Modification of germanium surface exposed to radiation of a nanosecond ultraviolet laser

Modification of the polished {111} surface of single-crystal germanium (n-type, resistivity 47 Ohm · cm), exposed to radiation of a focused frequency-pulse nanosecond ultraviolet Nd : YaG laser, was studied by optical profilometry, scanning electron and probe microscopy. It was revealed, that the threshold of plasma formation with generation of a crater on the surface, occurs at an energy density of E ~ 1.2—1.3 J/cm². When the sample was stationary, at E ~ 0.1 J/cm² irreversible damage to the surface occurred. When scanning the surface with radiation at E ~ 0.50—1.15 J/cm², in the absence of noticeable traces of crater formation, the generation of etching pits with a regular triangular shape was observed, the concentration of which was (3—5) · 10⁵ cm². The figures resemble dislocation-etching pits, usually obtained by selective chemical etching.
Dislocations were detected by ablation because of exposure to laser radiation. The centers of ablation nucleation are dislocations that come to the crystal surface. The transverse dimension of etching pits was ~ 5—10 µm and their overlap led to an alternating picture of trihedral pyramids, formed by the {111} planes. The presented images show the rounded edges and tops of the pyramids and the height of the profile of the figures ~ 1—2 μm. The linear dimensions of the pits testify a rapid flow of the process. Based on the total time of exposure to radiation on the surface ~ 200 ns, the rate of formation of flat faces in the pits equal to ~ 0.1—0.3 m/s, which is several orders of magnitude higher, than the rate of formation of the same faces during crystal growth was established. The depth of the surface layer, in which the structure was formed, was ~ 15 μm.

UV laser, nanosecond pulse, optical breakdown threshold, ablation, laser etching, germanium single crystal

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