Orientation dependence of intersubband absorption in AlAs/Ga1−xAlxAs X valley superlattices

Abstract

We present the first systematic studies of infrared absorption from interconduction subband transitions for AlAs/Ga1−xAlxAs X-valley superlattices grown in the [001], [115], [113], [112], [111], and [110] directions. In the AlAs quantum well material, electrons occupy X valleys with ellipsoidal constant energy surfaces. Due to the effective mass anisotropy of electrons in the ellipsoidal valleys, these structures can absorb normally incident radiation when the superlattice growth direction is not collinear with the principal axes of at least one of the ellipsoids (i.e., not grown along the <001≳ directions). For both parallel and normal incidence radiation at wavelengths of 12–20 μm, peak absorption coefficients of 3000–6000 cm−1 were obtained for the [113] and [112] superlattices with well widths in the range of 30–50 Å and sheet doping concentrations of 1012 cm−2. Their ability to detect normally incident light and to obtain absorption comparable to that in the GaAs/Ga1−xAlxAs superlattice detectors makes these novel structures promising for use as normal incidence infrared photodetectors.