Resistive switching in mixed conductors : Ag2S as a model system

Resistive switching memories have gained an increased interest due to the possibilities for downscaling of memory devices down to a few nanometers. These memories consist of a resistive material sandwiched between two metal electrodes, and applying a voltage between them induces resistance switching. In this thesis we study the specific case when switching is due to the reversible formation of a conductive path that connects and disconnects the electrodes. We investigate the electrical conductance properties and transport mechanisms in solid electrolyte memory devices, to gain a fundamental understanding of conductance switching. Our model system consists of Ag2S thin films with a Ag bottom electrode and a Pt AFM or STM tip as top electrode. We present a quantitative analysis of the steady state transport that leads up to resistance switching. We discuss the relation between stoichiometry and resistance changes in the material, and the necessity of Ag supersaturation prior to nucleation and switching. We also discuss the possible presence of two distinct switching mechanisms in Ag2S. Our findings could be extended to other semiconductor materials with mobile donors or acceptors.LEI Universiteit LeidenFOMQuantum Matter and Optic

Bulk and Surface Nucleation Processes in Ag2S Conductance Switches

We studied metallic Ag formation inside and on the surface of Ag2S thin films, induced by the electric field created with a STM tip. Two clear regimes were observed: cluster formation on the surface at low bias voltages, and full conductance switching at higher bias voltages (V > 70mV). The bias voltage at which this transition is observed is in agreement with the known threshold voltage for conductance switching at room temperature. We propose a model for the cluster formation at low bias voltage. Scaling of the measured data with the proposed model indicates that the process takes place near steady state, but depends on the STM tip geometry. The growth of the clusters is confirmed by tip retraction measurements and topography scans. This study provides improved understanding of the physical mechanisms that drive conductance switching in solid electrolyte memristive devices.Comment: In press for PR

Copper and Transparent-Conductor Reflectarray Elements on Thin-Film Solar Cell Panels

This work addresses the integration of reflectarray antennas (RA) on thin film Solar Cell (SC) panels, as a mean to save real estate, weight, or cost in platforms such as satellites or transportable autonomous antenna systems. Our goal is to design a good RA unit cell in terms of phase response and bandwidth, while simultaneously achieving high optical transparency and low microwave loss, to preserve good SC and RA energy efficiencies, respectively. Since there is a trade-off between the optical transparency and microwave surface conductivity of a conductor, here both standard copper and transparent conductors are considered. The results obtained at the unit cell level demonstrates the feasibility of integrating RA on a thin-film SC, preserving for the first time good performance in terms of both SC and RA efficiency. For instance, measurement at X-band demonstrate families of cells providing a phase range larger than 270{\deg} with average microwave loss of -2.45dB (resp. -0.25dB) and average optical transparency in the visible spectrum of 90% (resp. 85%) using transparent conductive multilayer (resp. a copper layer)

Impact of TCO Microstructure on the Electronic Properties of Carbazole-based Self-Assembled Monolayers

Carbazole-based self-assembled monolayers (PACz-SAMs), anchored via their phosphonic acid group on a transparent conductive oxide (TCO) have demonstrated excellent performance as hole-selective layers in inverted perovskite solar cells. However, the influence of the TCO microstructure on the work function (WF) shift after SAM anchoring as well as the WF variations at the micro/nanoscale have not been extensively studied yet. Herein, we investigate the effect of the Sn-doped In2O3 (ITO) microstructure on the WF distribution upon 2PACz-SAMs and NiOx/2PACz-SAMs application. For this, ITO substrates with amorphous and polycrystalline (featuring either nanoscale or microscale-sized grains) microstructures are studied. A correlation between the ITO grain orientation and 2PACz-SAMs local potential distribution was found via Kelvin probe force microscopy and electron backscatter diffraction. These variations vanish for amorphous ITO or when adding an amorphous NiOx buffer layer, where a homogeneous surface potential distribution is mapped. Ultraviolet photoelectron spectroscopy confirmed the ITO WF increase after 2PACz-SAMs deposition. Considering the importance of polycrystalline TCOs as high mobility and broadband transparent electrodes, we provide insights to ensure uniform WF distribution upon application of hole transport SAMs, which is critical towards enhanced device performance.Comment: 18 pages, 5 figure

Observing “Quantized” Conductance Steps in Silver Sulfide: Two Paralell Resistive Switching Mechanisms

014302Quantum Matter and Optic

Towards a quantitative description of solid electrolyte conductance switches

Quantum Matter and Optic

Impact of the TCO Microstructure on the Electronic Properties of Carbazole-Based Self-Assembled Monolayers

Carbazole-based self-assembled monolayers (PACz-SAMs), anchored via their phosphonic acid group on a transparent conductive oxide (TCO), have demonstrated excellent performance as hole-selective layers in perovskite/silicon tandem solar cells. Yet, whereas different PACz-SAMs have been explored, the role of the TCO, and specifically its microstructure, on the hole transport properties of the TCO/PACz-SAMs stack has been largely overlooked. Here, we demonstrate that the TCO microstructure directly impacts the work function (WF) shift after SAM anchoring and is responsible for WF variations at the micro/nanoscale. Specifically, we studied Sn-doped In2O3 (ITO) substrates with amorphous and polycrystalline (featuring either nanoscale- or microscale-sized grains) microstructures before and after 2PACz-SAMs and NiOx/2PACz-SAMs anchoring. With this, we established a direct correlation between the ITO crystal grain orientation and 2PACz-SAMs local potential distribution, i.e., the WF. Importantly, these variations vanish for amorphous oxides (either in the form of amorphous ITO or when adding an amorphous NiOx buffer layer), where a homogeneous surface potential distribution is found. These findings highlight the importance of TCO microstructure tuning, to enable both high mobility and broadband transparent electrodes while ensuring uniform WF distribution upon application of hole transport SAMs, both critical for enhanced device performance.</p

Cs-Doped and Cs-S Co-Doped CuI p-Type Transparent Semiconductors with Enhanced Conductivity

One hindrance in transparent electronics is the lack of high-performance p-type transparent conductors (TCs). The state-of-the-art p-type TC, CuI, has a conductivity two orders of magnitude lower than n-type TCs like ITO. While doping strategies have shown promise in enhancing the hole carrier density in CuI, they often come at the expense of hole mobility. Therefore, understanding how extrinsic dopants affect the mobility of CuI is critical to further improve the performance of CuI-based TCs. Here the structural and electronic properties of Cs-doped CuI are investigated. It is demonstrated that ≈4 at.% Cs doping in CuI increases the carrier density from 2.1 × 1019 to 3.8 × 1020 cm−3 while preserving the film microstructure and local coordination of Cu, as confirmed by HRTEM and XAS analysis. Introducing S as a co-dopant in Cs:CuI boosts the carrier density to 8.2 × 1020 cm−3, reaching a stable conductivity of ≈450 S cm−1. In all cases, the enhanced carrier density negatively affects the hole mobility with ionized impurity scattering and increased Seebeck hole effective mass as mobility limiting mechanisms. Nonetheless, the new Cs, S co-doped CuI exhibits high p-type conductivity, Vis–NIR transparency, and stability, presenting an attractive candidate for future transparent electronic devices.</p