Ohmic contact characterization of AlGaN/GaN device layers with spatially localized LEEN spectroscopy

Abstract

Low energy electron-excited nano-luminescence (LEEN) spectroscopy has been used to correlate higher intensities of deep level emissions with higher ohmic contact values on AlGaN/GaN device layers. Deep level defect emissions in the AlGaN layers have also been identified by LEEN as a signature of uncharacteristically high sheet resistances for a given wafer. The spectral features observed locally at the ohmic contact region are compared with direct electrical measurements of the same device structures. Ohmic contacts formed by annealing Ti/Al/Ni/Au at 800 °C for 30 s in nitrogen range from 1.0×10 −6 to 1.0×10 −4 Ω cm 2 (0.2–3.9 Ω mm), with average sheet resistance values ranging from 650 to 2275 Ω/□ as Al mole fraction decreases. The depth dependent characterization capacity of LEEN allows the density of defect emissions to be assigned to particular interfaces. For these samples, the deep level emissions found at 2.20 and 2.60–3.15 eV found near the surface of the sample while the defect emissions associated with high sheet resistance are found to be in the AlGaN layer itself. Unprocessed material shows that samples with the lowest amounts of midgap luminescence produce the lowest contact resistance. This information can be used to determine the outcome of device fabrication prior to contact formation.