Impact of Various Dopants on Thermoelectric Transport Properties of Polycrystalline GeSb2Te4

GeSb2Te4 (GST124), one of the well‐known phase‐change materials for nonvolatile memory and rewritable optical storage, has been recently found to be promising thermoelectric materials with low lattice thermal conductivity and high electrical conductivity. However, its thermoelectric performance is greatly restricted by the excessively high hole concentration. Herein, the impact of a series of group IIIA (Al, Ga, In) and group VIA (S, Se) dopants on the electrical transport properties of polycrystalline GST124 has been studied. It is found that element sulfur (S) has the best doping efficiency because the GeS bonds are very strong and ionic that are beneficial for suppressing Ge vacancies to reduce the carrier concentration. Meanwhile, element indium (In) also shows decent doping efficiency because its ionic radius is close to the Ge ion and the InTe bonds have moderate bonding strength. Moreover, In doping introduces a resonant level in the valence band, leading to enhanced Seebeck coefficient and power factor. A high figure of merit (zT)of 0.73 at 700 K and an average zT of 0.48 over 300–750 K are obtained in Ge0.92In0.08Sb2Te4, which are 26% and 66% higher than pristine GST124. This study will advance the understanding and development of high‐performance GeSbTe‐based thermoelectric materials.


Introduction
6] As the electrical properties of S, σ, and κ e are coupled with each other by the carrier concentration, there exists an optimal carrier concentration range to maximize the electrical properties and zT. [7,8]Therefore, it is of great and fundamental significance to carefully tune the carrier concentration to realize high thermoelectric performance.[16] These beneficial thermoelectric transport properties are related to the layered structure that partially decouples the thermal and electric properties, and the special metavalent bonding leading to both narrow bandgap and strong anharmonicity. [17,18]Among them, the GST225 compound is the most widely studied one.Upon resonant doping with In, [19] alloying with S or Se, [20] the excessively high concentration of hole carriers can be suppressed and a maximum zT above 0.7 has been realized.Very recently, another compound GST124 with a 21-layer complex unit cell has received more attention.Particularly, single-crystal GST124 has been synthesized and the maximum zT value of unity has been reached in doped/ alloyed samples. [21,22]The study on thermoelectric properties of GSTs will not only expand the material families in thermoelectrics but also offer new insight for the development of the design of low-power phase-change memory devices. [9,10,14]espite the rapid progress, the thermoelectric performance of GST materials (including GST124, GST225, and GST147) is far from the predicted maximum value due to the stubbornly high DOI: 10.1002/aesr.202300216GeSb 2 Te 4 (GST124), one of the well-known phase-change materials for nonvolatile memory and rewritable optical storage, has been recently found to be promising thermoelectric materials with low lattice thermal conductivity and high electrical conductivity.However, its thermoelectric performance is greatly restricted by the excessively high hole concentration.Herein, the impact of a series of group IIIA (Al, Ga, In) and group VIA (S, Se) dopants on the electrical transport properties of polycrystalline GST124 has been studied.It is found that element sulfur (S) has the best doping efficiency because the Ge─S bonds are very strong and ionic that are beneficial for suppressing Ge vacancies to reduce the carrier concentration.Meanwhile, element indium (In) also shows decent doping efficiency because its ionic radius is close to the Ge ion and the In─Te bonds have moderate bonding strength.Moreover, In doping introduces a resonant level in the valence band, leading to enhanced Seebeck coefficient and power factor.A high figure of merit (zT )of 0.73 at 700 K and an average zT of 0.48 over 300-750 K are obtained in Ge 0.92 In 0.08 Sb 2 Te 4 , which are 26% and 66% higher than pristine GST124.This study will advance the understanding and development of high-performance GeSbTe-based thermoelectric materials.hole concentrations. [13,19,22]Currently, the carrier concentration in GST124 compounds remains on the order of 10 20 cm À3 whereas the optimal range is approximately around 10 19 cm À3 , one order of magnitude lower than the experiment. [19,22]As predicted by theoretical calculation, [22] a higher zT value of 1.25 can be achieved in GST124 single crystals if the carrier concentration is optimally tuned.The high hole concentration is mainly caused by the low formation energy of Ge vacancy in GST. [23,24]Therefore, optimizing carrier concentration can be achieved by aliovalent doping or suppressing Ge vacancy concentration.In general, element doping is related to several factors, including the actual amount of dopants that enter the matrix and replace matrix atoms (solubility), the number of extra electrons that become free carriers, and the compensating defect. [25,26]Meanwhile, the formation of the intrinsic Ge vacancy is also affected by the chemical bonding strength and structure distortion degree, making the doping effect extremely complex. [27,28]Therefore, it is of great significance to explore the effect of different dopants in GST.
In this work, taking polycrystalline GST124 as a case study, we select a series of group IIIA (Al, Ga, In) and group VIA (S, Se) elements as dopants to optimize the carrier concentration and zT.It is found that these dopants exhibit distinct doping effects.Notably, doping In and alloying S are very effective in reducing the hole density while Al-doping seems insignificant.This difference is rationalized from the perspective of chemical bonding features.Moreover, doping indium introduces a resonant level in the electronic band, which can further enhance Seebeck coefficient and improve power factor (PF).A maximum zT value of 0.73 at 700 K has been achieved in polycrystalline Ge 0.92 In 0.08 Sb 2 Te 4 .This study provides insights into the impact of various dopants on the transport properties of GST124 which will be helpful to optimize thermoelectric performance not only in GST but also in other material systems.

Phase Structure and Microstructures
The X-ray diffraction (XRD) patterns of the pristine and doped GST124 materials are shown in Figure 1a and S1, Supporting Information, respectively.The diffraction peaks are indexed to the trigonal phase with the space group of R3m, and no impurity phases are detected.Due to the layered features, obvious anisotropy is observed in XRD patterns.The (00l) orientation is dominant along the pressing direction in the sintered polycrystalline sample.Consistently, the microstructure also shows a lamellar character in Figure 1b.All the elements are found to be homogeneously distributed in materials without agglomeration or precipitation (Figure S2, Supporting Information).Figure 1d shows the high-resolution transmission electron microscopy (HRTEM) image and corresponding fast Fourier transformed selected area electron diffraction pattern along [010] zone axis for pristine GST124.The lattice fringes are clearly seen, demonstrating a decent crystallinity.The plane distance of ≈0.31 nm is coincident with the theoretical distance of the (107) plane of the structure shown in Figure 1c.

Effect of Various Dopants
GST124 is anisotropic and the TE performance parallel to the pressiure direction is better than that perpendicular to the pressing direction as shown in Figure S3, Supporting Information.Therefore, only the data parallel to the pressing direction are presented here.The electrical transport properties at room temperature with different dopants are shown in Figure 2. The carrier concentration (p) decreases with increasing doping content except for Al.Particularly, In and S are found to give better doping efficiency compared with Ga and Se, yielding p = 3.49 and 3.19 Â 10 20 cm À3 at a doping amount of 10%, respectively.It is also noteworthy that for all dopants used here, the doping efficiency is far below the ideal predictions, which will be discussed in detail later.In general, the electrical conductivity decreases  and the Seebeck coefficient increases with the increasing dopant content.In-doped samples possess the largest descending slope of electrical conductivity with doping content, decreasing from 2.8 Â 10 5 S m À1 for x = 0 to 6.7 Â 10 4 S m À1 for x = 0.1.Correspondingly, the Seebeck coefficient increases significantly with the increase of the In content.When x = 0.1, the S reaches the highest value of 94 μV K À1 , which is twice larger than that of the pristine sample.Due to the enhanced Seebeck coefficients, the PFs of In-doped samples are much higher than that of pristine and other doped samples.The In-doped sample has the highest PF of 6.9 μW cm À1 K À2 at the doping content of 8%.
Here, the distinct doping effect on TE properties is preliminarily understood from the view of chemical bonding characteristics.Figure 3a shows the -ICOHP (a measure of bond strength) versus the relative charge transfer (a measure of bond ionicity) for the original Ge─Te bond and the new bonds.The local charge distributions of these bonds are also shown in Figure 3b-d.It is seen that the S/Se alloying effects well coincide with the chemical intuition: the Ge─S and Ge─Se bonds are more ionic than the Ge─Te bond as supported by the larger charge density around S atoms (Figure 3d) than Te atoms (Figure 3b).Moreover, the Ge─S bond is much stronger.The stronger and more ionic chemical bonding is beneficial for fixing Ge atoms in the crystal lattice, resulting in a correspondingly lower concentration of Ge vacancies, thus lower carrier concentration and higher doping efficiency.As a contrast, the doping efficiency at the cation site (doped by Al, Ga, In) is more complicated.The apparent result shown in Figure 3a is that the substitution of Al, Ga, and In for Ge increases both the cation─Te bond strength and ionicity.This should lead to a decent doping efficiency, but the fact is not the simple case.Al 3þ has the largest radius difference around 27% compared with Ge 2þ , while In 3þ has the smallest one making it easier to substitute the Ge atom.Thus, the Al element has a minor doping effect as the Seebeck coefficient and the carrier concentration fluctuate just slightly, which suggests a low doping efficiency and may be related to the large ionic size difference between Al 3þ and Ge 2þ .Due to the above factors, the In element has the highest doping efficiency among the three cation dopants.

Thermoelectric Transport Properties
The electrical transport properties of doped GST samples at room temperature are further analyzed by using the single-parabolic band (SPB) model. [29]The dashed line in Figure 4a is the Data from a previous work are also given. [21,22]heoretical Pisarenko curve at the density of states (DOS) effective mass m d * = 1.32 m e (m e is the mass of the electron).The effective mass of the doped GST124 samples scarcely or slightly changes except for In and Ga dopants, indicating the merely disrupted valence band maxima.The remarkably enhanced Seebeck coefficient of In-doped samples is probably due to the resonant doping effect.That is, the intensive hybridization of Te p and In s orbitals introduces the resonant level in the valence band and distorts the DOS near the Fermi level as shown in Figure 4b. [19,21,30,31]This effect has also been observed in the previously reported In-doped GST225 polycrystal and GST124 single-crystal samples. [19,21]The Ga doping also introduces the resonant level in the valence band as shown in Figure S9, Supporting Information, and the small extent of Seebeck coefficient increase may be limited by the low doping efficiency.
The hole mobility generally decreases with decreasing carrier concentration as shown in Figure 4c, indicating additional scattering processes alongside the acoustic phonon scattering (black dashed line).We then fitted the carrier mobilities by considering both acoustic phonon and alloying scatterings (see the violet and yellow dashed lines). [32,33]The calculation details are shown in the Supporting Information with μ ph = 39 cm 2 V À1 s À1 and an alloy scattering potential U of 0.35 and 0.40 eV for Se-and S-alloyed samples, respectively.Despite the slightly larger U, the dopant of S shows a better doping efficiency than Se, which means that a smaller amount of S is required to achieve the same level of carrier concentration.
The temperature-dependent electrical conductivity (σ), Seebeck coefficient (S), and PF measured for Ge 1Àx In x Sb 2 Te 4 and other doped samples are shown in Figure 5a-c and S4 and S5, Supporting Information, respectively.The electrical conductivity of all tested samples decreases with increasing temperature, indicating typical heavily doped semiconductor behavior.As shown in Figure 5d and S6, Supporting Information, the total thermal conductivity κ shows a decreasing trend with the increase of all dopant content.The κ of Ge 0.9 In 0.1 Sb 2 Te 4 is as low as 1.13 W m À1 K À1 at 300 K, less than half of the pristine GST124.The reduced κ is primarily attributed to the decrease in the carrier thermal conductivity.The temperature dependence of zT for doped samples is shown in Figure 5e and S7, Supporting Information.Figure 5f presents the best zT as a function of temperature for every dopant.The zT of the pristine GST124 sample at 300 and 750 K is 0.07 and 0.58, respectively.Owing to the enhanced Seebeck coefficient and reduced thermal conductivity, Ge 0.92 In 0.08 Sb 2 Te 4 exhibits a maximum zT of 0.17 at 300 K, which is about twice that of pristine GST124.A maximum zT value of 0.73 is achieved for Ge 0.92 In 0.08 Sb 2 Te 4 at 700 K. Ultimately, the significant suppression in κ and improvement in PF directly contributes to outstanding average zT values of 0.48 between 300 and 750 K.

Conclusion
In summary, we have studied the impact of group IIIA (Al, Ga, In) and group VIA (S, Se) dopants on thermoelectric transport properties of polycrystalline GST124 materials.It is found that In and S show better doping efficiency, reducing the hole carrier concentration from 4.97 Â 10 20 cm À3 to 3.49 and 3.19 Â 10 20 cm À3 at 300 K, respectively.The chemical bonding analysis reveals that the bond ionicity, strength, and the ionic size mismatch are important to the doping efficiency.Moreover, In-doping brings about a

Figure 1 .
Figure 1.a) XRD patterns, b) SEM morphology at different directions as well as elemental distribution, c) crystal structure, and d) HRTEM and the fast Fourier transformed selected area electron diffraction pattern as inset projected along [010] zone axis of pristine GST124 materials.

Figure 2 .
Figure 2. a) Hall carrier concentration, b) electrical conductivity, c) Seebeck coefficient, and d) PF as a function of doping content at 300 K.

Figure 3 .
Figure 3. a) -ICOHP versus relative charge transfer for pristine and doped GST124 samples.b-d) Charge density difference mapping for Ge─Te, In-Te, and Ge─S bonds in pristine, In-and S-doped GST124 samples, respectively.The yellow regions between the atoms represent the isosurface charge level larger than 0.004 bohr À3 (obtaining electrons) while blue ones are smaller than À0.004 bohr À3 (losing electrons).

Figure 4 .
Figure 4. a) Experimental and calculated Pisarenko curve (S vs p) at 300 K.The dashed line is calculated based on the SPB model.b) The calculated density of states for In-doped GST124.c) Hall mobility (μ) vs Hall carrier concentration at 300 K.The black dashed line is calculated assuming that the dominant scattering mechanism is the acoustic phonon scattering while the violet and yellow ones consider both acoustic phonon and alloying scatterings.d) Power factor (PF) vs Hall carrier concentration at 300 K.The dashed line is calculated based on the SPB model and acoustic phonon scattering.Data from a previous work are also given.[21,22]