Improvement of Al-polar AlN layer quality was accomplished by three-stage flow-modulation metalorganic chemical vapor deposition (FM-MOCVD). In this method, the unit of the FM-MOCVD sequence was composed of three stages; Stage I for simultaneous source supply, Stage II for trimethylaluminum supply, and Stage III for ammonia supply, which were cyclically repeated. The AlN quality revealed by X-ray diffraction strongly depended on the time of Stage I. A growth model was proposed considering the surface coverage of the islands nucleated during Stage I. Exciton fine structures were eventually observed by low-temperature cathodoluminescence reflecting the tremendously improved crystalline quality.
Large-area (∼1 cm2) laser lift-off (LLO) wafer separation of Al0.45Ga0.55N layers from AlN/sapphire templates has been demonstrated by using 200-period AlN/Al0.22Ga0.78N short-period superlattice (SPSL) sacrificial layers instead of conventional GaN photoabsorbing layers. The SPSL functions as the photoabsorbing and mechanically weakened layer in the LLO process. This SPSL-assisted LLO technique promises future progress of vertical-type deep ultraviolet light emitting diodes and freestanding AlN–AlGaN bulk substrates.
A three-dimensional plasmonic metamaterial absorber (3-D PMA) was theoretically investigated and designed for the performance enhancement of wavelength selective uncooled infrared (IR) sensors. All components of the 3-D PMA are based on thin layers of plasmonic metals such as Au. The post produces a narrow gap, such as a few hundred nanometers, between the micropatch and the metal plate. The absorption properties of the 3-D PMA were investigated by rigorous coupled-wave analysis. A strong wavelength selective absorption is realized by the plasmonic resonant mode of the micropatch and the narrow-gap resonant mode between the micropatch and the plate. The disturbance of the post for both resonance modes is negligible. The absorption wavelength is defined mainly by the size of the micropatch, regardless of the micropatch array period and is longer than the micropatch array period. The absorption mode can also be controlled by the shape of the micropatch. Through-holes can be formed on the plate area, where there is no gap resonance to the micropatch. The thickness of each component can be reduced considering the skin depth effect and there is no added absorption of materials such as SiO 2 . A small pixel size with reduced thermal mass can be realized using a 3-D PMA structure. The results obtained here will contribute to the development of high-performance uncooled IR sensors for multicolor imaging.
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