Photoluminescence from Porous Silicon Anodized with Monochromatic Light Illumination
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Takahiro Matsumoto*, Yasuaki Masumoto*, Go Arata**, and Hidenori Mimura*** *Single Quantum Dot Project, ERATO, Japan Science and Technology Corporation, 5-9-9 Tokodai, Tsukuba 300-26, Japan, [email protected] **Institute of Physics, University of Tsukuba, Ibaraki 305, Japan "***Research Institute of Electrical Communication, Tohoku University, Sendai 980-77, Japan
ABSTRACT The effect of monochromatic light during the anodization of porous silicon is investigated from the points of view of structural characterization and photoluminescence. A clear correlation is observed between the photoluminescence peak energy and the bandgap energy determined by the illuminating photon energy. The difference of these two energies is analyzed by a size-dependent Stokes shift. INTRODUCTION Porous Si (PS) formed by electrochemical anodization has been attracting a lot of interest because it exhibits strong visible photoluminescence (PL) at room temperature [1]. Although a great deal of effort has been made to elucidate the origin of the PL, the mechanism is still a topic of discussion.
It is generally known that holes are necessary for the electrochemical dissolution [2,3], therefore in the case of anodization for n-type substrates, light illumination is necessary to supply holes into Si substrates. A consequience of this process is that the size and the bandgap of the nanostructure should be controlled by the illumination photon energy; the etching process proceeds until the illumination photon energy becomes lower than the band gap of porous Si. In this paper, we aim to clarify the effect of monochromatic light illumination on the size of nanostructure and on the optical properties of porous Si. We have characterized the nanostructures fabricated by the various energies of monochromatic light using secondary electron microscope analysis (SEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The correlation between the nanostructure and the PL peak energy points to the existence of sizedependent Stokes shift. EXPERIMENTAL Porous silicon samples were prepared by anodizing 1-5 i2cm resistivity (100)-oriented ntype silicon wafers using HF-ethanol solution (HF:H 20:C 2H5 OH=I:l:2) for 15 min. Al was 535 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
deposited on the back of the wafers to ensure an enough current flow to the wafer. The sample preparations were carried out at a constant current density in the range of 16 mA/cm 2 under the illumination of monochromatic light ranging from 450 nm to 700 nm (450 nm, 500 nm, 600 nm, and 700 nm). (To avoid the illumination of unnecessary light, the anodization was performed in a dark room.) These wavelengths of the monochromatic light were made by using a combination of interference filters (the transmission band width of 10 nm) and a 1 kW xenon- or a 500 W halogen lamp. The power density through the interference filters is of the order of 100 mW/cm 2 . After the anodization, the samples were rinsed with ethanol and they were immediatel
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