Nanopores formation and shape evolution in Ge during intense ionizing irradiation

• We study the formation mechanism, alignment and elongation of isolated voids formed in damaged Ge after SHI irradiation.
• Dynamics of accumulation of open volume under successive ion impacts to form porous network and their alignment is studied.
• The shape change of the voids along strain direction is understood based on plastic deformation.
• This study is helpful to tune the shape, size and density of voids in damaged layer.

Open volume formation, their growth and shape evolution in Ge by two step ion beam processing are meticulously analyzed and presented in this paper. In the first step, a continuous damaged Ge (d-Ge) layer of ∼1.2 μm thickness is prepared by multiple energy Ar ion implantations in single crystalline Ge (c-Ge) with damage formation of ∼7 DPA. Doppler broadening spectroscopy (DBS) which is highly sensitive to vacancy type defects indicates the formation of 5% open volume in d-Ge layer as compared to c-Ge. The high resolution transmission electron microscopic (HRTEM) studies show that the d-Ge layer consists of nano-crystallites of size ∼1–2 nm. In the second step, d-Ge layer is further irradiated using 100 MeV Ag ions at room temperature. After this irradiation, a substantial volume expansion is detected with the help of a stylus profiler. The volume expansion is observed to be due to the formation of open volume which is restricted to d-Ge only as characterized by DBS at low irradiation fluences and by cross-section scanning electron microscopy (XSEM) and HRTEM at higher irradiation fluences. Microscopic analysis shows that the voids grow in size and change in shape from spherical to prolate spheroid with increasing ion fluence while void density ∼2 × 1012 cm−3 remains almost invariant of ion fluence. The major axis of prolate spheroid is observed to be aligned approximately along the direction of strain which is confirmed by irradiation at two different angles (0° and 45°). Relative swelling and aspect ratio increase with fluence and tend to saturate at higher fluences. These results are explained on the basis of thermal spike model which indicates that temperature within ion track reaches above melting point of Ge only in disordered phase. The voids are restricted to only d-Ge as formation of voids along ion track is due to melting and re-solidification. The shape change of voids and alignment in strain direction is a consequence of ‘ion hammering’ effect.

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