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

Abstract

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‐Si_1−xGe_x: 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, E_C, with a value of 2×10¹⁵ cm⁻³ eV⁻¹. For undoped a‐Si_1−xGe_x: H (E₀=1.44 eV), the same has been obtained at 0.70 eV below E_C, 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.