Nanostructured surfaces

Features of surface structuring of direct and indirect band gap semiconductors by femtosecond laser
Berezovska N.I.1, Dmitruk I.M.1, Hrabovskyi Ye.S.1, Degoda V.Ya.1, Kolodka R.S.1, Dmytruk A.M.2, Blonskyi I.V.2
1 Faculty of Physics, Taras Shevchenko National University of Kyiv. Volodymyrska Street, 64/13, Kyiv-01601, Ukraine. 
E-mail: n_berezovska@univ.kiev.ua
2 Photon Processes Department, Institute of Physics of NAS of Ukraine. Prospect Nauky, 46, Kyiv-03028, Ukraine.

Significant progress in the understanding of the formation mechanisms of femtosecond laser-induced periodic surface structures (LIPSS) on different materials inspires the practical interest to such kind modified surfaces [1]. Studies of the structural peculiarities of the laser processed surfaces, as well as spectroscopic characteristics of the treated areas are of research interest. 
In present report we study the impact of femtosecond (fs) laser impulses on semiconductors with direct (ZnSe, GaAs) and indirect (Si) band gap. The laser treatment of corresponding semiconductor has been performed in the multi-pulse regime in air environment. The influence of fs-laser radiation parameters (wavelength, power density) on surface morphology of the samples has been analysed by scanning electron microscopy (SEM) and 2D Fourier transform of SEM images. Under the treatment with the fundamental fs-laser radiation (800 nm, about 150 fs) we observed both low spatial frequency LIPSS and high spatial frequency LIPSS. The laser treatment with the second harmonic frequency resulted in LIPSS formation with one period. Other features at the surface (different defects in periodic structure, ablation products, etc.) have been also analysed.
Optical spectroscopy has been used for the analysis of the secondary emission spectra of studied semiconductors observed after ultra-short laser processing. The enhancement of some emission bands in the photoluminescence (PL) spectra of direct band gap semiconductors has been detected. The nature of the observed PL bands is discussed.

1.	Bonse J., Gräf S. Maxwell meets Marangoni – a review of theories on laser-induced periodic surface structures // Laser Photonics Rev.-2020.-14, N.10.-P. 2000215.