A method includes providing a semiconductor substrate having a front side surface and a back side surface opposite to the front side surface. A photosensitive region of the semiconductor substrate is etched to form a recess. A semiconductor material is deposited on the semiconductor substrate to form a radiation sensing member filling the recess. The semiconductor material has an optical band gap energy smaller than 1.77 eV. A device layer is formed over the front side surface of the semiconductor substrate and the radiation sensing member. A trench isolation is formed in an isolation region of the semiconductor substrate and extending from the back side surface of the semiconductor substrate.