A green worthy of envy
The crystalline semiconductor gallium nitride (GaN) has opened the door to realize green lasers thanks to its unique optoelectronic properties. Indium GaN (InGaN) devices operating in the green spectral range region (wavelengths around 510 to 570 nm) are the result. However, despite recent progress, high-quality InGaN light-emitting and laser diodes (LEDs) are very difficult to produce. EU-funded scientists tackled the technical basis for the deficiencies through the project SINOPLE. The team focused on the use of molecular beam epitaxy (MBE) to deposit In-rich InGaN active layers onto nearly dislocation-free GaN single crystals. Incorporating high levels of In, a prerequisite for the green emitters, had not been achieved previously by MBE and there was no detailed knowledge of In-rich InGaN/GaN heterostructures. Researchers achieved epitaxial growth of InGaN with high In content (up to 20 %) on various substrates using MBE. Scientists also had success exploiting InGaN on zinc oxide. On the road to device development, the team also produced a highly sensitive method to characterise In fluctuation based on transmission electron microscopy with unprecedented accuracy. Investigators delivered a variety of lasers in the ultraviolet, blue and green spectral ranges. The novel MBE system facilitated growth of higher-quality InGaN and produced a record for electrically pumped nitride MBE laser diodes with wavelength emission at 482 nm. Additionally, the team produced continuous-wave laser diodes with a wavelength of 450 nm, a power of 60 mW and a lifetime exceeding 5 000 h. The result shows that MBE can compete with conventional metal organic chemical vapour deposition (MOCVD). In addition, the technique facilitates flexibility in processing temperature and substrates that are not accessible with MOCVD. SINOPLE has significantly advanced the state of the art regarding production of green LED lasers with higher efficiencies, power and lifetimes. The application potential is enormous for this elusive coherent-light source. From information displays to televisions to biomedicine, the world is on the brink of an eye-opening surprise.
Keywords
Lasers, green light, gallium nitride, green lasers, InGaN, LEDs
