Laser Diode Failure Mechanisms

Facet COD Power Level

Catastrophic Optical Damage COD

COD is an irreversible process is in which the laser facet or the bulk of the semiconductor melts due to a strong absorption of light. This happens due to a high light density, a high density of non-radiative recombination centers or a combination of both. Due to the abovementioned effect of heat on the energy bands, this effect tends to reinvigorate itself quickly creating so much heat in a very small portion of the semiconductor crystal or the facet, that it actually physically melts, locally destroying the laser diode. To increase the COD power levels on a laser design is challenging and it includes beam expansion to reduce local light intensity. By placing special attention on the epitaxial growth process and the facets, significant improvements can be achieved.


Laser Diode Facet Treatment and Mirrors

After cleaving the semiconductor, a set of vacant bonds appear at the exposed surface caused by the interruption of the periodicity of the crystal lattice. Impurities (especially oxygen) readily attach to them. Their effect is to render the facet partly non-transparent for the laser photons absorption at the facet, generating heat and causing facet degradation. The depleted bands of the active region then become absorbing at the lasing wavelength. Beyond a critical point, the positive feedback of heat and increased absorption leads to an overheating of the facet and an COD. A facet treatment to reduce the facet absorption will likely increase the COD level. The facet treatment comes along with an effort to significantly improve the laser efficiency by applying two different etalon mirror coatings on both sides. This is achieved by a series of dielectric coatings with alternating diffraction indexes with a thickness that corresponds to the wavelength of the laser. The mirror on the backside ideally reflects all the photons back into the resonator increasing the stimulated emission and reducing the threshold current. The second mirror allows a precise amount of light to be bounced back into the cavity and the rest to exit the semiconductor. The later is the light emitting side of the laser. Both mirrors consist of non absorbing dielectrics.

Facet Passivation and Mirror Coating

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.


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