Min-Hwi Kim

Resistive switching characteristics of Si3N4-based resistive-switching random-access memory cell with tunnel barrier for high density integration and low-power applications

In this letter, a bipolar resistive-switching random-access memory (RRAM) in Ni/Si3N4/SiO2/p+-Si structure and its fabrication process are demonstrated. The proposed device with double-layer dielectrics consisting of Si3N4 layer (5 nm) as a resistive switching and SiO2 (2.5 nm) layer for the tunnel barrier is investigated in comparison with that having a single layer of Si3N4. Double-layer cell shows ultra-low power operation under a compliance current (ICOMP) of 500 nA, which ensures the reset current (IRESET) of sub-1 μA much lower than that of the single-layer cell. Also, large on/off ratio (∼105) has been obtained since the SiO2 layer efficiently suppresses the current in the high-resistance state. Moreover, maximum selectivity in double-layer cell is 122 when 1/2 read bias scheme is applied to the crossbar array. Highly nonlinear I-V characteristics of the double-layer Si3N4-based RRAM cell warrant the realization of selector-free RRAM cell in the crossbar array pursuing higher integration density.

Observation of sphere resonance peak in ferromagnetic GaN:Mn

We report temperature-dependent optical spectra of ferromagnetic Mn-doped GaN in a wide photon energy region of 5 meV–4 eV. Below the GaN gap, an absorption peak around 1.25 eV whose intensity increases at lower temperatures was observed. A composite medium theory, called the Maxwell–Garnett theory, shows that the absorption peak can be assigned to a sphere resonance from metallic particles embedded in a Mn-doped GaN matrix. We also report that the far-infrared absorption of Mn-doped GaN sample was very small. This result suggests that itinerant carrier-mediated ferromagnetism does not fully explain the observed magnetic properties.

Nano-cone resistive memory for ultralow power operation

SiN x -based nano-structure resistive memory is fabricated by fully silicon CMOS compatible process integration including particularly designed anisotropic etching for the construction of a nano-cone silicon bottom electrode (BE). Bipolar resistive switching characteristics have significantly reduced switching current and voltage and are demonstrated in a nano-cone BE structure, as compared with those in a flat BE one. We have verified by systematic device simulations that the main cause of reduction in the performance parameters is the high electric field being more effectively concentrated at the tip of the cone-shaped BE. The greatly improved nonlinearity of the nano-cone resistive memory cell will be beneficial in the ultra-high-density crossbar array.

Improved resistive switching characteristics in Ni/SiNx/p++-Si devices by tuning x

This letter studies the effect of the negative-set on the resistive switching performances of CMOS-compatible Ni/SiNx/p++-Si resistive memory devices by simply tuning x. A Ni/SiN1.07/p++-Si device showed lower power switching (20 μW) and better endurance cycles (103) compared to a Ni/SiN0.82/p++-Si device because of the improved negative set behavior and initially lower set and reset currents. In addition, we achieved fast switching speed for set (200 ns) and reset (100 ns) processes in the Ni/SiN1.07/p++-Si device. For the Ni/SiN1.07/p++-Si device, fine adjustment of resistance values is attainable by varying the pulse amplitude and width due to the gradual reset switching characteristics. The barrier-height-dependent conduction model is proposed to explain the change in the current level with the x value.

Analog Synaptic Behavior of a Silicon Nitride Memristor

In this paper, we present a synapse function using analog resistive-switching behaviors in a SiNx-based memristor with a complementary metal-oxide-semiconductor compatibility and expandability to three-dimensional crossbar array architecture. A progressive conductance change is attainable as a result of the gradual growth and dissolution of the conducting path, and the series resistance of the AlOy layer in the Ni/SiNx/AlOy/TiN memristor device enhances analog switching performance by reducing current overshoot. A continuous and smooth gradual reset switching transition can be observed with a compliance current limit (>100 μA), and is highly suitable for demonstrating synaptic characteristics. Long-term potentiation and long-term depression are obtained by means of identical pulse responses. Moreover, symmetric and linear synaptic behaviors are significantly improved by optimizing pulse response conditions, which is verified by a neural network simulation. Finally, we display the spike-timing-dependent plasticity with the multipulse scheme. This work provides a possible way to mimic biological synapse function for energy-efficient neuromorphic systems by using a conventional passive SiNx layer as an active dielectric.

Self-Compliant Bipolar Resistive Switching in SiN-Based Resistive Switching Memory

Here, we present evidence of self-compliant and self-rectifying bipolar resistive switching behavior in Ni/SiNx/n+ Si and Ni/SiNx/n++ Si resistive-switching random access memory devices. The Ni/SiNx/n++ Si device’s Si bottom electrode had a higher dopant concentration (As ion > 1019 cm−3) than the Ni/SiNx/n+ Si device; both unipolar and bipolar resistive switching behaviors were observed for the higher dopant concentration device owing to a large current overshoot. Conversely, for the device with the lower dopant concentration (As ion < 1018 cm−3), self-rectification and self-compliance were achieved owing to the series resistance of the Si bottom electrode.

Scaling Effect on Silicon Nitride Memristor with Highly Doped Si Substrate

A feasible approach is reported to reduce the switching current and increase the nonlinearity in a complementary metal-oxide-semiconductor (CMOS)-compatible Ti/SiNx /p+ -Si memristor by simply reducing the cell size down to sub-100 nm. Even though the switching voltages gradually increase with decreasing device size, the reset current is reduced because of the reduced current overshoot effect. The scaled devices (sub-100 nm) exhibit gradual reset switching driven by the electric field, whereas that of the large devices (≥1 µm) is driven by Joule heating. For the scaled cell (60 nm), the current levels are tunable by adjusting the reset stop voltage for multilevel cells. It is revealed that the nonlinearity in the low-resistance state is attributed to Fowler-Nordheim tunneling dominating in the high-voltage regime (≥1 V) for the scaled cells. The experimental findings demonstrate that the scaled metal-nitride-silicon memristor device paves the way to realize CMOS-compatible high-density crosspoint array applications.

Switching and conduction mechanism of Cu/Si3N4/Si RRAM with CMOS compatibility

In this work, we fabricated CMOS compatible Cu/Si3N4/Si RRAM, showing good resistive switching. This memory cell is stable for bipolar switching (BS) than unipolar switching (US). Trap-controlled space charge limited current (SCLC) conduction is observed. Low resistance state (LRS) shows semiconducting behavior according to the temperature dependency.

Concurrent events of memory and threshold switching in Ag/SiNx/Si devices

In this work, the simultaneous detection of threshold switching and bipolar memory switching in Ag/SiNx/p++-Si devices is investigated. In the DC sweep mode, threshold switching is observed with low compliance current limit (CCL) of 1 μA while memory switching is dominant when high CCL (1 mA) is applied. It is found that in the pulse switching mode, pulse amplitude is an important factor in determining the nature of switching. It has been proven that the strength of the Ag filament formed in the SiNx determines the nonvolatile property of the switching. The undirectional threshold switching behavior in low currents of Ag/SiNx/p++-Si devices could be used as a selector for a low-power unipolar memory. Moreover, operating in two modes in one device will provide more flexibility in device design.

Optimization and modeling of npn-type selector for resistive RRAM in cross-point array structure

In this paper, we investigate the characteristics of npn device as a candidate for RRAM selector. npn selector shows high current density and selectivity which are key metrics for the bidirectional select device. We confirm that length and doping concentration of base and emitter region can be varied to optimize the characteristic of the selector. In addition, we observe AC characteristic with 10 ns pulse width and interval. We confirm that I-V curve is well fitted with a combination of exponential and quadratic terms.