Imaging of edge channels in the integer quantum Hall regime by the lateral photoelectric effect

We have studied experimentally the widths and positions of edge channels in an inhomogeneous two-dimensional electron gas (2DEG) under quantum Hall conditions. The locations and widths of the edge channels are controlled by a low mobility two-dimensional hole gas (2DHG) in between the GaAs substrate and the 2DEG, which acts as an interior gate. This 2DHG-2DEG system works as a p-i-n photodiode. A lateral photovoltage develops across two contacts attached to the 2DEG, when the structure is illuminated locally. By modulating the illuminated area with respect to position, the differential lateral photovoltage is obtained. We present such measurements as a function of the spot position in a sample under quantum Hall conditions. Two-dimensional images made across large areas of the Hall bar clearly show the edge channels. The measurements are interpreted in terms of macroscopically wide compressible strips alternated with incompressible strips.