R. Soni

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...

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 double barrier memristive device.

We present a quantum mechanical memristive Nb/Al/Al2O3/NbxOy/Au device which consists of an ultra-thin memristive layer (NbxOy) sandwiched between an Al2O3 tunnel barrier and a Schottky-like contact. A highly uniform current distribution for the LRS (low resistance state) and HRS (high resistance state) for areas ranging between 70 μm2 and 2300 μm2 were obtained, which indicates a non-filamentary based resistive switching mechanism. In a detailed experimental and theoretical analysis we show evidence that resistive switching originates from oxygen diffusion and modifications of the local electronic interface states within the NbxOy layer, which influences the interface properties of the Au (Schottky) contact and of the Al2O3 tunneling barrier, respectively. The presented device might offer several benefits like an intrinsic current compliance, improved retention and no need for an electric forming procedure, which is especially attractive for possible applications in highly dense random access memories or neuromorphic mixed signal circuits.

Reliability analysis of the low resistance state stability of Ge0.3Se0.7 based solid electrolyte nonvolatile memory cells

We report on the low resistance state (LRS) stability analysis of Ge0.3Se0.7 based solid electrolyte nonvolatile memory cells under elevated temperature and bias current stress conditions. The activation energy was found to be about 1.02 eV, which is comparable to that of an electromigration-induced failure process. Experimental results also show that there is trade-off between the LRS stability and the thickness of Ge0.3Se0.7 layer.

Rate limiting step for the switching kinetics in Cu doped Ge0.3Se0.7 based memory devices with symmetrical and asymmetrical electrodes

We report on the comparison of the resistance switching properties and kinetic behavior of Cu doped Ge0.3Se0.7 solid electrolyte based dual layer memory devices integrated with asymmetrical (Pt and Cu) and symmetrical electrodes (only Cu). In spite of the fact that the observed resistance switching properties and its parameters are quite similar for both memory devices, the dependence of the SET-voltage on the voltage sweep rate suggests different microscopic rate limiting factors for the resistance switching behavior. Additionally, in order to alleviate the cross talk problem in passive crossbar arrays, a dual layer oxide stack (TiO2/Al2O3) is integrated with Ge0.3Se0.7 based dual layer memory devices to achieve a specific degree of non-linearity in the overall resistance of the low resistance state.

Voltage controlled biaxial strain in VO2 films grown on 0.72Pb(Mg1∕3Nb2∕3)-0.28PbTiO3 crystals and its effect on the transition temperature

Vanadium oxide thin films (VO2) were deposited on 0.72Pb(Mg1∕3Nb2∕3)-0.28PbTiO3 (PMN-PT) crystalline substrates using pulsed laser deposition method. Due to their huge piezoelectric coefficients in the order of 2500 pm/V, the PMN-PT substrates are used to impose additional amount of biaxial strain to the VO2 films by applying an external bias to the substrates. The influence of the biaxial strain on the transition temperature and on the conductive properties of the VO2 films is investigated in this work. Thus, a change in the biaxial strain of −0.8 × 10−3 applied in the (110) plane of the rutile cell of the VO2 lowered the metal-to-insulator transition temperature by 1.35 °C.

Low-temperature dynamics of ferroelectric domains in PbZr0.3Ti0.7O3 epitaxial thin films studied by piezoresponse force microscopy

Dynamics of domain boundaries is expected to change drastically at low absolute temperatures but direct experimental information for this temperature range is still lacking. To clarify the mechanism of low-temperature domain dynamics, we studied the growth of ferroelectric domains in the temperature range 4.2–295 K using the out-of-plane piezoresponse mode of a cryogenic atomic force microscope (AFM). Nanoscale 180° domains were created in epitaxial PbZr0.3Ti0.7O3 films by applying short voltage pulses between the conductive AFM tip brought into contact with the bare film surface and the bottom LaSr0.7Mn0.3O3 electrode. A quantitative analysis of acquired piezoresponse images enabled us to determine the in-plane domain size as a function of the writing voltage and pulse duration. It is found that at all studied temperatures the dependence of this size on the pulse duration can be fitted by a logarithmic function, which indicates that the domain-wall velocity exponentially depends on the driving electric f...