Structural modulations in Bi2Te3

Abstract

Bi 2 Te 3 is known for its large thermopower, a low thermal conductivity, and thereby a large thermoelectric figure of merit ZT at room temperature. Particularly, the low thermal conductivity is attributed to a high structural disorder. Electron probe microanalysis, transmission electron microscopy (TEM), and energy dispersive x-ray spectrometry in the TEM were applied on n-type Bi2(Te,Se)3 and p-type (Bi,Sb)2Te3 bulk materials for structural and chemical analysis. Significant variations in stoichiometry were found on the micrometer scale. Bulk materials showed a texture, grain sizes of 1−10 μm, and dislocations in the basal plane with a density of 109 cm−2 having a high mobility at room temperature. A structural modulation [natural nanostructure (nns)] was found on the nanometer scale. This nns was also observed in Bi2Te3 thin films and Bi2Te3/Bi2(Te,Se)3 superlattices and turned out to be of general character for Bi2Te3 materials. The nns was analyzed in detail by stereomicroscopy in the TEM and by image simulation. The nns was found to be a pure sinusoidal displacement field with (i) a displacement vector parallel to ⟨5,−5,1⟩ and an amplitude of about 10 pm and (ii) a wave vector parallel to {1,0,10} and a wavelength of 10 nm. Bi2Te3 samples from different batches produced under similar conditions showed different characteristics with respect to the nns: none, one, or two superimposed nns were observed. The nns is present in all parts of the sample with the same orientation. The formation of the nns is either bound to a certain stoichiometry range or to the thermal history of a sample and is related to the tendency of Te compounds to form amorphous phases. An ordered network of dislocations a few nanometers apart and chemical fluctuations on the nanometer scale as origins of the nns were ruled out. The displacement field of the nns is superimposed to the average structure and should significantly affect the thermoelectric properties. Particularly, the lattice thermal conductivity should be decreased due to phonon scattering on the sinusoidal strain field of the nns. Also, the nns should yield a one-dimensional or zero-dimensional behavior of the phonons and anisotropic transport coefficients in the basal plane.