a substrate having a front surface and a rear surface opposite to each other;

a first tunnel layer formed directly on the front surface of the substrate and a first doped conductive layer formed over a side of the first tunnel layer facing away from the substrate, wherein the first tunnel layer and the first doped conductive layer are each aligned with a metal pattern region on the front surface, and the first doped conductive layer comprises a first doping element of a same type as that of a doping element in the substrate; and

a second tunnel layer formed on the rear surface of the substrate and a second doped conductive layer formed over a side of the second tunnel layer facing away from the substrate, wherein the second doped conductive layer comprises a second doping element of a different type from that of the first doping element in the first doped conductive layer, and wherein a full width at half maximum near a first peak of a Raman spectrum for the first doped conductive layer is not greater than a full width at half maximum near a first peak of a Raman spectrum for the second doped conductive layer, and a crystallite size of the first doped conductive layer is not less than a crystallite size of the second doped conductive layer.