On the generation of bulk microdefects in phosphorus-diffused monocrystalline silicon solar wafers after a high-thermal treatment studied by X-ray topography

Abstract

Oxygen is the most important impurity in free dislocation Czochralski silicon single crystals incorporated interstitially during the growth. The knowledge of oxygen behavior after thermal processes is of great technological importance, since different kinds of bulk microdefects such us SiO2 precipitates, dislocation loops and stacking faults can be generated. In monocrystalline silicon solar cell manufacturing fabrication, there are several high-thermal treatments. The first is the diffusion process at 850–900 °C. Three different kinds of phosphorus diffusion wafers, standard PO3Cl liquid, spray-on and screen printing, were comparatively studied by X-ray topography showing that phosphorus diffusion improves the crystal quality by a gettering process whose best efficiency is in PO3Cl-diffused wafers. Later, another fabrication high-thermal step is for instance the rear surface passivation taking place at temperatures from 800 to 1,050 °C. For this reason, it is important to study how a high-thermal treatment at 1,000 °C affects the different phosphorus-diffused wafers mentioned above. To evaluate and characterize the possible defects induced by the oxygen precipitation, X-ray topography has been employed. Results show that annealed wafers are not perfect crystals; the oxygen precipitation induces the generation of bulk microdefects whose kind, size and density depend on the diffusion method employed. In PO3Cl and spray-on diffused wafers, retardation in the oxygen precipitation process takes place after annealing, while in screen printing this process is recovered and a kind of mixed defects between dislocation loops and platelet precipitates is generated.