Nature of the 3.28 eV photoluminescence peak in unintentionally doped GaN microdisk grown by Na flux method

This Ultraviolet luminescence (UVL) peak near 3.28 eV, as a common defect-induced luminescence phenomenon, has been a subject of extensive study as its origin is linked to the impurity incorporation during growth, which is crucial for material quality control in GaN-based optoelectronic and microelectronic devices. However, the UVL-related transition mechanism is controversial. GaN microdisks grown by Na-flux method with c-plane and (101¯1)-plane, can provide what we believe is a new perspective to reveal UVL-related mechanism. The low-temperature photoluminescence (PL) demonstrates the stronger UVL on the c-plane compared to the (101¯1)-plane. With increasing power and temperature, there is a significant blue shift of UVL in the c-plane. Therefore, the UVL-related mechanism may be the DAP transition. In unintentionally doped GaN, we suggest that the shallow acceptor associated with this DAP defect may be Mg-related defect. SIMS results show that c-plane has a high concentration of Mg impurities, which is consistent with the PL results that the c-plane has a significant UVL peak intensity. It is also consistent with the conclusion that the UVL-related shallow acceptor is Mg_Ga. This work reveals the origin of ultraviolet luminescence in Na-flux GaN, providing new insights into the optical properties of GaN crystals grown by the Na-flux method.