of the low temperature conductance to change from logarithmic to exponential and the magnetoresistance to change sign from positive to negative.

Minimum metallic conductivity in a thin crystal

Abstract Electrical conductivities of thin crystals of Bi 2 (Te,S) 3 measured from 4.2°K to 300°K fall into four regions: 1) σ −5 S with positive temperature coefficient of conductivity; 2) 1.3×10 −5 S −5 S with temperature independent conductivity; 3) 1.4×10 −5 S σ −5 S with negative temperature coefficient of conductivity, and 4) σ > 4×10 −5 S with hardly any temperature dependence. A disproportionately high fraction of samples falls into the second range; 1.3×10 −5 S −5 S.

Low field magnetoconductivity in thin crystals

Low field magnetoconductivity measurements have been performed on thin crystals of Bi2(Te1-xSx)3. Δσ(B) > 0 when σ σmin, the minimum metallic conductivity. Δσ(B) vs B/(T-Tc) curves are superimposed for different temperatures; Tc < 0. At high source-drain fields E Δσ(B) vs B/ (T-Tc + aE) curves are approximately superimposed for different fields; a is a positive coefficient. For very low conductivity samples, however, Δσ(B) increases with increasing E. At temperatures T < 3 °K a “reverse hysteresis” is observed—when scanning across B = 0 σ(rmB) reaches its maximum (or minimum) before B = 0.

Thin crystal as a 2d system

Abstract Cleaved thin crystals of Bi2(Te1−1Sx)3 are discussed as a 2D system. Their characteristics are compared to those of the more widely explored system of silicon inversion layers. There are many similarities and some differences, which might disappear when the proper experiment will be performed on both systems. The most basic difference seems to be the low effective mass and long relaxation time for silicon inversion layers and the homogeneity of the thin crystal film especially in field effect devices.

Peaked structure in a single-crystal film

Abstract A peaked structure, similar to that previously observed in the field effect mobility versus gate voltage experiments on silicon inversion layers, has been observed on single-crystal films. These films and contacts to them are the same continuous crystals; only the thicknesses are different. A double modulation circuit, which measures the field effect mobility and the conductivity simultaneously, is sensitive enough to measure field effect mobilities of less than 1 × 10 -5 m 2 V -1 s -1 . The fine structure is reproducible in great detail, and some regularity seems to appear at very low temperatures.