Although phase-change memory (PCM) offers promising features for a ‘universal memory’ owing to high-speed and non-volatility, achieving fast electrical switching remains a key challenge. In this work, a correlation between the rate of applied voltage and the dynamics of threshold-switching is investigated at picosecond-timescale. A distinct characteristic feature of enabling a rapid threshold-switching at a critical voltage known as the threshold voltage as validated by an instantaneous response of steep current rise from an amorphous off to on state is achieved within 250 picoseconds and this is followed by a slower current rise leading to crystallization. Also, we demonstrate that the extraordinary nature of threshold-switching dynamics in AgInSbTe cells is independent to the rate of applied voltage unlike other chalcogenide-based phase change materials exhibiting the voltage dependent transient switching characteristics. Furthermore, numerical solutions of time-dependent conduction process validate the experimental results, which reveal the electronic nature of threshold-switching. These findings of steep threshold-switching of ‘sub-50 ps delay time’, opens up a new way for achieving high-speed non-volatile memory for mainstream computing.

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Non-volatile phase-change photonic memory devices are of particular interest in recent years due to their highdensity storage with excellent scalability features. The optical chip employed in on-chip photonic memory devices has displayed outstanding performance operating at infrared wavelengths; however, realizing multilevel switching in the visible region is a key challenge owing to the limitations of a high absorption coefficient and the undesirable volume changes. In this study, 4-bit multilevel switching operation (16 levels) with a uniform reflectivity contrast (∼1.5% per level) in Ge 2 Sb 2 Te 5 and Ag 5 In 5 Sb 60 Te 30 (AIST) films operating at the visible wavelength (532 nm) is demonstrated by optimizing the pump beam (PB) diameter. The optimization of the PB diameter has illustrated its larger influence in determining the substantial reflectivity contrast, especially in growth-dominated AIST. Additionally, the role of PB diameter is corroborated through the calculation of crystal growth velocity in AIST. The simulation reveals the higher growth velocity of 110 m/s for smaller PB diameter (0.4 mm), whereas 84.16 and 7.94 m/s are obtained for diameters of 0.6 and 0.7 mm, respectively. Furthermore, the vibrational modes of individual optical levels have been systematically explored using Raman spectroscopy, and the underlying mechanism behind multilevel switching has been validated in technologically important nucleation and growth-dominated phase-change materials. The present experimental findings demonstrating the feasibility of 16 multilevel states in the visible region would be promising for designing future photonic memory devices.

Phase change materials including GeSbTe and AgInSbTe have successfully demonstrated multilevel switching capabilities, yet achieving precise controllability and reproducibility are crucial towards technological applications. In this study, we demonstrate nine distinct optical levels in Ge2Sb2Te5 (GST225) and Ag5In5Sb60Te30 (AIST) phase-change materials using pump-probe experiments under identical device conditions and the role of the crystallization mechanism is examined for realization of reliable multi-level programming. Nucleation-dominated GST225 material corroborates improved performance characteristics of low threshold fluence (6 mJ cm−2), least optical variation (±0.25%), and high reflectivity contrast (∼2.5%) between any two consecutive levels as compared to growth-dominated AIST material. Furthermore, the opto-thermal simulations depict a gradual change in the crystalline fraction in GST225 and an abrupt change in AIST, which further confirms the improved controllability in nucleation-dominated crystallization. Hence, these identical measurements along with the opto-thermal simulations elucidate that the role and nature of crystallization play a critical role in precise control of variation of reflectivity in multi-level states of GST225 and AIST, respectively. These findings will be useful towards the development of reliable multi-bit phase-change photonic memory devices.