Ferromagnetic Properties of Spark-Processed Photoluminescing Silicon
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Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
in sp-Si and those known in the literature for Si are possible. Magnetic susceptibility measurements were conducted to investigate interactions of the defects created during spark processing. Using a Superconducting Quantum Interference Device (SQUID) with an accuracy better than Ix10"8 emu. [6] EXPERIMENTAL PROCEDURE Commercial, p-doped silicon wafers having a purity of 5.5x1O's cm"3 were spark-processed by applying a unidirectional high voltage (15kV) discharge between an anode tip and the silicon wafer acting as the cathode. To exclude ferromagnetic contamination, the anode tip was chosen to be from the same silicon wafer as the substrate. A single-pulse spark event had a duration of about 10 ns. The repetition frequency was l6kIHz with the off time between two spark events lasting about 60 Ais. Moreover, fabrication was done in an isolated environment to avoid possible contamination from ferromagnetic elements. For magnetic susceptibility measurements, long Si strips 10.0 by 0.64 cm, were fashioned to eliminate possible stray fields seen by the magnetometer. Spark-processing was performed for 6 days. This led to a processed area which extended completely through the wafer. Specifically, a processed area of 0.5 cm in diameter on the top portion and 0.2 cm on the bottom of the wafer was observed. For grounding of the wafer, the ends of the long strips were adhered to an aluminum plate using a conductive, black polymer tape. The ends of the strips were cut off after fabrication to ensure no contamination. Moreover, susceptibility measurements of the conductive polymer were performed to check for any possible ferromagnetic response. None was found. The susceptibility measurements were conducted in a SQUID magnetometer at temperatures ranging from 4.3 to 300 K and fields varying from +200 to +50,000 Gauss. Annealing of the specimens were performed in UH.P nitrogen (99.999% purity). Samples for EPR measurements were produced similar to those above with some exceptions. All wafers were cut to 3 mm2 . Sp-Si samples were then spark-processed for 2 hours. They were produced using stagnant air, flowing air, and in a bell jar back filled with oxygen (99.99% purity). For the oxygen-prepared sample, a bell jar was evacuated and reflushed five times to minimize the presence of unwanted residual gases or contaminants. Again, samples were annealed in UHP nitrogen for 30 minutes. EPR experiments were conducted at room temperature. The samples were mounted on quartz rods to fix their position. Measurements were performed both parallel and perpendicular to the applied magnetic field. Silicon wafers used in the luminescence experiments were cut to 1 cm2 squares. The spark processing was initiated for two hours under differing atmospheric conditions (see above). Annealing of sp-Si occurred again in UHP nitrogen flowing across the samples for 30 minutes. The PL measurements were performed in air within less than 5 min. after heat treatment. Excitation was per
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