Subsurface modifications in indium phosphide induced by single and multiple femtosecond laser pulses: A study on the formation of periodic ripples

We use cross-sectional transmission electron microscopy to study the damage induced below the surface of indium phosphide (InP) samples by single and multiple femtosecond laser pulses with a photon energy lower than the InP band gap. Single-pulse irradiation creates a similar to 100 nm deep crater with a resolidified surface layer consisting of quasiamorphous indium phosphide. The resolidified layer has a thickness of similar to 60 nm at the center and extends laterally beyond the edge of the crater rim. Exposure to multiple femtosecond pulses of 2050 nm center wavelength results in the formation of laser-induced periodic surface structures (LIPSS) with two different periods, one (similar to 1730 nm) less than but close to the laser wavelength and one (similar to 470 nm) four times smaller. Segregation beneath both types of ripples leads to the formation of In-rich particles embedded in the resolidified surface layer. Extended defects are detected only below the center of the multiple-pulse crater and their distribution appears to be correlated with the LIPSS modulation. Finally, LIPSS formation is discussed in terms of the observed subsurface microstructures. (c) 2007 American Institute of Physics.