Transport properties of cubic crystalline Ge2Sb2Te5: A potential low-temperature thermoelectric material

Abstract

Ge2Sb2Te5 (GST) has been widely used as a popular phase change material. In this study, we show that it exhibits high Seebeck coefficients 200 -300 µV/K in its cubic crystalline phase (c-GST) at remarkably high p-type doping levels of ∼ 1×10 19 -6×10 19 cm −3 at room temperature. More importantly, at low temperature (T = 200 K), the Seebeck coefficient was found to exceed 200 µV/K for a doping range 1×10 19 -3.5×10 19 cm −3 . Given that the lattice thermal conductivity in this phase has already been measured to be extremely low (∼ 0.7 W/m-K at 300 K), 1 our results suggest the possibility of using c-GST as a low-temperature thermoelectric material.

Electronic Structures of Ge2Sb2Te5/Co2FeX (X: Al, Si) Interfaces for Phase Change Spintronics

“…The unit cell structure is relaxed by generalized gradient approximation (GGA) functional in CASTEP. , We employ ultrasoft pseudopotentials and 330 eV cutoff energy. The calculated band gap of c-Ge 2 Sb 2 Te 5 is 0.45 eV, slightly smaller than the hybrid functional result but close to the experimental value of order 0.5 eV. , We notice that several works have reported even smaller GGA band gaps for cubic Ge 2 Sb 2 Te 5 that range from 0.1 to 0.37 eV. ,, For Co 2 FeAl and Co 2 FeSi, they crystallize in the L2 1 structure. We relax these two unit cell structures by GGA functional and obtain lattice constants of 5.64 and 5.55 Å for Co 2 FeAl and Co 2 FeSi, respectively, close to the experimental values. , It has been reported that GGA functional fails to give the correct magnetic moment of Co 2 FeSi. , This is partially due to the nearly closed GGA band gap in the minority spin channel and the Fermi level lies in the conduction band.…”

“…36,37 We notice that several works have reported even smaller GGA band gaps for cubic Ge 2 Sb 2 Te 5 that range from 0.1 to 0.37 eV. 29,36,38 For Co 2 FeAl and Co 2 FeSi, they crystallize in the L2 1 structure. We relax these two unit cell structures by GGA functional and obtain lattice constants of 5.64 and 5.55 Å for Co 2 FeAl and Co 2 FeSi, respectively, close to the experimental values.…”

Glass-like Lattice Thermal Conductivity and Thermoelectric Properties of Incommensurate Chimney-Ladder Compound FeGeγ

“…Furthermore, the transport properties were evaluated using the BoltzTraP code, employing the Boltzmann transport theory within the constant relaxation time approximation that has been frequently used to explain and/or predict thermoelectric properties. ,− Assuming a constant relaxation time for an electron, each parameter can be calculated using the following equations: where e , k⃗ , f 0 , k B , ϵ, v⃗ , and μ are the carrier charge, the wave vector, the Fermi distribution function, the Boltzmann constant, the energy, the group velocity, and the chemical potential, respectively. Note that μ is identical to the Fermi level, E F at 0 K, and deviates from E F at finite temperatures.…”

Origin of Band Modulation in GeTe-Rich Ge–Sb–Te Thin Film

“…Most of the thermoelectric studies for GST-related compounds focused on bulk systems with the highest reported zT value of about 2.4. − In the case of bulk compounds, its thermoelectric properties are tunable by varying composition, microstructure, and vacancy content . The highest thermoelectric performance occurs when the material is GeTe-rich in composition and transforms into its higher-temperature rock-salt cubic structure. − This phase is interesting because, in addition to possessing a higher thermoelectric performance, thin films of a GeTe-rich GST phase also stabilize in this cubic state even at room temperature. , The stark contrast between the bulk and thin-film form of this compound motivated us to look into the latter form for a possible enhancement in its thermoelectric properties.…”