Imaging the edge channel structure in a quantum Hall device

In a magnetically quantised two dimensional electron gas (2DEG) of finite dimension, the Landau levels bend up at the boundaries due to the confining potential. Edge channels are formed where these intersect the Fermi level. We have used laser imaging with a spatial resolution of 5 μm to investigate the edge channel structure in a gallium arsenide Hall bar at temperatures between 1.5 K and 150 mK. The beam from an Ar⁺ laser is focused to a small spot on the top surface of the device and the induced Hall photovoltage is measured as a function of the spot position. The size of the photovoltage depends on the potential profile in the device and, at integer Landau level filling factors, is a maximum at the edges. In our device the edge regions turn out to be very wide compared to the magnetic length.