KEYWORDS: Silicon carbide, Metals, Crystals, Scanning electron microscopy, Chemical vapor deposition, Semiconducting wafers, Transmission electron microscopy, Nickel, Control systems, Nanowires
SiC nanowires (NWs) are attractive building blocks for the next generation electronic devices since silicon carbide is a wide bandgap semiconductor with high electrical breakdown strength, radiation resistance, mechanical strength, thermal conductivity, chemical stability and biocompatibility. Epitaxial growth using metal-catalyst-based vapor-liquid-solid mechanism was employed for SiC NW growth in this work. 4H-SiC substrates having different crystallographic orientations were used in order to control NW alignment and polytype. A new technique based on vapor-phase delivery of the metal catalyst was developed to facilitate control of the NW density. Both 4H and 3C polytypes with a strong stacking disorder were obtained. The 4H and 3C NWs had different orientations with respect to the substrate. 4H NWs grew perpendicular to the c-plane of the substrate. The stacking faults (SFs) in these nanowires were perpendicular to the [0001] nanowire axes. All 3C NWs grew at 20° with respect to the substrate c-plane, and their projections on the c-plane corresponded to one of the six equivalent ⟨101-0⟩ crystallographic directions. All six orientations were obtained simultaneously when growing NWs on the (0001) substrate surface, while only one or two NW orientations were observed when growing NWs on any particular crystallographic plane parallel to the c-axis of the substrate. Growth on {101-0} surfaces resulted in only one NW orientation, thereby producing well-aligned NW arrays. Preliminary measurements of the NW electrical conductivity are reported utilizing two-terminal device geometry.
Improvement of recombination properties is observed in EFG microcrystalline Si wafers subjected to consecutive solar cell processing steps. A dramatic increase of room- temperature band-to-band photoluminescence (PL) intensity (hvmax equals 1.1 eV) by a factor of two orders of magnitude occurs in the solar cell, to demonstrate a significant reduction in non-radiative recombination during the upgrading steps that benefit solar cell efficiency. Using spatially resolved PL mapping over 100 cm2 wafers, we study PL behavior in solar cell fabrication. We compared point-by-point PL mapping with distribution of minority carrier diffusion length in the same poly-Si wafer. A correlation between PL intensity and the diffusion length is documented using a statistically valid data-base. It is suggested that room-temperature PL mapping can be used for on-line monitoring of poly-Si solar cell quality.
Ultrasound treatment (UST) was applied to improve electronic properties of polycrystalline silicon films on glass. A strong decrease of sheet resistance was observed in plasma hydrogenated films at UST temperatures lower than 100 degrees C. An enhanced passivation of grain boundary defects after UST was directly measured by nano-scale contact potential difference with atomic force microscope. These strong UST effects are accompanied by improvement of hydrogenation homogeneity as confirmed by spatially resolved photoluminescence study. We also observed a dramatic increase of intra-red photoluminescence (PL) intensity by a factor of two orders of magnitude after a few minutes of UST at elevated temperatures up to 280 degrees C. IN films obtained by solid-phase crystallization of (alpha) -Si, UST activates a new PL maximum at 0.9eV related to the amorphous fraction of poly-Si films. A new mechanism of ultrasound stimulated hydrogenation of dangling bonds in polycrystalline and amorphous Si films is proposed. UST processing was also applied to plasma hydrogenated poly-Si thin film transistors. We found UST stimulated reduction of a leakage current and shift of a threshold voltage, which can be beneficial for AMLCD applications.
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