Green photonics holds the key to challenge the energy and environmental challenges we face. III-nitride semiconductors have unique properties that can be exploited to realize next generation green photonic devices and systems. However, conventional III-nitride epilayers have some fundamental issues. It is very difficult to incorporate high indium composition. As a result, the performance of GaN-based green LEDs is much worse than that operating in the blue spectrum, leading to the so-called “green gap” in photonics. On the same circumstances, there has no demonstration of efficient photovoltaic devices using GaN-based materials. Therefore, it is essential to develop novel III-nitride semiconductor materials to realize green photonic devices and systems.
In this thesis, we have investigated the molecular beam epitaxial (MBE) growth and fundamental structure, electrical and optical properties of InGaN/GaN heterostructures for applications in green light emitters and artificial photosynthesis. With the use of selective area epitaxy (SAG) techniques, InGaN/AlGaN core–shell dot-in-nanowire arrays has been grown with preciously controlled size, spacing, and morphology. We have demonstrated, for the first time, all epitaxial surface-emitting laser green diodes, which can exhibit a very low threshold current density ~400 A/cm2 at room temperature. We have further explored the potential of GaN-based nanocrystals for the development of artificial photosynthesis devices for the conversion of CO2 to clean fuels, including syngas, a combination of CO and H2, one of the prominent future solar fuels. We have investigated the design and performance of photoelectrochemical and photochemical cells for syngas production. A new benchmark record of 1.88% solar to syngas efficiency has been achieved in a photoelectrochemical cell. In order to utilize the abundant visible solar spectrum, we have engineered multi-band InGaN/GaN nanowire heterostructure, which exhibit a solar to-syngas conversion efficiency of ~0.73% in a simple, one-step photochemical cell. This work is the first ever report on photochemical CO2 reduction with efficiency above 0.5%.
This thesis establishes the use of III-nitride nanocrystals for energy efficient green light emitters and solar fuel production.