Underlying the a-Si pillar 101is a boron-doped or other p-type polysilicon electrode 130 that is in contact with a lower end face of the a-Si pillar 101 and that extends laterally away from the a-Si pillar 101 to accommodate an overlying metal electrode 125 that can be made of anysuitable metal, including, for example, a platinum group metal such as palladium or platinum.