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.

Rapid and reversible switching properties of Ag, In doped Sb 2 Te (AIST) phase change material is widely used in re-writable optical data storage applications. We report here a systematic evolution of optical band gap (E g ), local disorder (Tauc parameter, β), and Urbach energy (E U ) of AIST material during amorphous to crystalline transition using in situ UV-Vis-NIR spectroscopy. Unlike GeTe-Sb 2 Te 3 (GST) family, AIST material is found to show unique characteristics as evidenced by the presence of direct forbidden transitions. Crystallization is accompanied by a systematic reduction in E g from 0.50 eV (as-deposited amorphous at 300 K) to 0.18 eV (crystalline at 300 K). Moreover, decrease in E U (from 272 to 212 meV) and β is also observed during increasing the temperature in the amorphous phase, revealing direct observation of enhancement of the medium-range order and distortion in short range order, respectively. These findings of optical transition would be helpful for distinguishing the unique behavior of AIST material from GST family.

Chalcogenide-based Ge15Te85 thin films have recently been explored for ovonic threshold switching (OTS) selector devices for vertically stackable cross-point memory applications. Despite reasonable understanding over its crystallization kinetics and threshold switching properties, the structural stability and morphological acquaintance at elevated temperatures remain key challenges. In this paper, we investigate the thermal stability, surface morphology and local structure of as-deposited amorphous Ge15Te85 thin film starting from room temperature up to 325 °C. Our experimental results reveal that upon heating, the de-vitrification is initiated in the form of localized segregation of Te atoms at 120 °C, followed by crystallization of Te at ~220 °C and GeTe at ~263 °C as corroborated by temperature-dependent measurements of electrical resistance, x-ray diffraction and scanning electron microscopic studies. Furthermore, the crystalline areas of these films are characterized by the fine-grained morphology, which clearly distinguishes the segregation of crystallization of Te and GeTe microstructures. These findings elucidate a deeper understanding of the multi-phase crystallization process through morphological evidence, which will be useful towards optimization of materials for OTS selector applications.

Recent advancements in commercialization of high-speed non-volatile electronic memories including phase change memory (PCM) have shown potential not only for advanced data storage but also for novel computing concepts. However, an in-depth understanding on ultrafast electrical switching dynamics is a key challenge for defining the ultimate speed of nanoscale memory devices that demands for an unconventional electrical setup, specifically capable of handling extremely fast electrical pulses. In the present work, an ultrafast programmable electrical tester (PET) setup has been developed exceptionally for unravelling time-resolved electrical switching dynamics and programming characteristics of nanoscale memory devices at the picosecond (ps) time scale. This setup consists of novel high-frequency contact-boards carefully designed to capture extremely fast switching transient characteristics within 200 ± 25 ps using time-resolved current-voltage measurements. All the instruments in the system are synchronized using LabVIEW, which helps to achieve various programming characteristics such as voltage-dependent transient parameters, read/write operations, and endurance test of memory devices systematically using short voltage pulses having pulse parameters varied from 1 ns rise/fall time and 1.5 ns pulse width (full width half maximum). Furthermore, the setup has successfully demonstrated strikingly one order faster switching characteristics of AgInSbTe (AIST) PCM devices within 250 ps. Hence, this novel electrical setup would be immensely helpful for realizing the ultimate speed limits of various high-speed memory technologies for future computing.

Threshold switching is a unique characteristic feature in amorphous chalcogenide materials that establishes stable and fast switching between a high resistance OFF state and a conductive ON state in the amorphous phase, envisaging the electronic nature of two-terminal ovonic threshold switch (OTS) selectors in vertically stackable cross-point memory arrays. In this paper, we demonstrate voltage-dependent nanosecond threshold switching dynamics and stable OFF–ON transitions of co-sputtered thin Ge15Te85 film devices using ultrafast time-resolved current–voltage measurements. The time-resolved measurement of device current upon the application of voltage pulse reveals a stable threshold switching and OFF–ON transient characteristics of OTS devices and the measured delay time is found to decrease to few nanoseconds upon increasing the amplitude of the applied voltage pulse and such OTS characteristics are found to be stable even above 60% of the high value of threshold voltage. These experimental results found to be consistent with analytical solutions and also demonstrate a systematic trend in the voltage dependent switching properties enabling ultrafast threshold switching characteristics suitable towards designing reliable and stable OTS selector devices.