Si Region Research Articles

Imaging hot photocarrier transfer across a semiconductor heterojunction with ultrafast electron microscopy

Semiconductor heterojunctions have gained significant attention for efficient optoelectronic devices owing to their unique interfaces and synergistic effects. Interaction between charge carriers with the heterojunction plays a crucial role in determining device performance, while its spatial-temporal mapping remains lacking. In this study, we employ scanning ultrafast electron microscopy (SUEM), an emerging technique that combines high spatial-temporal resolution and surface sensitivity, to investigate photocarrier dynamics across a Si/Ge heterojunction. Charge dynamics are selectively examined across the junction and compared to far bulk areas, through which the impact of the built-in potential, band offsets, and surface effects is directly visualized. In particular, we find that the heterojunction drastically modifies the hot photocarrier diffusivities in both Si and Ge regions due to charge trapping. These findings are further elucidated with insights from the band structure and surface potential measured by complementary techniques. This work demonstrates the tremendous effect of heterointerfaces on hot photocarrier dynamics and showcases the potential of SUEM in characterizing realistic optoelectronic devices.

Change in the InSb nanocrystal growth direction at the Si/SiO2 interface during ion-beam synthesis

The effect of annealing time on the InSb nanocrystal formation in a silicon-on-insulator structure, containing, near the Si/SiO2 interface, Si and SiO2 regions implanted with Sb+ and In+ ions, respectively, was studied. The annealing temperature was 1100 °C. A change in the nanocrystal growth direction was obtained as the annealing time increased from 1 to 90 min. After the 1 min annealing, the InSb nanocrystals grew within the Si matrix and were faceted. As the annealing time increased to 90 min, the nanocrystals grew from the Si/SiO2 interface into the SiO2 matrix; they had a half-spherical shape. A respective change in the phonon mode was observed, too. The origin of the obtained effect is discussed.

Deep-level defects induced by implantations of Si and Mg ions into undoped epitaxial GaN

The properties and concentrations of deep-level defects induced by implantations of Si and Mg ions into unintentionally doped (UID) epitaxial GaN have been revealed by using the Laplace-transform photoinduced transient spectroscopy (LPITS) and molecular dynamics (MD) calculations. The material lattice damage, produced by the Si ions implanted at room temperature in the single process at the energies of 200 and 340 keV, is compared with that produced by the Mg ions implanted in the similar process at the energies of 150, 210, and 270 keV. The LPITS results indicate that the same deep traps with the activation energies of 396, 512, 531, 587, 635, and 736 meV are present in the tail regions of the semi-insulating Si- and Mg-implanted films. It is argued that the predominant implantation-induced point defects in the tail region of the Si-implanted films are nitrogen vacancies, whose concentration is 7.7 × 1017 cm−3. In the Mg-implanted films, the predominant implantation-induced point defects are gallium interstitials, whose concentration is 1.2 × 1 018 cm−3.

Generalized damage profile function for subsurface damage in SiC induced by helium focused ion beam under line scanning at different energies and doses

Silicon carbide is receiving increasing attention as a substrate due to its stability in harsh environments. At the same time, further minimization of sensors and micro devices requires new techniques for nanofabrication. In this context, the helium focused ion beam (He-FIB) is a promising candidate, due to its high resolution and capability of sub-10 nm fabrication, as well as its ability to modify the properties of the substrate via ion implantation. In this work, we consider previously reported experimental images of subsurface damage in both SiC and Si, as induced by He-FIB under line scanning, and provide a summary of the resulting amorphous and bubble regions as a function of the beam energy and dose. Based on empirical relations collectively encapsulated into a damage profile function (DPF) that was previously used to describe the contour of the amorphous region in Si, we focus on presenting a generalized damage profile function (GDPF) that enables describing the contours of both the amorphous and bubble regions in SiC simultaneously. In particular, the role of swelling, as a relatively small protruding region from the initial surface that eventually restricts the upward expansion of the bubble region, is fully considered. The parameters of the resulting mathematical model are related to measurements from the experimental cross-sectional images and are eventually optimized by using an evolutionary algorithm. Comparison to the experimental profiles concludes that the proposed GDPF captures both the amorphous and bubble region profiles with good precision.

Modeling and optimization of CuIn1-xGaxSe2/Si1-yGey structure for solar cells applications

A combination of a Copper Indium Gallium Selenide (CIGS) and Silicon (Si) layer has been recognized as an excellent choice for producing heterojunction based solar cells with improved efficiency and low cost processing techniques. The quaternary compound CIGS and silicon (Si) regions exhibit a lattice mismatch of about 5%, which induces a strain and impacts the electronic characteristics of the CIGS/Si heterojunction solar cell. A new viewpoint suggests the integration of a silicon germanium (Si1-yGey) layer in the CIGS/Si region to reduce the impact of lattice mismatch. The objective of this study is to investigate how different gallium and germanium concentrations (xGa and yGe) affect the following factors: lattice mismatch (ε), critical thickness (hc) and absorption coefficient (α)of CIGS/SiGe based solar cells. It also aims to analyze how these concentrations impact the primary parameters used to evaluate solar cell performance such as external quantum efficiency, short circuit current density, fill factor, open circuit voltage and conversion efficiency. The simulation results agree well with the existing theoretical and experimental literature data, confirming the suitability of the physical characteristics employed in this numerical study. By tuning the concentrations of Gallium and Germanium, it is feasible to attain an efficiency of 24% owing to the lattice compensation phenomenon in Si1-yGey layers. These findings hold significant implications for the development and advancement of solar cell technology, as well as for enhancing their conversion efficiency and commercialization.

Depth profile analysis of 100 keV Ni ions in Si 〈100〉 substrate

Nickel implanted silicon substrate shows potential applications for the fabrication of infrared detectors, solar cells, spintronic devices as well as for the synthesis of an embedded nickel silicide layer inside Si substrates. Emerging applications of the transition metal implanted silicon structures for the fabrication of intermediate band materials motivated us to study the in-depth distribution profile of Ni ions as well as the structural changes that occur at the top of silicon substrate surface before and after annealing process. In the present work, we have carried out detailed study on the crystalline silicon substrates, implanted with Ni− ions comprising ion dose value of 5 × 1016 ions/cm2 at 100 keV implantation energy followed with the post implantation annealing at 800 °C temperature for a time duration of 2 h. The distribution profile of nickel ions inside the silicon substrate was estimated using the non-destructive simultaneous XRR-GIXRF measurements and were also compared with results obtained from the RBS and SIMS measurements. The in-depth concentration profile of Ni ions inside Si substrate derived from XRR-GIXRF measurements was found to be in close agreement with that of obtained from the RBS measurements. Furthermore, our investigations clearly showed that the annealing process causes the inward and outward diffusion of the Ni atoms inside the Si substrate, thus significantly changing the in-depth concentration profile of Ni ions in the Si matrix. The XANES and EXAFS results also demonstrate the formation of NiSi2 phase around Ni atoms in the implanted region of Si 〈100〉 substrate.

Bound states in the continuum (BIC) in silicon nanodisk array on mirror structure: Perfect absorption associated with quasi-BIC below the bandgap

A structure composed of a hexagonal array of Si nanodisks having toroidal dipole resonances and a reflecting mirror separated by a SiO2 spacer is proposed as a platform that exhibits narrow-band perfect absorption in the Si sub-bandgap wavelength range for a CMOS compatible Si based photodetector operating below the bandgap range. The numerical simulation reveals that the structure possesses Fabry–Pérot bound states in the continuum at proper spacer thicknesses due to the interference between the toroidal dipole and its image dipole. By slightly detuning the spacer thickness to meet the critical coupling condition, narrow-band perfect absorption appears despite assumption of a very small extinction coefficient (5 × 10−4). The wavelength of the perfect absorption is controlled in a wide range by the structural parameters of a Si nanodisk hexagonal array and is insensitive to the fluctuation of the extinction coefficient and the choice of a metallic mirror. In the structure, over 90% of incident power can be absorbed in the Si region. This suggests that the structure can be used as a narrow-band photodetector operating in the Si sub-bandgap wavelength range. We also evaluate the sensing performance of the proposed structure as an intensity based refractive index sensor operating in the near-infrared range.

Deep UV transparent conductive Si-doped Ga2O3 thin films grown on Al2O3 substrates

β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices.

Physical studies of CuIn1-xGaxSe2/Si1-yGey

The importance of combining the Copper Indium Gallium Selenide (CIGS) and Silicon (Si) for solar cell technologies outruns the limitations for their commercialization and the conversion efficiency limits. The lattice mismatch (about 5%) between chalcopyrite quaternary (CIGS) and silicon (Si) regions induces strain, affecting the electronic properties of the CIGS/Si hetero-junction solar cell, an innovative view suggests inserting a silicon germanium (Si1-yGey) layer into CIGS/Si interface to reduce the lattice mismatch effects. The impact of varying gallium and germanium concentrations (xGa and yGe) on the lattice mismatch, critical thickness and absorption coefficient of CIGS/SiGe based solar cells is investigated. As well as their effects on the main parameters used to characterize the performance of solar cells such as external quantum efficiency, open circuit voltage, short current density, fill factor and the conversion efficiency. Good agreement between simulation results and both existing theoretical and experimental literature data proves adequacy of the physical properties used in this numerical investigation. Optimizing Gallium and Germanium concentrations makes it possible to achieve an efficiency of 24% due to the lattice compensation effect in Si1-yGey layers.

On the Band-Gap Width of NiSi2 Nanocrystals Created in the Surface Region of Si Using Ion Implantation

On the Band-Gap Width of NiSi2 Nanocrystals Created in the Surface Region of Si Using Ion Implantation

CMOS-compatible manufacturability of sub-15 nm Si/SiO2/Si nanopillars containing single Si nanodots for single electron transistor applications

This study addresses the complementary metal-oxide-semiconductor-compatible fabrication of vertically stacked Si/SiO2/Si nanopillars (NPs) with embedded Si nanodots (NDs) as key functional elements of a quantum-dot-based, gate-all-around single-electron transistor (SET) operating at room temperature. The main geometrical parameters of the NPs and NDs were deduced from SET device simulations using the nextnano++ program package. The basic concept for single silicon ND formation within a confined oxide volume was deduced from Monte-Carlo simulations of ion-beam mixing and SiO x phase separation. A process flow was developed and experimentally implemented by combining bottom-up (Si ND self-assembly) and top-down (ion-beam mixing, electron-beam lithography, reactive ion etching) technologies, fully satisfying process requirements of future 3D device architectures. The theoretically predicted self-assembly of a single Si ND via phase separation within a confined SiO x disc of <500 nm3 volume was experimentally validated. This work describes in detail the optimization of conditions required for NP/ND formation, such as the oxide thickness, energy and fluence of ion-beam mixing, thermal budget for phase separation and parameters of reactive ion beam etching. Low-temperature plasma oxidation was used to further reduce NP diameter and for gate oxide fabrication whilst preserving the pre-existing NDs. The influence of critical dimension variability on the SET functionality and options to reduce such deviations are discussed. We finally demonstrate the reliable formation of Si quantum dots with diameters of less than 3 nm in the oxide layer of a stacked Si/SiO2/Si NP of 10 nm diameter, with tunnelling distances of about 1 nm between the Si ND and the neighboured Si regions forming drain and source of the SET.

Interplay of Quantum Confinement and Strain Effects in Type I to Type II Transition in GeSi Core-Shell Nanocrystals.

The electronic properties of hydrogenated, spherical SiGe and GeSi core-shell nanocrystals, with a diameter ranging from 1.8 to 4.0 nm, are studied within density functional theory. Effects induced by quantum confinement and strain on the near-band-edge state localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that SiGe core-shell nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other hand, our results point out that a type II offset cannot be observed in small (diameter less than 3 nm) GeSi core-shell nanocrystals. In these systems, quantum confinement and strain drive the near-band-edge states to be mainly localized on Ge atoms, i.e., in the core region. In larger GeSi core-shell nanocrystals, instead, the formation of a type II offset can be engineered by playing with both core and shell thickness. The factors which determine the band-offset character at the Ge/Si interface are discussed in detail.

The silicon active sites driven by oxygen vacancies in iron silicate for activating peroxymonosulfate.

Most metal sites and some non-metallic sites such as carbon and nitrogen are usually considered to be traditional active sites during peroxymonosulfate (PMS) activation. However, as an important non-metallic element, the actual role of silicon (Si) in PMS activation still remains unclear. In this work, taking iron silicate (FeSi) as an example, the role of the Si region in PMS activation was clearly revealed. The experiments and density functional theory (DFT) calculation results showed that besides the traditional Fe sites, the Si also played a non-negligible role during PMS activation. In FeSi containing oxygen vacancies (Ovac), Fe-Si was the active site instead of Fe-Fe. The Bard charge results implied that the presence of Ovac tuned the electronic properties of FeSi, making the Si participate in PMS activation. This work deepened understanding of the role of Si in silicates for PMS activation and provided a theoretical basis for the development of excellent Si-based catalysts.

Compositionally graded aluminum-silicon alloy fabricated via friction extrusion

A novel solid-phase gradient alloying technique was developed for a high throughput composition-microstructure-mechanical property assessment. Friction extrusion of a two-piece billet setup facilitated the formation of a seamless compositionally graded aluminum (Al)-silicon (Si) extrudate rod, with Si concentration in Al matrix gradually increasing from ~1 at. % at one end to ~7 at. % at the other end of the rod. We observed a strong dependence of the Al grain size, morphology, and texture on the Si concentration. The Si particles pinned the recrystallized grain boundaries resulting in refined (< 10 um) and textured grains in the high Si content region, while the Al grains in the low Si region are equiaxed, randomly textured, and with a larger average size (~50 um). The hardness of Al-Si gradient extrudate increases by ~50% percent with a 6 at.% increase in Si content due to Hall Petch and particle strengthening effects. Our approach demonstrates the use of a solid phase processing technique to successfully produce a defect-free continuously compositionally graded bulk components thus overcoming the limitations of porosity and material heterogeneities commonly observed in other techniques such as by additive laser deposition. • A novel solid-phase gradient alloying technique is developed. • Current method provides a microstructure applicable to structural applications. • Effect of Si content on microstructure and performance of Al-Si alloy is displayed.

The effects of low intensity focused ultrasound on neuronal activity in pain processing regions in a rodent model of common peroneal nerve injury

BackgroundNon-invasive, external low intensity focused ultrasound (liFUS) offers promise for treating neuropathic pain when applied to the dorsal root ganglion (DRG). ObjectiveWe examine how external liFUS treatment applied to the L5 DRG affects neuronal changes in single-unit activity from the primary somatosensory cortex (SI) and anterior cingulate cortex (ACC) in a common peroneal nerve injury (CPNI) rodent model. MethodsMale Sprague Dawley rats were divided into two cohorts: CPNI liFUS and CPNI sham liFUS. Baseline single-unit activity (SUA) recordings were taken 20 min prior to treatment and for 4 h post treatment in 20 min intervals, then analyzed for frequency and compared to baseline. Recordings from the SI and ACC were separated into pyramidal and interneurons based on waveform and principal component analysis. ResultsFollowing CPNI surgery, all rats (n = 30) displayed a significant increase in mechanical sensitivity. In CPNI liFUS rats, there was a significant increase in pyramidal neuron spike frequency in the SI region compared to the CPNI sham liFUS animals beginning at 120 min following liFUS treatment (p < 0.05). In the ACC, liFUS significantly attenuated interneuron firing beginning at 80 min after liFUS treatment (p < 0.05). ConclusionWe demonstrate that liFUS changed neuronal spiking in the SI and ACC regions 80 and 120 min after treatment, respectively, which may in part correlate with improved sensory thresholds. This may represent a mechanism of action how liFUS attenuates neuropathic pain. Understanding the impact of liFUS on pain circuits will help advance the use of liFUS as a non-invasive neuromodulation option.

Effect of Rapid Heating and Cooling Conditions on Microstructure Formation in Powder Bed Fusion of Al-Si Hypoeutectic Alloy: A Phase-Field Study.

Al alloy parts fabricated by powder bed fusion (PBF) have attracted much attention because of the degrees of freedom in both shapes and mechanical properties. We previously reported that the Si regions in Al-Si alloy that remain after the rapid remelting process in PBF act as intrinsic heterogeneous nucleation sites during the subsequent resolidification. This suggests that the Si particles are crucial for a novel grain refinement strategy. To provide guidelines for grain refinement, the effects of solidification, remelting, and resolidification conditions on microstructures were investigated by multiphase-field simulation. We revealed that the resolidification microstructure is determined by the size and number of Si regions in the initial solidification microstructures and by the threshold size for the nucleation site, depending on the remelting and resolidification conditions. Furthermore, the most refined microstructure with the average grain size of 4.8 µm is predicted to be formed under conditions with a large temperature gradient of Gsol = 106 K/m in the initial solidification, a high heating rate of HR = 105 K/s in the remelting process, and a fast solidification rate of Rresol = 10−1 m/s in the resolidification process. Each of these conditions is necessary to be considered to control the microstructures of Al-Si alloys fabricated via PBF.

The Impact of Nanostructured Silicon and Hybrid Materials on the Thermoelectric Performance of Thermoelectric Devices: Review

Nanostructured materials remarkably improve the overall properties of thermoelectric devices, mainly due to the increase in the surface-to-volume ratio. This behavior is attributed to an increased number of scattered phonons at the interfaces and boundaries of the nanostructures. Among many other materials, nanostructured Si was used to expand the power generation compared to bulk crystalline Si, which leads to a reduction in thermal conductivity. However, the use of nanostructured Si leads to a reduction in the electrical conductivity due to the formation of low dimensional features in the heavily doped Si regions. Accordingly, the fabrication of hybrid nanostructures based on nanostructured Si and other different nanostructured materials constitutes another strategy to combine a reduction in the thermal conductivity while keeping the good electrical conduction properties. This review deals with the properties of Si-based thermoelectric devices modified by different nanostructures and hybrid nanostructured materials.

Increase of the photoconductivity quantum yield in silicon irradiated by neutrons to extremely high fluences

An enhanced quantum yield observed in silicon ionizing radiation detectors, neutron-irradiated to extremely high fluences, could be attributed to impact ionization via deep levels. The quantum yield was investigated by the intrinsic photoconductivity optical spectroscopy in silicon irradiated by neutrons to a wide range of fluences up to 1 × 1017 neutron cm−2. An increase of quantum yield was observed in highly irradiated samples. We have demonstrated that the quantum yield enhancement could be attributed to the impact ionization via deep levels, this process being presumably related to disordered defect clusters regions in Si. The proposed mechanism explains the observed decrease of the impact ionization energy by at least an order of magnitude at low temperature. The impact ionization energy values of up to 0.30–0.36 eV and less, and 0.38–0.40 eV were determined at T ∼ 21–33 K and at T = 195 K, respectively.

Mapping of the mechanical response in Si/SiGe nanosheet device geometries

The performance of next-generation, nanoelectronic devices relies on a precise understanding of strain within the constituent materials. However, the increased flexibility inherent to these three-dimensional device geometries necessitates direct measurement of their deformation. Here we report synchrotron x-ray diffraction-based non-destructive nanoscale mapping of Si/SiGe nanosheets for gate-all-around structures. We identified two competing mechanisms at different length scales contributing to the deformation. One is consistent with the in-plane elastic relaxation due to the Ge lattice mismatch with the surrounding Si. The second is associated with the out-of-plane layering of the Si and SiGe regions at a length scale of film thickness. Complementary mechanical modeling corroborated the qualitative aspects of the deformation profiles observed across a variety of nanosheet sample widths. However, greater deformation is observed in the SiGe layers of the nanosheets than the predicted distributions. These insights could play a role in predicting carrier mobilities of future devices.

Efficient friction and wear reduction of Al-Si alloy via tribofilms generated from synergistic interaction of ZDDP and chemically functionalized h-BN additives

Zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) are conventional lubricant additives for reducing friction and wear of metallic components. However, their efficacy for lightweight aluminum (Al)-based alloys is limited due to the wear of soft Al matrix regions. The present work reports significant improvement in the tribological properties of Al-Si alloys (ADC12) under liquid lubricants containing a combination of ZDDP and hexagonal boron nitride (h-BN) nanosheets. In situ monitoring of tribofilm growth, friction, and wear was carried out using atomic force microscopy (AFM)-based measurements. At high temperature (110 °C), experiments were carried out at the Al matrix and Si phase of ADC12 alloy under the lubricant containing ZDDP (1 wt%) and tert-butoxy (TBO) functionalized h-BN nanosheets [(h-BN-TBO), (0.02 mg/mL and 0.05 mg/mL)]. The addition of h-BN-TBO in ZDDP containing lubricant significantly improved the tribological properties due to the synergistic interaction between h-BN-TBO and ZDDP; leading to the formation of lubricious and robust antiwear tribofilms on Si and Al phase regions. A synergistic combination of h-BN-TBO and ZDDP as lubricant additive showed superior performance compared to ZDDP or a combination of ZDDP and MoDTC-based lubricant systems.