Sumit Mondal

Growth and electrical characterization of Al0.24Ga0.76As/AlxGa1−xAs/Al0.24Ga0.76As modulation-doped quantum wells with extremely low x

We report on the growth and electrical characterization of modulation-doped Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As quantum wells with mole fractions as low as x=0.00057. Such structures will permit detailed studies of the impact of alloy disorder in the fractional quantum Hall regime. At zero magnetic field, we extract an alloy scattering rate of 24 ns-1 per %Al. Additionally we find that for x as low as 0.00057 in the quantum well, alloy scattering becomes the dominant mobility-limiting scattering mechanism in ultra-high purity two-dimensional electron gases typically used to study the fragile nu=5/2 and nu=12/5 fractional quantum Hall states.

Exploration of the limits to mobility in two- dimensional hole systems in GaAs/AlGaAs quantum wells

We report on the growth and electrical characterization of a series of two-dimensional hole systems (2DHSs) used to study the density dependence of low temperature mobility in 20-nm GaAs/AlGaAs quantum wells. The hole density was controlled by changing the Al mole fraction and the setback of the δ-doping layer. We varied the density over a range from 1.8 × 10 10 cm −2 to 1.9 × 10 11 cm −2 finding a nonmonotonic dependence of mobility on density at T = 0.3 K. Surprisingly, a peak mobility of 2.3 × 10 6 cm 2 /Vs was measured at a density of 6.5 × 10 10 cm −2 , with further increase in density resulting in reduced mobility. We discuss possible mechanisms leading to the observed nonmonotonic density dependence of the mobility. Relying solely on interface roughness scattering to explain the observed drop in mobility at high density requires roughness parameters that are not consistent with measurements of similar electron structures. This leaves open the possibility of contributions from other scattering mechanisms at high density.