At block 240 of FIG. 2, n-dopant may be implanted to the semiconductor structure 301. Referring to FIGS. 4A and 4B, n-dopant may be implanted to the semiconductor structure 301 to form n-doped semiconductor in the semiconductor structure 301. The n-doped semiconductor may be Si doped with antimony (Sb), arsenic (As) and/or phosphorous (P), and the n-doped semiconductor may be heavily doped such as, for example, n+-Si. During the ion implantation process, a mask layer, for example, a photoresist, may be formed to cover the rest of the substrate except the area above the semiconductor structure 301. In the ion implantation process, ions are implanted at a high enough energy and dose into the semiconductor structure 301 to create the n+-Si region. In some instances, a tilted angle ion implantation process may be used to implant the n-dopant to the semiconductor structure 301.
