Keywords:AlGaN UV-B LED, Relaxed n-AlGaN, Light output power
Eco-friendly, smart and high-power DUV and UV-B LED light sources on AlN template are strongly demanded for both medical and agricultural applications, including vitamin D3 production in the human body, immunotherapy, and enriching phytochemicals in the plants. AlN template-based n-AlGaN buffer layer (BL) and n-AlGaN electron injection layer (EIL) require a low dislocation densities (TDDs) and cracks free surface underneath the multiple quantum wells (MQWs) for the fabrication of LEDs. The crystal structure of AlN template grown on c-(0001)-sapphire substrates was improved using a well-known technique of “ammonia (NH3) pulsed-flow multilayer (ML) growth” in Riken, where FWHM values of the XRCs for the (0002) and (10-12) planes approximately 200 and 350 arcsec, respectively (TDDs ~5×108 cm−2) were achieved [Hirayama, H. et al. Phys. Status Solidi A 206, 1176–1182 (2009)]. But still the growth of Al0.40Ga0.60N BL on AlN template, with x~0.40 Al-content for UV-B emission, can have a lattice mismatch >1.7% and subsequently can generate a huge number of vertically propagating TDDs in the n-AlGaN EIL underneath the MQWs and can deteriorate the internal quantum efficiency (IQE).
In this work, using the same growth condition as given in Ref [Khan and Hirayama et al., J. Mater. Chem. C 7 (2019)143] for 295nm UV-B emission, except the Si-doping level variation as well as thickness variation of n-AlGaN BL were revisited to investigate the effect of both on emission efficiency from UV-B MQWs. Thanks to the 4μm-thick and 50% relaxed n-AlGaN EIL layer, where a record PL emission intensity of 5×107 [a.u] were achieved at low temperature in MQWs and finally the light power of UV-B LED at 308nm emission were improved from 8mW(previous) to 12mW. Single peak EL spectra under different dc drive were confirmed.