S. Binetti

Effect of compensation and of metallic impurities on the electrical properties of Cz-grown solar grade silicon

In this work we present a study of a p-type Czochralski-grown Si ingot which was grown using 10% solar grade silicon (SoG-Si). As the SoG-Si contains a relatively high concentration of impurities including phosphorus, the electrical properties of the as-grown wafers from this ingot are affected by both the compensating dopants and other impurities. Measurements of the minority charge carrier lifetime in the as-grown material reveal very low values (4–8μs). The Hall mobilities at room temperature correspond to normal values for Czochralski silicon in the upper part of the ingot (which solidifies first) and decrease significantly toward the bottom of the ingot. Segregation leads to an accumulation of impurities toward the lower parts of the ingot as well as to a stronger increase in phosphorus than of boron, the latter of which results in a high compensation level (i.e., an increasing resistivity). A priori, both effects could be responsible for the degradation of the electrical properties in the lower part...

Optical properties of oxygen precipitates and dislocations in silicon

Photoluminescence (PL) and deep level transient spectroscopy (DLTS) measurements were used to study the origin of optical emissions in the 0.8–1.0 eV region of selected oxygen precipitated and dislocated silicon samples. It was shown that the D1 band, present in both types of samples, is the convolution of different sub-bands, narrowly spaced between 0.802 and 0.820 eV. The emission at 0.807 eV, assigned in the literature to dislocations, was found only in samples where dislocations were intentionally generated by plastic deformation or induced by the clustering of self-interstitials generated during the growth of oxide precipitates. A comparison of the results of PL, DLTS, and optical DLTS measurements allows the assignment of levels involved in the radiative recombination processes.

Optimized luminescence properties of Mn doped ZnS nanoparticles for photovoltaic applications

Mn2+ doped ZnS nanoparticles (ZnS:Mn2+ NPs) are non-toxic systems known for their attractive light emitting properties. This paper discusses the luminescence properties of ZnS:Mn2+ NPs prepared by wet chemical synthesis with the objective of using them as down-shifters. A modification of the incident solar spectrum inducing improved exploitation of the UV region was expected to increase the efficiency of single junction cells with an optimal absorber band gap around 1.1 eV. The potential of ZnS:Mn2+ NPs as down-shifters was therefore demonstrated on both Si and Cu(In,Ga)Se2 solar cells.

Effect of nitrogen contamination by crucible encapsulation on polycrystalline silicon material quality

Abstract This paper reports some results about a recently discovered source of lifetime degradation found in polycrystalline silicon ingots grown in silicon-nitride-lined silica crucibles. A systematic analysis of the low lifetime areas, which preferentially are found corresponding to the ingot edges and corners, carried out using different structural, spectroscopical and electrical techniques, shows that the lifetime drop is associated with the presence of a high density of silicon nitride and iron silicide submicrometre particles, revealed by transmission electron microscopy and electron diffraction measurements. An at least partial recovery of the lifetime is obtained by P gettering at 900°C, which seems to be effective in dissolving the iron silicide from the nitride-rich areas.

Fast LBIC in-line characterization for process quality control in the photovoltaic industry

The photovoltaic industry asks for fast, non-destructive techniques for in-line characterization tools in solar cells production. We shall show in this paper that the use of the light beam induced current technique (LBIC) is capable to get in a few seconds time photocurrent maps of large area solar cells and to correlate these data with the cell efficiency. The samples analysed in this study are industrial 10 � 10 cm 2 multicrystalline silicon solar cells. The LBIC setup works with three laser sources at 633, 780 and 830 nm, taking the laser beam power below 1 mW. The laser beams are moved on the wafer surfaces using a galvanometer x2y scanner system and the beam size on the focus has a diameter of about 65mm. We demonstrated the possibility to obtain quantitative information about the cell quality in o3 s from the photocurrent maps with a pretty good correlation with the efficiency data. r 2002 Elsevier Science B.V. All rights reserved.

Dislocation luminescence in nitrogen-doped Czochralski and float zone silicon

This paper reports the results of a study of the effect of nitrogen on the optical properties of dislocations in nitrogen-doped Czochralski and nitrogen-doped float zone silicon samples where the nitrogen doping was carried out by adding Si3N4 in the molten silicon charge or by nitrogen gas dissolution. Dislocations were introduced by plastic deformation at 650?C. In nitrogen-doped plastically deformed samples, emissions in the range of the D1?D4 bands of dislocations are present with a significant shifting from the energies and intensities of the corresponding bands in nitrogen-free samples. It has been shown that the main effect of nitrogen could be the enhancement of the oxygen precipitation. The results confirm the suggestion of some of the present authors that luminescence at 0.830 eV is associated with some intrinsic properties of oxygen precipitates.

Effect of oxygen aggregation processes on the recombining activity of 60° dislocations in Czochralski grown silicon

The interaction of dislocations with light elements like oxygen and carbon presents a variety of aspects which are of basic interest for the understanding of gettering processes, as well as for a deeper knowledge of the electrical and optical properties of dislocations. We report the results of a systematic investigation of the influence of the dislocations on the segregation of oxygen and of the effect of oxygen segregation on their electrical activities. The experiments were carried out on p‐type Czochralski silicon, in the 700–1100 °C temperature range. It has been shown that not only a direct oxygen‐dislocation interaction occurs, but also that a competition between homogeneous and heterogeneous segregation processes occurs, which influences the overall electrical properties of the samples as well.

Donor−acceptor pair luminescence in compensated Si for solar cells

This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

Subband gap photoresponse of nanocrystalline silicon in a metal-oxide-semiconductor device

In this paper we report on the photoconduction and photovoltaic properties of nanocrystalline silicon. Silicon nanocrystals (Si-ncs) have been prepared by using plasma-enhanced chemical vapor deposition on a p-type silicon substrate. The Si-ncs have been formed into the dielectric of a metal-oxide-semiconductor device. I-V characteristics of the devices have been studied under dark and illumination. Illumination was performed with light in the wavelength range of 350–1630 nm. A photovoltaic effect has been observed in the illuminated I-V characteristics in the range of 350–1100 nm. For longer wavelengths no measurable photovoltaic effect has been observed, but considerable photocurrent has been measured for 1300–1630 nm light under reverse bias condition. This photoresponse is attributed to absorption through subband gap states at the Si-nc and silicon oxynitride matrix interface.

Electrical and optical characterization of Er-doped silicon grown by liquid phase epitaxy

We have carried out deep level transient spectroscopy (DLTS), optical DLTS, and capacitance–voltage measurements on liquid phase epitaxy-grown Er-doped Si to characterize the deep levels present in the material and to identify those related to dislocations and involved in the luminescence activity. The optical properties of the material have been studied by photoluminescence, cathodoluminescence investigations, and two emission lines, labeled A and B, have been found, line A being located at 0.806 eV. We have observed that an actual interaction occurs between dislocations and Er-related emitting centers.