Doping schemes in AlGaAs heterostructures for studies of fractional quantized Hall states

Stefan Fält (1), Christian Reichl (1), Benedikt Friess (2), Johannes Nübler (2), Jürgen Smet (2), Werner Wegscheider (1)

(1) Solid State Physics Laboratory, ETH Zürich, Switzerland
(2) Max-Planck-Institute for Solid State Research,Stuttgart, Germany 

Two-dimensional electron gases (2DEGs) in AlGaAs heterostructures can have ultra-high mobilities and are of great interest for several research topics in solid states physics. The prominent fractional quantized Hall state at ν=5/2 features several interesting properties. As this state is very fragile and it requires samples with minimized crystal and potential disorder.

We have investigated the design of 2DEGs made from AlGaAs heterostructures. The main focus is on the different doping schemes in order to reduce potential disorder stemming from randomly distributed ionized donors. This reduction can be realized by different means of creating screening layers of weakly bound electrons near the donor-atoms, capable of adapting to the irregular coulomb potential the ions generate. Devices with optimized doping schemes were studied and pronounced minima reaching zero length resistance where found in Shubnikow-de-Haas measurements at magnetic fields that correspond to the 5/2 fractional quantized Hall state.

These 2DEG structures did not require any illumination to exhibit the 5/2 state. The doping scheme consists of δ-Si doping layers in AlAs-GaAs-AlAs quantum wells. With the right thickness of these AlAs- and GaAs-layers, the X-states in the AlAs are at a slightly lower energy than the Γ-states in the GaAs-layer. The X-states are populated by quasi-bound electrons, allowing screening of the potential disorder from the Si-donors. Well-developed minima were also observed with screening electrons provided via illumination.