Engineering ultraclean GaAs quantum wells at high densities

High-density, ultraclean two-dimensional electron systems in GaAs quantum wells (QWs) enable exploration of exotic many-body phases such as even-denominator fractional quantum Hall states (FQHSs) at Landau-level fillings ν = 5/2 and 7/2, as well as stripe and bubble phases in higher Landau levels. Conventionally, achieving higher densities in MBE-grown, modulation-doped GaAs QWs requires reducing the setback between the δ-doped Si layer and the GaAs QW. At densities above ~4 × 10¹¹ cm⁻², however, the short setbacks enhance remote ionized impurity scattering, degrading mobility. Employing a doping-well structure (DWS) [1] offers a route to raise density by narrowing the AlAs cladding layers without reducing the setback. Our latest DWS incorporates 8.5 Å (3 monolayer) AlAs cladding layer, compared to 19.8 Å (7 monolayer) previously used [2], forming a shallow X-band feeder well that efficiently supplies carriers to the main QW. This design achieves mobilities approaching 20 × 10⁶ cm²/Vs at densities up to 6 × 10¹¹ cm⁻². These samples exhibit a robust ν = 5/2 FQHS with an energy gap of 300 mK, 2.5 times larger than previously reported in the standard δ-doped samples at similar densities [3]. We also observe numerous bubble phases between ν = 2 and 4, which were absent at such high densities in previous samples. Our results mark a new frontier for high-density GaAs heterostructures.

[1] Y. J. Chung, et al., PRM 4, 044003 (2020).

[2] Y. J. Chung, et al., Nat. Mater. 20, 632 (2021).

[3] C. Zhang, et al., PRL104, 166801 (2010).