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▲ [13p-A23-5] Voltage-Pulse-Induced Expansion of Chalcogenide Superlattices Measured by Scanning Probe Microscopy
Keywords:phase change material, scanning probe microscopy, electric measurements
Interfacial phase-change memory topological materials such as (GeTe)/(Sb2Te3) chalcogenide superlattices (CSL) have been the promising candidate material for next generation of nonvolatile memory. [1] As the resistivity switching in the CSL films involves a structural phase transition,[1] evaluation of CSL deformation by voltage pulses are of particular importance for using CSL films in sensors, switches and memory elements. Here, we measured the expansion of SL films caused by voltage pulses employing the multimode scanning probe microscope (MSPM).[2,3]
We measured films consisting of 8-nm-thick [(GeTe)2/(Sb2Te3)]4 and 7-nm-thick GeTe prepared by sputtering on Sb2Te3(3 nm)/Si substrates at 240oC. In our setup [Fig.1(a)] the metal SPM probe was placed in a pre-defined location, and the structural phase transition was activated by voltage pulses (Vwrite) with an amplitude of 2 - 5 V and a duration of 10 ms - 3 s applied to the sample. The probe height position (Z) and the current were recorded as a function of time during the local excitation. To minimize film deformation by the SPM probe, a small gap maintained in the non-contact SPM mode by keeping a shift of the SPM sensor resonance frequency (Δf) in ultrahigh vacuum at room temperature.[2].
Fig. 1(b) shows a typical CSL response where the CSL expansion (Z-jump) coincided with the conductance state change at 96.1 s. The CSL expansion varied from -150 pm to +1000 pm for different pulse voltage amplitudes and positions along the granular film [Fig. 1(c)]. On average, the CSL expansion was +3.5% in accord with the solid-solid transition mechanism.[1,3] Interestingly, at Vwrite <+2.5 V the switching delay was >10 ms, suggesting a ferroelectric-like behavior where the structural phase transition and corresponding low-to-high conductance switching activated by strong electric field. Though, details of GeTe and Sb2Te3 layer stacking are responsible for observed variations. The work was supported by Japan Science and Technology Agency (JST/CREST).
[1] J. Tominaga et al., Adv. Mater. Interfaces 1 (2014) 1300027; [2] L.Bolotov et al., JJAP 50, 04DA04 (2011); [3] L.Bolotov et al., JJAP 55, 04EK02 (2016).
We measured films consisting of 8-nm-thick [(GeTe)2/(Sb2Te3)]4 and 7-nm-thick GeTe prepared by sputtering on Sb2Te3(3 nm)/Si substrates at 240oC. In our setup [Fig.1(a)] the metal SPM probe was placed in a pre-defined location, and the structural phase transition was activated by voltage pulses (Vwrite) with an amplitude of 2 - 5 V and a duration of 10 ms - 3 s applied to the sample. The probe height position (Z) and the current were recorded as a function of time during the local excitation. To minimize film deformation by the SPM probe, a small gap maintained in the non-contact SPM mode by keeping a shift of the SPM sensor resonance frequency (Δf) in ultrahigh vacuum at room temperature.[2].
Fig. 1(b) shows a typical CSL response where the CSL expansion (Z-jump) coincided with the conductance state change at 96.1 s. The CSL expansion varied from -150 pm to +1000 pm for different pulse voltage amplitudes and positions along the granular film [Fig. 1(c)]. On average, the CSL expansion was +3.5% in accord with the solid-solid transition mechanism.[1,3] Interestingly, at Vwrite <+2.5 V the switching delay was >10 ms, suggesting a ferroelectric-like behavior where the structural phase transition and corresponding low-to-high conductance switching activated by strong electric field. Though, details of GeTe and Sb2Te3 layer stacking are responsible for observed variations. The work was supported by Japan Science and Technology Agency (JST/CREST).
[1] J. Tominaga et al., Adv. Mater. Interfaces 1 (2014) 1300027; [2] L.Bolotov et al., JJAP 50, 04DA04 (2011); [3] L.Bolotov et al., JJAP 55, 04EK02 (2016).