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Improved carrier concentration control in Zn-doped Ca5Al2Sb6
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) (a) CaAlSb orthorhombic unit cell () viewed along the [001] direction. (b) Covalent substructure formed from infinite chains of corner sharing AlSb tetrahedra connected via Sb–Sb bonds. Sb atoms are orange, Ca are green, and Al are blue (Ref. 12 ).

Image of FIG. 2.
FIG. 2.

CaAl ZnSb x-ray diffraction patterns ( = 0, 0.20), Reitveld fit to the  = 0 sample, and associated difference profile. No secondary phases are observed for the range investigated (0 0.20).

Image of FIG. 3.
FIG. 3.

With zinc doping, the carrier concentration at 300 K is directly proportional to the level predicted using simple electron counting (dashed line). This is in contrast to the lower doping efficiency with sodium (Ref. 13 ).

Image of FIG. 4.
FIG. 4.

(a) High temperature Hall coefficient measurements of CaAl ZnSb yield the carrier concentration. Doped samples exhibit extrinsic behavior to 800 K. (b) Undoped CaAlSb exhibits a mobility dominated by phonon scattering across the entire temperature range investigated, whereas Zn-doped samples show evidence of additional scattering mechanisms at low temperature. (c) The high temperature resistivity decreases with increasing Zn doping level.

Image of FIG. 5.
FIG. 5.

Similar Seebeck coefficient values for Na- and Zn-doped CaAlSb at equivalent carrier concentrations suggest similar band mass. The curve was generated using a single parabolic band approximation and an effective mass of 1.8 m (Ref. 13 ).

Image of FIG. 6.
FIG. 6.

High temperature Seebeck coefficients of CaAl ZnSb show degenerate behavior for the extrinsically doped, p-type compositions. Similar behavior is seen in the Na-doped analog (Ref. 13 ).

Image of FIG. 7.
FIG. 7.

(Color online) (a) The calculated CaAlSb band structure shows a direct gap at . The valence bandedge consists of two nested hole pockets with band masses between 0.25 and 2 , depending on the direction. The dashed lines at −0.08 and −0.13 eV correspond to 1 and 5 × 1020 + cm−3, respectively. (b) The density of states of CaAlSb shows that the material is a semiconductor with Sb states as the dominant influence near the valence bandedge.

Image of FIG. 8.
FIG. 8.

(Color online) (a) The total thermal conductivity of CaAl ZnSb increases with increasing doping level and thus electronic conductivity. (b) The lattice contribution () decays with −1 and approaches a minimum value at high temperature, consistent with prior Na-doped compositions (Ref. 13 ). To calculate the Lorenz coefficients (inset) were calculated using the single parabolic band approximation.

Image of FIG. 9.
FIG. 9.

As compared to sodium doped CaAlSb, the reduced mobility of the samples in this study lead to a lower Within the Zn series, the highest doped composition exhibits the largest


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Improved carrier concentration control in Zn-doped Ca5Al2Sb6