Uncovering Thermal and Electrical Properties of Sb₂Te₃/GeTe Superlattice Films

Abstract

Superlattice-like phase change memory (SL-PCM) promises lower switching current than conventional PCM based on Ge2Sb2Te5 (GST). However, a fundamental understanding of SL-PCM requires detailed characterization of the interfaces within such a SL. Here, we explore the electrical and thermal transport of SLs with Sb2Te3/GeTe alternating layers of various thicknesses. We find a ~3× reduction of the effective crossplane thermal conductivity of Sb2Te3/GeTe (4/1 nm/nm) SL compared to crystalline GST due to the thermal interface resistances within the SL. Thermal measurements with varying periods of our SLs show a signature of phonon coherence with a transition from wave-like to particle-like phonon transport, further described by our modeling. Electrical resistivity measurements of such SLs reveal strong anisotropy (~2000×) between the in-plane and cross-plane directions due to the weakly interacting van der Waals gaps. This work uncovers electro-thermal transport in Sb2Te3/GeTe SLs, for improved design of low-power PCM.

Exploring “No Man's Land”—Arrhenius Crystallization of Thin‐Film Phase Change Material at 1 000 000 K s−1 via Nanocalorimetry

“…This feature is typical for incongruent melting of alloy with non-eutectic composition and possibly a useful signature of cycle failures in PCM devices with antimony migration. [55] NanoDSC characterization of the melting peak is also important for novel phase-change materials including superlattice Sb 2 Te 3 /GeTe [12,13] and Ge 4 Sb 6 Te 7 . [56,57] GST can be completely evaporated after first melting without a capping layer.…”

“…[6] Ge-Sb-Te ternary chalcogenides [7,8] used in PCM are located on the pseudobinary line between GeTe and Sb 2 Te 3 and have extraordinary properties including 1) metal-insulator transition in the crystalline phase, [9,10] 2) phase separation during incongruent melting, [11] 3) large density of ordered/disordered vacancies (>10%), [10] and 4) tunable thermal conductivity via van der Waals interfaces. [12,13] Among them, Ge 2 Sb 2 Te 5 (GST) has been intensively studied and commercialized [14] given its fast crystallization as a poor glass former, dramatic change of electrical/optical properties after switching, [15] and high stability in amorphous phase. However, the fundamental mechanism of GST crystallization remains mysterious in many aspects, such as whether it crystallizes from amorphous states and whether GST is a strong or fragile glass-forming liquid.…”

Ultralow–switching current density multilevel phase-change memory on a flexible substrate

“…Earlier work had suggested that Ge atom movement may be responsible for switching in similar superlattice PCM on rigid substrates ( 21 ). However, we have recently found that the superlattice has low cross-plane thermal conductivity and very high electrical anisotropy ( 35 ) owing to its numerous parallel vdW-like interfaces. Thus, we exploited these properties to conceive our design, which includes confinement of the superlattice PCM cell in a pore-like geometry on a substrate with ultralow thermal conductivity (the flexible PI).…”

Unveiling the Effect of Superlattice Interfaces and Intermixing on Phase Change Memory Performance

“…Recently, superlattice (SL) phase change materials with ultrathin alternating layers (e.g., TiTe 2 /Sb 2 Te 3 , GeTe/Sb 2 Te 3 , or GeSb 2 Te 4 /Sb 2 Te 3 ) have been reported to lower the switching current density ( J reset ), resistance drift coefficient ( v ), and switching time in PCM. ,− Low cross-plane thermal conductivity and high cross-plane electrical resistivity of the SL films generate electro-thermal confinement in SL-PCM, driving the energy-efficiency in such devices. ,− The electro-thermal properties of the SL materials stem from the van der Waals (vdW)-like interfaces within the SL, − which have also been studied in other material systems; , the vdW-like interfaces are qualitatively similar but weaker than in covalently bonded SLs such as SiGe. Thus, SL interfaces, in particular their interface density and their intermixing (i.e., loss of vdW-like gaps, stacking faults, and disordering), can play a key role in the SL-PCM device performance.…”

“…We measured the effective cross-plane thermal conductivity ( k eff ) of our superlattices (Figure f) with varying period thickness using a time domain thermoreflectance (TDTR) technique described in detail elsewhere. , Our measured k eff of the ST/GST superlattices decreases with increasing number of interfaces, that is, with decreasing period thickness from 16/14.4 nm/nm (4 interfaces) to 2/1.8 nm/nm (32 interfaces). A minimum k eff ≈ 0.33 ± 0.01 Wm −1 K −1 is measured for the 2/1.8 nm/nm stack.…”