Thin Silicon Oxide Layer Research Articles

Vertical silicon nanowedge formation by repetitive dry and wet anisotropic etching combined with 3D self-aligned sidewall nanopatterning

Three-dimensional (3D) stacking of nano-devices is an effective method for increasing areal density, especially as downscaling of lateral device dimensions becomes impractical. This stacking is mainly achieved through plasma processing of stacked layers on top of a silicon (Si) substrate, which offers process flexibility but poses challenges in obtaining vertical sidewalls without plasma induced damage. A novel wafer-scale fabrication method is presented for realizing sub-200 nm vertically stacked Si nanowedges at the wafer scale, using iterative dry etching, wet anisotropic etching, and thermal oxidation. This approach forms nanowedges by the slow etching {111} Si planes, resulting in smooth surfaces at well-defined angles. A silicon nitride (Si3N4) hard mask is used in an iterative (etch-and-deposit) process, with its thickness determining the number of process iterations. By optimizing etch selectivity during dry etching and/or increasing the initial Si3N4 thickness, the number of process iterations can be increased. The periodicity of the nanowedges can be adjusted by varying the etch time of both dry and wet anisotropic etching. A thin silicon dioxide (SiO2) layer (∼6 nm) is grown on the nanowedges during each iteration. 3D sidewall patterning at the sub-20 nm scale is achieved using corner lithography and local oxidation of Si to selectively open the concave corners. Rhombus-shaped structures are formed at each concave corner after wet anisotropic etching of Si. This novel technology platform will allow for the 3D fabrication of high-density nanodevices for electronic, fluidic, plasmonic, and other applications.

Demonstration of 4H-silicon carbide on an aluminum nitride integrated photonic platform.

The existing silicon-carbide-on-insulator photonic platform utilizes a thin layer of silicon dioxide under silicon carbide (SiC) to provide optical confinement and mode isolation. Here, we replace the underneath silicon dioxide layer with 1-µm-thick aluminum nitride and demonstrate a 4H-silicon-carbide-on-aluminum-nitride integrated photonic platform for the first time to our knowledge. Efficient grating couplers, low-loss waveguides, and compact microring resonators with intrinsic quality factors up to 210,000 are fabricated. In addition, by undercutting the aluminum nitride layer, the intrinsic quality factor of the silicon carbide microring is improved by nearly one order of magnitude (1.8 million). Finally, an optical pump-probe method is developed to measure the thermal conductivity of the aluminum nitride layer, which is estimated to be over 30 times of that of silicon dioxide.

Three-Step Process for Efficient Solar Cells with Boron-Doped Passivated Contacts

Crystalline silicon (c-Si) solar cells with passivation stacks consisting of a polycrystalline silicon (poly-Si) layer and a thin interfacial silicon dioxide (SiO2) layer show high conversion efficiencies. Since the poly-Si layer in this structure acts as a carrier transport layer, high doping of the poly-Si layer is crucial for high conductivity and the efficient transport of charge carriers from the bulk to a metal contact. In this respect, conventional furnace-based high-temperature doping methods are limited by the solid solubility of the dopants in silicon. This limitation particularly affects p-type doping using boron. Previously, we showed that laser activation overcomes this limitation by melting the poly-Si layer, resulting in an active concentration beyond the solubility limit after crystallization. High electrically active boron concentrations ensure low contact resistivity at the (contact) metal/semiconductor interface and allow for the maskless patterning of the poly-Si layer by providing an etch-stop layer in an alkaline solution. However, the high doping concentration degrades during long high-temperature annealing steps. Here, we performed a test of the stability of such a high doping concentration under thermal stress. The active boron concentration shows only a minor reduction during SiNx:H deposition at a moderate temperature and a fast-firing step at a high temperature and with a short exposure time. However, for an annealing time tanneal = 30 min and an annealing temperature 600 °C ≤ Tanneal≤ 1000 °C, the high conductivity is significantly reduced, whereas a high passivation quality requires annealing in this range. We resolve this dilemma by introducing a second, healing laser reactivation step, which re-establishes the original high conductivity of the boron-doped poly-Si and does not degrade the passivation. After a thermal annealing temperature Tanneal = 985 °C, the reactivated layers show high sheet conductance (Gsh) with Gsh = 24 mS sq and high passivation quality, with the implied open-circuit voltage (iVOC) reaching iVOC = 715 mV. Therefore, our novel three-step process consisting of laser activation, thermal annealing, and laser reactivation/healing is suitable for fabricating highly efficient solar cells with p++-poly-Si/SiO2 contact passivation layers.

Creation of color centers in diamond by recoil implantation through dielectric films

The need of near-surface color centers in diamond for quantum technologies motivates the controlled doping of specific extrinsic impurities into the crystal lattice. Recent experiments have shown that this can be achieved by momentum transfer from a surface precursor via ion implantation, an approach known as “recoil implantation.” Here, we extend this technique to incorporate dielectric precursors for creating nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers in diamond. Specifically, we demonstrate that gallium focused-ion-beam exposure to a thin layer of silicon nitride or silicon dioxide on the diamond surface results in the introduction of both extrinsic impurities and carbon vacancies. These defects subsequently give rise to near-surface NV and SiV centers with desirable properties after annealing.

Activation of Al2O3 surface passivation of silicon: Separating bulk and surface effects

Understanding surface passivation arising from aluminium oxide (Al2O3) films is of significant relevance for silicon-based solar cells and devices that require negligible surface recombination. This study aims to understand the competing bulk and surface lifetime effects which occur during the activation of atomic layer deposited Al2O3. We demonstrate that maximum passivation is achieved on n- and p-type silicon with activation at ∼ 450 °C, irrespective of annealing ambient. Upon stripping the Al2O3 films and re-passivating the surface using a superacid-based technique, we find the bulk lifetime of float-zone and Czochralski silicon wafers degrade at annealing temperatures > 450 °C. By accounting for this bulk lifetime degradation, we demonstrate that the chemical passivation component associated with Al2O3 remains stable at activation temperatures of 450─500 °C, achieving an SRV of < 1 cm/s on n- and p-type silicon. In conjunction with the thermal stability, we show that films in the range of 3–30 nm maintain an SRV of < 1 cm/s when annealed at 450 °C. From atomic-level energy dispersive X-ray analysis, we demonstrate that, post deposition, the interface has a structure of Si/SiO2/Al2O3. After activation at > 300 °C, the interface becomes Si/SixAlyO2/Al2O3 due to diffusion of aluminium into the thin silicon oxide layer.

Facile and Low-Cost Fabrication of SiO2-Covered Au Nanoislands for Combined Plasmonic Enhanced Fluorescence Microscopy and SERS.

An easy and low-cost way to fabricate monometallic Au nanoislands for plasmonic enhanced spectroscopy is presented. The method is based on direct thermal evaporation of Au on glass substrates to form nanoislands, with thicknesses between 2 and 15 nm, which are subsequently covered by a thin layer of silicon dioxide. We have used HR-SEM and AFM to characterize the nanoislands, and their optical transmission reveals strong plasmon resonances in the visible. The plasmonic performance of the fabricated substrates has been tested in fluorescence and Raman scattering measurements of two probe materials. Enhancement factors up to 1.8 and 9×104 are reported for confocal fluorescence and Raman microscopies, respectively, which are comparable to others obtained by more elaborated fabrication procedures.

Using Vertically Aligned Carbon Nanofiber Arrays on Rigid or Flexible Substrates for Delivery of Biomolecules and Dyes to Plants.

The delivery of biomolecules and impermeable dyes to intact plants is a major challenge. Nanomaterials are up-and-coming tools for the delivery of DNA to plants. As exciting as these new tools are, they have yet to be widely applied. Nanomaterials fabricated on rigid substrate (backing) are particularly difficult to successfully apply to curved plant structures. This study describes the process for microfabricating vertically aligned carbon nanofiber arrays and transferring them from a rigid to a flexible substrate. We detail and demonstrate how these fibers (on either rigid or flexible substrates) can be used for transient transformation or dye (e.g., fluorescein) delivery to plants. We show how VACNFs can be transferred from rigid silicon substrate to a flexible SU-8 epoxy substrate to form flexible VACNF arrays. To overcome the hydrophobic nature of SU-8, fibers in the flexible film were coated with a thin silicon oxide layer (2-3 nm). To use these fibers for delivery to curved plant organs, we deposit a 1 µL droplet of dye or DNA solution on the fiber side of VACNF films, wait 10 min, place the films on the plant organ and employ a swab with a rolling motion to drive fibers into plant cells. With this method, we have achieved dye and DNA delivery in plant organs with curved surfaces.

EXTRAORDINARY MAGNETORESISTANCE OF LASERANNEALED NANO BORON DEPOSITEDON OXIDIZED POROUS SILICON

This study explores the impact of laser annealing on the electrical and magnetic properties of nano boron deposited on oxidized porous silicon (n-B/PSiO2) and its potential for spintronic applications. The Nd: YAG laser was used at varying energies to anneal the n-B thin films. Increasing the laser energy increased grain size and more ordered grain structures. It also increased surface roughness due to forming new grain boundaries and secondary phases. The electrical properties of the material were also affected by the laser annealing, with an increase in forward and reverse current and an increase in electrical resistivity with increased annealing temperature. The study also found that the magnetoresistance of the material increased with increasing laser temperature, attributed to tunnel injection through the thin silicon dioxide layer, and could be up to 7 times higher than non-annealed n-B/PSiO2in a magnetic field. The study highlights the importance of controlling materials’ grain size and structure for their physical and electrical properties. In addition, it provides insights into the electronic properties of n-B/PSiO2and the behavior of charge carriers in a magnetic field.

In Situ Reflectometry and Diffraction Investigation of the Multiscale Structure of p-Type Polysilicon Passivating Contacts for c-Si Solar Cells.

The integration of passivating contacts based on a highly doped polycrystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) layer has been identified as the next step to further increase the conversion efficiency of current mainstream crystalline silicon (c-Si) solar cells. However, the interrelation between the final properties of poly-Si/SiOx contacts and their fabrication process has not yet been fully unraveled, which is mostly due to the challenge of characterizing thin-film stacks with features in the nanometric range. Here, we apply in situ X-ray reflectometry and diffraction to investigate the multiscale (1 Å–100 nm) structural evolution of poly-Si contacts during annealing up to 900 °C. This allows us to quantify the densification and thinning of the poly-Si layer during annealing as well as to monitor the disruption of the thin SiOx layer at high temperature >800 °C. Moreover, results obtained on a broader range of thermal profiles, including firing with dwell times of a few seconds, emphasize the impact of high thermal budgets on poly-Si contacts’ final properties and thus the importance of ensuring a good control of such high-temperature processes when fabricating c-Si solar cells integrating such passivating contacts. Overall, this study demonstrates the robustness of combining different X-ray elastic scattering techniques (here XRR and GIXRD), which present the unique advantage of being rapid, nondestructive, and applicable on a large sample area, to unravel the multiscale structural evolution of poly-Si contacts in situ during high-temperature processes.

Plasmon Wave Propagation Property of Metal Wedge Plasmonic Waveguides Covered by a Protective Oxide Layer

Guiding plasmon waves is based on dielectric/metal interfaces. The wedge-shaped interface shows an excellent capacity in the tight lightwave confinement at deep-subwavelength propagation mode size. Several types of metals have also been investigated for guiding plasmon waves. Among them, the Ag metal shows a plasmon wave guiding ability superior to other metals, however, it is sensitive to the operating medium and is easily oxidized. To overcome these drawbacks, the Ag wedge covered by a protective thin oxide layer is proposed. Numerically investigated results show that the propagation length of the Ag wedge covered by a protective thin silicon dioxide layer can be enhanced by a factor of 7.5 while its figure of merit is at least 1.7 times larger than that of the Au wedge waveguide. The advantage of the proposed interface is potential for developing plasmonic waveguide components.

Classification of Two Volatiles Using an eNose Composed by an Array of 16 Single-Type Miniature Micro-Machined Metal-Oxide Gas Sensors.

The artificial replication of an olfactory system is currently an open problem. The development of a portable and low-cost artificial olfactory system, also called electronic nose or eNose, is usually based on the use of an array of different gas sensors types, sensitive to different target gases. Low-cost Metal-Oxide semiconductor (MOX) gas sensors are widely used in such arrays. MOX sensors are based on a thin layer of silicon oxide with embedded heaters that can operate at different temperature set points, which usually have the disadvantages of different volatile sensitivity in each individual sensor unit and also different crossed sensitivity to different volatiles (unspecificity). This paper presents and eNose composed by an array of 16 low-cost BME680 digital miniature sensors embedding a miniature MOX gas sensor proposed to unspecifically evaluate air quality. In this paper, the inherent variability and unspecificity that must be expected from the 16 embedded MOX gas sensors, combined with signal processing, are exploited to classify two target volatiles: ethanol and acetone. The proposed eNose reads the resistance of the sensing layer of the 16 embedded MOX gas sensors, applies PCA for dimensional reduction and k-NN for classification. The validation results have shown an instantaneous classification success higher than 94% two days after the calibration and higher than 70% two weeks after, so the majority classification of a sequence of measures has been always successful in laboratory conditions. These first validation results and the low-power consumption of the eNose (0.9 W) enables its future improvement and its use in portable and battery-operated applications.

Numerical Simulation and Experiment of a High-Efficiency Tunnel Oxide Passivated Contact (TOPCon) Solar Cell Using a Crystalline Nanostructured Silicon-Based Layer

We report on the tunnel oxide passivated contact (TOPCon) using a crystalline nanostructured silicon-based layer via an experimental and numerical simulation study. The minority carrier lifetime and implied open-circuit voltage reveals an ameliorated passivation property, which gives the motivation to run a simulation. The passivating contact of an ultra-thin silicon oxide (1.2 nm) capped with a plasma enhanced chemical vapor deposition (PECVD) grown 30 nm thick nanocrystalline silicon oxide (nc-SiOx), provides outstanding passivation properties with low recombination current density (Jo) (~1.1 fA/cm2) at a 950 °C annealing temperature. The existence of a thin silicon oxide layer (SiO2) at the rear surface with superior quality (low pinhole density, Dph &lt; 1 × 10−8 and low interface trap density, Dit ≈ 1 × 108 cm−2 eV−1), reduces the recombination of the carriers. The start of a small number of transports by pinholes improves the fill factor (FF) up to 83%, reduces the series resistance (Rs) up to 0.5 Ω cm2, and also improves the power conversion efficiency (PEC) by up to 27.4%. The TOPCon with a modified nc-SiOx exhibits a dominant open circuit voltage (Voc) of 761 mV with a supreme FF of 83%. Our simulation provides an excellent match with the experimental results and supports excellent passivation properties. Overall, our study proposed an ameliorated knowledge about tunnel oxide, doping in the nc-SiOx layer, and additionally about the surface recombination velocity (SRV) impact on TOPCon solar cells.

SiO2:P,Dy nano-thin film phosphor: Synthesis, structure and luminescence properties

A nano-sized thin film layer of phosphorus-dysprosium doped silicon oxide (SiO2:P,Dy) was successfully synthesized using the sol-gel method combined with spin coating technique. The surface morphology, structure, and luminescence properties of the synthesized thin film were characterized using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR), and Photoluminescence (PL). The average surface roughness was measured as 22.04 nm. The optical band gaps, Eg, of the samples were estimated using the Tauc model. A relatively good tensile strength between layers of the thin film of 10-SiO2:P,Dy at 450 °C was observed. The characteristic main emission lines of Dy3+ for the 10 and 15 times coated samples were measured at 2.16 eV and 1.98 eV originated from the 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 transitions, respectively. The thermoluminescence of 10- SiO2:P,Dy thin films annealed at 1000 °C exhibits generally broad TL glow curves peaking at 260 °C.

Polysilicon passivated junctions: The next technology for silicon solar cells?

<h2>Summary</h2> Despite the maturity of crystalline silicon photovoltaics (c-Si PV), the last 6 years have seen a string of efficiency improvements, most of which are centered around reducing the losses related to the directly metallized, heavily doped regions found in conventional c-Si solar cells. Among these advancements, polysilicon (poly-Si) passivated junctions, formed by embedding a thin silicon oxide (SiO₂) layer between the c-Si wafer and a highly doped poly-Si layer, are emerging as one of the most promising alternatives, and efficiencies above 26% have already been demonstrated. The excellent performance of this junction architecture has been found to be remarkably independent of the deposition and/or doping technique used—even extending to techniques already prevalent in industry. This greatly reduces the capital and retraining expenditure needed to integrate the new technology into mainstream production lines, allowing it to be an evolutionary, rather than disruptive advancement. This has led to the rapid demonstration of large-area cells featuring poly-Si contacts by multiple PV manufacturing companies, with efficiencies above 24.5%. Although a bright future for poly-Si junctions is anticipated, as supported by the predictions of the International Technology Roadmap of Photovoltaics, several issues remain to be resolved, including those associated with the cost of and damage to the poly-Si layers due to the cell's metallization process. This paper provides a perspective of the remaining challenges and potential of poly-Si junctions to transform the PV industry.

Theoretical design of a triple-band perfect metamaterial absorber based on graphene with wide-angle insensitivity

This paper proposes a square ring-shaped triple-band perfect absorber based on graphene with silicon as the substrate, gold as the reflector and a thin dielectric layer of silicon dioxide. After simulation by the FDTD method, the numerical results show that the absorption spectrum based on the periodic array structure of graphene exhibits three perfect absorption peaks in the terahertz band, corresponding to 25.53 μm, 36.44 μm and 53.44 μm, respectively, and they all reach 99% absorption rate. By changing the gate voltage, the material parameters of graphene can be easily adjusted, so as to achieve dynamic control of the absorption characteristics of the absorber. Due to the high symmetry of the structure, it has a good angular polarization tolerance in the far infrared band. In addition, the obvious spectral shift caused by the change of surrounding environment refractive index and the values of sensitivity (S) and figure of merit (FOM) are all indicates that our absorber has certain application prospects in the field of sensing. Furthermore, our absorber has shown great application potential in light detection in infrared and terahertz band as a consequence of the strong tunability.

Graphene-assisted infrared plasmonic metamaterial absorber for gas detection

In this paper, we propose a graphene-assisted plasmonic metamaterial absorber to operate as an ultra-compact optical gas sensor in the infrared (IR) region. The metamaterial absorber is comprised of a thin silicon-dioxide layer that is middling between a gold film and an array of graphene-coated gold nano-disks. Using the finite element method, we investigate the resonance characteristics of the proposed plasmonic metamaterial and show that the metamaterial has three distinct resonances in visible and infrared regions. We demonstrate that the strongest absorption peak of the proposed metamaterial is attributed to a fundamental localized surface plasmon resonance in IR, which has also the highest spectral sensitivity to the refractive index (RI) variations of the surrounding medium. The sensing mechanism is based on evaluation of gaseous medium RI surrounding the nano-disks, by illuminating the metamaterial with IR radiation and subsequently measuring the reflection or absorption spectra of the structure. We show that graphene coating of the gold nano-disks improves the sensitivity of the metamaterial to the gas RI by more than two times. Using the optimized design parameters, an average linear spectral sensitivity of 720 nm/RIU and an approximate detection limit of 1 × 10−5 RIU has been obtained for gas RI variations between 1 and 1.05. We also show that the proposed sensor has a linear response even for extremely small variations in gas RI in the order of 10−5 RIU. The proposed metamaterial configuration is relatively easy to fabricate and can be used to monitor low concentrations of various gases in different applications ranging from environmental monitoring to home safety monitoring systems.

Drain current characterization of dielectric modulated split gate TFET for bio-sensing application

This paper presents an analytical surface potential and drain current model of split gate (SG) Tunnel Field Effect Transistor (TFET) for label free detection of biomolecules e.g. protein, biotine, DNA, etc., utilizing the concept of underlap and dielectric modulation approach. A portion of the device channel is left open in between the splitted gates with a thin layer of silicon dioxide as adhesive layer. This open cavity acts as the binding site for the biomolecules to be detected which alters the effective capacitance of the region and hence modulates the device characteristic trend. The device electrostatics are developed by solving two dimensional Poisson’s equation and Young’s parabolic approximation and consequent drain current is then formulated applying Kane’s model. To assess the sensing ability of the device, both neutral and charged biomolecules with different dielectric values are immobilized within the cavity and the impact of variation in dielectric constant, charge density and height of biomolecules on device characteristics is analyzed. In addition to this, drain current sensitivity of the device is also investigated by varying the permittivity, charge density (both positive and negative) and height of the bio-species. All the analytical results are evaluated with corresponding Sentaurus TCAD simulated data to corroborate the accuracy of the derived model.

A Low Spring Constant Piezoresistive Microcantilever for Biological Reagent Detection.

This paper introduces a piezoresistive microcantilever with a low spring constant. The microcantilever was fabricated with titanium (Ti) as the piezoresistor, a low spring constant polyimide (PI) layer, and a thin silicon oxide (SiO2) layer as the top and bottom passive layers, respectively. Excellent mechanical performances with the spring constant of 0.02128 N/m and the deflection sensitivity of 1.03 × 10−7 nm−1 were obtained. The output voltage fluctuation of a Wheatstone bridge, which consists of four piezoresistive microcantilevers, is less than 3 μV@3 V in a phosphate buffered saline (PBS) environment. A microcantilever aptasensor was then developed through functionalizing the microcantilevers with a ricin aptamer probe, and detections on ricin with concentrations of 10, 20, 50 and 100 ng/mL were successfully realized. A good specificity was also confirmed by using bovine serum albumin (BSA) as a blank control. The experiment results show that the Ti and PI-based microcantilever has great prospects for ultrasensitive biochemical molecule detections with high reliability and specificity.

A New Silicon Dioxide-Coated MALDI-ToF Sample Plate for Peptide Analysis.

In this report, we describe the development and testing of a new coated plate which improves the sensitivity and accuracy in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS). The coated plate was covered with a thin layer of hydrophobic silicon dioxide, which enabled sample enrichment due to the water repellent nature of the silicon dioxide surface. Sensitivity and required laser strengths were tested using peptide standards, with the results that these coated plates required lower laser power and showed increased sensitivity than that of common plates. Accuracy was tested using bacteria, saliva, and serum samples. The coated plates showed significantly increased degrees of accuracy through their capacity to reduce mass shift. The importance and necessity of accuracy analysis in the assessment of new sample plates, which is rarely described in other papers, is also discussed.

Wet Chemical Oxidation to Improve Interfacial Properties of Al2O3/Si and Interface Analysis of Al2O3/SiOx/Si Structure Using Surface Carrier Lifetime Simulation and Capacitance–Voltage Measurement

A thin silicon oxide (SiOx) layer (thickness: 1.5–2.0 nm) formed at an Al2O3/Si interface can enhance the interface properties. However, it is challenging to control the characteristics of thin SiOx layers because SiOx forms naturally during Al2O3 deposition on Si substrates. In this study, a ~1.5 nm-thick SiOx layer was inserted between Al2O3 and Si substrates by wet chemical oxidation to improve the passivation properties. The acidic solutions used for wet chemical oxidation were HCl:H2O2:H2O, H2SO4:H2O2:H2O, and HNO3. The thicknesses of SiOx layers formed in the acidic solutions were ~1.48, ~1.32, and ~1.50 nm for SiOx-HCl, SiOx-H2SO4, and SiOx-HNO3, respectively. The leakage current characteristics of SiOx-HNO3 were better than those of the oxide layers formed in the other acidic solutions. After depositing a ~10 nm-thick Al2O3 on an SiOx-acidic/Si structure, we measured the effective carrier lifetime using quasi steady-state photoconductance and examined the interfacial properties of Al2O3/SiOx-acidic/Si using surface carrier lifetime simulation and capacitance–voltage measurement. The effective carrier lifetime of Al2O3/SiOx-HNO3/Si was relatively high (~400 μs), resulting from the low surface defect density (2.35–2.88 × 1010 cm−2eV−1). The oxide layer inserted between Al2O3 and Si substrates by wet chemical oxidation helped improve the Al2O3/Si interface properties.