Strained Silicon Inversion Layers Research Articles

A Model for Neutral Defect Limited Electron Mobility in Strained-Silicon Inversion Layers

On the strained silicon metal–oxide–semiconductor field-effect transistors (MOSFETs), we show how to derive a formalism dealing with the scattering of a 2-D electron by a neutral defect. The corresponding neutral defect limited inversion-layer electron mobility, $\mu _{n}$ , is calculated in the momentum relaxation time approximation. The calculated results lead to a new analytical model: $\mu _{n} =cN_{n} ^{-1}$ , where $N_{n}$ is the neutral defect density per unit area and ${c}$ is the coefficient independent of the inversion-layer density, the strain, and the temperature. The validity and applicability of the model are confirmed by citing three independent experiments on strained silicon MOSFETs undergoing different implantation sources and different annealing budgets. Importantly, this paper clarifies for the first time that strain will not change neutral defect limited mobility unless changing the neutral defect density. This reasonably explains the two experimental observations during implantation and annealing: 1) the implantation-induced strain relaxation in strained sample does not occur and 2) the neutral defect density is much higher in strained sample than in unstrained sample.

Physical insight into reduced surface roughness scattering in strained silicon inversion layers

A seemingly anomalous enhancement of electron mobility in strained silicon inversion layers at high sheet densities has exposed a conspicuous gap between device physics theory and experiment in recent years. We show that the root of this discrepancy is due to a bulging effect in the electron \Delta 4 wavefunction at the silicon surface. This renders \Delta 4 electrons more susceptible to perturbations in surface structure thereby increasing surface roughness scattering for these states. Strain engineering utilized by the CMOS industry reduces the relative occupancy of the \Delta 4 states resulting in less overall surface roughness scattering in the channel. We show that the origin of this effect can be explained by moving beyond the effective mass approximation and contrasting the properties of the \Delta 2 and \Delta 4 wavefunctions in a representation that comprehends full crystal and Bloch state symmetry.

Effect of strain on the electron effective mobility in biaxially strained silicon inversion layers: An experimental and theoretical analysis via atomic force microscopy measurements and Kubo-Greenwood mobility calculations

Recently, in order to explain the long-channel electron effective mobility at a high sheet carrier density in strained silicon channel transistors, it has been suggested by [M. V. Fischetti, F. Gamiz, and W. Hansch, J. Appl. Phys. 92, 7230 (2002)] that biaxial tensile strain should smooth the Si/SiO2 interface. To address this topic, the roughness properties of biaxial strained silicon-on-insulator (s-SOI) films are investigated by means of atomic force microscopy. Through in-depth statistical analysis of the digitalized surface profiles, the roughness parameters are extracted for unstrained and strained SOI films, with 0.8% biaxial tensile strain. Especially, it is found that strain significantly reduces the roughness amplitude. Then, mobility calculations in SOI and s-SOI inversion layers are performed in the framework of the Kubo-Greenwood formalism. The model accounts for the main scattering mechanisms that are dominant in the high electron density range, namely phonon and surface roughness. Special attention has been paid to the modeling of the latter by accounting for all the contributions of the potential which arise from the deformed rough interface, and by using a multisubband wavelength-dependent screening model. This model is then applied to study the influence of the surface morphology on the mobility in s-SOI inversion layers. In this context, the mobility gain between s-SOI and unstrained SOI layers is found to agree significantly better with experimental data if the strain-induced decrease of the roughness amplitude is taken into account.

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Laudatio for Gerhard Abstreiter

Inherent point defects in thermal biaxially tensile strained-(1 0 0)Si/SiO 2 probed by electron spin resonance

Electron spin resonance studies are reported on (1 0 0)Si/SiO 2 entities grown by thermal oxidation of biaxial tensile strained-(1 0 0)Si layers epitaxially grown on relaxed virtual substrates, with main focus on P b-type interface defects, in particularly the electrically detrimental P b0 variant. In the as-grown state a significant decrease (>50%) in interface defect density compared to the standard (1 0 0)Si/SiO 2 interface was observed. As compared to the latter, this inherent decrease in electrically active interface trap density establishes strained Si/SiO 2 as a superior device entity for all electrical properties in which (near) interface traps may play a detrimental role. For one, it may be an additional reason for the commonly reported mobility enhancement in strained silicon inversion layers and the reduction in 1/ f noise. The data also confirm the admitted relationship between inherent incorporation of the P b related interface defects and the Si/SiO 2 interface mismatch.

Monte Carlo study of electron transport in strained silicon inversion layers

The effect of degeneracy both on the phonon-limited mobility and the effective mobility including surface-roughness scattering in unstrained and biaxially tensile strained Si inversion layers is analyzed. We introduce a new method for the inclusion of the Pauli principle in a Monte Carlo algorithm. We show that incidentally degeneracy has a minor effect on the bulk effective mobility, despite non-degenerate statistics yields unphysical subband populations and an underestimation of the mean electron energy. The effective mobility of strained inversion layers slightly increases at high inversion layer concentrations when taking into account degenerate statistics.

On the enhanced electron mobility in strained-silicon inversion layers

The recently reported large enhancement of the electron mobility in strained-Si inversion layers at large carrier concentrations cannot be easily explained: The strong carrier confinement in inversion layers removes the sixfold degeneracy of the conduction-band minima, much as tensile in-plane strain does, so that the effect of strain should become irrelevant at large sheet carrier densities. The problem is studied by calculating the electron mobility accounting for scattering with phonons and interface roughness. Surprisingly, the latter process is found to be significantly stronger in strained layers for a given interface roughness. Only the ad hoc assumption of increasingly smoother interfaces with increasing strain seems to explain the data.

Strained-Si/SiGe-on-insulator inversion layers: The role of strained-Si layer thickness on electron mobility

We show by Monte Carlo simulation that electron mobility is greater when strained-silicon inversion layers are grown on SiGe-on-insulator substrates than when unstrained-silicon-on-insulator devices are employed (as experimentally observed). However, the electron mobility in strained-Si/SiGe-on-insulator inversion layers is strongly dependent on the strained-silicon layer thickness, TSi, due to an increase of the phonon scattering, which partially counteracts the increase in the mobility achieved by the strain. This effect is less important as the germanium mole fraction, x, is reduced, and as the value of TSi increases.

Strained Si/SiGe Heterostructures at Low Temperatures. A Monte Carlo Study

Monte Carlo simulations are used to study the transport properties of electrons in strained silicon inversion layers in Si/SiGe MOSFETs at low temperature. The strain produces an enhancement of electron mobility due to the reduction of intervalley scattering and a lower effective conduction mass as a consequence of the valley splitting. In this work, mobility curves for different Ge concentrations are obtained by Monte Carlo simulation at both low and room temperatures. It has been observed that at room temperature mobility enhancement saturates at a Ge concentration close to x=0.3, while the mole fraction of Ge at which the electron mobility curves saturate is much lower at low temperature. In addition, the relative enhancement at low temperatures is much less than at room temperature. Finally, mobility curves show a tendency to coincide at high transverse electric fields, regardless of the Ge concentration, as a consequence of the greater concentration of electrons in the ground subband.