Efficiency Improvement of Deep Ultraviolet Leds via 2-Dimensional P-Type Boron Nitride

This project aims to develop high transparent and high conductive p-type boron nitride (p-BN) epilayer, serving as the p-type contact layer for 260-nm UVC LEDs. p-BN is adopted to replace the mostly used p-type AlGaN, which has been haunted by the trade-off between transparency and conductivity. The low UV transparency or low p-type conductivity of AlGaN is the main culprit for the very low (< 10 %) external quantum efficiencies (EQEs) of UVC LEDs. The development of UVC LED is mainly challenged by the incapability of p-type AlGaN. Depending on the Al composition, the hole activation energy (EA) in AlGaN is usually above 300 meV, leading to low free hole concentrations. Although the concentration can be increased with a reduced bandgap (Eg), the high optical absorption in the p-type layer can severely sacrifice the output optical power in the UV regime, resulting in the low EQEs of UVC LEDs. p-BN is a unique two-dimensional material, like graphite, but with a very high Eg, being close to that (6.1 eV) of AlN. Moreover, the hole activation energy (EA) in p-BN is reported to be merely 30 meV, much less than that (510 meV) of AlN. The high Eg and low EA of p-BN make it the only material that can serve as a high-UV-transparent, yet low-resistive, p-type semiconductor, which is extremely attractive for the applications on UVC LEDs.Since Jun. 2017, we have been growing p-BN with MOCVD, and have learned some characteristics of this unique material. According to the preliminary results, the p-BN produced in our lab exhibits the transmittance above 80 % for the wavelengths above 220 nm. And the Hall measurement showed that the 8-nm-thick p-BN contains the free hole concentration above 1018 cm-3, with the sheet resistance of 1200 ohm/sq. These results indicate the high transparency and high conductivity of p-BN, being consistent with the prediction by other research groups. We plan to integrate the growth of p-BN with that of the 260-nm UVC LED, aiming to develop new p-type semiconductor for UVC LEDs.