Study of structural-, compositional-, and thickness-dependent thermoelectric and electrical properties of Bi93Sb7 alloy thin films

Abstract

We have used the melt-quenching technique to prepare the bulk material and vapor-quenching technique to prepare the thin films of Bi93Sb7 alloy. The Bi93Sb7 alloy thin films of different thicknesses were grown onto well-cleaned glass and silicon substrates. The films were annealed at 150 ° C for 4 h in a vacuum of the order of 10−6torr in order to remove the defects and to increase the grain size. The bulk and thin-film x-ray diffraction results agree with the transmission electron microscopy results and the compositional analysis of bulk by particle-induced x-ray emission and of thin films by Rutherford backscattering. The thickness and temperature dependences of thermoelectric power and electrical resistivity have been analyzed. The negative temperature coefficient of resistivity confirmed that the material is semiconducting in nature. The negative thermoelectric power confirmed that the present bismuth-rich material is a n type. In this paper we have made an attempt to study the thermoelectric properties of bulk as well as thin films of Bi93Sb7, maintaining the same composition. The scattering index parameter was calculated from the experimental data and was compared with the theoretical predictions of the size effect theory.

Changes in the Crystal Lattice Parameters of Bismuth Films on Substrates with Different Thermal Expansion

“…As was shown earlier, [16][17][18][19][20][21][22] all films have a predominant direction of the trigonal axis of crystallites, which is perpendicular to the film plane. Therefore, the largest observed maxima resembles the maxima in the diffraction pattern of a single crystal of bismuth that is cleaved along the plane of perfect cleavage (trigonal plane) (Figure 2a), which is used as a reference.…”

“…Numerous studies have demonstrated that bismuth films produced under the given conditions had a block-textured structure with crystallite sizes larger than the film thickness, [13][14][15] and the predominant orientation of the blocks' trigonal axis perpendicular to the film surface. [16][17][18][19][20][21][22] Since in this work, the emphasis was on crystallites whose trigonal plane lies in the plane of the film, the values of the hexagonal lattice constant c were obtained for them. For this, the Wulff-Bragg equation was solved:…”

“…As shown in multiple studies, [13][14][15][16][17][18][19][20][21][22] bismuth films have a block (or crystallite) structure, with the trigonal axis predominantly perpendicular to the film plane. The size of these crystallites is equal to or greater than the film thickness.…”

Constructing Anisotropic Single-Dirac-Cones in Bi1–xSbx Thin Films

“…The classical confinement effect comes from the film boundary scattering mechanism, which can be described by the empirical relation σ l z /σ bulk = C 1 · l z ·( C 2 −ln l z ), ,− where σ l z is the electrical conductivity for the thin film with thickness l z , σ bulk is the bulk electrical conductivity, and C 1 and C 2 are empirical constants that can be measured by experiments. This simple relation is very successful in describing the Bi thin films and Bi 1– x Sb x thin films. ,− On the basis of the above, we know that the main factor that determines the electrical conductivity in a Bi 1– x Sb x thin film is also the carrier group velocity, and because the L (1) -point Dirac fermions have much larger group velocities than the T -point parabolically dispersive fermions, we believe that the electrical conductivity is dominated by the L (1) -point Dirac cone.…”

“…For the L -point Dirac fermions, ν x ( E f ) [Dirac] ∼ 10 –2 · c light , whereas for the T -point parabolically dispersive fermions, ν x ( E f ) [ T ] ∼ 10 3 m/s, thereby explaining why σ xx [Dirac] ≫ σ xx [ T ] . The difference of electrical conductivity between the thin film Bi 1– x Sb x and the bulk Bi 1– x Sb x can be explained by the classical confinement effect and the quantum confinement effect. ,− The quantum confinement effect is accounted for by the band structure itself . The classical confinement effect comes from the film boundary scattering mechanism, which can be described by the empirical relation σ l z /σ bulk = C 1 · l z ·( C 2 −ln l z ), ,− where σ l z is the electrical conductivity for the thin film with thickness l z , σ bulk is the bulk electrical conductivity, and C 1 and C 2 are empirical constants that can be measured by experiments.…”