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.
On-chip multicomponent system made with vertical structure quantum well diode
A vertical-structure quantum well (QW) diode based monolithic multicomponent system is for the first time implemented by integrating a light-emitting diode (LED), waveguide, and photodiode (PD) on a metal-bonded III-nitride-on-silicon platform. Two identical QW-diodes that are manufactured using all existing standard fabrication processes function as the LED and PD respectively, and waveguides are formed to establish on-chip optical link between the LED and PD. The PD absorbs the light from the adjacent LED coupled through the waveguide to induce a photocurrent that is proportional to the injection current of the LED, leading to an on-chip power monitoring system. One PD can monitor multiple LEDs simultaneously by identifying individual differences of the system from the superimposed signals using signal extraction approach.
Cathodoluminescence study of InGaN MQW laser diodes using laser lift‐off technique
In this paper, spatially integrated or spatially resolved cathodoluminescence (CL) spectroscopy were employed to evaluate the structural and optical changes of InGaN multiple quantum well (MQW) structure before and after Laser lift-off (LLO) process. As for a partially strain-relaxed In0.1Ga0.9N MQW laser diode (LD) structure with cracking in underlying epitaxial layers, LLO did not cause the blue shift in CL spectra as expected, but rather the red shift induced by the indium diffusion from its abundant regions around cracks. This result could be proved by the monochromated scanning CL images taken at 390 nm and 408 nm not having the same obvious complementary as those of original ones. Compared with calculated laser fluence for LLO, this phenomenon could be mainly attributed to the easy penetration of laser energy via cracks and consequent irradiation absorption within MQWs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)