Pouya Hashemi

Gate-All-Around n-MOSFETs With Uniaxial Tensile Strain-Induced Performance Enhancement Scalable to Sub-10-nm Nanowire Diameter

The effects of high-level uniaxial tensile strain on the performance of gate-all-around (GAA) Si n-MOSFETs are investigated for nanowire (NW) diameters down to 8 nm. Suspended strained-Si NWs with ~2-GPa uniaxial tension were realized by nanopatterning-induced unilateral relaxation of ultrathin-body 30% strained-Si-directly-on-insulator substrates. Based on these NWs, GAA strained-Si n-MOSFETs were fabricated with a Si thickness of ~8 nm and NW widths in the range of 50 nm down to 8 nm. The GAA strained-Si MOSFETs show excellent subthreshold swing and cutoff behavior, and approximately two times current drive and intrinsic transconductance enhancement compared to similar unstrained Si devices.

Thin Film a-Si/c-Si1-xGex/c-Si Heterojunction Solar Cells: Design and Material Quality Requirements

a-Si:H/crystalline-Si 1-x Ge x /c-Si heterojunction solar cells (HIT cells) are simulated and fabricated for the first time. Cells with junction layers consisting of Si, Si 0.75 Ge 0.25 , and Si 0.59 Ge 0.41 are compared to study the effect of increasing Ge concentration. The results show a V oc drop from 0.6V for Si cells to 0.4V for Si 0.59 Ge 0.41 , consistent with the reduction in bandgap. The measured J sc increases from ~18.5 mA/cm 2 for Si cells to 20.3 mA/cm 2 for the Si 0.59 Ge 0.41 cells, for one light pass. Simulations suggest that the measured J sc for the Si 0.59 Ge 0.41 based solar cells is limited by a low lifetime. In order for Si 1-x Ge x based cells to exceed the efficiency of Si, simulations indicate that Ge percentages larger than 40% and lifetimes above 1 ∝s are required.

Electron transport in Gate-All-Around uniaxial tensile strained-Si nanowire n-MOSFETs

The intrinsic performance and electron effective mobility of uniaxially strained-Si gate-all-around (GAA) NanoWire (NW) n-MOSFETs are investigated, for the first time. Suspended strained-Si NWs show very high stress (up to ~2.1 GPA) as confirmed by Raman, with no bending of the wires. GAA strained-Si NW n-MOSFETs exhibit excellent subthreshold swing, and current drive and transconductance enhancement of ~2X over unstrained Si control NW devices. The mobility enhancement of these devices over unstrained planar and GAA MOSFETs as well as their scalability to circular NWs with radius of ~4 nm are also demonstrated.

Enhanced Hole Mobility in High Ge Content Asymmetrically Strained-SiGe p-MOSFETs

The hole mobility characteristics of 〈110〉 /(100)-oriented asymmetrically strained-SiGe p-MOSFETs are studied. Uniaxial mechanical strain is applied to biaxial compressive strained devices and the relative change in effective hole mobility is measured. The channel Ge content varies from 0 to 100%. Up to -2.6% biaxial compressive strain is present in the channel and an additive uniaxial strain component of -0.06% is applied via mechanical bending. The hole mobility in biaxial compressive strained-SiGe is enhanced relative to relaxed Si. It is observed that this mobility enhancement increases further with the application of 〈110〉 longitudinal uniaxial compressive strain. The relative change in mobility with applied stress is larger for biaxial compressive strained-SiGe than for Si and increases with the amount of biaxial compressive strain present in the channel.

Asymmetric strain in nanoscale patterned strained -Si/strained -Ge/strained-Si heterostructures on insulator

The engineering of asymmetric strain is demonstrated in nanoscale patterned strained-Si/strained-Ge/strained-Si heterostructure on insulator with body thickness of 15nm. Starting material has layers with symmetric in-plane strain, including biaxial strained Si (∼1.8%, tension) and biaxial strained Ge (∼1.8%, compression). Micro-Raman spectroscopy is utilized to characterize the stress in heterostructures patterned into 10-μm-long bars with widths ranging from 300to30nm. Raman measurements are consistent with the transformation from biaxial to uniaxial compressive strain in the Ge for 30-nm-wide bars, as predicated by simulations. Measurements also demonstrate enhanced asymmetric relaxation in the tensile strained Si cap as its thickness is increased.

Detection of charge storage on molecular thin films of tris(8-hydroxyquinoline) aluminum (Alq3) by Kelvin force microscopy: a candidate system for high storage capacity memory cells.

Retention and diffusion of charge in tris(8-hydroxyquinoline) aluminum (Alq(3)) molecular thin films are investigated by injecting electrons and holes via a biased conductive atomic force microscopy tip into the Alq(3) films. After the charge injection, Kelvin force microscopy measurements reveal minimal changes with time in the spatial extent of the trapped charge domains within Alq(3) films, even for high hole and electron densities of >10(12) cm(-2). We show that this finding is consistent with the very low mobility of charge carriers in Alq(3) thin films (<10(-7) cm(2)/(Vs)) and that it can benefit from the use of Alq(3) films as nanosegmented floating gates in flash memory cells. Memory capacitors using Alq(3) molecules as the floating gate are fabricated and measured, showing durability over more than 10(4) program/erase cycles and the hysteresis window of up to 7.8 V, corresponding to stored charge densities as high as 5.4 × 10(13) cm(-2). These results demonstrate the potential for use of molecular films in high storage capacity nonvolatile memory cells.

High Hole-Mobility Strained-

Low-field effective hole mobility of highly strained (~2.4%, biaxial) germanium-channel (7.8 nm-thick) p-MOSFETs with high-K/metal gate stack has been experimentally investigated. Devices with various ultrathin strained-Si cap layer thicknesses, as thin as ~8 Å, show excellent capacitance-voltage characteristics with no hysteresis or frequency dispersion and hole mobility enhancement of more than 6.5X over Si universal and 2.3X over similar devices with no strained-Si cap, at Eeff = 0.6 MV/cm. The influence of the strained-Si cap thickness on the hole mobility is also studied. The mobility increases with increasing Si cap thickness up to ~1.8 nm (with a peak mobility of 940 cm2/Vs at this cap thickness) consistent with a reduction in remote Coulombic scattering.

\hbox{Ge/Si}_{0.6} \hbox{Ge}_{0.4}

Suspended strained-Si nano-wires (NWs) were fabricated from a highly biaxially strained-Si substrate (with an initial stress of 2.16 GPa). Using e-beam lithography, ~25nm thick NWs with the widths in the range of 20 to 80 nm were fabricated and the stress was investigated by UV micro-Raman spectroscopy. Suspended NWs are strained to an average uniaxial tensile stress level of ~2.1 GPa which is almost independent of NW width, in the range studied in this work. Ultra-dense (25 NWs per micron) sub-20 nm suspended strained-Si NWs were fabricated using resolutionenhanced lithography to improve the Raman signal-to-noise ratio. A tensile in-plane stress level of 1.7GPa was measured for 18 nmwide NWs at 40 nm pitch. Gate-all-around n-MOSFETs were fabricated based on these strained-Si NWs. Electrical measurements on these MOSFETs demonstrate near ideal subthreshold behavior, very high on-to-off ratio and current drive and transconductance enhancement of ~2X over unstrained NWs.

P-MOSFETs With High-K/Metal Gate: Role of Strained-Si Cap Thickness

The onset of misfit dislocation formation, i.e., the critical thickness for heteroepitaxy, is studied for selective epitaxial growth of high Ge-content, strained SiGe on oxide-patterned Si wafers. Misfit dislocation spacing was analyzed as a function of film thickness using plan-view transmission-electron microscopy. For selective epitaxial growth at 450u2009°C, the critical thickness for Si0.33Ge0.67 is found to be 8.5 nm. This is a twofold increase compared to the 4.0 nm theoretical equilibrium critical thickness and the 4.5 nm critical thickness measured for growth on bare Si wafers. The misfit dislocation density for selective epitaxial growth is strongly influenced by the shape and orientation of the growth area.

Fabrication and Characterization of Suspended Uniaxial Tensile Strained-Si Nanowires for Gate-All-Around Nanowire n-MOSFETs

Thin film a-Si(n⁺)/c-Si_1-xGe_x(p)/c-Si(p⁺) heterojunction solar cells are fabricated with Ge content up to 56 atomic percent. Solar cells with junction layers consisting of Si, Si_0.75Ge_0.25, Si_0.59Ge_0.41, and Si_0.44Ge_0.56 are compared to study the effect of increasing Ge concentration. The measured short-circuit current (J_sc) increases from ~14 mA/cm² for Si cells to 21 mA/cm² for the Si_0.44Ge_0.56 cells, for one light pass and a 2 μm-thick SiGe layer. The results show an open-circuit voltage (V_oc) of 0.61 V for Si cells, dropping to 0.32 V for Si_0.44Ge_0.56, consistent with the reduction in band-gap. Quantum efficiency measurements highlight the improved spectral response for higher Ge percentages. Physics based TCAD simulations combined with the experimental results are used to extract lifetime and interface velocity.