Heterogeneously structured phase-change materials and memory

Phase-change memory (PCM), a non-volatile memory technology, is considered the most promising candidate for storage class memory and neuro-inspired devices. It is generally fabricated based on GeTe-Sb2Te3 pseudo-binary alloys. However, natively, it has technical limitations, such as noise and drift in electrical resistance and high current in operation for real-world device applications. Recently, heterogeneously structured PCMs (HET-PCMs), where phase-change materials are hetero-assembled with functional (barrier) materials in a memory cell, have shown a dramatic enhancement in device performance by reducing such inherent limitations. In this Perspective, we introduce recent developments in HET-PCMs and relevant mechanisms of operation in comparison with those of conventional alloy-type PCMs. We also highlight corresponding device enhancements, particularly their thermal stability, endurance, RESET current density, SET speed, and resistance drift. Last, we provide an outlook on promising research directions for HET-PCMs including PCM-based neuromorphic computing

Nonvolatile memory characteristics associated with oxygen ion exchange in thin-film transistors with indium-zinc oxide channel and HfO2-x gate oxide

Non-charge-storage-based nonvolatile memory characteristics associated with oxygen ion exchange are demonstrated in a thin-film transistor (TFT) composed of an indium-zinc oxide (IZO) channel and an oxygen-deficient HfO2???x gate oxide. A nonvolatile increase in drain current and a reduced threshold voltage are obtained upon application of positive gate voltage, with the opposite characteristics upon application of negative voltage. The device shows nonvolatile retention properties and suitable endurance properties after repeated operations. Modulation of channel conductance occurs as a results of oxygen ion exchange between the HfO2???x gate oxide and the IZO channel, which consequently alters the oxygen vacancy concentration in the IZO channel; these vacancies act as n-type dopants. For comparison, a device with a thin SiO2 layer inserted between the HfO2???x gate oxide and the IZO channel to prevent oxygen ion exchange shows only the increased threshold voltage upon application of a positive gate voltage as a result of electron charging. These results verify the conductance modulation mechanism associated with oxygen ion exchange at the interface of the HfO2???x gate oxide and the IZO channel. In addition, the nonvolatile memory characteristics of the device are indicative of its potential for non-charge-storage-based nonvolatile memory application

Reset Current Reduction with Excellent Filament Controllability by Using Area Minimized and Field Enhanced Unipolar Resistive Random Access Memory Structure

We firstly propose a novel resistive random access memory (RRAM) cell structure, which makes it possible to minimize the switching area and to maximize the electrical field where resistive switching occurs, resulting in the improvement of resistive switching characteristics. With excellent structural advantages, resistive switching characteristics such as reset current and set voltage fluctuation are improved through the enhancement of conductive filament (CF) controllability. A simple fabrication process is delivered and the device performance from the viewpoints of the forming voltage, set voltage, and reset current is investigated. Conducting defect effects are also investigated in comparison with the conventional RRAM cell structure. Numerical simulation is performed using a random circuit breaker (RCB) model to confirm the proposed structure

Areal and Structural Effects on Oxide-Based Resistive Random Access Memory Cell for Improving Resistive Switching Characteristics

A new technical improvement in understanding the resistive switching characteristics of unipolar resistive random access memory (RRAM) is investigated. It is possible to minimize reset current (I-RESET), set voltage variation, and forming voltage (V-FORMING), which results in a wide sensing margin and high density applications by using a conducting filament (CF) minimized structure up to a 10 nm technology node. Its structural advantages enable I-RESET to be tuned with excellent manufacturability. Numerical simulation is also performed using a random circuit breaker (RCB) model, showing that the proposed structure elucidates the resistive switching improvement

Novel U-Shape Resistive Random Access Memory Structure for Improving Resistive Switching Characteristics

We firstly propose a novel U-shape resistive cell structure which is the best fit for generating low power resistive random access memory (RRAM) with forming-less process. We find that irregular resistive switching behavior in the initial transition and the characteristics associated with it. Controlling the conducting filament (CF) dimension and deposition orientation of resistive material are expected to reduce the distribution and forming voltage, which enables low power RRAM to be feasible without forming state. Simple fabrication flow and device performances are also evaluated in the aspect of forming-less process. Numerical simulation is performed using random circuit breaker model (RCB) to confirm the proposed structure

Interface-Modified Unipolar Resistive Random Access Memory (RRAM) Structure for Low-Power Application

An interface-engineered resistive random access memory (RRAM) using bilayer transition metal oxide (TMO) is presented for improving unipolar resistive-switching characteristics. The experiment and simulation data show that better resistive switching characteristics and superb uniformity can be realized by inserting a thin AlOx insertion layer between the Ir/NiO interface. To elucidate the uniformity improvement of our bilayer structure, the conducting-defect effects in the resistive cell were also investigated using a random circuit breaker (RCB) simulation model. It has been verified that the forming and set characteristics are more effectively improved because the conducting-defect ratio in the insertion layer region is low, therefore making it more advantageous for a filament path controllability. Using the optimal oxygen contents in both the insertion layer and the resistive cell, it was confirmed that a significant reduction of up to 0.15 mA of the reset current (/(RESET)) is possible compared to the conventional cell. These results indicate that new Al insertion has a large contribution to the reset and forming processes

Effect of number of laser pulses on p(+)/n silicon ultra-shallow junction formation during non-melt ultra-violet laser thermal annealing

We investigate the effect of the number of laser pulses on the formation of p(+)/n silicon ultra-shallow junctions during non-melt ultra-violet laser (wavelength, 355 nm) annealing. Through surface peak temperature calculating by COMSOL Multiphysics, the non-melt laser thermal annealing is performed under the energy density of 130 mJ/cm(2). We demonstrate that increasing the number of laser pulses without additional pre-annealing is an effective annealing method for achieving good electrical properties and shallow junction depth by analyzing sheet resistance and junction depth profiles. The optimal number of laser pulses is eight for achieving a high degree of activation of dopant without further increase of junction depth. We have also explained the improved electrical characteristics of the samples on the basis of fully recovered crystallinity as revealed by Raman spectroscopy. Thus, it is suggested that controlling the number of laser pulses with moderate energy density is a promising laser annealing method without additional pre-annealing

Understanding on the current-induced crystallization process and faster set write operation thereof in non-volatile phase change memory

We experimentally demonstrate that the crystallization process of Ge-Sb-Te crystallites during the set operation in non-volatile phase change memory commences after threshold switching event. It is also shown that the nucleation and growth rates have opposite behaviors with the increase of set operation power: the incubation time in nucleation stage can be minimized at higher power, whereas the percolation time in growth stage is smaller at lower power. Based on these results, we introduce a two-step set pulse of high-power nucleation and low-power growth making the set write operation much faster than conventional simple rectangular or slow-quenched form

Novel Protruded-Shape Unipolar Resistive Random Access Memory Structure for Improving Switching Uniformity through Excellent Conductive Filament Controllability

Resistive random access memory (RRAM) with a new structure which can effectively control switching area and electric field is proposed. It has been verified that the decrease in area of resistive material with the new structure increases electric field of switching area, and that such increased electric field makes initial forming at unipolar switching rather easier, resulting in effective decrease in forming voltage. Also, as the area in switching area is effectively reduced, decrease in reset current and set voltage in a limited area has also been verified. Excellent resistive switching characteristics are possible by decrease of conductive filament (CF) area in our structure. Random circuit breaker (RCB) simulation model which can effectively explain percolation switching similar to unipolar switching verifies such structural effect

Crystallinity of silicon films grown on carbon fibers by very high frequency plasma enhanced chemical vapor deposition

In this work, silicon films were deposited on carbon fibers using very high frequency (60 MHz) plasma enhanced chemical vapor deposition (VHF-PECVD) to form Si/carbon fiber hybrid structures for the applications to the flexible solar cell. The effect of deposition conditions of VHF-PECVD such as hydrogen flow rate and R.F. power on the microstructure of Si films was investigated with Raman spectroscopy, X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopy. The crystallinity of Si films on carbon fibers showed strong dependence on the hydrogen flow rate and was changed from amorphous structure to partially crystallized structure with increasing hydrogen flow rate. Increasing R.F. power enhanced the crystallinity for amorphous Si films while deteriorated the crystallinity for partially crystallized Si films on carbon fibers. And it was observed that the crystallinity of silicon films on carbon fibers was increased drastically by thermal annealing above 500 degrees C