Influence of the hydrogen level in (InAlGa)N-based laser diodes on the stability of the device's operating voltage

The impact of hydrogen impurities in the semiconductor heterostructure of (InAlGa)N-based laser diodes on the stability of the device's operating voltage is investigated. Diodes emitting at a wavelength of around 400 nm with different hydrogen concentrations and different p-contact metals, here Pt and Pd, were operated under constant current stress. After specific operation times, current-voltage measurements from the p- to the n-side and between two adjacent p-contacts were performed. During continuous-wave (cw) operation, three effects can be observed, namely (a) an increase of the operating voltage originating from degradation of the p-contact, (b) an increase of the voltage due to degradation of the heterostructure, and (c) a reduction of the voltage caused by a reduction of the series resistance. Degradation of the p-contact only occurs when Pt is used as the p-contact metal. In particular, Pd as the p-contact metal leads to stable p-contacts, regardless of the hydrogen concentration. Operating-voltage variations related to changes in the heterostructure may be attributed to the migration of residual hydrogen during operation. A reduction of the hydrogen level to a ratio of 20 between magnesium and hydrogen atoms, in combination with Pd p-contacts, led to stable forward voltages during cw operation for up to 100 h.