Hyunsik Im

Temperature dependence of high-and low-resistance bistable states in polycrystalline NiO films

The resistance switching current-voltage (I-V) characteristics in polycrystalline NiO films were investigated in the temperature range of 10K<T<300K. Very clear reversible resistive switching phenomena were observed in the entire temperature range. An analysis of the temperature dependence of the resistance switching transport revealed additional features, not reported in previous studies, that weak metallic conduction and correlated barrier polaron hopping coexist in the high-resistance off state and that relative dominance depends on the temperature and defect configuration. In addition, the authors propose that metallic Ni defects, existing near polycrystalline (or granular) boundaries, play a key role in the formation of a metallic channel.

Impact of Drain Induced Barrier Lowering on Read Scheme in Silicon Nanocrystal Memory with Two-Bit-per-Cell Operation

The threshold voltages (Vths) and read schemes of silicon nanocrystal memories with two bits per cell are examined by experiments and simulations. It is found that the drain induced barrier lowering (DIBL) has a marked effect on Vths in the four states and thus on read schemes for detecting the four Vths. It is also shown that the read scheme can be selected by controlling DIBL using device parameters including gate length, injected charge fraction, and injected charge density. Suitable read schemes for low-voltage and low-power applications are discussed.

Hysteretic bipolar resistive switching characteristics in TiO2/TiO2−x multilayer homojunctions

TiO2 (oxygen rich, region 1)/TiO2−x (oxygen poor, region 2) multilayer homojunctions were studied as alternative resistive switching structures for both high and low resistance transitions. Stable bipolar resistive switching characteristics, including stable switching speeds (microseconds) and endurance behaviors, as well as long retention times (>104 s) were demonstrated. The nature of the resistive switching phenomenon in multilayer structures seems to be a combination of the conduction path and the redox reaction, resulting from the oxygen ions drifting between the oxygen rich and poor regions of the multilayer structures. A possible conduction sketch for bipolar switching behaviors is also discussed.

Al electrode dependent transition to bipolar resistive switching characteristics in pure TiO2 films

Stable bipolar resistive switching was demonstrated in polycrystalline TiO2 films involving two different top and bottom Al electrodes of two different structures (Al/TiO2/Pt and Pt/TiO2/Al) after a forming process. With an Al electrode, the transition to bipolar resistive switching was clearly observed, together with counterclockwise and clockwise switching directions, which depended on the position of the Al electrode. The transition from unipolar to bipolar resistive switching seems to be attributable to the redox reaction and trap/detrap at the interfaces between the Al electrode and TiO2 layer due to the migration of oxygen ions and electrons. However, current level analysis of devices reveals that the forming process method basically leads to the formation of conducting paths inside the TiO2 layers. The electrical device properties of the two different structures, the effects of compliance currents, and the operation voltages are also analyzed.

Effects of Ultrathin Al Layer Insertion on Resistive Switching Performance in an Amorphous Aluminum Oxide Resistive Memory

We prepared resistive switching Al–AlOx multilayered junctions and observed considerably improved endurance properties. The mechanism of the observed resistance switching basically reflects the filament model. The temperature dependence of the transport in each resistance state revealed additional features, that is a well-defined thermal activation behavior in the high-resistance state is not observed in the layered device and the metallic conduction in the low-resistance state is not affected. The improved endurance properties are discussed in terms of the increased effective number of active regions, where the Reset and Set processes probably occur before a permanent dielectric breakdown.

Resistance switching characteristics in Li-doped NiO

We investigated the effects of lithium (Li)-doping on bi-stable resistance switching in polycrystalline NiO film in the temperature range of 10 K<T<300 K. Compliance-dependent resistive switching transport revealed some distinctive and interesting features not observed in undoped NiO films previously studied. An analysis of the temperature dependence of the resistive switching transport showed that Li-doping could modify the thermal properties of the off-state leading to a stable on/off switching operation. It is clearly shown that doping Li in NiO can improve NiO’s retention properties and stability of on/off switching voltages.

Effect of additional nonmagnetic acceptor doping on the resistivity peak and the Curie temperature of Ga1-xMnxAs epitaxial layers

We have investigated the effect of additional doping by Be on the properties of Ga1−xMnxAs (x=0.03). For this relatively low value of x, the Curie temperature is observed to increase with increasing Be concentration. We show that the temperature dependence of the resistivity at zero magnetic field, including the resistivity maximum near the Curie temperature, can be successfully described by the magnetoimpurity scattering model proposed by Nagaev [Phys. Rep. 346, 387 (2001)] in both the paramagnetic and the ferromagnetic temperature regions. Quantitative analysis of the data in terms of this model yields the value of the p–d exchange energy |N0β|≈1.6 eV for Ga0.97Mn0.03As.

Effect of Be doping on the properties of GaMnAs ferromagnetic semiconductors

We have studied two series of molecular beam epitaxy grown Ga1−xMnxAs epilayers with several different Be doping levels. Two Mn concentrations x were chosen for this study: 0.03 and 0.05, and these values were maintained constant in each series. These samples were characterized by using SQUID and magnetotransport measurements. A systematic increase of the Curie temperature TC was observed in SQUID measurements on the series of Ga1−xMnxAs with x=0.03. The resistivity measured at zero magnetic field shows a local maximum near the Curie temperature, reflecting the effects of critical scattering near TC. The observed increase of TC in Ga1−xMnxAs for this low range of x can be explained by the increase of the free carrier concentrations in the system arising from Be doping. However, in the series of Ga1−xMnxAs with the higher concentration of Mn (x=0.05), the measurements reveal that the TC systematically decreases with increasing Be doping level. We discuss this effect in terms of a fundamental limitation of ...

Cell reprogramming into the pluripotent state using graphene based substrates

Graphene has been attracting considerable interest in the field of biomedical engineering because graphene and its derivatives are considered to be ideal platforms for supporting cell growth and differentiation. Here we report that graphene promotes the reprogramming of mouse somatic fibroblasts into induced pluripotent stem cells (iPSCs). We constructed a layer of graphene film on a glass substrate and characterized it as a monolayer using Raman spectroscopy. We found that the graphene substrate significantly improved cellular reprogramming efficiency by inducing mesenchymal-to-epithelial-transition (MET) which is known to affect H3K4me3 levels. Thus, our results reveal that a graphene substrate directly regulates dynamic epigenetic changes associated with reprogramming, providing an efficient tool for epigenetic pluripotent reprogramming.

Electrically induced conducting nanochannels in an amorphous resistive switching niobium oxide film

Metallic nanostructures that act as electrical switches between bistable resistance states are created electrically in an insulating amorphous niobium oxide thin film. The physical formation of the metallic nanostructures are probed using in situ focused ion beam scanning electron microscopy equipped with a current-voltage measurement system. While the electroforming process changes the film, dramatically inducing metallic nanochannels across it, significant changes in the film do not occur during repeated resistance switching afterward. A qualitative resistive switching model is proposed taking into account the gradual forming process.