Highly-doped Silicon Research Articles

Enhanced Sensitivity of Gas Sensor Based on Poly(3-hexylthiophene) Thin-Film Transistors for Disease Diagnosis and Environment Monitoring.

Electronic devices based on organic thin-film transistors (OTFT) have the potential to supply the demand for portable and low-cost gadgets, mainly as sensors for in situ disease diagnosis and environment monitoring. For that reason, poly(3-hexylthiophene) (P3HT) as the active layer in the widely-used bottom-gate/bottom-contact OTFT structure was deposited over highly-doped silicon substrates covered with thermally-grown oxide to detect vapor-phase compounds. A ten-fold organochloride and ammonia sensitivity compared to bare sensors corroborated the application of this semiconducting polymer in sensors. Furthermore, P3HT TFTs presented approximately three-order higher normalized sensitivity than any chemical sensor addressed herein. The results demonstrate that while TFTs respond linearly at the lowest concentration values herein, chemical sensors present such an operating regime mostly above 2000 ppm. Simultaneous alteration of charge carrier mobility and threshold voltage is responsible for pushing the detection limit down to units of ppm of ammonia, as well as tens of ppm of alcohol or ketones. Nevertheless, P3HT transistors and chemical sensors could compose an electronic nose operated at room temperature for a wide range concentration evaluation (1–10,000 ppm) of gaseous analytes. Targeted analytes include not only biomarkers for diseases, such as uremia, cirrhosis, lung cancer and diabetes, but also gases for environment monitoring in food, cosmetic and microelectronics industries.

Nano-photonic crystal formation on highly-doped n-type silicon

We present a novel electrochemical technique for the fabrication of nano-photonic crystal structures. Based on a specially designed electrolyte, porous silicon (PSi) layers with different porosities are possible to be produced on highly-doped n-type silicon substrate by varying the applied current density which determines the size and the morphology of pores. By applying an alternative current density modulation during anodization, porous silicon photonic crystals are obtained using HF-containing electrolyte without oxidizing components. The current burst model (CBM) is employed to interpret the mechanism of the formation of the macropore porous silicon.

Quantitative electrostatic force microscopy with sharp silicon tips

Electrostatic force microscopy (EFM) probes are typically coated in either metal (radius ∼ 30 nm) or highly-doped diamond (radius ∼ 100 nm). Highly-doped silicon probes, which offer a sharpened and stable tip apex (radius ∼ 1–10 nm) and are usually used only in standard atomic force microscopy, have been recently shown to allow enhanced lateral resolution in quantitative EFM and its application for dielectric constant measurement. Here we present the theoretical modelling required to quantitatively interpret the electrostatic force between these sharpened tips and samples. In contrast to a sphere-capped cone geometry used to describe metal/diamond-coated tips, modelling a sharpened silicon tip requires a geometry comprised of a cone with two different angles. Theoretical results are supported by experimental measurements of metallic substrates and ∼10 nm radius dielectric nanoparticles. This work is equally applicable to EFM and other electrical scanned probe techniques, where it allows quantifying electrical properties of nanomaterials and 3D nano-objects with higher resolution.

Using the Effect of Mechanical Stress on Doped Silicon as an Angular Movement Sensor for MOEMS/MEMS Micro Mirrors

The<b> </b>effect of elastic strain of moderate magnitude using high doped silicon substrate can change the conductivity of the substrate. The commonly used metal (strain) gage has a magnitude factor of between 2 ÷ 4 while high doped silicon (strain) gage factor magnitude is between 150 ÷ 200, thus improving the substrate sensitivity considerably. Using those physical attributes allow us to create a MOEMS sensor resolving accuracy issues and saving space in any future MOEMS device design. Those devices will be able to measure any mechanical movement connected to the high doped silicon substrate by converting the physical strain created from the movement stress to current/voltage change in the substrate device. The simplicity of the device is that the device could measure movement without any need to implement an outer sensor to it. By measuring the device's strain change it would feel the movement and convert it to an analog value, thus creating a strain gage built in the MOEMS device surface.

Study of Bilayer Porous Structure for Efficient Light Absorption

In this paper, we present a novel structure for bio-inspired photochemical light absorption based on porous silicon formation technology. Macro- and microporous samples were obtained by anodic etching on highly-doped n-type silicon substrate. TiO2 nanoparticles were synthesized by the sol-gel hydrolyzing method by immersing the samples in the precursor. After rinse and calcinations, light absorption effects were characterized by the reflective spectrums. It can be easily found that the two samples play different roles in the light absorption by comparing the spectrums before and after TiO2 synthesization which can enhance the efficiency of energy transformation.

Terahertz surface plasmon polaritons on a semiconductor surface structured with periodic V-grooves

We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with V-shaped grooves. A one-dimensional array of V-grooves is fabricated on a highly-doped silicon surface, using anisotropic wet-etching of crystalline silicon, thereby forming a plasmonic waveguide. Terahertz time domain spectroscopy is used to characterize the propagation of waves near the corrugated surface. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of the V-grooves. The surface modes are confirmed through both numerical simulations and experimental measurements. Not only does the V-groove geometry represent a new and largely unexplored structure for supporting surface waves, but it also enables the practical fabrication of terahertz waveguides directly on semiconductor surfaces, without relying on reactive-ion etching or electroplating of sub-millimeter metallic surfaces.

Thermoelectric Properties of High-Doped Silicon from Room Temperature to 900 K

Silicon is investigated as a low-cost, Earth-abundant thermoelectric material for high-temperature applications up to 900 K. For the calculation of module design the Seebeck coefficient and the electrical as well as thermal properties of silicon in the high-temperature range are of great importance. In this study, we evaluate the thermoelectric properties of low-, medium-, and high-doped silicon from room temperature to 900 K. In so doing, the Seebeck coefficient, the electrical and thermal conductivities, as well as the resulting figure of merit ZT of silicon are determined.

Compact modeling of extremely scaled graphene FETs

In this work, compact current modeling of field-effect transistors (FETs) with transferred graphene channel grown by using chemical vapor deposition is presented. A highly-doped silicon substrate is used as a back gate, channels are defined by using electron-beam lithography, and the channel length of the transistor is scaled down to 20 nm. The DC characteristics of the scaled graphene transistors are observed by considering the source/drain series resistances. In compact modeling of graphene FETs, an electron-hole puddle existing near the charge-neutral region (Dirac point) is considered at a low carrier density while the velocity saturation effect due to surface polar phonon scattering is included at a high carrier density.

Formation of macro–meso–microporous multilayer structures

In this study, we present a novel electrochemical technique for the fabrication of macro–meso–microporous multilayer structures. Based on systematic investigation of HF/H2O2/ethanol electrolyte system, all three types of pores (smooth macropores, branched mesopores and spongy micropores) can be produced respectively on highly-doped n-type silicon substrate by varying the applied current density, which determines the pore sizes and morphologies. By applying a multi-step current modulation during anodization, a multilayer consisting of three different pore morphologies (macro-, meso-, and micro-pores), as well as several types of multilayer structures with two different pore morphologies, was fabricated on the same silicon substrate using the same electrolyte. A wide range of porosities (30–82%) can be easily achieved with this approach. The current-burst-model is employed to interpret the mechanism concerning such three types of pores formation.

Influence of the compensation level on the performance of p-type crystalline silicon solar cells: Theoretical calculations and experimental study

We have investigated the influence of the compensation level on the performance of p-type crystalline silicon solar cells by combining theoretical calculations and experiments. With the compensation level increasing, the fraction of ionized boron (B) atoms increases. The hole mobility is significantly reduced due to the increased ionized impurities scattering, while the electron mobility almost keeps constant due to the inverse contributions of ionized impurities scattering and electron–hole scattering. The carrier lifetime in compensated silicon, mainly dominated by high concentration of dopants, could get improved. Correspondingly, the minority carrier diffusion length increases, causing a higher short-circuit current. However, the open-circuit voltage first decreases and then increases with the increase of the compensation level, owing to the competition effect of the reduced net doping concentration and the increased excess carrier concentration. All these factors could finally result in a higher efficiency solar cell. These results suggest that intentional dopant compensation might be utilized for the improvement of solar cell efficiency, which is especially interesting for those based on highly-doped silicon feedstock.

A study of atomic orbital basis sets for doped silicon nanowires

First-principle calculations are performed within Density Functional Theory to determine the accuracy of numerical atomic orbital basis sets used for studies of highly-doped silicon nanowires. Boron, phosphorous, gallium and arsenic atoms are used as dopants. Structural and electronic properties are studied and the performance of optimised basis sets is benchmarked. A good compromise between efficiency and accuracy is offered by optimised double zeta polarised basis sets. The transferability of numerical atomic orbitals for studies of silicon nanowires is tested by applying in nanowires with different orientations.

Voltage-dependent Mobility Characterization of MDMO-PPV Thin-Film Transistors for Flexible Sensor Applications

We complement our previous published results on MDMO-PPV thin-film transistors by treating data according to different models that consider charge transport by hopping. Devices were processed on highly-doped silicon and ITO-covered glass substrates with dielectrics such as thermal SiO2 and PECVD deposited SiOxNy. Charge carrier mobility and threshold voltage on polymeric thinfilm transistors can be better estimated considering transport dependence on carrier density, Poole-Frenkel effect and interface states. Best results were achieved on HMDS-treated SiO2, but a completely new voltage-dependence is attained. These models can be employed for a better understanding of the polymeric semiconductor behavior when in a TFT-based sensor.

Marshmallowing of nanopillar arrays by field emission

We fabricated nanoscale field electron emitters formed by highly-doped silicon nanopillars on a silicon membrane. Electron-beam induced deposition of carbon-based contaminants is employed as a probe of the spatial activity of electron emission from the nanopillars. In stark contrast to the general assumption that field emission only occurs at the tips of nanoscale emitters, we found strong emission from the sidewalls of the nanopillars. This is revealed by the deposition of carbon contaminants on these sidewalls, so that the nanopillars finally resemble marshmallows. We conclude that field emission from nanostructured surfaces is more intricate than previously expected.

Fast speed nano-sized macropore formation on highly-doped n-type silicon via strong oxidizers

The growth of nano-sized macropores at high speed is studied in this work. Nice macropores with diameters of 60–100 nm and aspect ratio up to 2500 were formed by anodic etching on highly-doped n-type silicon without illumination. The HF-containing electrolytes were modulated with strong oxidizer, H 2O 2, which was also attempted by a few other researchers, but did not lead to the expected macropore formation. Our findings reveal that the pore morphology and etching speed are on dependence of HF concentration and the applied current density. The parameter window of macropore formation, corresponding to the HF concentration ranging from 33% to 67% (by volume) in our experiment, is rather large. In addition, the growth speed can be driven up to 1800 μm/h, while the pores are straight, cylindrical and rather smooth. The current-burst-model is applied to interpret the mechanism of such nano-sized macropore formation.

Photocatalytic properties of porous silicon nanowires

Porous silicon nanowires are synthesized through metal assisted wet-chemical etch of highly-doped silicon wafer. The resulted porous silicon nanowires exhibit a large surface area of 337 m(2)·g(-1) and a wide spectrum absorption across the entire ultraviolet, visible and near infrared regime. We further demonstrate that platinum nanoparticles can be loaded onto the surface of the porous silicon nanowires with controlled density. These combined advancements make the porous silicon nanowires an interesting material for photocatalytic applications. We show that the porous silicon nanowires and platinum nanoparticle loaded porous silicon nanowires can be used as effective photocatalysts for photocatalytic degradation of organic dyes and toxic pollutants under visible irradiation, and thus are of significant interest for organic waste treatment and environmental remediation.

Marshmallowing of nanopillar arrays by field emission

We have fabricated mechanically flexible field electron emitters formed by highly-doped silicon nanopillars on a silicon membrane. Electron beam induced deposition of carbon-based contaminants is employed to probe the spatial activity of electron emission from the nanopillars. The experimental configuration provides a powerful tool to investigate the physics of the field electron emission (FEE). In contrast to the general assumption that field emission only occurs at the tips of nanoscale emitters, we found that the emission from the nanopillars' sidewalls is as strong as from their tips.

Silicon–air batteries

A new “metal”–air battery based on silicon–oxygen couple is described. Silicon–air battery employing EMI·2.3HF·F room temperature ionic liquid (RTIL) as an electrolyte and highly-doped silicon wafers as anodes (fuels) has an undetectable self-discharge rate and high tolerance to the environment (extreme moisture/dry conditions). Such a battery yields an effectively infinite shelf life with an average working voltage of 1–1.2 V. Silicon–air battery can support relatively high current densities (up to 0.3 mA/cm 2) drawn from flat polished silicon wafers anodes. Such batteries may find immediate applications, as they can provide an internal, built-in autonomous and self sustained energy source.

Self-oscillation conditions of a resonant nanoelectromechanical mass sensor

This article presents a comprehensive study and design methodology of cointegrated oscillators for nanoscale mass sensing applications based on resonant nanoelectromechanical system (NEMS). In particular, a comparison is provided between the capacitive and the piezoresistive transduction schemes in terms of overall sensor performance. The developed model is clearly in accordance with the general experimental observations obtained for NEMS-based mass detection. The piezoresistive devices are more sensitive (up to 10 zg/√Hz) than capacitive ones (close to 100 zg/√Hz) since they can work at higher frequency. Moreover, the high doped silicon piezoresistive gauge, which is of great interest for very large scale integration, shows similar theoretical resolution to that of the metallic gauge already used experimentally.

Comparison between the electrical properties of atomic layer deposited thin ZrO 2 films processed from cyclopentadienyl precursors

Electrical characterization of zirconium oxide (ZrO 2) based metal-oxide-semiconductor (MOS) structures has been carried out. ZrO 2 films have been atomic layer deposited (ALD) by using novel cyclopentadienyl-based precursors, which have recently revealed themselves as very adequate in terms of thermal stability and high permittivity of the dielectrics deposited. Our results demonstrate good quality of the films, especially when mixed alkylamido-cyclopentadienyl precursors are used on SiO 2/Si substrates. Conduction mechanisms in these MIS capacitors were studied, with moderately or highly-doped silicon used as substrate.

Development of mid-infrared surface plasmon resonance-based sensors with highly-doped silicon for biomedical and chemical applications

Biomedical and chemical sensors utilizing surface plasmon resonance (SPR) in the mid-infrared range were developed with the aid of highly doped silicon owing to its tailored optical constants. SPR may be excited by light incident on a periodic doping profile embedded in an intrinsic silicon film without constraints on the flow of chemical solutions or activities of biomedical samples. General guidance for tuning SPR wavelengths based on dispersion curves to catch different target materials in free space or water was also provided. The feasibility of sensors was demonstrated with a sharp spectral-directional reflectance dip, which shifted with optical constants variation. The effects of doping concentration, doping profile, and angle of incidence on sensor performance were numerically studied with a rigorous coupled-wave analysis algorithm. Developed sensors could work well for a real target and show superiority in sensitivity over existing sensors.