The effect of thermal annealing of Ni∕AlxGa1−xN∕GaN structures on electric properties of AlxGa1−xN∕GaN heterostructures has been studied by means of temperature-dependent Hall measurements and deep level transient spectroscopy. It is found that the mobility of the two-dimensional electron gas (2DEG) decreases from 1530to986cm2∕Vs at room temperature (RT) after annealing the Al0.25Ga0.75N∕GaN heterostructure with a 10nm thick Ni cap layer at 600°C. The density of the 2DEG is also reduced by 2.0×1012cm−2 at RT after the annealing, and decreases with increasing temperature between 100 and 460K. It is determined that an acceptorlike deep level with an activation energy of 1.23eV and apparent capture cross section of 2.8×10−13cm2 is introduced into the heterostructures. We believe that the acceptorlike deep level is induced by Ni diffusion during the annealing, and it results in the significant degradation of the transport properties of the 2DEG in the heterostructures.

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