Resistive random access memory (RRAM) devices are receiving increasing extensive attention due to their enhanced properties such as fast operation speed, simple device structure, low power consumption, good scalability potential and so on, and are currently considered to be one of the next-generation alternatives to traditional memory. In this review, an overview of RRAM devices is demonstrated in terms of thin film materials investigation on electrode and function layer, switching mechanisms and artificial intelligence applications. Compared with the well-developed application of inorganic thin film materials (oxides, solid electrolyte and two-dimensional (2D) materials) in RRAM devices, organic thin film materials (biological and polymer materials) application is considered to be the candidate with significant potential. The performance of RRAM devices is closely related to the investigation of switching mechanisms in this review, including thermal-chemical mechanism (TCM), valance change mechanism (VCM) and electrochemical metallization (ECM). Finally, the bionic synaptic application of RRAM devices is under intensive consideration, its main characteristics such as potentiation/depression response, short-/long-term plasticity (STP/LTP), transition from short-term memory to long-term memory (STM to LTM) and spike-time-dependent plasticity (STDP) reveal the great potential of RRAM devices in the field of neuromorphic application.

Ellipsometry was used to measure the amplitude ratio and phase difference of light undergoing a phase shift as it interacts with a thin film of organic-inorganic hybrid perovskite CH3NH3PbI3 (MAPI) deposited onto a (100) silicon wafer. The refractive index and extinction coefficient was extracted from a multi-oscillator model fit to the ellipsometry data, as a function of wavelength, from 300 to 1500 nm.

A vapour-phase reaction process has been used to deposit smooth and uniform CH3NH3PbI3 perovskite material to enable the measurement of its optical dispersion relations, n and k, by ellipsometry. Fitting was achieved with a combination of Tauc–Lorenz, critical point parabolic band (CPPB) and harmonic oscillators. We have used the dispersion relations in an all-optical model of new planar device architectures in order to establish design rules for future materials choices to maximize the short-circuit current (Jsc) performance. For 500nm of MAPI with no window layer, the maximum performance expected from the model is Jsc=21.63mAcm−2. The ability of thin layers (in the range 20–60nm) of a range of window layer materials (TiO2, WO3, ZnO, Nb2O5, CdS, and Cd0.4 Zn0.6S) to enhance the short-circuit current of the devices was investigated. The performance of the oxides showed interference behaviour, with the first maxima in their Jsc curves exceeding the value achievable without a window layer. However, after the first maximum, the performance generally fell off with increasing thickness. The only material to stay greater than the no-window condition for the entire investigated range is WO3. The highest performance (Jsc of 22.47mAcm−2) was obtained with 59nm of WO3, with that of TiO2, ZnO, and Nb2O5 being marginally lower. Parasitic absorption in CdS window layers caused the Jsc to decrease for all non-zero thicknesses – it gives no interference enhancement and its use cannot be recommended on optical grounds. Use of the wider gap alloy Cd0.4Zn0.6S gave higher currents than did CdS but its performance was not so high as for the oxides. Observations are made on the practicalities of fabricating the target structures in the fabrication of practical PV devices

A study into the optimal deposition temperature for ultra-thin La₂O₃/Ge and Y₂O₃/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides (La₂O₃ and Y₂O₃) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La₂O₃ has been found to be more reactive to Ge than Y₂O₃, forming LaGeO_x and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400 °C. In contrast, Y₂O₃/Ge deposited at 400 °C allows for an ultra-thin GeO₂ layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeO_x/Ge interface. The Y₂O₃/Ge gate stack deposited at lower temperature shows a sub-band gap absorption feature fitted to an Urbach tail of energy 1.1 eV. The latter correlates to a sub-stoichiometric germanium oxide layer at the interface. The optical band gap for the Y₂O₃/Ge stacks has been estimated to be 5.7 ± 0.1 eV from Tauc-Lorentz modelling of VUV-VASE experimental data. For the optimal deposition temperature (400 °C), the Y₂O₃/Ge stack exhibits a higher conduction band offset (>2.3 eV) than the La₂O₃/Ge (∼2 eV), has a larger band gap (by about 0.3 eV), a germanium sub-oxide free interface, and leakage current (∼10⁻⁷ A/cm² at 1 V) five orders of magnitude lower than the respective La₂O₃/Ge stack. Our study strongly points to the superiority of the Y₂O₃/Ge system for germanium interface engineering to achieve high performance Ge Complementary Metal Oxide Semiconductor technology.

Resistive random access memory (RRAM), which is considered as one of the most promising next-generation non-volatile memory (NVM) devices and a representative of memristor technologies, demonstrated great potential in acting as an artificial synapse in the industry of neuromorphic systems and artificial intelligence (AI), due its advantages such as fast operation speed, low power consumption, and high device density. Graphene and related materials (GRMs), especially graphene oxide (GO), acting as active materials for RRAM devices, are considered as a promising alternative to other materials including metal oxides and perovskite materials. Herein, an overview of GRM-based RRAM devices is provided, with discussion about the properties of GRMs, main operation mechanisms for resistive switching (RS) behavior, figure of merit (FoM) summary, and prospect extension of GRM-based RRAM devices. With excellent physical and chemical advantages like intrinsic Young’s modulus (1.0 TPa), good tensile strength (130 GPa), excellent carrier mobility (2.0 × 10⁵ cm²∙V⁻¹∙s⁻¹), and high thermal (5000 Wm⁻¹∙K⁻¹) and superior electrical conductivity (1.0 × 10⁶ S∙m⁻¹), GRMs can act as electrodes and resistive switching media in RRAM devices. In addition, the GRM-based interface between electrode and dielectric can have an effect on atomic diffusion limitation in dielectric and surface effect suppression. Immense amounts of concrete research indicate that GRMs might play a significant role in promoting the large-scale commercialization possibility of RRAM devices.