Investigating the effect of carbon on oxygen behavior in n-type Czochralski silicon for PV application

• Published in: Journal of crystal growth 2015-02, Vol.411, p.63-70
• Main Authors: Zhang, Song, Juel, Mari, Øvrelid, Eivind Johannes, Tranell, Gabriella
• Format: Article
• Language: English
• Subjects: Carbon; Copper decoration; Crystal defects; Cz silicon; Czochralski process; Density; Etching; Formations; Heat treatment; Minority carriers; Oxidation; Oxygen related defects; Silicon; Solar cells
• ISSN: 0022-0248; 1873-5002
• DOI: 10.1016/j.jcrysgro.2014.11.002

The objective of the current work was to understand the effect of carbon as an impurity in silicon in terms of the formation of as-grown oxygen defects and the subsequent behavior of these defects in n-type Czochralski (Cz) silicon during heat treatment. Three n-type Cz ingots with different carbon levels were used in the investigation. Copper decoration was used to quantify the number of as-grown defects, while a two-step oxidation process (4 h at 750°C and 16h at 1050°C) was used to study the evolution of as-grown defects, that is, the formation and morphology of oxygen precipitates (stacking faults and smaller precipitates) in the silicon during heat treatment. Carrier Density Imaging (CDI) revealed the defect distribution and distinguished their states. Results from the study show that substitutional carbon enhances the etch pit density on the copper decorated samples; indicating an enhanced concentration of defects when the carbon level in the material increases. The higher number density but smaller size as-grown oxygen defects is concluded to be induced by the presence of substitutional carbon, given the oxygen precipitate formation pattern and morphology. Vacancies introduced by carbon did not, however, affect the density of voids significantly and we hence conclude that vacancies were largely consumed by the formation of oxygen complexes, as illustrated by the presence of a higher number density of as-grown oxygen defects in samples with a high carbon concentration. The highest effective minority carrier lifetime of as-grown wafers after amorphous-Si (a-Si) passivation was found on the sample with the highest carbon concentration, and the lifetime in the all samples showed stronger dependence on the oxygen concentration than on the carbon content. •We show that substitutional carbon enhances the formation of oxygen related defects and consequently promotes Cu precipitation after copper decoration at solar cell processing temperatures.•It was experimentally verified that the presence of high [Cs] enhances the growth of spheroidal oxygen precipitates by reducing their interfacial energy•We observed suppressed growth of larger oxygen precipitates surrounded by stacking faults, and tentatively attributed these results to the presence of substitutional carbon consuming most of supersaturated interstitial oxygen•Based on results from copper decoration and two-step oxidation experiments, it was concluded that high [Cs] results in a higher number density