An apparent metal-insulator transition in high-mobility two-dimensional InAs heterostructures

We report on the first experimental observation of an apparent metal-insulator transition in a two-dimensional electron gas confined in an InAs quantum well. At high densities we find that the carrier mobility is limited by background charged impurities and the temperature dependence of the resistivity shows a metallic behavior with resistivity increasing with increasing temperature. At low densities we find an insulating behavior below a critical density of $n_c=5\times {10}^{10}{\mathrm{cm}}^{-2}$ with the resistivity decreasing with increasing temperature. We analyze this transition using a percolation model arising from the failure of screening in random background charged impurities. We also examine the percolation transition experimentally by introducing remote ionized impurities at the surface. Using a bias during cooldown, we modify the screening charge at the surface, which strongly affects the critical density. Our study shows that transition from a metallic to an insulating phase in our system is due to percolation transition.

Figure 1

(a) Schematic of the quantum well containing InAs layer. (b) Band diagram and charge distribution of the corresponding structure shown in (a).

Figure 2

(a) Electron mobility plotted as a function of density (by varying the top gate bias). An exponent of $\alpha =0.8$ is derived from fitting the data to $\mu =n^{\alpha }$ at 2 K. (b) Corresponding magnetoresistance data measured at different densities from top to bottom: $n=2.1$ (red), 2.4, 2.7, 2.9, 3.2, 3.5, 3.8, 4.0, 4.3, 4.6, and 4.9 (blue) $\times {10}^{11}$ ${\mathrm{cm}}^{-2}$. The lowest (and highest) densities are marked in red (and blue) in (a) and (b).

Figure 3

Resistivity vs temperature at (a) $n=30\times {10}^{10}$ ${\mathrm{cm}}^{-2}$. A metallic behavior is observed. (b) For $n=3.5$, 5, and $6\times {10}^{10}{\mathrm{cm}}^{-2}$ resistivity increases with decreasing temperature.

Figure 4

(a) Conductivity measured as a function of electron density (squares) and fit to Eq. (1) (solid curve). (b) Magnetoresistance in the low magnetic field regime is shown. For clarity the zero magnetic field values are subtracted from each trace. (c) Mobility is plotted as a function of density. The slope changes near the metal-insulator transition.

Figure 5

Conductivity measured as a function of density for a structure that supports a conductive channel on the surface of the InGaAs layer (shown as inset). The sample is cooled down from room temperature with a positive bias, $V_c=+3$ V (red) and +5 V (blue). The fits (solid curves) are shown for both cases near the insulating phase.