A simulation study has been performed to analyze resistive switching (RS) phenomena in valence change memories (VCM) based on a HfO ₂ dielectric. The kernel of the simulation tool consists of a 3D kinetic Monte Carlo (kMC) algorithm implemented self-consistently with the 3D Poisson and heat equations. These VCM devices show filamentary conduction, their RS operation is based on the destruction and regeneration of an ohmic conductive filament (CF) composed of oxygen vacancies. The physics underlying the RS operation is described by means of processes linked to generation of oxygen vacancies, oxygen ion migration and recombination between vacancies and oxygen ions that can be accurately described by using the electric field and temperature distributions in the dielectric. The studied devices consist of TiN/Ti/HfO ₂ /W stacks where the Ti capping layer plays the role of oxygen ion getter material. The simulation tool is useful for obtaining information of internal physical variables, explaining RS dynamics and the CFs evolution from the microscopic viewpoint in terms of their size and shape under different electrical input signals; particularly, the pulsed operation regime has been studied in depth. Furthermore, interesting phenomena, such as partial SETs within overall RESET processes can be accurately reproduced.

In this letter, we focus on the cycle-to-cycle variability of the low resistive state in Ni/HfO ₂ -based resistive switching structures. The results show that several discrete current levels can individually last hundreds of cycles. They are a result of the random nature of the reversible conductive path formation through percolation processes, which could be attributed to a different shape, size, or number of conductive filaments. After successive cycles, the same or new filaments will nucleate in the weaker zones of the dielectric. In addition, the switching voltages related to the creation and dissolution of localized conductive paths are found to be statistically associated.

An in-depth study of reset processes in RRAMs (Resistive Random Access Memories) based on Ni/HfO₂/Si-n⁺ structures has been performed. To do so, we have developed a physically based simulator where both ohmic and tunneling based conduction regimes are considered along with the thermal description of the devices. The devices under study have been successfully fabricated and measured. The experimental data are correctly reproduced with the simulator for devices with a single conductive filament as well as for devices including several conductive filaments. The contribution of each conduction regime has been explained as well as the operation regimes where these ohmic and tunneling conduction processes dominate.

A new RRAM simulation tool based on a 3D kinetic Monte Carlo algorithm has been implemented. The redox reactions and migration of cations are developed taking into consideration the temperature and electric potential 3D distributions within the device dielectric at each simulation time step. The filamentary conduction has been described by obtaining the percolation paths formed by metallic atoms. Ni/HfO ₂ /Si-n ⁺ unipolar devices have been fabricated and measured. The different experimental characteristics of the devices under study have been reproduced with accuracy by means of simulations. The main physical variables can be extracted at any simulation time to clarify the physics behind resistive switching; in particular, the final conductive filament shape can be studied in detail.

Nanobeam electron diffraction (NBD) is a transmission electron microscopy (TEM) technique that allows determining strain in crystalline materials with a high spatial resolution. In particular, it can be used to measure local strain in the Si channel of metal oxide semiconductor (MOS) devices. In this work, the NBD technique is evaluated with the purpose to optimize its different parameters, such as the thickness of the TEM specimen and the width of the energy filtering window, and to further estimate its precision. Strain is then measured in positive-channel MOS (PMOS) and negative-channel MOS (NMOS) devices with different gate length and Ge or C concentration in the source/drain respectively. The results are compared with optical and electrical characterization, finite-element simulations and to other strain measuring TEM-based techniques.