Influence on the Porous Silicon Photoluminescence By Magnetic Nanostructures

Abstract

In this framework luminescent porous silicon (PSi) is filled with a magnetic metal (e.g. Ni) to influence the photoluminescence due to the presence of metal deposits. This happens in two ways. On the one hand the surface plasmon of the metal deposits is exploited to modify the luminescence and on the other hand it is influenced by an external magnetic field. Due to the metal filling of the porous silicon 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 nanocomposits is presented. The porous silicon is fabricated by anodization of a moderately doped p-silicon wafer. The electrolyte consists of hydrofluoric acid, Ethanol and distilled water (1:2:1). The metal deposition has been performed by pulsed electrodeposition in using a NiCl2 and NiSO4 solution. First the optical properties of the luminescent PSi are investigated, second Ni is deposited within the porous silicon samples and subsequently the nanocomposite specimens are characterized optically again and also magnetically. The samples are structurally characterized by SEM, EDX and TEM. The optical properties are investigated with respect to the shift of the photoluminescence 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 and the luminescence intensity is increased. Figure 1 shows the comparison of the photoluminescence of two Ni-filled samples with different metal deposition time. Concerning the magnetic properties of the nanocomposite the embedded Ni structures can be superparamagnetic from the size of the pore diameters but due to the branched morphology the achieved deposits tend to be interconnected and thus do not offer necessarily a superparamagnetic behavior. Also temperature dependent magnetization measurements give no hint for a superparamagnetic behavior. Field dependent magnetization measurements performed with the magnetic field applied perpendicular and parallel to the sample surface show a high magnetic anisotropy. It can be clearly seen that the samples offer a film-like behavior due to the interconnected Ni structures which is represented by the easy axis parallel to the surface. In the frame of this work the optical characterization of luminescent PSi with respect to its 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 are promising candidates for applications in optoelectronics and also for magneto optical integrated devices. Figure 1. Photoluminescence spectra of porous silicon with incorporated Ni for two different deposition times. Figure 1