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Deep-ultraviolet (DUV) light-emitting diodes (LEDs) and laser diodes (LDs) have a lot of potential applications, such as, sterilization, water purification, medicine and biochemistry, light sources for high density optical recording, white light illumination, air purification equipment, and so on.We demonstrated aluminum gallium nitride (A1GaN)-based DUV-LEDs with emission wavelengths in the range of 222-351 nm fabricated on low threading dislocation density (TDD) A1N template on sapphire substrate.Low TDD A1N layers were developed by using an ammonia (NH3) pulse-flow multilayer (ML) growth technique, with metalorganic chemical vapor deposition (MOCVD) crystal growth system.We achieved significant increase of internal quantum efficiency (IQE) of AlGaN quantum wells (QWs) from about 0.5% to more than 60% by reducing the TDD.Also, an electron injection efficiency (EIE) was markedly improved (from 20% to more than 80%) due to the suppression of leakage-electron flow by using multi-quantum barrier (MQB).We also improved light-extraction efficiency (LEE) by using 2-dimensional (2D) photonic crystal.The maximum cw output power and external quantum efficiency (EQE) obtained for 270 nm LED were 33 mW and 3.8%, respectively.