V. Aubry-Fortuna

On the Ability of the Particle Monte Carlo Technique to Include Quantum Effects in Nano-MOSFET Simulation

In this paper, we report on the possibility of using particle-based Monte Carlo (MC) techniques to incorporate all relevant quantum effects in the simulation of semiconductor nanotransistors. Starting from the conventional MC approach within the semiclassical Boltzmann approximation, we develop a multisubband description of transport to include quantization in ultrathin-body devices. This technique is then extended to the particle simulation of quantum transport within the Wigner formulation. This new simulator includes all expected quantum effects in nanotransistors and all relevant scattering mechanisms, which are taken into account the same way as in Boltzmann simulation. This paper is illustrated by analyzing the device operation and performance of multigate nanotransistors in a convenient range of channel lengths and thicknesses to separate the influence of all relevant effects: Significant quantization effects occur for thickness smaller than 5 nm and wave-mechanical-transport effects manifest themselves for channel length smaller than 10 nm. We also show that scattering mechanisms still have an important influence in nanoscaled double-gate transistors, both in the intrinsic part of the channel and in the resistive lateral extensions.

Band offset predictions for strained group IV alloys: Si1-x-yGexCy on Si(001) and Si1-xGex on Si1-zGez(001)

The band offsets for strained Si1-x-yGexCy layers grown on Si(001) substrate and for strained Si1-xGex layers grown on fully relaxed Si1-zGez virtual substrates are estimated. The hydrostatic strain, the uniaxial strain and the intrinsic chemical effect of Ge and C are considered separately. Unknown material parameters relative to the latter effect are chosen to give the best agreement with the available experimental results for Si1-xGex and Si1-yCy layers on Si. As a general trend concerning carrier confinement opportunities, it is found that a compressive strain is required to obtain a sizeable valence band offset, while a tensile strain is needed to obtain a conduction band discontinuity. In most cases the strain is responsible for a bandgap narrowing with respect to that of the substrate. The obtained results are in very good agreement with available experimental determinations of band offsets and bandgap changes for ternary alloys on Si(001).

Effect of the Ion Mass and Energy on the Response of 70-nm SOI Transistors to the Ion Deposited Charge by Direct Ionization

The response of SOI transistors under heavy ion irradiation is analyzed using Geant4 and Synopsys Sentaurus device simulations. The ion mass and energy have a significant impact on the radial ionization profile of the ion deposited charge. For example, for an identical LET, the higher the ion energy per nucleon, the wider the radial ionization track. For a 70-nm SOI technology, the track radius of high energy ions (> 10 MeV/a) is larger than the transistor sensitive volume; part of the ion charge recombines in the highly doped source or drain regions and does not participate to the transistor electric response. At lower energy (<; 10 MeV/a), as often used for ground testing, the track radius is smaller than the transistor sensitive volume, and the entire charge is used for the transistor response. The collected charge is then higher, corresponding to a worst-case response of the transistor. Implications for the hardness assurance of highly-scaled generations are discussed.

Ultra-short n-MOSFETs with strained Si : device performance and the effect of ballistic transport using Monte Carlo simulation

Semi-classical Monte Carlo simulation is used to study the electrical performance of 18-nm-long n-MOSFETs including a strained Si channel. In particular, the impact of extrinsic series resistance on the drive current Ion is quantified: we show that the large on-current improvement induced by the strain is preserved, even by including an external parasitic resistance. The importance of ballistic transport is also examined and its influence on Ion is highlighted.

Electron transport properties in high-purity Ge down to cryogenic temperatures

Electron transport in Ge at various temperatures down to 20 mK has been investigated using particle Monte Carlo simulation taking into account ionized impurity and inelastic phonon scattering. The simulations account for the essential features of electron transport at cryogenic temperature: Ohmic regime, anisotropy of the drift velocity relative to the direction of the electric field, as well as a negative differential mobility phenomenon along the ⟨111⟩ field orientation. Experimental data for the electron velocities are reproduced with a satisfactory accuracy. Examples of electron position in the real space during the simulations are given and evidence separated clouds of electrons propagating along different directions depending on the valley they belong to.

Structural properties and stability of Zr and Ti germanosilicides formed by rapid thermal annealing

Because of their good ohmic and rectifying properties, silicides are routinely used in Si technology. This approach has been recently extended to the novel devices produced using Si1−xGex alloys. Here, we study the Zr and Ti germanosilicides produced in the low thermal budget contact formation during Si/Si1−xGex heterodevice processing. Phase formation was monitored by combining a range of spectrometries with electron microscopy and x-ray diffraction techniques, while sheet resistance measurements allowed correlation of phase formation with film conductance. After completion of the reaction, the final crystalline phase was either C49–Zr(Si1−yGey)2 in the entire Ge composition (x) range, or C54–Ti(Si1−yGey)2 in the Ge composition range 0–0.47. In the Zr–Si–Ge system, the C49–Zr(Si1−yGey)2 formation temperature (Tf) decreases as x increases, and films formed at this temperature are continuous. Excess heating (above Tf) produces islanded films with embedded grains. A most significant feature of the results w...

Phase formation and strain relaxation during thermal reaction of Zr and Ti with strained Si1−x−yGexCy epilayers

Silicides are often used in Si technology for both their ohmic and rectifying properties. In this work, we have compared Zr and Ti germanosilicides as possible metallic contacts on SiGeC alloys in terms of phase formation and stability of the unreacted SiGeC alloy. The germanosilicides are obtained after rapid thermal annealings of Zr or Ti with strained SiGeC layers. The interactions of the metal films with these alloys have been investigated by sheet resistance measurements, x-ray diffraction (XRD), cross-sectional transmission electron microscopy (TEM), and energy dispersive spectroscopy in situ in the TEM. Four crystal x-ray diffraction was performed to measure the residual strain of the unreacted SiGeC epilayer after reaction. The analyses indicate that the final compounds are the C49–Zr(SiGe)2 and C54–Ti(SiGe)2 phases, respectively: In both cases, the compound is formed by monocrystalline grains with various orientations. Nevertheless, neither XRD, nor sheet resistance measurements give any clear in...

Comparison of Monte Carlo Transport Models for Nanometer-Size MOSFETs

This paper presents the results of a comparison among five Monte Carlo device simulators for nano-scale MOSFETs. These models are applied to the simulation of the I–V characteristics of a 25 nm gate-length MOSFET representative of the high-performance transistor of the 65 nm technology node. Appreciable differences between the simulators are obtained in terms of simulated ION. These differences are mainly related to different treatments of the ionized impurity scattering (IIS) and pinpoint a limitation of the available models for screening effects at very large carrier concentrations.

Fermi level position at metal Si1−x−yGexCy interfaces

In this work, we have investigated the Schottky barrier heights on n- and p-type Si1−x−yGexCy alloys with Zr, Ti, W, Ni and Pt as metals (ΦBn and ΦBp, respectively). Contacts on Si1−xGex alloys showed various behaviors depending on the metal work function Φm. For low-Φm metals (Zr, Ti), ΦBn increases with x, while ΦBp(x) decreases. For higher Φm metals (Pt), ΦBn strongly decreases with x. In the particular case of W (intermediate Φm value), ΦBp follows exactly the decrease of the SiGe band gap with x, while ΦBn remains constant. Nevertheless, whatever the metal, the reduction of the sum ΦBn+ΦBp gives the band-gap variation as a function of x, and the Fermi level is located at the same position for both n and p-type layers. A weaker effect of Φm on the Schottky barrier heights is observed compared to pure Si: the position of the Fermi level tends to remain in the range 0.60–0.65 eV below the conduction band, as soon as Ge is adding in Si. W contacts on Si1−x−yGexCy alloys evidenced the strong effect of C o...

A metastable (√3 × √3)R30° reconstruction of the Si(111) surface, induced by silicon adatoms

Abstract Reflection high-energy electron diffraction (RHEED) studies of Si(111) growth using silane reveal that the growing surface exhibits two equilibrium structures as a function of growth temperature: a 1 × 1:H structure for temperatures below 600°C, and (7 × 7) for temperatures higher than 600°C. Starting from the (1 × 1):H surface, two structural pathways are identified upon H desorption when silane flux is interrupted. In the growth temperature range 570–600°C, the (1 × 1):H surface transforms directly to the (7 × 7) surface. At lower growth temperatures, a metastable √3 × √3)R30° structure is observed as an intermediate step during the transformation from the (1 × 1) to the (7 × 7) surface. The formation of the (√3 × √3)R30° structure can be explained by the redistribution of excess Si adatoms present on the surface during growth.