Red GaN microLEDs gain an efficiency boost from a surrounding reflector
UC Santa Barbara researchers have demonstrated red InGaN/GaN microLEDs on sapphire with 20.8% peak external quantum efficiency. The devices emit at 619 nm, a result that brings efficient red emission closer to the GaN material family already used for blue and green LEDs.
Getting InGaN to emit red light requires more indium in the active material. Maintaining efficiency becomes harder as that concentration rises, so the quality of the epitaxial growth is an important part of the UCSB result. The team then improved how much of the generated light escapes by surrounding each emitter with a annular distributed Bragg reflector. The reflector redirects light that would otherwise remain trapped. Its contribution appears in the external quantum efficiency figure, which measures photons escaping the device relative to injected electrons.
“One material can emit every visible color now,” says doctoral researcher Roark Chao.
For display integration, the attraction is a common semiconductor family across red, green and blue. Different epitaxial designs would still be needed for the colours, but the prospect is a closer match between the material systems being integrated into the array. The published work concerns individual microLEDs, with the reflector built around the emitter.
The group also tested operation at 200°C. At high current, the devices retained more than 74% of their light output at room temperature, while the wavelength moved by less than 8 nm. Those thermal measurements describe the brightness and colour stability at elevated temperature; the 20.8% figure is the separately reported peak efficiency.
