Probing carriers in two-dimensional systems with high spatial resolution by scanning spreading resistance microscopy

In this work, cross-sectional scanning spreading resistance microscopy (SSRM) is used to profile carriers in quantum wells (QWs). The investigated structures consist of InGaAs wells of different widths sandwiched between Si-doped InP barriers. It is demonstrated that SSRM is indeed capable of detecting electrons in the quantum wells with high lateral resolution and that the SSRM signal shows a systematic trend for the different wells. Clear dips in the resistance signal are observed at the quantum wells and imply accumulated electron densities higher than in the surrounding barriers. Carrier density in the QW is found by using the calibration curve obtained from the resistance measurements on reference layers sample. It is also shown that only at certain appropriate tip-sample bias conditions the depletion regions in the barriers adjacent to the wells are resolved. Finally, we demonstrate that under very low forward biases the full width at half maximum of the observed resistance dips in SSRM data is nearly equal to the geometric QW widths.