Zlatan Stanojevic

Exploring the design space of non-planar channels: Shape, orientation, and strain

We conduct a comprehensive simulation study of non-planar n-type channels based on consistent, physical models containing measurable quantities rather than fit-parameters. This contrasts empirical thin-body models used in classical/quantum-corrected TCAD. The method involves the self-consistent solution of the two-dimensional Schrödinger-Poisson system, combined with linearized Boltzmann transport in the third dimension. We advance the art of simulation by (i) introducing quantum simulation on unstructured meshes for arbitrary geometries, (ii) providing an efficient framework for rapid evaluation of device designs, and (iii) contributing a surface roughness scattering model for arbitrarily shaped surfaces. Consistent modeling allows us to make reliable assertions with respect to device performance.

On the validity of momentum relaxation time in low-dimensional carrier gases

The momentum relaxation time (MRT) is widely used to simplify low-field mobility calculations including anisotropic scattering processes. Although not always fully justified, it has been very practical in simulating transport in bulk and in low-dimensional carrier gases alike. We review the assumptions behind the MRT, quantify the error introduced by its usage for low-dimensional carrier gases, and point out its weakness in accounting for inter-subband interaction, occurring specifically at low inversion densities.

Efficient simulation of quantum cascade lasers using the Pauli master equation

A transport model for quantum cascade lasers based on the Pauli master equation is presented. An efficient Monte Carlo solver has been developed. The numerical methods to reduce the computational cost are discussed in detail. Finally, the simulator is used to obtain current-voltage characteristics as well as microscopic quantities of a mid infrared QCL structure.

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.

Vertically stacked nanowire MOSFETs for sub-10nm nodes: Advanced topography, device, variability, and reliability simulations

Using an advanced simulation framework we analyze a recent sub-10 nm technology demonstration based on stacked nanowire transistors (NW-FETs). The study encompasses (i) topography simulation which realistically reproduces the fabricated device, (ii) device simulation based on the subband Boltzmann transport equation (iii) a comprehensive set of scattering models for the gate stack, (iv) physical models for time-zero variability and BTI device degradation. We find that (i) the fabrication process introduces parasitic capacitances not present in a comparable FinFET, (ii) the device performance is significantly affected by interface-charge-induced Coulomb scattering resulting in up to 50% reduction in drain current compared to an ideal device, (iii) device time-zero variability is increased due to a lower amount of dopant atoms per device, (iv) the device is more affected by BTI than a comparable FinFET. Using physics-based TCAD for technology path-finding and device optimization, we are able to point out critical improvements required for the stacked NW-FET to surpass current FinFET technology.

A versatile finite volume simulator for the analysis of electronic properties of nanostructures

We present a novel semantic approach to modeling and simulation of nanoelectronic devices. The approach is based on a finite volume spatial discretization scheme. The scheme was adapted to accurately treat material anisotropy. It is thus capable of capturing orientation and strain effects both of which are prominent in the nanoscale regime. We also demonstrate the methods simplicity and power with a three-dimensional simulation study of a quantum dot using a six band k · p Hamiltonian for holes as model.

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.

Transport properties of spin field-effect transistors built on Si and InAs

We investigate the properties of ballistic spin field-effect transistors (SpinFETs). First we show that the amplitude of the tunneling magnetoresistance oscillations decreases dramatically with increasing temperature in SpinFETs with the semiconductor channel made of InAs. We also demonstrate that the [100] orientation of the silicon fin is preferred for practical realizations of silicon SpinFETs due to stronger modulation of the conductance as a function of spin-orbit interaction and magnetic field.

Phase-space solution of the subband Boltzmann transport equation for nano-scale TCAD

We present a comprehensive simulation framework for transport modeling in nano-scaled devices based on the solution of the subband Boltzmann transport equation (BTE). The BTE is solved in phase space using a k·p-based electronic structure model and includes all relevant scattering processes. The BTE solver is combined with a conventional drift-diffusion-based simulator using a novel iteration approach. The pairing between BTE, DD, and Poisson results in a flexible toolkit which converges quickly in any mode of operation, allows large-scale parallelization, and to include near-equilibrium transport outside the BTE region, i.e. the contacting regions. The toolkit is commercially available as part of the GTS Nano Device Simulator (NDS). We examine realistic NMOS and PMOS devices, including transport at the microscopic scale and possible numerical approximations.

Experimental and Simulation Results of Magnetic Modulation of Gate Oxide Tunneling Current in Nanoscaled MOS Transistors

An experimental-simulation methodology to explore the spatially nonhomogeneous properties of the tunneling current in nanoscaled MOSFET is introduced. The magnetic field B is introduced into the Schrödinger-Poisson system, which allows simulating the effect of the B field on the gate oxide tunneling current and be compared with experimental data. We found out that sweeping the B field from negative to positive values is equivalent to scan or map the tunneling mechanism along the channel from source to drain. The proposed methodology is useful for studying nonhomogeneous space distributed conductive properties, and it was validated with a 28-nm n-type Si MOSFET.