Antimony Telluride Thin Films Research Articles

Thermoelectric properties of nanocrystalline Sb2Te3 thin films: experimental evaluation and first-principles calculation, addressing effect of crystal grain size

The effect of crystal grain size on the thermoelectric properties of nanocrystalline antimony telluride (Sb2Te3) thin films was investigated by experiments and first-principles studies using a developed relaxation time approximation. The Sb2Te3 thin films were deposited on glass substrates using radio-frequency magnetron sputtering. To change the crystal grain size of the Sb2Te3 thin films, thermal annealing was performed at different temperatures. The crystal grain size, lattice parameter, and crystal orientation of the thin films were estimated using XRD patterns. The carrier concentration and in-plane thermoelectric properties of the thin films were measured at room temperature. A theoretical analysis was performed using a first-principles study based on density functional theory. The electronic band structures of Sb2Te3 were calculated using different lattice parameters, and the thermoelectric properties were predicted based on the semi-classical Boltzmann transport equation in the relaxation time approximation. In particular, we introduced the effect of carrier scattering at the grain boundaries into the relaxation time approximation by estimating the group velocities from the electronic band structures. Finally, the experimentally measured thermoelectric properties were compared with those obtained by calculation. As a result, the calculated thermoelectric properties were found to be in good agreement with the experimental results. Therefore, we can conclude that introducing the effect of carrier scattering at the grain boundaries into the relaxation time approximation contributes to enhance the accuracy of a first-principles calculation relating to nanocrystalline materials.

Empirical modelling and optimization of pre-heat temperature and Ar flow rate using response surface methodology for stoichiometric Sb2Te3 thin films prepared by RF magnetron sputtering

In this work, a flexible antimony telluride thin film was deposited by RF magnetron sputtering. The response surface methodology based on central composite design was used to study the influence of the Ar flow rate (A) and the pre-heat temperature of the substrate (T) on the Te content (%Te). The %Te of the thin film tended to increase with an increase in both the Ar flow rate and the pre-heat temperature. Stoichiometric Sb2Te3 thin films were obtained that agreed with the model equation of %Te = 59.22 + 0.070 A - 0.098 T + 5.580 × 10−4AT – 7.212 × 10−4A2 + 1.05 × 10−4T2. Micro-strainand dislocation density were enhanced using high Ar flow rate and low pre-heat temperature. The dislocation density and stoichiometry contributed to the substantially enhanced Seebeck coefficient and electrical conductivity of the films, respectively. The temperature dependence of the power factor is strongly dominated by electrical conductivity, leading to the highest value for a stoichiometric film of 2.0 × 0−3 W/m.K2 at 250 °C. During process optimization, several conditions can be prepared by RF magnetron sputtering with an Sb2Te3 target in order to obtain stoichiometric Sb2Te3 films. However, the expected condition to obtain highest power factor was the highest Ar gas flow rate and the lowest pre-heat temperature.

Electrochemical Nucleation and Growth of Binary Antimony Telluride Compounds

Whereas antimony telluride based compound materials have been demonstrated to be suitable for phase change memory devices,[1] among other applications, and are well suited to be fabricated via an electrochemical route, the ability to electrodeposit high quality thin films of these materials requires a good understanding of the nucleation and growth behavior that defines the initial stages of growth of these films.[2-4] Although electrochemical nucleation has been studied for decades,[5] little attention has been paid to the behavior of codeposition systems, where two or more materials are deposited simultaneously from a single electrolyte bath. In this study, the nucleation of Sb-Te compound is studied on gold and glassy carbon electrodes, to determine how the nucleation behavior of each element contributes to that of the overall system. Additionally, the effect of applied potential and electrolyte concentrations on the nucleation behavior and deposit composition is studied. Cyclic voltammetry coupled with quartz crystal microbalance measurements are used to extensively study the deposition processes that occur during compound electrodeposition. It is seen that deposited Te is further reduced to a soluble species near the potentials required to deposit Sb, however the presence of Sb in the deposited film seems to hinder the further reduction to a soluble species, enabling compound film growth (figure 1). Additionally, the free energy released by compound formation allows the compound to be formed at potentials positive to those normally required for Sb deposition. Chronoamperometry, along with TEM-EDS analysis of the composition gradient of individual nucleation sites will be presented and discussed in conjunction with the CVs to understand the nucleation and growth of binary Sb-Te compounds. REFERENCES 1. Raoux, S.; Wuttig, M., Phase Change Materials: Science and Applications. New York : Springer Dec. 2008: 2008. 2. Huang, Q.; Kellock, A. J.; Raoux, S., Electrodeposition of SbTe Phase-Change Alloys. Journal of The Electrochemical Society 2008, 155(2), D104-D109. 3. Catrangiu, A. S.; Sin, I.; Prioteasa, P.; Cotarta, A.; Cojocaru, A.; Anicai, L.; Visan, T., Studies of antimony telluride and copper telluride films electrodeposition from choline chloride containing ionic liquids. Thin Solid Films 2016, 611, 88-100. 4. Jung, H.; Myung, N. V., Electrodeposition of antimony telluride thin films from acidic nitrate-tartrate baths. Electrochimica Acta 2011, 56(16), 5611-5615. 5. Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B., Electrochemical nucleation. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1982, 138 (2), 225-239. Figure 1

Enhancing thermoelectric properties of Sb2Te3 flexible thin film through microstructure control and crystal preferential orientation engineering

Preparation of high performance flexible thermoelectric thin films would promote applications of flexible thermoelectric device. In this work, antimony telluride (Sb2Te3) thin films were directly deposited on polyimide substrate. The crystalline structures and morphologies of the thin films were analyzed, and the mechanism of crystal growth influenced by sputtering pressure was discussed. We also investigated the effects of microstructure on their thermoelectric properties, where Hall effect measurement was conducted to provide further insight into the enhancement of thermoelectric properties. The mean free path of the carrier was calculated on the basis of carrier concentration and mobility. Our results showed that with (015) crystal preferential orientation, the electrical conductivity and Seebeck coefficient of Sb2Te3 thin films were simultaneously increased, and a maximum power factor of 6.0μWcm−1K−2 was achieved, which was increased by 75% compared with the ordinary thin film. Meanwhile, due to the reduced lattice thermal conductivity and increased power factor, the estimated figure of merit (ZT) value was largely enhanced to 0.42.

Microstructure and Electrical Properties of Antimony Telluride Thin Films Deposited by RF Magnetron Sputtering on Flexible Substrate Using Different Sputtering Pressures

The microstructural, electrical, and thermoelectric properties of antimony telluride (Sb2Te3) thin films have been investigated for thermoelectric applications. Sb2Te3 thin films were deposited on flexible substrate (polyimide) by radiofrequency (RF) magnetron sputtering from a Sb2Te3 target using different sputtering pressures in the range from 4 × 10−3 mbar to 1.2 × 10−2 mbar. The crystal structure, [Sb]:[Te] ratio, and electrical and thermoelectric properties of the films were analyzed by grazing-incidence x-ray diffraction (XRD) analysis, energy-dispersive x-ray spectroscopy (EDS), and Hall effect and Seebeck measurements, respectively. The XRD spectra of the films demonstrated polycrystalline structure with preferred orientation of (015), (110), and (1010). A high-intensity spectrum was found for the film deposited at lower sputtering pressure. EDS analysis of the films revealed the effects of the sputtering pressure on the [Sb]:[Te] atomic ratio, with nearly stoichiometric films being obtained at higher sputtering pressure. The stoichiometric Sb2Te3 films showed p-type characteristics with electrical conductivity, carrier concentration, and mobility of 35.7 S cm−1, 6.38 × 1019 cm−3, and 3.67 cm2 V−1 s−1, respectively. The maximum power factor of 1.07 × 10−4 W m−1 K−2 was achieved for the film deposited at sputtering pressure of 1.0 × 10−2 mbar.

Effects of Cr interlayer thickness on adhesive, structural, and thermoelectric properties of antimony telluride thin films deposited by radio-frequency magnetron sputtering

We investigated the effects of Cr interlayer thickness on adhesive, structural, and thermoelectric properties of p-type antimony telluride (Sb2Te3) thin films. Prior to depositing Sb2Te3 films with a thickness of 1μm using radio-frequency magnetron sputtering, a Cr film with a thickness ranging from 0 to 216nm was deposited on a glass substrate using the same sputtering equipment. The obtained samples were thermally annealed at 300°C for 1h. Adhesiveness of the films was examined by a cross-cut method. Adhesion of Sb2Te3 thin films could be improved by incorporating the Cr interlayer, independent of the thickness of the interlayer. The surface morphology of the Sb2Te3 thin films became flat with a dense structure as the thickness of the Cr interlayer increased. Atomic composition analysis indicated that Te atoms might be diffused into the Cr interlayer and formed Cr-based alloys when the Cr interlayer thickness was increased to >100nm. X-ray diffraction analyses indicated that the crystal orientation, crystallite size, and strain of the films reached their maximum values at Cr interlayer thicknesses of 46–70nm. Thermoelectric properties of the films were improved by incorporating the Cr interlayer of appropriate thickness. In particular, a power factor of 18.7μW/(cmK2) was achieved at a Cr interlayer thickness of 46nm, which was approximately 4 times larger than that of the film with no Cr interlayer.

Analysis of Thermal Conductivity of Antimony Telluride Thin Films by Modified Callaway and Sondheimer Models

Analysis of Thermal Conductivity of Antimony Telluride Thin Films by Modified Callaway and Sondheimer Models

Effect of thermal annealing on the structural and thermoelectric properties of electrodeposited antimony telluride thin films

We investigated the effect of thermal annealing on structural and thermoelectric properties of p-type antimony telluride (Sb2Te3) thin films fabricated on a stainless steel (SUS304) substrate by electrodeposition. Antimony telluride thin films were annealed for 1 h at temperatures between 200 and 400 °C. We observed that the as-deposited thin film and thin films annealed at temperatures less than 250 °C possessed a stoichiometric atomic composition (Sb:Te = 40:60) with no impurities from the substrate. At the annealing temperature of 300 °C, we observed a certain amount of impurities (Fe, Cr, Ni) in the thin film. The Seebeck coefficient was decreased but the electrical conductivity was increased for films treated at 300 °C. As a result, the thin film exhibited a maximum power factor of 13.6 μW/(cm·K2). On further increasing the annealing temperature to 400 °C, the film structure and thermoelectric properties were drastically changed. The impurity concentration in the thin film reached approximately 50 at.%. The crystal structure of Sb2Te3 completely disappeared, and instead, other chemical compounds formed by alloying the elements of the thin film and the substrate were observed.

Bismuth antimony telluride thin films with unique crystal orientation by two-step method

P-type bismuth antimony telluride thin films have been prepared by flash evaporation and a subsequent two-step synthesis method of homogeneous electron beam (EB) irradiation and thermal annealing. EB irradiation of the thin films was performed in a N2 atmosphere at room temperature and an acceleration voltage of 0.17 MeV. Thermal annealing was performed at 300°C for 1 h under Ar/H2 (95/5%). Thin films prepared using the two-step method exhibit a unique crystal orientation that provides improved electrical conductivity and an improved Seebeck coefficient. With an EB irradiation dose of 0.43 MGy, the power factor of the thin films reached 38.4 μW/(cm·K2). Thermal conductivity was calculated using a phonon transport model and the Wiedemann–Franz law, providing a dimensionless figure of merit (ZT). The ZT was expected to enhance significantly by a two-step method compared with alternate synthesis techniques.

Optimization of Thermoelectric Properties of p-type AgSbTe2 Thin Films via Electrochemical Synthesis

Silver antimony telluride (AgSbTe2) thin films were fabricated by a simple and cost-effective electrodeposition method for the first time. To investigate the effects of crystallinity and composition on the films’ thermoelectric (TE) properties, amorphous AgxSbyTez solid solutions of various compositions were electrodeposited by tailoring the [Ag+] concentration in the electrolytes, followed by a thermally driven solid-state amorphous-to-nanocrystalline phase transition. Because of the lower carrier concentration in the nanocrystalline phases, higher Hall mobility and Seebeck coefficients, almost twice as high as those observed in the bulk materials, resulted in enhanced power factors of 90–553μW/mK2. The maximum power factor was obtained from stoichiometric AgSbTe2 thin films. The reported power factors are highly comparable to other values reported for electrodeposited p-type V–VI compounds. The improvement achieved in the TE properties of electrodeposited AgSbTe2 thin films demonstrates the feasibility of the use of these films as p-type legs in room-temperature operations.

アンチモン・テルル熱電薄膜の斜め蒸着法による配向性制御

We prepared p-type antimony telluride thin films by an oblique deposition at the incident angle ranging from 0º to 80º. We investigated the relationship between the structural and electrical properties of the thin films. As the structural properties, we analyzed cross-section morphology by scanning electron microscope (SEM), and the crystal orientation and crystallite size by x-ray diffraction (XRD) analysis. As the electrical properties, we measured in-plane electrical conductivity, Seebeck coefficient and power factor at room temperature. As a result, we found the thin film at the incident angle of 40° obtained the highest crystal orientation. As the incident angle increased, the crystallite size were enhanced but the power factor decreased. The power factor of thin film at the incident angle of 0° was 1.26 μW / ( cm·K2 ) and thin film at the incident angle of 80° was 0.38 μW / ( cm·K2 ).

Investigation of the effects of compressive and tensile strain on n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films for use in flexible thermoelectric generators

To assess the potential performances of flexible thin film thermoelectric generators, we prepared n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films on flexible polyimide substrates using a RF magnetron sputtering method. We applied various compressive and tensile strains to the films by changing the bending radii of the flexible substrates prior to deposition. The structural and thermoelectric properties of the completed samples were analyzed. The bismuth telluride and antimony telluride thin films had dense granular and a chaff-like surface morphologies, respectively. The shapes of both types of films did not greatly change when strains were applied. Both types of thin films demonstrated higher electrical conductivities when under compressive strain than when under tensile strain. The absolute Seebeck coefficients of the bismuth telluride thin films were slightly higher when under tensile strain than when under compressive strain. In contrast, the antimony telluride thin films demonstrated the opposite trend, having higher values when under compressive strain than when under tensile strain. The power factors of both types of thin films were found to decrease with increasing tensile stain. Therefore, we expect that the thermoelectric performances of flexible thin film generators are not largely changed under compressive strain but they decrease under tensile strain.

Thickness and Grain Size Effects on Thermal Transport of Post-Annealed Antimony Telluride Thin Films

Thickness and Grain Size Effects on Thermal Transport of Post-Annealed Antimony Telluride Thin Films

Effect of grain size on thermal transport in post-annealed antimony telluride thin films.

The effects of grain size and strain on the temperature-dependent thermal transport of antimony telluride (Sb2Te3) thin films, controlled using post-annealing temperatures of 200°C to 350°C, were investigated using the 3-omega method. The measured total thermal conductivities of 400-nm-thick thin films annealed at temperatures of 200°C, 250°C, 300°C, 320°C, and 350°C were determined to be 2.0 to 3.7 W/m · K in the 20 to 300 K temperature range. We found that the film grain size, rather than the strain, had the most prominent effect on the reduction of the total thermal conductivity. To confirm the effect of grain size on temperature-dependent thermal transport in the thin films, the experimental results were analyzed using a modified Callaway model approach.

30am2-PN-52 めっき法を用いたビスマステルライド系薄膜及び熱電モジュールに及ぼす熱処理の影響

N-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) thin films have been prepared by electrodeposition, followed thermal annealing. The electrodeposition was performed by changing the mole ratios of Te/(Bi+Te) and Sb/(Sb+Te). The thermal annealing was performed at 300 ℃ for 1 hour under Ar/H_2 (95/5%). We measured the in-plane Seebeck coefficient and electrical conductivity at room temperature, and then the power factor was estimated using both the measured properties. For n-type films, with the Te/(Bi+Te) of 0.5, the power factor of the Bi-Te thin film reached 4.5 μW/(cm・K^2). For p-type films, with the Sb/(Sb+Te) of 0.35, the power factor of the Sb-Te thin film reached 8.5 μW/(cm・K^2). The XRD peaks of both types of thin films exhibited well crystalized regardless of the mole ratios. Finally, we fabricated thermoelectric generators on flexible substrates by electrodeposition and transfer method, and observed that the generators worked.

Optimization of the thermopower of antimony telluride thin film by introducing tellurium nanoparticles

Antimony telluride (Sb2Te3) is one of the best thermoelectric materials at room temperature. Low dimension has the potential to improve the material thermoelectric properties. Herein, we demonstrate experimental evidence of the positive influence of tellurium (Te) nanoparticles on thermopower of Sb2Te3 thin film. Sb2Te3 films with Te nanoparticles are prepared by alternate growth of Sb2Te3 layers and Te layers. When the single Te layer thickness is 1 nm, Te nanoparticle diameter is about 5 nm and areal density is approximately 160 µm−2, the Seebeck coefficient increases by ~25 %, thermoelectric power factor by ~50 %, and thermal conductivity decreases by ~26 % compared with Te layer free Sb2Te3 film without Te nanoparticles. The enhancement of thermoelectric thermopower could be attributed to carrier energy filtering effect induced by Te–Sb2Te3 nanocrystal boundary, and the thermal conductivity reduction may be ascribed to enhance phonon scattering by Te nanoparticles. The results show the Te nanoparticles have the ability to improve Sb2Te3 material thermoelectric properties.

Evaluation of Specific Heat, Sound Velocity and Lattice Thermal Conductivity of Strained Nanocrystalline Bismuth Antimony Telluride Thin Films

To investigate the effect of strain on specific heat, sound velocity and lattice thermal conductivity of nanocrystalline bismuth antimony telluride thin films, we performed both experimental study and modeling. The nanocrystalline thin films had mostly preferred crystal orientation along c-axis, and strains in the both directions of c-axis and a-b-axis. It was found that the thermal conductivity of nanocrystalline thin films decreased greatly as compared with that of bulk alloys. To gain insight into the thermal transport in the strained nanocrystalline thin films, we estimated the lattice thermal conductivity based on the phonon transport model of full distribution of mean free paths accounting for the effects of grain size and strain which was influenced to both the sound velocity and the specific heat. As a result, the lattice thermal conductivity was increased when the strain was shifted from compressive to tensile direction. We also confirmed that the strain was influenced by the lattice thermal conductivity but the reduction of the lattice thermal conductivity of thin films can be mainly attributed to the nano-size effect rather than the strain effect. Finally, it was found that the measured lattice thermal conductivities were in good agreement with modeling.

Effect of electronic contribution on temperature-dependent thermal transport of antimony telluride thin film

We study the theoretical and experimental characteristics of thermal transport of 100nm and 500nm-thick antimony telluride (Sb2Te3) thin films prepared by radio frequency magnetron sputtering. The thermal conductivity was measured at temperatures ranging from 20 to 300K, using four-point-probe 3-ω method. Out-of-plane thermal conductivity of the Sb2Te3 thin film was much lesser in comparison to the bulk material in the entire temperature range, confirming that the phonon- and electron-boundary scattering are enhanced in thin films. Moreover, we found that the contribution of the electronic thermal conductivity (κe) in total thermal conductivity (κ) linearly increased up to ∼77% at 300K with increasing temperature. We theoretically analyze and explain the high contribution of electronic component of thermal conductivity towards the total thermal conductivity of the film by a modified Callaway model. Further, we find the theoretical model predictions to correspond well with the experimental results.

Strain and grain size effects on thermal transport in highly-oriented nanocrystalline bismuth antimony telluride thin films

We investigated the effects of strain and grain size on the thermal transport of highly-oriented nanocrystalline bismuth antimony telluride thin films using both experimental studies and modeling. The fabricated thin films had preferred crystal orientation along the c-axis, average grain sizes of 30<d<100nm, and strain of −0.8%<ε<−1.4% in the c-axis direction, whereas the strain in the a–b-axis direction was constant at 1.7%. The thermal conductivities were measured to be 0.32<κ<0.52W/m/K using a 3ω method at room temperature. To gain insight into the thermal transport in the strained nanocrystalline thin films, we estimated the lattice thermal conductivities from the measured thermal conductivities. We then calculated the lattice thermal conductivity using a simplified phonon transport model that accounts for the strain effect based on the Christoffel equation and uses Lennard–Jones potentials, and the grain size effect based on the full distribution of mean free paths. The theoretical calculations were in good agreement with our experimental results, and we conclude that the decrease of the lattice thermal conductivity of nanocrystalline thin films can be mainly attributed to the nano-size effect rather than the strain effect.

Effect of Annealing on the Properties of Antimony Telluride Thin Films and Their Applications in CdTe Solar Cells

Antimony telluride alloy thin films were deposited at room temperature by using the vacuum coevaporation method. The films were annealed at different temperatures in N2ambient, and then the compositional, structural, and electrical properties of antimony telluride thin films were characterized by X-ray fluorescence, X-ray diffraction, differential thermal analysis, and Hall measurements. The results indicate that single phase antimony telluride existed when the annealing temperature was higher than 488 K. All thin films exhibited p-type conductivity with high carrier concentrations. Cell performance was greatly improved when the antimony telluride thin films were used as the back contact layer for CdTe thin film solar cells. The dark current voltage and capacitance voltage measurements were performed to investigate the formation of the back contacts for the cells with or without Sb2Te3buffer layers. CdTe solar cells with the buffer layers can reduce the series resistance and eliminate the reverse junction between CdTe and metal electrodes.