Understanding the effects of pressure on the contact angle of water on a silicon surface in nitrogen gas environment: Contrasts between low- and high-temperature regimes

HYPOTHESIS

Pressure dependence of contact angle is expected to be influenced by temperature. Nevertheless, the correlation of water contact angle with pressure is rarely investigated at high temperatures (over 100 ℃).

EXPERIMENTS

In this work, measurements of the contact angle and interfacial tension of water in N₂ atmosphere were conducted at various pressures and temperatures (up to 17 MPa and 300 ℃). The experimental observations were elucidated based on the theory of surface thermodynamics.

FINDINGS

It was shown that the water-N₂ interfacial tension linearly decreases with increasing the pressure, and that the pressure coefficient declines as temperature rises. The pressure dependence of the water contact angle was found to be different for the low- and high-temperature regimes: the water contact angle increases below 100 ℃, whereas an inverse variation occurs over 100 ℃. According to the theoretical analysis, the pressure dependence of both the water interfacial tension and contact angle is attributed to N₂ adsorption on the surfaces of water and silicon. The variations in the water contact angle with pressure, including both the sign and magnitude, are actually the consequence of the changes of water-N₂ and Si-N₂ interfacial tensions manipulated by pressure and temperature.

Theoretical and Experimental Study on AlGaN/GaN Schottky Barrier Diode on Si Substrate with Double-Heterojunction

An AlGaN/GaN Schottky barrier diode (SBD) with double-heterojunction is theoretically and experimentally investigated on the GaN/AlGaN/GaN/Si-sub. The two-dimensional hole gas (2DHG) and electron gas (2DEG) are formed at the GaN-top/AlGaN and AlGaN/GaN interface, respectively. At the off-state, the 2DEH and 2DHG are partially depleted and then completely disappear. There remain the fixed positive and negative polarization charges, forming the polarization junction. Therefore, a flat electric field in the drift region and a high breakdown voltage (BV) are obtained. Moreover, the anode is recessed to reduce turn-on voltage (V_ON). The low-damage ICP etching process results in the improved Schottky contacts, and a low leakgae current and a low V_ON is obtained. The fabricated SBD exhibits a BV of 1109 V with anode-to-cathode distance (L_AC) of 11 μm. The fabricated SBDs achieve a low V_ON of 0.68 V with good uniformity, a high on/off current ratio ∼ 10¹⁰ at room temperature, a low specific on-resistance (R_ON,SP) of 1.17 mΩ cm², and a high Baliga's figure-of-merit (FOM) of 1051 MW/cm².

Strain-Controlled Recombination in InGaN/GaN Multiple Quantum Wells on Silicon Substrates

This paper reports the photoluminescence (PL) properties of InGaN/GaN multiple quantum well (MQW) light-emitting diodes grown on silicon substrates which were designed with different tensile stress controlling architecture like periodic Si δ-doping to the n-type GaN layer or inserting InGaN/AlGaN layer for investigating the strain-controlled recombination mechanism in the system. PL results turned out that tensile stress released samples had better PL performances as their external quantum efficiencies increased to 17%, 7 times larger than the one of regular sample. Detail analysis confirmed they had smaller nonradiative recombination rates ((2.5~2.8)×10^-2 s^-1 compared to (3.6~4.7)× 10^-2 s^-1), which was associated with the better crystalline quality and absence of dislocations or cracks. Furthermore, their radiative recombination rates were found more stable and were much higher ((5.7~5.8) ×10^-3 s^-1 compared to [9~7] ×10^-4 s^-1) at room temperature. This was ascribed to the suppression of shallow localized states on MQW interfaces, leaving the deep radiative localization centers inside InGaN layers dominating the radiative recombination.

Surface thiolation of silicon for antifouling application

Thiol groups grafted silicon surface was prepared as previously described. 1H,1H,2H,2H-perfluorodecanethiol (PFDT) molecules were then immobilized on such a surface through disulfide bonds formation. To investigate the contribution of PFDT coating to antifouling, the adhesion behaviors of Botryococcus braunii (B. braunii) and Escherichia coli (E. coli) were studied through biofouling assays in the laboratory. The representative microscope images suggest reduced B. braunii and E. coli accumulation densities on PFDT integrated silicon substrate. However, the antifouling performance of PFDT integrated silicon substrate decreased over time. By incubating the aged substrate in 10 mM TCEP·HCl solution for 1 h, the fouled PFDT coating could be removed as the disulfide bonds were cleaved, resulting in reduced absorption of algal cells and exposure of non-fouled silicon substrate surface. Our results indicate that the thiol-terminated substrate can be potentially useful for restoring the fouled surface, as well as maximizing the effective usage of the substrate.

Silicon Substrate Strained and Structured via Cavitation Effect for Photovoltaic and Biomedical Application

A hybrid structure, which integrates the nanostructured silicon with a bio-active silicate, is fabricated using the method of MHz sonication in the cryogenic environment. Optical, atomic force, and scanning electron microscopy techniques as well as energy dispersive X-ray spectroscopy were used for the investigation of the morphology and chemical compound of the structured surface. Micro-Raman as well as X-ray diffraction, ellipsometry, and photovoltage spectroscopy was used for the obtained structures characterization. Ellipsometer measurements demonstrated the formation of the layer with the thicknesses ~700 nm and optical parameters closed to SiO2 compound with an additional top layer of the thicknesses ~15 nm and the refractive index ~1. Micro-Raman investigation detects an appearance of Ca-O local vibrational mode, and the stretching vibration of SiO4 chains characterized the wollastonite form of CaSiO3. A significant rise in the value and an expansion of the spectral range of the surface photovoltage for silicon structured via the megasonic processing was found. The concept of biocompatible photovoltaic cell on the base of Si\CaSiO3 structure for the application in bioelectronics was proposed.