Structural characteristics of GeTe-rich GeTe–Sb2Te3 pseudobinary metastable crystals

Abstract

The (GeTe)1−γ–(Sb2Te3)γ pseudobinary system has, over almost its entire composition range, two kinds of crystalline phase: one is a metastable phase with a NaCl-type structure and the other is a spectrum of stable phases with homologous structures. In the metastable phase, Ge/Sb atoms and intrinsic vacancies occupy the Na sites; on the other hand, Te atoms are located at the Cl sites. These vacancies are produced by following γ/1+2γ to ensure the stoichiometry of the metastable pseudobinary compound. This metastable phase obstinately holds its NaCl-type structure and resists transformation to stable homologous structures, even at high temperatures on the GeTe-rich side of the system. In GeTe (γ=0), the NaCl-type atomic configuration itself is the stable structure. GeTe has, as is well known, a high-temperature cubic phase and a low-temperature rhombohedral phase. This GeTe and the pseudobinary compounds containing a small quantity of Sb2Te3 have their single-phase regions not on the GeTe–Sb2Te3 tie line but at Ge-poor sides off the line: in other words, the Na sites of these off-stoichiometric compounds have some excess vacancies besides the intrinsic vacancies. As Sb2Te3 is further added to GeTe, however, the structural transformation temperature continuously falls and the single-phase region converges on the tie line as the excess vacancies at the Na site disappear, which change its electrical property from metallic to semiconducting conductivity. The low-temperature rhombohedral phase is present up to near γ=0.14. The NaCl-type metastable phase becomes unstable with increased Sb2Te3; after subjecting the compound Ge8Sb2Te11 (γ=0.11) to heat treatment for 15 days at 773 K, a stable homologous structure appeared.

Structure of amorphousGe8Sb2Te11:<

“…These measurements do not distinguish between Ge-Te and Sb-Te bonds and give 2.88 and 3.10 Å for the shorter and longer bonds, most of which are Ge-Te. 4 These values are 1%-2% shorter than the PBE results for GST-225. The calculated cohesive energy of crystalline GST-8,2,11 is 3.02 eV, and the melt-quenched amorphous sample is 0.084 eV/atom higher in energy.…”

“…It is astonishing that almost nothing is known about the amorphous structure of GST-8,2,11, a material now used for the third generation optical storage, BD. 4 Until very recently, this observation applied to the amorphous phases of almost all PCMs.…”

“…GeTe-rich alloys with small values of x are better and it has been found that the optical contrast between the two phases increases monotonically as x → 0. 4 On the other hand, it has been known for many years that the most rapid crystallization occurs for larger values of x, as in GeSb 4 Te 7 ͑x = 2 3 ͒. Compromises are necessary, and Ge 8 Sb 2 Te 11 ͑GST-8,2,11, x = 1 9 ͒ is a material of choice for BD.…”

“…Compromises are necessary, and Ge 8 Sb 2 Te 11 ͑GST-8,2,11, x = 1 9 ͒ is a material of choice for BD. 4 The amorphous-crystalline phase change cannot be understood without knowing the structures involved. The crystalline phases of GeTe-rich alloys in the family ͓͑GeTe͒ 1−x ͑Sb 2 Te 3 ͒ x , x Յ 1 3 ͔ have been studied for many years.…”

“…In most cases there is a metastable phase with a distorted NaCl structure and a higher temperature phase with homologous structures. 4 Vacancies play an essential role in stabilizing the NaCl structure over a wide range of x. 5 Density-functional calculations have been performed for crystalline structures with compositions ͑GeTe͒ 1−x ͑Sb 2 Te 3 ͒ x and nearby stoichiometries.…”