Umesh Kumar Bhaskar

Piezoresistance of nano-scale silicon up to 2GPa in tension

The piezo-resistance of 100 nm-thick, [110] oriented, p-type, mono-crystalline Si beams has been investigated under large uniaxial tension up to 2 GPa using an original on-chip tensile testing set-up. The piezo-resistance coefficient (π) was found to increase by a factor of 6 compared with ∼1.5 for Si bulk, when decreasing the dopant concentration from Na ∼ 1 × 1019 cm−3 down to Na ∼ 5 × 1017 cm−3. Reduction of resistance by a factor of 5.8, higher than theoretical maximum of 4.5, is reported for Na ∼ 5 × 1017 cm−3 under a stress of 1.7 GPa, without any sign of saturation.

High-Throughput On-Chip Large Deformation of Silicon Nanoribbons and Nanowires

An on-chip internal stress-based testing device has been developed in order to deform silicon nanoribbons and nanowires up to large strains enabling high throughput of data. The fracture strain and survival probability distribution have been generated for 50-nm-thick and 50- or 500-nm-wide specimens with lengths varying between 2.5 and 10 . Fracture strains reaching up to 5% are attained in the smallest specimens, whereas 90% of the specimens survive 2.5% deformation. This testing platform opens an avenue to investigate and use electromechanical couplings appearing under large mechanical stress or large deformation.

Strain in silicon nanowire beams

In this work, strain in silicon free standing beams loaded in uniaxial tension is experimentally and theoretically investigated for strain values ranging from 0 to 3.6%. The fabrication method allows multiple geometries (and thus strain values) to be processed simultaneously on the same wafer while being studied independently. An excellent agreement of strain determined by two non-destructive characterization techniques, Raman spectroscopy and mechanical displacement using scanning electron microscopy (SEM) markers, is found for all the sample lengths and widths. The measured data also show good agreement with theoretical predictions of strain based upon continuum mechanical considerations, giving validity to both measurement techniques for the entire range of strain values. The dependence of Youngs modulus and fracture strain on size has also been analyzed. The Youngs modulus is determined using SEM and compared with that obtained by resonance-based methods. Both methods produced a Youngs modulus value close to that of bulk silicon with values obtained by resonance-based methods being slightly lower. Fracture strain is analyzed in 40 sets of samples with different beam geometries, yielding values up to 3.6%. The increase in fracture strain with decreasing beam width is compared with previous reports. Finally, the role of the surface on the mechanical properties is analyzed using UV and visible lasers having different penetration depths in silicon. The observed dependence of Raman shift on laser wavelength is used to assess the thermal conductivity of deformed silicon.

Note: Fast and reliable fracture strain extraction technique applied to silicon at nanometer scale

Simple fabrication process and extraction procedure to determine the fracture strain of monocrystalline silicon are demonstrated. Nanowires/nanoribbons in silicon are fabricated and subjected to uniaxial tensile stress along the complete length of the beams. Large strains up to 5% are measured for nanowires presenting a cross section of 50 nm × 50 nm and a length of 2.5 μm. An increase in fracture strain for silicon nanowires (NWs) with the downscaling of their volume is observed, highlighting the reduction of the defects probability as volume is decreased.

Surface states and conductivity of silicon nano-wires

The transport characteristics of low dimensional semiconductors like silicon nano-wires (SiNWs) rarely conform to expectations from geometry and dopant density, exhibiting significant variations as a function of different surface terminations/conditions. The association of these mechanisms with surface states and their exact influence on practical SiNW devices still remains largely unclear. Herein, we report on the influence of surface state charge distributions on SiNW transport characteristics. For this study, p-type SiNW devices with widths of 50, 100, and 2000 nm are fabricated from 25, 50, and 200 nm-thick SOI wafers. A five order difference in effective carrier concentration was observed in the initial SiNWs characteristics, when comparing SiNWs fabricated with and without a thermal oxide. The removal of the surface oxide by a hydrogen fluoride (HF) treatment results in a SiNW conductance drop up to six orders of magnitude. This effect is from a surface depletion of holes in the SiNW induced by positive surface charges deposited as a result of the HF treatment. However, it is observed that this charge density is transient and is dissipated with the re-growth of an oxide layer. In summary, the SiNW conductance is shown to vary by several orders of magnitude, while comparing its characteristics for the three most studied surface conditions: with a native oxide, thermal oxide and HF induced H-terminations. These results emphasize the necessity to interpret the transport characteristics of SiNWs with respect to its surface condition, during future investigations pertaining to the physical properties of SiNWs, like its piezo-resistance. As a sequel, prospects for efficiently sensing an elementary reduction/oxidation chemical process by monitoring the variation of SiNW surface potential, or in practice the SiNW conductance, is demonstrated.

Experimental observations of surface roughness in uniaxially loaded strained Si microelectromechanical systems-based structures

Surface roughness in uniaxially loaded strained Si has been studied experimentally using high-resolution atomic force microscopy and a microelectromechanical systems-based on-chip loading device. A reduction in rms roughness from 0.29 nm to 0.07 nm has been identified as strain increases from 0 to 2.8% (stress from 0 to 4.9 GPa). The correlation length of the roughness, also known to affect carrier mobility, increases with increasing strain up to 1.7% before reducing at larger levels of strain. These results partly explain the high-field mobility observed in strained Si, indicating that a modified correlation length should also be considered in transport modelling of strained Si.

Nanomechanical testing of free-standing monocrystalline silicon beams

A fabrication process to characterize single crystalline silicon microbeams under uniaxial tensile stress is presented. The microbeams subjected to uniaxial tensile strain are successfully released without any stiction by the use of critical point drying tool. Based on the deformation measured using scanning electron microscope (static measurement) images, the corresponding strain and stress are calculated to plot the uniaxial tensile characteristics curve of monocrystalline silicon. Dynamic stress determination based on the measurements of flexural resonance frequency of the released beams is discussed. Finally, comparison of stress values obtained using the two methods is shown.

Note: Size effects on the tensile response of top-down fabricated Si nanobeams

The tensile response of top-down fabricated sc-Si nanobeams is inferred from the fitting of stress-strain data obtained under tensile loading conditions over a large range of deformation. The testing is performed using MEMS structures consisting of two connected beams; a highly stressed silicon-nitride (SiN) beam connected to a sc-Si specimen beam. The high tensile stress component present upon the deposition of the SiN loads the sc-Si beam once the entire structure is released. The strain and stress values are extracted independently, respectively, by scanning electron microscopy inspection and vibration frequency measurement of the released tensile MEMS structures. The tensile tests are undertaken for six thicknesses to determine the dependence of the elastic response on dimensions. The Youngs modulus shows a variation of 40% for thicknesses ranging from 200 to 30 nm.

On-chip tensile testing of the mechanical and electro-mechanical properties of nano-scale silicon free-standing beams

A simple and versatile on-chip tensile testing method is proposed for the statistical evaluation of size effects on the mechanical strength of silicon thin films along with the simultaneous study of (from low to ultra) strain effects on the carrier transport. Mechanical results are presented on the fracture strength of micro-nano scale silicon beams, followed with a discussion on interface states and problems facing reliable nano-electronic and nano-electromechanical characterizations.