Photoconductivity of ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ parabolic quantum wells in the optical-phonon regime

We present far-infrared photoconductivity (PC) measurements for a two-dimensional electron gas (2DEG) in an ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ parabolic quantum well grown on a GaAs substrate. By applying a magnetic field, we observe a response in the longitudinal resistance of the 2DEG caused by the bolometric effect of the cyclotron resonance (CR). We can tune the CR across the optical phonon regimes of both ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ and GaAs. In this regime, we observe both optical effects from the multilayered structure and effects of the coupling between the 2DEG and the longitudinal optical phonons. We compare our results to dielectric calculations and find that the PC signal arises from the absorption in the 2DEG but not from the total absorption of the multilayers.

Figure 1

(Color online) (a) Experimental photoconductivity spectrum of the parabolic quantum well structure for a magnetic field of 8 T. (b) Calculated spectrum of the absorption of the electron gas $\left(A_{2\mathrm{DEG}}\right)$. (c) Calculated total absorption spectrum $\left(A_{\mathrm{tot}}\right)$. The vertical lines indicate the positions of the TO phonon modes of ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ and GaAs and the InAs-like LO phonon mode of ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$. The dotted curve in (b) shows the absorption of the CR mode at this magnetic field without including the phonon modes.

Figure 2

(Color online) (a) Experimental photoconductivity spectrum of the parabolic quantum well structure for a magnetic field of 12 T. (b) Calculated spectrum of the absorption of the electron gas $\left(A_{2\mathrm{DEG}}\right)$. (c) Calculated total absorption spectrum $\left(A_{\mathrm{tot}}\right)$. The vertical lines indicate the positions of the TO phonon modes of ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$ and GaAs. The dotted curve in (b) shows the absorption of the CR mode at this magnetic field without including the phonon modes.

Figure 3

(Color online) Magnetic field dispersion of the resonance peaks of the FIR-PC spectra measured on the parabolic quantum well structure. The solid lines show the results of the dielectric calculations. The gray areas indicate the regions of the TO phonon modes in ${\mathrm{In}}_x{\mathrm{Al}}_{1-x}\mathrm{As}$. The dashed lines represent the LO phonon modes and the TO phonon mode of the GaAs substrate.