Largely enhanced Seebeck coefficient and thermoelectric

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Largely enhanced Seebeck coefficient and thermoelectric performance by the distortion of electronic density of states in Ge2Sb2Te5 Ping Hu, Tian-Ran Wei, Pengfei Qiu, Yan Cao, Jiong Yang, Xun Shi, and Lidong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12854 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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ACS Applied Materials & Interfaces

Largely Enhanced Seebeck Coefficient and Thermoelectric Performance by the Distortion of Electronic Density of States in Ge2Sb2Te5 Ping Hu,†,‡, Tian-Ran Wei,*,‖ Pengfei Qiu,† Yan Cao,⊥ Jiong Yang,⊥ Xun Shi,*,† and Lidong Chen† †State

Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ‡Center

of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ‖State

Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China ⊥Materials

Genome Institute, Shanghai University, Shanghai 200444, China

KEYWORDS: thermoelectric, Ge2Sb2Te5, electronic density of states, resonant level, bonding character

ABSTRACT: As one of the state-of-the-art phase-change materials, the stable Ge2Sb2Te5 hexagonal compound also exhibits decent thermoelectric performance with high electrical conductivity and low thermal conductivity. Nonetheless, the excessively high carrier concentration and low Seebeck coefficient are the bottlenecks to achieve high zT values. In this work, with the intention to optimize the electrical properties, indium was introduced as a potentially donor-like dopant in a series of Ge2-xInxSb2Te5 samples. The substitution of indium for germanium lowers the density of hole carriers and enhances the Seebeck coefficient. Noticeably, the room-temperature Seebeck coefficient of the doped samples can be three times as large as that of the pristine one, which obviously departures from the theoretically predicted Pisarenko relation based on the single parabolic band model. By virtue of DFT calculations and modeling, the remarkable enhancement of Seebeck coefficient was attributed to the doping-induced local distortion in the electronic density of states. Further insight reveals that indium doping amplifies the bonding character of Ge-Te adjacent to indium, and enhances the atomic interaction along c-axis. Due to the optimized electrical properties as well as the suppressed thermal conductivity, a maximal zT value of 0.78 was achieved in Ge1.85In0.15Sb2Te5 at 700 K, which is about 40% higher than that of the pristine sample.

Introduction The rising concerns on energy exhaustion and environmental degradation have made thermoelectrics a cutting-edge research topic, which is believed to provide an alternative solution to green and sustainable energy utilization.1,2 The performance of a thermoelectric (TE) material is characterized by the figure of merit, zT = S2σT/κ, where S, σ, T, and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively.3,4 In keeping with the rising zT values in conventional material systems, searching for new TE materials with novel structures and transport mechanisms has always been a fascinating topic.5-10 As the pseudo-binary alloys of GeTe and Sb2Te3, Ge-Sb-Te (GST) compounds have been widely investigated as phase-

change materials (PCMs).11,12 The fast phase transformation and remarkable contrasts in electrical resistivity between the stable and metastable phases can realize the reading and storage of digital information. It has been reported that GST compounds are degenerate p-type semiconductors with high electrical conductivity and low thermal conductivity.1315 Recently, we found that three typical GST compounds (Ge2Sb2Te5, GeSb2Te4, and GeSb4Te7) exhibit decent zT values of 0.4-0.6 at 750 K with strong anisotropy in Seebeck coefficient as well as electrical and thermal conductivities.16 The main barrier limiting the TE performance of GST compounds is the excessively high carrier concentration and low Seebeck coefficient. Taking Ge2Sb2Te5 as an example, the room-temperature hole density is about 6.01020 cm-3, which leads to a low Seebeck coefficient of 18-

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30 V/K along different directions.16 It is well recognized that elemental doping is an effective and fundamental approach to optimize the carrier concentration, which has been successfully implemented in nearly all the known TE systems.17-23 Conventional doping can tune the carrier concentration by shifting the Fermi level with little effect on the band structure. In some special cases, the orbitals of the dopant may strongly interact with the host atomic orbitals, and modify the shape or the position of the electronic band, which will introduce additional effects to charge transport.24-26 In this work, focusing on the typical compound Ge2Sb2Te5,

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the best established PCM with excellent phase change properties, we demonstrate the successful attempts to optimize the electrical properties and TE performance for GST with the stable, hexagonal structure. It is found that indium (In) doping at Ge site is effective to enhance the Seebeck coefficient. Different from conventional doping, a large deviation of the experimental data from the theoretical Pisarenko relation was observed for indium-doped samples, which was analyzed by DFT calculations on the band structure and bonding characters. By virtue of the enhanced electrical performance and the reduced thermal conductivity, a maximum zT value of 0.78 was obtained in Ge1.85In0.15Sb2Te5.

Figure 1. (a) Powder XRD patterns of Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2). (b) lattice parameters (a and c) as a function of indium content. (c) Crystal structure of pristine Ge2Sb2Te5. (d) Calculated distance between specific layers in pristine and indium-doped Ge2Sb2Te5.

Experimental details Synthesis: Bulk samples of polycrystalline Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2) were synthesized by the meltingannealing process. High purity elements Ge (pieces, 99.999%, Alfa Aesar), Sb (shots, 99.999%, Alfa Aesar), Te (shots, 99.999%, Sigma-Aldrich), and In (shots, 99.999%, Alfa Aesar) were weighted, and then sealed in quartz tubes under vacuum. The tubes were heated to 1273 K in 10 h, held at 1273 K for 12 h, and then quenched in cold water. Afterwards, the ingots were annealed at 853 K for 72 h. The obtained ingots were crushed into fine powder and sintered using spark plasma sintering (SPS, Dr Sinter SPS-2040) at 763 K for 5 min under a pressure of 50 MPa. Finally, highdensity (>98% of theoretical densities) samples in a diameter of 10 mm and thickness of ~10 mm were obtained.

Characterization: Powder X-ray diffraction (Rigaku Rint 2000) was used to analyze phase purity and crystal structure. The lattice constants were calculated by Rietveld refinements. The microstructures of fractured surface and chemical compositions of bulk samples were examined by using the field emission scanning electron microscopy (FESEM, ZEISS Supra 55) equipped with energy dispersive Xray analysis (EDS, Oxford). The electrical conductivity (σ) and Seebeck coefficient (S) were measured using a ZEM-3 system in a helium atmosphere from 300 K to 800 K. The temperature dependence of thermal diffusivity (D) was measured by the flash laser method (Netzsch LFA 457) in an argon atmosphere. The heat specific (Cp) of samples was estimated by using Dulong-Petit value, and the density (ρ) was measured using the Archimedes method. The total thermal conductivity (κ) was calculated using the equation κ = D × Cp × ρ. The low-temperature Hall coefficient (RH) and


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electrical conductivity measurements were performed using a physical property measurement system (PPMS-9, Quantum Design) by sweeping the magnetic field up to ± 3 T in the temperature range of 5-300 K. The carrier concentration (pH) was calculated via pH = 1/eRH, where e is the elementary charge. The carrier mobility (μH) was calculated according to the relation μH = σ/pHe. Both the electrical and thermal transport properties were measured along the pressure direction of the sample.27 Computation: All the calculations were based on density functional theory (DFT)28 and carried out with the projector augmented wave (PAW) method as implemented in the Vienna ab initio Simulation Package (VASP).29,30 We used the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) as the exchange-correlation functional.31 The DFT-D2 method for the correction of van der Waals interactions32 could accurately capture the lattice parameters of Ge2Sb2Te5 as shown in our previous study,16 and thus was adopted in this work. The plane-wave energy cutoff was 400 eV and energy convergence criterion was 104 eV. The electronic structure of pristine Ge Sb Te and 2 2 5 indium-doped Ge2Sb2Te5 (3×3×1 primitive cell of Ge2Sb2Te5) were all carried out under fully relaxed structures. We introduced one indium atom in the supercell to mimic the In-doping, i.e., the doping content is around Ge1.89In0.11Sb2Te5. The chemical bonding analysis, as well as the densities of states, were carried out by the Local Orbital Basis Suite Towards Electronic-Structure Reconstruction (LOBSTER) package.33-36

morphology, which agrees with the layered crystal structure. The electrical conductivity (σ), Seebeck coefficient (S), power factor (PF), and thermal conductivity (κ) measured in the direction parallel to the sintering pressure for Ge2xInxSb2Te5 samples are shown in Figure 3(a)-(d). The electrical conductivity decreases and Seebeck coefficient increases with increasing temperature for all the samples, which is the typical behavior for heavily doped semiconductors.38 The electrical conductivity at room temperature monotonously decreases with increasing indium content from 3.06  105 S m-1 for x = 0 to 6  104 S m-1 for x = 0.2. Correspondingly, the Seebeck coefficient obviously increases from 30 to 96 μV K-1, tripling that for pristine Ge2Sb2Te5. The S-T peak gradually emerges for indium-doped samples at lower temperatures, which may be related to the thermal activation of minority carriers.39 Due to the enhanced Seebeck coefficients, the power factors (PFs) of indium-doped samples are much higher than that of pristine sample, especially at room temperature. The maximum room-temperature PF of 6.6 μW cm-1 K-2 was obtained for x = 0.1. At 800 K, the maximum PF reaches 15.2 μW cm K-2 for x = 0.05. As shown Figure 3(d), the total thermal conductivity  decreases with indium doping, reaching 1.02 W m-1 K-1 for Ge1.8In0.2Sb2Te5 at 300 K, which is only one third of that for pristine Ge2Sb2Te5. The reduction of  is mainly due to the decrease of the electronic part, e, which accounts for 80% of the total  as calculated using the single parabolic band (SPB) model.40

Results and discussion

Table 1. Lattice constants (a & c) obtained by DFT calculations and experiment in pristine and indium-doped Ge2Sb2Te5. For doped samples, the doping content is x = 1/9 for calculation and nominally x = 0.10 in experiment.

Figure 1(a) shows the powder XRD patterns at room temperature for all polycrystalline Ge2-xInxSb2Te5 samples. The diffraction peaks were well indexed to the P3m1 space group (hexagonal lattice system, trigonal crystal system)37 as shown in Figure 1(c), and no other phases were detected. The refined lattice parameters are shown in Figure 1(b). Indium doping leads to the lattice expansion along a-axis and shrinkage along c-axis. Considering the larger atomic radius of In than Ge, the decrease of c seems out of intuitive comprehension. Nonetheless, the variation of the lattice parameters was captured by the DFT calculations. As shown in Table 1, there is a good consistence between the experimental and calculation results. As seen in the relaxed crystal structures demonstrated in Figure 1(d), In doping at Ge sites indeed increases the Te-In/Ge-Te distance (Space A’) due to the larger In atoms and the longer In-Te bonds. Nonetheless, the adjacent Te-Ge-Te (Space A) or Te-Sb-Te (Space B’) layers and the van der Waals gap (Space C) are apparently shrunk by several picometers. All these variations contribute to a slightly shorter c axis. Such a subtle decrease of the lattice parameter c suggests an enhanced interaction along this direction, which may affect the band structure and transport properties. As shown in Figure 2(a) and (c)-(f), all the elements are homogeneously distributed, and no secondary phase or precipitation was observed. The elemental ratio of Ge: In: Sb: Te of the Ge1.8In0.2Sb2Te5 sample is 20.7: 2.3: 22.7: 54.3, which is close to the nominal composition. The fractured surface microstructure of Ge1.8In0.2Sb2Te5 is shown in Figure 2(b). The bulk samples exhibit a typical lamellar

Ge2Sb2Te5 Ge1.89In0.11Sb2Te5

a (Å) Cal. Expt. 4.208 4.218 4.214 4.220

Cal. 17.180 17.169

c (Å) Expt. 17.241 17.235

Figure 2. (a) Backscattered electron microscopy (BSE) image. (b) Fractured surface secondary electron (SE2) image. EDS mapping for (c) Ge, (d) Sb, (e) Te and (f) In of Ge1.8In0.2Sb2Te5.



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Figure 3. Temperature dependence of (a) electrical conductivity σ, (b) Seebeck coefficient S, (c) power factor PF and (d) total thermal conductivity κ for Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2) samples.

Figure 4. Temperature dependence of (a) Hall carrier concentration (pH) and (b) Hall carrier mobility (H) for Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2).

The carrier concentration (pH) and mobility (H) for all samples from 5-300 K are shown in Figure 4. pH scarcely changes with increasing temperature for all samples, which is commonly seen in heavily doped semiconductors and metals. With indium doping, pH at 300 K gradually decreases from 5.97 × 1020 cm-3 to 4.45 × 1020 cm-3 when x reaches 0.1, indicating a weak, n-type doping. The relatively low doping efficiency and the stabilization of pH at x=0.1 suggest a complicated role of indium doping and possible competition between InGe and other defects. As shown in Figure 4(b), the carrier mobility is nearly independent on temperature below 30 K, suggesting that carriers are primarily scattered by impurities or crystal defects.41 The mobilities of the pristine and the doped samples scale roughly with T-1 around room temperature, which is a clear indication that carriers are mainly scattered by acoustic phonons in degenerate semiconductors.42 As shown in Figure 5(a), the change of the carrier concentration by indium doping is moderate, but the Seebeck coefficients significantly increase and largely

deviate from the Pisarenko plot (S vs pH) calculated by the SPB model.40 A remarkable increase of the apparent densityof-state effective mass md* is seen from 1.02 me for x = 0 to 2.87 me for x = 0.2. Since md* is related to the effective mass of the single band (mb*) and the band degeneracy NV, the increase of either parameter will cause the enhancement of md*.43,44 Multiband conduction (increase of NV) is expected when the Fermi level crosses the deeper valence or conduction band edge as demonstrated in various thermoelectric materials.45,46 For GST materials, however, indium doping decreases the hole concentration and pushes the Fermi level towards the band gap, which is not likely to intensify the effect of multiband transport. Instead, it is speculated that indium doping modifies the band dispersion and leads to the local distortion of DOS, which can enhance the Seebeck coefficient via25


8 k BT 2

S

3eh

2

2



    3p 

md 

2 3

(1)

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8 ( md ) 

DOS  g ( E ) 

3/ 2

h3

2E

(2)

Figure 5. (a) Pisarenko plot (Seebeck coefficients as a function of carrier concentration) for Ge2-xInxSb2Te5 samples at 300 K. The dashed lines are calculated from the single parabolic band (SPB) model with the effective masses of 1.02 me and 2.87 me, respectively. (b) Seebeck coefficients (S) as a function of md*.

Figure 6. (a) Electronic band structure and (b) Projected density of states (pDOS) for pristine Ge2Sb2Te5. (c) Projected crystal orbital Hamilton population (pCOHP) between Te and cations for pristine Ge2Sb2Te5. (d) pDOS for indium-doped Ge2Sb2Te5. (e) -pCOHP between In-Te, Ge-Te neighboring to In-Te in indium-doped Ge1.89In0.11Sb2Te5. (f) Schematic depiction of Ge-Te bonds neighboring to the In-Te bonds.

As shown in Figure 5(b), the increase of Seebeck coefficient well follows the enhancement of the effective mass. To explicitly understand the effect of indium doping on the structure and transport properties of Ge2Sb2Te5, we calculated the electronic band structures of pristine Ge2Sb2Te5 and Ge1.89In0.11Sb2Te5. As shown in Figure 6(a), Ge2Sb2Te5 is a semiconductor with a narrow band gap of ~ 0.24 eV, which is consistent with our previous study.16 The valence band maximum (VBM) is located at the Γ point and conduction band minimum (CBM) is along the K-Γ line. Based on the projected density of states (DOS), the VBM is

mainly contributed by Te 5p orbitals (Figure 6(b)). The crystal orbital Hamilton population (COHP) is an effective indicator to chemical bonding, which identifies the orbital pair contributions based on band structure energy partitioning. The negative and positive signs of the project value (pCOHP) correspond bonding and anti-bonding, respectively.35 As shown in Figure 6(c), there are strong anti-bonding states around VBM between cations and Te. The electronic structure dramatically changes when partial Ge is replaced by In. A local rise of DOS was observed around the Fermi level as illustrated in Figure 6(d), and the



distortion is also mainly composed of Te 5p orbitals. Correspondingly, the COHP calculation reveals a strong antibonding state at the Fermi level for the In-Te bond (Figure 6(e)). This scenario is known as the resonant doping.25,47,48 It states that when the impurity was introduced into the host, the extra carriers weakly bonded to impurity can be easily excited into the electronic band. Then, the impurity levels fall inside the electronic band and the energy states with a certain width is formed.47 Resonant doping effects have been reported in many IVA-VIA semiconductors doped with IIIA elements such as Ga/In/Tl: PbTe49, Al: PbSe50, In/Ga: GeTe51, In: SnTe52. Here in Ge2Sb2Te5, the strongly anti-bonded In-Te also affects the bonding states for other interatomic pairs. Most remarkably, the Ge-Te bond neighboring to the In-Te bond (marked in Figure 6(f)) shows a nontrivial bonding character at the Fermi level (Figure 6(e)), which is not seen in pristine GST. Consistently, the length of this Ge-Te bond is 2.912 Å, being noticeable smaller than that of 2.967 Å in pristine GST. Therefore, the bonding characters strengthen the Ge-Te interaction, which agrees with the reduced spacing between Te-Ge-Te (Space A) shown in Figure 1(d). Figure 7 shows the zT values for all Ge2-xInxSb2Te5 samples. The zT values of 0.03 and 0.56 at room temperature and 800 K were achieved for undoped samples. Because of the enhanced Seebeck coefficients by the indium-induced resonant states, the peak zT value reaches 0.78 in Ge1.85In0.15Sb2Te5 compound at 700 K, being about 40% higher than that of the pristine sample at the same temperature. The results demonstrate that indium is an effective yet unconventional doping in Ge2Sb2Te5 compound. Since the change of the carrier concentration by indium doping is not as large as expected (~20% as seen in Figure 4(a)), we have also tuned the chemical composition with the attempt to introduce other possible donor-like defects such as interstitial Ge atoms, Te vacancies, SbGe anti-site defects, but all in vain (Figure S1 in SI). Apart from the dopability issues due to the formation energy, our DFT calculations did not find resonant levels in Ge2Sb2Te5 with Te deficiencies or SbGe defects (Figure S2 in SI). It is interesting to notice that in GeTe materials also with high concentration of hole carriers, only Bi and Sb have been found to be efficient ntype dopants.53,54 However, ternary Ge2Sb2Te5 and similar GST compounds are the intermediate phases of GeTe and Sb2Te3, and resemble the crystal structure of the latter. Considering that both GeTe and Sb2Te3 are strongly p-type semiconductors due to the intrinsic cation vacancies and/or antisite defects, GST compounds will possibly inherit these easily formed defects and exhibit similar degenerate behaviours. These attempts and results suggest that it is difficult to realize the conventional doping in intrinsically degenerate semiconductors. On the contrary, unconventional doping aiming at regulating the bonding character and band structure rather than merely suppressing or compensating the intrinsic defects could be more effective in tuning the electrical properties of these metallic semiconductors.

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Figure 7. Temperature dependence of zT for Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2).

Conclusions In summary, thermoelectric properties of indium-doped Ge2Sb2Te5 were investigated. Being different from conventional doping, the substitution of indium for germanium moderately reduces the carrier concentration from 6.0 × 1020 cm-3 to 4.5 × 1020 cm-3 yet significantly enhances the Seebeck coefficient from 30 to 96 μV K-1 at room temperature. Such a dramatic increase of Seebeck coefficient largely deviates from the Pisarenko relation and was correlated to the enhanced apparent density-of-state effective mass. Based on DFT calculations, this phenomenon was attributed to the doping-induced modification of band structure and local distortion in the density of states. It was also found that indium doping also induces the bonding character and enhances the interaction of the Ge-Te pair adjacent to indium, which is consistent with the experimentally observed decrease of the lattice parameter c. Due to the optimized electrical properties as well as the suppressed thermal conductivity, a maximal zT value of 0.78 at 700 K was achieved in Ge1.85In0.15Sb2Te5, which is about 40% higher than the pristine sample. These findings suggest that unconventional doping involving the modification of the band structure could be effective to tune the electrical properties in highly degenerate semiconductors. This work should be helpful to advance the understanding and optimization of the thermoelectric properties of GST compounds.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. Electrical conductivity and Seebeck coefficient as a function of x for Ge2+xSb2Te5, Ge2-xSb2+xTe5 and Ge2Sb2Te5-x, respectively; projected density of states (pDOS) for Ge1.89Sb2.11Te5 and Ge2Sb2Te4.89.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]


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Notes The authors declare no competing financial interest.

Acknowledgements This work is supported by the National Key Research and Development Program of China (2018YFB0703600), the National Natural Science Foundation of China (51625205 and 51802333), the Key Research Program of Chinese Academy of Sciences (KFZD-SW-421) and Shanghai Sailing Program (18YF1426700).

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Table of Contents


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Figure 1. (a) Powder XRD patterns of Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2). (b) lattice parameters (a and c) as a function of indium content. (c) Crystal structure of pristine Ge2Sb2Te5. (d) Calculated distance between specific layers in pristine and indi-um-doped Ge2Sb2Te5. 149x125mm (300 x 300 DPI)



Figure 2. (a) Backscattered electron microscopy (BSE) image. (b) Fractured surface secondary electron (SE2) image. EDS mapping for (c) Ge, (d) Sb, (e) Te and (f) In of Ge1.8In0.2Sb2Te5. 80x75mm (300 x 300 DPI)


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Figure 3. Temperature dependence of (a) electrical conductivity σ, (b) Seebeck coefficient S, (c) power factor PF and (d) total thermal conductivity κ for Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2) samples. 149x111mm (300 x 300 DPI)



Figure 4. Temperature dependence of (a) Hall carrier concentration (pH) and (b) Hall carrier mobility (H) for Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2). 149x55mm (300 x 300 DPI)


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Figure 5. (a) Pisarenko plot (Seebeck coefficients as a function of carrier concentration) for Ge2-xInxSb2Te5 samples at 300 K. The dashed lines are calculated from the single parabolic band (SPB) model with the effective masses of 1.02 me and 2.87 me, respectively. (b) Seebeck coefficients (S) as a function of md*. 149x57mm (300 x 300 DPI)



Figure 6. (a) Electronic band structure and (b) Projected density of states (pDOS) for pristine Ge2Sb2Te5. (c) Projected crystal orbital Hamilton population (pCOHP) between Te and cations for pristine Ge2Sb2Te5. (d) pDOS for indium-doped Ge2Sb2Te5. (e) -pCOHP between In-Te, Ge-Te neighboring to In-Te in indiumdoped Ge1.89In0.11Sb2Te5. (f) Schematic depiction of Ge-Te bonds neighboring to the In-Te bonds. 149x92mm (300 x 300 DPI)


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Figure 7. Temperature dependence of zT for Ge2-xInxSb2Te5 (x = 0, 0.05, 0.1, 0.15, 0.2). 80x62mm (300 x 300 DPI)


Largely enhanced Seebeck coefficient and thermoelectric

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