A. Sassella

In situ Cr gettering in polycrystalline silicon sheets obtained by edge-defined film-fed growth

Gettering of Cr during the growth of silicon sheets from a Cr‐doped melt is observed when the solid/liquid interface region is exposed to CO or CO2 gases. The gettering occurs within a region about 1‐μm wide at the surface of the crystal, where a large accumulation of carbon and oxygen is detected. Mechanisms for carbon and oxygen participation in forming gettering sites for Cr are examined.

Polarization effect on infrared absorption of oxygen precipitates in silicon

High‐spatial resolution Fourier transform infrared spectroscopy permitted us to study the effects of incident light polarization on the absorption band related to aggregates of SiO2 disk‐shaped precipitates present in annealed (100) Czochralski silicon samples. This is the first report documenting the strong dependence of such a band intensity on light polarization. Experimental results were compared with the simulation obtained using the effective medium theory. From this comparison we deduce that the precipitates only lie on the planes among {100} which are parallel to the wafer surface. This result can be related to the difference between the 〈100〉 direction and those crystallographically equivalent, introduced by the ingot growth process.

Stoichiometry of oxygen precipitates in silicon

Abstract We studied oxygen precipitation in silicon upon brief sample annealing at 1100°C by high resolution Fourier transform infrared technique. This technique permits the detection of small agglomerates of disc-shaped precipitates formed in the bulk of annealed silicon wafers. Precipitates are characterized by a peak at 1230 cm -1 in the infrared absorption spectrum. It is further demonstrated that only the disc-shaped precipitates can contribute to the absorption band mentioned. Moreover, the exact wavenumber position of the peak can give additional information about the chemical composition of the oxide constituting the discs.

Non-doping light impurities in silicon for solar cells

The impurities present in silicon for solar cells directly influence its electrical characteristics and therefore solar cells performance. Light non-doping impurities like oxygen, carbon, hydrogen, and nitrogen are of particular importance since they exhibit both a positive and a negative impact when present in silicon substrates. In this overview we illustrate the positive and negative effects of impurities on silicon material properties. It is also shown that their contemporary presence leads to a mutual interaction and hence to the formation of more complex structures.

Homogeneity of carbon microdistribution in edge-defined film-fed grown polycrystalline silicon

Abstract Edge-defined film-fed grown polycrystalline silicon sheets are known to contain carbon impurities in a state of extremely high supersaturation. During very rapid growth, carbon is incorporated with an unusually high effective segregation coefficient close to unity; therefore it is of interest to study its microdistribution over the sheet. We performed a scanning IR absorption microanalysis over macroscopic distances in order to evaluate the homogeneity of carbon microdistribution in as-received samples grown in an inert or oxidizing atmosphere, as well as on samples annealed for 72 h in air at different temperatures, up to 1150 °C. As-received samples revealed a very good homogeneity of carbon distribution, which is not even disturbed by the presence of oxygen. However, the homogeneity of carbon microdistribution was found to be significantly disturbed upon annealing. This must be taken into consideration when studying the electrical characteristics of solar cells produced from such material upon various thermal treatments.

Highly Sensitive Optical Method for the Characterization of SiO2 Films in Bonded Wafers

The thickness of very thin films (1-10 nanometers thick) and the frequency of the longitudinal optical mode of the material composing the film (related to its chemical and structural properties) are demonstrated to be measurable with high sensitivity by means of infrared transmission. The particular optical configuration proposed permits the analysis of the SiO2 film in bonded wafers. It is shown that the sensitivity of this new method increases as the films are made thinner.

Surface mode excitation in platelet SiOx precipitates in silicon

Infrared absorption spectra of very thin SiO 2 films embedded in silicon are reported and analyzed. From these results a new interpretation of the characteristic absorption band of platelet precipitates in silicon can be proposed, based on the excitation of surface modes. On this basis, the quantitative evaluation of precipitated oxygen is demonstrated to be unreliable.

Boron accumulation at epi-substrate silicon interface during epitaxial growth

Abstract Boron accumulation was observed close to the interface between an epitaxially grown silicon layer and a silicon substrate wafer and then analyzed. It was concluded that boron contamination interacting with the surface oxide on wafers led to boron accumulation close to the interface. Such accumulation is shown to occur for epilayers of standard thickness (approximately 10 μm), with boron being electrically unactive.