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Sample Preparation The PS layers were formed by anodizing nondegenerate p- and n-type Si wafers in an HF2 solution composed of 50% HF : ethanol = 1 : 1 by volume at current densities of 10-100 mAIcm for 5-60 min in the dark or under illumination by a 500 W tungsten lamp from a distance of 20 cm. In some cases, post-anodization illumination was employed in order to enhance the PL intensity and/or to control the PL peak energy. The thickness of anodized PS layers is 3 to 50 gim. Free standing PS films were also prepared by electrochemical separation of anodized PS layers from Si substrates. 695 0 Mat. Res. Soc. Symp. Proc. Vol. 358 1995 Materials Research Society

Measurements The experiments were carried out cocerning the following aspects. (a) Characteristics of PC, PL, PLE, and EL Prepared PS samples were transferred into the vacuum chamber, and semitransparent thin Au films were evaporated onto the PS layer surface. The PS layer was photoexcited through the thin Au film using a 500 W Xe lamp as a light source, and vertical photoconduction was measured under the condition that a positive or negative bias voltage was applied to the Au contact with respect to the ohmic back contact. This PC cell also operates as a visible EL diode. The temperature dependence of the dark- and photocurrents of these diodes were measured, including the spectral photoresponse, P1, PLE spectra, and the corresponding EL emission spectra. For PL measurements, the PS samples were excited with a 325 nm He-Cd laser, a 514 nm Ar laser, or other monochromatic light. (b) Polarization Properties of PL The polarization of PL from PS was investibated using linearly polarized light as an excitation source. The polarization degree P was measured as functions of crystallographic orientation, PL excitation and emission wavelength, and temperature. The P value is defined as (Ill - I.L )/(Il, + I±), where ll and It. are the PL intensity components polarized parallel and perpendicular to the polarization plane of the excitation light, respectively. (c) Dynamic Behavior of Electrical PL Quenching and EL Effects of the external electric field on the PL characteristics were studied for PS diodes with the same structure as the PC cells. The PL spectra and intensity were measured in a N 2 gas atmosphere as a function of positive or negative dc bias voltage. Dynamic response of the PL and EL intensities was also measured under the pulsed bias condition, and compared to that of the PL decay time. (d) Analyses of EL Diodes with a Large-Area Contact After anodization, the PS samples formed on p-type wafers were rinsed in ethanol and then immersed in an electrochemical cell consisting of an electrolytic solvent and a counter Au electrode. Some anodized PS samples were lightly oxidized by a photo-induced or thermal processing before immersion into the cell. Polypyrrole films were synthesized by electropolymerization in the same way as that described previously [7]. After deposition of PP, thin Au films were evaporated in order to provide an electrical contact. The