Understanding efficiency of deep UV LEDs
Japanese team shows that steps that form during LED fabrication improve their efficiency by creating tiny paths of electric current
AlGaN-based DUV-LEDs are receiving much research attention due to their potential use in sterilisation, water purification, phototherapy, and sunlight-independent high-speed optical communication.
Now Kazunobu Kojima of Tohoku University and colleagues have used a variety of specialised microscopic techniques to understand how the structure of AlGaN-based LEDs affects their efficiency.
They have discovered that deep ultraviolet LEDs made from AlGaN efficiently transfer electrical energy to optical energy due to the growth of one of its bottom layers in a step-like fashion. They think that this finding, published in the journal Applied Physics Letters, can lead to the development of even more efficient LEDs.
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 subatomic particles called 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 localisation 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.
