A method includes forming an emitter at the first side of a semiconductor substrate by doping, wherein the dopant concentration is higher in the emitter than in the edge region; growing an oxide layer on the first side by annealing, wherein the oxide layer has a first thickness in a first region covering the emitter, and a second thickness in a second region covering the edge region. The first thickness is larger than the second thickness. Heavy metal ions are implanted through the first side with a first energy, and with a second energy, wherein the first energy and the second energy are different, such that the implanted heavy metal concentration in the edge region is higher than in the emitter due to an absorption of the oxide layer covering the emitter, resulting in a lower charge carrier lifetime in the edge region than in the emitter.