Mehmet O. Baykan

Strain effects on three-dimensional, two-dimensional, and one-dimensional silicon logic devices: Predicting the future of strained silicon

Using a classification scheme based on carrier confinement type (electrostatic and spatial) and the degrees of freedom of the mobile carriers (3DOF, 2DOF, and 1DOF), strain effects on 3DOF to 1DOF silicon logic devices are compared from quantum confinement and device geometry perspectives. For these varied device geometries and types, the effects of strain-induced band splitting and band warping on the modification of the average conductivity effective mass and carrier scattering rates are evaluated. It is shown that the beneficial effects of strain-induced band splitting are the most effective for devices with little or no initial band splitting and become less so for devices with already large built-in band splitting. For these devices with large splitting energy, the potential for strain-induced carrier conductivity mass reduction through repopulation of lower energy bands and the suppression of optical intervalley phonon scattering are limited. On the other hand, for all devices without spatial confin...

Strained SiGe and Si FinFETs for high performance logic with SiGe/Si stack on SOI

In this work, we report high performance (I<inf>on</inf> ∼1 mA/µm at Ioff 100nA/µm @ 1V Vcc) short channel p-type SiGe/Si FinFETs combining high mobility, low T<inf>inv</inf> (scaled High-k w/o Si cap), low R<inf>sd</inf>, and process-induced strain. A dual channel scheme for high mobility CMOS FinFETs is demonstrated.

Impact of Fin Doping and Gate Stack on FinFET (110) and (100) Electron and Hole Mobilities

Double-gate FinFET (110) (110) and (100) (100} electron mobility (μ_e) and hole mobility (μ_h) are experimentally investigated for the following: 1) a wide range of boron and phosphorus fin doping concentrations and 2) a wide variety of gate stacks combining HfO₂, SiO₂, or SiON insulators with TiN or poly-Si electrodes. It is found out that, irrespective of fin doping and gate stack, (110) (110) μ_e is competitive with the (100)(100) μ_e, while (110)(110) μ_h is ≥ 2× higher than (100) (100) μ_h. Inversion μ_e and μ_h are independent of doping as long as the effective field/doping combination enables the screening of the depletion charge. Mobility degradation with doping is significantly lower in accumulation mode (AM) than in inversion mode (IM) such that, for heavily B-doped fins, AM hole mobility exceeds the IM electron mobility even in (100) FinFETs. In undoped fins, ALD TiN gate stress is observed to improve μ_e for both orientations without degrading μ_h.

Performance and variability in multi-V T FinFETs using fin doping

The impact of fin doping (B, P, As) on FinFET device parameters is studied for high-K/midgap metal gate SOI FinFETs. For a fin width of ~25 nm, >;1 V VT modulation is demonstrated from accumulation mode (AM) to inversion mode (IM). IM FinFETs improve short channel FinFET electrostatics, on-off ratio, and VT variability compared to their undoped counterparts. The same parameters degrade in accumulation mode FinFETs. A VT modulation of ±0.25 V using fin B and P doping comes at the expense of 24% and 14% high field mobility penalty for NFET and PFET, respectively. For the same dose, Arsenic is found to modulate the VT more effectively than does Phosphorus. Basic modeling results show that for aggressively scaled (5 nm-wide) fins, the impact of single dopant atom on VT can be as high as 25 mV, severely challenging the viability of the technique towards the end of roadmap.

Strain Effects in AlGaN/GaN HEMTs

Since stress is a major factor in the operation, performance, and reliability in AlGaN/GaN HEMT devices, a thorough understanding of the impact of stress on performance and reliability can lead to improvements in device design. Mechanical wafer bending is a cost-effective method to investigate the effects of stress on semiconductor devices which has been extensively used to isolate and study the effect of stress in strain-engineered Si MOSFETs. In this chapter, a systematic study of the effects of externally applied mechanical stress on the AlGaN/GaN HEMT channel resistance and gate current is presented to provide insights into the physical mechanisms responsible for stress-related performance and reliability issues.

Physical insights on comparable electron transport in (100) and (110) double-gate fin field-effect transistors

We have investigated the physical mechanisms that result in comparable electron mobility measured from (100) and (110) sidewall double-gate fin field-effect transistors (FinFETs). Using a self-consistent Schrodinger-Poisson simulator coupled with a sp3d5s* tight-binding bandstructure, we have shown that the (100)/〈100〉 and (110)/〈110〉 average conductivity effective mass values are similar. This is explained by the much heavier non-parabolic confinement mass for Δ2 valley of (110) FinFETs, which leads to lower Δ2 energy than Δ4. Thus, for both (100) and (110), the majority of electrons occupy the Δ2 valley with 0.19m0 conductivity effective mass, resulting in comparable electron mobility.

Understanding the FinFET Mobility by Systematic Experiments

The impact of the surface orientation, strain, fin doping, and gate stack on SOI double-gate FinFET mobility is systematically investigated. Impact of channel material, temperature, and fin width were also touched upon to better understand the trends. For the unstrained case, the (110) sidewall electron mobility is very close to the (100) sidewall electron mobility irrespective of the fin doping level and gate stack. This weak dependence of electron mobility to surface orientation distinguishes the FinFETs from the bulk planar MOSFETs, where (100) electron mobility is systematically reported to be much higher than that of (110). On the other hand, the (110) sidewall hole mobility is substantially higher than the (100) sidewall hole mobility in FinFETs, as in the planar case. Both the (100)/ and (110)/ FinFET electron mobility can be improved with tensile strain. It is also confirmed that the (110)/ FinFET hole mobility can be significantly improved with compressive strain while the (100)/ hole mobility is sensitive to neither compressive nor tensile strain. Compared to Si, the use of a SiGe channel increases the hole mobility drastically, and even further improvement is achievable by external compressive stress. Overall, the experimental results in this chapter suggest that the (110)/ Si FinFETs conventionally built on standard (100) wafers offer simultaneously high electron and hole mobility, which can be further improved by tensile and compressive stress, respectively.

Size- and Orientation-Dependent Strain Effects on Ballistic Si p-Type Nanowire Field-Effect Transistors

The size- and orientation-dependent uniaxial strain effects on ballistic hole transport in nanowire field-effect transistors are investigated using an sp3d5s*-based tight-binding formalism coupled with a compact electrostatics model and a semiclassical transport model. It is found that the strain-induced reduction of the valence band density of states leads to an increased ballistic hole current. This is explained by the product of a small reduction in hole density and a significant increase in the average ballistic hole velocity under uniaxial compression. While uniaxial compressive strain is beneficial for both 〈110〉 and 〈100〉 devices, the strain response of 〈110〉 nanowires is much larger than their 〈100〉 counterparts. Ultrascaled 〈110〉 nanowires have the highest hole drive current under both strained and unstrained conditions, despite the reduction of strain-induced ballistic hole current enhancement for narrower devices.