  Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs
heterostructures are often realized by annealing of AuGe/Ni/Au that is
deposited on its surface. We studied how the quality of this type of ohmic
contact depends on the annealing time and temperature, and how optimal
parameters depend on the depth of the 2DEG below the surface. Combined with
transmission electron microscopy and energy-dispersive X-ray spectrometry
studies of the annealed contacts, our results allow for identifying the
annealing mechanism and proposing a model that can predict optimal annealing
parameters for a certain heterostructure.
