Influence of the bubbler temperature on PSG and emitter formation during the POCL3 diffusion process

Abstract

In this work we focus on the emitter formation and its dopant source, the PhosphoSilicate Glass (PSG), formed by POCl3 diffusion. An optimum emitter requires an exact adjustment of diffusion parameters, such as process temperature, time, POCl3-N2 gas flow and O2 gas flow. Another important process parameter which is normally kept constant but also has a strong influence on PSG and emitter formation, is the temperature of the POCl3 bubbler. We characterise PSG and emitter in dependence of the bubbler temperature, within a temperature range from 15.5°C to 24.5°C. PSG thickness varies in respect to bubbler temperature from 29 nm to 33 nm, which is a remarkably narrow range. Further, the total amount of phosphorus in the PSG is measured using ICP-OES. By combination of measured PSG thickness and its P dose, we determine a lower limit for P concentration in the PSG for different bubbler temperatures. On the other hand, the emitter is characterised by the active doping profile, measured by ECV. The plateau depth of these profiles depend clearly on the bubbler temperature. In addition, we show the influence of bubbler temperature on the emitter saturation current density. Finally, the effect of the bubbler temperature has been transferred to industrial screen-printed 6-inch solar cells.