Emerging nonvolatile memory technologies such as phase change memory, spin-transfer torque-magnetic memory, and resistive switching memory (ReRAM) have been investigated as next-generation technology to replace conventional flash memory. Among them, ReRAM has been extensively studied for nonvolatile memory applications owing to its excellent retention, endurance, and high on/off ratio. Furthermore, ReRAM has a simple two-terminal structure, fast switching speed, and low power consumption with excellent scalability. ReRAM cells can be integrated into a cross-point array to obtain an area-efficient structure. In addition to planar cross-point arrays, 3D stackable cross-point arrays have been recently considered to maximize the ReRAM density. Among various materials for use in ReRAM inorganic metal oxides have been investigated widely. Recently, active research has been done on the fabrication and characterization of ReRAM devices utilizing emerging materials. Especially, hybrid organic-inorganic perovskite materials have been used as the resistive switching layer in ReRAM devices. In this presentation, a strategy toward design of high-density memory devices utilizing metal-halide perovskite materials will be discussed in detail with an emphasis on practical applicability, scalability, and reliability.

As an emerging class of light-responsive semiconductors, hybrid organo-metal perovskites seamlessly marry the characteristics of organic and inorganic materials. Perovskites have been on the radar screen of material scientists for over a century, before the reign of silicon, but only a decade ago they started receiving a strong wave of renewed interests thanks to their extraordinary photovoltaic performance. The hybrid nature of organo-metal perovskites leads to a unique combination of highly tunable physical properties such as superior visible light absorption, decent charge transport, defect tolerance, solution processing, and even ferroelectric-like polarization. In this talk, I will share our view on the opportunities and challenges of hybrid perovskites and their heterostructures, with a focus on the potential electronic applications. I will discuss an effective strategy towards enhancing the device performance of hybrid perovskites via coupling with low-dimensional materials. We found that combining 3D hybrid perovskites with 1D carbon nanotubes or 2D two-dimensional metal dichalcogenides significantly enhances charge transport and device performance. I will also discuss perovskite-based opto-ion-electronics, with photo-memories as an example, which leverages on highly correlated electrons, ions, and photons in halide perovskites.

The development of internet of things and artificial intelligence induces the rapid growing of sensory nodes which generates a large portion of unstructured and redundant raw data. In the conventional design, the analogue sensory data are initially transformed into digital data with analogue-to-digital conversion, then stored in memory unit. The computational task is further performed by transferring the digital data between memory and local computation unit. The architecture of separated sensor, memory and data processing units results in the data-accessing latency and relatively high-power consumption. Alternatively, near-sensor computing with accelerator or processing unit reside besides sensor to execute computational task and in-sensor computing with individual sensors or multiple connected sensor to directly process information have been proposed to improve energy, area and time efficiency [1, 2].

However, transistor-based chips and single devices to implement near-/in-sensor computing make them bulky, energy-inefficient and complicated. The devices with relatively compact structure and simple operation mode are highly required. Memristor is a two-terminal electronic device featured by nanometer size, storage capacity and dynamic continuous variable resistance. In a typical bipolar resistive switching device, the switching of high resistance state (HRS) and low resistance state (LRS) is driven by the application of voltage above the set voltage. The rich dynamic properties of ion migration and electronic-ionic coupling ensures that memristor is the promising candidate for near-/in-sensor computing. In additon, the multi-field controlled memristor is expected to further scale down the chip size. In this presentation, we will discuss the benefit of functional memristor technologies in the application of near-/in-sensor computing [3-6].

References

[1] Kagawa, K. et al. IEEE J. Sel. Top. Quantum Electron. 2004, 10, 816

[2] Mennel, L. et al. Nature 2020, 579, 62

[3] S.-T. Han* et al. Matter 2021, 4, 1702

[4] S.-T. Han* et al. Nature Commun. 2021, DOI 10.1038/s41467-021-26314-8.

[5] S.-T. Han* et al. Adv. Funct. Mater. 2021, 31, 2100144

[6] S.-T. Han* et al. Adv. Mater. 2018, 30, 1802883

A memristor is a device that has different metastable states at a voltage V. It has a resistance that depends on the history of the system, and the states can be switched by applied voltage. A memristor is a device with very large hysteresis. Neurons have the same ingredients as memristors plus at least one negative resistance. The neuron can undergo a Hopf bifurcation that passes the dynamics from rest to a spiking state. We provide impedance spectroscopy criteria to identify these features. First, we show the impedance model of a halide perovskite memristor, that exhibits inductive behaviour due to inverted hysteresis. We introduce the concept of a chemical inductor and explain the general kinetic model that generates such behaviour in a variety of systems. Next, we show the full dynamical regimes of a FitzHugh-Nagumo model, that is a representative minimal model of a spinning neuron

Hybrid organic-inorganic perovskite (HOIP) memristors are being studied extensively in recent times as a potential alternative to conventional memory technologies as well as to mimic the function of biological synapse in neuromorphic circuits [1]. In this work, the conduction mechanism and performance of MAPbI₃-based memristors have been studied. These devices exhibited an excellent ON/OFF ratio of greater than 10³. However, it has been found that measurement procedure brings significant variations in their performance [2]. While the RESET voltage and current significantly increase with scan rate, the ON/OFF ratio increases significantly with compliance current. Also, the forming voltage and ON/OFF ratio decrease with device scaling. In the case of voltage pulse characterization, the ON/OFF ratio can be enhanced by increasing the pulse amplitude and width. Hence, the selection of proper pulse amplitude and width is of utmost importance in achieving high endurance with desired noise margin. Based on these results, a protocol has been identified for the characterization of HOIP memristors to achieve their best possible switching parameters. This protocol is quite general in nature and can be employed for any other memristor structure based on HOIPs as well as on other related materials. Furthermore, the experimental data fitting to a standard SPICE based analytical model indicates that the current conduction is dominated by a tunnelling mechanism in the OFF state and is ohmic in the ON state [3].

Field-effect transistors based on hybrid metal halide perovskite semiconductors provide a controlled means of studying the charge transport physics of these materials and are also of interest for a broad range of applications in electronics, optoelectronics or bioelectronics. In this talk we will provide a general overview of the current understanding of the main factors that govern and limit the charge transport properties of these materials and provide specific examples of recent approaches that aim to enhance FET performance and stability. We will discuss in particular approaches to understand the effects of ion migration on the transport of electronic charges and approaches to minimze effects of ion migration.  

Perovskites have been intensively investigated for their use in solar cells and light-emitting diodes. However, research on their applications in thin-film transistors (TFTs) has drawn less attention despite their high intrinsic charge carrier mobility. In this study, we report the universal approaches for high-performance and reliable p-channel lead-free phenethylammonium tin iodide TFTs. These include self-passivation for grain boundary by excess phenethylammonium iodide, grain crystallisation control by adduct, and iodide vacancy passivation through oxygen treatment. We found that the grain boundary passivation can increase TFT reproducibility and reliability, and the grain size enlargement can hike the TFT performance; thus, enabling the first perovskite-based complementary inverter demonstration with n-channel indium gallium zinc oxide (IGZO) TFTs. In addition, we applied the same transistors for photosensors to detect green light. Details of performance will be discussed in my presentation. The inverter exhibits a high gain over 30 with an excellent noise margin. This work aims to provide widely applicable and repeatable methods to make the gate more open for intensive efforts towards high-performance printed perovskite TFTs.

In this talk, I will cover our investigations on halide perovskite memristors for computing and hardware security. The first part of the talk will focus on designing diffusive and drift halide perovskite memristive barristors as nociceptive and synaptic emulators for neuromorphic computing [1]. Here, we will discuss the role of interfaces that play pivotal roles in determining the switching characteristics of perovskite memristors, demonstrations of nociceptors and synapses using halide perovskite memristors and their integration with robotics for in-sensory computing. In the second part of the talk, we will discuss the use case of perovskite memristors for Physical Unclonable Functions (PUFs)- a security primitive exploiting the switching physics of one-dimensional halide perovskites as excellent sources of entropy for secure key generation and device authentication [2].

Understanding the capacitive response in hybrid perovskite devices has been the focus of much research in recent years as it is connected with hysteresis and degradation of solar cells. Negative capacitance and loops have been observed during the analysis of Impedance Spectroscopy results. ^1,2-4 The origin of these features seems to be related to the dynamic interaction of migrating ions with external interfaces and play a role in memristor switching between high and low resistance states. Here we present the dynamic state transition in a 2D Ruddlesden-Popper perovskite-based memristor device, measured via impedance spectroscopy. The spectral evolution of the transition exhibits a significant transformation of the low frequency arc to a negative capacitance arc. The capacitance-frequency evolution of the device indicates that the appearance of the negative capacitance is intimately related to a slow kinetic phenomenon due to ionic migration and further interaction with the external contacts. We discuss the internal mechanism on the basis of the device configuration and the electrical response. The switching mechanisms of devices containing a perovskite/metal contact are due to the interface chemical transformation and devices containing an organic buffer layer follow a filamentary formation.

2D layered perovskites have recently emerged as energy materials due to their improved stability against atmospheric degradation and low-cost solution processing methods.^1,2 These materials have been investigated optically and computationally to determine their electron structure and carrier mobility,³ as well as in tandem with traditional 3D perovskites (MAPbI₃) for improved interface passivation increasing solar harvesting efficiencies.⁴ However, up until now, little focus has been on these materials for use as memory storage devices and within IoT.

Herein we report the fabrication of a 2D layered perovskite (PEA)₂PbI₄ in-situ device, where large single crystals are grown vertically between device contacts using a peripheral evaporation technique.⁵ This achieves an easy route for encapsulation and improved stability. We show through a series of voltage scans that the devices have two resistance states that are stably switched between over 1000 voltage cycles with the HRS being five times higher than the LRS. Furthermore, we show that these devices are photoresponsive with the ON/OFF ratio decreasing due to increasing light intensity from darkness to 1 Sun intensity, which is restored once returned into the dark. We have also shown that carrier transport varies with the measurement orientation of the device when measured either vertically or laterally. Lateral devices are fabricated via spin-coating in which allows control of the crystallite size via the addition of Tetrahydrothiophene 1-oxide (THTO) thus changing the number of grain boundaries. By performing electrical characterisation during light- and temperature-dependant measurements we observe a photoresistive response in these lateral devices, for the first time allowing the characterisation of ionic transport behaviour in this material.

Halide perovskites, a family of printable materials with coupled ionic-optoelectronic properties provide a multitude of mechanisms for controlling memristive behaviour. Perovskites are well known for solar cells and light emission, but its ionic activity is unexploited. Due to both ionic and electronic transport within them, a myriad of mechanisms can be exploited to form diffusive and drift perovskite memristors - reversible doping, interfacial reactions, conductive filament formation as well as carrier trapping-detrapping. These mechanisms can be modulated by choice of perovskites, interfacial layers and morphologies. Our group’s efforts in investigation of new materials, their characterisation and deployment in two terminal and three terminal architectures will be covered.

Ordered nanostructured crystals of thin perovskites films are of great interest to researchers because of the dimensional-dependence of their photoelectronic properties for developing the perovskites with novel properties [1]. In this presentation, both top-down and bottom-up approaches for fabricating nanostructured perovskite films are demonstrated. First, a variety of micro/nanopatterns of a perovskite film are fabricated by either micro/nano-imprinting or transfer-printing a thin spin-coated precursor film in soft-gel state with a topographically pre-patterned polymer mold, followed by thermal treatment for complete conversion of the precursor film to a perovskite one [2]. Second, we also demonstrate a simple and robust route, involving the controlled crystallization of the perovskites templated with a self-assembled block copolymer (BCP), for fabricating nanopatterned perovskite films with various shapes and nanodomain sizes [3]. The nanopatterned perovskites showed significantly enhanced photoluminescence with high resistance to both humidity and heat due to geometrically confining crystals in the BCP domains. We also demonstrate an artificially intelligent photonic synapse based on a floating-gated field effect transistor with area-density-tunable perovskite nano-cone arrays templated in a BCP [4]. Our device is capable of electric charge (de)trapping and photo-excited charge generation, and it exhibits versatile synaptic functions of the nervous system, including paired-pulse facilitation and long-term potentiation, with excellent reliability. The area-density variable perovskite floating gate developed by off-centered spin coating process allows for emulating the human retina with a position-dependent spatial distribution of cones.