Laser Diode Failure Mechanisms

When the facets are cleaved and the periodicity of the crystal lattice is interrupted, the open bonds of the atoms react very quickly with the atmosphere. Especially oxygen leads to a large increase in the nonradiative recombination rate at the surface. In other words, the laser light absorption at the facet can be significant if left untreated. This effect is specially significant for high power GaAs lasers. For InP the recombination velocity at the surface is about two orders of magnitude smaller and and the absorption is lower. There is a number of various processes that are individual for the different materials and types of edge emitting lasers but all come down to the removal or neutralization of oxygen and the passivation with a component that neutralizes the absorption at the facet. The neutralization of the oxygen can be done by applying a thin layer or Al that will react with the oxygen to build the insulating and transparent Al2O3. Some report that a layer of 5nm is not too thin nor too thick. Another method is to mechanically remove the oxygen with ion milling and passivate the facet with nitrogen. The nitride layers seal the surface from further oxidation and form nitride compounds that have higher bandgaps than the bulk crystal and cannot absorb the laser photons. The simplest way is probably to ion-mill the oxygen away and immediately apply the dielectric mirror coating in the same vacuum chamber. The coating process is usually carried out in a PECVD (Plasma Enhanced Chemical Vapor Deposition) or IBD (Ion Beam Deposition) sputter system depending on the actual coating material and passivation technique used. Selection of the material and thickness of the passivation layer is also determined by the fact that the PECVD process involves ions with much lower energies than the IBD process. High-energy ions can damage irreversibly on impact a thin passivation layer a few nanometers thick.