Nonalloyed ohmic contacts on GaN using InN/GaN short-period superlattices

It is well known that ohmic contacts on GaN, a highly promising material for electronic and optoelectronic devices with a wide band gap of about 3.4 eV, constitute a major obstacle to further development of devices based on this material. We demonstrated a novel scheme of nonalloyed ohmic contacts on GaN using a short-period superlattice (SPS), composed of GaN and narrow band-gap InN, sandwiched between the GaN channel and an InN cap layer. Comparison with a similar layer without the SPS structure indicates that quantum tunneling through the SPS conduction band effectively reduces the potential barrier formed by the InN/GaN heterostructure leading to low contact resistivities. From the transmission-line-method measurements, specific contact resistances as low as 6×10−5 Ω cm2 with GaN doped at about 5×1018 cm−3 have been obtained without any post-annealing. Theoretical estimation based on the SPS tunneling model is consistent with the experiment.