Filament-to-dielectric band alignments in $$\hbox {TiO}_{2}$$ and $$\hbox {HfO}_{2}$$HfO2 resistive RAMs

The next-generation nonvolatile memory storage may well be based on resistive random access memories (RRAMs). \(\hbox {TiO}_{2}\) and \(\hbox {HfO}_{2}\) have been widely used as the resistive switching layer for RRAM devices. However, the electronic properties of the filament-to-dielectric interfaces are still not well understood yet, compared to those of the electrodes and the dielectric. In this work, we study the electronic structures of three typical filament and dielectric structures, \(\hbox {Ti}_{4}\hbox {O}_{7}/\hbox {TiO}_{2}\), \(\hbox {Hf}_{2}\hbox {O}_{3}/\hbox {HfO}_{2}\) and \(\hbox {Hf}/\hbox {HfO}_{2}\), using ab initio calculations. We implement the GGA-1/2 method, which rectifies the band gaps of GGA through self-energy correction. Our calculation predicts an ohmic contact for the \(\hbox {Ti}_{4}\hbox {O}_{7}/\hbox {TiO}_{2}\) interface, where the defective \(\hbox {Ti}_{4}\hbox {O}_{7}\) phase was experimentally identified as the filament composition in \(\hbox {TiO}_{2}\). However, there is a finite Schottky barrier existing in either \(\hbox {Hf}_{2}\hbox {O}_{3}/\hbox {HfO}_{2}\) interface (1.96 eV) or \(\hbox {Hf}/\hbox {HfO}_{2}\) interface (0.61 eV), the two probable filament–dielectric configurations in hafnia-based RRAM. Our results suggest that the distinct filament-to-dielectric band alignments in \(\hbox {TiO}_{x}\) and \(\hbox {HfO}_{x}\) systems account for the much larger resistance window for the latter.     

Transport studies of $$\hbox {CeO}_{2}$$ molecular device and adsorption behavior of CO on $$\hbox {CeO}_{2}$$ device: a first-principles investigation

Using density functional theory and the non-equilibrium Green’s function formalism, the transport and CO adsorption properties of \(\hbox {CeO}_{2}\) molecular device are studied. The band structure shows that \(\hbox {CeO}_{2}\) nanostructure exhibits semiconducting nature. The electron density is found to be more in oxygen sites rather than in cerium sites along \(\hbox {CeO}_{2}\) nanostructure. The density of states spectrum shows the variation in density of charge upon adsorption of CO on CeO\(_2\) device. The transmission spectrum provides the insights on the transition of charge in \(\hbox {CeO}_{2}\) molecular device upon adsorption of CO along the scattering region. I–V characteristics confirm the adsorption of CO with the variation of current along \(\hbox {CeO}_{2}\) molecular device. The findings show that \(\hbox {CeO}_{2}\) two probe molecular device can be efficiently used for CO detection in the atmosphere.     

Structural,electrical and optical properties of $$\hbox {InGaZnO}_{4}$$ and $$\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}$$In29Sn3O48: a first-principles study

First-principles calculations were performed to investigate the electrical and optical properties of \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) with Sn-doped \(\hbox {In}_{2}\hbox {O}_{3}\) and \(\hbox {InGaZnO}_{4}\) (IGZO). The band structure, density of states, optical properties including dielectric function, loss function, reflectivity and absorption coefficient are calculated. The calculated total energy shows that the most stable crystal structures are type III for \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and type II for \(\hbox {InGaZnO}_{4}\). The band structure indicates the both \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and \(\hbox {InGaZnO}_{4}\) are direct gap semiconductors. The intrinsic band gap of \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) is much narrower than that of \(\hbox {InGaZnO}_{4}\), and results in a better electrical conductivity for \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\). The density of states shows the main hybridization occurring between In-4d and O-2p states for \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) while between In-4d In-5p, Zn-4s and O-2p states for \(\hbox {InGaZnO}_{4}\) near the valence band maximum. The reflectivity index \(R({\omega })\) shows that the peak value of \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and \(\hbox {InGaZnO}_{4}\) appears only in the ultraviolet range, indicating that these two materials have all excellent transparency. In addition, the absorption coefficient \({\alpha }({\omega })\) of both \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and \(\hbox {InGaZnO}_{4}\) is high in the ultraviolet frequency range, and therefore they show, a high UV absorption rate.     

First-principles investigation on transport properties of $$\hbox {Zn}_{2}\hbox {SnO}_{4}$$ molecular device and response toward $$\hbox {NO}_{2}$$NO2 gas molecules

The transport properties of a \(\hbox {Zn}_{2}\hbox {SnO}_{4}\) device along with adsorption properties of \(\hbox {NO}_{2}\) gas molecules on \(\hbox {Zn}_{2}\hbox {SnO}_{4}\) (ZTO) molecular devices are investigated with density functional theory using the non-equilibrium Green’s function technique. The transmission spectrum and device density of states spectrum confirm the changes in HOMO–LUMO energy level due to transfer of electrons between the ZTO-based material and the \(\hbox {NO}_{2}\) molecules. I–V characteristics demonstrate the variation in the current upon adsorption of \(\hbox {NO}_{2}\) gas molecules on the ZTO device. The findings of the present study clearly suggest that ZTO molecular devices can be used to detect \(\hbox {NO}_{2}\) gas molecules in the trace level.     

Comparative study on interfacial properties of Mo,Nb, and W contacts with monolayer $$\hbox {MoS}_{2}$$

In this work, we make a comparative study on the interfacial properties of top contact for Mo, Nb, and W metals with monolayer \(\hbox {MoS}_{2 }\,(\hbox {mMoS}_{2})\) by employing first-principles based on density functional theory (DFT) calculations. We evaluate the heights of Schottky barrier (SBH) and orbital overlap of the three models by carefully observing band structure and the density of the states relative to the Fermi level. Also, the tunnel barriers and electron densities of the three systems are analyzed. In accordance with the DFT simulations, \(\hbox {mMoS}_{2}\) forms an n-type Schottky contact with Mo, Nb, and W electrodes with electron SBH of 0.28, 0, and 0.6 eV, respectively. Besides, \(\hbox {Nb-mMoS}_{2}\) contact exhibits higher average electron density and lower tunneling barriers, demonstrating that Nb can form a better top contact with \(\hbox {mMoS}_{2}\) and should have prior electron injection efficiency and backgated regulation of current compared to the \(\hbox {mMoS}_{2}\) contacts with Mo and W.     

Numerical analysis of transmission coefficient,LDOS, and DOS in superlattice nanostructures of cubic $$\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}$$ resonant tunneling MODFETs

Numerical analysis of the transmission coefficient, local density of states, and density of states in superlattice nanostructures of cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) resonant tunneling modulation-doped field-effect transistors (MODFETs) using \(\hbox {next}{} \mathbf{nano}^{3}\) software and the contact block reduction method is presented. This method is a variant of non-equilibrium Green’s function formalism, which has been integrated into the \(\hbox {next}\mathbf{nano}^{3}\) software package. Using this formalism in order to model any quantum devices and estimate their charge profiles by computing transmission coefficient, local density of states (LDOS) and density of states (DOS). This formalism can also be used to describe the quantum transport limit in ballistic devices very efficiently. In particular, we investigated the influences of the aluminum mole fraction and the thickness and width of the cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N}\) on the transmission coefficient. The results of this work show that, for narrow width of 5 nm and low Al mole fraction of \(x = 20\,\%\) of barrier layers, cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) superlattice nanostructures with very high density of states of 407 \(\hbox {eV}^{-1}\) at the resonance energy are preferred to achieve the maximum transmission coefficient. We also calculated the local density of states of superlattice nanostructures of cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) to resolve the apparent contradiction between the structure and manufacturability of new-generation resonant tunneling MODFET devices for terahertz and high-power applications.     

First-principle calculations of electronic and ferromagnetic properties of $$\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}$$

We have used the first-principle calculations of density functional theory within full-potential linearized augmented plane-wave method to investigate the electronic and ferromagnetic properties of \(\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}\) alloys. The electronic structures of \(\hbox {Al}_{0.25}\hbox {V}_{0.75}\hbox {Sb}, \hbox {Al}_{0.5}\hbox {V}_{0.5}\hbox {Sb}\) and \(\hbox {Al}_{0.75}\hbox {V}_{0.25}\hbox {Sb}\) exhibit a half-metallic ferromagnetic character with spin polarization of 100 %. The total magnetic moment per V atom for each compound is integral Bohr magneton of 2 \(\mu _{\mathrm{B}}\), confirming the half-metallic feature of \(\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}\). Therefore, these materials are half-metallic ferromagnets useful for possible spintronics applications.     

Optimal design of the multiple-apertures-GaN-based vertical HEMTs with $$\hbox {SiO}_{2}$$ current blocking layer

Gallium nitride (GaN) based vertical high electron mobility transistor (HEMT) is very crucial for high power applications. Combination of advantageous material properties of GaN for high speed applications and novel vertical structure makes this device very beneficial for high power application. To improve the device performance especially in high drain bias condition, a novel GaN based vertical HEMT with silicon dioxide \((\hbox {SiO}_{2})\) current blocking layer (CBL) was reported recently. In this paper, effects of the thickness of CBL layer and the aperture length on the electrical and breakdown characteristics of GaN vertical HEMTs with \(\hbox {SiO}_{2}\) CBL are simulated by using two-dimensional quantum-mechanically corrected device simulation. Intensive numerical study on the device enables us to optimize and conclude that devices with \(0.5\hbox {-}\upmu \hbox {m}\)-thick \(\hbox {SiO}_{2}\) layer and \(1\hbox {-}\upmu \hbox {m}\)-long aperture will be beneficial considerations to improve the device performance. Notably, using the multiple apertures can effectively reduce the on-state conducting resistance of the device. On increasing the number of apertures, the drain current is increased but the breakdown voltage is decreased. Therefore, device with four apertures is taken as an optimized result. The maximum drain current of 84 mA at \(\hbox {V}_\mathrm{G}= 1\,\hbox {V}\) and \(\hbox {V}_\mathrm{D}= 30\,\hbox {V}\), and the breakdown voltage of 480 V have been achieved for the optimized device.     

DFT studies of electronic structure and dielectric properties in layered perovskite $$\hbox {LaSrAlO}_{4}$$

The structural, electronic, dielectric and optical properties of tetragonal \(\hbox {LaSrAlO}_{4}\) are studied in detail using density functional theory calculations. The energy band structures and density of states are predicted by generalized gradient approximation (GGA) and local density approximation (LDA) respectively. The fundamental band gaps of \(\hbox {LaSrAlO}_{4}\) are all indirect by GGA (2.860 eV) and LDA (2.863 eV) calculations. The complex dielectric function was calculated. There are two peaks in the real part \(\varepsilon _{1}(\omega )\) and three peaks in the imaginary part \(\varepsilon _{2}(\omega )\). The optical spectra are assigned to the interband transition from O valence to La and Sr conduction bands in the low energy region. In addition, the electron energy-loss spectrum, optical conductivity, reflectivity spectrum, and refractive index, are given to support the potential applications for microwave dielectric ceramics.     

Analog and RF performance of doping-less tunnel FETs with $$\hbox {Si}_{0.55} \hbox {Ge}_{0.45}$$ source

This paper reports studies of a doping-less tunnel field-effect transistor (TFET) with a \(\hbox {Si}_{0.55} \hbox {Ge}_{0.45}\) source structure aimed at improving the performance of charge-plasma-based doping-less TFETs. The proposed device achieves an improved ON-state current (\(I_{{\mathrm{ON}}} \sim {4.88} \times {10}^{-5}\,{\mathrm{A}}/\upmu {\mathrm{m}}\)), an \(I_\mathrm{ON}/I_\mathrm{OFF}\) ratio of \({6.91} \times {10}^{12}\), an average subthreshold slope (\(\hbox {AV-SS}\)) of \(\sim \) \({64.79}\,{\mathrm{mV/dec}}\), and a point subthreshold slope (SS) of 14.95 mV/dec. This paper compares the analog and radio of frequency (RF) parameters of this device with those of a conventional doping-less TFET (DLTFET), including the transconductance (\(g_{{\mathrm{m}}}\)), transconductance-to-drain-current ratio \((g_\mathrm{m}/I_\mathrm{D})\), output conductance \((g_\mathrm{d})\), intrinsic gain (\(A_{{\mathrm{V}}}\)), early voltage (\(V_{{\mathrm{EA}}}\)), total gate capacitance (\( C_{{\mathrm{gg}}}\)), and unity-gain frequency (\(f_{{\mathrm{T}}}\)). Based on the simulated results, the \(\hbox {Si}_{0.55}\hbox {Ge}_{0.45}\)-source DLTFET is found to offer superior analog as well as RF performance.     

Structural,electronic, and optical properties of C-type $$\hbox {Gd}_{2}\hbox {O}_{3}$$: a density functional theory investigation

Recently, \(\hbox {Gd}_{2}\hbox {O}_{3}\) has gained considerable interest in industry, and its optical applications have been of interest in optoelectronic. The band structure and optical properties of cubic \(\hbox {Gd}_{2}\hbox {O}_{3}\) are investigated using the density functional theory framework. Calculations are performed within the local density approximation and generalized gradient approximation, adding the empirical Hubbard potential U. Calculation of the electronic band structure indicates a direct \({\Gamma }\) band gap. Further, the total and partial densities of states were presented, and the contribution of different orbitals is analyzed. Moreover, the behavior of optical spectra such as real and imaginary part of dielectric function, refractive index, extinction coefficient, optical conductivity, and electron energy-loss function is analyzed. There is a good agreement between the computed results and reported experimental data.     

First-principles simulation of oxygen vacancy migration in $$\hbox {HfO}_{ x}$$, $$\hbox {CeO}_{ x}$$CeOx,and at their interfaces for applications in resistive random-access memories

Transition metal-oxide resistive random-access memories seem to be a viable candidate as the next-generation storage technology because transition metals have multiple oxidation states and are good ionic conductors. A wide range of transition metal oxides have recently been studied; however, fundamental understanding of the switching mechanism is still lacking. Migration energies and diffusivity of oxygen vacancies in amorphous and crystalline \(\hbox {HfO}_{2}\) and \(\hbox {CeO}_{2}\) and at their interface are investigated by employing density functional theory. We found that oxygen dynamics is better in \(\hbox {CeO}_{2}\) compared to \(\hbox {HfO}_{2}\), including smaller activation energy barriers and larger diffusion pre-factors, which can have implications in the material-selection process to determine which combination of materials offer the most efficient switching. Furthermore, we found that motion of vacancies is different at the interface of these two oxides as compared to that within each constituents, which provided insight into the role of the interface in vacancy motion and ultimately using interface engineering as a way to tune material properties.     

Computational study of a new resonant tunneling diode based on an $$\hbox {MoS}_{2}$$ nanoribbon with sulfur line vacancies

Recent experimental studies have shown that sulfur vacancies in monolayer \(\hbox {MoS}_{2}\) are mobile under exposure to an electron beam and tend to accumulate as sulfur line vacancies (Komsa in Phys Rev B 88: 035301, 2013). In this work, we designed a new resonant tunneling diode (RTD) based on this natural property. Two rows of sulfur vacancies are introduced into armchair \(\hbox {MoS}_{2}\) nanoribbons (\(\hbox {A-MoS}_{2}\) NRs) to tune the nanoribbons’ bandgap to obtain the double-barrier quantum well structure of the resonant tunneling diode. This arrangement has a unique benefit that will result in very little physical distortion. A tight-binding (TB) model, with five 4d-orbitals of the Mo atom and three 3p-orbitals of the S atom, is employed for calculations. In the TB model, which is described in terms of Slater–Koster parameters, we also incorporate the changes of edge bonds. Density functional theory is used to determine all the necessary parameters of the TB model. They are obtained by an optimization procedure which achieves very fine parameter values, which can regenerate the most important energy bands of \(\hbox {A-MoS}_{2}\) NRs of different widths, with highly satisfactory precision. The introduction of these new parameters is another contribution of this work. Lastly, the nonequilibrium Green’s function formalism based on the TB approximation is used to explore the properties of the new RTD structures based on \(\hbox {A-MoS}_{2}\) NRs. Negative differential resistance with peak to valley ratio (PVR) of about 78 at room temperature is achieved for one RTD, having peak current \(I_\mathrm{p}=90\) nA. We show that the PVR can exceed 120 when increasing the barrier length of the RTD at the expense of lower \(I_\mathrm{p}\).     

Simulation of dielectric behavior in RFeO$$_{}$$ orthoferrite ceramics (R = rare earth metals)

Due to their colossal dielectric constant (CDC), \(\hbox {RFeO}_{3}\), orthoferrite ceramics (R = rare earth metal) have recently attracted much attention. In the present research, the dielectric constants of \(\hbox {RFeO}_{3}\) orthoferrite ceramics, whether with or without CDC, have been simulated. The type of synthesis method, the type of R material, temperature, and frequency as the effective parameters on the dielectric behavior are introduced to the model. Another input parameter is the ratio of \(\hbox {Fe}^{+2}/\hbox {Fe}^{+3}\) peak area (in the XPS diagram), which is the most important parameter that affects the CDC behavior. Initially, a colossal database is formed by means of WebPlotDigitizer software and 2930 experimental data, and then the simulation is carried out through gene expression programming. Two case studies are also performed on \(\hbox {PrFeO}_{3}\) and \(\hbox {NdFeO}_{3}\) orthoferrite ceramics to validate the accuracy of the presented model. \(\hbox {PrFeO}_{3}\) exhibits significant CDC behavior whereas the \(\hbox {NdFeO}_{3}\) ceramic samples possess little CDC property, both of which were precisely simulated by the model. Two-dimensional tenth-degree equations resulting from the model predict the dielectric constant variations accurately.     

Composition dependence of fundamental properties of $$\hbox {Cd}_{\mathrm{1-x}}\hbox {Co}_\mathrm{x}$$Te magnetic semiconductor alloys

Ab initio calculations based on density functional theory have been performed using the full-potential augmented-plane-wave method so as to investigate the composition dependence of the electronic structure and fundamental properties of hypothetical zinc-blende \(\hbox {Cd}_{\mathrm{1-x}}\hbox {Co}_{\mathrm{x}}\hbox {Te}\) magnetic semiconductor alloys at low Co concentrations. To treat the exchange and correlation energies, the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof has been used. In addition, the modified Becke–Johnson exchange potential with the GGA approach is used for the band structure providing high accuracy. It is found that the addition of a small amount of Co atoms in the \(\hbox {Cd}_{\mathrm{1-x}}\hbox {Co}_{\mathrm{x}}\hbox {Te}\) makes the latter less compressible, ferromagnetic and exhibiting a half metallic character. Besides, the composition dependence of the real and imaginary parts of the dielectric function has been examined and discussed. The information derived from the present study may be useful for spintronics technological applications.     

Investigation on the structural,elastic, electronic,and magnetic properties of half-metallic $$\hbox {Co}_{2}\hbox {MnSi}$$ and CoMnIrSi via first-principles calculations

We investigated the structural, elastic, electronic, and magnetic properties of \(\hbox {Co}_{2}\hbox {MnSi}\) and CoMnIrSi full-Heusler compounds by means of density functional theory based on the full-potential linearized augmented plane wave (FP-LAPW) approach. The generalized gradient approximation as proposed by Wu and Cohen (GGA-WC) was employed to treat the exchange-correlation effect. The results show that both alloys are structurally and mechanically stable. \(\hbox {Co}_{2}\hbox {MnSi}\) is almost elastically isotropic, while CoMnIrSi is anisotropic, and both alloys are ductile. The studied compounds have perfect spin polarization of 100 %, with down-spin bandgap of 0.796 eV and 0.728 eV, respectively. The calculated magnetic properties indicate that the Slater–Pauling rule is satisfied in both cases. Finally, the effect of strain on the half-metallic properties of \(\hbox {Co}_{2}\hbox {MnSi}\) and CoMnIrSi was also investigated by varying the lattice constant over a wide range.     

Theoretical insights and experimental characterization of $$\hbox {HfO}_2$$-based OxRRAMs operation

The intensive research in resistive random access memories (RRAM) field has brought in significant improvements in the performance, optimization and reliability of the devices as well as more understanding on their operation. This was made possible through the combination of different tools starting from material engineering to device characterization, modeling and simulations. In this review, we bring an overview of our recent work on RRAM through experimental characterization and first-principles calculations. We explore the effects of metal electrodes on the switching performance and conductive filament (CF) stability of \(\hbox {HfO}_2\) oxide-based RRAM (OxRRAM). With the insight gained from the experimental data, we employ first-principles calculations to have a better microscopic understanding on OxRRAM operation. We show that CF stability and device operating voltages strongly depend on the electrode material. Ti being an electrode material of high interest, we investigate the type of \(\hbox {Ti/HfO}_2\) interface that may be formed and propose a probable composition. We also study the formation and migration of extended Frenkel-pair (EFP) defect in \(\hbox {HfO}_2\) which we consider to be the prototype defect responsible for OxRRAM degradation leading to CF formation. This EFP emission occurs through a cascading migration of O atoms inside \(\hbox {HfO}_2\) lattice. Based on EFP formation and diffusion, we present a simplified CF formation model. Finally, we study low resistance data retention failure in OxRRAM through \(\hbox {HfO}_2\), \(\hbox {Hf}_{1x}\hbox {Al}_{2x}\hbox {O}_{2+x}\) (HfAlO) and \(\hbox {Hf}_{1-x}\hbox {Ti}_{x}\hbox {O}_{2}\) (HfTiO) type of cells. We link its origin to the lateral diffusion of oxygen vacancies at the constriction/tip of the conductive filament in \(\hbox {HfO}_2\)-based RRAM.     

Ambipolar leakage suppression in electron–hole bilayer TFET: investigation and analysis

In this paper, we propose and simulate two new structures of electron–hole bilayer tunnel field-effect transistors (EHBTFET). The proposed devices are n-heterogate with \(\hbox {M}_{1}\) as overlap gate, \(\hbox {M}_{2}\) as underlap gate and employs a high-k dielectric pocket in the drain underlap. Proposed structure 1 employs symmetric underlaps (Lgs = Lgd = Lu). The leakage analysis of this structure shows that the lateral ambipolar leakage between channel and drain is reduced by approximately three orders, the OFF-state leakage is reduced by one order, and the \(I_{\mathrm{ON}}/I_{\mathrm{OFF}}\) ratio is increased by more than one order at \(V_\mathrm{{GS}}=V_{\mathrm{DS}} =1.0\) V as compared to the conventional Si EHBTFET. The performance is improved further by employing asymmetric underlaps (\(\hbox {Lgs}\ne \hbox {Lgd}\)) with double dielectric pockets at source and drain, called as proposed structure 2. The pocket dimensions have been optimized, and an average subthreshold swing of 17.7 mV/dec (25.5% improved) over five decades of current is achieved with an ON current of \(0.23~\upmu \hbox {A}/\upmu \hbox {m}\) (11% improved) in proposed structure 2 in comparison with the conventional EHBTFET. Further, the parasitic leakage paths between overlap/underlap interfaces are blocked and the OFF-state leakage is reduced by more than two orders. A high \(I_{\mathrm{ON}}/I_{\mathrm{OFF}}\,\hbox {ratio}~>10^{9}\) (two orders higher) is achieved at \(V_{\mathrm{DS}} =V_{\mathrm{GS}} =1.0~\hbox {V}\) in the proposed structure 2 in comparison with the conventional one.     

$${ SIM}^2{ RRAM}$$: a physical model for RRAM devices simulation

In the last few years, resistive random access memory (RRAM) has been proposed as one of the most promising candidates to overcome the current Flash technology in the market of non-volatile memories. These devices have the ability to change their resistance state in a reversible and controlled way applying an external voltage. In this way, the resulting high- and low-resistance states allow the electrical representation of the binary states “0” and “1” without storing charge. Many physical models have been developed with the aim of understanding the mechanisms that control the resistive switching. In this work, we have compiled the main theories accepted as well as their corresponding models for the conduction characteristics. In addition, simulation tools play a very important role in the task of checking these theories and understanding these mechanisms. For this reason, the simulation tool called \(\hbox {SIM}^{2}\hbox {RRAM}\) has been presented. This simulator is capable of replicating the global behavior of RRAM cell based on \(\hbox {HfO}_{x}\).     

A simulation study of voltage-assisted low-energy switching of a perpendicular anisotropy ferromagnet on a topological insulator

We present a novel memory device that consists of a thin ferromagnetic layer of Fe deposited on topological insulator thin film, \(\hbox {Bi}_{2}\hbox {Se}_{3}\). The ferromagnetic layer has perpendicular anisotropy, due to MgO deposited on its top surface. When current is passed on the surface of \(\hbox {Bi}_{2}\hbox {Se}_{3}\), the surface of the \(\hbox {Bi}_{2} \hbox {Se}_{3}\) becomes spin polarized and strong exchange interaction occurs between the d electrons in the ferromagnet and the electrons conducting the current on the surface of the \(\hbox {Bi}_{2}\hbox {Se}_{3}\). Part of the current is also shunted through the ferromagnet, which generates spin transfer torque in the ferromagnet. The exchange interaction torque along with voltage-controlled magnetic anisotropy allows ultralow-energy switching of the ferromagnet. We perform micromagnetic simulations and predict switching time of the order of 2.5 ns and switching energy of the order of 0.88fJ for a ferromagnetic bit with thermal stability of \(43\,k_\mathrm{{B}}T\). Such ultralow-energy and high-speed switching of a perpendicular anisotropy ferromagnet on a topological insulator could be utilized for energy-efficient memory design.