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 Alₓ{Ga}_{1{{-}}x}$As and ${Al}ₓGa_1-xSb alloys, and both materials have the advantage over GaAs of being able to absorb light incident normal to the quantum-well plane. For a wavelength of 10 {μ}m, sheet concentration of 7.6×{}10¹¹ cm^-2, and a linewidth of 11 meV, the calculated fractional absorptions for normally incident light on a (111) Al0.5{Ga}0.5$As and (100) ${Al}0.3Ga0.7Sb quantum well are 0.000 73 and 0.0022, respectively, independent of the azimuthal angle of polarization. The absorption is stronger for Alₓ{Ga}_{1{{-}}x}$Sb than for ${Al}ₓGa_1-xAs because the ellipsoid eccentricity is greater for L valleys than for X valleys in these materials.