III-nitride based deep-ultraviolet light-emitting diodes (DUV LEDs) have been recognized as one of the ideal candidates to replace the toxic mercury-based UV-lamps for the application of air/water purification, disinfection/sterilization and biochemical environmental sensing. However, the poor optical performance of the DUV LEDs is still limited by the low hole injection efficiency and the strong quantum confined Stark effect (QCSE). To address those issues, we proposed two unique multiple quantum well (MQW) architectures including graded quantum wells (QWs) for mitigating the QCSE and graded quantum barriers (QBs) for enhancing the electron blocking capability as well as hole injection efficiency.

As a result, we found that the DUV LED with graded QWs by linearly increasing the Al-composition (IAC) along the growth direction could significantly enlarge the wave function from 40.4% to 58.6% and thus boosting the optical power by 67.04%. In order to optimize the IAC grading structure, we further calculated the evolution of optical power as well as the wave function overlap when the Al-composition at the end of the QWs increases from 0.5 to 0.6 while maintaining the same value of beginning Al-composition (0.5) in the QWs. We found that the value of optical power increases with the ending Al-composition increasing from 0.5 to 0.6 because of the rapid increase of the wave function overlap. At last, we concluded that IAC grading from 0.6 to 0.5 shows the best optical performance.

Besides the research on the QWs, a proper design of the QBs can also dramatically affect the device performance. We have already utilized band-engineered graded QB structure to attain a higher output of optical power. In this part, we proposed a DUV LED structure that includes five unique Al~x~Ga~1−x~N QBs. Each QB has a linear-increment of Al-composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Then, the optical power was significantly improved by 65.83% and the external quantum efficiency (EQE) was remarkable enlarged by 67.05% at the injection current of 60 mA. Additionally, we found that the thickness of the proposed graded QBs plays a critical role in high-efficiency UV illumination because it not only affects the electron/hole concentration and distribution, and their transport property in the active region, but also determines QCSE in the QWs, which is directly related to radiative recombination rate. Therefore, it is crucial to choose a proper thickness of the graded QB to maximize carrier concentrations and minimize the QCSE in the QWs, aiming to improve the LOP and EQE of DUV LEDs.

In summary, we investigated the optical characteristics of DUV LEDs by incorporating graded QWs or graded QB structures in the active region. The results suggest that DUV LEDs with those unique MQW structure exhibit better hole injection, and increased electron and hole wave function overlap, thus achieving an enhanced optical power and IQE performance. The proposed structure with such promising graded QWs or graded QBs provides us an effective solution to boost optical power in the pursuit of high-performance DUV emitters.


Optimized Quantum Structures for Efficient Deep Ultraviolet Light Emitting Diodes

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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