Planar Si Research Articles

Recent Advances in Silicon FET Devices for Gas and Volatile Organic Compound Sensing

Highly sensitive and selective gas and volatile organic compound (VOC) sensor platforms with fast response and recovery kinetics are in high demand for environmental health monitoring, industry, and medical diagnostics. Among the various categories of gas sensors studied to date, field effect transistors (FETs) have proved to be an extremely efficient platform due to their miniaturized form factor, high sensitivity, and ultra-low power consumption. Despite the advent of various kinds of new materials, silicon (Si) still enjoys the advantages of excellent and reproducible electronic properties and compatibility with complementary metal–oxide–semiconductor (CMOS) technologies for integrated multiplexing and signal processing. This review gives an overview of the recent developments in Si FETs for gas and VOC sensing. We categorised the Si FETs into Si nanowire (NW) FETs; planar Si FETs, in which the Si channel is either a part of the silicon on insulator (SOI) or the bulk Si, as in conventional FETs; and electrostatically formed nanowire (EFN) FETs. The review begins with a brief introduction, followed by a description of the Si NW FET gas and VOC sensors. A brief description of the various fabrication strategies of Si NWs and the several functionalisation methods to improve the sensing performances of Si NWs are also provided. Although Si NW FETs have excellent sensing properties, they are far from practical realisation due to the extensive fabrication procedures involved, along with other issues that are critically assessed briefly. Then, we describe planar Si FET sensors, which are much closer to real-world implementation. Their simpler device architecture combined with excellent sensing properties enable them as an efficient platform for gas sensing. The third category, the EFN FET sensors, proved to be another potential platform for gas sensing due to their intriguing properties, which are elaborated in detail. Finally, the challenges and future opportunities for gas sensing are addressed.

Temperature-dependent photoluminescence of InAs quantum dots Grown on Silicon

This paper presents detailed studies on the temperature dependent photoluminescence (PL) of self-assembled InAs/GaAs quantum dots grown on planar on-axis Si (001) substrates. Through the analyze, we have a better understanding of carrier transport processes into InAs/GaAs quantum dots system. Before 160K, the carrier transport from wetting layer to excited states (ES) and ground states (GS) is domination. As the temperature keeps rising, carrier thermal escape from GS to ES is domination.

Design Principles for Efficient and Stable Water Splitting Photoelectrocatalysts

ConspectusPhotoelectrochemical water splitting is a promising avenue for sustainable production of hydrogen used in the chemical industry and hydrogen fuel cells. The basic components of most photoelectrochemical water splitting systems are semiconductor light absorbers coupled to electrocatalysts, which perform the desired chemical reactions. A critical challenge for the design of these systems is the lack of stability for the majority of desired semiconductors under operating water splitting conditions. One strategy to address this issue is to protect the semiconductor by covering it with a stabilizing insulator layer, creating a metal-insulator-semiconductor (MIS) architecture, which has demonstrated improved stability. In addition to enhanced stability, the insulator layer may significantly affect the electron and hole transfer, which governs the recombination rates. Furthermore, the insertion of an insulator layer leads to the introduction of additional insulator/electrocatalyst and insulator/semiconductor interfaces. These interfaces can impact the system's performance significantly, and they need to be carefully engineered to optimize the efficiencies of MIS systems. In this Account, we describe our recent progress in shedding light on the critical role of the insulator and the interfaces on the performance of MIS systems. We discuss our findings by focusing on the concrete example of planar n-type Si protected by a HfO2 insulator layer and coupled to a Ni or Ir electrocatalyst that performs the oxygen evolution reaction, one of the water splitting half-reactions. To improve our fundamental understanding of the insulator layer, we precisely control the HfO2 insulator thickness using atomic layer deposition (ALD), and we perform a series of rigorous electrochemical experiments coupled with theory and modeling. We demonstrate that by tuning the insulator thickness, we can control the flux and recombination of photogenerated electrons and holes to optimize the generated photovoltage. Despite optimizing the thickness, we find that the maximum generated photovoltage in MIS systems is often significantly lower than the upper performance limit, i.e., there are additional losses in the system that could not be addressed by optimizing the insulator thickness. We identify the sources of these losses and describe strategies to minimize them by a combination of improving the semiconductor light absorption, removing nonidealities associated with interfacial defects, and finding alternative insulators with improved charge carrier selectivity. Finally, we quantify the improvements that can be obtained by implementing these specific strategies. Our collective work outlines strategies to analyze MIS systems, identify the sources of efficiency losses, and optimize the design to approach the fundamental performance limits. These general approaches are broadly applicable to photoelectrochemical materials that utilize sunlight to produce value-added chemicals.

Silicon Waveguide Analysis

The Maxwell’s equations are a basic formulation using for solve propagation equations within a waveguide. In this research, the wave propagation inside the waveguide was studied and some basic parameters of wave propagation were measured through MATLAB 2013. Silicon with a refractive index 3.46 was chosen to study this propagation and the use of core and cladding with a lower refractive index 2.46 and some parameters were extracted for the waveguide including propagation constant, cutoff wavenumber, cutoff frequency, cutoff thickness and normalized parameter. These parameters are studied at the wavelength 0.6 μm for transvers electric TE and transvers magnetic TM we changed the core thickness at wavelength constant and found that the number of modes increases as the increase of core thickness and the waveform is thinner in TM than it is in TE. We note from the measurements that the value of cutoff frequency and cutoff thickness are equal in the two types TE and TM. Moreover, when two modes (m=0, 1) and core thickness 0.24 μm the value of propagation constant in TE is equal (34.8901, 30.7737) while in TM (30.6440, 26.1967) respectively, we find that the values in TE are greater than TM. Also, the results obtained from Finite Difference Method were compared with the method used (Maxwell’s equation through wave equation solutions). This work represents a short pathway for the theoretical analysis concerning the electromagnetic waves propagating through Si planar waveguide with both modes (TE and TM) considered.

The past and future of multi-gate field-effect transistors: Process challenges and reliability issues

This work reviews the state-of-the art multi-gate field-effect transistor (MuGFET) process technologies and compares the device performance and reliability characteristics of the MuGFETs with the planar Si CMOS devices. Owing to the 3D wrapped gate structure, MuGFETs can suppress the SCEs and improve the ON-current performance due to the volume inversion of the channel region. As the Si CMOS technology pioneers to sub-10 nm nodes, the process challenges in terms of lithography capability, process integration controversies, performance variability etc. were also discussed in this work. Due to the severe self-heating effect in the MuGFETs, the ballistic transport and reliability characteristics were investigated. Future alternatives for the current Si MuGFET technology were discussed at the end of the paper. More work needs to be done to realize novel high mobility channel MuGFETs with better performance and reliability.

Integration of Fe3O4 nanospheres and micropyramidal textured silicon wafer with improved photoelectrochemical performance

Silicon (Si)-based composites have attracted extensive attention for photoelectrochemical (PEC) application. Herein, micropyramidal textured Si wafer was constructed by wet chemical etching method. Hydrothermally synthesized Fe3O4 nanospheres were further deposited on the micropyramids by a simple dip-coating approach. The integrated Fe3O4 nanospheres and Si micropyramids (denoted as Fe3O4@SiMPs) revealed 20 times of higher PEC efficiency than planar Si wafer, without obvious photocurrent decay and crystalline structure change during chronoamperometry test. The greatly enhanced PEC performance of Fe3O4@SiMPs is largely attributed to the seamless integration of Si micropyramids and Fe3O4 nanospheres. The micropyramidal textured structure can enhance light absorption by reducing the surface reflectance and enhancing the light trapping effect. The micropyramids and Fe3O4 nanospheres on the surface of planar Si wafer offer more specific surface area for the contact of electrode with electrolyte. The Fe3O4 nanospheres not only protect the micropyramids from corrosion, but also accelerate the PEC kinetics by promoting charge transfer between the electrode and the electrolyte. This study can inspire the optimal design of Si wafer-based nanocomposites as efficient PEC catalysts for overall water splitting.

3D InGaN nanowire arrays on oblique pyramid-textured Si (311) for light trapping and solar water splitting enhancement

We study Ⅲ-nitride nanowire (NW) anti-reflection structures employed as photoelectrocatalyst for solar water splitting. We found that 1D vertical InGaN NW arrays tilted by 73° exhibit maximized photocurrent. Therefore, we grow 3D InGaN NW arrays on the facets of oblique pyramid-textured Si (311) (OPSi/InGaN) by plasma-assisted molecular beam epitaxy, exhibiting facets tilted by 73°. In addition, using finite difference time domain simulations we find the crucial impact of the asymmetry of the oblique pyramid InGaN NW arrays on the light trapping. Compared with InGaN NWs grown on a planar Si substrate (Si/InGaN), the OPSi/InGaN photoanode exhibits ~500% enhancement of the photocurrent due to various light trapping effects attributed to: 1D vertical NW arrays, their tilt, 3D arrangement and asymmetry. Decorated with NiOOH co-catalyst, the OPSi/InGaN photoanode exhibits photocurrent densities in the mA cm−2 range. The present research provides a rational design for the fabrication of nanostructured photoelectrocatalyst with enhanced light absorption.

Laser CVD growth of graphene/SiC/Si nano-matrix heterostructure with improved electrochemical capacitance and cycle stability

Si-based micro-supercapacitors are considered to be promising in energy storage due to their compatibility to modern electronics and high energy densities. However, the capacitance and cycle stability of the electrode remains to be further improved. In this work, we fabricated high-crystalline graphene/SiC (G/SiC) nano-matrix on the surface of planar Si and Si nanowires (SiNWs) array by laser chemical vapor deposition. The surface of SiNWs was fully covered with the G/SiC passivation layers, which composed of highly <001>−orientated 3C–SiC with graphene distributed in the films and the outermost layer. Compared to the SiNWs and planar G/SiC/Si structures, the G/SiC/SiNWs nano-matrix exhibited a low charge transfer resistance of 3.1 Ω, and a high areal capacitance value of ∼3.2 mF/cm2 achieved at a CV scan rate of 50 mV/s, nearly 10 times and 75 times higher than those of SiNWs and G/SiC/Si, respectively. The capacitance of the G/SiC/SiNWs nano-matrix was increased by 115% after 10,000 cyclic voltammetry cycles at 100 mV/s, showing excellent cyclic stability. This type of G/SiC/SiNWs nano-matrix heterostructure electrode paves the way for Si-based electrochemical applications.

Defect-Tolerant TiO2-Coated and Discretized Photoanodes for &gt;600 h of Stable Photoelectrochemical Water Oxidation

Arrays of GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition (MOCVD) onto photoactive planar Si substrates. This tandem, vertical-wire-array-on-planar absorber...

Improvement of electrical characteristics of SnSe/Si heterostructure by integration of Si nanowires

In this study, the effects of the nanowire geometry on Si wafer substrate were investigated for the SnSe/Si-nanowire heterojunction device and the obtained results were compared with the one fabricated on planar Si surface. Nanowires on Si surface were produced by metal-assisted etching method and the SnSe film layer was deposited by thermal evaporation technique. On both Si and glass surfaces, deposited film shows polycrystalline and single SnSe phase. From optical transmission measurements, optical band gap of this film was determined as 1.36 eV in a good agreement with the literature. All SnSe/Si heterostructures were found in a p-n diode behavior and the ideality factor and series resistance values were calculated as 2.40, 547 Ω, and 3.71, 1.57 × 103 Ω, for SnSe/Si-nanowire and SnSe/Si heterojunctions, respectively. As a result, an improvement in device characteristics concerning the planar Si structure was found by utilizing Si nanowire structure.

Fractal Web Design of a Hemispherical Photodetector Array with Organic-Dye-Sensitized Graphene Hybrid Composites.

The vision system of arthropods consists of a dense array of individual photodetecting elements across a curvilinear surface. This compound-eye architecture could be a useful model for optoelectronic sensing devices that require a large field of view and high sensitivity to motion. Strategies that aim to mimic the compound-eye architecture involve integrating photodetector pixels with a curved microlens, but their fabrication on a curvilinear surface is challenged by the use of standard microfabrication processes that are traditionally designed for planar, rigid substrates (e.g., Si wafers). Here, a fractal web design of a hemispherical photodetector array that contains an organic-dye-sensitized graphene hybrid composite is reported to serve as an effective photoactive component with enhanced light-absorbing capabilities. The device is first fabricated on a planar Si wafer at the microscale and then transferred to transparent hemispherical domes with different curvatures in a deterministic manner. The unique structural property of the fractal web design provides protection of the device from damage by effectively tolerating various external loads. Comprehensive experimental and computational studies reveal the essential design features and optoelectronic properties of the device, followed by the evaluation of its utility in the measurement of both the direction and intensity of incident light.

Heterogeneous optoelectronic characteristics of Si micropillar arrays fabricated by metal-assisted chemical etching

Recent progress achieved in metal-assisted chemical etching (MACE) has enabled the production of high-quality micropillar arrays for various optoelectronic applications. Si micropillars produced by MACE often show a porous Si/SiOx shell on crystalline pillar cores introduced by local electrochemical reactions. In this paper, we report the distinct optoelectronic characteristics of the porous Si/SiOx shell correlated to their chemical compositions. Local photoluminescent (PL) images obtained with an immersion oil objective lens in confocal microscopy show a red emission peak (≈ 650 nm) along the perimeter of the pillars that is threefold stronger compared to their center. On the basis of our analysis, we find an unexpected PL increase (≈ 540 nm) at the oil/shell interface. We suggest that both PL enhancements are mainly attributed to the porous structures, a similar behavior observed in previous MACE studies. Surface potential maps simultaneously recorded with topography reveal a significantly high surface potential on the sidewalls of MACE-synthesized pillars (+ 0.5 V), which is restored to the level of planar Si control (− 0.5 V) after removing SiOx in hydrofluoric acid. These distinct optoelectronic characteristics of the Si/SiOx shell can be beneficial for various sensor architectures.

Continuous crossover from two-dimensional to one-dimensional electronic properties for metallic silicide nanowires

In a joint experimental and theoretical study on metallic TbSi2 nanowires, we observe a continuous crossover from a two-dimensional (2D) to a quasi-one-dimensional (1D) electronic structure by reduction of the nanowire width. The nanowires were grown by self-organization on vicinal Si(111) substrates denoted by the Miller indices (hhk). Their electronic structure was analyzed by angle-resolved photoemission spectroscopy (ARPES) and calculated using density functional theory (DFT). In ARPES, the TbSi2 nanowires show basically the 2D electronic structure of the TbSi2 film on planar Si(111) with an increasing momentum broadening for decreasing nanowire widths, consistent with Heisenberg's uncertainty principle. In contrast, DFT calculations predict a purely 1D electronic structure for TbSi2 nanowires. Unfolding this 1D electronic structure onto the Brillouin zone of the TbSi2 film leads to a Fermi surface appearing similar to the one of the 2D TbSi2 film, but with an additional 1D contribution from nanowire edges. Such an additional 1D signature is also observed in ARPES for narrow nanowires. These results indicate a continuous transition to a 1D electronic structure for decreasing nanowire widths. (Less)

Comparison of passivation properties of plasma-assisted ALD and APCVD deposited Al2O3 with SiNx capping

In this paper, we report on the direct comparison of Al2O3 films deposited by plasma-assisted atomic layer deposition (ALD) and atmospheric pressure chemical vapor deposition (APCVD) techniques on the passivation properties of both bare and boron diffused Si surfaces. It is found that ALD and APCVD deposited Al2O3 layers with SiNx capping have very similar chemical composition, hydrogen concentration at the Al2O3/Si interface, and interface defect density (Dit), but the ALD film has slightly higher negative fixed charge (Qf) after a high temperature (~750 °C) contact firing cycle typically used for solar cell fabrication. Both films showed excellent surface recombination velocities of <5 cm/s on planar and textured un-diffused Si wafers, but the ALD films gave 1 to 8 fA/cm2 lower emitter saturation current densities (J0e) on implanted boron emitters with sheet resistance in the range of 90–180 Ω/□. This is attributed to the observed slightly higher negative charge-induced field-effect passivation in the ALD films. However, SiNx/Al2O3/p+/n/n+ passivated emitter rear totally diffused (PERT) cell structures fabricated with 110 and 150 Ω/□ emitter in this study showed no appreciable difference in cell efficiency. Device analysis and simulations showed that even for >23% high-performance solar cells design, with very low bulk and rear contact recombination, the observed small difference in J0e has negligible effect on cell efficiency. Thus, APCVD Al2O3 passivation of B emitter can produce comparable efficiencies to their counterpart ALD Al2O3 passivated n-type Si solar cells.

Simulation research of high-efficiency unidirectional vertical coupling grating couplers for optical through-silicon vias in 3D optoelectronic integrated circuits

A unidirectional vertical coupling structure is designed based on optical through-silicon vias (TSVs) and gratings to couple and steer the optical interconnects in 3D optoelectronic integrated circuits. The optical TSVs provide high-speed vertical optical data transmission over short distances. The grating coupler structure achieves efficient coupling between an optical TSV and a planar Si waveguide. Through the action of a mirror that covered the end of the optical TSV cladding and a distributed Bragg reflector(DBR) positioned on the far side away from the waveguide, coupling efficiency of 80% at 1550 nm is obtained in simulations. Our proposed vertical coupling structure represents a high-efficiency solution for 3D optical interconnects and can play an important role in 3D optoelectronic integrated circuits.

Vertically aligned InGaN nanowire arrays on pyramid textured Si (1 0 0): A 3D arrayed light trapping structure for photoelectrocatalytic water splitting

To boost photoelectrocatalytic water splitting, we synthesize InGaN nanowires (NWs) on pyramid textured Si (100) substrates (InGaN/PSi) by plasma-assisted molecular beam epitaxy which act as a novel anti-reflection 3D nano-grating light trapping structure. An InGaN/PSi photoanode exhibits 200% enhancement of the photocurrent compared to that of InGaN NWs on planar Si (InGaN/Si), which is mainly ascribed to the light trapping effects from pyramid-like NW arrays and the tilt structure of the NW arrays, number of InGaN NWs and better wettability. An ultra-low reflectivity around 1.5% due to the pyramid texture and tilt of the NWs implies enhanced absorption. Finite Difference Time Domain (FDTD) simulations verify the light trapping of the 3D nano-grating structure. Furthermore, the inner n-InGaN/n-Si heterostructure fosters photocarrier separation, driving holes to the surface to enhance water oxidation. This research provides a vital approach, exploiting a novel 3D nano-grating light trapping structure to enhance photoelectrocatalytic water splitting by the combined effects of light management, heterostructure formation, doping, and maximizing surface wetting and surface area, which also has significance in other photoelectrocatalytic fields.

Embossed Mie resonator arrays composed of compacted TiO2 nanoparticles for broadband anti-reflection in solar cells

Mie resonator arrays formed by embossing titanium dioxide (TiO2) nanoparticles (NPs) from solution are investigated as optical coatings for anti-reflection applications. Compacted nanoparticle assemblies offer unique possibilities to tailor the effective refractive index (RI). Here, we demonstrate a simple table-top, low pressure, and low temperature method to fabricate structured optical coatings. TiO2 nanostructures in the form of nanodisks support Mie resonances in the visible wavelength spectrum and exhibit strong forward scattering into the high index substrates, making them suitable as broadband anti-reflection coatings for solar cells. TiO2 NP-based nanodisk arrays are designed, fabricated, and characterized regarding their anti-reflection properties on Si, GaAs, and InP substrates and solar cells. Detailed finite-difference time-domain simulations are performed to optimize the TiO2 NP-based Mie resonator arrays for the broadband anti-reflection as well as to explain the measured reflectance spectra. The solar-weighted reflectance is used as a figure of merit (FoM). TiO2 nanodisk arrays on Si show a FoM of ~ 7% in the 400–1,100 nm wavelength spectrum; similar values are obtained for GaAs and InP substrates. TiO2 nanodisk arrays embossed directly on prefabricated planar single-junction Si, GaAs, and InP solar cells result in an appreciable increase (~ 1.3 times) in the short-circuit current densities.

Decoupling Strategy for Enhanced Syngas Generation from Photoelectrochemical CO2 Reduction.

SummaryPhotoelectrochemical CO2 reduction into syngas (a mixture of CO and H2) provides a promising route to mitigate greenhouse gas emissions and store intermittent solar energy into value-added chemicals. Design of photoelectrode with high energy conversion efficiency and controllable syngas composition is of central importance but remains challenging. Herein, we report a decoupling strategy using dual cocatalysts to tackle the challenge based on joint computational and experimental investigations. Density functional theory calculations indicate the optimization of syngas generation using a combination of fundamentally distinctive catalytic sites. Experimentally, by integrating spatially separated dual cocatalysts of a CO-generating catalyst and a H2-generating catalyst with GaN nanowires on planar Si photocathode, we report a record high applied bias photon-to-current efficiency of 1.88% and controllable syngas products with tunable CO/H2 ratios (0–10) under one-sun illumination. Moreover, unassisted solar CO2 reduction with a solar-to-syngas efficiency of 0.63% is demonstrated in a tandem photoelectrochemical cell.

Solar Cell Based on Hybrid Structural SiNW/Poly(3,4 ethylenedioxythiophene): Poly(styrenesulfonate)/Graphene.

Solar energy is considered as a potential alternative energy source. The solar cell is classified into three main types: i) solar cells based on bulk silicon materials (monocrystalline, polycrystalline), ii) thin‐film solar cells (CIGS, CdTe, DSSC, etc.), and iii) solar cells based on nanostructures and nanomaterials. Nowadays, commercial solar cells are usually made by bulk silicon material, which requires not only high fabrication costs but also limited performance. In this study, the fabrication of high‐performance solar cells based on hybrid structure of silicon nanowires/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/graphene (SiNW/PEDOT:PSS/Gr) is focused upon. SiNWs with different lengths of 125, 400, 800 nm, and 2 µm are fabricated by a metal‐assisted chemical etching method, and their influence on the performance of the hybrid solar cells is studied and investigated. The experimental results indicate that the suitable SiNW length for the fabrication of the hybrid solar cells is about 400 nm and the best power conversion efficiency obtained is about 9.05%, which is about 2.1 times higher than that of the planar Si solar cell.

MOCVD growth of InP-based 1.3 μ m quantum dash lasers on (001) Si

Quantum dot and quantum dash (QDash) lasers exhibit lower threshold, less temperature sensitivity, and larger modulation bandwidths than the conventional quantum well lasers. For III–V lasers monolithically grown on Si, the stronger carrier confinement and the discrete distribution of these three-dimensional (3D) quantum structures add to their immunity to material defects resulted from hetero-epitaxy. In this study, we report InAs/InAlGaAs/InP QDash lasers emitting at 1.3 μm directly grown on compliant InP/Si substrates by metalorganic chemical vapor deposition. Room-temperature lasing has been demonstrated on both nano-V-groove patterned and unpatterned planar (001) Si under pulsed electrical pumping, with a low threshold current density of 1.05 kA/cm2. A comparison of lasers grown on these two categories of InP/Si templates in terms of material quality and device performance is presented. Results presented in this work demonstrate the possibility of integrating both datacom and telecom lasers on Si, using the same InAs/InP quantum dash material system on a developed InP-on-Si virtual substrate.