Anbarasu Manivannan

The process-induced variability in nanoscale phase-change memory (PCM) devices is of utmost importance for the development of reliable single-bit/multi-bit data storage devices. In this study, the influence of structural and interfacial parameters on the performance of Ge2Sb2Te5 (GST) PCM device is systematically investigated using Plackett-Burman design of experiment method. Five important structural parameters, (i) heater (TiN) radius (HR), (ii) heater height, (iii) GST radius (W GST), (iv) GST thickness, and (v) top electrode thickness, and along with three interfacial parameters namely, (i) thermal boundary resistance (TBR) between GST and TiN interface (ii) TBR between GST and SiO2 interface, and (iii) electrical interface resistance (EIR) between GST and TiN interface are considered for the study. Furthermore, to understand the impact of scaling, the performance metrics i.e. RESET resistance (RRESET ), SET resistance (RSET ), RESET power (PRESET ) and SET power (PSET ) of an isotropically scaled-down device with a HR of 10 nm are extracted and compared against the reference device of 50 nm HR. The TCAD simulation results reveal that HR and W GST are the most dominant structural parameters for the output metrics and the analysis shows that ratio should be maintained between 2.7 and 4.5 to offer reliable RESET operation. Among the interfacial parameters, GST/TiN EIR is the most significant controlling parameter for PRESET /PSET metrics, whereas GST/TiN TBR plays an important role in achieving better RRESET / RSET . Hence, our findings of the most and least sensitive input parameters can be effectively used for the better optimization of RESET/SET pulse parameters to achieve reliable programming of PCM devices in the future technology nodes.

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.

Interfacial strain plays a vital role in determining the coupling strength between the magnetic and electrically ordered phases in magnetoelectric (ME) nanostructures. The interfacial strain and its gradient size in a polycrystalline trilayer ME composite with a specific microstructure were estimated by grazing incident X-ray diffraction (GI-XRD). The average interfacial strain was estimated to have a maximum value of ∼7%, and was found to be relaxed at a length scale of 25-35 nm away from the interface. The optimized gradient size estimated from the trilayer ME composite was utilized to fabricate multilayers with specific periodicities ("Δ") and tested for the inverse piezomagnetic effect to estimate the optimum periodicity required to have enhanced ME coupling. Multilayers with periodicity (∼40 nm) compared to multilayers with relaxed/partial interfacial strain exhibited ∼25 to 26% increment in piezoelectric coefficient (d33) in the presence of a magnetic field. The constraint imposed on polarization by interfacial strain reflects on the enhancement of stiffness and introduces a quicker linear response to the piezoelectric displacement. In contrast, the partially strained and/or strain-relaxed layers exhibited nonlinear responses in polarization switching. The linear piezoelectric displacement in these strain-engineered ME composites makes them a potential candidate for device applications like actuators and transducers.

Multilevel storage in chalcogenide-based phase-change materials is one of the desired characteristics to design neuromorphic and in-memory computing applications. However, precisely controlling the crystalline and amorphous fraction to achieve reliable multilevel states is one of the key challenges in multilevel switching. Herein, multilevel switching is focused on the aspect of optical domain, where it enjoys the benefits of higher bandwidth with low delay connectivity suitable for non-von Neumann architecture. The essential requirements for multilevel optical switching are discussed in terms of programming techniques, novel device structures, and emerging materials for its better optimization. Furthermore, the impact of nature of crystallization mechanism on the multilevel switching for different families of phase-change materials is reviewed. In addition, the multilevel switching for neuromorphic engineering and in-memory computation based on the integrated photonic memory devices are assessed. Finally, several challenges and different strategies to improve the performance of multilevel switching in phase-change materials are discussed and thereby signify its importance for the design of future on-chip phase-change photonic memory devices.

Conventional Ge-Sb-Te based phase change memory (PCM) devices offer promising attributes, including fast non-volatile memory operations, higher endurance and sub-ns re-crystallization/re-amorphization capabilities for the next generation high speed non-volatile memory. The threshold switching process in the amorphous phase of the PCM device enables breakdown of electrical resistance followed by rapid flow of device current, leading to Joules heating induced crystallization, known as SET operation. The speed of SET operation is, therefore, governed by voltage dependent threshold switching dynamics and the delay time (t d) characteristics at the timescale of nanoseconds. Although GeSb2Te4 offers faster crystal growth velocity, realization of fast threshold switching capabilities remains unexplored. In this work, the ultrafast threshold switching dynamics of a prototypical GeSb2Te4 PCM device are investigated. The present experimental results demonstrate an exponential reduction of t d upon increasing the applied voltage (V A) above its threshold voltage (V T). The t d has been reduced to a strikingly lower value of 0.5 ns for V A of 1.6 times of V T, which enables faster SET operation within 3 ns as evidenced by a permanent change in the device resistance. These experimental findings of sub-nanosecond threshold switching dynamics and faster SET operation in the GeSb2Te4 device pave a way for designing a PCM device with ns programming speed for future electronics.

The study focuses on the polarization dynamics of the ferroelectric phase under an external magnetic field in a trilayered magnetoelectric (ME) composite of 0.94(Na0.5Bi0.5TiO3)-0.06BaTiO3 (NBT-BT)/CoFe2O4(CFO)/NBT-BT. With the estimation of gradient size of the strain across the interface, the thin films with varying top layer (NBT-BT) thicknesses were fabricated. The piezoelectric displacement curves revealed the linear characteristics for the 30 nm NBT-BT ME composite due to the presence of dominant interfacial strain. Time-resolved polarization switching studies confirmed the role of interfacial strain on the time scale of polarization switching of the ferroelectric phase. Magnetic field-assisted piezoresponse force microscopy studies confirmed the presence of nonlinear contribution like polarization rotation in the 100 nm NBT-BT ME composite. The interfacial strain was found to operate in a way that imposes constraints on the polarization rotation in a spatial region of ∼20-30 nm away from the interface. However, at a spatial region >30 nm, the interfacial strain was found to supplement the field-induced strain and assisted the polarization rotation to happen. The spatial-dependent behavioral analysis of the interface strain on the polarization dynamics will help in using the ME composite for targeted device applications such as actuators or energy harvesters.

Highly reproducible and precisely controlled gradual variation in optical reflectivity or electrical resistance between amorphous and crystalline phases of phase change (PC) material is a key requirement for multilevel programming. Here we report high-contrast multilevel set and reset operations through accumulative switching in growth-dominated AgInSbTe PC material using a nanosecond laser-based pump-probe technique. The precise tuning of fractions of crystallized or re-amorphized region is achieved by means of controlling the number of irradiated laser pulses enabling six stable multilevels with high-reflectivity contrast of 2% between any two states. Furthermore, Raman spectra of irradiated spots validate the structural changes involved during multilevel switching between amorphous and crystalline phases.

Superhydrophobic and hydrophobic surfaces on Al and Cu have a wide range of applications in electronics, industrial devices, air conditioning, and refrigeration plants, and home appliances due to their excellent electrical and thermal characteristics. In many of these applications, the wettability of their surface is essential. This work presents the fabrication of superhydrophobic and hydrophobic surfaces on Al and hydrophobic surfaces on the Cu metal sheets via laser processing and aging without any additional chemical treatment. Texturing with laser spot overlap is used to generate naturally formed nanoscale textures by scanning the whole sample surface. The influence of laser parameters on the dimension and shape of the fabricated surface textures and their impact on wettability is analyzed along with the evolution of wetting behavior over time. The laser textured surfaces, which are initially hydrophilic, are found to transform hydrophobic over time upon exposure to atmospheric conditions. Experimental evidence using X-ray photoelectron spectroscopy (XPS) and ATR-FTIR spectroscopy corroborates this transition is owing to the adsorption of organic molecules present in ambient. The depth profiling using XPS reveals carbon contamination around 60–70 nm from the sample surface. Furthermore, textures exhibiting static contact angles of up to ~154° have been achieved with Al sheets, whereas contact angles up to ~122° have been attained in the Cu sample. These experimental findings enable control of wettability of Al and Cu sheets through precise tuning of laser parameters for desired properties.

Discovery of electrical switching in chalcogenide glasses by S.R. Ovshinsky paves a new path for developing high-speed nonvolatile electronic memory and high-performance computing solutions. This article presents a review on the systematic understanding of threshold switching (TS) properties in various chalcogenide materials, Ovonic threshold switching (OTS) and Ovonic memory switching (OMS), the nature of TS, voltage-dependent transient characteristics, and the role of TS in governing the programming speed based on research efforts over the last six decades. Furthermore, realization of TS in picosecond timescale, the commonalities between OTS and OMS, and the possible underlying mechanism has been explored. Furthermore, a scheme of material classification based on TS dynamics for ultrafast yet energy-efficient programming has been proposed for phase-change memory with SRAM-like programming speed for future electronics.

Phase change materials exhibit threshold switching (TS) that establishes electrical conduction through amorphous material followed by Joule heating leading to its crystallization (set). However, achieving picosecond TS is one of the key challenges for realizing non-volatile memory operations closer to the speed of computing. Here, we present a trajectory map for enabling picosecond TS on the basis of exhaustive experimental results of voltage-dependent transient characteristics of Ge2Sb2Te5 phase-change memory (PCM) devices. We demonstrate strikingly faster switching, revealing an extraordinarily low delay time of less than 50 ps for an over-voltage equal to twice the threshold voltage. Moreover, a constant device current during the delay time validates the electronic nature of TS. This trajectory map will be useful for designing PCM device with SRAM-like speed.