Carsten Kügeler

Probing Cu doped Ge0.3Se0.7 based resistance switching memory devices with random telegraph noise

The ultimate sensitivity of any solid state device is limited by fluctuations. Fluctuations are manifestations of the thermal motion of matter and the discreteness of its structure which are also inherent ingredients during the resistive switching process of resistance random access memory (RRAM) devices. In quest for the role of fluctuations in different memory states and to develop resistive switching based nonvolatile memory devices, here we present our study on random telegraph noise (RTN) resistance fluctuations in Cu doped Ge0.3Se0.7 based RRAM cells. The influence of temperature and electric field on the RTN fluctuations is studied on different resistance states of the memory cells to reveal the dynamics of the underlying fluctuators. Our analysis indicates that the observed fluctuations could arise from thermally activated transpositions of Cu ions inside ionic or redox “double-site traps” triggering fluctuations in the current transport through a filamentary conducting path. Giant RTN fluctuation...

Crossbar Logic Using Bipolar and Complementary Resistive Switches

Memristive switches are promising devices for future nonvolatile nanocrossbar memory devices. In particular, complementary resistive switches (CRSs) are the key enabler for passive crossbar array implementation solving the sneak path obstacle. To provide logic along with memory functionality, “material implication” (IMP) was suggested as the basic logic operation for bipolar resistive switches. Here, we show that every bipolar resistive switch as well as CRSs can be considered as an elementary IMP logic unit and can systematically be understood in terms of finite-state machines, i.e., either a Moore or a Mealy machine. We prove our assumptions by measurements, which make the IMP capability evident. Local fusion of logic and memory functions in crossbar arrays becomes feasible for CRS arrays, particularly for the suggested stacked topology, which offers even more common Boolean logic operations such as and and nor .

On the stochastic nature of resistive switching in Cu doped Ge0.3Se0.7 based memory devices

Currently, there is great interest in using solid electrolytes to develop resistive switching based nonvolatile memories (RRAM) and logic devices. Despite recent progress, our understanding of the microscopic origin of the switching process and its stochastic behavior is still limited. In order to understand this behavior, we present a statistical “breakdown” analysis performed on Cu doped Ge0.3Se0.7 based memory devices under elevated temperature and constant voltage stress conditions. Following the approach of electrochemical phase formation, the precursor of the “ON resistance switching” is considered to be nucleation — the emergence of small clusters of atoms carrying the basic properties of the new phase which forms the conducting filament. Within the framework of nucleation theory, the observed fluctuations in the time required for “ON resistance switching” are found to be consistent with the stochastic nature of critical nucleus formation.

A Fundamental Analysis of Nano-Crossbars with Non-Linear Switching Materials and its Impact on TiO2 as a Resistive Layer

While materials with a linear IV-characteristic yield in a practically unusable voltage swing when used in crossbar arrays, materials with a nonlinear IV-curve were expected to yield better results. With a fundamental analytical approach, we can show that the gain is theoretically limited to the square root of the number of word lines used in the crossbar. Furthermore the degree of nonlinearity must not exceed a certain value, otherwise the voltage swing decreases. TiO2 with its nonlinear IV-characteristic outperforms any possible material with a linear IV-characteristic but the voltage swing for large crossbars is still below 10%VDD and would demand for high costly sense amplifiers.

Nano-Crossbar Arrays for Nonvolatile Resistive RAM (RRAM) Applications

We present a fast and flexible method for the fabrication of nano-crossbar arrays with a feature size of 100 nm. TiO2 is integrated and electrically characterized as the nonvolatile resistively switching material. This structure serves as a key component for the investigation of novel high density nonvolatile resistive RAM cores.

A Simulation Model of Resistive Switching in Electrochemical Metallization Memory Cells (ECM)

The storage principal of the Electrochemical Metallization Memory Cell is based on change of cell resistance induced by electro-chemical driven growth and rupture of a cupric or silver filament in an insulating matrix. This kind of switching was found in several materials as AgGeSe, CuGeS, silicon oxide or tungsten oxide [1]. During write operation copper or silver is oxidized at the corresponding electrode and copper or silver ions are driven out of the copper or silver anode into the insulating matrix due to the applied field, whereas the insulating matrix serves as solid electrolyte. The silver or copper ions migrate towards the cathode. At the cathode electrochemical reduction occurs, and deposition of metallic copper or silver takes place. Fast diffusion paths in the solid electrolyte matrix or preferred nucleation sites (seeds) at the boundary lead to filamentary growth. This growing cupric or silver dendrite finally reaches the anode and switches the device to a low resistance state. Based on this switching mechanism a FEM simulation model was set up. To simplify the model space charges due to silver or copper migration are neglected. It is further assumed, that the conductivity in the solid electrolyte is only ionic. Hence, it is sufficient to solve the well-known Laplace equation to address the electric properties as well as ion migration. A “Level Set” method is used to track the boundary of the growing filament. The velocity of this boundary is proportional to the ionic current density calculated by Laplace equation. Based on this model simulations are applied to cell structures with multiple fast diffusion paths and seeds. Simulation results show that just one filament reaches the anode. In a second step, Butler-Vollmer boundary conditions are introduced. This nonlinearity leads to an exponential dependence between switching time and switching voltage. As switching voltage increases, switching time decreases. A simulation model capable of simulating ECM memory cells is presented. The model enables to simulate the behaviour of different cell geometries or different materials as solid electrolyte. Furthermore it gives deeper insight into the switching mechanism. This work was supported by the European project EMMA “Emerging Materials for Mass storage Architectures” (FP6-033751).

Liquid Injection Atomic Layer Deposition of TiO x Films Using Ti [ OCH ( CH3 ) 2 ] 4

TiO x films were prepared by liquid injection atomic layer deposition using titanium tetraisopropoxide (TTIP), Ti[OCH(CH 3 ) 2 ] 4, dissolved in ethylcyclohexane (ECH). We analyzed the residual water content and the reaction with the TTIP for several solvents, choosing ECH for dissolving the TTIP because of the lowest residual water level and no ligand exchange reaction with the TTIP. TiO x films were deposited at 240°C with a wide range of the TTIP solution injections per cycle. However, an ideal self-regulated growth was not achieved for the TiO x films due to a slow catalytic decomposition of the TTIP molecules followed by the exchange reaction with the underling layer. The contribution of the catalytic decompositions to the deposition rates was suppressed by increasing the injection frequency of the TTIP solution into the vaporizer. A rather independent deposition rate of the input of the TTIP solution was achieved by increasing the injection frequency to 4 Hz, while TiO x films deposited with a low injection frequency of 0.25 Hz showed almost linear film growth rate to the input of the TTIP solution. The deposited TiO x films were amorphous and clearly showed both unipolar and bipolar resistive switching behaviors, which are applicable to nonvolatile memory applications.

A multilayer RRAM nanoarchitecture with resistively switching Ag-doped spin-on glass

Next generation memory materials and novel memory architectures are in the focus of investigations because CMOS based systems are supposed to reach their physical limits during the next decade. To enhance the potential for very high integration density we fabricated multilayer crossbar memory architectures with integrated spin-on glass (methyl-silsesquioxane - MSQ). UV nanoimprint lithography, a widely proposed future technology, was used to structure down to 100 nm feature sizes. Electrical measurements on silver doped MSQ devices show the potential for the use of the material composition in the field of future memory applications.

Phenomenological considerations of resistively switching TiO 2 in nano crossbar arrays

Within this paper we present the fabrication of nano crosspoint junctions and arrays with electron beam direct writing (EBDW). Reactively sputtered TiO2 was incorporated as a resistively switching thin film and electrically characterized concerning its performance. These devices are suitable for novel non-volatile storage systems in form of resistive random access memories (RRAM). All used materials as well as the fabrication processes for the functional thin film are in good accordance with current and future CMOS technology and provide a way to achieve low cost, high density non-volatile memory. The experiments were performed with 100 ⋅ 100 nm2 small single junctions and arrays with 200 nm wide wires. The results of the former prove a non-volatility for more than 105 s and a switching speed better than 10 ns for the SET- and RESET operation (from high to low resistance state and in reverse direction). The latter prove the direct addressability of junctions within an array.

A Novel Dual-Layered Electrolytic Resistance Memory with Enhanced Retention

A novel dual-layered electrolytic resistance memory is proposed and its high retention ability at room and elevated temperature is successfully demonstrated. This dual-layered memory cell was fabricated as cross-point structure with active material, Cu containing Ge0.3Se0.7 based solid electrolyte and a thin insulator layer stack, sandwiched between an inert Pt bottom electrode and an oxidizable Cu top electrode. The reduction of the cell programming current up to few nA achieved with this dual- layered memory cell is promising for low power consumption applications.