Orientation dependence of normal incidence absorption in ellipsoidal-valley quantum wells

Abstract

In this article, we present the first systematic studies of normal incidence infrared absorption from interconduction subband transitions for different orientations in ellipsoidal-valley quantum wells. Due to the effective-mass anisotropy of electrons in the ellipsoidal valleys, normal incidence absorption is allowed when the growth direction is not collinear with the principal axes of the ellipsoidal valley. From the relationships between the reciprocal effective mass tensor of the x valley and absorption coefficient, we found that in the AlAs quantum well system the absorption is near optimal for such low-index structures as [210], [320], and [113] quantum wells, contrary to the intuitive conclusion that the optimum absorption will be along [111]. For normal incidence radiation at wavelengths of 12–20 μm, peak absorption coefficients of 3000–6000 cm−1 could be obtained with well widths in the range of 30–50 Å and sheet doping concentrations of 1012 cm−2. Infrared absorption measurements were performed on samples grown on a few selected orientations including [113], [115], [111], and [100] with normal incidence radiation at wavelengths of 5–20 μm. The results showed that their absorption properties are consistent with our theoretical predictions. Long-wavelength broadband normal incidence infrared photodetectors (on [113] and [115]) were demonstrated. High infrared absorption quantum efficiency (α=2000 cm−1, η=17%), wide-band peak photoresponsivity of Rp=0.54 A/W and low dark current (ID=10−6 A at 68 K) have been achieved. These structures’ good performance make them promising for use as normal incidence infrared photodetectors.