Linear Polarization of Porous Si Photoluminescence
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M. BEN CHORINt, J. DIENERI, and F. KOCHt * General Physics Institute, RAS, Vavilova Street 38, Moscow 117333, Russia, [email protected] t Beam Theory Section, Naval Research Laboratory, Washington, DC 20375, USA t Technische Universiteit Miinchen, Physik-Department E16, D-85747 Garching, Germany ABSTRACT We demonstrate experimentally that linear polarization of porous Si photoluminescence depends significantly on the excitation geometry and describe this effect within the framework of a dielectric model in which porous Si is considered as an aggregate of slightly deformed, elongated and flattened, dielectric elliptical Si nanocrystals with preferred orientation in the [100] direction. The theoretical best-fit analysis of the experimental data allows us to get certain information concerning the shapes and orientation of the ellipsoids. INTRODUCTION An early model of porous silicon as a system of isolated quantum wires, proposed in the first papers [1,2], is probably far from reality. Recent TEM [3] have shown that it is a conglomeration of Si nanocrystal chunks, more or less aligned in the [100] direction. We have demonstrated recently [4] that the unusual behavior of the porous Si photoluminescence polarization can be related to the nonsphericity of the nanocrystals in porous silicon. The effect arises from the large difference between the dielectric constants of the anisotropic semiconductor nanocrystals and the surrounding medium. Similar effects due to this difference were first demonstrated for free standing semiconductor quantum well wires [5]. In this paper we analyze a possibility to reconstruct some parameters of the shape and orientation distribution of Si nanocrystals from the experimental data on the anisotropy of the porous
Si photoluminescence. EXPERIMENT Microporous silicon layers are prepared from p-type (100), boron doped substrates with typical resistivity of 5 f2 cm. Before anodization, the substrates are provided with an ohmic back contact by aluminum evaporation, followed by 20 minutes of sintering at 550 *C. The electrochemical anodization is carried out in the dark. The etching solution is a 1:1 by volume mixture of hydrofluoric acid (49 wt. % in water) and ethanol. The etch current density of 30 mA/cm 2 and anodization time is tuned to get a layer thickness of 20 pm. After anodization, the samples are removed from the etching cell, rinsed with propanol and dried in a nitrogen gas flow. The PL is excited using the 442 nm line of a He-Cd laser which is polarized by a glass polarizer. The light is deflected using a small mirror and is incident onto the sample normal to the investigated surface. The emitted light is collected by two lenses in the direction normal to the surface and focused onto the entrance slit of a 22 cm Spex single monochromator through a depolarizer. The polarization of the detected light is determined by a second polarizer placed between the lenses. PL spectra are normalized for the spectral response of the detection optical system. Measurements are done at room temperature. 203
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