First-Principles Calculations to Establish the Functionality of Self-Connected Point-Defect Migrations in n-ZnO- and p-CuO-Based Memristive Devices
M. Julie Therese, Vikash Mishra, M. S. Ramachandra Rao, Tejendra Dixit
Abstract
Memristors, which are utilized in the development of memory devices, exhibit remarkable scalability, enhanced switching speed, and reduced power consumption. Native point defects regulate resistive switching, and therefore, memristive devices’ atomic-level conduction processes demand investigation. The feasibility of memristive behavior is expected to be dependent on the growth environment of materials, which actually controls the stability of specific types of defects. Here, we have carried out the analysis of resistive switching mechanism in ZnO- (n-type) and CuO (p-type)-based native point defects under various growth conditions to elucidate the resistive switching mechanism. The activation energy of the defects and the self-connected point-defect migration paths for the formation of filaments have been investigated using density functional theory (DFT) calculations. In the case of ZnO, oxygen vacancy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> ) defects under O-poor conditions exhibit low formation energy, whereas our investigations also demonstrate that copper vacancy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {Cu}}$ </tex-math></inline-formula> ) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> defects in CuO are the most favorable under O-rich and O-poor conditions, respectively. In ZnO, threefold and fourfold <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> -sites contribute significantly in resistive switching, while only fourfold coordinated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> -sites are critical for CuO. It is evident that under O-poor conditions, ZnO and CuO have activation energies of 0.65 eV and 0.42 eV for +2q charged <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> , respectively. Finally, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> characteristics have been plotted for all cases where it is found that <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {O}}$ </tex-math></inline-formula> in O-poor conditions provides the highest resistive window in ZnO- and CuO-based memristive devices. The impact of defect concentration on the transition from analog-to-digital switching behavior is found to play a substantial role in memristive behavior.