Electrical characterization of ion implantation induced defect states in gallium nitride

Gallium nitride layers grown by molecular beam epitaxy on sapphire were implanted with different ion species, such as magnesium, carbon, helium, and oxygen. After implantation without a following thermal annealing procedure the layers exhibited still n-type conductivity, which is caused by additionally generated defect states. The impact of the implantation process on the deep level spectrum was analyzed by temperature dependent conductivity and thermal admittance spectroscopy measurements. As a result, we observed three implantation induced electron traps with thermal activation energies of about 300, 650, and 700 meV. Furthermore, we performed optical admittance investigations to detect transitions between very deep levels and the corresponding bands. Here, we found an implantation induced enhancement of already existing midgap states between 1.8 and 2.5 eV and a new deep trap at 2.1 eV. This implantation induced defect generation is independent of the implanted ion species, and so we attribute all these generated deep levels to intrinsic defects.