Ultrathin Silicon Research Articles

Comparison of random upright pyramids and inverted pyramid photonic crystals in thin crystalline silicon solar cells: An optical and morphological study

The potential of enabling a wide range of applications with lightweight, flexible, thin silicon photovoltaic (PV) devices has led to research interest in various light-trapping technologies for thin silicon absorbers. Herein, using both experiments and photonic modelling, we present a detailed comparison between random pyramid texturing and inverted pyramid photonic crystal patterning in thin silicon vis-à-vis light trapping. In particular, we investigate the potential of uniform single-dimension periodic structures in contrast to conventional multi-dimensional random structures for high broadband absorption efficiency in ultra-thin silicon. We find that inverted pyramid photonic crystals cause a large increase in optical path length especially for long wavelength photons due to strong wave interference producing exceptionally high absorption and so does random pyramids though with slightly lower absorption than that of photonic crystals. On the other hand, the random pyramids, fabricated via a one-step etching process with feature sizes ranging from sub-micron to 4 µm, have higher absorption at short wavelengths due to their multi-dimensional size distribution. Overall, we find that random textured pyramids appropriately fabricated can yield a comparable ideal photocurrent density to that by photonic crystals for 10 µm thick silicon.

Ultrasonic resonance-based inspection of ultra-thin nickel sheets bonded to silicone

In the field of non-destructive testing (NDT), The detection of bonding defects between ultra-thin metal and silica gel is a difficult problem. In this study, In this study, ultrasonic resonance method was used to evaluate the bonding strength of ultra-thin metal to silica gel bonding structure. The composite parts of ultra-thin nickel sheet and silicon sheet with three different bonding states were studied. The bonding state of nickel sheet and silica gel is different, and the absorption of ultrasound is different. Using the resonance generated by high-frequency ultrasound in ultra-thin nickel sheet, the acoustic attenuation of the combination of ultra-thin nickel sheet and silicon rubber sheet was analyzed by resonance signal, and the bonding state between ultra-thin nickel sheet and silicon rubber sheet was characterized by bonding coefficient. Through experimental comparison, the results showed that the attenuation of ultrasonic signal in the nickel sheet and silicon film with different adhesive states characterize the adhesive state of ultra-thin nickel sheet and silicon film by the bonding coefficient, the bonding coefficient of good parts, weak adhesive parts and debonded parts is reduced successively. By setting an appropriate determination threshold value, the bonding state between the ultra-thin nickel sheet and the silicon film can be accurately determined according to the bonding coefficient obtained by detection.

Optimization of activated phosphorus concentration in recrystallized polysilicon layers for the n-TOPCon solar cell application

The pivotal structure of n-TOPCon (tunnel oxide passivating contact) solar cells is the passivating contact structure composed of a heavily doped polysilicon (poly-Si) layer and an ultrathin silicon oxide (SiOx), which can provide excellent selectively carrier transport. The activated phosphorus concentration in the passivating contact structure plays a crucial role in the passivation quality and contact resistivity performance. By adjusting PH3 flow rate, SiOx thickness and activation temperature, three different activated phosphorus concentrations in poly-Si were achieved in this work, the phosphorus concentration from SiOx to silicon bulk decreased at the same rate. It can be found that the passivation quality decreases with increasing concentration and the contact resistivity decreases significantly. With the aid of ellipsometry, we successfully extracted the refractive index n and extinction coefficient k of three poly-Si with different activated phosphorus concentrations. The extracted n and k values can well feedback the influence of concentration change on short-circuit current density in the simulation. Due to the high concentration led to the decrease of the open circuit voltage and short circuit current of the final TOPCon solar cell, but the obvious filling factor benefit can be obtained, only moderate doping concentration can obtain the best efficiency performance. Finally, the activated phosphorus concentration of about 3 × 1020 cm−1 can obtain the best efficiency, >24.8% on average.

Light absorption enhancement of silicon solar cell based on horizontal nanowire arrays

The effects of geometry and horizontal positioning of ultrathin triangular silicon nanowires (SiNWs) on the function of solar cells (SCs) have been investigated. Two differentiated methods have been presented to improve the current density of SiNW-SCs. First, a semiperiodic array is used for better utilization of the optical antenna effect. In the second method, several NWs with different geometries are used to achieve broadband absorption. The key to increase absorption is irregularity. Therefore, maximum light absorption is achieved by introducing irregular NWs. A set of optimized irregular NWs have better properties compared to other broadband structures. Using irregular NW arrays, the increase in the absorption of SCs can be improved without applying more absorbent materials. Optimization processes are used to obtain the maximum current density of SCs.

A computer vision approach for analyzing label free leukocyte trafficking dynamics on a microvascular mimetic.

High-content imaging techniques in conjunction with in vitro microphysiological systems (MPS) allow for novel explorations of physiological phenomena with a high degree of translational relevance due to the usage of human cell lines. MPS featuring ultrathin and nanoporous silicon nitride membranes (µSiM) have been utilized in the past to facilitate high magnification phase contrast microscopy recordings of leukocyte trafficking events in a living mimetic of the human vascular microenvironment. Notably, the imaging plane can be set directly at the endothelial interface in a µSiM device, resulting in a high-resolution capture of an endothelial cell (EC) and leukocyte coculture reacting to different stimulatory conditions. The abundance of data generated from recording observations at this interface can be used to elucidate disease mechanisms related to vascular barrier dysfunction, such as sepsis. The appearance of leukocytes in these recordings is dynamic, changing in character, location and time. Consequently, conventional image processing techniques are incapable of extracting the spatiotemporal profiles and bulk statistics of numerous leukocytes responding to a disease state, necessitating labor-intensive manual processing, a significant limitation of this approach. Here we describe a machine learning pipeline that uses a semantic segmentation algorithm and classification script that, in combination, is capable of automated and label-free leukocyte trafficking analysis in a coculture mimetic. The developed computational toolset has demonstrable parity with manually tabulated datasets when characterizing leukocyte spatiotemporal behavior, is computationally efficient and capable of managing large imaging datasets in a semi-automated manner.

Effect of Grain Size Before Cold Rolling on Microstructure, Texture and Magnetic Properties of Ultra-Thin Low-Si Non-oriented Silicon Steel

Effect of Grain Size Before Cold Rolling on Microstructure, Texture and Magnetic Properties of Ultra-Thin Low-Si Non-oriented Silicon Steel

Characterizing an Optically Induced Sub-micrometer Gigahertz Acoustic Wave in a Silicon Thin Plate.

Optically induced GHz-THz guided acoustic waves have been intensively studied because of the potential to realize noninvasive and noncontact material inspection. Although the generation of photoinduced guided acoustic waves utilizing nanostructures, such as ultrathin plates, nanowires, and materials interfaces, is being established, experimental characterization of these acoustic waves in consideration of the finite size effect has been difficult due to the lack of experimental methods with nm × ps resolution. Here we experimentally observe the sub-micrometer guided acoustic waves in a nanofabricated ultrathin silicon plate by ultrafast transmission electron microscopy with nm × ps precision. We successfully characterize the excited guided acoustic wave in frequency-wavenumber space by applying Fourier-transformation analysis on the bright-field movie. These results suggest the great potential of ultrafast transmission electron microscopy to characterize the acoustic modes realized in various nanostructures.

Chronic Stability of Local Field Potentials Using Amorphous Silicon Carbide Microelectrode Arrays Implanted in the Rat Motor Cortex.

Implantable microelectrode arrays (MEAs) enable the recording of electrical activity of cortical neurons, allowing the development of brain-machine interfaces. However, MEAs show reduced recording capabilities under chronic conditions, prompting the development of novel MEAs that can improve long-term performance. Conventional planar, silicon-based devices and ultra-thin amorphous silicon carbide (a-SiC) MEAs were implanted in the motor cortex of female Sprague-Dawley rats, and weekly anesthetized recordings were made for 16 weeks after implantation. The spectral density and bandpower between 1 and 500 Hz of recordings were compared over the implantation period for both device types. Initially, the bandpower of the a-SiC devices and standard MEAs was comparable. However, the standard MEAs showed a consistent decline in both bandpower and power spectral density throughout the 16 weeks post-implantation, whereas the a-SiC MEAs showed substantially more stable performance. These differences in bandpower and spectral density between standard and a-SiC MEAs were statistically significant from week 6 post-implantation until the end of the study at 16 weeks. These results support the use of ultra-thin a-SiC MEAs to develop chronic, reliable brain-machine interfaces.

Single-crystalline van der Waals layered dielectric with high dielectric constant.

The scaling of silicon-based transistors at sub-ten-nanometre technology nodes faces challenges such as interface imperfection and gate current leakage for an ultrathin silicon channel1,2. For next-generation nanoelectronics, high-mobility two-dimensional (2D) layered semiconductors with an atomic thickness and dangling-bond-free surfaces are expected as channel materials to achieve smaller channel sizes, less interfacial scattering and more efficient gate-field penetration1,2. However, further progress towards 2D electronics is hindered by factors such as the lack of a high dielectric constant (κ) dielectric with an atomically flat and dangling-bond-free surface3,4. Here, we report a facile synthesis of a single-crystalline high-κ (κ of roughly 16.5) van der Waals layered dielectric Bi2SeO5. The centimetre-scale single crystal of Bi2SeO5 can be efficiently exfoliated to an atomically flat nanosheet as large as 250 × 200 μm2 and as thin as monolayer. With these Bi2SeO5 nanosheets as dielectric and encapsulation layers, 2D materials such as Bi2O2Se, MoS2 and graphene show improved electronic performances. For example, in 2D Bi2O2Se, the quantum Hall effect is observed and the carrier mobility reaches 470,000 cm2 V-1 s-1 at 1.8 K. Our finding expands the realm of dielectric and opens up a new possibility for lowering the gate voltage and power consumption in 2D electronics and integrated circuits.

High-performance thin-film lithium niobate electro-optic modulator based on etching slot andultrathin silicon film.

An electro-optic modulator (EOM) is an indispensable component to connect the electric and optical fields. Here, we propose a high-performance, thin-film lithium niobate-based EOM, where the modulation waveguide is formed by an etching slot on the lithium niobate film and the deposit of an ultrathin silicon film in the slot region. Therefore, a small mode size and high mode energy can be simultaneously achieved in the LN region with a high EO coefficient, which will be beneficial to increase the EO overlap and gradually decrease in the mode size. Further, we employed a waveguide structure to construct a typical Mach-Zehnder interference-type EOM. According to the requirements of high-speed traveling wave modulation, we conduct the index matching, impedance matching, and low-loss operation. From the results, the key half-wave voltage length product and 3dB modulation bandwidth are, respectively, 1.45Vcm and 119GHz in a modulation length of 4mm. Moreover, a larger 3dB bandwidth also can be achieved by shortening the modulation length. Therefore, we believe the proposed waveguide structure and EOM will provide new ways to enhance the performance of LNOI-based EOMs.

High‐Performance Transparent Silicon Nanowire Thin Film Transistors Integrated on Glass Substrates via a Room Temperature Solution Passivation

AbstractCatalytic synthesized ultrathin silicon nanowires (SiNWs) are ideal 1D channel materials to fabricate high‐performance transparent and low‐cost thin film transistors (TFTs) that are widely needed for flexible electronics and displays. In this work, a scalable integration of orderly array of SiNW array, with a uniform diameter of only 52 ± 4 nm, grown directly upon glass/wafer substrates, via a guided in‐plane solid–liquid–solid (IPSLS) process, and passivated by a new solution oxidizing/etching cycling technique is demonstrated. This has enabled an all‐low‐temperature (<350 °C) fabrication of high‐performance SiNW‐TFTs, achieving Ion/Ioff current ratio and subthreshold swing (SS) of >106 and 120 mV dec−1 respectively, with excellent negative and positive bias stabilities. Importantly, the SiNW‐TFTs fabricated on glasses with ITO/or metal electrodes demonstrate a high transparency of 90% or 73% respectively, making them ideal candidates for building the next generation of high aperture displays, transparent electronics, and augmented reality applications.

Engineering Heterointerface for High-efficiency Silicon Solar Cells

High-performance passivating contacts are crucial to realizing high-efficiency crystalline silicon (c-Si) solar cells. The passivating contacts can efficiently transport photogenerated carriers from c-Si to metal electrodes. A representative passivating contact is the stack of ultra-thin silicon oxide (SiOx) and doped polycrystalline Si. In the structure, the layer thickness of silicon oxide is limited to 1～2 nm to collect photogenerated carriers. For further improvements, we have developed NAnocrystal Transport path in Ultra-thin dielectrics for REinforced passivating contact（NATURE contact）as a new passivating contact. The NATURE contact consists of Si nanocrystals (NCs) and SiOx matrix. The Si NCs and the SiOx matrix function as carrier transport paths and passivation layers for the c-Si surface, respectively. The NATURE contact can attain good passivation performance and conductive property. In this paper, we briefly explain the fundamentals of the passivating contacts and introduce the properties of the NATURE contact.

Application of Si-related Ultrathin (∼1 nm) Films to Crystalline Silicon Solar Cells

Ultrathin (∼1 nm) films have been widely used for high-efficiency crystalline silicon (c-Si) solar cells such as tunnel oxide passivated contact (TOPCon) solar cells. In this article, we present our recent results for the applications of ultrathin silicon oxide (SiOx) and silicon nitride (SiNx) films to c-Si solar cells. The following topics are reviewed. 1) Ultrathin SiNx can be utilized for passivating contacts instead of SiOx in the TOPCon structure. 2) The addition of SiOx between c-Si and thick catalytic-chemical-vapor-deposited (Cat-CVD) SiNx significantly improves the quality of surface passivation. 3) Ultrathin Al-doped SiOx films formed just by dipping in Al(NO3)3 solution on c-Si provide strong upward band bending due to negative fixed charges, which can be used for hole-selective contacts.

A Comparison of Biaxial Bending Strength of Thin Silicon Dies in the Ball-on-Ring and PoEF Tests

Abstract This paper aims to demonstrate the biaxial bending strength test on thin silicon dies using the classic ball-on-ring (BoR) test and discuss it in detail by comparing those results with the newly-proposed point load on elastic foundation (PoEF) test. The geometric linear and nonlinear solutions to the BoR test are reviewed and also provided with theoretical and numerical formulations, respectively. Three different thicknesses (t = 42 μm, 57 μm, and 82 μm) of the thin silicon dies (with a size of 10 mm × 10 mm) are tested in the BoR test, and their bending strengths, load–displacement curves and failure modes are presented and thoroughly discussed with a comparison of the published data from the PoEF test. It is found that, for the bending strengths of 57 μm and 82 μm-thick dies, the data from both the BoR and PoEF tests are very consistent, but not for 42 μm-thick dies with a relatively lower value in the BoR test. This lower strength value in the BoR test is attributed to the more pronounced local buckling effect than the PoEF test. Based on that, it can concluded that the PoEF test is better and more reliable than the conventional BoR as for testing the ultrathin silicon dies, even though both tests are identical for the regularly thin dies.

Plasmon-enhanced parabolic nanostructures for broadband absorption in ultra-thin crystalline Si solar cells.

Sub-wavelength plasmonic light trapping nanostructures are promising candidates for achieving enhanced broadband absorption in ultra-thin silicon (Si) solar cells. In this work, we use finite-difference time-domain (FDTD) simulations to demonstrate the light harvesting properties of periodic and parabola shaped Si nanostructures, decorated with metallic gold (Au) nanoparticles (NPs). The active medium of absorption is a 2 μm thick crystalline-silicon (c-Si), on top of which the parabolic nanotextures couple incident sunlight into guided modes. The parabola shape provides a graded refractive index profile and high diffraction efficiencies at higher order modes leading to excellent antireflection effects. The Au NPs scatter light into the Si layer and offer strong localized surface plasmon resonance (LSPR) resulting in broadband absorption with high conversion efficiency. For wavelengths (λ) ranging between 300 nm and 1600 nm, the structure is optimized for maximum absorption by adjusting the geometry and periodicity of the nanostructures and the size of the Au NPs. For parabola coated with 40 nm Au NPs, the average absorption enhancements are 7% (between λ = 300 nm and 1600 nm) and 28% (between λ = 800 nm and 1600 nm) when compared with bare parabola. Furthermore, device simulations show that the proposed solar cell can achieve a power conversion efficiency (PCE) as high as 21.39%, paving the way for the next generation of highly efficient, ultra-thin and low-cost Si solar cells.

Solid phase crystallization of amorphous silicon at the two-dimensional limit.

The epitaxy of silicene-on-Ag(111) renewed considerable interest in silicon (Si) when scaled down to the two-dimensional (2D) limit. This remains one of the most explored growth cases in the emerging 2D material fashion beyond graphene. However, out of a strict silicene framework, other allotropic forms of Si still deserve attention owing to technological purposes. Here, we present 2D Solid Phase Crystallization (SPC) of Si starting from an amorphous-Si on Ag(111) in atomic coverage to gain a crystalline-Si layer by post-growth annealing below 450 °C, namely Complementary Metal Oxide Semiconductor (CMOS) Back-End-of-Line (BEOL) thermal budget limit. Moreover, considering the benefit of the 2D-SPC scheme, we managed to write crystalline-Si pixels on the amorphous-Si matrix. Our in situ and ex situ analyses show that an in-plane interface or a lateral heterojunction between amorphous and crystalline-Si is formed. This amorphous-to-crystalline phase transformation suggests that 2D silicon may stem from an epitaxially grown layer and thermal self-organization/assembling.

Nanostripe-Confined Catalyst Formation for Uniform Growth of Ultrathin Silicon Nanowires

Uniform growth of ultrathin silicon nanowire (SiNW) channels is the key to accomplishing reliable integration of various SiNW-based electronics, but remains a formidable challenge for catalytic synthesis, largely due to the lack of uniform size control of the leading metallic droplets. In this work, we explored a nanostripe-confined approach to produce highly uniform indium (In) catalyst droplets that enabled the uniform growth of an orderly SiNW array via an in-plane solid-liquid-solid (IPSLS) guided growth directed by simple step edges. It was found that the size dispersion of the In droplets could be reduced substantially from Dcatpl = 20 ± 96 nm on a planar surface to only Dcatns = 88 ± 13 nm when the width of the In nanostripe was narrowed to Wstr= 100 nm, which could be qualitatively explained in a confined diffusion and nucleation model. The improved droplet uniformity was then translated into a more uniform growth of ultrathin SiNWs, with diameter of only Dnw= 28 ± 4 nm, which has not been reported for single-edge guided IPSLS growth. These results lay a solid basis for the construction of advanced SiNW-derived field-effect transistors, sensors and display applications.

An SOI-Structured Piezoresistive Differential Pressure Sensor with High Performance.

This paper presents a piezoresistive differential pressure sensor based on a silicon-on-insulator (SOI) structure for low pressure detection from 0 to 30 kPa. In the design phase, the stress distribution on the sensing membrane surface is simulated, and the doping concentration and geometry of the piezoresistor are evaluated. By optimizing the process, the realization of the pressure sensing diaphragm with a controllable thickness is achieved, and good ohmic contact is ensured. To obtain higher sensitivity and high temperature stability, an SOI structure with a 1.5 µm ultra-thin monocrystalline silicon layer is used in device manufacturing. The device diaphragm size is 700 µm × 700 µm × 2.1 µm. The experimental results show that the fabricated piezoresistive pressure sensor has a high sensitivity of 2.255 mV/V/kPa and a sensing resolution of less than 100 Pa at room temperature. The sensor has a temperature coefficient of sensitivity (TCS) of -0.221 %FS/°C and a temperature coefficient of offset (TCO) of -0.209 %FS/°C at operating temperatures ranging from 20 °C to 160 °C. The reported piezoresistive microelectromechanical systems (MEMS) pressure sensors are fabricated on 8-inch wafers using standard CMOS-compatible processes, which provides a volume solution for embedded integrated precision detection applications of air pressure, offering better insights for high-temperature and miniaturized low-pressure sensor research.

Unique characteristics of the novel-GTAW process for the butt joint of ultra-thin silicon steel sheets

Butt joining of ultra-thin silicon steel sheets is a major challenge because of the ultra-thin thickness, high silicon content, and the coating layer on their surfaces. For the first time, welding butt-joint of the 0.2 mm ultra-thin silicon steel sheets has been performed successfully and reported in this paper. The results show that the weld beads fabricated by the novel-GTAW process are straight, stable, and without defects. They have a narrower width on the top and a larger width on the bottom side compared to the conventional-GTAW process. The mechanical properties of the welding joints also overmatch that of the base metal, confirming good qualities of the weldment. The success of the novel GTAW process in butt weld of ultra-thin silicon steel sheets is because of its unique characteristics: First, the mechanism of the novel-GTAW torch allows removing and blowing the insulation coating layer to prevent its penetration into the melting point and its accumulation on the electrode during the welding process; thus, eliminating welding defects and electrode wear and maintaining the arc stability. Second, the novel-GTAW process enables superiorly reducing the heat input and increasing the cooling rate, which ensures the high quality of the butt weld of ultra-thin silicon steel sheets. Lastly, the shielding efficiency of the melting pool is enhanced to prevent silicon oxidation and welding defects.

Ultrathin (3.7 nm) Silicon Oxide Layers with a Low Concentration of Broken Bonds on the Contact with a Semiconductor

Ultrathin (3.7 nm) Silicon Oxide Layers with a Low Concentration of Broken Bonds on the Contact with a Semiconductor