Origin of electronic localization in metal-insulator transition of phase change materials

Abstract

Tellurium based phase change materials are unique 3D-solids proposed to undergo Anderson type metal-insulator transition. However, the origin of this transition is not unambiguously understood. Here, we report combined high energy resolution photoemission spectroscopy and high k-resolution X-ray diffraction measurements on a reversibly phase switched Ge 2 Sb 2 Te 5 film. The results resolve the ambiguity between previous spectroscopic data and the proposed theoretical model for the origin of Anderson localization in these materials. Furthermore, by switching between the metallic state to insulating and back to metallic, we probe the electronic structure evolution in the phase change material.

Interface Formation during the Growth of Phase Change Material Heterostructures Based on Ge-Rich Ge-Sb-Te Alloys

“…After deposition of GGST, chemical shifts of the Te 4d and Ge 3d core levels toward higher BEs were clearly visible in the XPS spectra of both the heterostructures, while the opposite trend could be observed for Sb 4d ( Figure 2 b,c). To understand the behavior of the chemical shifts after the deposition of the GGST layers, it is useful to consider the expected BE shifts after the formation of binary compounds [ 21 , 22 , 23 ]. The Sb-Te bonding shifts the Sb 4d core levels toward higher BE (+0.6 eV with respect to metallic Sb) and the Te 4d toward lower BE (−0.55 eV with respect to metallic Te).…”

“…The Sb-Te bonding shifts the Sb 4d core levels toward higher BE (+0.6 eV with respect to metallic Sb) and the Te 4d toward lower BE (−0.55 eV with respect to metallic Te). Conversely, the Ge-Te bonding moves the Te 4d core levels of around 0.3 eV towards lower BE and the Ge 3d core levels to higher BE with a shift of +0.55 eV [ 21 , 22 , 23 ]. Therefore, after the formation of a ternary Ge-Sb-Te alloy, the BE of Te 4d core levels should be in the middle between the values expected for metallic Te and Sb 2 Te 3 .…”

Revealing the Impact of Prestructural Ordering in GaSb Thin Films

“…Similar to the electronic structure of GST, there is a steep increase in the density of states (DOS) near the E F due to vacancy-related disorder localization (see Figure a,b) . The amorphous sample has a valence band offset of ∼0.1 eV, which justifies the high resistance state.…”

“…Similar to the electronic structure of GST, there is a steep increase in the density of states (DOS) near the E F due to vacancy-related disorder localization (see Figure 8a,b). 37 The amorphous sample has a valence band offset of ∼0.1 eV, which justifies the high resistance state. The UPS spectra of the crystalline and amorphous samples follow each other very closely, except for two regions which are presented in the insets.…”

“…This has been observed in the prototype materials like GST and has been theoretically predicted for GaSb too. 37,38 This "dip" is generally attributed to the sharpening of the DOS between the orbitals expected in the crystalline form. 38 As expected, the valence band maximum falls at E F in both the crystalline samples, indicating that the annealed samples are in the metallic state (see Figure 8, insets ② and ④).…”