Nanocomposites and nanomaterials

Photoluminescence polarization anisotropy of porous silicon nanocrystals array
I.B. Olenych1, L.S. Monastyrskii1, A.P. Luchechko1, A.M. Kostruba2
1 Ivan Franko National University of Lviv. 50 Dragomanov Str., Lviv-79005, Ukraine. E-mail: iolenych@gmail.com
2 Lviv University of Trade and Economics, 9 Samtshuka Str., Lviv-79011, Ukraine

The intense visible photoluminescence of porous silicon (PS) has stimulated great interest to study of optical properties of silicon nanocrystals due to the wide range of possible applications. In particular, LEDs, photodetectors, multilayer Bragg reflectors and waveguides based on the PS have been successfully proposed [1]. An interesting feature of the PS structures is photoluminescence polarization anisotropy due to the wire-like structure of silicon nanocrystals aligned mainly along [100] direction [2]. 
In this work, the arrays of PS nanocrystals formed on the substrates with [100] and [111] orientations were obtained in an epoxy resin. The use of epoxy resin provides the preservation of the PS nanostructure orientation after separation from the silicon substrate. Systems of PS nanoparticles were characterized by scanning electron microscopy, atomic-force microscopy, ellipsometry and luminescent spectroscopy. The influence of the PS nanostructures orientation on optical and photoluminescent properties of the obtained composite was investigated. 
It has been found by ellipsometry technique that effective refractive index of composites depends on the angle between the orientation of nanocrystals and the test laser beam. In addition, we have observed a partial polarization of photoluminescent radiation in the PS nanosystems. The photoluminescence intensity with polarization perpendicular to the polarization of the excitation light was significantly lower. Moreover, the PS nanocrystals with the [111] orientation slightly enhanced the polarization anisotropy. The obtained results show the influence of morphological anisotropy of silicon nanosystems to optical properties.

1.	Bisi O., Ossicini S., Pavesi L. Porous silicon: a quantum sponge structure for silicon based optoelectronics // Surf. Sci. Rep.-2000.-38.-P.1-126.
2.	Kovalev D., Ben Chorin M., Diener J., Koch F., Kux A., Efros A.L., Rosen M., Gippius N.A., Tikhodeev S.G. Polarization of porous silicon photoluminescence: alignment and built-in anisotropy // Thin Solid Films.-1996.-276.-P. 120-122.