Strong electron intersubband infrared absorption is observed for Sb δ-doped Ge/Si1-xGex multiple-quantum-well structures grown on Si (001) substrates. The intersubband absorption occurs in the Ge wells and is shown to be allowed for the optical field components both perpendicular and parallel to the quantum wells. The latter is due to the tilted ellipsoids of constant energy surfaces. An absorption coefficient of 15 000 cm-1 is measured by a Fourier transform infrared spectrometer using a waveguide structure with ten internal reflections and absorption peaks ranging from 5 to 10 μm are observed. The peak transition energy is shown to be tunable by changing the Ge composition in Si1-xGex barriers and/or the doping concentration in the Ge quantum wells.
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