Fabrication and Characterization of Si Substrate-Free InGaN Light-Emitting Diodes and Their Application in Visible Light Communications
Visible light communications with InGaN-based light-emitting diodes (LEDs) grown on large-diameter (6-inch) and cost-effective Si (111) substrates are investigated experimentally. During epitaxial growth, the transition layers consisted of the step-graded AlGaN buffers incorporated with three low-temperature-grown (∼900 °C) AlN interlayers on AlN/Si substrates that are used to compensate for thermally induced tensile stress and to maintain a reasonable crystalline quality of GaN-on-Si LEDs. Strong light absorption from Si can be prevented by fabricating a Si substrate-free InGaN LED with a composite metal coating of Al/Ag/Al multilayer, providing improved adhesive strength and reflectivity comparable to the unitary Ag film. In comparison with GaN-on-Si LEDs, stripping Si substrates combined with the use of a highly reflective bottom mirror (Al/Ag/Al multilayer) reflected a more intense emission pattern corresponding to a 2.2 times (@ 190 mA) increase in light output power in thin-film LEDs. In addition, a 1.8 times (@ 160 mA) increase in optical channel bandwidth is achieved by using thin-film LEDs as optical transmitters. A direct line-of-sight optical link using the proposed thin-film LEDs achieved data transmission rates of up to 100 Mb/s over a distance of 100 cm, indicating that the proposed LEDs have potential for use as optical transmitters in indoor visible light communications.
SU-8 Planarized InGaN Light-Emitting Diodes With Multipixel Emission Geometry for Visible Light Communications
The potential for visible light communications with SU-8 planarized InGaN light-emitting diodes (LEDs) is investigated experimentally. For large-size LEDs, current crowding occurring near the n-contact is addressed by shrinking the dimensions of the emitters/pixels, along with the use of parallel-connected schemes to achieve multipixel emissions. Through improved heat dissipation, current uniformity, and light extraction efficiency, the resulting LED matrices fabricated with 6 <inline-formula> <tex-math notation="TeX">$\times$</tex-math></inline-formula> 6 pixels outperform conventional LEDs in terms of light output power and current-induced spectral shift. It was also found that good control of the SU-8 planarization process and optimizing the number of pixels facilitates the fabrication of high-efficiency LED matrices. In addition, the presence of large junction capacitance caused by the parallel connection of the individual pixels prevents these LED matrices with 6 <inline-formula> <tex-math notation="TeX">$\times$</tex-math></inline-formula> 6 pixels from operating at high speed. After eliminating the slow-responding phosphorescent components emitting from the phosphor-converted white LEDs, an open eye diagram at 80 Mb/s is demonstrated over a distance of 100 cm in directed line-of-sight optical links.
A New Vacuum Pressure Infiltration CFRP Method and Preparation Experimental Study of Composite
In order to prepare a carbon-fiber-reinforced polymer composite (CFRP) with ideal microstructure and properties, a new vacuum pressure infiltration CFRP method is proposed based on an analysis of existing CFRP preparation process methods. Research on composite material preparation systems was carried out by using this new method principle. The system mainly includes a fiber pre-forming module, a vacuum heating infiltration module, a hot-press curing molding module, and a data acquisition control module. Under the conditions of natural curing at 0 MPa + 6 h + 25 °C, vacuum heating curing at –0.05 MPa + 30 min + 80 °C, and hot-press curing at 0.7 MPa + 5 min + 50 °C, a two-dimensional (2D) CFRP with excellent microstructure and properties was successfully prepared. Observing the microstructure of the prepared composite material, it can be found that the inside of the composite material was sufficiently and uniformly infiltrated, and common preparation defects such as holes and delamination were effectively controlled. Through the performance test, the bending strength of the material reached 790 MPa.