A novel method for determining the gap-state profile and its application to amorphous Si1−xGex:H films

A novel technique has been proposed for determining the density-of-state (DOS) distribution in the energy gap of highly resistive amorphous semiconductors, using amorphous/crystalline heterojunction structures. The technique has been tested and applied to undoped hydrogenated amorphous silicon (a-Si:H) films and silicon-germanium (a-Si1−xGex:H ) alloy films, covering the optical gap (E0) range of 1.3–1.7 eV. For undoped a-Si:H with E0=1.7 eV, the peak of the midgap DOS distribution has been located at 0.84 eV below the conduction-band mobility edge, EC, with a value of 2×1015 cm−3 eV−1. For undoped a-Si1−xGex:H (E0=1.44 eV), the same has been obtained at 0.70 eV below EC, with a magnitude of 7×1016 cm−3 eV−1. Those midgap DOS have been found to be correlated with singly occupied dangling bonds, representative of a homogeneous bulk property of the material, unaffected by interface states.