G. Scardera

Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells

We report work progress on the growth of Si quantum dots in different matrices for future photovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantum dots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PL decay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength.

Fourier transform infrared spectroscopy of annealed silicon-rich silicon nitride thin films

A correlation between bonding changes in silicon-rich silicon nitride films, subjected to high temperature annealing under N2 ambient, and the formation of silicon nanocrystals is presented. The postannealing appearance of a shoulder between 1000 and 1100 cm−1 in the Fourier transform infrared (FTIR) spectra of silicon-rich silicon nitride films is attributed to a reordering in the films toward an increased SiN4 bonding configuration resulting from the precipitation of silicon nanocrystals. The FTIR monitoring of bonding changes in these films allows for the indirect verification of silicon nanocrystal formation.

Investigating Large Area Fabrication of Silicon Quantum Dots in a Nitride Matrix for Photovoltaic Applications

The suitability of using dual-mode PECVD, in conjunction with high temperature annealing, to fabricate arrays of silicon nanocrystals in a nitride matrix over large areas is investigated. The formation of nanocrystals is verified using TEM, XRD, micro-Raman and FTIR. Initial results show reliable growth of a superlattice of Si nanocrystals over an area of 14 by 10.5 cm

Effect of annealing temperature on the formation of silicon nanocrystals in a nitride matrix

Superlattices of silicon nanocrystals or quantum dots (QDs) are fabricated by depositing alternating layers of stoichiometric and sub-stoichiometric silicon nitride by dual-mode PECVD and subsequent high temperature annealing. NH3, SiH4 and Ar are used as processing gases. The formation of QDs is monitored for varying annealing temperatures using TEM and GI-XRD. Samples composed of 50 bi-layers are grown under the same conditions and annealed for two hours at temperatures ranging between 600 and 1150°C. A 50 bi-layer superlattice structure of silicon nanocrystals with an estimated average grain size of approximately 4 nm was achieved at 1000°C. The use of FTIR spectroscopy as a complementary technique for verifying the formation of silicon nanocrystals in a nitride matrix is investigated. The IR absorbance spectra for samples containing silicon nanocrystals show a distinct shoulder at 1080 cm-1 corresponding to the Si-O-Si stretching mode possibly due to oxidation. Preliminary evidence is also presented showing the possible formation of α-Si3N4 nanocrystals at 1100 and 1150°C.

Effects of silicon nanocrystallite density on the Raman-scattering spectra of silicon quantum dot superlattices

Silicon nanostructures based on silicon quantum dots (SiQDs) in a silicon dielectric are being investigated for application to Si based tandem cells. The main challenge for such a structure is to obtain sufficient carrier mobility and hence a reasonable conductivity. It is believed that the conductivity of such novel SiQDs embedded in a silicon dielectric matrix is controlled by the close spacing of the SiQDs. In this study we grew a-SiOx/a-SiO2 ordered arrays by reactive RF magnetron co-sputtering. The composition of the SiOx (12. The Raman scattering spectra presented in this study suggest a dot size-dependent peak below 520 cm-1 (Inc) and an inter-dot spacing-dependent shoulder between 495 and 500 cm-1(Is). The correlation between crystalline silicon density and ratio of the relative integrated intensity of SiQDs and its shoulder bands are presented. The size of the SiQDs is also confirmed by structural analysis through transmission electron microscopy (TEM) and X-ray diffraction (XRD). Initial analysis of the relationship between the relative integrated intensity (Inc/Is) and conductivity of SiQD superlattices with various compositions of the SiOx are presented.