Comparison of morphology and interfacial composition of Pd ultrathin films on 6H–SiC and 4H–SiC at different annealing temperatures

Abstract

Pd/SiC Schottky diode has been applied as a chemical sensor for hydrogen and hydrocarbon gases at high temperatures. The diffusion and interfacial reactions between the metal thin film and SiC substrate are known to alter the electrical properties of the device. In this work, the morphology and interfacial composition of Pd ultrathin films on 6H–SiC and 4H–SiC are investigated after thermal annealing using atomic force microscopy and x-ray photoelectron spectroscopy. The Pd ultrathin films of about 3 nm thickness are deposited by radio frequency sputtering. The SiC substrates are 3.5° tilted, Si-face, and n-type, 6H–SiC and 4H–SiC, and the Pd thin film deposition and annealing conditions are identical for both samples. The samples are annealed consecutively from 100 to 600 °C in air for 30 min. The Pd ultrathin film on 6H–SiC and 4H–SiC has a good uniformity as deposited, and there are no significant morphological changes for either sample at the low annealing temperatures. At 300 °C, a nanofeatured layer and Pd2Si were formed on both samples. The irregularly shaped cracks on the Pd nanofeatured layer are observed on 4H–SiC substrate, while the Pd on 6H–SiC exhibits a uniform nanofeatured layer. The interaction between Pd and 4H–SiC is more enhanced than between Pd and 6H–SiC. At 500 and 600 °C, Pd on both 4H–SiC and 6H–SiC substrates becomes nanosize clusters. Pd2Si reacted with SiC to form PdSi at 500 °C for Pd/4H–SiC, and at 600 °C for Pd/6H–SiC. The surface composition and morphology of Pd/6H–SiC and Pd/4H–SiC are almost identical after annealing at 600 °C. The Schottky barrier heights are 1.4 and 1.7 eV for Pd as deposited on 6H–SiC and 4H–SiC, respectively. No significant changes in Schottky barrier height are found on either Pd/6H–SiC or Pd/4H–SiC after annealing.