Icosahedron  Helmut Werheit | Prof. Dr. rer. nat website:   http://www.werheit.mynetcologne.de/
e-mail:   helmut.werheit@koeln.de
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Object of Research

Semiconductor and Solid State Properties of
Boron and Boron-Rich Solids

Object of the research ist the elucidation of the interrelations between the electronic and the structural properties of solids with complex crystal structures, in particular of boron-rich solids. This knowledge is a necessary requisite to tailor materials for specific technical applications.

For some boron-rich solids, whose structures are determined by icosahedra, we were able to prove the quantitative correlation between the concentrations of structural defects and the densities of gap states (split-off valence states ), which determine the electronic properties of these solids [119,120].

Fundamental research on the modifications of elementary boron and of boron-rich compounds of boron:

  • Determination of energy band structure parameters of boron-rich solids by experimental methods.
  • Determination of electrical, thermoelectrical, magnetoelectrical and photoelectrical transport properties.
  • Investigation of the soliton character of the drift mechanism of optically generated electrons and holes.
  • Research of the spectra of IR-active and Raman-active phonons of complex structures to determine details of structure, that are not detectable by x-ray or NMR studies.
Applied research for the systemtic developement of boron compounds for specific applications:
  • Determination of the interrelations between structural and physical properties of boron-rich solids by the systematic variation of the specific composition and of the preparation parameters.
  • Optimizing of the electronic transport properties of specific boron compounds for technical application.
  • Optimizing of the thermoelectric properties, in particular of boron carbides, for direct thermoelectric energy conversion.

Research on the semiconductor properties of other refractory solids.
  • Comparative investigation of the electric properties of icosahedral boron-rich solids and icosahedral quasicrystals.

Investigated Solids:

Boron (elementary): alpha-rhombohedral, beta-rhombohedral, amorphous
Boron carbide: whole homogeneity region  B 4.3C to B~11C
Strukture group of alpha-rhombohedral boron
Strukture group of beta-rhombohedral boron
Strukture group of alpha-tetragonal boron
Strukture group of beta-tetragonal boron
Strukture group of the  MgAl B 14  type
Strukture group of the YB66 type 
Metal hexaborides (metallic, semiconducting, intermediate valency)

Optical properties:

        Interband transitions
        Transitions in connection with structural defects
        Phonons (IR and Raman)
        Plasma vibrations
Electronic transport properties:
Elektrical conductivity
Dynamical conductivity
Hall effect
Thermoelectric properties
Diffusion and drift of optically excited carriers 

Structure properties:

Elucidation of structural details, which cannot be detected by X-ray or NMR investigations because of their statistical distribution in the structures. Main object is the distribution of compound atoms and defects in boron compounds. For example in the case of boron carbide, we succeeded in determining the quantitative distribution of B12 and B11C icosahedra, C-B-C, C-B-B, C-C-C, and B-[vacancy]-B arrangements  throughout the homogeneity range.
In the case of RE-metal dodecaborides was proved that boron vacancies, small shifts of the metal atoms from their idealized position, and even the distribution of the boron isotopes in boron lift the selection rules sufficiently to detect them with Raman spectroscopy [136].

     Investigations on SiC:

 Investigation of the properties of single crystal and sintered SiC [128, 134]