Carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN platform with ultra-low resistivity

In this work, the carrier transport mechanism of Mg/Pt/Au Ohmic contact on p-GaN/AlGaN/GaN with a very low specific contact resistivity of 1.98 × 10−5 Ω cm2 is investigated. Secondary ion mass spectroscopy measurement results show that the Mg concentration near the p-GaN surface increases form 8 × 1019 to 7 × 1020/cm3 for Mg/Pt/Au contact after annealing. It indicates that Mg atoms from the Mg/Pt/Au metal stack diffuse into the p-GaN during annealing, forming a heavily Mg doped p++-GaN layer with a depth of about 3 nm. The sheet resistance Rsh depends on temperature for Mg/Pt/Au contacts on p-GaN/GaN, indicating that the influence of 2DHG on carrier transport mechanisms at the metal/p-GaN interface was eliminated. For Mg/Pt/Au contacts at ≥360 K, specific contact resistivity reasonably follows T−1, which indicates that the band conduction of Schottky theory dominates the carrier transport. For Mg/Pt/Au contacts at 200–360 K, the electrical resistivity reasonably follows T−1/4, indicating variable-range hopping (VRH) conduction through Mg-related deep-level defects (DLD). Based on the VRH conduction model, the effective barrier height (qφ) of the Mg-related DLD band is extracted as 0.265 eV, which well matches the excellent Ohmic contact.