CARRIER TRANSPORT MECHANISMS OF P-TYPE AMORPHOUS-N-TYPE CRYSTALLINE SILICON HETEROJUNCTIONS

Measured current-voltage characteristics of undoped and p-type hydrogenated amorphous silicon (a-Si:H)/n-type crystalline silicon (c-Si) heterojunctions are used to discuss the carrier transport mechanisms. The forward current was characterized by two parts: The forward current increased with applied voltage exponentially (region 1), and nonexponentially (region 2). In region 1, it was found that the current was dominated by the tunneling process in which electrons tunneled from the c-Si into gap states in the a-Si:H and recombinated holes captured by the gap states in the a-Si:H. In region 2, the current was found to be a space-charge-limited current due to both electrons injected from the c-Si and holes injected from an ohmic contact. The carrier transport mechanism of reverse currents depended on the magnitude of boron doping in the a-Si:H. The reverse current was considered to be mainly generated in the depletion layer of the a-Si:H for the heterojunction with undoped a-Si:H, generated in the depletion layer of both the a-Si:H and the c-Si for that with boron-doped a-Si:H of B2H6/SiH4 = 1 x 10(-5), and generated in the depletion layer of the c-Si for that with boron-doped a-Si:H of B2H6/SiH4 = 1 x 10(-4).