Optical Properties of Nanoporous Silicon in the Presence of Magnetic Nanostructures
Authors: Granitzer Petra , Rumpf Klemens , Poelt Peter , Reissner Michael 
Published: February 2020

This work aims to combine luminescent microporous silicon with a ferromagnetic metal to create a nanocomposite system with modifiable photoluminescence due to the presence of metal deposits as well as to influence the optical properties by an external magnetic field. Due to the metal filling of the porous silicon (PSi) the peak of the photoluminescence is blue-shifted and furthermore an increase of the intensity is observed. The influence of the magnetic metal filling on the optical properties is discussed and the magnetic characterization of the samples is presented.
The microporous silicon is fabricated by anodization of a moderately doped p-silicon wafer. The electrolyte consists of HF, distilled water and ethanol in the ratio 1:1:2. The deposition of the magnetic metal has been performed by pulsed electrodeposition in using in the case of Ni the so called Watts electrolyte (NiCl2 and NiSO4). First the optical properties of the luminescent PSi are investigated, second the magnetic metal is electrochemically deposited within the porous silicon and subsequently the nanocomposite specimens are characterized optically and magnetically. The samples are structurally characterized by SEM, EDX and TEM.
The optical properties are investigated with respect to the shift of the photoluminescence peak due to the metal filling. Photoluminescence spectra of bare PSi show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm depending on the metal deposition time and furthermore the luminescence intensity is increased. The deposition of the metal is crucial since the metal has to be incorporated within the nanopores but has to be avoided on the sample surface to still enable the luminescence. Furthermore the oxide layer between PSi and metal is essential to guarantee a photoluminescence of the specimens. Concerning the magnetic properties of the nanocomposites the embedded Ni structures offer the same branched morphology as the porous silicon template and thus these interconnected Ni-structures do not offer a superparamagnetic behavior which is confirmed by temperature dependent magnetization measurements. Generally superparamagnetic behavior is possible due to the pore diameters in the nanometer range. Field dependent magnetization measurements have been performed with a magnetic field applied perpendicular and parallel to the sample surface showing a high magnetic anisotropy. Due to the interconnected Ni-structures the samples offer a film-like behavior with the easy axis parallel to the surface. Figure 1 shows the film-like behavior of a Ni-filled sample.
In the frame of this work the optical characterization of luminescent PSi with respect to the peak position of the photoluminescence compared with Ni filled samples is discussed in detail as well as the corresponding magnetic properties of the nanocomposites. Furthermore the influence of an external magnetic field on the optical properties is elucidated. The presented systems give not only rise to optoelectronic applications but also to magneto optical integrated devices.
Figure 1: Field dependent magnetization measurements performed in two directions, showing a high magnetic anisotropy with the easy axis for an applied field parallel to the surface.

Figure 1