ICT2014 Abstracts

Pseudobinary GeTe-Sb₂Te₃ compounds are widely used as phase-change optical materials for DVD-RAM [1]. Pseudobinary compounds there are claimed to be good thermoelectric materials due the large number of intrinsic structural vacancies [2].Ge₂Sb₂Te₅ (GST-225) is one of the most used materials for phase-change optical devices even though the stacking sequence of the stable cell is motive of debate. The stable crystal structure is rhombohedral with space group #164 (P-3m1) and nine atoms in the primitive cell. This structure is characterized by a stacking of layers along the c-axis. We present thermoelectric properties for two stacking sequences of GST-225. The one proposed by Kooi and De Hosson which consist in a layer sequence Te-Ge-Te-Sb-Te-Te-Sb-Te-Ge-, and the one proposed by Petrov et al which derives from the previous one by the exchange of Sb and Ge layers. For our purpose, we use DFT [3, 4] and Boltzmann transport equation in the constant relaxation time approximation. We compute phonons using DFPT and no dynamic instabilities were found in the entire irreducible brillouin zone for either of the proposed stacking sequences. The structure proposed by Kooi and De Hosson shows semiconductor-like density of states (DOS) with a computed gap of 190 meV unlike the other stacking sequence which has a metallic-like DOS. Thermoelectric properties calculation reveals that semiconductor-like structure has the highest value of Seebeck coefficient (SC) (at 650K S = 100 against 45 μV/K of the metallic-like). Our theoretical results of SC are in good agreement with experimental data [5].

References

[1] Kolobov, et al., Nature Materials 3 (2004) 703.
[2] Konstantinov, et al., Inorganic Materials 37 (2001) 662.
[3] Hohenberg, P. and Kohn, W., Phys. Rev. 136 (1964) B864.
[4] Kohn, W. and Sham, L. J., Phys. Rev. 140 (1965) A1133.
[5] Yan F., et al., Applied Physics A 88, (2007) 425.