Disorder Control in Crystalline GeSb₂Te₄ and its Impact on Characteristic Length Scales

Abstract

Crystalline GeSb 2 Te 4 (GST) is a remarkable material, as it allows to continuously tune the electrical resistance by orders of magnitude without involving a structural phase transition or stoichiometric changes. While wellordered specimen are metallic, increasing amounts of disorder eventually lead to an insulating state with vanishing conductivity in the 0 K limit, but a similar number of charge carriers. Hence, GST provides ideal grounds to explore the impact of disorder on transport properties. Here, a sputterdeposition process is employed that enables growing biaxially textured GST films with large grain sizes on mica substrates. The resulting films exhibit a systematic variation between metallic and truly insulating specimen upon varying deposition temperature. Transport measurements reveal that their electron mean free path can be altered by a factor of 20, while always remaining more than an order of magnitude smaller than the lateral grain size. This proves unequivocally that grain boundaries play a negligible role for electron scattering, while intra-grain scattering, presumably by disordered vacancies, dominates. These findings underline that the insulating state and the system's evolution toward metallic conductivity are intrinsic properties of the material.

Phase change thin films for non-volatile memory applications

“…charge transport and optical reectivity, in crystalline GST materials. 58,61,[81][82][83] In GST225 phase I the vacancies are randomly distributed and are necessary to stabilize the structure as indicated by a negative formation energy. 65 For example, the creation of vacancies in the Ge 2 Sb 2 Te 4 compound reduces the average number of valence electrons and thus, the occupation of antibonding states.…”

“…85 The high degree of structural disorder in cubic phase I leads to large electron scattering and, as a consequence, to electron low mobility, thus resulting in insulating behaviour. 58,61,82 The combination of directional bonding, lattice distortions and high vacancy concentrations causes charge carrier localization. 60 The microscopic origin of the electron localization was directly attributed to the presence of vacancy clusters on cation sites, which have a high impact on the nature of the wave functions.…”

Persistence of spin memory in a crystalline, insulating phase-change material

“…1c, e) described by R % R 0 expðð T TMott Þ À 1 4 Þ; the associated carrier localization length of 2.1 nm 8,9 also confirms that the sample shows strong-localization. However, the occurrence of a separate Efros-Shklovskii hopping regime indicative of a soft Coulomb gap 9 and usually present at lowest temperatures 19,21,26,27 cannot be unambiguously detected (see Supplementary Note 6).…”

“…A metal-insulator transition (MIT) in bulk PCMs (like GeSb 2 Te 4 ) arises through the ordering of the intrinsic vacancies in the metastable rock-salt phase Fig. 1a, [17][18][19][20][21][22] preceded by hallmark phase-coherent effects on the metallic side of the MIT 21,23 driven by strong spin-orbit interactions. To investigate charge transport beyond the breakdown of the diffusive picture, we consider whether spin-or phase-coherence lifetimes still play a prominent role in the insulating state of phase-change materials as the disorder is continuously increased.…”

The impact of vacancies on the stability of cubic phases in Sb–Te binary compounds

“…The crystalline phase utilized in GeSbTe-based PCMs that are currently in use is the cubic metastable phase, which contains large quantities of vacancies [6][7][8] . The vacancies in the Ge/Sb sublattice are intrinsic 9 and have been demonstrated to play an important role in phase transitions and related properties [10][11][12][13][14][15][16][17] .…”