Study of defects and lifetime of green InGaN laser diodes

InGaN quantum wells for direct green lasers with indium concentrations of about 30% are studied in respect to defect structure and influence of defects on device lifetime. A new test structure enables us to determine strain relaxation in In-rich layers by X-ray diffraction even at typical well thicknesses of few nanometers. We show that there is a high risk to generate screw and edge dislocations in the In-rich quantum wells in non-optimized structures as verified by transition electron microscopy. Such defects strongly influence the device lifetimes of green lasers. Non-optimized devices have a drop of output power on a time scale of 100 h. The threshold current increases proportional to the square root of time. Additionally, a linear correlation of slope efficiency versus one over laser threshold current is observed. The degradation process is explained as reduced carrier injection efficiencies related to the generation of additional charged defects in the active layers. Optimized active layers already enable stable operation over a period of more than 1000 h at 50 mW constant power at 517 nm emission wavelength with an extrapolated increase of operation current by <30% within this time.