Metal oxide resistive switching memories have been a crucial component for the requirements of the Internet of Things, which demands ultra-low power and high-density devices with new computing principles, exploiting low cost green products and technologies. Most of the reported resistive switching devices use conventional methods (physical and chemical vapor deposition), which are quite expensive due to their up-scale production. Solution-processing methods have been improved, being now a reliable technology that offers many advantages for resistive random-access memory (RRAM) such as high versatility, large area uniformity, transparency, low-cost and a simple fabrication of two-terminal structures. Solution-based metal oxide RRAM devices are emergent and promising non-volatile memories for future electronics. In this review, a brief history of non-volatile memories is highlighted as well as the present status of solution-based metal oxide resistive random-access memory (S-RRAM). Then, a focus on describing the solution synthesis parameters of S-RRAMs which induce a massive influence in the overall performance of these devices is discussed. Next, a precise analysis is performed on the metal oxide thin film and electrode interface and the recent advances on S-RRAM that will allow their large-area manufacturing. Finally, the figures of merit and the main challenges in S-RRAMs are discussed and future trends are proposed.

Recent Progress in Solution-Based Metal Oxide Resistive Switching Devices.

Neuromorphic computation based on resistive switching devices represents a relevant hardware alternative for artificial deep neural networks. For the highest accuracies on pattern recognition tasks, an analog, linear, and symmetric synaptic weight is essential. Moreover, the resistive switching devices should be integrated with the supporting electronics, such as thin-film transistors (TFTs), to solve crosstalk issues on the crossbar arrays. Here, an a-Indium-gallium-zinc-oxide (IGZO) memristor is proposed, with Mo and Ti/Mo as bottom and top contacts, with forming-free analog switching ability for an upcoming integration on crossbar arrays with a-IGZO TFTs for neuromorphic hardware systems. The development of a TFT compatible fabrication process is accomplished, which results in an a-IGZO memristor with a high stability and low cycle-to-cycle variability. The synaptic behavior through potentiation and depression tests using an identical spiking scheme is presented, and the modulation of the plasticity characteristics by applying non-identical spiking schemes is also demonstrated. The pattern recognition accuracy, using MNIST handwritten digits dataset, reveals a maximum of 91.82% accuracy, which is a promising result for crossbar implementation. The results displayed here reveal the potential of Mo/a-IGZO/Ti/Mo memristors for neuromorphic hardware. 

https://aip.scitation.org/doi/pdf/10.1063/5.0073056

Tailoring the synaptic properties of a-IGZO memristors for artificial deep neural networks

The present paper deals with the numerical optimization of CIGS based thin film solar cells using solar cell capacitance simulator (SCAPS). In the present work we have tried to study the variation of electrical parameters such as: Voc, Jsc, η and FF with the variation in material parameter particularly the absorber layer thickness and the operating temperature. Conversion efficiency of 23.58% was observed using 1.7 μm thickness and 1.35eV bandgap of CIGS layer under the AM1.5 G spectrum having a constant illumination of 1000W/m2 and 1 sun, also the simulation results inferred that the efficiency increases on increasing the CIGS layer thickness which is due to the large number of photon absorption.

Numerical modelling for enhancement of output performance of CIGS based thin film solar cell using SCAPS 1-D simulation software

Sunlight is arguably the most promising continuous and cheap alternative sustainable energy source available at almost all living places of the human world. Photovoltaics (PV) is a process of direct conversion of sunlight into electricity and has become a technology of choice for sustainable production of cleaner and safer energy. The solar cell is the main component of any PV technology and transparent conducting oxides (TCO) comprising wide band gap semiconductors are an essential component of every PV technology. In this research, transparent conducting thin films were prepared by solution combustion synthesis of metal oxide nitrates wherein the use of indium is substituted or reduced. Individual 0.5 M indium, gallium and zinc oxide source solutions were mixed in ratios of 1:9 and 9:1 to obtain precursor solutions. Indium-rich IZO (A1), zinc-rich IZO (B1), gallium-rich GZO (C1) and zinc-rich GZO (D1) thin films were prepared through spin coating deposition. In the case of A1 and B1 thin films, electrical resistivity obtained was 3.4 × 10−3 Ω-cm and 7.9 × 10−3 Ω-cm, respectively. While C1 films remained insulating, D1 films showed an electrical resistivity of 1.3 × 10−2 Ω-cm. The optical transmittance remained more than 80% in visible for all films. Films with necessary transparent conducting properties were applied in an all solution-processed solar cell device and then characterized. The efficiency of 1.66%, 2.17%, and 0.77% was obtained for A1, B1, and D1 TCOs, respectively, while 6.88% was obtained using commercial fluorine doped SnO2: (FTO) TCO. The results are encouraging for the preparation of indium-free TCOs towards solution-processed thin-film photovoltaic devices. It is also observed that better filtration of precursor solutions and improving surface roughness would further reduce sheet resistance and improve solar cell efficiency.

https://www.mdpi.com/2071-1050/12/24/10423/pdf

Solution Combustion Synthesis of Transparent Conducting Thin Films for Sustainable Photovoltaic Applications

This article presents the results of experimental studies of the impact of electrode material and the effect of nanoscale film thickness on the resistive switching in forming-free nanocrystalline ZnO films grown by pulsed laser deposition. It was demonstrated that the nanocrystalline ZnO film with TiN, Pt, ZnO:In, and ZnO:Pd bottom electrodes exhibits a nonlinear bipolar effect of forming-free resistive switching. The sample with Pt showed the highest resistance values RHRS and RLRS and the highest value of Uset = 2.7 ± 0.4 V. The samples with the ZnO:In and ZnO:Pd bottom electrode showed the lowest Uset and Ures values. An increase in the number of laser pulses from 1000 to 5000 was shown to lead to an increase in the thickness of the nanocrystalline ZnO film from 7.2 ± 2.5 nm to 53.6 ± 18.3 nm. The dependence of electrophysical parameters (electron concentration, electron mobility, and resistivity) on the thickness of the forming-free nanocrystalline ZnO film for the TiN/ZnO/W structure was investigated. The endurance test and homogeneity test for TiN/ZnO/W structures were performed. The structure Al2O3/TiN/ZnO/W with a nanocrystalline ZnO thickness 41.2 ± 9.7 nm was shown to be preferable for the manufacture of ReRAM and memristive neuromorphic systems due to the highest value of RHRS/RLRS = 2307.8 ± 166.4 and low values of Uset = 1.9 ± 0.2 V and Ures = −1.3 ± 0.5 V. It was demonstrated that the use of the TiN top electrode in the Al2O3/TiN/ZnO memristor structure allowed for the reduction in Uset and Ures and the increase in the RHRS/RLRS ratio. The results obtained can be used in the manufacturing of resistive-switching nanoscale devices for neuromorphic computing based on the forming-free nanocrystalline ZnO oxide films.

/pdf/nanoscale-resistive-switching-in-forming-free-zinc-oxide-1flw7bpt.pdf

Nanoscale-Resistive Switching in Forming-Free Zinc Oxide Memristive Structures

In this study, molybdenum tungsten/amorphous InGaZnO (a-IGZO)/TiO2/n-type Si-based resistive random access memory (ReRAM) is manufactured. After deposition of the a-IGZO, annealing was performed at 200, 300, 400, and 500 °C for approximately 1 h in order to analyze the effect of temperature change on the ReRAM after post annealing in a furnace. As a result of measuring the current-voltage curve, the a-IGZO/TiO2-based ReRAM annealed at 400 °C reached compliance current in a low-resistance state, and showed the most complete hysteresis curve. In the a-IGZO layer annealed at 400 °C, the O1/Ototal value increased most significantly, to approximately 78.2%, and the O3/Ototal value decreased the most, to approximately 2.6%. As a result, the a-IGZO/TiO2-based ReRAM annealed at 400 °C reduced conductivity and prevented an increase in leakage current caused by oxygen vacancies with sufficient recovery of the metal-oxygen bond. Scanning electron microscopy analysis revealed that the a-IGZO surface showed hillocks at a high post annealing temperature of 500 °C, which greatly increased the surface roughness and caused the surface area performance to deteriorate. Finally, as a result of measuring the capacitance-voltage curve in the a-IGZO/TiO2-based ReRAM in the range of -2 V to 4 V, the accumulation capacitance value of the ReRAM annealed at 400 °C increased most in a nonvolatile behavior.

/pdf/filamentary-resistive-switching-and-capacitance-voltage-3tbfjpj7bi.pdf

Filamentary Resistive Switching and Capacitance-Voltage Characteristics of the a-IGZO/TiO 2 Memory

Most of the existing copper indium gallium diselenide (CIGS) thin film solar cells are based on a cadmium sulfide (CdS) buffer layer fabricated using a chemical bath deposition (CBD) process. However, due to environmental pollution caused by material toxicity and the unique wet process's incompatibility with the vacuum process, many studies are now being actively carried out on nontoxic buffer layers. In this study, to replace CdS buffer layers, zinc sulfide (ZnS) buffer layers with a big band gap and a low optical loss at a short wavelength were fabricated using a magnetron sputtering system. For comparative analysis, this study also fabricated CdS buffer layers using the CBD process. Then, the conversion efficiency of CIGS thin film solar cells deposited with ZnS and CdS thin film as buffer layers was measured. The light conversion efficiency of ZnS buffer layer-based CIGS was measured at 14.44%, while that of the CdS buffer layer-based CIGS was measured at 15.71%. Given that both are higher than the minimum conversion efficiency required for commercialization (10%), ZnS buffer layer-based solar cells could have a competitive edge over the existing CdS buffer layer-based solar cells.

High-Efficiency Cu(In,Ga)Se₂ Thin Film Solar Cells Using ZnS and CdS Buffer Layers.

Enhanced Electrical Performance of Structurally Engineered Memristor Devices with Multi‐Stacked Indium Zinc Oxide Films

In this study, the effects of femtosecond laser postannealingwere experimentally confirmed by applying it to amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistor (TFT) with enhanced electrical characteristics. We confirmed that a-IGZO TFTs fabricated by applying femtosecond laser annealing for 50 and 100 s improved more than as-deposited a-IGZO TFTs before annealing. However, a-IGZOTFTs irradiated formore than 150 s degraded in overall performance compared with the as-deposited a-IGZO TFTs. The result of the X-ray photoelectron spectroscopy measurement of the O1s peak on the a-IGZO thin-film indicated that the ${O}_{{1}}/{O}_{\text {total}}$ value dropped to approximately 44.3%, and the ${O}_{{3}}/{O}_{\text {total}}$ value increased to approximately 17.6% when the a-IGZO thin film was irradiated for 150 s. As a result, oxygen vacancies remained without sufficient recovery of the metal–oxygen bond, and as the electron concentration increased, the conductivity increased, and the performance of the TFT decreased due to an increase in leakage current. A resistive load inverter was fabricated by connectinga TFT irradiatedwith the femtosecond laser at the best-performing time of approximately 100 s. The result of measuring the reversal characteristics confirmed a large inversion at the rising and falling edges.

Effect of Femtosecond Laser Postannealing on a-IGZO Thin-Film Transistors

We fabricated zinc sulfide (ZnS) buffer layers with a great band gap and small light loss at a short wavelength, and then applied them to copper indium gallium sulphur-selenide (CIGS) thin film solar cells. A CIGS evaporation system was used for fabrication of the CIGS thin films, and a thickness monitor was used to check the evaporation rate at each source. The evaporation rate and deposition time were adjusted to change the composition ratio of the thin films. Also, CIGS thin films were deposited by changing the temperature of the substrates from room temperature (RT) to 150 °C, 250 °C, and 350 °C during ZnS deposition, and among them, the optimal substrate temperature was selected to measure the light conversion efficiency of ZnS-deposited CIGS thin film solar cells. The grown ZnS thin films were analyzed for crystallinity and composition by using X-ray diffraction, and by using a scanning electron microscope, the cross section and surface shape of the thin films were examined. When we applied the ZnS thin film that was fabricated at a temperature of 150 °C with a thickness of 50 nm as a buffer layer for the CIGS solar cells, we obtained a light conversion efficiency of 14.48% without an antireflection layer.

Han-Sang Kim

Papers

Filamentary Resistive Switching and Capacitance-Voltage Characteristics of the a-IGZO/TiO 2 Memory

High-Efficiency Cu(In,Ga)Se₂ Thin Film Solar Cells Using ZnS and CdS Buffer Layers.

Enhanced Electrical Performance of Structurally Engineered Memristor Devices with Multi‐Stacked Indium Zinc Oxide Films

Effect of Femtosecond Laser Postannealing on a-IGZO Thin-Film Transistors

Improvement of the Electrical Properties of a Cu(In,Ga)Se₂ Solar Cell Based on a ZnS Buffer Layer from Radio Frequency Magnetron Sputtering.