Nanocrystalline silicon thin film growth and application for silicon heterojunction solar cells: a short review

Doped nanocrystalline silicon thin films, in which silicon nanocrystallites are embedded in an amorphous silicon matrix, are emerging as carrier-selective contacts for next-generation silicon heterojunction solar cells.

Formation of Nanocrystalline Silicon in Tin-Doped Amorphous Silicon Films

The process of crystalline silicon phase formation in tin-doped amorphous silicon (a-SiSn) films has been studied. The inclusions of metallic tin are shown to play a key role in the crystallization of researched a-SiSn specimens with Sn contents of 1–10 at% at temperatures of 300–500 ∘C. The crystallization process can conditionally be divided into two stages. At the first stage, the formation of metallic tin inclusions occurs in the bulk of as-precipitated films owing to the diffusion of tin atoms in the amorphous silicon matrix. At the second stage, the formation of the nanocrystalline phase of silicon occurs as a result of the motion of silicon atoms from the amorphous phase to the crystalline one through the formed metallic tin inclusions. The presence of the latter ensures the formation of silicon crystallites at a much lower temperature than the solid-phase recrystallization temperature (about 750 ∘C). A possibility for a relation to exist between the sizes of growing silicon nanocrystallites and metallic tin inclusions favoring the formation of nanocrystallites has been analyzed.

Elucidating the Effect of Etching Time Key-Parameter toward Optically and Electrically-Active Silicon Nanowires

In this work, vertically aligned silicon nanowires (SiNWs) with relatively high crystallinity have been fabricated through a facile, reliable, and cost-effective metal assisted chemical etching method. After introducing an itemized elucidation of the fabrication process, the effect of varying etching time on morphological, structural, optical, and electrical properties of SiNWs was analysed. The NWs length increased with increasing etching time, whereas the wires filling ratio decreased. The broadband photoluminescence (PL) emission was originated from self-generated silicon nanocrystallites (SiNCs) and their size were derived through an analytical model. FTIR spectroscopy confirms that the PL deterioration for extended time is owing to the restriction of excitation volume and therefore reduction of effective light-emitting crystallites. These SiNWs are very effective in reducing the reflectance to 9–15% in comparison with Si wafer. I–V characteristics revealed that the rectifying behaviour and the diode parameters calculated from conventional thermionic emission and Cheung’s model depend on the geometry of SiNWs. We deduce that judicious control of etching time or otherwise SiNWs’ length is the key to ensure better optical and electrical properties of SiNWs. Our findings demonstrate that shorter SiNWs are much more optically and electrically active which is auspicious for the use in optoelectronic devices and solar cells applications.

Vertical-Aligned Silicon Nanowire Arrays with Strong Photoluminescence Fabricated by Metal-Assisted Electrochemical Etching

Metal-assisted chemical etching of silicon is a commonly used method to fabricate vertical aligned silicon nanowire arrays. In this report we show that if in the above method the chemical etching is replaced by the electrochemical one, we can also produce silicon nanowire arrays, but with a special characteristic-extremely strong photoluminescence. Further research showed that the huge photoluminescence intensity of the silicon nanowire arrays made by metal-assisted electrochemical etching is related to the anodic oxidation of the silicon nanowires which has occurred during the electrochemical etching. It is most likely that the luminescence of the silicon nanowire arrays made with metal-assisted electrochemical etching is the luminescence of silicon nanocrystallites (located on the surface of silicon nanowire fibers) embedded in a silicon oxide matrix, similar to that in a silicon rich oxide system.

Interplay Between Silicon Nanocrystal Size and Local Environment Within Porous Silicon on the Analyte-Dependent Photoluminescence Response

Porous silicon (pSi) exhibits strong photoluminescence (PL) and its PL is often exploited for chemical sensor development. However, the sensor response is not uniform across a pSi specimen. We use co-localized confocal PL and Raman scattering mapping to establish a relationship between the analyte-induced PL response and the silicon nanocrystallite size, size distribution, and amorphous silicon (aSi) contribution across a pSi specimen. Using toluene as a model analyte, high analyte-induced PL response is associated with areas within the specimen that have (i) low aSi content, (ii) silicon nanocrystallites having diameters between 2 and 5 nm, and (iii) silicon nanocrystallites that exhibit a narrow size distributions (≤1% relative standard deviation).

Photoluminescence Spectroscopy of Si:SiO2 Films Fabricated by Radio Frequency Sputtering

We fabricated Si:SiO2 films including silicon nanocrystallites as one of nanoscale materials, which were prepared by co-sputtering of silicon and silicon dioxide. Optical properties of these films were analyzed by photoluminescence spectroscopy. Specimens fabricated at higher pressure between 0.3 Pa to 0.9 Pa emitted photoluminescence strongly. Photoluminescence was strong at shorter wavelength. In case fabricated in same Ar gas pressure, specimens that had composition ratio, 0.04, of the silicon for target emitted the photoluminescence strongly. Peaks existed on some PL spectra of specimen fabricated by the Ar gas pressure 0.3, 0.5 Pa and the composition ratio 0.04. Suppose that silicon nanocrystallites were fabricated, these size were few nm.