Thermoelectric Properties of Sb-doped GaGeTe C.Drasar, V. Kucek, P. Lostak Fakulty of Chemical Technology, University of Pardubice, Studentska 573, 532 10 Pardubice, Czech Republic 1 2

[email protected] [email protected] 3 [email protected]

Abstract — The polycrystalline samples of composition GaGeTe1xSbx (x=0–0.07) were synthesized from elements of 5N purity using solid state reaction. The products of synthesis were identified by x-ray diffraction. The samples for transport measurements were prepared using hot-pressing. They were characterized by the measurement of electric conductivity, Hall coefficient, thermal conductivity, and Seebeck coefficient in the temperature range 100 – 450K. The samples show all p-type conductivity and we observe an increase in hole concentration with increasing Sb content. We discuss the temperature dependence of figure of merit ZT; ZT increases with temperature in the studied temperature range. Keywords— GaGeTe, preparation, thermoelectric properties, doping, hot-pressing

I. INTRODUCTION In the GaTe - Ge system there is GaGeTe compound with peritectic melting point 1073 K. It crystallizes in the layered crystal lattice, space group with lattice parameters a=4.05Å, c=34.65Å [1]. In the literature, there are studies of crystal structure and an investigation of chemical bonding using Raman spectroscopy [2] only. The aim of this contribution is to present original information on transport properties of GaGeTe and the influence of Sb-impurity on them. II. EXPERIMENTAL Polycrystalline GaGeTe and GaGeTe1-xSbx (x=0.01-0.07) were synthesized from elements Ga, Ge, Sb and Te of 5N purity. The mixture of elements was heated in evacuated quartz ampoule at 1220 K for 75 minutes and quenched in air. The ampoules were further annealed at 770 K for 3 days to reach equilibrium. Products were powdered for one minute in vibrating mill and identified by x-ray diffraction (XRD). Bulk samples with dimensions of 15x3.5x3.5 mm3 were prepared from the powder using high pressure/high temperature (710K/65MPa) technique in a graphite die. Density of the samples was always higher than 95% of its theoretical value. The transport parameters we report on include electrical conductivity, Seebeck coefficient, Hall coefficient and thermal conductivity measured over a temperature range from 100K to 450K. Details concerning the experimental techniques used for first three parameters are described elsewhere [3]. Thermal conductivity was measured by means of Laser Flash Apparatus 457 (NETZSCH) using pyroceram as heat capacity standard. ! [" -1.m-1]

S [µV.K-1]

ZT [-]

128

# [W.m-1.K-1] 1.14

1880

GaGeTe0.99Sb0.01

4590

99

1.13

0.012

GaGeTe0.97Sb0.03

10460

94

1.70

0.017

GaGeTe0.95Sb0.05

190

210

1.37

0.002

GaGeTe0.93Sb0.07

270

199

0.99

0.003

Sample GaGeTe

0.008

III. RESULTS AND DISCUSSION Lattice parameters of all the samples are summarized in Table I. The parameters a=0.40479 nm and c=3.4784 nm of undoped GaGeTe are close to them published in [1]. It is clear that upon Sb-doping/alloying the unit cell volume V grows suggesting that Sb enter the structure of GaGeTe. Fig. 1 presenting dependence of unit cell volume on content of Sb shows that the solubility limit of Sb in GaGeTe1-xSbx is close to x=0.05. All the diffraction patterns also evidence presence of some foreign phases. While there are negligible peaks attributable to Ga2Te3 for x=0-0.03, for higher content of Sb we observe also other peaks of unknown phases. Since we start with stoichiometric mixture the presence of Ga2Te3 indicates that the GaGeTe deviates from stoichiometry. This in turn suggests presence of defects in the crystal lattice. TABLE I LATTICE PARAMETERS OF GAGETE1-XSBX SAMPLES

Sample

a [nm]

c [nm]

c/a

V [nm3]

GaGeTe

0.40479

3.4784

8.593

0.49358

GaGeTe0.99Sb0.01

0.40505

3.4758

8.581

0.49384

GaGeTe0.97Sb0.03

0.40510

3.4762

8.581

0.49402

GaGeTe0.95Sb0.05

0.40506

3.4808

8.593

0.49461

GaGeTe0.93Sb0.07

0.40504

3.4810

8.594

0.49462

Fig. 1 Unit cell volume V of GaGeTe1-xSbx as a function of Sb content x

TABLE II ELECTRICAL CONDUCTIVITY CONDUCTIVITY

, SEEBECK COEFFICIENT S, THERMAL

AND FIGURE OF MERIT ZT OF GAGETE1-XSBX SAMPLES AT

300K.

The room temperature values of electrical conductivity !, Seebeck coefficient S, and thermal conductivity " are presented in Table II. We do not present the measurements of Hall coefficient as the voltage signals were too small. We got reasonable signal just for two samples; Hall coefficient RH(T=300K)=0.30 cm3.C-1 for GaGeTe and 3 -1 RH(T=300K)=0.15 cm .C for GaGeTe0.97Sb0.03. These data qualitatively indicate that GaGeTe is p-type semiconductor (p ≈1019cm-3) and Sb impurities show acceptor like behavior. We neglect minority carriers for such low values of Hall coefficient. In accordance we observe increase in electrical conductivity and decrease in Seebeck coefficient with increasing Sb content up to x=0.03. Decrease of electrical conductivity and increase in Seebeck coefficient at samples with higher content of Sb can be attributed to occurence of other phases. It is clear that figure of merit ZT(=!S2T/") shows its maximum also at x=0.03. Thus the doping of GaGeTe with Sb results in the increase in hole concentration. However, a comparison of the Hall hole mobility µ~RH.! (µ=6 cm2.V-1.s-1for GaGeTe and µ=16 cm2.V-1.s-1 for GaGeTe0.97Sb0.03) leads us to striking result that Sb-doping increases both the concentration as well as mobility of holes. We give a qualitative account in the next paragraph. The starting point of the account is the idea that structure of GaGeTe compound prepared from stoichiometric mixture contains in our case a considerable concentration of defects due to segregation of tiny amount of Ga2Te3 as evidenced by XRD. As a first approximation we can consider formation of negatively charged vacancies in cation sublattice (unspecified) and positively charged vacancies in Te-sublattice ( ). Regarding the p-type conductivity negatively charged defects must dominate and their excess gives concentration of holes much like in Bi2Te3. The Sb atoms entering tellurium sublattice form negatively charged substitutional defect and thus increase the concentration of holes. The healing effect of Sb might be attributed to decrease of the concentration of tellurium vacancies due to Sb doping. Since the scattering magnitude of vacancies is lager compare to substitutional defects the Hall mobility of holes increases. The situation can be much more complex especially at samples with higher x due to presence of other phases. The healing effect of Sb on GaGeTe structure is however evident.

Fig. 2 Electrical conductivity !, Hall coefficient RH, Seebeck coefficient S, and thermal conductivity " as a function of temperature for GaGeTe0.97Sb0.03 sample

We present results for GaGeTe0.97Sb0.03 sample in Fig.2 to illustrate the temperature dependence of transport parameters. Figure of merit as a function of Sb content for three temperatures is presented in Fig. 3. It is clear that we find the highest values of ZT parameter at GaGeTe0.97Sb0.03 sample. It is due to enhanced mobility and concentration of holes.

Fig. 3 ZT parameter of GaGeTe1-xSbx samples as a function of x for three temperatures

Abrupt drop of ZT parameter for samples with x≥0.05 can be probably ascribed to higher concentration of foreign phases in the samples. We see that the value ZT=0.055 is approximately 20 times lower than those of commercial materials [4]. However this material can be further optimized through carrier concentration variations. Also, it is evident that the ZT parameter steeply increases (Fig. 4) and that the region of optimal thermoelectric efficiency fall in higher temperatures. Since the decomposition temperature of GaGeTe is 1073 K [1] there is enough space for considerable improvement. The high temperature thermoelectric properties are currently under investigation.

samples with x≥0.05 other unidentified phase appears and its concentration increases with Sb doping. -the incorporation of Sb atoms in GaGeTe crystal lattice led to an increase in concentration and mobility of holes (in concentration range x=0-0.03). -we find the highest value of ZT=0.055 for sample GaGeTe0.97Sb0.03 at the highest temperature measured T=420K. Since it grows with temperature notably higher values of ZT are expected. ACKNOWLEDGMENT This research was supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project number MSM0021627501. REFERENCES Fig. 4 ZT parameter as a function of temperature for GaGeTe0.97Sb0.03 sample

IV. CONCLUSIONS The polycrystalline samples of GaGeTe1-xSbx (x=0-0.07) prepared by hot pressing were characterized by the measurements of XRD patterns, electrical conductivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. The results led to following conclusions: -the prepared samples up to x=0.03 contain a negligible amount of Ga2Te3 as foreign phase, while in the

[1]

[2] [3] [4]

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