Sub-surface damage in indium phosphide caused by micromachining of grooves with femtosecond and nanosecond laser pulses

Grooves laser-micromachined in InP using 130 fs and 8 ns pulses with fluences ≈2 and 0.7 J/cm2 are investigated by cross-sectional transmission electron microscopy. At the fluence of 2 J/cm2, irradiation with both femtosecond and nanosecond laser pulses yield substantial resolidified layers with a maximum thickness of ≈0.5 μm. In contrast, at the fluence of 0.7 J/cm2, irradiation with nanosecond pulses leads to a layer of similar thickness, while femtosecond irradiation produces laser induced periodic surface structures with minimal resolidified material. For both fluences, femtosecond pulses generate substantial densities of defects extending over a few microns in depth, while nanosecond laser irradiation leads to no observable damage beneath the resolidified layer. The high peak power density and the stress confinement obtained from femtosecond pulses, along with incubation effects, are identified as the major factors leading the observed plastic deformations.