Short‐Range Order Investigation of Cu_xGe_50−xTe₅₀ Phase‐Change Materials

Abstract

The data storage stability and energy consumption of so-called phase-change materials (PCM) are determined by the stability of the amorphous and crystalline phases. The melting temperature T m of the crystal is a measure for the energy needed to transform from the crystalline to the amorphous phase. On the other hand, the stability of the amorphous phase is governed by the onset of crystallization at temperature T c . T m of the crystalline phase and the crystallization temperature T c are therefore important material properties.T m should be as low as possible to minimize power consumption and T c should be as high as possible to avoid spontaneous crystallization of the amorphous phase. [1] To optimize the stability of PCM materials towards long-lifetime storage applications, it is therefore crucial to understand the relationship between these parameters and the structural correlations in the amorphous phases.GeTe is a typical PCM which adopts at room temperature a distorted rock salt structure in which Ge is surrounded by three Te atoms with short bonds and three Te atoms with longer bonds. [2] Even though the crystalline phase is known for long, the amorphous phase is still subject of intense discussion. In the amorphous phase, the Ge atoms seem to be fourfold coordinated following the 8 À N rule. [3] Hence, the short range order between the two phases differs considerably. [4] The value of T c of the trigonal GeTe phase is %190 C. [5] As a PCM, it would therefore not be suitable for use at higher temperatures because the amorphous phase would no longer be stable. It was however found that substituting Ge atoms by Cu, to give Cu-Ge-Te compositions CGT Cu x Ge 50Àx Te 50 , enhances the crystallization temperature. [5] In this study CGT compositions are labeled by attaching the ratios of the elements Cu:Ge:Te (e.g., Cu 33 Ge 17 Te 50 equals CGT-213). Saito et al. investigated the phase-change characteristics for CGT compounds with different Cu contents. [5] They found an increasing T c up to 250 C at x % 15, while a further rise in Cu content leads to a decrease in T c . Compounds with lower Cu content, up to x % 20, crystallize into the GeTe structure. At higher Cu contents (26 ≤ x ≤ 38), they crystallize into the CGT-213 structure. [6] Saito et al. and also other authors point out that CGT-213 promises the best properties for PCM applications. Even though the T c of CGT-213 is lower than that for CGTs with less Cu

Structural origins of the unusual thermal stability of amorphous Cu x Ge50−x Te50 (0 ⩽ x ⩽ 33.3)

“…It can be described as Ge 50−x Te 50 + Cu x . A more detailed analysis of the Cu clusters and their embedding in the Ge-Te network is also given in [18]. Similar observations were recently reported for Cr clusters in Cr 2 Ge 2 Te 6 (Cr-Ge-Te) [32,33].…”

“…This is related to the Ge deficiency in both systems: In Ge-Te, the Ge deficiency leads a higher statistical change of finding Te around Ge than other Ge atoms, or in other words a decreasing Ge-Ge coordination number in favor of an increasing Ge-Te coordination [35]. The same situation occurs in Cu-Ge-Te, where the Ge deficiency is correlated with the formation of the Cu nanoclusters, which are embedded in the Ge 50−x Te 50 matrix mainly by Cu-Te bonds [18]. The decrease at larger x, on the other hand, is in both cases characterized by the transformation to a different network structure: In the Ge 50−x Te 50+x system, it was shown to be correlated with a transformation from a random covalent network to a chemically ordered network for x > 26 [35,36].…”

“…An overview of the partial coordination numbers for all correlations i-j obtained from RMC modeling of AXS and EXAFS data [7,17] and from the direct fitting of the EXAFS data [18], respectively, is given in figure 2 (with i-j denoting the environment of j atoms around i). A clear transformation is observed around 20 at% Cu content, where the Cu-Cu coordination number suddenly drops from around 4.2 to 2.6, and the Cu-Te coordination number increases from about 1.6 to 2.6.…”

“…The sputtering was conducted with a RF power of 50 W to sample thickness of about 1 µm. More details on the sample preparation are given elsewhere [2,4,18].…”

“…We have analyzed details of the structure of CuTe-GeTe glasses with varying concentrations of Cu in previous studies, for x = 0 [17], 5 < x < 25 [18] and x = 33.3 [7]. Note that though the pure GeTe system (x = 0) has been widely studied (a concise overview of experimental results in comparison with ab initio simulations is given for example by Micoulaut et al [19].…”