Slow‐mode degradation mechanism and its control in new bright and long‐lived ZnSe white LEDs

Abstract

AbstractThis paper presents slow‐mode degradation mechanism of ZnSe‐based white LEDs. A systematic study has been made from a viewpoint of microscopic point defect reaction such as generation and migration in both device active layer (ZnCdSe/ZnSe MQW) and p‐type ZnMgSSe cladding layer utilizing DLTS/ ICTS, SSRM (scanning spreading resistance microscope), and EL (electroluminescence)‐imaging techniques, coupled with device aging experiments. We have found two different degradation stages (1st and 2nd stages) in the slow‐mode degradation, which are caused by quite different microscopic point defect species. The 1st stage is induced by the long‐diffusion of H0‐center (nitrogen‐complex deep hole trap in p‐cladding layer), forming high‐density dark‐spots in the MQW active layer. This active center is generated only in the stress‐stimulated condition such as thermal or device fabrication process. After controlling the initial concentration of the H0 center, we have observed no detectable new dark‐spots during device operation, leading to fairly long device‐lifetime (∼1000 h). This 2nd stage has appeared as a carrier (hole) reduction in the p‐type cladding layer. This final degradation stage is found to take place by an increase of shallow compensating donor‐like centers in p‐type cladding layer (ZnMgSSe). Based on these insights on the microscopic point defect reaction, we have developed (new) double cladding i‐ZnMgBeSe/p‐ZnMgSSe white‐LEDs, which has exhibited long device lifetime of over 10000 h. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)