Growth of compound semiconductors in nanometer sized channels of polymers
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Growth of compound semiconductors in nanometer sized channels of polymers R. Engelhardt and R. Könenkamp, Hahn-Meitner Institut Berlin, Glienicker Str. 100, 14109 Berlin, Germany
ABSTRACT We report about the electrodeposition of a transparent, p-type semiconductor in etched channels of a thin polyester foil. The embedded semiconductor columns are robust against mechanical stress exerted on the foil, as the soft polyester matrix exerts only small forces on the embedded material. Dissolving the polyester results in freestanding semiconductor columns with height-to-diameter ratios in excess of 100.
INTRODUCTION Growth of semiconductors in confined volumes is of interest for a number applications in sensing, photonics and, to some extent, photovoltaics. In this paper we show that well-defined semiconductor columns with diameters as low as 30 nm and aspect ratios in excess of 100 can be grown in electrodeposition in etched ion tracks of polymer foils. We have deposited a quarternary, transparent p-type semiconductor, β-CuSCN, in the vertical etch cylinders of polyethylenterephthalat (PET) foils. While similar approaches have previously been employed for the growth of metal micro-wires (1-3), there has been no work on the preparation of compound semiconductor wires by electrodeposition. β-CuSCN has a bandgap of 3.6 eV and has recently been characterized optically and electrically (4), since it has aroused interest as a transparent p-type contact material to solar cells (5-7).
Both, the
fabrication of the polymer foils and the growth of the semiconductor columns are compatible with low cost and large area applications, such as solar cells and sensors.
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EXPERIMENTAL DETAILS Narrow channels of circular cross-section and diameters varying between 30 nm to 3 µm can be prepared in thin polyethylenterephthalat (PET) foils using irradiation of high energetic heavy ions (8). The passage of these heavy energetic ions induces amorphization of the polymer structure in a narrow cylinder of a few Å diameter (9). After irradiation the polymer foil is exposed to an aqueous solution of NaOH (8). This treatment induces fast etching of the amorphized ion track and isotropic slow etching of the intact polyester structure. The process results in well-defined cylindrical etch holes whose geometry can be accurately controlled in the etching process and, to some extent, by the irradiation. The lateral distribution of the etched channels in the foil is random, unless the ion beam is guided. Filling these channels with metals using galvanostatic techniques has been demonstrated earlier by Vetter et al. (2). CuSCN has previously been deposited in our lab in nano-porous substrates (5). CuSCN thin films exhibit hole conduction below ~100°C, and mixed ionic and electronic conduction for higher temperatures. At room temperature the dark conductivity is typically in the 10-3 (Ωcm)-1-range, while the hole mobility lies between 10 and 20 cm2/Vs, as determined recently from Hall measurements (10). For the electrodeposition process we have coat
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