Electrical Characteristics of Ag-Pd-Cu Alloy Schottky Contacts on n-Type Al_0.6Ga_0.4N

Ultrawide bandgap (UWBG) semiconductors are of significant interest for their wide applications in optoelectronic devices such as ultraviolet light emitting diodes (UV LEDs), UV lasers, Schottky barrier diodes, solar-blind UV photodetectors, and high-electron-mobility transistors (HEMTs). Poor Schottky contacts, for example, in III-nitrides based photodetectors or HEMTs resulted in severe leakage current, high power consumption, and device failure. Furthermore, considering that AlGaN with a high aluminum content is relatively rarely studied compared to GaN, research on developing a high quality Schottky contacts with superior reliability on AlGaN is required to improve the performance of AlGaN-based optoelectronic devices.<br/>In this study, we investigated the electrical properties of Ag-Pd-Cu (APC) alloy Schottky contacts on n-AlGaN under various annealing temperature. V/Al/Ti/Au (30/200/10/50 nm) were deposited on n-AlGaN as ohmic contacts, and annealed at 900 °C for 1 min in N₂ atmosphere. APC films as Schottky contacts were deposited on the inner Schottky region, followed by annealing at 300, 500, 700 °C for 1min in air. For comparison, conventional Ni/Au (20/20 nm) Schottky contacts were also applied after annealing at 500 °C for 1 min in air.<br/>The Schottky barrier heights (SBHs) were measured using the current-voltage (I-V) method, the barrier inhomogeneity model, and the capacitance-voltage (C-V) method. The SBHs of APC contacts obtained by I-V method were in the range of 0.822-0.972 eV, depending on the annealing temperature. The SBHs of APC contacts measured by using the barrier inhomogeneity model and C-V method were in the range of 1.62-1.80 eV and 1.81-2.19 eV, respectively. On the other hand, the SBHs of Ni/Au contacts measured by I-V method, the barrier inhomogeneity model, and the C-V method were 0.788-0.820 eV, 1.43-1.69 eV, and 1.48-1.66 eV, respectively. In addition, we observed that APC samples showed more uniform leakage current properties compared to Ni/Au samples. The relative standard deviation (RSD) of leakage current at reverse bias region of annealed APC samples exhibited lower values (〈0.3) than that of Ni/Au samples (〉0.5), which means APC can provide a better reliability in chip fabrication process.<br/>The X-ray photoemission spectroscopy (XPS) were used to observe the shift of the Ga 3d core levels of the annealed APC samples toward lower binding energies after annealing, as compared to that of as-dep samples. The outdiffusion of Ga atoms from n-AlGaN was observed by the scanning transmission electron microscope (STEM) elemental mapping. The scanning electron microscope (SEM) images showed the enhanced thermal stability of APC films. The better electrical properties and reliabilities of APC Schottky contacts with various annealing temperature will be discussed based on XPS, STEM and SEM results.