Morphology of ion irradiation induced nano-porous structures in Ge and Si1−xGex alloys

Abstract

Crystalline Ge and Si 1Àx Ge x alloys (x ¼ 0.83, 0.77) of (100) orientation were implanted with 140 keV Ge À ions at fluences between 5 Â 10 15 to 3 Â 10 17 ions/cm 2 , and at temperatures between 23 C and 200 C. The energy deposition of the ions leads to the formation of porous structures consisting of columnar pores separated by narrow sidewalls. Their sizes were characterized with transmission electron microscopy, scanning electron microscopy, and small angle x-ray scattering. We show that the pore radius does not depend significantly on the ion fluence above 5 Â 10 15 ions/cm 2 , i.e., when the pores have already developed, yet the pore depth increases from 31 to 516 nm with increasing fluence. The sidewall thickness increases slightly with increasing Si content, while both the pore radius and the sidewall thickness increase at elevated implantation temperatures.

Nanostructuring Germanium Nanowires by In Situ TEM Ion Irradiation

“…Typically, the elongated nanopores in germanium can be several hundred nm long. [ 22,26,29 ] In this work, as such elongation was limited due to the reduced dimension of the target material such long cylindrical nanopores were not formed and the nanostructuring has instead led to an increase of the nanopores diameter as a function of fluence. The tilt series as well as the BF images and the in situ high resolution TEM (HR‐TEM) video (Video S1, Supporting Information) showed that the nanovoids grew whilst remaining relatively circular.…”

“…This is in contrast to previous works in the literature in which the nanopores diameter did not increase continuously with the fluence. [ 22,26 ] For example, Alkhadi et al., [ 26 ] during self‐ion irradiation of bulk germanium, did not observe an increase of the pore diameters but, instead, an evolution of the nanostructures from spherical, when they emerged, to nanopores of relatively small radius (10–20 nm), although elongated in the direction of the ion beam at higher fluences.…”

“…For this purpose, it has been proposed in the literature to use ion beams to nanostructure several semiconductor materials such as gallium antimonide, indium antimonide, gallium nitride, and germanium. [ 22–29 ] Indeed, it is reported that these materials can become nanoporous when subjected to ion bombardment. [ 22–29 ] The use of ion beams to induce nanopores in germanium is attractive as nanoporous germanium is interesting for integration in photovoltaic cells, batteries, microelectromechanical systems or gas and bio sensors.…”

“…[ 22–29 ] Indeed, it is reported that these materials can become nanoporous when subjected to ion bombardment. [ 22–29 ] The use of ion beams to induce nanopores in germanium is attractive as nanoporous germanium is interesting for integration in photovoltaic cells, batteries, microelectromechanical systems or gas and bio sensors. [ 22,30,31 ]…”

“…Whilst the exact characteristics of the mechanisms are still being investigated, it is generally accepted that the nanoporosity is the result of void formation by the agglomeration of vacancies. [ 22–29 ] The voids are first observed in germanium when the fluence is above 10 15 ions cm −2 as the irradiated material gradually takes on a nanoporous form (also referred to as a honeycomb or foam‐like structure). [ 22,32 ] The vacancies which may form the nanopores are induced during ion irradiation in the collision cascade.…”