V. Sverdlov

Stochastic model of the resistive switching mechanism in bipolar resistive random access memory: Monte Carlo simulations

A stochastic model of the resistive switching mechanism in bipolar metal-oxide-based resistive random access memory (RRAM) is presented. The distribution of electron occupation probabilities obtained is in good agreement with previous work. In particular, it is shown that a low occupation region is formed near the cathode. Our simulations of the temperature dependence of the electron occupation probabilities near the anode and the cathode demonstrate a high robustness of the low and high occupation regions. This result indicates that a decrease in the switching time with increasing temperature cannot be explained only by reduced occupations of the vacancies in the low occupation region, but is rather related to an increase in the mobility of the oxide ions. A hysteresis cycle of a RRAM simulated with the stochastic model is in agreement with experimental results.

Electronic band structure modeling in strained Si-nanowires: Two band k · p versus tight binding

The subband structure of silicon nanowires has gained much interest recently. Nanowires with diameters below 10 nm are predicted to have a significantly altered subband structure compared with bulk silicon. The effective mass approximation fails to describe these alterings correctly, and so far the semiempirical tight binding method and first principles calculations were used to investigate them. In this paper we present an approach based on a two band k · p description of the conduction band minima. The method excels in simplicity of modeling and versatility including the ability to model strain effects on the subband structure.

Reliability-Based Optimization of Spin-Transfer Torque Magnetic Tunnel Junction Implication Logic Gates

Recently, magnetic tunnel junction (MTJ)-based implication logic gates have been proposed to realize a fundamental Boolean logic operation called material implication (IMP). For given MTJ characteristics, the IMP gate circuit parameters must be optimized to obtain the minimum IMP error probability. In this work we present the optimization method and investigate the effect of MTJ device parameters on the reliability of IMP logic gates. It is shown that the most important MTJ device parameters are the tunnel magnetoresistance (TMR) ratio and the thermal stability factor Δ. The IMP error probability decreases exponentially with increasing TMR and Δ.

Properties of Silicon Ballistic Spin Fin-Based Field-Effect Transistors

We investigate the properties of ballistic fin-structured silicon spin field-effect transistors. The spin transistor suggested first by Datta and Das employs spin-orbit coupling to introduce the current modulation. The major contribution to the spin-orbit interaction in silicon films is of the Dresselhaus type due to the interface-induced inversion symmetry breaking. The subband structure in silicon confined systems is obtained with help of a two-band k·p model and is in good agreement with recent density functional calculations. It is demonstrated that fins with [100] orientation display a stronger modulation of the conductance as function of spin-orbit interaction and magnetic field and are thus preferred for practical realizations of silicon SpinFETs.

Valley splitting and spin lifetime enhancement in strained thin silicon films

Spintronics attracts much attention because of the potential to build novel spin-based devices which are superior to nowadays charge-based microelectronic devices. Silicon, the main element of microelectronics, is promising for spin-driven applications. We investigate the surface roughness and electron-phonon limited spin relaxation in silicon films taking into account the coupling between the relevant valleys through the Γ-point. We demonstrate that applying uniaxial stress along the [110] direction considerably suppresses the spin relaxation.

Modeling of Low Concentrated Buffer DNA Detection with Suspend Gate Field-Effect Transistors (SGFET)

The experimental data of a suspend gate field-effect transistor (SGFET) have been analyzed with three different models. A SGFET is a MOSFET with an elevated gate and an empty space below it. The exposed gate-oxide layer is biofunc- tionalized with single stranded DNA, which is able to hybridize with a complementary strand. Due to the intrinsic charge of the phosphate groups (minus one elementary charge per group) of the DNA, large shifts in the transfer characteristics are induced. Thus label-free, time-resolved, and in-situ detection of DNA is possible. It can be shown that for buffer concentrations below mmol/l the Poisson-Boltzmann description it is not valid any- more. Because of the low number of counter ions at small buffer concentrations, the screening of the oligo-deoxynucleotides/DNA is more appropriately described with the Debye-H ¨ uckel model. Additionally we propose an extended Poisson-Boltzmann model which takes the closest possible ion distance to the oxide surface into account, and we compare the analytical soultion of this model with the Poisson-Boltzmann and the Debye-H ¨ uckel model.

Stochastic modeling of the resistive switching mechanism in oxide-based memory

We have investigated a stochastic model of the resistive switching mechanism in resistive random access memory (RRAM) based on electron hopping. The distribution of electron occupation probabilities analyzed with our approach is in good agreement with previous work. In particular, a low occupation region is formed near the cathode for bipolar behavior or near the anode for unipolar behavior. Our simulation of the temperature dependence of the electron occupation probability near the anode and the cathode shows an amazing stability of the low occupation region. This result indicates high robustness of failure-free RRAM switching from a state with low resistance to a state with high resistance for elevated temperature.

Frequency dependence study of a bias field-free nano-scale oscillator

Oscillators belong to the group of fundamental building blocks and are ubiquitous in modern electronics. Especially spin torque nano oscillators are very attractive as cost effective on-chip integrated microwave oscillators, due to their nano-scale size, frequency tunability, broad temperature operation range, and CMOS technology compatibility. Recently, we proposed a micromagnetic structure capable of operating as non-volatile flip flop as well as a spin torque nano oscillator. The structure consists of three anti-ferromagnetically coupled stacks (two for excitation A, B and one for readout Q) and a shared free magnetic layer. Micromagnetic simulations show a current regime, where the structure exhibits large, stable, and tunable in-plane oscillations in the GHz range without the need of an external magnetic field or an oscillating current. In this work the dependence of these oscillations on the shared free layer geometry at a fixed input current is studied. It is shown that the precessional frequency can be controlled by the dimensions of the shared free layer. Most efficient is to utilize the layer thickness to control the precessional frequency, but also changing the layer length can be exploited.

Electron Momentum and Spin Relaxation in Silicon Films

Semiconductor spintronics is promising, because it allows creating microelectronic elements which are smaller and consume less energy than present charge-based devices. Silicon is the main element of modern charge-based electronics, thus, understanding the peculiarities of spin propagation in silicon is the key for designing novel devices. We investigate the electron momentum and the spin relaxation in thin (001) oriented SOI films using a k ⋅ p-based approach with spin degree of freedom properly included. We demonstrate that shear strain routinely used to enhance the electron mobility can boost the spin lifetime by an order of magnitude.

Concept of a Bias-Field-Free Spin-Torque Oscillator Based on Two MgO-MTJs

We propose a novel spin-torque oscillator based on two MgO-MTJs with a shared free layer. By performing extensive micromagnetic modeling we found that the structure exhibits a wide tunability of oscillation frequencies from a few GHz to several ten GHz.