Optical Property Characterization of Silicon Quantum Wires
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silicon has large heterogeneity due to the intricate mechanism of formation and complicated morphology. We have recently succeeded in bulk synthesis of pure SiQW's [6,7]. Using these real freestanding SiQW's, we have been able to evaluate the size-dependence of the PL spectrum, and to elucidate the origins of the different emission bands. EXPERIMENTAL The synthesis of the SiQW samples used here are described elsewhere [6]. Sponge-like SiQW samples were pressed into slabs, and were oxidized in air in a quartz tube furnace at about 700'C for different times to minimize and control the crystalline core diameter of the wires. The morphology of the SiQW's was observed using an AMRAY scanning electron microscope (SEM) with a field emission gun, or a JEOL-200CX transmission electron microscope (TEM). The atomic structure analysis of the as-grown and partially oxidized SiQW's was performed by a Hitachi H-9000-NAR high resolution electron microscope (HREM). The PL measurements were performed at room temperature using photoexcitation under a second harmonic generation of a femtosecond dye laser with wavelength around 313.5 nrm. Electron paramagnetic resonance (EPR) signals were recorded in an S-band Bruker ER-200D spectrometer.
19 Mat. Res. Soc. Symp. Proc. Vol. 571 © 2000 Materials Research Society
RESULTS AND DISCUSSIONS The main feature of the SiQW's is their high purity and uniformity of diameter. At the macroscopic scale the deposit was grown into millimeter-sized bundles, as is revealed in the SEM micrograph in Fig. l(a). The TEM image shown in Fig. l(b) reveals the general morphology of the SiQW's in the tip of a single bundle. The SiQW's have an average diameter of 13 ± 3 nm, and lengths of up to hundreds of micrometers. The inset shows one quarter of the selected-area electron diffraction (SAED) pattern, which is characterized by sharp spotty rings similar to that of bulk polycrystalline silicon. It should be further noted that though X-ray diffraction taken from a bulk quantity of the SiQW samples revealed typical crystalline silicon peaks without any impurity phase, structural disorder and lattice distortion are characterized by obvious peak broadening compared to that of a bulk silicon crystal.
Fig. 1. (a) This typical SEM micrograph reveals that the sample grew into bundles which consist of pure SiQW's. (b) Representative TEM image showing the morphology of the extremely pure SiQW's of average diameter around 13 ± 3 nm. Inset shows one quarter of a SAED pattern revealing the crystalline silicon structure of the SiQW's. The as-grown SiQW sample was photoexcited using an ultraviolet laser source at 313.5 nm, and the corresponding room-temperature PL spectrum was measured. A surprising feature is that multiple emission bands corresponding to dark red [8], green and blue light were observed at 816 nm, 517 rim, and 420 rim, respectively. To evaluate the origin of the different emission bands, the dependence of the PL on the size of the SiQW's was characterized. The diameter of the SiQW's was reduced controllably
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