Metal - Insulator Transition Driven by Vacancy Ordering in GeSbTe Phase Change Materials

Abstract

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows.

Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

“…Both samples are characterized by a clear metallic behavior, as Sb 2 Te 3 and GST featuring at least a partial degree of vacancy ordering. 14,22 Besides the expected decrease of the mobility as the temperature increases, the GST-SL sample has a factor ∼2 higher mobility compared to the GeTe-SL (gray lines and triangles). At the same time, the 3D carrier concentration, which does not change significantly as a function of temperature, is higher in the case of the GeTe-SL (n GeTe-SL = 5.91 × 10 20 cm 3 and n GST-SL = 2.78 × 10 20 cm 3 at room temperature).…”

“…Notably, none of the satellites is found exactly at the Q z position of Sb 2 Te 3 peaks, as generally expected in the case of SLs. The average composition of the GST reference sample, measured from the separation between the main GST peak and the corresponding 1st order vacancy layer peak, 14 is GST225. The broadening of the vacancy layer peak and the smearing out of higher order peaks is related to the coexistence in the film of different GST blocks and both cubic and rhombohedral stackings.…”

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

“…Here, we explore STM and STS on the prototype phasechange material GST-225 [1,11], grown epitaxially by molecular beam epitaxy (MBE) [21][22][23][24][25]. This provides the first STM study on single crystalline phase-change materials.…”

Carbon doping induced Ge local structure change in as-deposited Ge2Sb2Te5 film by EXAFS and Raman spectrum

“…On the contrary, the TCR of C6.7-GST is -2.2×10 -3 , showing a remarkable semiconductor behavior. Given that the transition to the metallic state is driven by vacancy ordering in GST, 25,26 however, C6.7-GST, annealed at 420 • C, hasn't undergone metalinsulator transition (MIT). We can reasonable infer that carbon atoms may occupy the vacancy site and/or the interstitial site of crystalline CGST to impede the ordering of vacancy process in CGST, which will be discussed further below.…”