Piezo-Raman measurements and anharmonic parameters in silicon and diamond

Advancements and challenges in strained group-IV-based optoelectronic materials stressed by ion beam treatment

In this perspective article, we discuss the application of ion implantation to manipulate strain (by either neutralizing or inducing compressive or tensile states) in suspended thin films. Emphasizing the pressing need for a high-mobility silicon-compatible transistor or a direct bandgap group-IV semiconductor that is compatible with complementary metal-oxide-semiconductor technology, we underscore the distinctive features of different methods of ion beam-induced alteration of material morphology. The article examines the precautions needed during experimental procedures and data analysis and explores routes for potential scalable adoption by the semiconductor industry. Finally, we briefly discuss how this highly controllable strain-inducing technique can facilitate enhanced manipulation of impurity-based spin quantum bits (qubits).

Electronic Transport Modulation in Ultrastrained Silicon Nanowire Devices

In this work, we explore the effect of ultrahigh tensile strain on electrical transport properties of silicon. By integrating vapor-liquid-solid-grown nanowires into a micromechanical straining device, we demonstrate uniaxial tensile strain levels up to 9.5%. Thereby the triply degenerated phonon dispersion relation at the Γ-point of silicon disentangle and the longitudinal phonon modes are used to precisely determine the extent of mechanical strain. Simultaneous electrical transport measurements showed a significant enhancement in the electrical conductance. Aside from considerable reduction of the Si bulk resistivity due to strain-induced band gap narrowing, comparison with quasi-particle GW calculations further reveals that the effective Schottky barrier height at the electrical contacts undergoes a substantial reduction. For these reasons, nanowire devices with ultrastrained channels may be promising candidates for future applications of high-performance silicon-based devices.

Carbon and Neon Ion Bombardment Induced Smoothing and Surface Relaxation of Titania Nanotubes

Titania nanotube arrays with their enormous surface area are the subject of much attention in diverse fields of research. In the present work, we show that not only 60 keV and 150 keV ion bombardment of amorphous titania nanotube arrays yields defect creation within the tube walls, but it also changes the surface morphology: the surface relaxes and smoothens in accordance with a curvature-driven surface material's transport mechanism, which is mediated by radiation-induced viscous flow or radiation-enhanced surface diffusion, while the nanotubes act as additional sinks for the particle surface currents. These effects occur independently of the ion species: both carbon and neon ion bombardments result in comparable surface relaxation responses initiated by an ion energy of 60 keV at a fluence of 1 × 1016 ions/cm2. Using atomic force microscopy and contact angle measurements, we thoroughly study the relaxation effects on the surface topography and surface free energy, respectively. Moreover, surface relaxation is accompanied by further amorphization in surface-near regions and a reduction in the mass density, as demonstrated by Raman spectroscopy and X-ray reflectivity. Since ion bombardment can be performed on global and local scales, it constitutes a versatile tool to achieve well-defined and tunable topographies and distinct surface characteristics. Hence, different types of nanotube arrays can be modified for various applications.

Effect of Ion Irradiation on Nanoindentation Fracture and Deformation in Silicon Carbide

Silicon carbide is desirable for many nuclear applications, making it necessary to understand how it deforms after irradiation. Ion implantation combined with nanoindentation is commonly used to measure radiation-induced changes to mechanical properties; hardness and modulus can be calculated from load-displacement curves, and fracture toughness can be estimated from surface crack lengths. Further insight into indentation deformation and fracture is required to understand the observed changes to mechanical properties caused by irradiation. This paper investigates indentation deformation using high-resolution electron backscatter diffraction (HR-EBSD) and Raman spectroscopy. Significant differences exist after irradiation: fracture is suppressed by swelling-induced compressive residual stresses, and the plastically deformed region extends further from the indentation. During focused ion beam cross-sectioning, indentation cracks grow, and residual stresses are modified. The results clarify the mechanisms responsible for the modification of apparent hardness and apparent indentation toughness values caused by the compressive residual stresses in ion-implanted specimens.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11837-021-04636-8.

Residual stress analysis of thin film photovoltaic cells subjected to massive micro-particle impact

Micro-particle impact experiments and the residual stress tests by Raman spectrum.

Polarized Raman spectroscopy-stress relationship considering shear stress effect

This Letter uses polarized Raman spectroscopy as a "strain rosette" to quantitatively determine all the in-plane components of the stress tensor for (110) silicon. Through polarized Raman spectroscopy, Raman wavenumber shifts can be obtained at the same point with different polarization directions of the incident and/or scattered light. With at least three measured Raman shifts in different polarized directions, the three stress components of a surface that contains two non-equal normal stresses and one shear stress can be calculated accordingly. We develop an analytical and linear Raman wavenumber shift-stress relationship when shear stress is considered. The experimental results verify the theoretical predictions. It shows that the simple stress condition assumption may lead to erroneous results.

Photo-induced cubic-to-hexagonal polytype transition in silicon nanowires

Crystalline phase transformation in silicon nanowires from cubic diamond to hexagonal diamond under strong laser excitation, caused by inhomogeneous heating-induced mechanical stresses.

Enhanced Electron Mobility in Nonplanar Tensile Strained Si Epitaxially Grown on SixGe1-x Nanowires

We report the growth and characterization of epitaxial, coherently strained Si_xGe_1-x-Si core-shell nanowire heterostructure through vapor-liquid-solid growth mechanism for the Si_xGe_1-x core, followed by an in situ ultrahigh-vacuum chemical vapor deposition for the Si shell. Raman spectra acquired from individual nanowire reveal the Si-Si, Si-Ge, and Ge-Ge modes of the Si_xGe_1-x core and the Si-Si mode of the shell. Because of the compressive (tensile) strain induced by lattice mismatch, the core (shell) Raman modes are blue (red) shifted compared to those of unstrained bare Si_xGe_1-x (Si) nanowires, in good agreement with values calculated using continuum elasticity model coupled with lattice dynamic theory. A large tensile strain of up to 2.3% is achieved in the Si shell, which is expected to provide quantum confinement for electrons due to a positive core-to-shell conduction band offset. We demonstrate n-type metal-oxide-semiconductor field-effect transistors using Si_xGe_1-x-Si core-shell nanowires as channel and observe a 40% enhancement of the average electron mobility compared to control devices using Si nanowires due to an increased electron mobility in the tensile-strained Si shell.

Approaching the intrinsic quality factor limit for micromechanical bulk acoustic resonators using phononic crystal tethers

We systematically demonstrate that one-dimensional phononic crystal (1-D PnC) tethers can significantly reduce tether loss in micromechanical resonators to a point where the total energy loss is dominated by intrinsic mechanisms, particularly phonon damping. Multiple silicon resonators are designed, fabricated, and tested to provide comparisons in terms of the number of periods in the PnC and the resonance frequency, as well as a comparison with conventional straight-beam tethers. The product of resonance frequency and measured quality factor (f×Q) is the critical figure of merit, as it is inversely related to the total energy dissipation in a resonator. For a wide range of frequencies, devices with PnC tethers consistently demonstrate higher f×Q values than the best conventional straight-beam tether designs. The f×Q product improves with increasing number of PnC periods, and at a maximum value of 1.2 × 10¹³ Hz, approaches limiting values set by intrinsic material loss mechanisms.

Manufacturing Te/PEDOT Films for Thermoelectric Applications

In this work, flexible Te films have been synthesized by electrochemical deposition using PEDOT [poly(3,4-ethylenedioxythiophene)] nanofilms as working electrodes. The Te electrodeposition time was varied to find the best thermoelectric properties of the Te/PEDOT double layers. To show the high quality of the Te films grown on PEDOT, the samples were analyzed by Raman spectroscopy, showing the three Raman active modes of Te: E₁, A₁, and E₂. The X-ray diffraction spectra also confirmed the presence of crystalline Te on top of the PEDOT films. The morphology of the Te/PEDOT films was studied using scanning electron microscopy, showing a homogeneous distribution of Te along the film. Also an atomic force microscope was used to analyze the quality of the Te surface. Finally, the electrical conductivity and the Seebeck coefficient of the Te/PEDOT films were measured as a function of the Te deposition time. The films showed an excellent thermoelectric behavior, giving a maximum power factor of about 320 ± 16 μW m^-1 K^-2 after 2.5 h of Te electrochemical deposition, a value larger than that reported for thin films of Te. Qualitative arguments to explain this behavior are given in the discussion.

Assessing strain mapping by electron backscatter diffraction and confocal Raman microscopy using wedge-indented Si

The accuracy of electron backscatter diffraction (EBSD) and confocal Raman microscopy (CRM) for small-scale strain mapping are assessed using the multi-axial strain field surrounding a wedge indentation in Si as a test vehicle. The strain field is modeled using finite element analysis (FEA) that is adapted to the near-indentation surface profile measured by atomic force microscopy (AFM). The assessment consists of (1) direct experimental comparisons of strain and deformation and (2) comparisons in which the modeled strain field is used as an intermediate step. Direct experimental methods (1) consist of comparisons of surface elevation and gradient measured by AFM and EBSD and of Raman shifts measured and predicted by CRM and EBSD, respectively. Comparisons that utilize the combined FEA-AFM model (2) consist of predictions of distortion, strain, and rotation for comparison with EBSD measurements and predictions of Raman shift for comparison with CRM measurements. For both EBSD and CRM, convolution of measurements in depth-varying strain fields is considered. The interconnected comparisons suggest that EBSD was able to provide an accurate assessment of the wedge indentation deformation field to within the precision of the measurements, approximately 2×10(-4) in strain. CRM was similarly precise, but was limited in accuracy to several times this value.

The fingerprint of Te-rich and stoichiometric Bi2Te3 nanowires by Raman spectroscopy

We unambiguously show that the signature of Te-rich bismuth telluride is the appearance of three new peaks in the Raman spectra of Bi2Te3, located at 88, 117 and 137 cm(-1). For this purpose, we have grown stoichiometric Bi2Te3 nanowires as well as Te-rich nanowires. The absence of these peaks in stoichiometric nanowires, even in those with the smallest diameter, shows that they are not related to confinement effects or the lack of inversion symmetry, as stated in the literature, but to the existence of Te clusters. These Te clusters have been found in non-stoichiometric samples by high resolution electron microscopy, while they are absent in stoichiometric samples. The Raman spectra of the latter corresponds to the one for bulk Bi2Te3. The intensity of these Raman peaks are clearly correlated to the Te content. In order to ensure statistically meaningful results, we have investigated several regions from every sample.

Coherently Strained Si-SixGe1-x Core-Shell Nanowire Heterostructures

Coherently strained Si-SixGe1-x core-shell nanowire heterostructures are expected to possess a positive shell-to-core conduction band offset, allowing for quantum confinement of electrons in the Si core. We report the growth of epitaxial, coherently strained Si-SixGe1-x core-shell heterostructures through the vapor-liquid-solid mechanism for the Si core, followed in situ by the epitaxial SixGe1-x shell growth using ultrahigh vacuum chemical vapor deposition. The Raman spectra of individual nanowires reveal peaks associated with the Si-Si optical phonon mode in the Si core and the Si-Si, Si-Ge, and Ge-Ge vibrational modes of the SixGe1-x shell. The core Si-Si mode displays a clear red-shift compared to unstrained, bare Si nanowires thanks to the lattice mismatch-induced tensile strain, in agreement with calculated values using a finite-element continuum elasticity model combined with lattice dynamic theory. N-type field-effect transistors using Si-SixGe1-x core-shell nanowires as channel are demonstrated.

The Origin of High Thermal Conductivity and Ultralow Thermal Expansion in Copper-Graphite Composites

We developed a nanocomposite with highly aligned graphite platelets in a copper matrix. Spark plasma sintering ensured an excellent copper-graphite interface for transmitting heat and stress. The resulting composite has superior thermal conductivity (500 W m(-1) K(-1), 140% of copper), which is in excellent agreement with modeling based on the effective medium approximation. The thermal expansion perpendicular to the graphite platelets drops dramatically from ∼20 ppm K(-1) for graphite and copper separately to 2 ppm K(-1) for the combined structure. We show that this originates from the layered, highly anisotropic structure of graphite combined with residual stress under ambient conditions, that is, strain-engineering of the thermal expansion. Combining excellent thermal conductivity with ultralow thermal expansion results in ideal materials for heat sinks and other devices for thermal management.

Effect of Mn doping on structural, morphological and dielectric properties of EuFeO3 ceramics

The conductivity measurements show that Mn doped EuFeO₃ compounds obey Jonscher's universal power law.

Direct optical mapping of anisotropic stresses in nanowires using transverse optical phonon splitting

Strain engineering is ubiquitous in the design and fabrication of innovative, high-performance electronic, optoelectronic, and photovoltaic devices. The increasing importance of strain-engineered nanoscale materials has raised significant challenges at both fabrication and characterization levels. Raman scattering spectroscopy (RSS) is one of the most straightforward techniques that have been broadly utilized to estimate the strain in semiconductors. However, this technique is incapable of measuring the individual components of stress, thus only providing the average values of the in-plane strain. This inherit limitation severely diminishes the importance of RSS analysis and makes it ineffective in the predominant case of nanostructures and devices with a nonuniform distribution of strain. Herein, we circumvent this major limitation and demonstrate for the first time the application of RSS to simultaneously probe the two local stress in-plane components in individual ultrathin silicon nanowires based on the imaging of the splitting of the two forbidden transverse optical phonons.

Power-dependent Raman analysis of highly strained Si nanobridges

Strain analysis of complex three-dimensional nanobridges conducted via Raman spectroscopy requires careful experimentation and data analysis supported by simulations. A method combining micro-Raman spectroscopy with finite element analysis is presented, enabling a detailed understanding of strain-sensitive Raman data measured on Si nanobridges. Power-dependent measurements are required to account for the a priori unknown scattering efficiency related to size and geometry. The experimental data is used to assess the validity of previously published phonon deformation potentials.

Correlation between matrix structural order and compressive stress exerted on silicon nanocrystals embedded in silicon-rich silicon oxide

Silicon nanocrystals embedded in a silicon oxide matrix were deposited by radio frequency reactive magnetron sputtering. By means of Raman spectroscopy, we have found that a compressive stress is exerted on the silicon nanocrystal cores. The stress varies as a function of silicon concentration in the silicon-rich silicon oxide layers varies, which can be attributed to changes of nanocrystal environment. By conducting the Fourier transform infrared absorption experiments, we have correlated the stresses exerted on the nanocrystal core to the degree of matrix structural order. PACS: 78.67.Bf, 78.67.Pt, 73.63.Bd, 78.47.D, 74.25.Nd.

Silicon nanowire polytypes: identification by Raman spectroscopy, generation mechanism, and misfit strain in homostructures

Silicon nanowires with predominant 9R, 27T, 2H and other polytype structures with respective hexagonalities of 50, 40 and 35.3% were identified by Raman microscopy. Transmission electron microscopy indicates that intrinsic stacking faults form the basic building blocks of these polytypes. We propose a generation mechanism in which polytypes are seeded from incoherent twin boundaries and associated partial dislocations. This mechanism explains observed prevalence of polytypes and trends in stacking for longer period structures. The percentage of hexagonal planes in a polytype is extracted from its Raman spectrum after correcting the zone-folded phonon frequencies to account for changes of the in-plane lattice parameter with respect to diamond cubic (3C) Si. The correction is found to be (i) of the same order of magnitude as frequency differences between modes of low period polytypes and (ii) proportional to the hexagonality. Corrected phonon frequencies agree with experimentally found values to within 0.4 cm(-1). Homostructures in which a central polytype region is bounded by 3C regions, with the planes (111)(3C)║(0001)(polytype) parallel to the nanowire axis, are found in <linear span>112<linear span> oriented nanowires. Strain-induced shifts of the Raman modes in such structures enable a rough estimation of the lattice misfit between polytypes, which compares favorably with first-principles calculations. Considerations presented here provide a simple and quantitative framework to interpret Raman frequencies and extract crystallographic information on polytype structures.

Mapping the "forbidden" transverse-optical phonon in single strained silicon (100) nanowire

The accurate manipulation of strain in silicon nanowires can unveil new fundamental properties and enable novel or enhanced functionalities. To exploit these potentialities, it is essential to overcome major challenges at the fabrication and characterization levels. With this perspective, we have investigated the strain behavior in nanowires fabricated by patterning and etching of 15 nm thick tensile strained silicon (100) membranes. To this end, we have developed a method to excite the "forbidden" transverse-optical (TO) phonons in single tensile strained silicon nanowires using high-resolution polarized Raman spectroscopy. Detecting this phonon is critical for precise analysis of strain in nanoscale systems. The intensity of the measured Raman spectra is analyzed based on three-dimensional field distribution of radial, azimuthal, and linear polarizations focused by a high numerical aperture lens. The effects of sample geometry on the sensitivity of TO measurement are addressed. A significantly higher sensitivity is demonstrated for nanowires as compared to thin layers. In-plane and out-of-plane strain profiles in single nanowires are obtained through the simultaneous probe of local TO and longitudinal-optical (LO) phonons. New insights into strained nanowires mechanical properties are inferred from the measured strain profiles.

Experimental determination of third-order elastic constants of diamond

To determine the nonlinear elastic response of diamond, single crystals were shock compressed along the [100], [110], and [111] orientations to 120 GPa peak elastic stresses. Particle velocity histories and elastic wave velocities were measured by using laser interferometry. The measured elastic wave profiles were used, in combination with published acoustic measurements, to determine the complete set of third-order elastic constants. These constants represent the first experimental determination, and several differ significantly from those calculated by using theoretical models.

Direct Measurements of Thermal Stress Distributions in Large Die Bonds for Power Electronics

Large semiconductor devices can be subject to significant mechanical stress resulting from thermal expansion mismatch between the devices and packaging materials. This stress can either cause mechanical failure or change the operational characteristics of the device. Furthermore, this stress governs the reliability of the system during use. Therefore, a complete understanding is required of the nature of stress evolution during packaging, and its time and temperature dependence during subsequent service. In the present investigation, the absolute magnitudes and spatial distributions of time-dependent thermal residual stress are measured directly by piezospectroscopic techniques. These measurements are performed with high stress (±15 MPa) and spatial (1 μm) resolution in silicon specimens attached to substrates. Measurements are performed at room temperature on 25 mm square specimens, with continuous, layered solder joints in model specimens of silicon/solder/copper. Room temperature creep relaxation is also investigated. The stress data are then analyzed according to fundamental micromechanical models.