Application of p and n-Type Silicon Nanowires as Human Respiratory Sensing Device.

Accurate and fast breath monitoring is of great importance for various healthcare applications, for example, medical diagnoses, studying sleep apnea, and early detection of physiological disorders. Devices meant for such applications tend to be uncomfortable for the subject (patient) and pricey. Therefore, there is a need for a cost-effective, lightweight, small-dimensional, and non-invasive device whose presence does not interfere with the observed signals. This paper reports on the fabrication of a highly sensitive human respiratory sensor based on silicon nanowires (SiNWs) fabricated by a top-down method of metal-assisted chemical-etching (MACE). Besides other important factors, reducing the final cost of the sensor is of paramount importance. One of the factors that increases the final price of the sensors is using gold (Au) electrodes. Herein, we investigate the sensor's response using aluminum (Al) electrodes as a cost-effective alternative, considering the fact that the electrode's work function is crucial in electronic device design, impacting device electronic properties and electron transport efficiency at the electrode-semiconductor interface. Therefore a comparison is made between SiNWs breath sensors made from both p-type and n-type silicon to investigate the effect of the dopant and electrode type on the SiNWs respiratory sensing functionality. A distinct directional variation was observed in the sample's response with Au and Al electrodes. Finally, performing a qualitative study revealed that the electrical resistance across the SiNWs renders greater sensitivity to breath than to dry air pressure. No definitive research demonstrating the mechanism behind these effects exists, thus prompting our study to investigate the underlying process.

Piezoresistance Characterization of Silicon Nanowires in Uniaxial and Isostatic Pressure Variation.

Silicon nanowires (SiNWs) are known to exhibit a large piezoresistance (PZR) effect, making them suitable for various sensing applications. Here, we report the results of a PZR investigation on randomly distributed and interconnected vertical silicon nanowire arrays as a pressure sensor. The samples were produced from p-type (100) Si wafers using a silver catalyzed top-down etching process. The piezoresistance response of these SiNW arrays was analyzed by measuring their I-V characteristics under applied uniaxial as well as isostatic pressure. The interconnected SiNWs exhibit increased mechanical stability in comparison with separated or periodic nanowires. The repeatability of the fabrication process and statistical distribution of measurements were also tested on several samples from different batches. A sensing resolution down to roughly 1m pressure was observed with uniaxial force application, and more than two orders of magnitude resistance variation were determined for isostatic pressure below atmospheric pressure.

Oblique angle deposition of nickel thin films by high-power impulse magnetron sputtering.

Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy. Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS. Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.

Power regulation and electromigration in platinum microwires.

We introduce a new experimental setup with a biasing circuit and computer control for electrical power regulation under reversing polarity in Pt microwires with dimensions of 1×10 µm(2). The circuit is computer controlled via a data acquisition board. It amplifies a control signal from the computer and drives current of alternating polarity through the sample in question. Time-to-failure investigations under DC and AC current stress are performed. We confirm that AC current stress can improve the life time of microwires at least by a factor of 10(3) compared to the corresponding time-to-failure under DC current stress.

Sub-wavelength bolometers: uncooled platinum wires as infrared sensors.

We present characterization results of microscopic platinum wires as bolometers. The wire lengths range from 16 µm down to 300 nm. Thus they are in many cases significantly smaller in size than the wavelength of the radiation from the 1200 K blackbody source they were exposed to. We observe a steep rise in both responsivity ℜ and detectivity D* with decreasing wire size, reaching ℜ = 3.1×10(4) V/W and D* = 2.7×10(9) cm Hz(1/2)/W at room temperature for a 300×300 nm(2) device. Two significant advantages of such small wires as bolometers are their low power requirement and fast response time. Our numerical estimations suggest response times in the order of nanoseconds for the smallest samples. They could help improve resolution and response of thermal imaging devices, for example. We believe the performance may be further improved by optimizing the design and operating parameters.

Changing the emission of polarized thermal radiation from metallic nanoheaters.

The polarization of the thermal radiation emitted from individual nanoheaters is investigated for nanoheaters with widths ranging from 500 nm to 2000 nm. The polarization is oriented along the long axis of the nanoheater for widths below 600 nm and rotates by 90 degrees and becomes perpendicular for widths above 900 nm. For certain width nanoheaters the orientation of the polarization of the thermal emission can be rotated from parallel to perpendicular by changing the temperature of the nanoheater. The change in the direction of the emitted thermal radiation is explained by thermally excited transverse plasmon modes.

Enhanced thermal emission from individual antenna-like nanoheaters.

Here we report polarization-sensitive, thermal radiation measurements of individual, antenna-like, thin film Platinum nanoheaters. These heaters confine the lateral extent of the heated area to dimensions smaller (or comparable) to the thermal emission wavelengths. For very narrow heater structures the polarization of the thermal radiation shows a very high extinction ratio as well as a dipolar-like angular radiation pattern. A simple analysis of the radiation intensities suggests a significant enhancement of the thermal radiation for these very narrow heater structures.