Laser ablation XPS depth profiling of indium phosphide: optimisation of ablation rate and effects of laser pulse duration on stoichiometry

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Original abstract

Abstract Femtosecond laser ablation XPS (fs-LA XPS) is a recently developed approach to XPS depth profiling. Developing an understanding of laser pulse duration variation in retaining or modifying the true material stoichiometry during profiling is crucial to the advancement of this new field. Laser ablation XPS depth profiles have been performed on bulk InP using a 1030 nm laser employing a top-hat beam shaper and pulse duration from 160 fs to 6 ps. Per ablation level in the profile, both single pulse (above the single-pulse ablation threshold) and multi-pulse (below the single-pulse ablation threshold) profiling approaches have been studied. XPS depth profiles performed using single pulses showed a clear P enrichment. Following multiple ablations, SEM images from the ablation crater exhibited many globules present on the surface. Energy dispersive X-ray spectroscopy (EDS) analysis identified the globules as being In rich, whilst Auger electron spectroscopy (AES) confirmed the presence of a P enriched general surface. The true InP stoichiometry during XPS depth profiling could be preserved using a multi-pulse regime for a pulse duration of 160 fs but deviation from the true stoichiometry occurred for pulse durations ≥ 1 ps, indicative of a transition to a thermal ablation process. The InP laser-induced ablation threshold (LIAT) was determined using a volume regression method for single- and various multi-pulse regimes. For the latter, the accumulated LIAT fluence increased linearly with the number of laser pulses. The ablation depth and ablation rate were studied as a function of laser fluence for different multi-pulse regimes. At low accumulated fluences, both parameters can be approximated to a logarithmic relationship, particularly for lower pulse numbers per irradiation sequence. For fluences above 0.52 J/cm 2 , the dependence follows a linear relationship. The ablation depth for a total accumulated fluence of 1.03 J/cm 2 per ablation level obtained using different values of laser fluence and numbers of pulses was found to be independent of the parameters varied and increased linearly with the number of ablation levels.

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