Effect of Structural Change on Thermoelectric Properties of the Chalcogenide Ge₂Sb₂Te₅Thin Films

Abstract

Ge 2 Sb 2 Te 5 (GST) films grown onto SiO 2 (250 nm)/Si substrate at room temperature were annealed at various temperatures to investigate the structural variations. The GST films changed from amorphous to face-centered cubic (FCC) and hexagonal closed packed (HCP) crystalline phases as the annealing temperature increases and the samples annealed at 380 • C showed co-existence of the FCC and the HCP. Samples annealed at 380 • C showed the highest charge carrier concentration, the lowest resistivity, and the highest power factor. © The Author ( Highly efficient thin thermoelectric films have recently gained acceptance for potential applications in the microelectronics industry as sensors, microcoolers, and as low-power generators.1,2 Thermoelectric transport can have a large impact on the performance of semiconductor devices and related nanostructures.3-5 Compared with the conventional mechanical devices, thermoelectric devices have the advantages such as maintenance-free, better transient response. 6,7 The primary candidate technology for miniaturizing thermoelectric devices is to apply semiconductor-processing technologies including thin film fabrication. The efficiency of conversion is described by the dimensionless, thermoelectric figure-of-merit, ZT = S 2 T/ρκ, where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and κ is the total thermal conductivity. A high ZT may be achieved by reducing κ and by enhancing the thermoelectric power factor, P = S 2 /ρ. The interrelated Seebeck coefficient, electrical conductivity, and thermal conductivity make it difficult to improve one transport property without affecting the others. 8 The key issue of micro-thermoelectric devices is how to fabricate thin films with high performance by semiconductor-processing technologies.Various chalcogenide materials reported that the holes are responsible for their positive Seebeck coefficient. 9 The Seebeck coefficient is strongly depended on the phase and the processing conditions. The Seebeck coefficients for the Ge 2 Sb 2 Te 5 (GST) films of various thicknesses and for the phase purity such as amorphous and crystalline phases have been reported. 10,11 There are two types of crystal structure in the GST compounds. One is the metastable rocksalt structure with Te atoms occupying sites on one face-centered-cubic (FCC) sublattice, and Ge and Sb randomly forming the other FCC sublattice. The other is the hexagonal structure (HCP), which is composed of Te layers forming a complicated cubic close packed structure with Ge and Sb atoms randomly occupying the octahedral interstices. Friedrich et al. reported that the temperature dependent measurements of the electrical resistance have been employed to study structural changes in direct current (dc) sputtered GST films.12 Using the films with a thickness between 80 and 250 nm, they reported that the pronounced changes of film resistance due to structural changes enabled a precise determination of transition temperature and activation energies. T...

Thermoelectric properties of c -GeSb 0.75 Te 0.5 to h -GeSbTe 0.5 thin films through annealing treatment effects

“…Further annealing up to 673 and 723 K seemed the Ge 2 Sb 2 Te 5 hexagonal structure (164/P-3m) with (013) and (203) peaks corresponding to number: 089-2233 [28]. The XRD intensity indicated the highest peak from the sample annealed at 673 K, and then decreased from the sample annealed at 723 K. It is noted that the annealing treatment of the sample at 623 K showed the XRD patterns that indicated a mixture between cubic structure and hexagonal structure, similar to the publications from Lee et al (2012) [16] and Hong and Yoon (2014) [17].…”

“…The Seebeck coefficient reduced from 371 mV K À1 to 206 mV K À1 as the crystalline fraction increased by a factor of four as quantified using X-ray diffraction. Recently, Hong and Yoon [17] reported phase change from amorphous to FCC and HCP crystalline phases, similar to ref. [16].…”

“…In addition, the results of the resistivity compared well with those from the XRD results, when the electrical resistivity changed greatly due to the phase change from the amorphous to FCC and finally to HCP structure. The decrease of the resistivity with the annealing treatment was closely related to the decrease of the lattice constant in the FCC to HCP structure [17]. However, by increasing the annealing temperature from 573 to 623 K, the carrier concentration was increased and the mobility was decreased because of the reduction of the anti-structural defects.…”

“…However, at 723 K annealing treatment, both thermoelectric properties of the GeSbTe film was suddenly decreased when the film thickness and the film composition changed, as already discussed in the FE-SEM and AES analyses. In order to investigate the performance of the prepared GeSbTe thin films, the authors compared the experimental data with reference data of (25 nm-thick) Ge 2 Sb 2 Te 5 ultra-thin film [16] and (500 nm) Ge 2 Sb 2 Te 5 thick film [17], as shown in Fig. 8(b) and (c).…”

Power factor investigation of RF magnetron sputtered c-GeSbTe thin film

“…The Ge-Sb-Te system has studied the several possible compositions of GeSb 2 Te 4 , GeSb 4 Te 7 , and Ge 2 Sb 2 Te 5 shown in a Ge-Sb-Te phase diagram [8][9]. The Ge 2 Sb 2 Te 5 has been the preferred thermoelectric material with high Seebeck coefficient and low electrical resistivity [10][11]. Nevertheless, other GeSbTe compositions, for example, Sb 2 Te 3 -GeTe and Ge x Sb y Te 1-(x + y) are also promising materials for thermoelectric applications [8], but have not been fully explored.…”