I. Introduction
Amorphous silicon (a-Si:H) has been widely used in large-area image-sensing applications, including indirect-conversion X-ray imaging and photovoltaic energy harvesting in view of its excellent light absorption characteristics [1]. In terms of device architecture, a 2-D vertical p-i-n-type structure was initially proposed in early 1990 [2] and has been widely adopted ever since. To get around the difficulties associated with deposition of the p-doped a-Si:H layer, a much simpler indium tin oxide (ITO)/a-Si:H Schottoky photodiode was reported [3].