Imaging electron density variations in a high mobility GaAs modulation-doped quantum well*

We use micro-photoluminescence (micro-PL) to measure the local electron density in a GaAs MBE quantum well under a 40 µm diameter laser excitation spot^[1]. When the laser excitation spot is raster-scanned across the sample, we observe in the PL signal, local variations in the Fermi filling that correspond to spatial variations in the electron density of the 2D system.
We find we can control the amplitude of these density fluctuations by varying the thickness of an insulating buffer layer located between the GaAs substrate and the quantum well. For a 200 nm buffer thickness the peak amplitudes of the density fluctuations are typically about 5% of the 2.6 x 10¹¹/cm² nominal electron density. Moreover, these fluctuation amplitudes decrease by a factor of ten when the buffer thickness increases by a factor of ten to 2000 nm. This suggests that the fluctuations are due to charged disorder patches located at the substrate/MBE interface. We believe these charged patches locally gate the 2D system in the nearby quantum well. The length-scale of these density fluctuations ranges from smaller than our 40 µm laser spot size to lengths as large as 700 µm.
[1] Appl. Phys. Lett. 110, 262104 (2017)