Si Plane Research Articles

Structural Anisotropy in Stretchable Silicon

AbstractPatterned planar silicon (Si) semiconductor structures able to conform around 3D surfaces are promising candidates for wearable devices from solar cells to displays. Despite the known anisotropic material properties of crystalline semiconductors, prior evaluations have assumed isotropic stretchable mechanical behavior. The effect of structural anisotropy on the mechanical stretching behavior of {100} single crystalline Si planes is demonstrated through models and experiments. 3D finite element analysis results show serpentines fabricated and strained loaded on the dense {111} family of planes will have maximum von Mises peak stresses that are 20% higher than those fabricated in the <100> direction on the {100} family of planes. A fabrication process is presented to release in‐plane Si serpentine test structures from (100) silicon‐on‐insulator wafers aligned parallel to the <110> and <100> crystallographic orientations. The released test structures can accommodate strains to 84%. Raman spectroscopy is used to characterize anisotropic effects on the internal stresses of Si serpentines. Raman measurements confirm two different maximum stress locations on <110> and <100> Si serpentines, resulting in two different break locations. All results show anisotropy plays a critical role in the mechanical behavior of stretchable Si serpentines and must be considered during design of stretchable semiconductor structures.

Effect of Seed Layer and Thermal Annealing on Structural and Optical Properties of Silicon Layers Deposited by PECVD

Hydrogenated silicon (Si:H) layers have been deposited on crystalline silicon substrate by radiofrequency plasma-enhanced chemical vapor deposition technique (rf-PECVD) from SiH4/H2 gas mixture using different deposition times and after a seed layer with thickness of 70 nm (to improve the nucleation of nanocrystallites). The effect of the deposition time on the as-deposited and annealed layers was investigated. Ultraviolet–visible–infrared (UV–Vis–IR) spectroscopy was used to determine the thickness of the layers. Before annealing, the layer thickness showed a linear variation with deposition time in the range from 450 s to 1500 s. On the other hand, for deposition times of 450 s, 600 s, and 1500 s, the layers showed an x-ray diffraction (XRD) peak corresponding to the (111) crystal plane of Si. Raman spectroscopy confirmed the appearance of the crystalline phase in these layers. Raman scattering studies showed that, when using the short deposition time for the top layer, the crystalline peak dominated. We deduce that insertion of a seed layer improves the crystallinity of the top layer. In addition, the crystallinity increased slightly with the deposition time. The structural properties of the layers annealed at 900°C were studied by Raman spectroscopy and XRD analysis. We noted an increase in the crystalline volume fraction and the appearance of (111) crystal plane of silicon, confirming the transition from amorphous to nanocrystalline phase for all the annealed films.

Dynamic processes in Si and Si/C anodes in lithium-ion batteries during cycling

Si is one of the most attractive anode materials because of its extremely high theoretical capacity. In this study, we find that the (111) plane of crystalline Si in the anode is still present after 100cycles and infer that it is the intercalation site of Li+ ions. We believe that Li+ ions are inserted into the tetrahedral sites of crystalline Si, and the (111) plane of Si collapses; meanwhile, expansion parallel to 〈220〉 crystal orientation occurs. After full lithiation, the crystalline Si is converted to amorphous silicon. In a Si/ composite anode, although the addition of amorphous carbon may improve the cycling performance of Si anode, the activation process is prolonged as the amount of amorphous carbon increased. We determine that amorphous carbon may reduce the diffusion of Li+, which increases the overpotential and results in the lower formation of the saturated Li/Si alloy.

Effective light polarization insensitive and omnidirectional properties of Si nanowire arrays developed on different crystallographic planes

In this paper, fabrication of vertical Si nanowire arrays (SiNWAs) by a facile metal assisted chemical etching approach on different crystallographic planes of Si has been reported. A very low specular reflectance (Rspec) of 0.04% and 0.03% has been achieved in the whole visible range for SiNWAs grown on Si(100) and Si(111) oriented substrates, respectively. High broadband enhancement has been detected for vertical SiNWAs due to multiple scattering paths inside the nanowire arrays. On the other hand, inclined nanowires showed a fascinating behavior at the longer wavelength regime, where light gets the longer path to reflect back-forth and ease to reflect back outward at normal incidence. Moreover, for [100] SiNWAs, transverse electric field component demonstrates the strong polarization insensitive properties at the expense of transverse magnetic field component with a minimum reflectance of <2% up to 1200 nm. The [100] SiNWAs demonstrates extraordinary omnidirectional properties at θB ≥ 58°. Theoretical validation of COMSOL with an effective medium approach reveal the effective dipole coupling and the presence of strong absorption modes for vertical SiNWs at a typical wavelength regime. The highly bound states of the particle tunneling through classical forbidden region shows a strong dependence on the gradient in the refractive index (mi) from 1 to 3.4. The high order scattering effect is observed at ∼520 cm−1 in a disordered optical medium. This novel finding of light localization properties for SiNWAs with different orientation gives a new route to support various photonic applications.

Magnetoelectric coupling tailored by the orientation of the nanocrystals in only one component in percolative multiferroic composites.

Novel 1–3 type multiferroic composite thin films were prepared via a simple 0–3 composite fabrication process. The orientation of the Ni0.5Zn0.5Fe2O4 (NZFO) nanocrystals in the BaTiO3–Ni0.5Zn0.5Fe2O4 (BTO–NZFO) composite thin films was controlled by magnetron sputtering, and the controlling effect of such orientation on magnetoelectric coupling was investigated in detail. The NZFO lattice in the BTO–NZFO composite thin films grew with (100) orientation under the induction of the (111) plane of Si. The transfer of substrate stress between the closely contacted grains also contributes to the orientation of the NZFO nanocrystals. The 0.6BTO–0.4(100)NZFO multiferroic composite thin film exhibited a magnetization strength of 9.2% at the Curie point of the BTO phase, showing extremely strong magnetoelectric coupling characteristics. This work provides an effective strategy for the development of high-performance multiferroic composites, and brings about new conception of realizing strong magnetoelectric coupling effect from the perspective of physical chemistry.

Molecular dynamic simulation of orientation-dependent effect on silicon crystalline during sputtering process of focused ion beam

In this paper, experiments and molecular dynamic simulation are performed to investigate the crystal orientation-dependent effects in a focused ion beam (FIB) sputtering etching process on silicon crystal planes. The experimental results indicate that different silicon orientation planes have different etched profiles, aspect ratios on microstructures, and sputtering yields under the same process conditions, which brings difficulty in deciding the process parameters for the fabrication of high-resolution nanostructures. Molecular dynamic simulation based on empirical potential is carried out to explain this effect on an atomic scale. The sputtered profiles of different silicon crystal planes after hundreds of impacts by Ga+ ions are analyzed. Both the simulation and experimental results show that with the same FIB etching conditions, structures on an Si (111) plane have the maximum opening width and minimum depth. Compared with the etched profile on an Si (100) plane, trenches on Si (110) have a smaller width, but greater depth. The Si (111) plane has the minimum sputtering yield. Furthermore, simulation results on the distribution of Ga+ ions remaining inside the substrate, number of ions sputtered out of the surface, and common neighbor analysis (CNA) can explain the orientation-dependent effect during FIB sputtering etching.

Multicolored-light emission from InGaN/GaN multiple quantum wells grown by selective-area epitaxy on patterned Si(100) substrates

GaN-based light-emitting diodes (LEDs) structures with microstripes of InGaN/GaN quantum wells were grown on the Si(111) planes of microscale V-grooved Si(100) substrate by metal–organic vapor-phase epitaxy. This microstriped microfacet structure is composed of two semipolar (1 $$ \bar{1} $$ 01) planes and two polar (0001) planes. The indium (In) composition and thickness of quantum wells grown on semipolar (1 $$ \bar{1} $$ 01) plane differ from polar (0001) plane, causing the variation of local emission wavelength along the planes. Using the cathodoluminescence (CL) techniques, it was found that the emission wavelength is longer at the top of the (0002) planes, whereas it is shorter at the bottom of the (0002) plane and the (1 $$ \bar{1} $$ 01) plane. The continuous change in the properties of the quantum wells results in a uniform multicolored emission band. The results show that unique structures could uniformly activate multicolored light to realize the integrated light emission over the most visible spectrum. In some situations, this arrangement can emit white light. This method can be used to produce integrated white-light sources for monolithically integrated white-light-emitting diodes.

Improving the Performance of the Optical Antenna for Integrated LIDAR with Optical Phased Arrays through High Contrast Grating Structure on SOI Substrate

With the development of automatic driving and remote sensing technology, light detection and ranging (LIDAR) has attracted more attentions. The optical phased arrays in silicon photonics have became one of the major solutions of LIDAR due to its low material cost, high integration, and compatibility with CMOS technology. However, most of the optical antennas used by LIDAR are directly etched second-order gratings on the array waveguides, which makes the emission efficiency very low, and a large part of the energy is wasted by the side lobes and grating lobes. Therefore, improving the antenna emission efficiency and its energy utilization has become the key solution to improve the performance of phased-array LIDAR systems. In this paper, a novel optical antenna for integrated LIDAR with optical phased arrays is proposed and simulated. The high contrast grating (HCG) structure is used to achieve extremely efficient emission. The basic structure of the optical antenna consists of a 220nm Si plane waveguide at the top-Si layer of SOI substrate, a 110nm SiO2 gap layer, and a 463nm Polysilicon layer. The HCG period is 1.228μm and duty cycle is 0.388, and it is etched at the Polysilicon layer. The proposed optical antenna has improved emission efficiency, energy utilization, and achievable scanning range. It is possible to increase emission efficiency significantly by the resonance properties of the HCG. 81.6% emission efficiency of the optical antenna is obtained with background suppression of 28.4 dB at 1550 nm. Moreover, the optical antenna can achieve more than 50% emission efficiency in the wavelength range of 1484 nm to 1609 nm. ±34.82°×18.99° scanning range was realized in lateral direction and longitudinal direction, respectively, by changing the phase difference between adjacent waveguides and the wavelength from 1480 nm to 1580 nm. In lateral direction, the scanning range achieved by the proposed optical antenna is 1.54 times larger than that of an arrayed waveguide grating with the same size. In longitudinal direction, the proposed optical antenna can achieve substantially the same scanning range as the arrayed waveguide grating. Figure 1

EUV photofragmentation and oxidation of a polyarylene – Sulfonium resist: XPS and NEXAFS study

Secondary electrons play a vital role in Extreme Ultraviolet Lithography (EUVL). However, the mechanisms and chemical reactions after photon absorption in this high energy region only recently have been studied in more detail. The present work is an attempt to obtain more information about the photofragmentation process when EUV radiation is used in polymers. EUV irradiation on a non-chemically amplified resist (non-CAR), poly[methyl (4-(phenyl thio) - phenyl) – sulfonium trifluoromethanesulfonate] (PAS), was studied using monochromatic synchrotron radiation at 103.5 eV of excitation energy. XPS and NEXAFS spectroscopy were used to follow the surface chemical changes after the samples were irradiated and oxidized. The results showed the fast desorption of the sensitive sulfonium triflate with important rearrangements that kept the sulfur contents and polymer backbone with minor changes. This was confirmed by the detection of new SC and SS signals after irradiation. New carbonyl signals were also detected and it was also observed that the aromatic rings resisted the irradiation of EUV photons. The photofragmentation process showed a different mechanism in relation with similar resists that we have studied in the past. Angle resolved-NEXAFS results and semiempirical calculations showed that the PAS film adopted a random configuration in relation to the Si(100) plane. It may allow a strong interaction after EUV irradiation between the excited species or radicals formed and the polymer chains favoring rearrangement reactions. The present investigation shows the need to understand the photochemical mechanisms under EUV photon excitations which are different to UV light. This knowledgement may lead to prepare better resist formulations for EUVL. Based on our finding a simple mechanism was proposed.

Production of high throughput nano-porous silicon (NPS) powder with different architectures

In this study, a powder technology manufacturing route as a pioneering, low cost, simple and safe method is used for the fabrication of nano-porous silicon (NPS) powder. It is a promising one as its high throughput for different applications due to their supposed low toxicity and good compatibility. The NPS has been prepared using a combination between high energy ball milling technique and wet alkali chemical etching based technique through the utilization of commercial silicon powder; with high yield efficiency (86.3%). Preparation of several NPS shapes (nanorods, nanoplates, nanwalls, nanospheres and nanobelts) are fabricated, which could be used in different applications. The most significant reduction of the crystallite size was observed by increasing the milling time. Transformation of crystallographic plane of the commercial powder Si (400) to the NPS {(111), (220), (211), (312) and (400)} is resulted, using several concentrations of (KOH) as the etchant and wetting agent {n-propanol (NPA)}.

SERS based detection of glucose with lower concentration than blood glucose level using plasmonic nanoparticle arrays

Surface Enhanced Raman Scattering (SERS) based detection of glucose, without using binder molecule, is difficult due to very small Raman scattering cross section of glucose and its very low affinity/binding with metal nanoparticles. This was overcome by using coupled plasmonic arrays of metal nanoparticles, which produced large field enhancement responsible for higher SERS intensity. It has been observed that the Raman intensity of glucose deposited on patterned Si with silver nanoparticle arrays is higher compared to that on plane Si and plane Si with randomly ordered nanoparticles. The nanoparticle arrays on patterned Si can detect glucose concentration as low as 5 × 10−5 g/ml, which is much lower than blood glucose level. The method can be used for the development of low concentration detection technologies for glucose.

Difference in anisotropic etching characteristics of alkaline and copper based acid solutions for single-crystalline Si

The so called inverted pyramid arrays, outperforming conventional upright pyramid textures, have been successfully achieved by one-step Cu assisted chemical etching (CACE) for light reflection minimization in silicon solar cells. Due to the lower reduction potential of Cu2+/Cu and different electronic properties of different Si planes, the etching of Si substrate shows orientation-dependent. Different from the upright pyramid obtained by alkaline solutions, the formation of inverted pyramid results from the coexistence of anisotropic etching and localized etching process. The obtained structure is bounded by Si {111} planes which have the lowest etching rate, no matter what orientation of Si substrate is. The Si etching rate and (100)/(111) etching ratio are quantitatively analyzed. The different behaviors of anisotropic etching of Si by alkaline and Cu based acid etchant have been systematically investigated.

On the atomic model of Guinier-Preston zones in Al-Mg-Si-Cu alloys

Structural feature of equiaxed Guinier-Preston zones with the diameter of ∼2 nm in an Al-Mg-Si-Cu alloy has been systematically investigated by three-dimensional atom probe analyses, high-resolution electron microscopy and first-principles calculations. The results suggest that Si and Mg planes alternate along [001]Al with Cu atoms diversely substituting for some Si sites. An fcc-based Mg1(Si,Cu)1 model was then proposed for the first time, with expanded a, b and compressed c lattice parameters compared to Al. Meanwhile the structural evolution of Mg1(Si,Cu)1 → (Mg,Cu)1(Si,Cu)1 → β″-(Mg,Cu)1Mg4(Al,Cu)2Si4 is suggested.

Formation of 45° Silicon (110) Surface Using Triton X-n Surfactants in Potassium Hydroxide for Infrared Applications

Silicon (Si) micromirrors are an integral feature for many micro-optomechanical systems (MOEMS). Such mirrors are generally wet etched in alkaline solution at elevated temperature. For 90° beam steering applications, 45° slanted Si (110) plane is the prime choice fabricated with the incorporation of tensioactive surfactants. Here, Triton-Si and Triton-hydroxide (OH−)/H2O interaction using varying hydrophilic chain length Triton (X-45, X-100 and X-405) were investigated. The surfactant concentration was varied from 0 to 1000 ppm in potassium hydroxide (KOH). Triton molecules were shown to adsorb preferentially on (110) than on (100) surface. Longer chain length Triton hampered OH− access to Si surface resulting in slower etch rate. In contrast, contact angle measurement suggested that shorter Triton interfaced better with Si surface. Later, Si wafers etched in Triton 10 ppm – KOH were examined. The measured output for (110)X-45, (110)X-100, (110)X-405 and polished Si wafer reference (Rq < 5Å) mirrors were 0.58, 0.76, 0.72 and 1.25 mW, respectively. Subsequently, Si-SiO2 thin film in [HLHL]2-substrate configuration was fabricated. Broadband micromirror for use in 3.0–5.5 μm spectrum range was experimentally realized with reflected efficiency of 73%.

High-performance MoS2/Si heterojunction broadband photodetectors from deep ultraviolet to near infrared.

Polycrystalline 2D layered molybdenum disulfide (MoS2) films were synthesized via a thermal decomposition method. The MoS2/Si heterostructures were constructed in situ by synthesis MoS2 on plane Si substrates. Such MoS2/Si heterostructures exhibited high sensitivity to light illumination with wavelengths ranging from the deep ultraviolet to the near infrared. Photoresponse analysis reveals that a high responsivity of 23.1A/W, a specific detectivity of 1.63×1012 Jones, and a fast response speed of 21.6/65.5 μs were achieved. Notably, the MoS2/Si heterojunction photodetector could operate with excellent stability and repeatability over a wide frequency range up to 150kHz. The high performance could be attributed to the high-quality heterojunction between MoS2 and Si obtained by the in situ fabrication process. Such high performance with broadband response suggests that MoS2/Si heterostructures could have great potential in optoelectronic applications.

Metal-like Band Structures of Ultrathin Si {111} and {112} Surface Layers Revealed through Density Functional Theory Calculations.

Density functional theory calculations have been performed on Si (100), (110), (111), and (112) planes with tunable number of planes for evaluation of their band structures and density of states profiles. The purpose is to see whether silicon can exhibit facet-dependent properties derived from the presence of a thin surface layer having different band structures. No changes have been observed for single to multiple layers of Si (100) and (110) planes with a consistent band gap between the valence band and the conduction band. However, for 1, 2, 4, and 5 Si (111) and (112) planes, metal-like band structures were obtained with continuous density of states going from the valence band to the conduction band. For 3, 6, and more Si (111) planes, as well as 3 and 6 Si (112) planes, the same band structure as that seen for Si (100) and (110) planes has been obtained. Thus, beyond a layer thickness of five Si (111) planes at ≈1.6 nm, normal semiconductor behavior can be expected. The emergence of metal-like band structures for the Si (111) and (112) planes are related to variation in Si-Si bond length and bond distortion plus 3s and 3p orbital electron contributions in the band structure. This work predicts possession of facet-dependent electrical properties of silicon with consequences in FinFET transistor design.

Arrangement of GaN nanowires on Si(001) substrates studied by X-ray diffraction: Importance of silicon nitride interlayer

X-ray diffraction measurements with the use of both laboratory and synchrotron X-ray sources and transmission electron microscopy have been applied to study two types of GaN nanowires grown by plasma assisted molecular beam epitaxy on Si (001) substrates. Detailed structural analysis of the first type of nanowires reveals wurtzite type of GaN grown with the c-axis perpendicular to the substrate surface and with good in-plane correlation with the substrate. A thin silicon nitride film is found between these nanowires and Si (001). Second group contains GaN nanowires of a novel type that are aligned perpendicular to the Si (111) planes, so are inclined to the Si (001) substrate surface. They grow without any interlayer in a direct contact with the substrate, first as rooflet-like zb-GaN islands and then transform to more stable wurtzite phase by the growth of pure hexagonal wz-GaN nanowires. Our findings show a crucial role of silicon nitride interlayer for growth mode and crystallographic alignment of GaN nanowires on silicon substrate.

Anisotropic relaxation behavior of InGaAs/GaAs selectively grown in narrow trenches on (001) Si substrates

Selective area growth of InGaAs inside highly confined trenches on a pre-patterned (001) Si substrate has the potential of achieving a high III-V crystal quality due to high aspect ratio trapping for improved device functionalities in Si microelectronics. If the trench width is in the range of the hetero-layer thickness, the relaxation mechanism of the mismatched III-V layer is no longer isotropic, which has a strong impact on the device fabrication and performance if not controlled well. The hetero-epitaxial nucleation of InxGa1-xAs on Si can be simplified by using a binary nucleation buffer such as GaAs. A pronounced anisotropy in strain release was observed for the growth of InxGa1-xAs on a fully relaxed GaAs buffer with a (001) surface inside 20 and 100 nm wide trenches, exploring the full composition range from GaAs to InAs. Perpendicular to the trench orientation (direction of high confinement), the strain release in InxGa1-xAs is very efficiently caused by elastic relaxation without defect formation, although a small compressive force is still induced by the trench side walls. In contrast, the strain release along the trenches is governed by plastic relaxation once the vertical film thickness has clearly exceeded the critical layer thickness. On the other hand, the monolithic deposition of mismatched InxGa1-xAs directly into a V-shaped trench bottom with {111} Si planes leads instantly to a pronounced nucleation of misfit dislocations along the {111} Si/III-V interfaces. In this case, elastic relaxation no longer plays a role as the strain release is ensured by plastic relaxation in both directions. Hence, using a ternary seed layer facilitates the integration of InxGa1-xAs covering the full composition range.

Formation of crystalline heteroepitaxial SiC films on Si by carbonization of polyimide Langmuir–Blodgett films

High-quality crystalline nano-thin SiC films on Si substrates were prepared by carbonization of polyimide (PI) Langmuir–Blodgett (LB) films. The obtained films were characterized by Fourier transform-infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, Raman spectroscopy, transmission electon microscopy (TEM), transmission electron diffraction (TED), and scanning electron microscopy (SEM). We demonstrated that the carbonization of a PI film on a Si substrate at 1000 °C leads to the formation of a carbon film and SiC nanocrystals on the Si substrate. It was found that five planes in the 3C-SiC(111) film are aligned with four Si(111) planes. As a result of repeated annealing of PI films containing 121 layers at 1200 °C crystalline SiC films were formed on the Si substrate. It was shown that the SiC films (35 nm) grown on Si(111) at 1200 °C have a mainly cubic 3C-SiC structure with small amount of hexagonal polytypes. Only 3C-SiC films (30 nm) were formed on the Si(100) substrate at the same temperature. It was shown that the SiC films (30–35 nm) can cover the voids with size up to 10 µm in the Si substrate. The current–voltage (I–V) characteristics of the n-Si/n-SiC heterostructure were obtained by conductive atomic force microscopy.

Domain formation mechanism of the Si(110) “ 16×2 ” reconstruction

The main factor that determines which of the two domains form upon reconstruction of the Si(110)"16 x 2" surface has been investigated. LEED and STM images showed that the domain orientation was independent of the heating current direction used to induce the Si(110)"16 x 2" reconstruction. Reciprocal-space lattice models of the reconstruction allowed for the correct identification of the domain orientations in the LEED images and confirm that the reconstruction is 2D-chiral. It is proposed that the domain orientation upon surface reconstruction is determined by the direction of monoatomic steps present on the Si(110) plane. This is in turn determined by the direction at which the surface is polished off-axis from the (110) plane.