Infrared emission of free carriers in semiconductors below the fundamental absorption edge

Abstract

It is shown that the IR emission spectrum of free carriers in semiconductors below the fundamental absorption edge ( ω < E g h ; E g is the forbidden band gap) can take the form of a curve with a maximum whose position is determined by the sample thickness, doping level and scattering mechanism in the material. In optically thick crystals ( K λ d > 1; K λ, is absorption coefficient and d is the semiconductor plate thickness) the emissive flux power at the maximum, allowing for reflection, reaches the black body emissive power. With decreasing crystal thickness or doping level (at a fixed temperature) the position of the maximum in the spectrum shifts to the long-wavelength region; at the same time the emission flux power falls off, while the spectral characteristic becomes less selective, mainly due to a decreasing contribution of high-frequency emission. The experiment was carried out on n-type germanium plates (5 × 10 16 ⩽ N d − N a ⩽ 5 × 10 18cm −3; 0.2 ⩽ d ⩽ 10mm) in a spectral range 5 ⩽ λ ⩽ 25 μm at 310 ⩽ T ⩽ 450 K.