Temperature-dependent contact resistivity of the nonalloyed ohmic contacts to p-GaN

Temperature-dependent contact resistivity of nonalloyed Pd/Pt/Au contacts to p-GaN films as well as temperature-dependent sheet resistivity of p-GaN films has been investigated in order to understand anomalously low contact resistivity (∼10−4 Ω cm2) considering the large work-function difference between the Pd and p-GaN. As the measured temperature decreases from 300 to 100 K, the contact resistivity increases by more than one order of magnitude. In addition, the sheet resistivity increases linearly with exp(To/T)1/4, implying variable-range hopping conduction via deep-level defects (DLDs). The density of the DLDs in the p-GaN films is estimated to be over 1019 cm−3, which suggests that the carriers may flow from the Pd directly to the dense DLDs, resulting in the anomalously low contact resistivity.