Picosecond Electric-Field-Induced Threshold Switching in Phase-Change Materials

Many chalcogenide glasses undergo a breakdown in electronic resistance above a critical field strength. Known as threshold switching, this mechanism enables field-induced crystallization in emerging phase-change memory. Purely electronic as well as crystal nucleation assisted models have been employed to explain the electronic breakdown. Here, picosecond electric pulses are used to excite amorphous ${\mathrm{Ag}}_4{\mathrm{In}}_3{\mathrm{Sb}}_{67}{\mathrm{Te}}_{26}$. Field-dependent reversible changes in conductivity and pulse-driven crystallization are observed. The present results show that threshold switching can take place within the electric pulse on subpicosecond time scales—faster than crystals can nucleate. This supports purely electronic models of threshold switching and reveals potential applications as an ultrafast electronic switch.

Figure 1

THz pulses with peak field strength of up to $100\mathrm{kV}/\mathrm{cm}$ and picosecond duration (a) interact with an array of electrodes (b), whose gaps are filled with the PCM AIST. The electrodes enhance the electric field (c) over that observed in a bare film (capping layer not included).

Figure 2

Evidence for filamentary crystallization of AIST after repetitive THz exposure (ex situ): (a) Mapping of optical white light reflectivity reveals brighter marks within the gap, (b) microwave dissipation (in-phase) signal shows a local decrease in resistance within the slit, (c) and (d) x-ray diffraction intensity in specific $q$ ranges for gold (c) and AIST (d), respectively, reveals local lattice ordering.

Figure 3

(a) THz pulses are absorbed in the PCM and induce temperature jumps that modulate its optical transmission after carrier relaxation (blue curve with red refined exponential decay). This long-lived transmission change is due to Joule heating of the PCM. (b) At low THz fluences this temperature jump also scales linearly with the square of the electric field, dominated by a phonon-absorption mechanism. At higher peak fields, however, a 1 order of magnitude increase in slope (conductivity) marks the onset of TS. (c) The instantaneous field-induced modulation due to electroabsorption scales linearly with the square of the electric field (intensity) and can be subtracted from the data at highest fluence (thick curve), revealing the ultrafast onset of the nonlinear Joule heating mechanism (red curve).