GaN quantum dot LEDs move into the UV
QDs are attracting interest because of the ability to fabricate highly efficient optoelectronic devices that are more stable with temperature than their quantum well relations.The group describes the fabrication and measurement of its LED in the latest issue of the Japanese Journal of Applied Physics (Vol.42 part 2, 8A, L885).
The GaN/AlGaN layers were grown using a Nippon EMC GMC210 MOCVD reactor onto low-temperature buffer layers deposited on 2-inch sapphire substrates. The AlGaN surface was then exposed to a silane/ammonia gas mixture to modify the surface states. The presence of silicon on the AlGaN surface has an anti-surfactant effect, preventing the subsequent GaN deposition by 2D step-flow growth and leading to 3D growth, which results in quantum dot (QD) formation. The layer of QDs was buried beneath an AlGaN capping layer and a final p-GaN layer improved the ohmic characteristics of the contact-metal interface.
Samples of the GaN QD layers were examined by atomic force microscopy (AFM) prior to the capping layer deposition in order to determine the QD size and distribution for various growth conditions. Typically, dots were 6.5 nm high and 50 nm wide with a density of about 5x1010 /cm2. The dot height was found to be controllable by adjusting the growth time while keeping density constant.
Room temperature CW electroluminescence spectra showed a well-defined emission peak at 360 nm (3.44 eV). As the injection current was increased from 5-100 mA, the emission shifted slightly towards the blue. The researchers attributed this to heating as the current increased because of the poor thermal conductivity of sapphire.
