Intraband transitions in quantum dot–superlattice heterostructures

We present a scheme of adjusting the mid-infrared absorption properties to desired energy transitions in quantum dot-based photodetectors by combining band gap engineering with the self-organized growth of quantum dots. Embedding the self-organized InAs quantum dots into an $\mathrm{AlAs}∕\mathrm{GaAs}$ superlattice enables us to tune the optical transition energy by changing the superlattice period as well as by changing the growth conditions of the dots. Using a one-band envelope function framework, we are able, in a three-dimensional calculation, to predict the absorption spectra of these devices as well as their polarization properties. These calculations further predict a strong impact of the dots on the superlattice minibands. By comparing aligned, periodic dot stacks with nonperiodic dot arrangements within the superlattice, we can experimentally confirm this prediction.

Figure 1

(Color online) Interband photoluminescence spectra of the investigated devices. On the right: Sketches of the different device layouts. The arrow denotes $z$, the growth direction.

Figure 2

Schematic representation of the supercell including the dot and its WL. $L_1=10\mathrm{nm}$ (MD-A, MD-B) or 13 nm (MD-C) and $L_{\mathrm{Al}\mathrm{As}}=0$ (MD-A) or 1 nm (MD-B, MD-C).

Figure 3

Density of states of even-even symmetry (even wave function in $x$ and $y$) for an electron with $L=12\mathrm{nm}$ (for $k_z=0$). The energy zero is taken at the onset of the GaAs conduction band edge.

Figure 4

Absorption coefficient for in-plane polarization with $L_1=10\mathrm{nm}$ (MD-A and MD-B). The inset shows a zoom of the bound-to-continuum absorption.

Figure 5

Absorption coefficients for $x$- and $y$-oriented polarizations with $L_1=10\mathrm{nm}$ (MD-B), AlAs barrier, and a slightly elliptical dot $(R_y∕R_x=0.9)$.

Figure 6

Absorption coefficient for a $z$-oriented polarization. Full line: $L=10\mathrm{nm}$ (MD-A). Dashed line: $L=11\mathrm{nm}$ (MD-B). Dotted line: $L=14\mathrm{nm}$ (MD-C). The same scale for the vertical axis is used for the three samples (the same arb. units as in the inset of Fig. 4).

Figure 7

Current-voltage (IV) characteristics of devices MD-A, MD-B, and MD-C (inset: MD-B, DD, SD). All devices show an asymmetric diodelike behavior. The IV shape changes drastically between MD, DD, and SD devices.

Figure 8

(a) Photocurrent spectra in the mid-infrared region. (b) Far-infrared spectra: The double peak structure is attributed to the shape asymmetry of the QDs. For a comparison, the calculated spectra are plotted as dotted lines.

Figure 9

Polarization dependence of the photocurrent peak maximum. The inset shows the waveguide structure of the device with corresponding polarization axes.