Calculation of the Intersubband Absorption Strength in Ellipsoidal-Valley Quantum Wells

A new calculation is made of the quantum-well intersubband absorption by electrons that occupy valleys with ellipsoidal constant-energy surfaces. Application of the results to the special case of spherical-valley materials yields excellent agreement with recent experimental results for GaAs quantum wells. According to the calculation, strong intersubband absorption is exhibited by ellipsoidal-valley quantum wells in indirect-band-gap AlxGa1-xAs and AlxGa1-xSb alloys, and both materials have the advantage over GaAs of being able to absorb light incident normal to the quantum-well plane. For a wavelength of 10 m, sheet concentration of 7.6×1011 cm-2, and a linewidth of 11 meV, the calculated fractional absorptions for normally incident light on a (111) Al0.5Ga0.5As and (100) Al0.3Ga0.7Sb quantum well are 0.000 73 and 0.0022, respectively, independent of the azimuthal angle of polarization. The absorption is stronger for AlxGa1-xSb than for AlxGa1-xAs because the ellipsoid eccentricity is greater for L valleys than for X valleys in these materials.