Optical tuning of the charge carrier type in the topological regime of InAs/GaSb quantum wells

We study the optical tunability of the charge carrier type in InAs/GaSb double quantum wells with its type-II broken band alignment and inverted band structure. Under constant optical excitation, the majority charge carrier type switches from electron to hole. Within the majority charge carrier type transition, the coexisting minority charge carrier contribution indicates electron-hole hybridization with a nontrivial topological insulating phase. The optical tuning is attributed to the negative photoconductivity of antimonide materials in combination with a persistent charge carrier buildup of photogenerated charges at the surface and substrate side of the device, respectively. Our study of the tuning of an InAs/GaSb double quantum well heterostructure reveals that an electro-optical switching is possible and paves the way to an optical control of the phase diagram of InAs/GaSb topological insulators.

Figure 1

(a) Scanning electron microscopy image of the full heterostructure. The active quantum well region consists of the InAs/GaSb DQW embedded between two $\mathrm{AlA}{\mathrm{s}}_{0.11}\mathrm{S}{\mathrm{b}}_{0.89}$ barriers. The quantum well thicknesses are 10 and 12 nm for GaSb and InAs, respectively. (b) Numerical simulation of the valence and conduction band profile of the heterostructure, including the GaSb quasisubstrate, the AlSb/GaSb superlattice (SL), and the AlAsSb/GaSb/InAs/AlAsSb region with its type-II band alignment between InAs and GaSb. The localized electron $\left(e^{-}\right)$ and hole $\left(h^{+}\right)$ states are depicted in the zoom in the active region. CB and VB denote the conduction and valance band, respectively.

Figure 2

(a) $R_{xy}$ (black solid line) and $R_{xx}$ (blue solid line) versus the illumination time of the Red-LED, recorded at a constant magnetic field of 2 T and $T=4.2\mathrm{K}$. $R_{xx}$ peaks with a maximum resistance $R_{xx,max}=33.5\mathrm{k}\mathrm{\Omega }$ at $t\approx 28\mathrm{s}$, at which the Hall resistance $R_{xy}$ depicts a sign reversal from positive to negative values. Below and above the illumination time $t\approx 28\mathrm{s}$, the majority charge carrier switches from electrons to holes. (b) Hall resistance versus the magnetic field, measured for the illumination times indicated in the vertical lines and corresponding numbers from panel (a). The Hall resistance traces are color coded. In the electron regime, corresponding to low illumination times, the Hall resistance curves show well pronounced integer quantum Hall plateaus. In the hole regime, corresponding to high illumination times, the Hall resistance shows distinct two-carrier transport with a sign reversal of the Hall resistance at about $B=0.5\mathrm{T}$.

Figure 3

Charge carrier densities [in (a)] and mobilities [in (b)] of the electron and hole gases as derived from a two-carrier model versus the illumination time. For comparison, $R_{xx}$ (blue squares) for $B=0\mathrm{T}$ is also shown.

Figure 4

(a) Illumination time $\left(\mathrm{\Delta }t\right)$ required to reach $R_{xx,max}$ as a function of the Red-LED light power. For low light powers, the maximum of the longitudinal resistance is obtained at about 100 s. $\mathrm{\Delta }t$ drops to 0.6 s as the LED power increases by four orders of magnitude. The slope is determined to be −0.58. (b) Comparison of $R_{xx}$ versus the illumination time for the Red- and IR-LED. Under illumination with the IR-LED, the system switches to $R_{xx,max}$ at much longer timescales but remains high as the illumination time increases. (c) Scheme of the band profile and the light absorption and accumulation processes involved. At the GaSb cap surface region, illumination with energies above 1.55 eV leads to hole accumulation and successive electron-hole recombination at the InAs quantum well. This process only occurs for the Red-LED. On the backside of the DQW, electron-hole pairs are created for both LEDs, leading to an electron accumulation on the backside of the device. (d) Scheme of the phase diagram of a dual gated InAs/GaSb double quantum well. Via the back and front gate voltage, the system can be switched between the electron regime, the hole regime, the trivial insulating (NI) regime, and the topological insulating regime. In the presented device and under illumination, the Red-LED enables both a front and a back gating of the device, while the IR-LED only enables a back gating of the structure.