Conducting electrophysiological measurements from human brain function provides a medium for sending commands and messages to the external world, as known as a brain–computer interface (BCI). In this study, we proposed a smart helmet which integrated the novel hygroscopic sponge electrodes and a combat helmet for BCI applications; with the smart helmet, soldiers can carry out extra tasks according to their intentions, i.e., through BCI techniques. There are several existing BCI methods which are distinct from each other; however, mutual issues exist regarding comfort and user acceptability when utilizing such BCI techniques in practical applications; one of the main challenges is the trade-off between using wet and dry electroencephalographic (EEG) electrodes. Recently, several dry EEG electrodes without the necessity of conductive gel have been developed for EEG data collection. Although the gel was claimed to be unnecessary, high contact impedance and low signal-to-noise ratio of dry EEG electrodes have turned out to be the main limitations. In this study, a smart helmet with novel hygroscopic sponge electrodes is developed and investigated for long-term usage of EEG data collection. The existing electrodes and EEG equipment regarding BCI applications were adopted to examine the proposed electrode. In the impedance test of a variety of electrodes, the sponge electrode showed performance averaging 118 kΩ, which was comparable with the best one among existing dry electrodes, which averaged 123 kΩ. The signals acquired from the sponge electrodes and the classic wet electrodes were analyzed with correlation analysis to study the effectiveness. The results indicated that the signals were similar to each other with an average correlation of 90.03% and 82.56% in two-second and ten-second temporal resolutions, respectively, and 97.18% in frequency responses. Furthermore, by applying the proposed differentiable power algorithm to the system, the average accuracy of 21 subjects can reach 91.11% in the steady-state visually evoked potential (SSVEP)-based BCI application regarding a simulated military mission. To sum up, the smart helmet is capable of assisting the soldiers to execute instructions with SSVEP-based BCI when their hands are not available and is a reliable piece of equipment for strategical applications.

Extended microtunnels with triangular cross sections in thick GaN films were demonstrated using wet chemical etching on specially designed epitaxial lateral overgrowth structures. For tunnels along the ͗1100͘ and ͗1120͘ directions of GaN, the ͕1122͖ and ͕1011͖ facets are the etch stop planes with activation energies of 23 kcal/mol determined by wet chemical etching. The axial etching rate of the tunnels in the ͗1100͘ direction is twice as large than that along the ͗1120͘ direction. The highest etching rate of the tunnels in the axial direction is 1000 m/h. GaN and its alloys are widely employed on optoelectronic devices, such as light-emitting diodes ͑LEDs͒ and laser diodes ͑LDs͒, because they have a wide direct bandgap, high thermal stability, and unusual chemical stability. However, their excellent chemical stability also makes wet chemical etching difficult. Most methods of GaN etching involve dry etching processes such as reactive ion etching or inductively coupled plasma etching. 1-3 While dry etching has favorable characteristics, including a high etching rate and the ability to yield vertical sidewalls, it also has several disadvantages, including the damage caused by ion bombardment and the difficulty of obtaining smoothly etched sidewalls. Furthermore, tunnel structures buried in semiconductors cannot be realized by dry etching or photoenhanced electrochemical ͑PEC͒ techniques. Numerous research groups demonstrated PEC etching techniques, 4-16 but in most cases the etched surfaces of GaN were roughened.In this article, extended microtunnels ͑EMTs͒ in GaN were prepared by wet chemical etching with an average etching rate of more than 15 m/min. The etch stop facets of GaN EMTs are the ͕1122͖ or ͕1011͖ crystal planes, depending on the direction of the designed patterns of epitaxial lateral overgrowth ͑ELOG͒ directions. Several groups have tried to grow semipolar LEDs or LDs on ͕1122͖, ͕1011͖, and ͕1011͖ GaN crystal facets, which could result in higherpower or lower-threshold devices due to the reduced internal piezoelectric polarization field, 17-24 but the properties of these facets are still not clearly understood to this date. GaN EMTs are demonstrated in this report to further understand the etching properties of these facets. These microtunnels offer a channel for microfluid studies, especially in the applications of microelectromechanical systems. 25 All of the samples herein consist of several tens of micrometers of GaN thickness grown by hydride vapor phase epitaxy ͑HVPE͒ on sapphire substrates. In the first growth process, a 4 m thick GaN template was grown by metallorganic chemical vapor deposition on a ͑0001͒ c-plane sapphire substrate. The second growth process was the deposition of a 300 nm thick SiO 2 layer by plasma-enhanced chemical vapor deposition. Then, standard photolithography was performed to fabricate the stripes of a 5 m wide SiO 2 mask separated by 5 m wide windows in the ͗1100͘ and ͗1120͘ directions of GaN. Subsequently, the GaN layer with the patterned SiO 2 mask was used as an EL...

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To prevent the cracking of GaN thick films grown on a sapphire substrate by hydride vapor phase epitaxy (HVPE), a novel technique without complex processes is developed. By adding a temperature ramping step in the HVPE GaN epitaxy process, more than 300-mm-thick high-quality crack-free GaN thick films on sapphire substrate can be obtained by this technique. After separation by a conventional laser-induced lift-off process, a 1.5 in. 300 mm freestanding GaN wafer with a dislocation density of approximately 1 Â 10 7 cm À2 could be fabricated without any cracks. No additional designed-patterned or stress-reduced structures were applied in these samples to reduce the dislocation density and thermal stress.

As one of the most mature techniques for manufacturing free-standing GaN substrates, hydride vapor phase epitaxy (HVPE) always encounters problems associated with residue thermal stress, such as GaN bending and cracking during and after growth. This work presents a patterning approach and a non-patterning approach to reduce stress in thick GaN films grown on sapphires by HVPE. The patterning approach, forming dot air-bridged structures, adopted standard photolithography to fabricate hexagonally aligned patterns of dots on GaN templates. Following HVPE growth, regular voids were formed and buried in the GaN thick-films. These voids helped to relax the stress in the GaN thick-films. In the non-patterning approach, thick GaN films were simply grown at a specially set sequence of ramping temperatures during HVPE growth without any patterned structure. This temperature-ramping technique, gives crack-free high-quality 2"-diameter GaN films, thicker than 250 μm, on sapphires in high yields. These thick GaN films can be separated from sapphire using conventional laser-induced lift-off processes, which can be followed by subsequent HVPE regrowths. A 600 μm-thick free-standing GaN films has a typical dislocation density of around 4×10 6 cm -2 with a full width at half maximum (FWHM) in the high resolution X-ray diffraction (HRXRD) spectrum of GaN (002) of around 150 arcsec.The residual stress in the thick GaN films was analyzed by micro-Raman spectroscopy. The effectiveness of the patterning and the non-patterning techniques in reducing the strain in GaN films is discussed. The advantages and weaknesses of the patterning and the non-patterning techniques will be elucidated.