Porous silicon (PSi) gained by electrochemical etching of a silicon wafer can be fabricated with various morphologies ranging from pore diameters of a few nanometers to tens of micrometers. In the nanoporous regime the material offers a branched structure with pore diameters of 2 – 5 nm. Such microporous silicon (IUPAC notation) is known to emit light in the visible regime since 1990 and is still under investigation [1].

The aim of this work is to combine luminescent porous silicon with a ferromagnetic metal (Ni) to influence on the one hand the photoluminescence by the presence of metal deposits and on the other hand the optical properties by an external magnetic field. The porous silicon samples are fabricated by anodization of moderately doped p-type silicon in an aqueous hydrofluoric acid solution under a constant current density of 10 mA/cm² for 20 minutes. The metal deposition within these small pores is challenging but succeeded. First the optical properties of the luminescent PSi are investigated, second Ni is electrochemically deposited in the porous silicon sample under cathodic conditions and subsequently the nanocomposite specimens are characterized optically and magnetically. Furthermore the structural characterization of the samples is carried out by SEM and TEM.

Photoluminescence spectra of bare porous silicon show a maximum around 620 nm whereas in the case of Ni filled samples the peak is blue-shifted to around 580 nm and the luminescence intensity is increased. Concerning the magnetic properties of the nanocomposite the embedded Ni structures are superparamagnetic from the size of the pore diameters but due to the branched morphology the achieved deposits are interconnected and thus do not offer necessarily a superparamagnetic behavior. Field dependent magnetization measurements with the magnetic field applied perpendicular to the sample surface show a low coercivity of about 200 Oe and a squareness (magnetic remanence/saturation magnetization) of about 0.2 which is indicative for a film like behavior due to the interconnected Ni structures (figure 1). Temperature dependent magnetization measurements give no hint for a superparamagnetic behavior.

The optical characterization of luminescent PSi with respect to photoluminescence intensity and decay times compared with Ni filled samples is discussed in detail as well as the corresponding magnetic properties of the nanocomposites. Furthermore the influence of a magnetic field on the optical properties is elucidated. The presented systems give not only rise to optoelectronics but also to magneto optical integrated devices.

[1] J. Joo, T. Defforge, A. Loni, D. Kim, Z.Y. Li, M.J. Sailor, G. Gautier, L.T. Canham, Appl. Phys. Lett. 108, 153111 (2016).

Figure 1: Field dependent magnetization curve of microporous silicon filled with Ni.