High Efficiency of Deep Ultraviolet LEDs

Prototype of a deep-ultraviolet LED. (Source: K.Kojima, Tohoku U.)

Deep ultra­violet light-emitting diodes (DUV-LEDs) made from aluminium gallium nitride (AlGaN) effi­ciently transfer electrical energy to optical energy due to the growth of one of its bottom layers in a step-like fashion. This finding can lead to the develop­ment of even more efficient LEDs.

AlGaN-based DUV-LEDs are receiving much research attention due to their potential use in sterili­zation, water puri­fication, photo­therapy, and sunlight-inde­pendent high-speed optical communi­cation. Scientists are inves­tigating ways to improve their efficiency in converting electrical energy into optical energy. Kazunobu Kojima of Tohoku Univer­sity specia­lizes in quantum opto­electronics, which studies the quantum effects of light on solid-state semi­conductor materials. He and colleagues in Japan used a variety of specialized micro­scopic techniques to understand how the structure of AlGaN-based LEDs affects their effi­ciency.

They fabricated an AlGaN-based LED by growing a layer of aluminium nitride on top of a sapphire substrate with a very small one degree off-angle. Next, they grew a cladding layer of AlGaN with silicon impurities on top of the aluminium nitride layer. Three AlGaN quantum wells were then grown on top of this. Quantum wells are very thin layers that confine electrons and holes within the dimension that is perpen­dicular to the layers’ surface, without restricting their movement in the other dimensions. The top quantum well was finally covered with an electron-blocking layer formed of aluminium nitride and AlGaN with magnesium impu­rities.

The micro­scopic investi­gations revealed that terraced steps form between the bottom aluminium nitride and AlGaN layers. These steps affect the shapes of the quantum well layers above them. Gallium-rich stripes form that connect the bottom steps to the small distortions they cause in the upper quantum well layers. These stripes represent micro­paths of electric current in the AlGaN cladding layer. These micropaths, together with a strong locali­zation of movement of electrons and holes within the quantum well layers, appears to increase the LEDs’ efficiency in converting electrical energy to optical energy, the researchers say. The team next plans to use this information to fabricate more effi­cient AlGaN-based deep ultra­violet LEDs, says Kojima. (Source: Tohoku U.)

Reference: K. Kojima et al.: Carrier localization structure combined with current micropaths in AlGaN quantum wells grown on an AlN template with macrosteps, Appl. Phys. Lett. 114, 011102 (2019); DOI: 10.1063/1.5063735

Link: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

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