Tuan-Khoa Nguyen

Single-Crystalline 3C-SiC anodically Bonded onto Glass: An Excellent Platform for High-Temperature Electronics and Bioapplications

Single-crystal cubic silicon carbide has attracted great attention for MEMS and electronic devices. However, current leakage at the SiC/Si junction at high temperatures and visible-light absorption of the Si substrate are main obstacles hindering the use of the platform in a broad range of applications. To solve these bottlenecks, we present a new platform of single crystal SiC on an electrically insulating and transparent substrate using an anodic bonding process. The SiC thin film was prepared on a 150 mm Si with a surface roughness of 7 nm using LPCVD. The SiC/Si wafer was bonded to a glass substrate and then the Si layer was completely removed through wafer polishing and wet etching. The bonded SiC/glass samples show a sharp bonding interface of less than 15 nm characterized using deep profile X-ray photoelectron spectroscopy, a strong bonding strength of approximately 20 MPa measured from the pulling test, and relatively high optical transparency in the visible range. The transferred SiC film also exhibited good conductivity and a relatively high temperature coefficient of resistance varying from -12 000 to -20 000 ppm/K, which is desirable for thermal sensors. The biocompatibility of SiC/glass was also confirmed through mouse 3T3 fibroblasts cell-culturing experiments. Taking advantage of the superior electrical properties and biocompatibility of SiC, the developed SiC-on-glass platform offers unprecedented potentials for high-temperature electronics as well as bioapplications.

Environment-friendly carbon nanotube based flexible electronics for noninvasive and wearable healthcare

Flexible and stretchable electronics have a wide variety of wearable applications in portable sensors, flexible electrodes/heaters, flexible circuits and stretchable displays. Spinnable carbon nanotubes (CNTs) constructed on flexible substrates are potential materials for wearable sensing applications owing to their high thermal and electrical conductivity, low mass density and excellent mechanical properties. Here, we demonstrate a wearable thermal flow sensor for healthcare using lightweight, high strength, flexible CNT yarns as hotwires, pencil graphite as electrodes, and lightweight, recyclable and biodegradable paper as flexible substrates, without using any toxic chemicals. The CNT-based sensor which could be utilized to monitor respiratory diseases is comfortably affixed to human skin and detects real-time human respiration. We also successfully demonstrate the temperature detecting functionality integrated in the same sensor, which can measure body temperature using a non-contact mode. The results indicate that the CNT yarn can be used to develop a wide range of environment-friendly, low-cost and lightweight paper-based flexible devices for wearable applications in temperature and respiratory monitoring, and personal healthcare.

Nano strain-amplifier: Making ultra-sensitive piezoresistance in nanowires possible without the need of quantum and surface charge effects

This paper presents an innovative nano strain-amplifier employed to significantly enhance the sensitivity of piezoresistive strain sensors. Inspired from the dogbone structure, the nano strain-amplifier consists of a nano thin frame released from the substrate, where nanowires were formed at the centre of the frame. Analytical and numerical results indicated that a nano strain-amplifier significantly increases the strain induced into a free standing nanowire, resulting in a large change in their electrical conductance. The proposed structure was demonstrated in p-type cubic silicon carbide nanowires fabricated using a top down process. The experimental data showed that the nano strain-amplifier can enhance the sensitivity of SiC strain sensors at least 5.4 times larger than that of the conventional structures. This result indicates the potential of the proposed strain-amplifier for ultra-sensitive mechanical sensing applications.

The Piezoresistive Effect in Top–Down Fabricated p-Type 3C-SiC Nanowires

This letter reports on the piezoresistive effect of top-down fabricated 3C-SiC nanowires (NWs). Focused ion beam was utilized to create p-type 3C-SiC NWs from a 3C-SiC thin film with a carrier concentration of 5 × 1018 cm-3 epitaxially grown on a Si substrate. The as-fabricated NWs were then subjected to tensile strains varying from 0 to 280 με. Experimental data showed that the p-type 3C-SiC NWs possess a large gauge factor of 35, which is at least one order of magnitude larger than that of other hard materials, such as carbon nanotubes and graphene. This large gauge factor and the linear relationship between the relative resistance change and the applied strain in the SiC NWs indicate their potential for nanoelectromechanical systems sensing applications.

Ultra-high strain in epitaxial silicon carbide nanostructures utilizing residual stress amplification

Strain engineering has attracted great attention, particularly for epitaxial films grown on a different substrate. Residual strains of SiC have been widely employed to form ultra-high frequency and high Q factor resonators. However, to date, the highest residual strain of SiC was reported to be limited to approximately 0.6%. Large strains induced into SiC could lead to several interesting physical phenomena, as well as significant improvement of resonant frequencies. We report an unprecedented nanostrain-amplifier structure with an ultra-high residual strain up to 8% utilizing the natural residual stress between epitaxial 3C-SiC and Si. In addition, the applied strain can be tuned by changing the dimensions of the amplifier structure. The possibility of introducing such a controllable and ultra-high strain will open the door to investigating the physics of SiC in large strain regimes and the development of ultra sensitive mechanical sensors.

Isotropic piezoresistance of p-type 4H-SiC in (0001) plane

In this work, the isotropic piezoresistance in the (0001) plane of p-type 4H-SiC was discovered by means of the hole energy shift calculation and the coordinate transformation. These results were also confirmed by the measurement of the piezoresistance using a bending beam method. The fundamental longitudinal and transverse piezoresistive coefficients π11 and π12 were found to be 6.43 × 10−11 Pa−1 and −5.12 × 10−11 Pa−1, respectively. The isotropy of the piezoresistance in the basal plane of p-type 4H-SiC is attributed to the isotropic hole energy shift under uniaxial strain. This interesting phenomenon in p-type 4H-SiC is promising for the design and fabrication of mechanical sensors and strain-engineered electronics since high sensitivity and consistent performance can be achieved regardless of the crystallographic orientation.

Self-sensing paper-based actuators employing ferromagnetic nanoparticles and graphite

Paper-based microfluidics and sensors have attracted great attention. Although a large number of paper-based devices have been developed, surprisingly there are only a few studies investigating paper actuators. To fulfill the requirements for the integration of both sensors and actuators into paper, this work presents an unprecedented platform which utilizes ferromagnetic particles for actuation and graphite for motion monitoring. The use of the integrated mechanical sensing element eliminates the reliance on image processing for motion detection and also allows real-time measurements of the dynamic response in paper-based actuators. The proposed platform can also be quickly fabricated using a simple process, indicating its potential for controllable paper-based lab on chip.

Highly sensitive p-type 4H-SiC van der Pauw sensor

This paper presents for the first time a p-type 4H silicon carbide (4H-SiC) van der Pauw strain sensor by utilizing the strain induced effect in four-terminal devices. The sensor was fabricated from a 4H-SiC (0001) wafer, using a 1 μm thick p-type epilayer with a concentration of 1018 cm−3. Taking advantage of the four-terminal configuration, the sensor can eliminate the need for resistance-to-voltage conversion which is typically required for two-terminal devices. The van der Pauw sensor also exhibits an excellent repeatability and linearity with a significantly large output voltage in induced strain ranging from 0 to 334 ppm. Various sensors aligned in different orientations were measured and a high sensitivity of 26.3 ppm−1 was obtained. Combining these performances with the excellent mechanical strength, electrical conductivity, thermal stability, and chemical inertness of 4H-SiC, the proposed sensor is promising for strain monitoring in harsh environments.

Superior Robust Ultrathin Single-Crystalline Silicon Carbide Membrane as a Versatile Platform for Biological Applications

Micromachined membranes are promising platforms for cell culture thanks to their miniaturization and integration capabilities. Possessing chemical inertness, biocompatibility, and integration, silicon carbide (SiC) membranes have attracted great interest toward biological applications. In this paper, we present the batch fabrication, mechanical characterizations, and cell culture demonstration of robust ultrathin epitaxial deposited SiC membranes. The as-fabricated ultrathin SiC membranes, with an ultrahigh aspect ratio (length/thickness) of up to 20 000, possess high a fracture strength up to 2.95 GPa and deformation up to 50 μm. A high optical transmittance of above 80% at visible wavelengths was obtained for 50 nm membranes. The as-fabricated membranes were experimentally demonstrated as an excellent substrate platform for bio-MEMS/NEMS cell culture with the cell viability rate of more than 92% after 72 h. The ultrathin SiC membrane is promising for in vitro observations/imaging of bio-objects with an extremely short optical access.

High-temperature tolerance of the piezoresistive effect in p-4H-SiC for harsh environment sensing

4H-silicon carbide based sensors are promising candidates for replacing prevalent silicon-based counterparts in harsh environments owing to their superior chemical inertness, high stability and reliability. However, the wafer cost and the difficulty in obtaining an ohmic contact in the metallization process hinders the use of this SiC polytype for practical sensing applications. This article presents the high-temperature tolerance of a p-type 4H-SiC piezoresistor at elevated temperatures up to 600 °C. A good ohmic contact was formed by the metallisation process using titanium and aluminium annealed at 1000 °C. The leakage current at high temperatures was measured to be negligible thanks to a robust p–n junction. Owing to the superior physical properties of the bulk 4H-SiC material, a high gauge factor of 23 was obtained at 600 °C. The piezoresistive effect also exhibits good linearity and high stability at high temperatures. The results demonstrate the capability of p-type 4H-SiC for the development of highly sensitive sensors for hostile environments.