Dmitry Osintsev

Predictive Hot-Carrier Modeling of n-Channel MOSFETs

We present a physics-based hot-carrier degradation (HCD) model and validate it against measurement data on SiON n-channel MOSFETs of various channel lengths, from ultrascaled to long-channel transistors. The HCD model is capable of representing HCD in all these transistors stressed under different conditions using a unique set of model parameters. The degradation is modeled as a dissociation of Si-H bonds induced by two competing processes. It can be triggered by solitary highly energetical charge carriers or by excitation of multiple vibrational modes of the bond. In addition, we show that the influence of electron-electron scattering (EES), the dipole-field interaction, and the dispersion of the Si-H bond energy are crucial for understanding and modeling HCD. All model ingredients are considered on the basis of a deterministic Boltzmann transport equation solver, which serves as the transport kernel of a physics-based HCD model. Using this model, we analyze the role of each ingredient and show that EES may only be neglected in long-channel transistors, but is essential in ultrascaled devices.

Fast Switching in Magnetic Tunnel Junctions With Two Pinned Layers: Micromagnetic Modeling

Spin transfer torque random access memory is one of the promising candidates for future universal memory. The reduction of the current density required for switching and the increase of the switching speed are the most important challenges in this area. In this paper, a penta-layer structure with two pinned magnetic layers is studied by means of extensive micromagnetic calculations. By numerically investigating the dynamics of the switching process, a methodology of how to achieve fast and symmetric switching without a compensating magnetic field is presented. Our simulations also highlight the importance of the field acting perpendicular to the plane, which facilitates switching.

Reduction of momentum and spin relaxation rate in strained thin silicon films

We investigate the surface roughness and phonon induced spin and momentum relaxation in ultra-scaled SOI MOSFETs. We show that the spin-flip hot spots characterized by strong spin relaxation can be efficiently removed by applying shear strain resulting in an increase of spin lifetime by orders of magnitude. In contrast, the momentum relaxation time in ultrathin films, which is mostly determined by surface roughness scattering can be only increased by a factor of two, in agreement with strain-induced mobility enhancement data.

Fast Switching in Magnetic Tunnel Junctions with Double Barrier Layer

Memory cells based on electric charge storage, such as flash memory, are rapidly approaching the physical limits of scalability. The spin transfer torque random access memory (STTRAM) is one of the promising candidates for future universal memory [1-6]. The reduction of the current density required for switching and the increase of the switching speed are among the most important challenges in this area. Measurements performed in [4] showed a decrease in the critical current density for the penta-layer magnetic tunnel junction (MTJ) compared with the tri-layer MTJ. To achieve symmetric switching in asymmetric MTJs an external in-plane compensating magnetic field has to be introduced [4]. By numerically investigating the dynamics of the switching process in a MTJ composed of five layers we present the methodology on how to achieve symmetric switching without an external magnetic field by properly engineering the nanopillar geometry. 2. Model Description Our micromagnetic simulations are based on the magnetization dynamics described by the Landau-Lifschitz-Gilbert equation:

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.

Dependence of spin lifetime on spin injection orientation in strained silicon films

The electron spin properties of semiconductors are of huge interest because of their potential for future spin-driven microelectronic devices. Modern charge-based electronics is dominated by silicon, and understanding the details of spin propagation in silicon structures is key for novel spin-based device applications. The peculiarities of the subband structure and details of the spin propagation in surface layers and thin silicon films in the presence of the spin-orbit interaction is under research. We investigate the influence of the spin injection direction on the spin relaxation. Beginning with the four-component wave functions, we show that the surface roughness induced spin intersubband relaxation matrix elements get reduced for an in-plane spin injection compared to perpendicular-plane spin injection, henceforth the corresponding spin relaxation rate (time) is diminished (enhanced). In order to explain this observation we point out that at the spin relaxation hot spots the perpendicular-plane spin injection results in a maximal spin randomization at any in-plane momentum, which increases the spin relaxation rate and decreases the spin lifetime as compared to an in-plane spin injection.

Acoustic Phonon and Surface Roughness Spin Relaxation Mechanisms in Strained Ultra-Scaled Silicon Films

We consider the impact of the surface roughness and phonon induced relaxation on transport and spin characteristics in ultra-thin SOI MOSFET devices. We show that the regions in the momentum space, which are responsible for strong spin relaxation, can be efficiently removed by applying uniaxial strain. The spin lifetime in strained films can be improved by orders of magnitude, while the momentum relaxation time determining the electron mobility can only be increased by a factor of two.

Enhancement of Electron Spin Relaxation Time in Thin SOI Films by Spin Injection Orientation and Uniaxial Stress

The electron spin properties of semiconductors are of immense interest for their potential in spin-driven applications. Silicon is a perfect material for spintronics due to a long spin lifetime. Understanding the peculiarities of the subband structure and details of spin propagation in thin silicon films in the presence of the spin-orbit interaction is under scrutiny. We have performed simulations to obtain the surface roughness limited, acoustic-and optical-phonon mediated spin relaxation time, when the film is under shear strain. The degeneracy between the non-equivalent valleys is lifted by strain, which in turn subdues the dominating inter-valley relaxation components and increases the spin lifetime. We also elaborate on the injection orientation sensitive spin relaxation model and predict that the spin relaxation time is maximum, when the spin is injected in-plane, relative to the (001) oriented silicon film.

Injection direction sensitive spin lifetime model in a strained thin silicon film

The electron spin properties are promising for future spin-driven applications. Silicon, the major material of microelectronics, also appears to be a perfect material for spintronic applications. The peculiarities of the subband structure and details of the spin propagation in ultra-thin silicon films in presence of the spin-orbit interaction and strain are investigated. The application of shear strain dramatically reduces the spin relaxation in such films. We investigate in detail, how spin injection in any arbitrary direction modifies the spin relaxation matrix elements, and finally the spin lifetime in the samples. We demonstrate a two-fold enhancement of spin lifetime, when spin is injected in-plane of the sample, compared to that, when injected along the perpendicular-plane direction.

Evaluation of Spin Lifetime in Thin-Body FETs: A High Performance Computing Approach

Silicon, the prominent material of microelectronics, is perfectly suited for spin-driven applications because of the weak spin-orbit interaction resulting in long spin lifetime. However, additional spin relaxation on rough interfaces and acoustic phonons may strongly decrease the spin lifetime in modern silicon-on-insulator and trigate transistors. Because of the need to perform numerical calculation and appropriate averaging of the strongly scattering momenta depending spin relaxation rates, an evaluation of the spin lifetime in thin silicon films becomes prohibitively computationally expensive. We use a highly parallelized approach to calculate the spin lifetime in silicon films. Our scheme is based on a hybrid parallelization approach, using the message passing interface MPI and OpenMP. The algorithm precalculates wave functions and energies, and temporarily stores the results in a file-based cache to reduce memory consumption. Using the precalculated data for the spin relaxation rate calculations drastically reduces the demand on computational time. We show that our approach offers an excellent parallel speedup, and we demonstrate that the spin lifetime in strained silicon films is enhanced by several orders of magnitude.