Electronic properties of co-doped nonstoichiometric germanium telluride
Introduction
In the last decades, nonrenewable energy resources (e.g. coal and natural gas), contributing to global climate changes, become a major environmental concern; accompanied with the dilution of conventional energy resources, the interest in the need for a renewable energy alternatives is ever-growing. Thermoelectricity, which can utilize waste heat into useable electrical energy, is one applicable solution; even a partial conversion of waste heat will bring us closer towards a cleaner and greener world. Such thermoelectric converters are successfully initiated by the development of various highly efficient thermoelectric material classes. Few examples are including the Bi2Te3 [1] for working temperatures of up to ~300 °C, telluride -based including lead telluride [2,3] tin telluride [4,5] and germanium telluride [6,7] for intermediate temperatures of up to 500 °C, and the silicides [8], half‐Heuslers [9,10] for higher temperatures. Those materials require an optimized combination of the electrical and lattice properties (Seebeck coefficient, α, electrical resistivity, ρ, and thermal conductivity, κ, while κ is contributed by both the electronic thermal conductivity, κe, and the lattice thermal conductivity, κl), which enable the highest possible energy conversion efficiency, via maximization of the thermoelectric figure of merit, ZT = α2T/[ρ(κe+κl)]. Such an optimized combination is not trivial since these parameters are strongly coupled; thus, the thermoelectric efficiency can be enhanced by either electronic properties improvement or thermal properties minimization. Approaches related to the latter, already succeeded reaching ultralow thermal conductivity [11], lightning the fact that approaches related to electronic optimization, are still required.
Among the IV-VI -based chalcogenides and their alloys which are highly efficient for medium working temperatures, are the well-known lead telluride compounds and its derivatives [12,13]. Nevertheless, wide usage in these compounds is very limited due to lead toxicity. The lead-free semiconducting GeTe compound has gained growing interest due to its lower rhombohedral crystal structure symmetry. This characteristic enables the reduction of the thermal conductivity due to enhanced phonon scattering and modulation of the band structure for superior electronic properties.
Germanium telluride -based alloys are characterized by a wide range of deviation of stoichiometry, extending only toward tellurium excess compositions [14]. Moreover, since germanium vacancies has the lowest formation energy compared to the other possible point defects of germanium telluride, it possess intrinsically high concentration of cation germanium vacancies; therefore the material behaves as a highly degenerate p-type compound with a very high hole concentration of 1020–1021 cm−3 [15]. The high hole concentration in turn, results in low electrical resistivity, but at the same time in a very low Seebeck coefficient. Several key strategies to improve its electronic properties were addressed in previous work [16], leading to increased ZTs which turned it a relevant compound in the thermoelectric field.
This research's focus is on the reduction of the high germanium vacancies by decreasing the high hole concentration towards the required 1019 cm−3 for thermoelectric applications; it is believed that impurity atoms introduced to the lattice will occupy the vacant sites in the germanium telluride sublattice, giving their valence electrons, and compensating the inherent high hole concentration [17].
While choosing the appropriate dopant element, one should consider its solubility in the matrix which depends on the original number of vacancies in germanium telluride. One can increase the original cation vacancies number artificially by introducing such an element which will occupy a cation site in one sublattice while simultaneously form a vacancy in the other one. In fact, considering the valence of germanium and bismuth, the substitution of germanium by bismuth gives one carrier in the conduction band and every excess tellurium atom creating an equivalent quantity of vacancies (two holes in the valence band) [17,18]. This phenomenon allows to increase the solubility of any other chosen effective dopant desired. Together with the fact that bismuth has been proven as one of the most effective donors to decrease drastically the holes concentration [19], bismuth was introduced as BiTe instead of the conventional Bi2Te3 owing to the higher bismuth concentration in the compound. In order to thermodynamically advance substitution of germanium, rather than occupying tetrahedral interstitial sites and creating of neutral clusters, bismuth needs to be introduced in a sufficient amount regarding the solubility limit in the matrix [20].
While the focus in previous studies was mainly on stoichiometric compounds, this work's strategy is to increase the original cation vacancies number also by adding an excessive amount of germanium/tellurium beyond the stoichiometric germanium telluride composition. This research aims to investigate the combination of both approaches, in order to increase the solubility limit as much as possible, for eventually increasing the amount of dopant that can be introduced to the matrix.
The effect of cationic and anionic impurity dopants was also studied while indium was chosen as a cationic impurity and iodine as an anionic impurity candidate. The former is known to enhance the Seebeck coefficient by creating resonant states close to Fermi level in the valence band [21], while the latter demonstrates an approach that as for the best of our knowledge, was less investigated in GeTe before. Both impurities were also chosen owing to their ionic radius values, favoring a higher solubility limit in the matrix; they are close enough to less interrupt the crystal structure and increase the solubility, yet small enough to increase their diffusivity in the matrix.
The present research investigates the synergy of the approaches explained above, and by correlating to the microstructural characteristics, the mechanisms affecting the transport properties are discussed.
Section snippets
Experimental
Four (GeyTe1-y)x (BiTe)1-x alloys, with different x and y values (Table 1), were synthesized from pure elements (5 N), mixed in the right ratio, and sealed in evacuated quartz ampoules under vacuum of 10−6 Torr. The ampoules were placed in a rocking furnace (Thermcraft Inc.) at 1000 °C for 15 min, then water quenched. The cast ingots were milled to a maximal powder particle size of ~250 μm using agate mortar and pestle. The sieved powder was hot pressed (HPW5 Hot Press, FCT System GmbH) under a
Results and discussion
GeTe with the rhombohedral crystal structure and space group R3m, melts congruently at 720 °C [14]; BiTe with a BiSe trigonal crystal structure and space group P-3m1, consists of 12 close-packed layers along the c axis, and melts at 540 °C [22].
As can be seen by the XRD diffractograms (Fig. 1a), the reflections, belonging to the germanium telluride matrix in the low-temperature rhombohedral structure appeared with the typical (003)/(021) and (024)/(220) doublets at ~25° and ~42.5°,
Conclusions
(GeyTe1-y)x (BiTe)1-x alloys, with different x and y values, and different second cationic/anionic impurity dopants obtaining excess tellurium/germanium respectively, were investigated. The mechanisms controlling both dopants (bismuth and indium/iodine) occupation in the lattice of nonstoichiometric alloys were discussed in detail. ZT enhancement throughout all temperature range, reaching a maximum of 60% for excess-germanium bismuth doped alloy, compared to the pristine GeTe, was achieved. By
Author statement
Dana Ben-Ayoun: Investigation, Validation, Formal analysis, Writing Yaniv Gelbstein: Supervision, Writing - Review & Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The work was supported by the Israel Science Foundation (ISF) Individual Research Grant No. 326/20. The authors would like to thank Mr. Yair George for the synthesis of the alloys and specimen's preparation.
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