Electron Paramagnetic Resonance, Electrical Conductivity, and Impurity Diffusion in Doped Boron

On excitons and other gap states in boron carbide

“…As worked out in [11] in some detail, the experimentally determined indirect band gap of 2.09 eV for boron carbide [20] yields the exciton bond energy of about 500 meV. Using the dielectric constant ε r ≈ 10 (see [2]), a carrier effective mass of the order of 10 m 0 is obtained, which agrees quite well with the evaluation of the dynamical conductivity of boron carbide [17,18], EPR [19], and several other experimental and theoretical investigations (see [2] and references therein).…”

Are there bipolarons in icosahedral boron-rich solids?

“…The band mass of free holes in boron carbide m p ∼ 10 m 0 estimated from the curvature of theoretical band structure calculations [9] was found to agree with the experimental value obtained from luminescence [51]. The effective masses estimated for mobile carriers from electrical conductivity and ESR (m * > 3 m 0 ) [30], from IR spectra (m * ∼ 3-5 m 0 , m * ∼ 10 m 0 ) [31,32], and from the dynamical conductivity (m * ∼ 1-10 m 0 ) [34,35] could be assigned to the (bi)polarons. However, obviously there is no indication that the effective mass of the mobile carriers in boron carbide is considerably larger than the band mass of free holes.…”

“…Obviously, without more detailed knowledge of the transport mechanisms of band type and hopping charge carriers, the low-field Hall effect does not yield quantitatively utilizable results. , spin density [42]; , [49,50] spin densities showing the strong spreading for differently prepared samples of nominally the same composition; properties for comparison; , free hole densities (Drude type), [34,35]; , free hole density, estimated from SdH oscillations (this work), experimental results [44]; , Hall density measured in high magnetic field (B = 0-15 T), [44]; •, Hall density [43]; •, Hall density [38].…”

“…The decrease in carrier mobility with increasing temperature fundamentally contradicts the (bi)polaron model. G -• -G, [49,50].…”

“…Boron carbide. Comparison of carrier densities determined by different experimental methods:, spin density[42]; ,[49,50] spin densities showing the strong spreading for differently prepared samples of nominally the same composition; properties for comparison; , free hole densities (Drude type),[34,35]; , free hole density, estimated from SdH oscillations (this work), experimental results[44]; , Hall density measured in high magnetic field (B = 0-…”